Scilight Press

1. Introduction

With the continuous increase in the number of vehicle and the continuous improvement of people’s environmental awareness, countries around the world have been tightening various environmental protection measures in recent decades. As an important source of gas and particulate matter pollution, vehicles have also been in the spotlight of public attention [1,2]. Chinese automobiles have been developing rapidly since 2000, with a continuous increase in the number of domestic vehicle. In 2018, the production and sales of vehicles exceeded 23 million, and in the first half of 2019, national vehicle ownership reached 250 million. The rapid growth of vehicle ownership provides convenience for people’s lives and effective support for economic development, but it also brings many problems to society, such as environmental pollution, traffic congestion, and an energy crisis. With the increasing awareness of environmental protection among people and the strengthening of national political air pollution, the control of automobile exhaust emissions has become a key factor in the technological development of the automotive industry [3,4]. From the implementation of Euro I in 1992 to Euro VI in 2013, it took 21 years for Europe, and from the implementation of the national standard in 2000 to the release of GB18352.6-2016 Emission Limits and Measurement Methods for Light-duty Vehicles (China’s Sixth Stage) in 2016 [5,6]. It took 16 years to catch up with European emission regulations, and increasingly stringent emission regulations have put forward stricter requirements (shown as Table 1 below) for vehicle emissions of pollutants [7].

GB18352.6-2016 also stipulates the implementation of RDE emission requirements and corresponding extension conditions from 1 July 2023, for detail please refer to Figure 1 below.

Figure 1. RDE emission requirements and expansion conditions.

There have been many studies on the emission performance characteristics of traditional vehicles under different altitude conditions, but there is currently limited research on emerging PHEV plug-in hybrid vehicles. The focus of this study is to investigate the ambient temperature emissions of PHEV vehicles in plateau environments, as well as the gas and PN emission characteristics of plateau RDE and plateau mountainous RDE [8,9].

2. Research Vehicle

This article takes a PHEV equipped with a 1.5 T direct injection engine as the research vehicle, and specific parameters of the vehicle are shown in Table 2 below.

3. Research on WLTC Cycle Emissions

Set different balance SOC values for the WLTC test cycle in the plateau condition to confirm the impact of different balance power settings on engine gas pollutants and PN emissions. The results are as follows:

The balance SOC setting of plug-in hybrid vehicles has a significant impact on the WLTC test cycle. In the low-speed range of WLTC, the vehicle speed is low, and after the engine starts for the first time and the SOC reaches the balance SOC, it often takes a long time for the SOC to drop to the starting SOC threshold. At this time, the temperature of the catalyst that has just started will drop again under the wind blowing condition, and the second engine start is equivalent to a cold start of the engine again [2]. Therefore, the balanced SOC setting results in two peaks for both gaseous pollutants (CO, THC, NO_x) and PN during low-speed engine cold start conditions, which greatly increases the emissions of the WLTC cycle (as shown in Figures 2–5) and increases the risk of exceeding emission standards. The engine balance SOC setting allows the engine to start after the low-speed range (as well as the medium speed range, high-speed range, and ultra-high speed range). Due to the high vehicle speed and high energy demand, the number of engine starts decreases, especially the interval between the first two engine starts becomes smaller, greatly reducing the cooling effect of the upstream wind on the catalyst. This is equivalent to only one cold start in the entire WLTC cycle, and the corresponding emissions of various pollutants will also be greatly reduced.

Different from plain emissions, plateau working conditions involve more climbing, and due to air density, the engine’s power performance decreases. It is necessary to improve the battery’s discharge capacity to compensate for the lack of engine power [10]. At this time, the balance power needs to be increased, and the corresponding engine start time will also be updated. If the engine start time is in the low-speed range, plateau emissions will be the same as plain emissions. Compared with balance SOC level B in the figure below, balance SOC level A has 51% more CO emissions, 27% more THC emissions, 13% more NO_x emissions, and 63% more PN emissions. Therefore, the PHEV plateau balance SOC setting needs to be confirmed according to the actual situation to ensure that plateau emissions meet development goals [11,12].

Figure 2. WLTC test cycle CO emissions (different SOC level).

Figure 3. WLTC test cycle THC emissions (different SOC level).

Figure 4. WLTC test cycle NO_x emissions (different SOC levels).

Figure 5. WLTC test cycle PN emissions (different SOC levels).

4. Research on RDE Cycle Emissions

The WLTC test cycle for China 6b I Type I test was implemented in China in 2019. It adopts fixed sliding resistance and a constant temperature of 23 degrees for testing. This test has its limitations as it cannot reflect the emission behavior of vehicles on actual roads. The RDE regulations have also been implemented since July 2023, and RDE can better reflect the real driving emission situation. The boundary conditions for RDE testing are specified in the China 6b regulations, including driving behavior, operating conditions (urban, suburban, highway), environmental factors (altitude, temperature), etc. A comparative study was also conducted on the engine operating area and pollutant emissions under normal and intense driving behaviors in plateau RDE and plateau mountainous RDE conditions.

In terms of the operating area of the engine, compared with the plain RDE condition, the air pressure decreases and the air density decreases under plateau conditions, resulting in poorer engine power performance. At the same speed, the engine torque is higher under plain conditions. During the plateau RDE process, in order to meet the power demand of the vehicle during driving, it can only be compensated by increasing the engine speed. Therefore, the engine speed of plateau RDE and plateau mountainous RDE is generally higher than that of plain RDE shown as Figure 6 below. Compared with RDE in plateau areas, RDE in plateau mountainous areas may experience more rapid acceleration under high load conditions due to road undulations and altitude fluctuations, resulting in greater power demand. Although the engine speed is at the same level, the engine torque in RDE conditions in plateau-mountainous areas is significantly higher than that in plateau RDE conditions. This will make the transient operating conditions (rapid acceleration and deceleration) of the engine more severe [13], and at the same time, the risk of increased exhaust temperature caused by high load conditions and ultimately leading to deterioration of (CO and PN) emissions, is greatly increased.

Figure 6. Comparison of RDE engine operating areas in plain-plateau-mountainous areas.

In terms of pollutant emissions, under the same driving behavior (such as normal driving and intense driving), the average travel emissions of RDE in the plateau mountainous areas are higher than those in the plateau RDE. This is because the higher the altitude, the slower the combustion, and the worse the combustion process, which will lead to an increase in PN and gas emissions. In addition, in plateau areas, the air is thin and the engine’s power performance deteriorates, which indirectly affects the shifting strategy and leads to the engine reaching the high-speed area [14]. The mixing time of the mixture in the high-speed area becomes shorter, resulting in a decrease in the uniformity of the mixture and the production of more gas and PN emissions (as shown in Figure 7 below), even though the overall travel emission seems no big difference, CO emissions under normal driving conditions in high-altitude and high-altitude mountainous areas are 6.3 and 6.6 times higher than those under plain conditions in urban conditions, respectively. NO_x is 1.1 and 1.6 times higher, and PN is 1.3 and 3.1 times higher.

Figure 7. RDE emission comparison for Plain-Plateau-Mountain conditions.

According to the emission standard GB18352-2016, the driving behavior of RDE can be measured by the [va_pos]95 index to determine the degree of aggressiveness of its driving behavior, as shown in the following figure, which divides the degree of aggressiveness of driving behavior. As shown in Figure 8, urban areas, suburbs, and highways are classified into three levels: normal, intense, and super intense. In any case, the emission results under the three types of intense driving behaviors must meet the requirements of RDE regulations for testing conditions.

Figure 8. Classification of driving behavior.

A study on emissions under intense driving behavior in the plateau and the plateau mountainous areas was conducted. The experimental results showed that regardless of gas or PN emissions, the urban and travel emissions under RDE conditions in the plateau mountainous areas were higher than those under plateau conditions. However, under the same driving conditions (such as plateau and plateau-mountainous areas), the urban and travel emissions under intense driving behavior were higher than those under normal driving behavior. This is because as the intensity of driving increases, the engine usage area expands, and the engine operation may enter the exhaust temperature enrichment zone, scavenging zone, and emission deterioration caused by transient rapid acceleration [15].

5. Conclusions

By analyzing the Worldwide Harmonized Light Vehicles Test Cycle emissions of Plug-in Hybrid Electrical Vehicle under different balance SOC levels, and comparing the results of different driving behaviors under plain-plateau-mountainous conditions, the following conclusions are drawn:

(1) The SOC level in plateau working conditions should be optimized separately. The results of plateau emissions vary greatly under different SOC level settings. The principle of setting the SOC level is to avoid the low-speed section of the WLTC test cycle with weak power demand as much as possible, reduce the number of cold starts, and improve the compliance of WLTC results.

(2) Under plateau conditions, the engine’s power performance deteriorates, and the usage area expands, resulting in higher RDE emissions compared to plains. Gas and PN emissions increase significantly under plateau and mountainous urban conditions.

(3) Compared with normal driving behavior, aggressive driving behavior will increase the range of engine usage, which in turn will increase the deterioration of gas and PN emissions caused by increased exhaust temperature, scavenging, and transient operating conditions.