Experimental Studies of the Effect of Air Filter Pressure Drop on the Composition and Emission Changes of a Compression Ignition Internal Combustion Engine

Abstract

This paper presents an experimental evaluation of the effect of air filter pressure drop on the composition of exhaust gases and the operating parameters of a modern internal combustion Diesel engine. A literature analysis of the methods of reducing the emission of toxic components of exhaust gases from SI engines was conducted. It has been shown that the air filter pressure drop, increasing during the engine operation, causes a significant decrease in power output and an increase in fuel consumption, as well as smoke emission of Diesel engines with the classical injection system with a piston (sectional) in-line injection pump. It has also been shown, on the basis of a few literature studies, that the increase in the resistance of air filter flow causes a change in the composition of car combustion engines, with the effect of the air filter pressure drop on turbocharged engines being insignificant. A programme, and conditions of tests, on a dynamometer of a modern six-cylinder engine with displacement V_ss = 15.8 dm³ and power rating 226 kW were prepared, regarding the influence of air filter pressure drop on the composition of exhaust gases and the parameters of its operation. For each technical state of the air filter, in the range of rotational speed n = 1000–2100 rpm, measurements of exhaust gas composition and emission were carried out, as well as measurements and calculations of engine-operating parameters, namely that of effective power. An increase in the pressure drop in the inlet system of a modern Diesel truck engine has no significant effect on the emissions of CO, CO₂, HC and NO_x to the atmosphere, nor does it cause significant changes in the degree of smoke opacity of exhaust gases in relation to its permissible value. An increase in air filter pressure drop from value Δp_f = 0.580 kPa to Δp_f = 2.024 kPa (by 1.66 kPa) causes a decrease in the maximum filling factor value from η_υ = 2.5 to η_υ = 2.39, that is by 4.5%, and a decrease in maximum power by 8.8%.

1. Introduction

There are over one and a half billion passenger cars and commercial vehicles in use in the world, and forecasts indicate that these numbers will continue to increase [1]. Road transport is the most common means of transport, and the most popular form of vehicle propulsion are internal combustion engines, among which compression ignition engines play a decisive role [2]. These engines are widely used in automotive, agricultural and industrial applications, as well as being used as stationary and military equipment, due to their high energy conversion efficiency and durability. Currently used compression ignition engines are fueled by mineral diesel derived from refined petroleum with minor additions of bio-components.

The main disadvantage of using Diesel engines is the emission of harmful substances, especially nitrogen oxides (NO_x), particulate matter (PM), carbon monoxide (CO) and unburned hydrocarbons (HC). The combustion of the fuel–air mixture is a process whose end results are often different from the desired ones. Although modern diesel vehicles provide lower fuel consumption, lower emissions and good driveability, NO_x and PM emitted by diesel engines are considered a serious public health problem and contribute to respiratory and cardiovascular diseases.

According to the authors of the paper [3], road transport is largely responsible for emissions of nitrogen oxides (30%), carbon monoxide (20%) and, to a lesser extent, for particulate matter emissions—a few percent. The authors of paper [4] analysed the actual driving conditions of passenger cars on five different routes in Delhi and showed that the average emission rates of CO, HC and NO_x were 3.99, 0.34 and 0.54 g/km for diesel vehicles, respectively. For petrol vehicles, the emission factors were higher and were: 7.26, 0.17 and 0.62 g/km, respectively. Emissions were shown to increase with increasing vehicle speed and acceleration. Moreover, the emissions were minimal at the speed of 40–60 km/h and acceleration, below 0.5 m/s². On the other hand, according to the authors of works [5,6], almost 30% of the world’s greenhouse gas emissions come from the transport sector, which leads to global warming, and compression ignition engines are the main source of airborne particulate matter.

A car engine exhaust is a mixture of many substances, molecules, compounds and groups of chemical compounds. For the most part, they are non-toxic gases normally contained in the air that humans breathe. Only a relatively small part of the exhaust gases is a burden on the environment (harmful substances), and an even smaller part has poisonous properties (toxic substances)—Figure 1. It is generally accepted that the toxic substances in exhaust gases are carbon monoxide CO, nitrogen oxides NO_x, hydrocarbons HC and particulate matter PM. The toxic components in the exhaust of an SI engine are about 1% and in Diesel engines only 0.3% [7].

PM (Particulate Matter) is soot that is a byproduct of complete and incomplete combustion of hydrocarbons caused by local oxygen deficiency. Exhaust particulate matter is not only concentrations of carbon atoms (soot), but also unburned hydrocarbons from fuel and lubricating oil, water vapor, abrasive wear products from metals, sulfur compounds and ash. Soot itself, as a chemically pure carbon, is not dangerous to the human body. It is the compounds on its surface that are dangerous.

The harmful effect of particulate matter on the environment and living organisms is due to the fact that: they persist for a long time in the atmosphere due to their small size (0.01–30 µm), so they are easily absorbed by the respiratory system. They enable heavy metals (lead), sulphur compounds, nitrogen compounds and other hydrocarbons to enter the body.

Nitrogen oxides NO_x are formed during combustion in the fuel chamber at high temperatures. They are the most toxic components of exhaust gases. Of the five different nitrogen oxides (N₂O, NO, N₂O₃, NO₂ and N₂O₄₎ in the atmosphere, NO nitric oxide and NO₂ nitrogen dioxide are most abundant. Nitric oxide NO, when absorbed into the human body, reacts rapidly with hemoglobin to form NO-hemoglobin (HBNO). Its affinity for hemoglobin is 1500 times higher than that of carbon dioxide (CO₂) [8]. The formation of NO_x is favoured by a high compression ratio, high engine load, high combustion temperature, early ignition or injection advance angle and a lack of air–fuel mixture turbulence.

Carbon monoxide (CO) is an odorless gas produced during unstable combustion processes caused by local oxygen deficiency. This compound binds easily with hemoglobin (200–300 times faster than oxygen). The formation of CO is favoured by a too-rich mixture in SI engines or a low excess air ratio in compression-ignition engines, low engine load, low temperature of cylinder walls (underheated engine, idling), late fuel injection advance angle or late ignition advance angle, small load turbulence and the use of exhaust gas recirculation [8].

Hydrocarbons (HC) emitted into the atmosphere by motor vehicles are primarily formed by complete and incomplete combustion of rich mixtures, or during local oxygen deficiency and by emissions from the fuel system. Large amounts of hydrocarbons are formed in the case of combustion of too-lean mixtures due to ignition loss and a prolonged combustion process resulting from a low combustion speed of such mixtures. The formation of hydrocarbons is favoured by the low temperature of the combustion chamber walls (underheated engine), late fuel injection or late ignition, non-uniform mixture and low charge turbulence.

In addition to fuel combustion products, motor vehicles emit particulate matter resulting from abrasive wear processes on tires and road surfaces [9,10,11,12,13,14], friction linings and brake discs [15,16,17], and clutch disc linings.

In order to minimise the problem of environmental pollution from car engine exhausts and to meet increasingly stringent emission standards, intensive research is being conducted that focuses on two directions: first, the development of technology to provide fuel economy; and second, the reduction in exhaust emissions. Research to reduce exhaust emissions is being conducted intensively in many areas. One of them is combustion management, mainly related to fuel injection control [18]. Higher emissions of particulate matter (PM) and nitrogen oxides (NO_x), and the resulting trade-off between them, are the main drawbacks of conventional diesel engines. Many researchers have shown that PM emissions from compression ignition engines are generally 10–100 times higher than those from gasoline-fueled spark ignition engines [19,20].

Combustion, exhaust emissions and performance characteristics of a diesel engine are directly influenced by several factors, including fuel injection pressure, time of injection start, amount of fuel injected, injection pattern, number of nozzles, spray pattern, etc. However, some of these parameters also have an indirect effect on engine power, and one such important parameter is heat transfer.

The parameters of the injection system and the injection characteristics have a very large influence on the combustion, emissions and performance of a compression ignition engine. The primary technique used in modern common rail injection diesel engines is the multiple injection method, which is used depending on the purpose to be served [21,22,23,24]. Depending on the engine speed and load, modern diesel engines have pilot injection, primary injection and secondary injection.

Typically, most of the fuel for the engine duty cycle is injected in the main injection cycle. Pilot injection reduces NO_x and PM emissions, reduces combustion noise and peak cylinder pressure and exhaust temperature. Final injection controls particulate emissions and exhaust gas temperature, the adequate value of which is necessary for the proper operation of exhaust after-treatment systems or turbochargers [25]. The authors of paper [26] found that final injection provides additional energy that improves mixing and accelerates soot oxidation at the end of the combustion process. They also found that end injection increases the temperature in the combustion chamber and the rate of soot oxidation. On the other hand, Kumar et al. [27] studied and compared the effect of injection start angle and intake air temperature on the combustion process and exhaust emissions in a compression ignition engine running on a mixture of ethanol and biodiesel (cottonseed oil product). An increased injection starting angle results in an earlier start of combustion relative to TDC. As the piston moves toward TDC, the charge in the cylinder is compressed, causing an increase in pressure and temperature. At the same time, there is an increase in pressure and temperature in the cylinder resulting from the progressive combustion of the mixture. As a result, there is a significant increase in the rate of heat release, which results in reduced HC emissions and increased NO_x emissions, due to the high combustion temperature. On the other hand, a delayed injection starting angle results in the opposite trend. For an extended injection timing and a higher intake of air temperature, soot and CO emissions show a decreasing trend due to the better reaction of fuel with oxygen. With shortened injection timing, soot and CO emissions show a reverse trend. Increased intake air temperature when using fuel blend containing ethanol and biodiesel resulted in an increased peak of in-cylinder temperatures. The increased charge temperature compensates for the higher heat of vaporisation of the ethanol fuel, resulting in reduced ignition delay. This results in increased NO_x emissions and decreased HC. As a result of improved in-cylinder evaporation and combustion, it can be concluded that preheating the intake air can potentially reduce CO emissions and smoke opacity.

Exhaust after-treatment technology is another area of research in reducing exhaust emissions from automobile engines [28,29,30]. There is also research work related to exhaust gas recirculation [31,32]. Mossa et al. [32] investigated the effect of hot exhaust gas recirculation (EGR) system on engine power and torque, average cylinder pressure, fuel consumption and exhaust emissions of a direct injection (DI) diesel engine. The test object was a single-cylinder, four-stroke engine with an air-cooled system with a rated speed of 3600 rpm and a displacement of 0.219 dm³. The tests were conducted in a speed range from 1600 to 3600 rpm with different percentages of EGR exhaust contributions (5%, 7%, 10% and 15%). The results showed that increasing the proportion of EGR exhaust decreased engine power and torque while increasing fuel consumption. The effect of EGR exhaust gas recirculation reduced NO_x from 800 to 240 ppm and CO₂ from 9% to 4%, while increasing CO from 2% to 4% and HC from 10 to 100 ppm.

Another method used to reduce exhaust emissions is the use of various alternative fuels to power engines. Currently, the most studied alternative fuels for compression ignition engines are biodiesel, vegetable oils, alcohols, dimethyl ether, dimethyl carbonate, natural gas, liquefied petroleum gas, methane, methane, propane, hydrogen and waste materials [33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50]. Some of these alternative fuels can be used in pure form while others are used as dual fuels, fuel blends and emulsions. For example, in [42], changes in exhaust emissions and effective parameters of an indirect injection compression ignition (IDI) engine were investigated when running on diesel fuel and in dual-fuel mode by adding compressed natural gas (CNG). The tests were carried out at three engine load steps of 100%, 75% and 50% and with different fixed CNG percentages of 20, 30 and 40%, and no gas. It was found that the addition of CNG has a positive effect on the thermal efficiency of the combustion process and a negative effect on incomplete combustion products, such as hydrocarbons (HC), which increase on average from 0 ppm to 320 ppm. Carbon monoxide (CO) increases on average by about 70% at high loads and by about 90% at medium and low loads compared to diesel fuel. Carbon dioxide (CO₂) emissions decrease by an average of 7%. Karczewski et al. [43] made an extensive analysis of the exhaust gas emission problem in the context of possibilities of its reduction by using fuels with increased H/C ratio. Such fuels may be a solution to the problem of increasing limitations of exhaust gas emission.

Kukharonak et al. [44] studied the use of n-butanol (biobutanol) in blends with gasoline in SI internal combustion engines. An experimental study of the effect of n-butanol (0%, 20% and 40% by volume) and gasoline blends on the parameters (M_o, N_e and g_e) of an SI engine was conducted. The results show that increasing the concentration of n-butanol in the mixture causes changes in engine performance. Power and torque decrease by 1.6 kW (12.7%) and 6.8 Nm (13.8%), respectively, and specific fuel consumption increases by 6.3 g/(kWh) (2%) when the n-butanol concentration is increased to 40%. The results show that, without changing the ignition advance angle αz, the gasoline mixture with 10% n-butanol has essentially no negative effect on engine power N_e, torque M_o and specific fuel consumption g_e.

Freitas et al. [45] evaluated the performance and NO_x, CO, and CO₂ content of a compression-ignition engine fueled with three blends of diesel, biodiesel, and ethanol: pure B7, B7E3 (B7 with 3% ethanol), and B7E10 (B7 with 10% ethanol). The experimental studies were performed for fixed engine speeds in the range of n = 1000–1750 rpm and for fixed loads in the range of 10–100% of maximum torque. B7 was taken as the base for the study and then ethanol additions (3% and 10% by weight) were evaluated. An increase in ethanol content in the diesel/biodiesel blend resulted in a 1.4-fold increase in exhaust emissions. An increase in engine speed resulted in a five- to seven-fold increase in exhaust emissions. As the ethanol content in the fuel blend increased, NO_x emissions were higher, with NO values being much higher than NO₂. CO and CO₂ concentrations increased with increasing ethanol content.

Yang et al. [46] believe that methanol in diesel impairs power performance but improves fuel economy and emissions in compression ignition engines. The methanol ratio should be kept at 10–15% to balance the power and emission performance.

Electric vehicle technologies are used to solve the problem of environmental pollution and to meet increasingly stringent emission standards. Pielecha et al. [48] compared exhaust emissions of two hybrid cars of the same manufacturer made in plug-in version and HEV version (petrol engine + electric motor). The comparative exhaust emission tests were carried out on a chassis dynamometer and in real conditions of use—in road traffic. The obtained values of emission of exhaust components: CO2, CO, NOx and HC were lower for the plug-in hybrid vehicle by 3%, 2%, 25% and 13%, respectively, compared to the HEV. Fuel consumption was 3% lower for the plug-in hybrid vehicle and particulate matter was 10% lower compared to the HEV. In real driving conditions, the differences were more pronounced in favour of the plug-in hybrid vehicle: CO₂ emissions in the road test were 30% lower, NO_x emissions were 50% lower, and particle counts were 10% lower.

According to the authors of the paper [50], dimethyl ether (DME) is a promising alternative to diesel in compression ignition (CI) engines used in various industrial applications. However, the high emission of nitrogen oxides (NO_x) during DME combustion limits its application. The main reason for high NO_x emission is high combustion temperature. In this study, high exhaust gas recirculation (EGR) was used while testing a CI engine with common rail direct injection, suitable (with minor modifications) for a passenger car. The modified fuel supply system provided high injection pressure during the combustion performance evaluation.

The use of alternative fuels in a compression ignition engine can have several undesirable consequences, such as increased fuel consumption and NO_x emissions, reduced engine power, piston ring jamming and sometimes cold engine starting problems [51,52,53]. These drawbacks can often be overcome by proper management of the combustion process and by using appropriate fuel additives. In recent years, nanomaterials are becoming very promising additives for diesel engine fuels. Their aim is to reduce harmful emissions from diesel engines and improve their performance [54,55,56,57,58,59,60,61,62,63]. Nanomaterials exhibit excellent properties so that they can be used as fuel additives to improve the performance of diesel engines.

For example, in [61], the effect of adding cerium oxide (CeO₂) nanoparticles, which were added to rapeseed oil (C30D) and diesel fuel at two concentrations (50 and 100 mg/L), on engine performance, NO_x and PM emissions was investigated. The addition of CeO₂ nanoparticles to C30D oil and diesel fuel was tested under a secondary fuel injection (PI) strategy. The results showed that when the engine was operated on a C30D + CeO₂ blend, the engine power increased by 9.42% compared to a diesel + CeO₂ operation with and without PI strategy. In addition, specific fuel consumption decreased by 16.46% for C30D + CeO₂ blends compared to CeO₂ nanoparticles in diesel. The results showed that the addition of CeO₂ nanoparticles with the same concentration (100 mg/L) to C30D and diesel fuel resulted in a decrease in NO_x emissions by 10.64% and 8.73%, respectively. The addition of CeO₂ nanoparticles at concentrations of 50 and 100 mg/L to C30D reduced the PM concentration in the engine exhaust by 28.62% and 42.72%, respectively, compared to engine operation on fuel without additives. In contrast, adding both concentrations of CeO₂ to diesel fuel reduced the PM concentration by 16.74 and 24.83%, respectively, compared to diesel fuel without additives. The data from this experiment showed that the introduction of the PI strategy and the addition of CeO₂ nanoparticles at 100 mg/L to the fuel has a positive effect of increasing engine performance and reducing NO_x emissions and PM concentration.

In work [62], an evaluation of specific fuel consumption and emission of selected components of exhaust gases from a six-cylinder compression-ignition engine with direct injection and a displacement of 11,051 dm³ was carried out. The engine was fed with fuel to which a catalyst containing 5% ferric chloride was added. The results obtained show that the component added to the fuel has a positive effect on exhaust emissions. There was an average decrease of 16.7% in particulate matter (PM), 10.1% in CO and 7.9% in hydrocarbons (HC). In contrast, there was a 1.2% increase in NO_x. The difference between the specific consumption of fuel to which ferric chloride was added and pure diesel fuel does not exceed the level of measurement error and amounts to 0.5%.

Adzmi et al. in [63] conducted an experimental evaluation of combustion characteristics, engine performance and exhaust emissions after addition and mixing of nanoparticles with palm oil methyl ester (POME) on a single-cylinder compression-ignition engine. Aluminum oxide (Al₂O₃) and silicon dioxide (SiO₂) nanoparticles at 50 ppm and 100 ppm were used. SiO₂ and Al₂O₃ were blended with POME and designated as PS50, PS100 and PA50, PA100, respectively. The test results for PS and PA fuels were compared with those for the POME test fuel. The tests were conducted at engine loads of: 7 Nm, 14 Nm, 21 Nm and 28 Nm and no load, at a constant engine speed n = 1800 rpm. The results show that the highest maximum in-cylinder combustion pressure obtained for the fuel containing nanoparticles is 16.3% higher compared to the POME fuel. Moreover, the peak engine torque and engine power show a significant increase of 43% and 44%, respectively, recorded in the test with PS50 fuel. For the fuel containing nanoparticles, NO_x emissions decreased by 10%, CO₂ emissions decreased by 6.3% and CO emissions decreased by 0.02%.

An important area of efforts to reduce carbon dioxide (CO₂) emissions into the atmosphere is the use of hydrogen fuel cell vehicles [64].

From the above analysis, there is a wide range of possibilities to reduce exhaust emissions from engine exhaust systems, but in many cases, this involves a decrease in engine performance.

One of the factors that determine the changes in exhaust emissions from internal combustion engines is the pressure drop of the intake system, and mainly the air filter pressure drop defined as the static pressure drop downstream of the filter. The air filter is an important component of an internal combustion engine that is the power unit of a motor vehicle, as its performance determines the reliability and life of the engine. Suitable purity of the air intake to the combustion engines of commercial vehicles is ensured by baffle air filters, where the filtering element is a pleated paper insert. A characteristic feature of baffle filters is that during operation, as a result of the deposition and accumulation of dust particles in the filter bed, its capacity decreases and the air filter pressure drop Δp_f increases steadily. The higher the value of the dust concentration in the air sucked into the engine, the faster the filter reaches the permissible value—Δp_fdop. The pressure drop of the air filter hinders the air flow to the engine cylinders, which results in a decrease in the engine filling and power, and an increase in specific fuel consumption. In the available literature, it is very difficult to find a sufficiently complete picture of the effect of air filter pressure drop on engine performance metrics, especially on changes in exhaust emissions. In the available literature, there is a full description of the study of the effect of air filter pressure drop on the performance of naturally aspirated carbureted and diesel internal combustion engines with a classic injection system with an in-line piston (sectional) injection pump [65,66,67,68]. The engines of modern passenger cars are equipped with gasoline direct injection systems with an electronic control system, while the engines of trucks are equipped with high-pressure diesel injection systems, such as common rail or systems with electronically controlled pump injectors. The ECU (Engine Control Unit) of the engine controls all the quantities which affect the value of the generated torque produced by the engine, while at the same time meeting the requirements in the area of exhaust emissions and fuel consumption throughout the life of the vehicle. In the available literature, the results of empirical investigations determining the influence of the flow resistance of the air supply system, including the filter, on the performance of a modern car engine and, in particular, on the composition of exhaust gases, are not encountered very often. Empirical tests are expensive and labour intensive, which explains the scarce number of available results. It is considered to be the research method that produces the most reliable results.

Dziubak and Karczewski in [69] carried out extensive experimental research on an engine dynamometer on the influence of air-filter flow resistance on the basic parameters of operation (filling factor, power and specific fuel consumption) of a modern Diesel engine equipped with the common rail fuel supply system. The investigated engine is a driving unit of a truck tractor. The present work is a continuation of the experimental research, the results of which will allow to fill the gap in the scope of the influence of the flow resistance of the air supply system on the change of the quantitative and qualitative composition of the exhaust gases from the modern Diesel engine.

2. Literature Analysis on the Effect of Intake System Pressure Drop on Engine Performance and Exhaust Emission Change

As dust settles on the surface of the filter material, the air flow decreases due to increasing resistance to air flow through the filtration system. The operation of an engine with an air filter of increased pressure drop causes a decrease in the cylinder filling with fresh charge and torque, resulting in a decrease in maximum engine power. The result is a reduction in the dynamic properties of the vehicle. A high air filter pressure drop (a significant vacuum created behind the filter) can cause the breakaway forces to exceed the grain adhesion forces to the filter substrate. The result of this phenomenon can be an avalanche of dust detachment (so-called secondary emission), and sucking it back together with the air into the engine cylinders will cause accelerated wear of P-PR-C (piston-piston ring-cylinder) components. In extreme cases the high pressure drop of the air filter can cause mechanical damage (breakage, rupture) of the filter insert, which will result in increased wear of engine components. For this reason, when the filter reaches a certain resistance value, it is necessary to service the air filter by replacing the filter cartridge. In passenger cars, due to low values of dust concentration in the air, and thus small increments of the filter pressure drop, this operation is performed depending on the mileage of the vehicle or its operation time.

Trucks, off-road vehicles, and special vehicles are operated with high and variable dust concentrations in the air. For this reason, the increase in pressure drop is significant (by 5–8 kPa), and occurs with varying intensity. If the systematic replacement of filter cartridges is not observed, an additional increase in air filter pressure drop occurs, resulting in a significant decrease in engine power, an increase in fuel consumption and a decrease in the dynamics of vehicle movement.

The paper [65] presents the characteristics of the filling ratio η_υ = f(n), power N_e = f(n) and torque M_o = f(n) o and specific fuel consumption g_e = f(n) of the six-cylinder naturally aspirated (V_ss = 6.842 dm³) 359M Diesel engine with a classic injection system, for three values of the air filter pressure drop Δp_f = 2.3, 6, 12 kPa [65]. As the engine speed increases in the range of n = 1200–2800 rpm, regardless of the value of the air filter pressure drop, the characteristics of filling η_υ = f(n), power N_e = f(n) and torque M_o = f(n) shift almost in parallel toward lower values, and specific fuel consumption g_e = f(n) toward higher values. The increase in the air filter pressure drop in the range of 2.3–12 kPa, when the engine is operated at engine speed n = 2800 rpm and at 100% load, causes: a decrease in fill factor by 25.7%, a decrease in power by 7.16% and an increase in specific fuel consumption by 8.49%. An increase in the air filter pressure drop by 1 kPa causes an average decrease in the filling factor by 2.65%, a decrease in power by 0.739% and an increase in specific fuel consumption by 0.876%. The author did not conduct any research on changes in exhaust emissions.

The results of investigations into the effect of the air filter pressure drop Δp_f on the characteristics of the external effective power N_e and specific fuel consumption g_e of the Diesel engine of a special vehicle are presented in [66]. A 12-cylinder (V-system) naturally aspirated engine with a displacement of 38.88 dm³ and rated power of 430 kW (580 hp) at n = 2000 rpm, with a classical injection system and multi-range speed controller (direct fuel injection) was studied. The increase in air filter pressure drop was modeled in the range Δp_f = 3–30 kPa. The effect of air filter pressure drop is not apparent until Δp_f = 6 kPa is reached. A further increase in air filter pressure drop already causes a significant decrease in engine power and an increase in specific fuel consumption, as well as a parallel shift in the external characteristics of power and specific fuel consumption toward lower values of N_e power and fuel consumption g_e with a simultaneous shift toward lower rotational speeds. For air filter pressure drop Δp_f = 26.7 kPa, the decreases in power N_e take the values: 11.75% at 2000 rpm, 20.6% at n = 1400 rpm and 32.7% for 1200 rpm. The author has not carried out any studies on changes in exhaust emissions.

Dziubak and Trawiński [67] presented experimental research on the effect of air filter pressure drop in the range of Δp_f = 3.1–24.7 kPa on the filling factor and smoke opacity of the turbocharged, six-cylinder (V_ss = 6 dm³) Diesel T359E engine with a classical injection system. The tests were carried out using the method of free acceleration of the engine loaded with its own resistance torque and its total moment of inertia, hereinafter referred to as the “dynamic characteristics” method, which makes it possible to determine the course of the instantaneous value of the engine torque without loading it on the dynamometer bench. In the final stage of the engine acceleration process for the air filter with a clean filter cartridge (Δp_f = 3.1 kPa), the filling factor has a value of 1.02. With the increase in the filter pressure drop, the filling factor takes on smaller and smaller values, respectively: 0.90; 0.81; 0.75. Thus, an increase in the air filter pressure drop Δp_f by 1 kPa causes a decrease in the filling ratio by 1.49%, 1.29% and 1.23% on average. The value of the air filter pressure drop Δp_f = 24.7 kPa, i.e., eightfold increase in the air filter pressure drop Δp_f over the value of the initial air filter pressure drop Δp_f0, causes a twofold increase in the smoke opacity of the T359E engine (increase in the light absorption coefficient) to k = 0.81 m⁻¹. This does not cause the T359E engine to exceed its maximum value–k_max = 3.0 m⁻¹.

The results of the study of the effect of the baffle filter on the characteristics of the UTD-20 internal combustion engine of a special vehicle are presented in [69]. A compression-ignition engine of displacement V_ss = 15.8 dm³ and rated power of 226 kW with a classical fuel injection system was used. The effect of the original air filter with a pressure drop of Δp_f = 13.2 kPa and the modernised filter–Δp_f = 4.9 kPa was studied, as well as the modernised filter working with the engine with an increased (about 7%) fuel dose. During the operation of the engine with an increased fuel dose and with the modernised air filter, a significant increase in power and torque was obtained in comparison with the basic variant of the filter: over 2% for 1600 rpm and over 10% for the 2200–2600 rpm range. However, a slight (2%) increase in specific fuel consumption was recorded. The increase in engine power occurred as a result of increased air mass resulting from reduced air filter pressure drop, an increase in engine fill and an increase in fuel delivery. The authors did not study the effect of air filter pressure drop on the composition of exhaust gases.

Yang et al. [70] studied the effect of two different air filter designs characterised by different pressure drop (A—standard filter, B—upgraded filter with lower pressure drop), on the speed characteristics of a single cylinder compression ignition engine. The effective power, torque, specific fuel consumption, smoke, oil temperature and engine exhaust temperature were determined for these two filters and compared with the engine operating parameters without air filter. The engine working without air filter obtained the highest power and torque, and the lowest fuel consumption. The operation of the engine with filter B and A successively causes a shift of the characteristics N_e = f(n) and M_o = f(n) almost in parallel toward smaller values, and the characteristic g_e = f(n) toward larger values, over the entire engine speed range. With the air filter type B, the torque increases its value and is M_omax = 73.6 Nm at 1500 rpm, and the fuel consumption decreases to g_e = 230.8 g/(kWh). This is 1.6% more torque and 1.5% less fuel consumption than with air filter type A (higher pressure drop air filter). Maximum power increases by 1.1% when a lower air filter pressure drop (type B) is used compared to an air filter type A. Over the entire power range, the exhaust emissions are naturally low without air filter and high with air filter type A. The maximum smoke value (light absorption coefficient) increases with an increasing load and during engine operation without air filter, with air filter type A and with air filter type B is: k = 1.7, 2.36 and 2.0 m⁻¹, respectively. After replacing type A air filter by type B filter, the reduction in smoke value by 11% was obtained.

Plotnikov et al. [71] presented the results of numerical simulations of the effect of geometrical characteristics of the intake duct on wave phenomena and pressure drop in the intake system of an eight-cylinder turbocharged diesel engine. Numerical studies of the effective parameters of the engine were carried out for the original intake duct length L = 1625 mm and diameter D = 156 mm and three other internal diameters: D₁ = 80 mm, D₂ = 250 mm and D₃ = 330 mm. It is shown that the geometric dimensions of the intake pipe have a significant effect on the dynamics and pressure drop of the engine intake system. Decreasing the inner diameter to 80 mm leads to significant pressure fluctuations in the intake system, an increase in pressure drop and a decrease in diesel engine power by 0.5–2.5%. Increasing the inside diameter to 250–330 mm leads to a slight smoothing of the pressure amplitude in the inlet pipe, a decrease in pressure drop and, thus, an increase in fill factor of 0.5% on average (Figure 2a). This leads to an increase in engine power by up to 0.7% and a decrease in effective specific fuel consumption by approximately 0.50–0.75 (Figure 2b).

Abdullah et al. [72] investigated the fuel consumption and exhaust emissions of a carbureted engine as a function of the pressure drop of the intake system. The study was conducted while the engine was running with and without air filter. The hourly fuel consumption with air filter increases from 0.687 dm³/h to 1.028 dm³/h, i.e., by 49.6%, when the engine is operated at a constant load in the speed range of 1500 rpm to 2500 rpm. In contrast, when the engine is operated under the same conditions but without an air filter, the hourly fuel consumption takes on a lower value and increases by only 35.2%.

The NO_x content of the exhaust, when the engine is operated without an air filter, is higher than that with an air filter (Figure 3). At speed n = 1500 rpm, the NO_x content of the exhaust without air filter is 51.7% higher than when the engine is operated with air filter. For higher speeds of 2000 and 2500 rpm, the NO_x content in the exhaust without air filter is 8.1% and 20.2% higher than that with air filter, respectively. The formation of NO_x is affected by the maximum temperature and pressure of the combustion process. Without air filter, the combustion process is more favourable, which leads to a higher temperature and higher charge pressure, which promotes NO_x formation. The higher pressure drop when the engine is operated with an air filter reduces NO_x formation.

Shannak et al. [73] measured the exhaust emissions of a four-cylinder, four-stroke gasoline engine as a function of the pressure drop of the intake system, the value of which was varied using different air inlet pipe diameters: 20, 25, 30, 35, 40 and 63 mm. The tests were conducted at engine speeds ranging from 1000 to 4000 rpm. The results showed that the amounts of hydrocarbons (HC) and carbon monoxide (CO) decrease with an increasing air intake pipe diameter, increasing engine speed and atmospheric pressure and decreasing the altitude at which the engine operates, while CO₂ and oxygen (O₂) remain constant. Increasing the diameter of the inlet pipe from 20 to 63 mm, which is equivalent to a decrease in pressure drop, and increasing the altitude at which the engine operates from 600 to 800 m above sea level, corresponding to a change in atmospheric pressure from 0.94 to 0.91 bar, leads to a reduction in hydrocarbons of about 60% and a reduction in carbon monoxide of about 40% while keeping CO₂ and oxygen constant at about 12.7% and 0.63%, respectively. Increasing the engine speed from 1000 to 4000 rpm leads to a HC reduction of about 45% and a CO reduction of about 25%, while keeping CO₂ and oxygen constant at about 12.7% and 0.63%, respectively.

Thomas et al. [74] studied the effect of air filter pressure drop on emission changes of three truck ZS engines. The results for a Dodge Ram 2500 Truck-6.7 L (2007) with a 6.7 dm³ inline six-cylinder engine with variable geometry turbocharging, six-speed automatic transmission, a diesel particulate filter (DPF) and a NO_x reduction system (LNT) are shown in Figure 4. An increase in air filter pressure drop from 0.3 kPa to 3.9 kPa and then to 7.6 kPa results in a slight increase in CO, HC and fuel consumption, and a slight decrease in CO₂ and NOx. A significant (from 0.3 to 7.6 kPa) increase in air filter pressure drop causes a slight change in exhaust emissions, which is due to the equipment that this car is equipped with, namely turbocharging to provide adequate airflow, and a diesel particulate filter (DPF) and NO_x mitigation system.

In order to protect the engine against excessive power reduction and increased exhaust emission caused by the increase in the air filter pressure drop, special sensors of the permissible pressure drop ∆p_fdop [75,76] are installed in the intake system. Reaching the set value of the flow resistance by the sensor (most often it is at the maximum air flow rate for a given engine) is the signal for air filter servicing—replacement of the filter insert. For trucks and special vehicles, it is assumed that the Δp_fdop values are from 6.25 to 7.5 kPa above the flow resistance of a clean air filter [77]. The value of Δp_fdop for passenger car engines is assumed to be from 2.5 to 4.0 kPa and, for truck engines, from 4 to7 kPa [78]. For special vehicle engines, the values range from 9 to 12 kPa [79].

The initial efficiency of the air filter (after replacing the filter cartridge with a new one) is low and, depending on the type of filter material, is about 96–98%, but at the end of the service life it increases significantly to over 99.9% [80]. Therefore, it is not advisable to replace the filter cartridge before reaching the established service life. On the other hand, frequent (unjustified) replacement of the filter element may result in premature engine wear. The operation of an air filter is a technical compromise between the increase in air filter pressure drop causing a decrease in engine power output, an increase in fuel consumption and exhaust emissions and the efficiency and accuracy of filtration, factors that determine wear and tear and the durability and reliability of a vehicle engine.

The above analysis shows that, as of today, in the available literature there are no results of investigations of the influence of the pressure drop in the inlet system on the performance of a modern ZS engine used for the propulsion of trucks/truck tractors currently on the roads and constituting the basic means of transport of goods. There is no sufficient data to identify in an unambiguous way the influence of the pressure drop in the inlet system on the composition of the exhaust gases and the emission of toxic compounds. Therefore, it is purposeful to determine experimentally the relation between the parameters of operation of the inlet system of a modern truck engine and the performance of this engine in terms of changes in the emission of individual components of exhaust gases.

3. Experimental Studies on the Influence of Air Filter Pressure Drop on the Performance of the Diesel Engine

3.1. Purpose and Focus of the Study

The aim of the research is an experimental evaluation of the influence of the air filter pressure drop Δp_f on the operation parameters represented by the composition and emission of particular exhaust components, as well as on the opacity of exhaust gases from a modern Diesel engine with an electronic fuel supply control system and a supercharging and air cooling system.

The test object was a six-cylinder, in-line Diesel engine with direct fuel injection—a supply system with electronically controlled injectors, of the Volvo D13C460 EURO V EEV truck with maximum power of 338 kW, being a driving unit of the Volvo FH13 truck tractor with the mileage of 790,500 km. The engine is equipped with four valves per cylinder, which are controlled by hydraulic tappets driven from the central camshaft located on the head. This shaft also drives electronically-controlled pump injectors using a piezo-quartz valve. The basic parameters of the engine are given in

Table 1. Basic parameters of the D13C460 EURO V engine [72].

Name of Device	Range
Engine type	D13C460 EURO V
Maximum power at 1400–1900 rpm	460 hp (338 kW)
Maximum engine speed	2100 rpm
Maximum torque at 1000–1400 rpm	2300 Nm
Number of cylinders	6
Cylinder diameter	131 mm
Piston stroke	158 mm
Displacement	12.8 dm3
Compression ratio	17.8:1

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The air supply system consists of an air intake located on the right side of the cabin at its highest height, an external intake duct of nearly rectangular cross-section located on the rear wall of the cabin, a rubber folding element connecting the external duct with the air filter intake duct and a single-stage (baffle) air filter. The filtering element is a cylindrical insert made of pleated filtering paper with active surface A_c = 13.72 m². On the outlet pipe from the air filter there is a sensor of acceptable pressure drop set at Δp_fdop = 4.8–5.0 kPa. Suitable filling of engine cylinders is assured by turbocharger and charge air cooler operating in an ″air-to-air″ system. A detailed description of the engine and intake system can be found in [69].

3.2. Test Methodology and Conditions

Tests were carried out on a standard dynamometer bench. The motor was loaded with a water brake type Zöllner PS1-3812/AE with a maximum power of 1250 kW. The torque M_o generated by the engine was measured with a strain gauge transducer connecting the swinging brake housing to the foundation. Hourly fuel consumption G_e was measured using an AVL fuel balance with a 5 s time interval and then averaged over a 60 s interval. The coolant temperature t_ch during engine testing was set equal to the operating temperature of t_ch = 87–92 °C and was maintained using an external heat exchanger. The opacity of the exhaust gases was determined with the AVL 439 OPACIMETER, which works on the principle of measuring the light absorption coefficient

Flue gas composition was measured using an Atmos FIR analyser operating on a hot 180 °C sample using Fourier Transform Infrared Spectroscopy (FTIR) and with a zirconia cell oxygen analyser built into the measuring chamber. The device has a Certificate confirming compliance with the requirements of the EN 15267-1:2009, EN 15267-2:2009, EN 15267-3:2007 standards. In the used configuration, the device allowed to measure the following components of the exhaust gases: NO, NO₂, N₂O, SO₂, CO, CH₄, CO₂, O₂, H₂O. The measurement results were automatically converted to normal conditions (p_n = 101 325 Pa, T_n = 273.15K). The spectral operating range of the analyser was 485–5500 cm⁻¹.

The volumetric air demand Q_s by the engine was recorded with a thermo-aerometric flow transducer. The air filter pressure drop Δp_f was determined as the pressure difference p₁ before and p₂ after the air filter, with the use of a TESTO 400 differential pressure gauge. The detailed test methodology was described in [69].

During the tests, for each speed of rotation, the following parameters of engine operation and parameters of the air flow in the intake system were measured directly:

• engine torque, M_o [Nm],
• engine rotational speed, n [rpm],
• hourly fuel consumption, G_e [kg/h],
• engine air demand, Q_s [m³/h],
• exhaust gas temperature, t_s [°C],
• exhaust gas opacity—light absorption coefficient, k [m⁻¹],
• air pressure before p₁ and after air filter p₂, [kPa],
• charge air pressure, p_d, [kPa],
• exhaust gas components: NO, NO₂, N₂O, SO₂, CO, CO₂, O₂, H₂O.

Based on the directly measured values of engine operating parameters, the following were determined:

• effective engine power, N_e [kW],
• specific fuel consumption, g_e [g/(kWh)],
• air filter pressure drop Δp_f [kPa].
• emission of individual exhaust components: NO, NO₂, N₂O, SO₂, CO, CO₂, O₂, H₂O,
• relative change in emission of exhaust gas components.

The emission of particular exhaust components was determined according to requirements contained in [81]. NO_x concentration was determined as the sum of NO+NO₂ components. All tests were repeated in duplicate to eliminate coarse errors that could lead to incorrect inference. Control of engine load, engine speed and brake load was performed from the control and measurement cabin, where gauges for measuring engine performance are also located. During the tests the engine operation was continuously controlled by using the diagnostic interface NAVIGATOR TXTs with the software IDC 5 TRUCK. The schematic diagram of the test stand is shown in Figure 5. The applied measurement equipment and its accuracy are presented in

Table 2. List of investigation equipment used during investigation.

No.	Name of Device/Measured Quantity	Type	Range	Accuracy
1.	Water dynamometer torque—Mo rotated speed—n	Zöllner PS1-3812/AE	Mo = (0–7000) Nm n = (0–3000) rpm Ne = (0–1250) kW	± 1 Nm ± 1 rpm ± 1 kW
2.	Fuel weight-meter (diesel)—Ge	AVL 733S Fuel Balance	(0–200) kg/h	± 0.005 kg/h
3.	Smoke concentration—extinction coefficient of light radiation—k	AVL Opacimeter 4390	(0.001–10.0) m−1	± 0.002 m−1
4.	Exhaust analyser—measuring of toxic elements concentration in exhaust gases carbon dioxide (CO2) carbon monoxide (CO) nitrogen oxides (NO) nitrogen diooxide (NO2) oxygen (O2) hydrocarbons (HC) water (H2O)	Atmos FIR emissions monitoring FTIR systems	CO2 (0.01-23) % CO (1.0–11,000) ppm NO (1.0–6000) ppm NO2 (1.0–6000) O2 (0.1–21) % HC (1.0–5000) H2O (0.25–25) %	± 0.1% measured quantity
5.	Thermocouple—measuring of exhaust temperature—ts	NiCr—NiAl (K)	(–50–1100) °C	± 1 °C
6.	Mass air consumption—Qs	SensyMaster FMT430 Thermal Mass Flowmeter	(100–6000) m3/h	± 1.0 m3/h
7.	Vacuum in the intake system	TESTO 400	(–100–200) hPa	0.3 Pa + 1% measured quantity

The results of engine operating parameters: power and hourly fuel consumption, obtained during the tests, were reduced to normal conditions in accordance with PN-ISO 15550:2009 [82].

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3.3. Analysis of Research Results

During the experimental research, the influence of four (New, A-33, B-66, C-90) technical states of the same air filter on the external characteristics of the Volvo D13C460 EURO V engine, differing in pressure drop, was determined. In each case the same parameters characterising the engine operation were measured.

An increase in the value of the filter pressure drop Δp_f was modelled by means of obscuring a part of the active filtration surface of the cylindrical cartridge. As a result, four technical conditions were obtained differing in the value of the pressure drop of the same air filter. At rotational speed n = 1900 rpm, the pressure drop of the air filter obtained the following values:

• technical condition ″New″—filter with clean, brand new, paper air filter cartridge, Δp_f = 0.580 kPa,
• technical condition A-33—air filter with an air filter insert that has had approx. 33% of its active filtration area obscured, Δp_f = 0.604 kPa,
• technical condition B-66—air filter with a filter insert, which is obscured by approx. 66% of the active filtering surface, Δp_f = 0.757 kPa,
• technical condition C-90—air filter with a filter insert, which has approx. 90% of its active filtering surface obscured, Δp_f = 2.024 kPa.

Figure 6 shows the characteristics Δp_f = f(n) and engine air demand Q_s = f(n) of the same air filter for four technical states (New, A-33, B-66, C-90) as a function of engine speed n of a Volvo D13C460 EURO V engine. As the engine speed increases in the range of n = 1000–2100 rpm, the pressure drop of the air filter, independently of the pressure drop (percentage obscuration of the active area of the cartridge), increases its value until the engine speed reaches n = 1900 rpm, which is related to the achievement of the maximum air demand of the engine (Figure 6) and the maximum power. Further increase in engine speed causes a decrease in filter pressure drop, which results from a decrease in the air flow rate Q_s. When the engine was operated with an air filter in A-33 condition (33% obscuration of the active surface of the cartridge), no significant differences were found in the pressure drop values compared to a new filter (New). The recorded differences of the pressure drop are within the limits of the measurement errors and do not significantly affect the other parameters of engine operation presented in the following figures. The analysis of the test results was carried out within the whole operating speed range of the engine, which is the driving unit of the truck tractor.

Figure 7 shows the charge air pressure pd in the intake manifold of the VOVLO DC13C460 engine as a function of rotational speed n for different technical states of the air filter: New, A-33, B-66, C-90. As the engine speed increases, regardless of the technical state of the filter, the charge air pressure of the engine increases quite rapidly and in the range n = 1400–1500 rpm reaches maximum values, after which it systematically decreases. The highest values of the decrease in the charge pressure in the intake manifold, caused by the increase in the air filter pressure drop, were recorded in the range of rotational speeds n = 1300–1900 rpm, with the characteristics p_d = f(n) shifted almost in parallel toward smaller values of pd. At rotational speed n = 1600 rpm, the decrease in the charge pressure pd, caused by the increase in the air filter pressure drop (A-33, B-66, C-90) over the value of the air filter pressure drop of the new “New” filter, assumes the following values: 0.389%, 1.95%, 4.28%.

Figure 8 shows the filling characteristics η_υ = (n) as a function of the speed n of the Volvo D13C460 EURO V engine, for four (New, A-33, B-66, C-90), differing in pressure drop, technical states of the air filter. As the engine speed increases, irrespective of the technical condition of the filter (percentage obscuration of the active surface of the cartridge), the engine filling factor increases quite rapidly and, in the range n = 1400–1500 rpm, reaches its maximum value (η_υ = 2.5), after which it systematically decreases. When the engine reaches n = 1900 rpm, the decrease in the engine fill factor is already steep, which is associated with a sharp drop in boost pressure. As the air filter pressure drop Δp_f increases, the filling characteristics η_υ = (n), in the speed range n = 1000–1900 rpm, are shifted almost parallel toward smaller values. The increase in the air filter pressure drop from a value Δp_f = 0.580 kPa (technical condition “New”) to Δp_f = 2.024 kPa (C-90) causes a decrease in the maximum value of the filling factor from η_υ = 2.5 do η_υ = 2.39, i.e., by 4.5%. The fill factor η_υ was determined from the engine speed, engine displacement, and flushing ratio. Due to the constant, angle of coverage (valve co-opening) independent from the rotational speed, the flushing coefficient was assumed to be constant at the level of 1.0. The valve co-opening angle is the angle of rotation of the crankshaft corresponding to the period during which the intake and exhaust valves of one cylinder are open at the same time.

The highest smoke opacity was recorded in the speed range n = 1000–1100 rpm (Figure 9). However, as the engine speed increased, the smoke level decreased rapidly, irrespective of the technical condition of the air filter, and in the speed range n = 1100–1700 rpm it remained stable, after which it increased slightly. However, the increase in air filter pressure drop does not cause significant changes in the degree of smoke opacity in relation to its permissible value, defined in the technical conditions of vehicle operation for this type of vehicles at 1.5 m⁻¹ [83].

Figure 10 shows the effect of four (New, A-33, B-66, C-90) air filter conditions, differing in pressure drop, on the characteristics of the effective power N_e = f(n) of the Volvo D13C460 EURO V engine. As the rotational speed increases, the effective engine power Ne, irrespective of the technical condition of the air filter, increases sharply in value until the engine reaches a rotational speed of n = 1400 rpm, and then decreases slightly until a rotational speed of n = 1900 rpm is reached, after which it loses its value sharply. The use of an air filter Δp_f with an increasing pressure drop, according to the technical states (A-33, B-66, C-90), causes a shift of the characteristics N_e = f(n) in the rotational speed range n = 1400–1900 rpm, almost in parallel toward the lower values of the engine power.

Figure 11 shows the relative decrease in engine power for each speed caused by successive states of the “k” air filter: A-33, B-66, C-90, in relation to the maximum power of the engine operating with air filter “New”. For the engine operating with the air filter “A-33” the power changes are practically imperceptible and oscillate at around 0.5%. Further increase in the resistance of the air filter flow significantly affects the relative decrease in the effective power of N_e engine. In the case of B-66 these changes reach about 4%, while in the case of operation of the engine with the filter in the state C-90, the relative decrease in power exceeds 9%. It can be emphasised that these changes are the greatest in the medium speed range (n = 1300–1900 rpm), in which engines of this type are most often used, which is a very unfavourable phenomenon. The decrease in power due to air filter contamination for a tractor-trailer type vehicle has a very negative effect on its traction characteristics. As the pressure drop increases and the power decreases, the traction properties of the vehicle deteriorate, in particular: the ability to climb a hill in individual gears, the change in the maximum speed value depending on the road inclination angle and the time and distance of acceleration of the vehicle-tractor-trailer combination [84].

Figure 12 shows the proportion of carbon dioxide CO₂ in the exhaust gas of the Volvo D13C460 EURO V engine as a function of the rotational speed n for four different air filter technical states k: New, A-33, B-66, C-90. As the engine speed increases, the CO₂ contribution, regardless of the technical state of the air filter, decreases (almost linearly) its value until the engine reaches the maximum speed n = 2100 rpm. The change of the engine rotational speed in the range of n = 1100–2100 rpm results in a significant decrease in the CO₂ share in the exhaust gases from 10.28% to 6.57%, which is connected to the increase in the mass of the air supplied to the cylinders.

The emission of CO₂ in the engine exhaust gas for particular rotational speeds caused by the technical states of the air filter k: New, A-33, B-66, C-90 is shown in Figure 13, while Figure 14 shows the relative change in the proportion of CO₂ in the engine exhaust for individual speeds caused by the technical states of the air filter k: A-33, B-66, C-90, relative to the emissions obtained when the engine was operated with air filter “New”.

Based on the information in Figure 12, Figure 13 and Figure 14, it can be concluded that air filter pressure drop (filter cartridge fouling) is quite important for CO₂ emissions. In the low-speed range of 1000–1200 rpm, the deterioration of the filter element causes a reduction in CO₂ emissions. This is due to a reduction in the amount of fuel fed to the cylinder by the ECU engine control system, due to a reduction in boost pressure.

As the engine speed increases above 1200 rpm, the effect of filter condition on CO₂ emissions is reduced. This is a result of the engine controller adjusting the fuel delivery to the current conditions in the engine intake system. The relative changes in CO₂ emissions do not exceed 1%. At high engine speeds n = 2000–2100 rpm a negative effect of the technical condition of the air filter on the CO₂ emissions can be observed. This phenomenon is a result of increasing damping of the air flow on the filter baffle. At high engine speeds, the engine control system tries to ensure the required charge pressure by changing the dose and angle of the fuel feed in such a way to generate a larger amount of exhaust gases necessary to increase the turbocharger rotational speed and consequently increase the charge air pressure. Such action is connected with the necessity of delivering a larger amount of fuel with the same mass of air to the cylinder, which results in increased CO₂ emission. This phenomenon is unfavourable from the point of view of atmosphere pollution; however, in the range of these rotational speeds, the engine does not run very often.

In the case of operation of the engine with the air filter in the technical condition A-33 (33% obscuration of the active surface of the cartridge), no significant differences were found (about 1.25%) in the change of the share of CO₂ in the exhaust gases. Increasing the pressure drop of the air filter (B-66, C-90) already causes a significant decrease in the proportion of CO₂ in the exhaust, but there is not such a significant decrease in CO₂ emissions (Figure 14).

This can be explained by the fact that the increase in air filter pressure drop causes a decrease in the air flow supplied to the engine Q_s and the boost pressure. Lower charge pressure is treated by the engine ECU as a signal to reduce (correct) the power generated by the engine by delivering less fuel, which is reflected in a decrease in hourly fuel consumption and power. The reduction in CO₂ concentration in the exhaust gases is correlated with the reduction in useful power resulting in small changes in CO₂ emissions. On the basis of the results presented in Figure 13 and Figure 14, it can be concluded that the increase in the pressure drop in the inlet system of a modern ZS truck engine has no significant effect on the CO₂ emission into the atmosphere.

Figure 15 shows the proportion of carbon monoxide CO in the exhaust gases of the Volvo D13C460 EURO V engine as a function of rotational speed n for four air filters differing in pressure drop (New, A-33, B-66, C-90). With increasing engine rotational speed, the concentration of carbon monoxide CO, regardless of the technical condition of the air filter, decreases its value until the engine reaches the maximum rotational speed n = 2100 rpm. The greatest changes (decrease) were registered in rotational speeds in the range of n = 1000–1200 rpm. The high concentration of carbon monoxide in the range of n = 1000–1100 rpm results from the low filling ratio of the engine (Figure 8), and the high dose of fuel fed to the cylinder resulting from the necessity to generate the necessary effective engine power required to overcome the vehicle motion resistance during the start-up and hill climbing.

Figure 16 shows the specific CO in the engine exhaust for each speed caused by air filter states k: New, A-33, B-66, C-90. The relative change in the proportion of CO in the engine exhaust for individual engine speeds is caused by air filter conditions k: A-33, B.-66, C-90, in relation to the emission obtained during operation of the engine with air filter ″New″ is shown in Figure 16.

When analysing the influence of the filter condition on CO emission, it should also be remembered that the measured values in the medium and high-speed range are very small—at the level of a dozen or so ppm, i.e., this is the measuring range of the analyser burdened with a large measurement uncertainty. Therefore, the nature of the changes should be interpreted qualitatively rather than strictly quantitatively.

Based on the information provided in Figure 15, Figure 16 and Figure 17, it can be concluded that the condition of the air filter (pressure drop) is important for carbon monoxide—CO emissions. In the low to medium speed range of 1000–1700 rpm, increasing the pressure drop of the filter element results in a decrease in CO emissions. This is related to a decrease in the dose of fuel fed to the engine cylinders by the engine control system ECU, as a result of reducing the boost pressure and in an effort to reduce the increase in emission of toxic components of the exhaust gases.

As the engine speed increases above 1600 rpm, the effect of air filter pressure drop on CO emissions is reduced. This is a result of actions resulting from the adjustment of the fuel dose by the engine controller to the currently prevailing conditions in the engine intake system—boost pressure. This problem is described in more detail during the analysis of the influence of the air filter pressure drop on the CO₂ emission.

In the case of operation of an engine with an air filter in the A-33 state (33% of the active surface of the filter cartridge is covered), the changes in the CO emission amount from 3 to 10%, depending on the rotational speed. Increasing the air filter pressure drop (B-66, C-90) already results in a significant decrease in CO concentration in the exhaust gases; moreover, there is a significant decrease in the CO emission—Figure 17. This phenomenon is similar to the one described for the CO₂ emission.

Figure 18 shows, for four air filters differing in pressure drop (New, A-33, B-66, C-90), NO nitric oxide concentration, and Figure 19 shows GASNO nitric oxide emissions as a function of speed n of the Volvo D13C460 EURO V engine. The relative variation of the NO contribution to the engine exhaust for different speeds due to air filter conditions k: A-33, B-66, C-90, in relation to the emission obtained during engine operation with air filter “New” is shown in Figure 20.

In the low-speed range, the NO concentration remains high at 830–880 ppm regardless of the filter condition. This phenomenon is a result of low boost pressure (Figure 7), low air filling of the engine (Figure 8) and quite a high fuel dose necessary to generate the set torque. Satisfying the above engine operating conditions results in increased exhaust temperature, which promotes the formation of oxides of nitrogen—Figure 18, Figure 19 and Figure 20. Increasing the engine speed increases the boost pressure and consequently increases the amount of oxygen in the fuel–air mixture, which results in a decreased combustion temperature. The reduction in combustion temperature results in a reduction in the concentration of NO (Figure 18), NO₂ (Figure 21), and NO_x as the sum of NO and NO₂ in the exhaust gas while increasing the generated useful power. The rate of increase in the generated effective power is greater than the decrease in the concentration of NO and NO₂ in the exhaust gas, resulting in a decrease in NO (Figure 19), NO₂ (Figure 22), and NO_x as the sum of NO and NO₂.

Based on the information in Figure 19 and Figure 22, it can be concluded that the condition of the air filter has no significant effect on NO, NO₂, NOx emissions. The changes in NO and NO_x emissions range from +2 to −3%, depending on the engine operating point (speed) and do not have a clearly identified nature of change. Changes in NO₂ emissions are slightly larger and range from +10 to −25%; however, NO₂ emissions are negligible relative to NO emissions, so its variations do not significantly affect total NO_x emissions (Figure 23).

In the low-speed range of 1000–1100 rpm, the deterioration of the filter element (increase in air filter pressure drop) results in reduced NO_x emissions. This is the result of a reduction in fuel delivery to the cylinder by the ECU engine management system, due to a reduction in boost pressure. In the mid-range, and as engine speed increases above 1200 rpm, the effect of air filter condition (effect of pressure drop) on NO_x emissions decreases. The relative changes in NO_x emissions do not exceed 1%.

When the engine was operated with the air filter being in the condition A-33 and B-66 (33% and 66% of the filter cartridge active area obscured), no significant differences (about 2.5%) were found in the change of NO_x concentration and emission. Further increasing the air filter pressure drop (C-90) only causes a significant decrease in the NO_x concentration in the exhaust gas; however, there is not such a significant decrease in NO_x emissions.

Based on the results in Figure 24, Figure 25 and Figure 26, it can be concluded that the increase in pressure drop in the intake system of a modern truck Diesel engine has no significant effect on NO_x emissions to the atmosphere.

Figure 27 shows the exhaust gas temperature t_s as a function of engine speed n of a Volvo D13C460 EURO V engine for four different air filter technical states (New, A-33, B-66, C-90). As the engine speed increases, the temperature of t_s, exhaust gas, irrespective of the technical condition of the air filter, slowly but systematically (almost linearly) decreases its value until the engine reaches the rotational speed n = 1800 rpm, after which it increases slightly and then decreases sharply. Operation of the engine with an air filter with increasing pressure drop Δp_f, according to the technical states (A-33, B-66, C-90), causes a shift of the characteristics t_s = f(n) almost in parallel toward lower values of the engine temperature. Technical states A-33 and B-66 do not cause significant changes in exhaust gas temperature. Operation of the engine with air filter with pressure drop Δp_f = 2.024 kPa (C-90) causes an already significant, drop of about 20 °C in exhaust temperature t_s in comparison with the operation of the engine with “New” filter. This phenomenon is connected with changes in boost pressure resulting from changes in pressure drop in the inlet system. Reducing the boost pressure causes a decrease in the pressure and temperature of the end of combustion, which results in a decrease in NO_x contributions (Figure 24). In a turbocharged compression ignition engine, the combustion is close to complete and total, reducing the boost pressure causes the ECU to reduce the maximum dose of fuel fed to the cylinder during the working stroke, which consequently contributes to a decrease in the combustion temperature.

Figure 28 shows the HC hydrocarbon concentration as a function of speed n of the Volvo D13C460 EURO V engine for four air filters differing in pressure drop (New, A-33, B-66, C-90). When analysing the effect of filter condition on HC emissions, we were limited to assessing the effect of the nature of HC concentration changes as a function of air filter condition. This action was dictated by the very low HC concentrations of several ppm which, given the high uncertainty of the analyser measurements in the range up to 25 ppm, could result in incorrect inferences.

At the same time, the study shows that the condition of the air filter (pressure drop) is not significant for HC concentration. A significant increase in pressure drop results in an increase in HC concentration by a few ppm. The determined average HC emission for the engine operating with the air filter in the “New” state (Δp_f = 0.58 kPa) was 0.0547 g/kWh, while for the filter in the C-90 state (Δp_f = 2.024 kPa) it was 0.0653 g/kWh, respectively. These values are small enough that they do not significantly affect the environmental performance of the tested engine.

4. Conclusions

• In the available literature there are no results of research on the influence of the inlet system flow resistance on the emission of toxic components of exhaust gases from a modern Diesel engine used for driving trucks (truck tractors) currently travelling on the roads and constituting the basic means of transporting goods.
• The conducted research has shown that the increase in flow resistance in the inlet system of the modern ZS truck engine has no significant effect on the NO_x emission into the atmosphere, and does not cause any significant changes in the degree of smoke opacity of exhaust gases in relation to its acceptable value specified in the technical conditions for this type of vehicle.
• The observed effect of the increase in air filter flow resistance of the modern ZS truck engine is a decrease in air demand by the engine, decrease in the boost pressure and, as a result, the decrease in the filling ratio η_υ which, in connection with the fuel dose reduction, causes the decrease in the engine power. An increase in air filter flow resistance by an average of 1 kPa results in a decrease in engine power of over 6%. In the conditions of vehicle use, this is associated with a reduction in the ability to climb a hill in individual gears and an increase in the time and distance of acceleration of the vehicle-tractor-trailer combination. As the flow resistance increases, the emissions of exhaust components change: NO, NO₂, NO_x, CO, HC and CO₂. These values are so small that they do not significantly affect the ecological properties of the tested engine.
• The results obtained show the effect of the air filter flow resistance of a modern truck engine on its performance, and in particular on the changes in the emission of the individual components of exhaust gases. It is advisable to continue the work; the final effect of which should be a determination of the maximum permissible flow resistance, the exceeding of which should eliminate the vehicle from further operation because of deterioration of the economic, energetic, ecological and traction properties of the vehicle.
• It is advisable to extend the presented research with the analysis of the influence of increased flow resistance in the air filtration system on the change of traction properties of the vehicle equipped with this type of inlet system.