Analysis of Emissions and Fuel Consumption in Freight Transport

Abstract

Currently in Europe, road freight transport is characterized by the most dynamic advancement. Year after year, we may observe an increase in the amount of transported goods. The paper presents the emissions of gaseous exhaust components such as CO, THC, and NO_x as well as fuel consumption in freight transport. The emission analysis was performed for the entire transport cycle covering the handling of the goods with forklifts and carriage with a heavy-duty truck. The investigations were performed under actual conditions of operation using a Portable Emission Measurement System (PEMS). The fuel mileage was determined using the carbon balance method. The test routes were designed so as to reproduce the transport-logistic system typical of small towns. The setting for the tests was a town located in central Poland near the A2 motorway constituting part of the trans-European logistic network with multiple locations of logistic centers. In order to present the real emissions during handling, two test variants were considered: an outdoor variant (on a nearby lot) and inside a warehouse. The test run of the heavy-duty truck involved transporting 24,000 kg of load on urban and extra-urban (local and intercity) roads. The exhaust emissions and fuel mileage were determined for each of the stages as well as for the entire research cycle.

1. Introduction

According to the data from the World Health Organization [1], over 90% of the population worldwide is exposed to polluted air, which contributes to the death of six million people annually. The main pollutants are CO, CO₂, THC, particulate matter, methane and other derivatives generated as a result of the combustion of fossil fuels. One of the anthropogenic sources of emissions is the exhaust gases of combustion engines used in vehicles and non-road equipment. As has been confirmed [2], transport is the largest consumer of oils and fossil fuel-derived products in the European Union. The greatest demand for these products comes from road transport that, in 2018, was responsible for the consumption of approx. 49% of crude oil in the EU member states. Numerous research works have confirmed the negative impact of combustion engines on the ecosystem. Transport, as one of the main sources of the emission of CO₂ (greenhouse gas) is responsible for a 30% burden of the atmosphere in the entire European Union, thus, contributing to the global increase in the temperature on the planet. The consequences of global warming are manifested by weather anomalies, extreme weather events, or rising of the sea levels, which can already be observed today [3,4]. (Many scientific investigations have also been devoted to the impact of combustion engines on the human health. Particulate matter, emitted mainly from diesel engines, is carcinogenic, causes diseases of the blood system and contributes to the development of other health conditions [5,6]. Exclusively in Europe in 2018, transport was responsible for the emission of approx. 48% NO_x, 3% SO_x, 13% PM2.5, 12% PM10 and 22% CO [7,8]. What is significant in this aspect is that the greatest share in the emission structure had road transport whose multiple modes are predominantly utilized in the presence of humans [7,8]. When analyzing and investigating the exhaust emissions from road transport one has to allow for the extensive range of vehicle types (passengers cars (PC), Heavy Duty Vehicles (HDV), Light Duty Vehicles (LDV), two-wheelers etc.) whose operating regimes and exhaust emissions vary widely. This is important when determining the actions aiming at reducing the emissions from road transport.

Road transport of goods is getting increasingly significant. When analyzing the statistical data (GUS Central Statistical Bureau), one may observe that out of all available modes of transport, road transport of cargo is characterized by the highest growth dynamics. Only in Poland, a 20% growth in the freight carriage was observed within a single year. In recent years, an increased number of heavy-duty vehicles in operation has been observed as well. What is important, however, is that the production of almost half of these vehicles dates back to 2003 [9,10,11]. The above data confirm the dynamic advancement of road transport of goods, which renders research on its impact on the ecosystem fully justified. Piscitelli, Valenzano et al. [12] in their work have undertaken to estimate the health cost related to the emission of PM and NO_x by road transport in Italy. The research has shown that the pollution generated by vehicles used in the road transport of goods has a huge impact on the cost of the healthcare.

Contrary to the carriage of passengers, the transport of goods is characterized by a greater level of complexity. It is because, aside from the transporting operation, the handling operations play an important part in the process [13]. Given the above, when determining the emission level and energy consumption in the carriage of goods, it is important to complete the investigations on each stage of the transport process (handling and transport). In the handling process, a variety of mechanical handling equipment (MHE) is used, fitted with different types of powertrains. One of the fundamental equipment is forklifts falling into the category of non-road mobile machinery, whose homologation process is based on tests performed under laboratory conditions. The powertrains of these machines are tested using two types of tests: Non-Road Transient Cycle (NRST) and Non-Road Stationary Cycle (NRTC). Homologation tests, performed on engine dynamometers, are also carried out on heavy-duty vehicles used in the process of the transport of goods. Heavy-duty vehicles (HDV) are tested according to the dynamic (World Harmonized Transient Cycle-WHTC) and stationary (World Harmonized Stationary Cycle-WHSC) cycles [14]. What is crucial, however, is that these tests are performed under precisely determined conditions and do not fully reflect the actual conditions of operation of these units. A multitude of research and papers [15,16,17,18] confirm that the emission of individual exhaust components during tests performed under actual operating conditions differs from those obtained under homologation conditions, sometimes even exceeding the legally imposed emission limits. The above publications show the real problem related to the testing of combustion engines whose emissions are sensitive to a variety of external factors such as the operator’s behavior, weather conditions or location of the operations. This problem pertains to both light-duty vehicles tested on chassis dynamometers and heavy-duty ones tested on engine dynamometers [19]. The solution to the above are tests that are based on a Portable Emission Measurement System (PEMS). Portable equipment allows obtaining true data and determining reliable emission levels of the tested units under actual conditions of their operation. This is particularly the case for non-road machinery whose operating regimes vary depending on their applications [8,20].

Currently worldwide, researchers conduct numerous investigations aiming at an accurate determination of emissions generated by road transport of goods, many of which were performed using portable measurement equipment. The study of available literature allows a statement that researchers utilize a wide spectrum of approaches to the problem of transport of goods devoting their works to the problem of fuel consumption, emission of CO₂ and other exhaust components or appropriate selection of the mode of transport. The majority of these works pertains to the problem of exhaust emissions exclusively during the carriage of freight with an HDV vehicle. Vierth, Lindgren and Lindgren [21] have undertaken to analyze the influence of the expansion of the Swedish fleet of longer and heavier light-duty vehicles (LHV) on the efficiency of transport and exhaust emissions. Based on statistical data, the authors have proved that the increase in the number of LDV vehicles did not have an impact on the exhaust emissions. Quite the contrary, a drop in the emissions of PM, SO₂, NO_x, NMVOC per ton-kilometer was observed, which was, inter alia, a consequence of the increased efficiency of this mode of transport. Liimatainen et al. [22] estimations have shown that the approval of LHV vehicles in the UK would contribute to the reduction of the emission of carbon dioxide (a greenhouse gas). Today, the PEMS-based measurements are becoming increasingly popular. This type of measurement provides extensive information related to the environmental performance of vehicles and machinery. One of the problems of the powertrain technology in heavy-duty vehicles is the increased emission of particulate matter and nitrogen oxides. Many works have been devoted to the determination of the actual emission of NO_x or the efficiency of selective catalytic reduction systems (SCR) [23,24,25]. Merkisz, Fuć et al. [26] have tested under actual operating conditions two heavy-duty vehicles of different specific power output indexes. The authors have shown that the selection of the vehicle with a higher index entails not only better dynamic properties but also higher fuel mileage, hence, lower exhaust emissions. Fuć et al. [27] present the influence of the cargo weight on the fuel consumption and emission of CO₂ and NO_x. The investigations have clearly shown an emission increase (several tens of percent) of a heavy-duty vehicle carrying a load compared to a vehicle performing an empty run. Merkisz, Andrzejewski, Kozak [28] focused on the dependence of the emission of CO₂ on the drive-off dynamics of a loaded heavy-duty vehicle. The investigations have shown that the driving technique has a significant impact on the emission of carbon dioxide, hence the energy consumption. The emission of individual exhaust components and the fuel consumption in a test run of a heavy-duty vehicle under actual operating conditions on roads of different classification have become an increasingly frequent subject of scientific research [29,30,31]. The application of the PEMS equipment allows a detailed assessment of the environmental performance of vehicles and machinery under almost any actual operating conditions. What is important is that majority of the publications pertain to passenger vehicles [32,33,34,35]. An increased number of investigations under actual operating conditions related to non-road machinery, railway vehicles and stationary engines has recently been observed [36,37,38]. There is, however, a relatively small number of works devoted to the measurement of exhaust emissions from handling equipment, forklifts in particular. The characteristics of the emission of CO, HC, CO₂, NO_x, PM2.5 from forklifts was a subject of the research of Pang, Zhang, Ma [39]. The authors have tested twelve different forklifts using the PEMS equipment. Fuć et al. [40] presented the emission of individual exhaust components and energy consumption for diesel and LPG-fueled forklifts. In the work of [41], the authors presented the relation between the exhaust emissions and the location of the forklift operation. Indoor and outdoor operation of diesel and LPG-fueled forklifts was investigated. The authors have clearly shown that the location and the condition of the infrastructure (the authors forced the operator to drive the machine with a preselected technique) has impact on the exhaust emissions and fuel consumption. The subject of the investigations of Al-Shaebi, Khadera et al. [42] was the influence of the operator’s behavior on the vehicle energy consumption and overall work efficiency.

One of the common features of the said publications is that their authors usually focus on a single stage of the process. Among the works devoted to determining the exhaust emissions and energy consumption during carriage of goods there are no investigations treating the process as a complex cycle, i.e., covering the handling as well as the actual road transport. The PEMS equipment equally allows an accurate determination of the exhaust emissions of the mechanical handling equipment (MHE) and tractor-trailers, which enables estimating the emission level and energy efficiency of the entire transport process. World literature does not provide the results of similar analyses. In this paper, the authors focus on the determination of the exhaust emissions and energy efficiency of the entire process of freight transport based on the investigations of the handling and transport events using the PEMS equipment

2. Materials and Methods

2.1. Research Objects

In order to determine the exhaust emissions level during the transport of goods, the authors tested a forklift and a heavy-duty truck under actual conditions of their operation. The road vehicle included a tractor and a curtain trailer (Figure 1).

The HDV vehicle was fitted with an 8-cylinder 15.6 dm³ diesel engine. Its maximum power output is 412 kW @1900 rpm. The vehicle was Euro V compliant. Additionally, the truck was fitted with a novel Scania Driving Support (SDS). The software provides information to the driver on the optimal driving style owing to the continuous monitoring of sensors fitted in various vehicle subcomponents. The task of the system is not only to improve road safety but also reduce fuel consumption and boost driver’s competence. The vehicle was carrying a load of 24 tons.

The investigations into the exhaust emissions of the handling equipment were performed on a forklift fitted with a 2.486 dm³ diesel engine of the maximum power output of 36 kW @2500 rpm (

Table 1. Research object specification.

Parameter	Forklift	Tractor-Trailer
Engine type	diesel	diesel
Displacement	2.486 dm3	15.6 dm3
Maximum power output	36 kW @2500 rpm	412 kW @1900 rpm
Maximum torque	175 Nm @2300 rpm	270 Nm @1000–1400 rpm
Number of cylinders	4	8
Emission standard	Stage III A	Euro V
Aftertreatment	-	SCR
Weight of transported cargo	1000 kg	24,000 kg
Curb weight	3560 kg	15,200 kg

). The vehicle payload was 2.5 tons. The vehicle carried a load of 1 ton at a time. The four-cylinder engine was Stage III compliant.

2.2. Measurement Equipment

For the investigations, the authors used SEMTECH DS, a portable emission measurement system (Figure 2). The equipment provided by an American supplier enables an analysis of the composition of the exhaust gas under the conditions of actual operation. The system can measure the concentration of CO, CO₂, NO_x, HC and O₂. Utilizing the on-board diagnostic and GPS systems, the equipment also records the engine parameters (engine speed, load). The method and accuracy of the measurement of each of the exhaust components have been presented in

Table 2. Semtech DS technical specifications.

Parameter	Measurement Method	Measurement Range	Measurement Accuracy
THC	Non-dispersive infrared	0–10,000 ppm	±2.5%
NOx	Non-dispersive ultraviolet	0–3000 ppm	±3%
CO	Flame ionization	0–10%	±3%
CO2	Non-dispersive	0–20%	±3%
O2	Electrochemical	0–20%	±1%
Exhaust gas flow	Mass flow Tmax up to 700 °C		±2.5% ±1%

. A gas sample taken directly from the exhaust system travels to the device through a flow meter and a heated line where, in the analyzers, the concentration of individual exhaust components is measured. The idea behind the heating of the line to the temperature of 191 °C is to counteract condensation of hydrocarbons. The gas filtered from particulate matter initially gets to the FID (Flame Ionization Detector) analyzer where the HC measurement is carried out. Then, upon chilling of the gas to the temperature of 4 °C, the concentration of NO_x (collectively NO and NO₂) is measured in the NDUV (Non-Dispersive Ultraviolet) analyzer. The last of the analyzers-NDIR (Non-Dispersive Infrared) is responsible for the measurement of CO and CO₂. The content of O₂ in the analyzed gas is measured using an electrochemical sensor.

2.3. Research Cycle

In order to carry out the investigations, the authors needed to develop a universal research cycle that would comprehensively cover each of the stages of the transport process, which is why the proposed test was divided into two parts: emission measurement during a simulation of the handling procedures using a forklift and the carriage of goods with a tractor-trailer. Today, we have many hubs, warehouses and logistic centers that vary widely in terms of their parameters such as the area, type of construction (indoor, outdoor, semi-outdoor) and store a variety of goods [43]. These factors force different variants of the handling procedures. For this reason, in order to perform an in-depth analysis of the entire transport process, the authors carried out tests in two variants. The first involved a simulation of the goods handled in an outdoor lot outside the warehouse. The second involved work inside the warehouse.

The forklift test was designed based on the VID2198 driving cycle, otherwise used to evaluate the energy demand for forklifts under conditions close to actual operation. The modification of the procedure enabled a reproduction of the actual handling works. This part of the research trial had the following stages:

• Loading, lifting the pallet (1 m), (lowering the palette at location A)
• Leaving location A
• Transport from location A to location B
• Dropping the load in location B (lowering the palette and unloading)
• Return to initial location

The weight of the loaded palette was 1 ton.

The tractor-trailer was tested under actual traffic conditions. The procedure covered a trip on the urban, local, and national roads whose total length was 27 km. The key roles were played by the test route and the location of the test. The authors paid particular attention to truly reproduce the transport system of goods with heavy-duty vehicles. For this reason, the test started and ended in the city’s industrial zone hosting a multitude of production facilities. The test of the collective duration of 30 min. included a drive through the city (A-B) and a trip to the motorway junction (B-C-D). The last stage was the return to the initial location. Figure 3 presents the test route of the tractor-trailer and forklift research cycle.

3. Results

The paper presents a detailed analysis of the exhaust emissions and fuel consumption by machines used in the process of freight transport. The energy demand was calculated based on the carbon balance method, which is currently the most accurate one in determining the exact value of the fuel mileage. The accuracy of the method results from the inclusion of the obtained emission level in the calculations.

The authors of this paper have analyzed the obtained results separately for the handling and the transport processes to estimate the emission level and energy consumption of the entire transport cycle. For the sake of an in-depth analysis of the handling process, the study included the lifting and lowering of the palette, a run with the load and without it. The results of the comparison of the road emission for the measurement performed inside the warehouse and on the lot considering individual stages of the forklift test have been presented in Figure 4. From the graph, it directly results that during the operation of the forklift on the lot, the emission of carbon monoxide increased by 19% for the forklift carrying a load and 87% for the empty run. The same trend was observed for nitrogen oxides. For the latter exhaust component, the authors observed a significant (approx. 50%) increase in its emission during an empty run on the lot. It results from a sudden increase in the vehicle speed and dynamics compared to a run with the load. Hydrocarbons had a different trend. The carriage of cargo inside the warehouse was characterized by a small increase compared to the empty run.

The overall analysis of the handling processes for 1 ton of freight has been presented in Figure 5. The investigations have clearly indicated an increase in the emissions of carbon monoxide by approx. 13% during the outdoor test. The situation for the nitrogen oxides was different. An increased emission thereof was observed during the indoor operation. These trends are directly related to the quality of the pavement, on which the vehicle operated. The good quality of the infrastructure inside the warehouse allowed a more dynamic operation. These conditions resulted in the increase of the emission of NO_x by 9% compared to the same test run performed outdoors where the lot was paved with concrete sett. The uneven surface forced the operator to adapt its driving style to the conditions to prevent damage to the handled goods. The authors also determined the average emission in time of individual exhaust components: 6.097 × 10³ g/h, 8.88 g/h, 29.57 g/h and 3.69 g/h for CO₂, CO, NO_x and THC, respectively. The obtained results are similar and remain within the range of average emissions obtained by Pang et al. [39] in their analysis. In the said publication, the testing of 12 diesel-fueled forklifts has shown that depending on the work cycle, the time emission indexes may reach 1.9–12.3 g/h, 11.2–78.0 g/h, 12.2–72.3 g/h, 12.2–72.3 g/h for CO₂, CO, NO_x and THC respectively.

Testing heavy-duty vehicles under actual traffic conditions is becoming increasingly popular when it comes to determining the environmental performance of vehicles of this category. Currently, works are under way to include RDE (Real Driving Emission) tests in the homologation procedure of HDV vehicles. The authors of this publication performed an analysis of the influence of the driving conditions on the emission of individual exhaust components. From the obtained data one may observe that, in the initial phase corresponding to the city drive, an increased emission of nitrogen oxides was recorded (on average 110 mg/s). An important factor influencing the formation of nitrogen oxides is the temperature of the flame front in the combustion chamber. It is noteworthy that using urban roads forces a more dynamic driving, which is conducive to the occurrence of such combustion chamber conditions. Additionally, as a matter of principle, the test was to reflect the real driving routines of heavy-duty trucks, which is why the authors decided to start the test from a cold-start condition. This resulted in a relatively low temperature of the exhaust gas, as a result of which the SCR system could not efficiently reduce the generated NO_x. Similar conclusions were drawn by Li and Lü [23] whose RDE tests also confirmed the highest emission of nitrogen oxides during a city drive. As opposed to NO_x, the emission of hydrocarbons had a different trend. The course of the emission of this component was characterized by significant variations on the level of 42.5 mg/s. As per the authors’ results, in the first phase of the test, a marginally lower emission of unburnt hydrocarbons was observed. The difference resulted from the lower speeds of the vehicle in this part of the test. The emission of carbon monoxide remained at a relatively constant level of slightly above 26 mg/s. When analyzing the obtained characteristics (Figure 6), one may observe several sudden emission peaks. The reason for their occurrence was sudden increases in the vehicle acceleration when the combustion chamber was supplied with a greater amount of fuel. The average emission of carbon dioxide during the test was 13.4 g/s. The increased emission of this component was observed when driving in the extra urban conditions. This was directly related to the greater speed of the vehicle, which resulted in a greater demand for fuel. One may, thus, infer that when carrying loads with tractor-trailers, the important emissions-impacting factors are mainly the vehicle speed and the driving dynamics resulting from the road conditions and the skills of the driver.

The performance of the above analyses enabled an estimation of the exhaust emissions and the fuel consumption in the entire transport cycle. The forklift test involved the transport of 1 ton of goods. During the investigations, the tractor-trailer carried 24 tons of cargo. In light of the above, in order to reproduce the full handling process, in the cycle analysis, the authors included 24 individual stages of the forklift operation. Given that the machines perform works of different nature, the emission indexes of individual exhaust components were calculated. Because it was impossible to record the operating parameters of the forklift working inside the warehouse, specific emission was not calculated. The authors calculated the emission referred to the distance covered by the research object. Additionally, the authors estimated an approximate emission referred to the distance covered in the entire transport cycle (

Table 3. Road emission of the transport cycle.

Transport Stage	CO2	CO	THC	NOx
	g/km	g/km	g/km	g/km
Loading of cargo inside the warehouse	2145.35	3.13	10.41	1.30
Loading of cargo on the lot	1820.23	3.38	9.55	0.99
Carriage of cargo with a tractor-trailer	1013.91	2.01	4.12	3.22

).

When comparing the obtained data, one can observe that the tractor-trailer was characterized by the lowest emissions. This results from a much greater distance covered during the test. Forklifts, due to their nature of the operation, covered a maximum distance of a few kilometers. In the case of the 24 test runs, this distance amounted to 1.5 km. Depending on the location of the performed forklift operations, it is estimated that during the performance of the entire transport process (including loading, transport, and unloading of cargo) approx. 30 kg of CO₂, 59 g of CO, 127 g of THC, and 89 g of NO_x were released to the atmosphere. The differences between the cycle when the loading was performed inside the warehouse and the outdoor cycle are minuscule and amount to approx. 1%. There are not many scientific works treating the real emissions in the process of transport of goods that would cover all its stages. It is, therefore, difficult to find data that would constitute a reference to the results obtained in the above analysis.

The authors have also calculated the fuel mileage (Figure 7). In the discussed investigations, the forklift operating outdoor on the lot was characterized by a lower fuel demand. During the loading of 1 ton of cargo under different conditions, an approx. 17% increase in the energy demand was observed that was eventually calculated to be approx. 81 dm³/km. In relation to HDV vehicles, the energy consumption mainly depends on the covered distance, vehicle speed, and weight of the transported cargo. From the performed measurements it results that the fuel demand of the tractor-trailer reached a level slightly higher than 38 dm³/100 km. On the other hand, tests performed on a vehicle of the same make but fitted with a Euro III compliant engine have shown a fuel mileage on the level of approx. 41 dm³/100 km [44]. In the test presented in Ziółkowski et al. [31], the fuel mileage of the investigated vehicle (the same vehicle was used for the investigations of the transport cycle described in this paper) was 30.9 dm³/100 km.

The exhaust emissions in the carriage of goods are tightly related to the weight of the transported cargo. Therefore, the authors decided to refer to the emissions of individual exhaust components in the performed transport work of the investigated vehicles. For the transport of 24 tons of cargo with a tractor-trailer (Figure 8), the following emission indexes were obtained: CO–0.08 g/tkm, NO_x–0.17 g/tkm and THC–0.13 g/tkm. Out of all the analyzed exhaust components, nitrogen oxides were characterized by the greatest index. The emission index of CO₂ in the tests was 42.25 g/tkm. Eickman [45] tells that the emission of carbon dioxide per ton-kilometer on the route Lehrte–Hameln (Germany) was 80 g/tkm for a tractor-trailer and 49 g/tkm for a train with 20 railroad cars. It is noteworthy that these values were estimated based on ecological databases of local and long-distance traffic.

In the case of a forklift, greater emission indexes of NO_x and THC were recorded for the test runs performed inside the warehouse. It reached a value greater by 8% and 35% respectively compared to the values of the same obtained during the outdoor test runs. An opposite trend was observed for carbon monoxide whose emission per ton-kilometer was over 8% higher when measured in the outdoor test (Figure 9). The emission of CO₂ in both cases was similar and amounted to over 3 g/tkm.

4. Conclusions

The measurement method of fuel consumption and exhaust emissions proposed by the authors allows a detailed analysis of the entire freight transport cycle including its individual stages. It is noteworthy that the analysis discussed in this paper is one of the first attempts to estimate the emissions level and energy consumption of the entire process of freight transport. However, we should bear in mind that both the energy demand and the emission of individual exhaust components depend on a variety of external factors such as: weather conditions, driving dynamics, driver’s skills, vehicle technical conditions, etc. The consumption of the fuel used to power internal combustion engines also has a significant impact on emissions. Therefore, it is important to understand that the conclusions drawn from the analysis pertain to a specific transport cycle. The method applied by the authors, however, gives a comprehensive view of the emission trends in the process of freight transport. Based on the performed research, the authors confirmed that a great deal of influence on the emissions from forklifts has the manner of their operation largely resulting from the quality of the infrastructure. A more dynamic driving inside the warehouse contributed to a greater emission of nitrogen oxides. Higher operating speeds of the machine resulted in greater fuel consumption. It was the forklift, to which the greatest fuel demand was attributed in the entire transport cycle. This mainly results from the different nature of operation of this vehicle. It is noteworthy that, in relation to the distance covered by both vehicles in the entire cycle under analysis, the share of distance covered by the tractor-trailer was 95%. One should note that transport of goods with HDV vehicles is carried out over a variety of distances. The test of the tractor-trailer has shown that driving on city roads resulted in a three-times greater emission of NO_x compared to other parts of the test. This directly results from the impossibility to activate the SCR system and increased driving dynamics. The observations and conclusions arising from the performed analyses may turn out useful in both the organization of the logistic processes and the design of the logistic infrastructure.