1. Introduction
For nearly fifty years, concerns about petroleum depletion have greatly affected the development of internal combustion engines, but nowadays, the anxiety about climate change and environmental degradation caused by vehicles or engines has even exceeded the threat of resource depletion [
1]. As a matter of fact, the exhaust emissions of vehicles do have extremely harmful effects on the atmospheric environment and human health [
2]. Diesel engine exhaust was classified as carcinogenic to humans (Group 1) by the International Agency for Research on Cancer (IARC) [
3]. It has also been reported that nitrogen oxides (NOx) and particulate matter (PM) with a diameter of smaller than 2.5 μm (PM2.5), abounding in the exhaust of diesel engines, may cause pathological changes in many important organs of the human body [
4,
5]. Nevertheless, the status of diesel engines in commercial vehicles, especially in the field of heavy-duty vehicles (HDV), still cannot be shaken at the wave of electrification. Because compared with the limited range and payload of electric vehicles [
6], diesel vehicles have a longer range, stronger load capacity with much more complete supporting facilities. Considering the huge market share of HDV, diesel engines will be one of the principal contributors to air pollution for a long time in the foreseeable future, particularly NOx and PM. Therefore, increasingly strict emission regulations have been made around the world [
7].
In fact, people gradually realize that fossil fuel is the core of the problem, not the engines themselves [
8]. Consequently, research on alternative fuel for engines has attracted wide attention. Blending oxygenated fuels with diesel is one of the potential methods to reduce the emission level of engines [
9,
10]. Alcohols, esters, and ethers were studied as fuel or diesel fuel additives and proved that these sorts of oxygenated fuels all have certain effects on reducing PM and gaseous emissions, while ethers are better than others in terms of combustion owing to the high cetane number (CN) and oxygen content [
11]. Among them, polyoxymethylene dimethyl ethers (PODE) have the most potential [
11]. With the success of carbon dioxide (CO
2) hydrogenation to methanol over catalysts [
12], the conversion of methanol into PODE as a diesel fuel additive becomes an effective way to reduce CO
2 emissions and even achieve carbon neutralization in the fuel life cycle.
Therefore, massive amounts of research have been carried out on PODE/diesel blends, including the measurement of fuel properties, studies of engine combustion and emission characteristics, etc. It has been proved that PODE blended fuels are beneficial to the whole engine combustion processes, such as spray atomization [
13], shorter ignition delay, more constant volume degree of combustion [
14]. The accelerated combustion rate has also been reported due to the high oxygen content and high volatility of PODE [
15]. It has also been proved in most research that the increased oxygen content of blended fuels makes engine carbon monoxide (CO), total hydrocarbon (THC), and soot decrease significantly with the punishment of a little increase in NOx emission [
16,
17,
18]. However, more feasible exhaust gas recirculation (EGR) strategies can be applied to reduce NOx emission without deterioration of the engine combustion and soot emissions [
15]. It has previously been observed that the reduction in PM and particle number (PN) emissions, because of the blending of PODE [
19]. The effect of PODE blending on brake thermal efficiency (BTE) has been reported that BTE increased with the rise of blending ratio [
14,
17]. Of course, the brake specific fuel consumption (BSFC) [
17] and the engine brake power could worsen due to the decline of the lower heating value (LHV) and the fuel injection strategy.
However, almost all of the research on PODE/diesel blends was based on laboratory conditions. Steady-state data obtained from the engine dynamometer experiments unable to capture transitory operation caused by start/stop events, transformations of traffic conditions, or turbocharger lag [
20]. In fact, there are significant differences between real driving emissions (RDE) and steady-state engine dynamometer experimental results [
21,
22,
23]. Therefore, portable emission measurement systems (PEMS) have been adopted to test the RDE of vehicles, which has also become a mandatory portion of the latest regulations in CHINA VI and Euro VI [
7,
24]. PEMS investigations can not only assess emission levels of a specific pollutant under real driving conditions but also exhibit RDE related vehicle features [
25], driving style [
26] or external environments [
27], and so on. To the best of our knowledge, there is currently no data on performances of PODE/diesel blended fuels under real driving conditions, which is crucial for the application of the PODE as a diesel additive. To fill this knowledge gap, this study investigated the emission characteristics and fuel economy performances of a CHINA VI heavy-duty engine fueled with PODE/diesel blends of different blending ratios under real driving conditions.
In this paper, on the basis of the CHINA VI emission regulation about the RDE test, a test road was selected, which consisted of urban, rural, and motorway segments connected together. A CHINA VI certificated semi-trailing tractor was used, and it ran on the fuels of pure CHINA VI 0
# diesel and PODE/diesel blends in mass proportions of 20% and 30%, respectively. The exhaust pipelines were modified to sample the original exhaust without any aftertreatment. Consequently, the pollutant emission characteristics of CO, CO
2, NOx, and PN were detected by PEMS. Furthermore, transient engine maps have been created directly from the RDE test data of the on-board diagnostic (OBD) system and PEMS [
20,
28,
29]. In addition to carrying out RDE tests under the conditions of using PODE/diesel blended fuels, the novelty of this paper mainly focuses on the following three aspects. Firstly, the identification of operating ranges with high brake specific emissions under real driving conditions when using the PODE/diesel blends. Secondly, the impact of the addition of PODE on emission characteristics and fuel economy performances under different road conditions (urban, rural, and motorway). Thirdly, the difference between the measured RDE and the result of the steady-state experiment of the engine of the same model. The results of this study offer support for the impact of PODE addition on engine transient emission performance and also provide important references for the application of PODE/diesel blends in diesel engines.
3. Results
In this paper, pure diesel named D100 and the PODE blended fuels of DP20 and DP30 were tested. Their emission and fuel economy characteristics were processed into colored maps. Because of the regulations, CO, NOx, and PN emissions were investigated, HC emission was not concerned; however, it is in the same or little lower level as CO [
38]. Based on the data, the brake power specific and distance specific parameters of the test and each road segment of the test were studied respectively in the following sections.
3.1. Engine Operation Condition Statistics
As shown in
Figure 5, the meshed operation map of the engine during the real driving test of D100 can give a direct impression on the engine conditions, and it can be seen that most of the engine operating conditions were within the speed range of 1150 to 1250 rpm. For a more detailed operation time distribution,
Figure 6 illustrates the proportion of time spent in different speed and torque operating ranges in three driving sections. Each row of subgraphs is for the same fuel condition, and from left to right in each row are the urban, rural, and motorway driving conditions under the corresponding fuel conditions. Each grid node represents the engine operating condition range of 250 rpm and 250 N·m. The percentage of time at each node for all three driving conditions under the corresponding fuel condition was marked and also reflected by the size of the point. It can be seen clearly that the engine operation pattern during the real driving test statistically. The engine ran in a similar range of operating conditions in the rural section and the motorway section. From the perspective of torque, the engine mainly ran in the range of 0–250 N·m during the urban section, while the common torque range was extended to 0–1000 N·m and 0–1250 N·m, respectively, in the rural and motorway sections. From the perspective of engine speed, the speed range of 1000–1250 rpm occupied about 80% of the total time, and the speed range greater than 1250 rpm only occupied around 3% of the total time.
3.2. CO2 Emission and Fuel Consumption Rates
There is no doubt that the CO
2 emission rate is closely related to the fuel consumption rate; therefore, their performance maps are quite similar, seen in
Figure 7. Though they look exactly alike, to analyze carefully, the regions representing higher fuel consumption rates (red, dark blue, and dark green) expand downwards to some extent in the maps of DP20 and DP30 compared with the map of D100, which indicates that more blended fuels were burned during the tests due to the decrease in the lower heating value of the blended fuels for the addition of PODE. The impact of the increase in fuel consumption rates, however, can be offset to some extent due to the decrease in the carbon content of the blended fuels [
39], resulting in no significant change in CO
2 emission rates.
3.3. Characteristics of BSFC and BTE
Due to the smaller LHV of PODE, more blended fuels could be consumed in order to produce the same engine power, which led to the rise of BSFC.
Figure 8a shows the calculated results; yellow bins become less while green and blue bins become more in the BSFC maps of DP20 and DP30. The BTE map of DP30 in
Figure 8b shows much better compared to the D100 case. The red region indicating higher BTE expands and moves downward to lower load conditions. When using the PODE/diesel blends, the improvement of the BTE mainly lies in the improvement of fuel spray, a faster burning speed, and more sufficient oxygen. The lower viscosity and smaller surface tension of the PODE are beneficial to reduce the Sauter mean diameter (SMD) of the blended fuel spray [
13]. The better performance of spray atomization contributes to the improvement of combustion conditions, which has an effect on improving brake thermal efficiency. Higher volatility and ignitability of the PODE increase the mixing rate and chemical reaction rate, respectively, which shortens the combustion duration [
17]. The rise in the degree of constant volume caused by the growth in the combustion rate increases the thermal efficiency of the engine. The oxygen-lack condition of the over-rich mixture is improved because of the intramolecular oxygen of the PODE [
14], which promotes the combustion more sufficient and improves the thermal efficiency.
The addition of PODE can improve the brake thermal efficiency of the vehicle.
Figure 9 illustrates the averaged brake thermal efficiencies in the different driving sections. The application of DP20 and DP30 increases the BTE by 9.9% and 15.7%, respectively, when driving in the urban section. The test averaged BTE of D100, DP20, and DP30 are 38.4%, 38.0%, and 40.3%, respectively. The abnormal BTE of DP20 can be attributed to driving conditions, for there is less high load condition recorded as shown in
Figure 8b. The importance is that 30% PODE addition promoted BTE nearly 2 percentage points.
3.4. Engine CO, PN and NOx Emission Characteristics
Figure 10 shows the transient engine performance maps of CO, PN, and NOx emissions. For the emission rates of CO and PN compared to pure diesel D100 operation, most areas in the maps of DP20 and DP30 are gray, implying that the addition of the PODE makes the engine emit quite lower. It is also obviously indicated that the more addition of PODE, the higher the reduction in PN emission. For their brake power specific emissions, D100 operation gives the highest BSCO and BSPN, the regions of the same level of BSCO and BSPN conditions come down significantly due to PODE addition in diesel, and higher BSCO and BSPN occur under low load conditions (0–250 N·m) for the three test fuels. The intramolecular oxygen and the absence of the C-C bond in PODE, as well as the increase in oxygen content in the blended fuel, can prompt the combustion within the engine cylinder, which results in the reduction in CO and PN formation and emission [
14]. The PODE, however, has higher volatility, which means the lean mixture is more likely to volatilize into unburned hydrocarbons and be further oxidized to CO when using the DP30. At low load conditions and around 1200 rpm, however, there are more bins with higher CO emission levels on the map of DP30 than on the map of DP20. Considering the high latent heat of vaporization and the low lower heating value of the PODE [
14], the main reason is that the combustion temperature is lower at low load conditions, and the increase in the PODE blending ratio further reduces the temperature of the in-cylinder gas, thereby inhibiting the oxidation of the CO to some extent.
The NOx emission rate is a little higher in the maps of DP20 and DP30 for the downward expansion of the red region. The differences of BSNOx maps are presented under higher load (>750 N·m) and engine speed conditions for PODE addition. The addition of PODE increases the oxygen content in the cylinder, which promotes the formation of NOx.
Figure 11 exhibits boxplots of the test results of CO, PN, and NOx emission rates in the three segments of the driving cycle. In each box, the star and the bar present the mean value and the median value of all data in the corresponding road section, respectively. Different from
Figure 6, which shows the engine operating conditions and related time,
Figure 11 exhibits the measured emissions statistically. Though there is a larger dispersity due to the transient operation, such as rapid acceleration, which is out of the scope of this paper,
Figure 11a–c still gives clear characteristics about CO, PN, and NOx emission levels. Diesel operation gives out more CO and PN emissions in the whole driving cycle, and PODE addition reduces about half of CO and PN emissions. Though a little increase can be seen observed, NOx emissions are at the same level for the test fuels.
From the perspective of distance,
Figure 12 presents the calculated distance-specific emissions, and they are compared with the brake specific ones under the same driving section. It can be seen that distance-specific parameters of CO, PN, and NOx emissions are quite similar to those of brake specific ones in their tendencies. The RDE limits of the CHINA VI regulation are marked on the figures (Except for the CO RDE limit, because the CO RDE limit, which is 6000 mg/kWh, is out of the axis range). The PN and NOx RDE measurement results were much larger than the limit since the raw emissions were measured in this study.
For the CO emissions, the heavy-duty engine used in the experiment can already control the original BSCO to a fairly low level. Because even if the diesel is used, the BSCO emissions of the rural section, the motorway section, and the whole test route under the real driving conditions are even lower than the steady-state limit of the CHINA VI regulation (1500 mg/kWh). The WHTC and WNTE limits of the CHINA VI regulation are also marked on the figure. The addition of PODE can even further reduce DSCO and BSCO by about 50%. Since the DP30 has less effect on CO emission reduction than the DP20 in the operating range with the largest proportion of time in the urban section (1000–1250 rpm, 0–250 N·m), shown in
Figure 6 and
Figure 10(a-1), the DP30 has a rise of DSCO and BSCO than DP20 in the urban segment.
The decreases in DSPN and BSPN are both positively correlated with the PODE blending ratio. DP20 and DP30 reduce DSPN and BSPN by approximately 40% and 55%, respectively.
For CO and PN emissions, the DSCO(PN) and BSCO(PN) of the urban section are significantly higher than those of the rural or the motorway section, indicating that the urban driving mode must be considered carefully both to the distance specific and brake specific parameters in order to control CO and PN emissions.
For the NOx emissions, the rural section and the motorway section reflect the strongest deteriorating effect of PODE addition on the DSNOx emission and the BSNOx emission. From the perspective of the entire test route, the usage of the DP20 could increase DSNOx and BSNOx by 18.0% and 19.3%, respectively. The usage of the DP30 could increase DSNOx and BSNOx by 17.9% and 14.7%, respectively.
5. Conclusions
In this paper, the real driving emissions of CO, PN, and NOx, as well as the vehicle fuel economy of a CHINA VI heavy-duty diesel vehicle, were measured with an AVL PEMS to study the effects of PODE addition. Experiments were carried out on the selected roads consisting of three segments of urban, rural, and motorway sections. The sampled data met the requirement of the RDE test of CHINA VI regulation. The data from the PEMS and the OBD system were statistically analyzed to illustrate engine transient operation maps. By studying characteristics of the engine operating condition and time, fuel consumption, BTE, and the averaged distance-specific and brake specific emissions, the following conclusions can be made:
- (1)
The addition of PODE had a fairly obvious inhibitory effect on the transient CO and PN emissions in a considerable range of operating conditions. However, even if PODE/diesel blends were used, the low load condition (0–250 N·m) was still the area with the highest BSCO and BSPN emissions.
- (2)
Under real driving conditions, the usage of PODE/diesel blends can increase BSNOx emissions in a wide range of operating conditions, which makes the high load condition (>750 N·m) high emission areas of BSNOx.
- (3)
Regardless of the fuel type, the CO and PN emissions in the urban section were the highest. From the perspective of the entire test route, the addition of PODE can reduce the emissions of CO and PN by about 50%. On the contrary, the NOx emissions were the highest in the motorway section. The usage of blended fuels can increase the overall NOx emissions by no more than 20%.
- (4)
The addition of PODE can lead to an increase in fuel consumption due to its lower LHV. The averaged BTE of the whole RDE test, however, reached 40.3%, which was better than 38.4% of the pure diesel operation when fueled with the DP30.
For the engine without EGR used in the experiment, the transient NOx emissions under real driving conditions were still increased when the PODE/diesel blends were used, even though the addition of PODE had an inhibitory on NOx emissions under certain steady-state operating conditions. The workload of the SCR system will increase, which means the matching and the optimization of the aftertreatment system for the application of the PODE/diesel blends may become a new challenge. However, the reduction in CO and PN emissions and the improvement of the vehicle BTE in the RDE tests are fascinating, creating greater tolerance for the EGR. Therefore, the application of PODE has considerable potential in controlling transient emissions. The impact of the addition of PODE on the aftertreatment system under real driving conditions needs to be investigated in detail in the future.