Analysis of Non-Road Mobile Machinery Homologation Standards in Relation to Actual Exhaust Emissions

Abstract

This article presents issues related to the current approval procedures in the group of off-road vehicles. Our research aimed to demonstrate significant differences between actual railway vehicle operation and stationary homologation tests regarding exhaust emissions. The research cycle consisted of analyzing emissions of toxic compounds from exhaust systems under real operating conditions, supplemented by a temporal share analysis based on the denormalized NRTC test upon which the tested object was homologated. Based on the conducted analyses, a significant difference was found between the actual operation of the tested railway vehicle and the stationary homologation test. By interpreting emission intensities within the parameter ranges of the propulsion unit’s operation, key areas with a significant impact on the vehicle’s overall emissions were identified. Based on the obtained results, a critical opinion is expressed regarding current homologation standards for the off-road vehicle group and the necessity for further empirical research in the area of actual operation of the tested vehicle group.

1. Introduction

Tests on emissions of harmful exhaust gases are carried out in laboratory conditions as dynamometric tests. However, such measurements are only intended to reflect the actual operating conditions of vehicles, and the results obtained are most often used as input data in the process of modeling the impact of road or track traffic on air quality. Measurements of this type are divided into two groups. The first group includes those measurements performed on an engine dynamometer, during which the engine installed directly on the test stand, equipped only with the equipment necessary for proper operation, is analyzed. This type of measurement is performed on heavy vehicles. The second group includes vehicle measurements carried out on a chassis dynamometer equipped with special rollers simulating road conditions. These tests are performed for light vehicles. The advantage of laboratory tests is primarily the ability to carry out many measurement cycles at relatively low unit costs. However, the data obtained are only approximate because the measurements do not include the impact of road conditions, the driver’s driving style, or the ambient air quality. Therefore, most test cycles performed on dynamometers do not reflect the actual operating conditions of vehicles in real conditions, which is confirmed by the literature [1,2,3].

Therefore, for several years, scientists have been developing a number of alternative methods that allow us to obtain more reliable results. The first step was to develop dynamic test cycles simulating specific driving conditions [4,5]. In a further stage, tests were carried out in real operating conditions. Currently, approval regulations require measurements of harmful exhaust gas compounds for passenger cars and heavy vehicles in road conditions in specially developed RDE tests using exhaust gas analyzers from the PEMS group. However, for rail vehicles, there are no specific regulations regarding emission tests of harmful exhaust gases, which is why they are carried out only in laboratory conditions. However, the latest regulations for non-healthy vehicles contain a provision informing us about the need for tests in real conditions, but no test procedure has been established and no limit values for individual harmful exhaust gas compounds have been provided.

The tests performed in real operating conditions are aimed at verifying the ecological indicators of vehicles in a wide range of operations of their drive systems. Measurements of this type are performed primarily to determine the impact of the used drive systems and vehicle movement parameters on the emission of harmful compounds and to indicate the differences between approval procedures and their actual operation. The development and minimization of measurement equipment for pollution testing allows for increasingly more accurate analyses in real operating conditions. Moreover, the application possibilities of this type of device are constantly increasing. Thanks to this, it is possible to take into account the specificity of the traffic of each type of vehicle (road, track, off-road).

2. Railway Passenger Rolling Stock in Poland

Air pollution is currently one of the biggest problems facing humanity, and continuous economic development contributes to the emergence of new sources of pollution. According to a World Health Organization (WHO) report published in 2023 [6], approximately 6.7 million people died prematurely in 2019 due to diseases of the respiratory and circulatory systems and cancers caused by air pollution.

One of the possible ways to counteract excessive exhaust emissions is to reduce the share of passenger vehicles in favor of the development of public transport. According to EEA data [7], in 2019, passenger vehicles were responsible for 60.6% of total CO₂ emissions in the European Union. In order to reduce the share of cars, city authorities are increasingly introducing restrictions that prevent or limit the use of combustion cars in city centers. Instead, public transport solutions are proposed, thanks to which citizens can move around cities. Due to rising housing prices and the continuous expansion of urban agglomerations, young people decide to live in nearby, smaller towns, which most often involves the need to travel by passenger car to their workplaces. A favorable solution in such a case is rail transport. Thanks to the use of rail transport, it is possible to connect the city center with nearby urban centers, which allows for quick access compared with travel by passenger vehicle. Such railway connections are most often implemented using electrified or diesel traction units when the railway line does not have an overhead contact line.

A diesel multiple unit (DMU) is a type of train that consists of at least two units that constitute one coherent whole during operation, and its main source of drive is the internal combustion engine. DMUs are used to transport passengers on main railway lines in order to deliver passengers from smaller towns to larger trans-shipment points, and they serve connections on regional and agglomeration railway lines, which are most often not electrified. Currently, multi-faceted research is being carried out on this type of vehicle and engine [8,9,10,11,12,13,14,15,16,17,18]. DMUs are also called rail buses; however, this concept also includes single-motor wagons, which are one separate unit equipped with a drive source. Diesel ignition engines are most often used in DMUs due to the nature of the operation of rail vehicles, which require high torque. In order to reduce the emission of toxic compounds from exhaust system pollutants, a design solution is usually used in which a selective reduction system is installed in the exhaust system for catalytic reduction (Selective Catalytic Reduction, SCR), which allows NO_x to be reduced by 90% to nitrogen (N₂) and water (H₂O) [19,20]. To carry out this process, AdBlue (trade name) is used, which consists of 32.5% urea and 67.5% distilled water. AdBlue is injected into the exhaust manifolds of diesel engines to remove impurities and neutralize NO_x [21,22].

According to data collected by the Office of Rail Transport, in 2021, 246 diesel multiple units were in operation throughout Poland [23]. A summary of changes in the number of DMUs in operation over the last 19 years is presented in Figure 1. Although efforts are being made to reduce exhaust emissions and reduce the number of combustion vehicles in order to replace them with electric versions, the number of combustion rail buses is constantly increasing. In just one year (2020–2021), the number of DMUs in operation increased by 37 units.

This is caused by an increase in the number of passengers, as people are more and more willing to choose to travel by train, which is confirmed by the data presented in Figure 2. It should be remembered that the decrease in the number of passengers in 2020–2021 was caused by restrictions related to movement caused by the COVID-19 pandemic (Coronavirus Disease). After the end of the pandemic in 2022, the highest number of passengers transported in 10 years was recorded, and the data from 2023 are even more optimistic, because in the period from January to July 2023 the number of passengers amounted to almost 213 million, while last year in this period it was only 188.60 million passengers, which significantly shows greater interest in rail transport. Over time, new regional connections are also being created, which increases the number of vehicles that need to serve passengers on railway lines.

3. Legal Regulations Regarding Testing and Approval of Non-Road Vehicles

The European Union pursues a policy aimed at reducing the negative impact of means of transport on the natural environment while ensuring the safety and comfort of their users. For this reason, regulations and directives are introduced that define guidelines for achieving the goals set by the European Union. Taking into account the regulations governing the requirements for engines used in diesel multiple units regarding permissible emissions of toxic compounds, two regulations can be distinguished. The first one is included in the report of the European Rail Research Institute (ERRI), formerly the Office de Recherches et d’Essais—Railway Research and Testing Office (ORE), while the second one is included in the Union Internationale des Chemins de fer—International Union of Railways (UIC) cards [25,26]. However, they do not have international legal status, and it is only an external obligation of all railways associated with the UIC. These regulations are still used to assess the engines used in diesel multiple units [27], and the permissible values are presented in

Table 1. Exhaust emission limits according to UIC 623-2 and UIC 624 [25,26].

Date of Introduction	Emission (g/kWh)
	CO	NOx	HC	PM
Until 31 December 2002 UIC I	3	12	0.8	1.6
From 1 January 2003 UIC II	2.5	6	0.6	0.25
p ≤ 560 kW		9.5 (n > 1000 rpm)
p > 560 kW	3	9.9 (n ≤ 1000 rpm)	0.8	0.25
From 1 January 2003
p ≤ 560 kW	2	4.6	0.5	0.15
p > 560 kW	2	6	0.5	0.2

.

The current EU regulations imposing requirements on emissions of toxic substances from exhaust gases are established in directives that have legal force. These directives specify in detail the standards regarding permissible emissions of harmful components from exhaust systems. These regulations apply both to vehicles traveling on roads and to railway rolling stock, which is equipped with various types of internal combustion engines, which is important in the case of the use of car engines in railway vehicles with internal combustion drives, e.g., in internal combustion multiple units. The most important directives relating to DMUs include the following:

• Directive 97/68/EC of 16 December 1997 (as amended), which contains provisions on emission standards and procedures for the approval of engines installed in non-road machines [19];
• Directive 2004/26EC of 21 April 2004 (as amended), which amends Directive 97/68/EC in order to bring members of the community closer to reducing emissions from combustion engines [27];
• Regulation 2016/1628 of the European Parliament and of the EU Council of 14 September 2016, which repeals Directive 97/68/EC, introducing the Stage V emission standard [28].

Table 2. Emission limits of toxic exhaust gases for non-road combustion engines with a power of 130 kW ≥ p ≤ 560 kW [29].

Date of Introduction		Emission (g/kWh)
		CO	HC	NOx	PM
Stage I	1999	5.0	1.3	9.2	0.54
Stage II	2002	3.5	1.0	6.0	0.2
Stage IIIA	2006	3.5	4.0		0.2
Stage IIIB	2011	3.5	0.19	2.0	0.025
Stage IV	2014	3.5	0.19	0.4	0.025
Stage V	2019	3.5	0.19	0.4	0.015

shows the emission limits of toxic exhaust gases from exhaust systems that apply to off-road vehicles, including rail vehicles. On 16 December 1997, the first EU directive was introduced regulating the permissible emission values of toxic exhaust gases for off-road vehicles. The regulations were implemented in two stages. In 1999, the Stage I standard was introduced, and in 2002, Stage II. Directive 2004/26/EC introduced further emission standards divided into Stage IIIA and Stage IIIB. The Stage IIIB standard introduced a stringent hydrocarbon emission standard, lowering the permissible value to 0.19 g/kWh compared with the previous Stage II standard, where the value was 1 g/kWh. In addition, in Stage IIIB, a particulate emission limit of 0.025 g/kWh was introduced, which was to force the use of particulate filters in vehicles. The Stage IV standard introduced in 2014 significantly reduced the permissible value of nitrogen oxide emissions to 0.4 g/kWh, which contributed to the increased use of SCR systems in rail vehicles. The latest emission standard for off-road vehicles, Stage V, was intended to contribute to increased control over particulate matter and limited the permissible PM emission to 0.015 g/kWh [29]. The permissible values of toxic compound emissions are determined using laboratory tests developed to measure emissions from engines for NRMM. The main test cycle for NRMM vehicles is the ISO 8178 test cycle, known as the Non-Road Steady Cycle (NRSC). This engine dynamometer test cycle includes several sequences of steady states with different weighting factors. The NRSC cycle for the Stage IIIA standard was the only test defining the allowable emission limits. Starting with the Stage IIIB standard, the requirement to conduct the Non-Road Transient Cycle (NRTC) test was introduced, and both test cycles (NRSC and NRTC) are now conducted in parallel during engine testing for NRMM vehicles.

The currently applicable worldwide approval test for non-road vehicles is the Non-Road Transient Cycle (NRTC) test. The test cycle is mandatory for engines used in NRMM vehicles approved for Stage IIIB and newer [30,31,32]. The main assumption of this test is to test the engine in such a way as to approximate its actual operating conditions. Two cycles are performed during the test (Figure 3). The first is a cold start cycle, which involves performing the test after conditioning the engine at laboratory temperature (20–30 °C). The second cycle is a hot start performed 20 min after warm conditioning beginning immediately after the first cycle is completed. To summarize, the test sequence includes a cold start cycle following natural or forced engine cooling, followed by warm conditioning and a hot start cycle. As a result of this study, we obtained the total emission of harmful compounds in exhaust gases. The work produced by the engine over the entire cycle is determined by integrating the power with respect to the cycle time, using torque and rotational speed feedback signals from the dynamometric brake. The concentration of gas components for the entire cycle can be determined in two ways:

• The concentration in the raw exhaust gas can be determined by integrating the exhaust gas analyzer signal in accordance with the procedure described in the Directive [33];
• In the undiluted exhaust gas in a Constant Volume Sample (CVS) system, diluting the total flow by integration or collecting a sample into bags in accordance with the procedure described in the Directive [33].

To determine the concentration of particulate matter, an exhaust gas sample is taken on an appropriate filter using the total or partial flow dilution method. The flow rate of the diluted or raw exhaust gas over the entire cycle shall be determined depending on the method used. To determine the mass of each tested pollutant per kilowatt hour, the mass emission value refers to the operation of the tested facility. Emissions expressed in g/kWh are measured during each cycle, both cold start and hot start. Total weighted emissions are determined by applying a weight of 10% for a cold start and 90% for a hot cycle. Total emissions of pollutants must not exceed the limit values [27,33,34].

To run an NRTC on a specific engine, engine torque and crankshaft speed reference levels must be determined in advance. It is therefore necessary to carry out the destandardization procedure in accordance with the procedure described in Commission Directive 2010/26/EU [19]. The engine crankshaft speed is determined based on the formula:

n = %n_nor(n_ref − n_idle)/100 + n_idle

(1)

where:

• n_nor is the normalized rotational speed,
• n_idle is the idle speed, and
• n_ref is the reference speed calculated according to the formula:

n_ref = n₅₀ + 0.95(n₇₀ + n₅₀)

(2)

where:

• n50 is the minimum speed at which the engine reaches 50% of its rated power, and
• n₇₀ is the maximum speed at which the engine reaches 70% of the rated power.
• Then, the torque is denormalized and determined based on formula:

  Mo = (%Mo_nor × Mo_max)/100

  (3)

  where:

• Mo_nor is the normalized torque, and
• Mo_max is the maximum torque.

4. Research Methodology

4.1. Research Object

A diesel multiple unit intended for operation on agglomeration and regional railway lines was used to test the emission of toxic exhaust gases and the operation of an internal combustion engine in real operating conditions [35,36,37,38,39,40]. The examined facility was selected in such a way that it could be compared whether the emission standards set by the European Union are realistically reflected in the actual operation of rail vehicles in the form of an improvement in the amount of harmful toxic compounds emitted into the atmosphere in exhaust gases. Therefore, a rail bus operating in Poland, meeting the Stage IIIB emission standard, was subjected to testing. The vehicle is equipped with two CI engines with a power of 390 kW and a maximum torque of 2300 Nm.

4.2. Measuring Equipment

Homologation tests of vehicles from the Non-Road Mobile Machinery (NNRM) group, including rail vehicles, are carried out exclusively on special engine dynamometers. The main reason for the approval of the engines of off-road vehicles is the difficulties in carrying out tests in real operating conditions, especially due to the dimensions of the vehicles [36]. Many scientists in their research works [41,42,43,44,45,46,47] indicate that standardized measurements on test benches are unable to reflect the impact of actual operating conditions on the emission results of harmful exhaust gases, as is the case with RDE tests. However, continuous technological development creates new opportunities to perform measurements during the daily operation of NNRM vehicles. One of them is the Axion R/S+ mobile measuring device, which was used during our research.

The equipment from the PEMS group (Axion R/S+, Figure 4), developed by the American company Global MRV, has passed a rigorous assessment of the Environmental Technology Verification (ETV) program conducted by the United States Environmental Protection Agency (USEPA). It is characterized by a weight of approximately 18 kg and compact dimensions, which allow us to conduct research in hard-to-reach places, including in non-road vehicles. The analyzers used in the equipment allow for the measurement of emissions of harmful compounds from exhaust systems, such as CO, CO₂, HC, NO_x, PM, and O₂.

A Nondipersive Infrared Sensor (NDIR) analyzer is used to measure CO, CO₂, and HC concentrations. The operation of this analyzer is based on the spectrometric method, in which the measurement photometer records the total absorption of radiation in a relatively small wavelength band specific to a specific gas compound. Measurements of particulate matter are made by a method based on laser scattering (Laser Scatter). NO_x and O₂ are checked using electrochemical analyzers, where an electrical signal is obtained after the electrochemical transformation of the measured substances. The equipment is also equipped with a Global Positioning System (GPS). The data measurement process takes place at a frequency of 1 Hz. The obtained test measurements are processed and corrected, and, on their basis, road and unit emissions of measured pollutants can be calculated.

In order to record vehicle operating parameters (rotational speed, torque, pressures, and temperatures) during the tests, the TEXA Navigator TXTs TRUCK (Treviso, Italy) diagnostic device was used. The measurement module is connected to the socket of the on-board diagnostic (OBD) system and to a computer with specially installed software IDC5, where data from the tested vehicle are recorded. Figure 5 shows the connection of research equipment to the research facility.

However, PEMS equipment has certain limitations. This primarily applies to the fact that the analyzers during testing activities must be installed on the test vehicle and the measurement probes must be placed directly in the exhaust system. Installing the equipment is usually very time-consuming, and it is not always possible to use exhaust gas flow meters, which make testing more efficient. For this reason, flow characteristics should be determined using information about the pressure in the intake manifold, the temperature behind the turbocharger, and the crankshaft rotational speed. Additionally, the device used for testing is characterized by a limited measurement range, as shown in

Table 3. Technical data on the Axion R/S+ device [48].

Exhaust Component	Measurement Range	Measurement Accuracy	Distribution	Type of Measurement	Measurement Time (s)
CO2	0–16%	±0.3% absolute ±4% relative	0.01 vol.%	NDIR	<3.5
CO	0–10%	±0.02% absolute ±3% relative	0.001 vol.%	NDIR	<3.5
HC	0–4000 ppm	±8 ppm absolute ±3% relative	1 ppm	NDIR	<3.5
NO *	0–4000 ppm	±25 ppm absolute ±3% relative	1 ppm	E-chem	<5
PM	0–300 mg/m3	±2%	0.01 mg/m3	Laser Scatter	2

* The NOx emissions value was estimated based on the assumption that the amount of nitrogen dioxide (NO₂) in exhaust gases is approximately 5% of the measured NO value for gasoline and approximately 10% for diesel oil.

. Another problem may be the relative measurement accuracy of ±2–4%, which is problematic mainly in the case of particulate matter, which has small values.

4.3. Research Route

Tests in real vehicle operating conditions took place on a route used for daily passenger transport by a railway operator from Greater Poland (Figure 6). The journey was carried out in the form of a standard driving style, i.e., one in which the driver tries to reach the maximum traveling speed as quickly as possible. The tests were conducted in clear weather conditions, which allowed for the elimination of external factors that could affect the increase in the emission of toxic compounds during the tests. Such factors mainly include precipitation, which causes the tracks to become wet, potentially leading to slippage between the rail vehicle and the infrastructure. In such cases, the train operator must brake and accelerate the vehicle cautiously, preventing the tests from being conducted in a standard driving style. The air temperature during the tests was approximately 20 °C (readings from meteorological stations), which was favorable for conducting the tests, as the temperature did not significantly impact the train’s operation, unlike in low temperatures during winter or high temperatures in summer, where additional systems such as air conditioning are activated for passenger comfort and safety.

The length of the route is 79.63 km and there are 19 stops. The total stop time during the measurements was 655 s, of which the longest single stop was 149 s. The railway line from Wolsztyn to Luboń near Poznań is a single-track line and the traffic on it is a shuttle, i.e., one train runs from Wolsztyn towards Poznań, and the second runs from Poznań to Wolsztyn. The result of this solution is the forced stoppage of one of the trains at the station in order to let the other train pass. In the case of the described research facility at work, the train of the opposite direction was waiting at the station, which allowed for the elimination of an excessive waiting time, which often reaches about 10 min on this route. Figure 7 shows the train’s driving profile. The largest height difference was 48 m.

The measurement run was carried out during the actual passenger service on the above-described test route. This allowed for maintaining realistic operating parameters of the rail vehicle, including stop times at stations during the journey according to the railway operator’s schedule, as well as the traffic characteristics on the given route. By conducting measurements during the actual rail service, realistic values of toxic compound emissions were obtained, which may differ slightly from those of an empty train set. During the measurements, the tested vehicle carried approximately 98 passengers.

5. Research Results

5.1. Introduction to Analysis

Density characteristics of their operation time have been used for many years for vehicle drive systems. This mathematical tool allows you to obtain information about phenomena occurring in specific speed and acceleration ranges while taking into account the variability in these parameters for the indicated range.

By determining the time share density characteristics, it is possible to determine the time shares of operation during the tests carried out, as well as to determine the emissions of harmful compounds in the torque and rotational speed ranges.

In the analysis of traffic parameters, certain assumptions were made regarding the determination of the characteristics of operating time shares for the tested vehicles. One-sided closed intervals were used in the time share characteristics, which allowed for greater readability and accuracy of the analyses performed. In order to properly analyze this object, the value of M = 10 Nm was assumed for the read negative torque values.

Exhaust emission standards for all groups of off-road vehicles are expressed as a unit of mass related to the work performed by the drive system. As shown in the works [41,42,43,44,45,46,47], for tests in real operating conditions this may result in underestimation of the obtained indicators by taking into account the internal combustion engine’s own starts. This means that the calculations take into account the indicated work, and it should be effective work. The reason for this is reading the torque value from the on-board diagnostic system, which is calculated based on the fuel injection time and pressure.

Non-road vehicles, due to their various purposes, cannot be assessed ecologically in terms of the distance traveled. This conclusion applies to the entire group of these machines in general. However, certain subgroups of these machines, e.g., passenger rail vehicles, can be expressed in such coefficients. This makes it easier to compare the environmental friendliness of this means of transport to the group of passenger cars, motorcycles, or suburban and intercity buses.

5.2. Analysis of Operating Parameters in the Aspect of Applicable Approval Tests

Based on the external characteristics of the tested engine, its characteristic reference speeds were determined, which are n_idle = 600 rpm, n₅₀ = 960 rpm, and n₇₀ = 2600 rpm. Then, the reference speed of the tested engine was determined, which was nref = 2518 rpm.

After denormalization, the NRTC cycle was obtained for the tested engine, which is shown in Figure 8.

In order to compare the operating conditions of the combustion engine in real operating conditions to the conditions of the NRTC homologation test, the shares of the drive unit’s operating time in the speed and torque ranges were determined. The tested combustion engine of a rail vehicle in the NRTC test most often operated at a crankshaft speed in the range of 2000 rpm to 2200 rpm in the full load range, where the total share of operating time was 32.71% (Figure 9). The time share of idling during the test was 3.88%. The total time share of the test in the actual operating area of the internal combustion engine was only 30.29%.

The profile of the instantaneous power generated by the internal combustion engine indicates that the test object frequently reached the engine’s rated power during the test, which influenced the obtained total work result of 50.36 kWh (Figure 10). Significant changes in engine speed and torque resulting from the test cycle characteristics contributed to dynamic changes in the power generated by the engine. High power values were achieved through the engine operating at high speeds and variable torque.

Similarly, Figure 11 shows the characteristics of the share of operating time for individual operating intervals of the combustion engine during travel in real operating conditions. In this case of the research object, the engine operating parameters were concentrated in three ranges. In the range of torque (… Nm, 400 Nm〉 and rotational speed (800 rpm, 1000 rpm〉 and (1000 rpm, 1200 rpm〉, 29.96% and 13.75% of the total operating time were recorded, respectively. When the vehicle was accelerating, the engine worked mainly at the rotational speed in the range of (1600 rpm, 1800 rpm〉 and (2000 Nm, … Nm), and its time share was 25.14%.

To complement the analysis, a comparison was made of the difference in the share of engine operating time in the NRTC cycle (subtractor) to actual operation (subtrahend). In Figure 12, the red line shows the range of engine operating parameters during its actual operation in a rail vehicle. During the RDE tests, the combustion engine operated in the rotational speed range from 800 rpm to 2000 rpm, which is a much narrower range of its possible operation. The main reason why the engine in a rail vehicle does not work to its maximum extent is the limitations resulting from the construction of the entire drive unit. The combustion engine must be synchronized with the main gear, which then transmits torque to the reversing gear via the cardan shaft. Therefore, in order for all elements of the drive unit of a rail vehicle to cooperate, the combustion engine must operate in a smaller range of rotational speeds.

The largest difference of −27.62% was observed in the range of rotational speed (800 rpm, 1000 rpm〉 and torque (0 Nm, 400 Nm〉. In this range, the combustion engine operated during stops, which in the characteristics of railway traffic on agglomeration lines is a significant part of the entire rail vehicle journey. An equally large difference (−24.89%) was obtained in the range of rotational speed (1600 rpm, 1800 rpm〉 and torque (2000 Nm, … Nm), where the diesel multiple unit accelerated to the maximum permitted speed on a given section of the route after a stop. The difference in the share of engine operating time during the NRTC cycle above the rotational speed range obtained during tests in real operating conditions was 59.45%.

Significant differences between the NRTC test and the actual operating conditions of the rail vehicle arise from the nature of the test object’s operation. The NRTC test cycle aims to reflect real driving conditions through dynamic changes in torque and engine speed. Consequently, a larger portion of the test is conducted at high engine speeds and variable torque. However, the actual conditions of the tested vehicle differ significantly from those presented in the homologation test. The tested object primarily operated within specific ranges of high engine speed and torque, resulting from the driver’s driving style and the vehicle’s design. The NRTC test cycle includes a small time proportion representing engine operation at low speeds and idle, whereas the tested vehicle in actual operating conditions frequently had to stop at railway stations, leading to a high percentage of idle operation.

5.3. Analysis of Emissions of Toxic Compounds in Real Operating Conditions from a Combustion Railbus

Section 4.2 of this article demonstrates significant differences in the time distribution between the NRTC test and actual operating conditions. It was shown that, compared with the actual operation of the vehicle, the NRTC test only accounts for 30.29% of the total time distribution. Such significant differences undermine the reliability of the laboratory test, which cannot accurately reflect the actual conditions of the tested vehicle, resulting in a lack of reliability in determining permissible values of toxic compound emissions. For this purpose, an analysis of the emission intensity of harmful compounds was conducted within the ranges of actual operating parameters of the tested object’s propulsion unit.

The emission values of harmful compounds were related to the real operating conditions of the rail vehicle engine. The analysis of the emission intensity of the tested toxic compounds in the ranges of torque and crankshaft rotational speed makes it possible to learn about the influence of engine operating parameters on the content of harmful compounds emitted into the atmosphere from the exhaust systems.

Analyzing the characteristics of the CO₂ emission intensity, a close relationship between the development of the value of this relationship and the engine operating parameters is noticeable (Figure 13). The area in which significant values of CO₂ emission intensity occurred is primarily the range of high torques (1600 Nm, … Nm) in the entire range of crankshaft rotational speeds. The maximum value (54.6 g/s) was recorded in the field described by the intervals (1800 rpm, … rpm) and (1600 Nm, 2000 Nm〉 for the crankshaft rotational speed and vehicle load, respectively. The average value of this compound in the entire driving test was 27.78 g/s.

A similar trend was obtained for the CO and HC emission rates (Figure 14 and Figure 15), which is undoubtedly related to the amount of fuel injected. Its increase causes a temporary global and local oxygen deficiency, which in turn leads to the values of these compounds being relatively higher. The maximum value of CO, which amounted to 249.00 mg/s, was recorded during the highest engine load in the range (2000 Nm, … Nm) and the rotational speed described in the range (1800 rpm, … rpm). The average value for this compound in the entire test along the measurement route was 135.10 mg/s. In the case of the HC emission intensity characteristics, the highest value (53.20 mg/s) was recorded in the same engine operating area. The average HC emission intensity in the entire test was 28.51 mg/s.

In the case of the NO_x emission intensity (Figure 16), much lower values of this compound can be observed at lower rotational speeds in the range (1000 rpm, 1200 rpm〉, where the average intensity was 135.5 mg/s. One of the factors influencing the EGR exhaust gas recirculation system was responsible for this distribution of flows. This system operates when the appropriate conditions are met, i.e., when the engine is operating in the medium speed range. Then, some of the exhaust gas enters the intake channel and less oxygen enters the cylinder for the combustion process. The SCR system also contributed to reducing nitrogen oxide emissions, as it does not cope well with high engine loads.

NO₂ is one of the two main pollutants in cities. Short-term exposure to this exhaust gas component in high concentrations leads to irritation of the respiratory tract and may cause inflammation, which is most often manifested by coughing, mucus production, and shortness of breath. It has also been shown that there is a relationship between NO₂ contained in the air and abnormal lung development and respiratory infections in children, as well as the impact on proper functioning in adulthood. Additionally, research is being conducted [49] on the adverse health effects of this compound, which ultimately lead to a reduction in life expectancy. However, it has not yet been confirmed whether these effects are caused by NO₂ itself or by other pollutants emitted at the same time during exhaust emissions [49].

Significant values of PM emission intensity (Figure 17) were observed primarily in the range of maximum rotational speed (1800 rpm, … rpm) and high torque (1600 Nm, 2000 Nm〉 and (2000 Nm, … Nm). The reason for the increased emission of particulate matter there is a DPF filter which, in order to burn them, must have appropriate conditions, i.e., the exhaust gas temperature must be high enough to burn the remaining carbon. It is very difficult to achieve high exhaust gas temperatures in a low speed range, as a result of which a large proportion of the particulate matter is deposited on the filter until a higher temperature occurs, which the test object reached in the above-mentioned crankshaft speed and torque range.

The obtained results on specific emissions of CO, HC, and NO_x during tests in real operating conditions can be related to the approval requirements for combustion engines specified in standards, in this case to the Stage III B standard. For comparison, the emission factor kj must be determined [50]. The emission factor is defined by the quotient:

k_j = e_real,j/e_limit,j,

(4)

where:

• j is a toxic compound for which an emission factor has been determined,
• e_real,j are specific emissions determined from tests in real operating conditions (g/kWh), and
• e_limit,j are permissible specific emissions in accordance with standards (g/kWh).

The presented results show that the most problematic emissions for a railbus in real operating conditions are the emissions of HC and NO_x (Figure 18). The k_HC coefficient was 1.51, which indicates that the permissible value for this compound has been exceeded. The DOC reactor installed in the vehicle had a positive effect on CO oxidation (k_CO < 1), but at the cost of exceeding the permissible value of hydrocarbons. In the case of the k_NOx coefficient, its value was 1.18, which also indicates that the permissible value established in the standards was exceeded. The k_PM factor, also when the k_CO is significantly below the permissible value, can be regarded as a positive impact of the use of additional exhaust gas treatment systems, such as a particulate filter, for newer generations of vehicles (Stage IIIB standard).

6. Conclusions

From our results, we can draw the following conclusions:

• The specific nature of the operation of rail vehicles, their large population, and the power of the drive units used in them play a significant role in the impact on the natural environment;
• The characteristics of the share of the research facility’s operating time in actual operation showed that the largest share in the traffic of these vehicles is its standstill, the value of which was 30.00% compared with the value of 2.34% for the NRTC approval test. The difference is therefore 27.7%, so steps are necessary to modify the applicable laboratory test for vehicles of this category, with particular emphasis on the engine operating characteristics during typical passenger transport (numerous stops at railway stops);
• The analysis of the determined differences between the values of the operating time share for real conditions and the NRTC approval test showed that the largest values of individual intervals were 27.62% (idle speed) and 24.08% for the average crankshaft rotational speed and the largest interval of torque values. Significant discrepancies prove that the applicable approval tests are unrepresentative and that they need to be modified for the group of machines in question or the testing legislation in real operating conditions;
• The measurements of rail vehicles were carried out in real operating conditions, which is an original research methodology for this group of vehicles. Current approvals for off-road vehicles are performed only for engines in special laboratory conditions, which makes it impossible to obtain reliable information on the impact of various external factors (Figure 19). This is confirmed by the results obtained. A comparative analysis of road emission factors determined on the basis of tests in real operating conditions with the permissible values of homologation tests showed significant exceedances of HC and NO_x limits;
• The presented scheme for carrying out measurements in real operating conditions should be continued in order to obtain the actual emissions of a vehicle that can operate in various conditions. Thanks to field tests, it is possible to measure and relate the obtained results of emissions of harmful compounds to atmospheric conditions, vehicle traffic parameters, the number of passengers, etc.