The real driving emission characteristics of light-duty diesel vehicle at various altitudes

Highlights

• The effect of altitude on vehicle emissions under RDE has been studied.

• There is great influence of after-treatment on the vehicle emissions.

• NOx emissions slightly decrease at 2990 m in whole test cycle.

• Different tendency occurs for NOx and PN emissions in specific test modals.

Abstract

High altitude shows a great influence on vehicle emissions because of lower pressure and shortage of oxygen concentrations. In addition, type approval test of light duty vehicle is usually carried out below the elevation of 1000 m. In order to address the effect of high altitude on vehicle emissions, real driving emission (RDE) test procedure has been introduced in China VI emission regulation. Thus, the effects of altitude on the emissions from a China IV certificated light-duty diesel vehicle under real road driving conditions were investigated in this work. Research results show that CO emissions increased with the elevated altitude. At the altitude of 2990 m, the CO emissions increased by 209% in comparison with that of near sea level in whole test cycle. At specific test modes, CO emissions follow the sequence of urban > rural > motorway. Both PN and NO_x emissions also rose with altitude while NO_x emissions at 2990 m showed a decreasing tendency. In addition, different tendency between high altitudes and relative low altitudes was observed under specific test modes, which was probably related to the function of after-treatment device.

Introduction

Great utilization of diesel vehicles contributes to the large-scale pollutant emissions to atmosphere, and thus results in the growing concern about civil ambient air and human health (Yang et al., 2017a). It is well known that some factors such as vehicle weight, fuel, driving pattern, exhaust after-treatment device and altitude will influence the exhaust emissions. Among them, altitude is pretty important because both the local air pressure and oxygen content decrease with the increasing altitudes. As a result, the power output reduces (Benjumea et al., 2009; He et al., 2011; Zhang et al., 2016a); the in-cylinder combustion of diesel engines deteriorates (Liu et al., 2017); the fuel consumption rises and thermal efficiency decreases (Wang et al., 2013b). Especially, in comparison with the operation at near sea level, the vehicle emissions are larger at high altitudes (Human et al., 1990). In addition, there are a lot of high altitude cities among the world. In China, the area of high altitude regions with altitude higher than 1000 m could be approximately 65% of territory. Among these regions, the altitude above 2000 m accounts for 33% (Yu et al., 2014). Hitherto, more than 15 million vehicles are registered within these regions. Moreover, the total vehicles registered population in some high altitude provinces, such as Yunnan, Qinghai and Tibet, occupies 20% of China's vehicle population. (Liu et al., 2017; MEP, 2017). Increasing vehicle emissions are generated every day. Thus, it is important to effectively control emissions in these regions.

Hitherto, there are numerous publications focused on the influence of altitude on the diesel vehicles performance. Wang et al., 2013a, Wang et al., 2013b investigated the effects of altitude on the thermal efficiency and emission characteristics of a Euro III diesel engine by mobile engine test bench and a portable emission measurement system (PEMS). The results showed that thermal efficiency decreased with altitude and regular pollutions, and particulate matter (PM) increased with altitude. Liu et al. (2014) reported that the soot emission decreased with the rising pressure by using simulated device. Chan et al. (2011) studied the gaseous and particle emission characteristics of a turbojet engine at simulated altitudes. They found that the emissions of CO₂, NO_x and particle number were lower and the emissions of CO and THC were larger at higher altitudes in comparison with that of lower altitudes. Benjumea et al. (2009) demonstrated the combustion process of a high speed direct injection automotive diesel engine operating at altitudes between 500 and 2400 m and found that both fuel consumption and combustion duration increased with altitude. From the aforementioned references, employing both test benches at actual altitude and altitude simulation system may be helpful. However, the simulation method may result in some unwanted uncertainties. In addition, there exists gaps between the emissions using test benches and real-world driving emissions, as a minor change in the ground speed of test vehicle will lead to great change in engine operating conditions (Yanowitz et al., 2002; Yin et al., 2013).

To avoid the disadvantages above, the measurement of the real road driving emissions was adopted by numerous reports about vehicle emission tests operating at high altitude. Yin et al. (2013) evaluated regulated emissions and PM distributions of a light duty diesel at different altitudes; however the test was performed at idle mode. Ramos et al. (2016) studied the effect of altitude on emissions from a light duty vehicle under real world driving conditions and found that NO_x emissions increased with altitude. However, their test operated at urban and extra-urban driving conditions without considering motorway driving condition. Liu et al. (2017) discussed the influence of altitude on the particle numbers and their size distribution in two Chinese cities, while the test vehicle did not incorporate after-treatment device. In addition, China VI regulation will be implemented from 2020, which will introduce real driving emission (RDE) test procedure and stipulate the altitude test requirements.

In this work, the emission characteristics from a China IV certificated light-duty diesel vehicle at different altitudes under real driving conditions were investigated. The related conclusions from this work can elucidate the emission characteristics under RDE conditions and provide useful information for policy-making process.