Diesel-piloted natural gas has been considered one of the most promising methods of using natural gas in a compression–ignition engine with few modifications, as this approach benefits from a high thermal efficiency resulting from a high compression ratio. This study experimentally investigated the impact of the natural gas substitution rate on the characteristics of combustion and emissions. Tests were performed under full-load operating conditions at a fixed speed of 1200 r/min with optimized injection timing, and the substitution rate of natural gas was varied with the fixed total fuel energy for different analyses. The in-cylinder pressure, pressure rise rate, heat release rate, cyclic variation of the maximum cylinder pressure (P_max) and emissions of HC, CO, NO_x, and smoke were analyzed. The results indicate that P_max and the maximum pressure rise rate initially increase and then decrease as the substitution rate of natural gas increases, whereas the heat release rate and standard deviation of P_max increase. Moreover, HC and CO increase as the substitution rate of natural gas increases, whereas NO_x and smoke emissions exhibit a trade-off trend.

Vehicle emissions standards and regulations remain weak in high-altitude regions. In this study, vehicle emissions from both the New European Driving Cycle and the Worldwide harmonized Light-duty driving Test Cycle were analyzed by employing on-road test data collected from typical roads in a high-altitude city. On-road measurements were conducted on five light-duty vehicles using a portable emissions measurement system. The certification cycle parameters were synthesized from real-world driving data using the vehicle specific power methodology. The analysis revealed that under real-world driving conditions, all emissions were generally higher than the estimated values for both the New European Driving Cycle and Worldwide harmonized Light-duty driving Test Cycle. Concerning emissions standards, more CO, NO_x, and hydrocarbons were emitted by China 3 vehicles than by China 4 vehicles, whereas the CO₂ emissions exhibited interesting trends with vehicle displacement and emissions standards. These results have potential implications for policymakers in regard to vehicle emissions management and control strategies aimed at emissions reduction, fleet inspection, and maintenance programs.

Intake throttling has been verified as an effective approach to increase the exhaust temperature of diesel engines, which could benefit the catalytic efficiency aftertreatment. To better understand the influence of intake throttling on the combustion characteristics and exhaust emissions of light-duty diesel engines operating under idle mode, a light-duty diesel engine was experimentally investigated. This study is a follow-on to previous studies on the effect of throttling on light-duty diesel engine exhaust temperatures and emissions. Tests were conducted at a fixed idle speed of 1100 rpm, and the throttle position and intake manifold air pressure (MAP) were varied. The in-cylinder pressure, pressure rise rate, heat release rate (HRR), in-cylinder temperature, exhaust temperature, and regular gaseous emissions were analyzed. The results indicated that under the influence of intake throttling, the MAP decreased from 101 kPa under wide-open-throttle (WOT) conditions to 52.5 kPa under the heaviest throttling conditions, and the exhaust temperature increased from 100 °C to 200 °C, with a fuel penalty associated with the increase in the pumping indicated mean effective pressure (IMEP). The in-cylinder pressure continuously declined with decreasing MAP, while the HRR generally increased with increasing MAP. Under WOT conditions, the ignition delay decreased, while the combustion duration decreased under heavier throttling conditions. The in-cylinder temperature with throttling was higher than that under WOT conditions, and after post-injection treatment, the in-cylinder temperature exhibited an increasing trend with decreasing MAP. The CO₂, CO, NO_x, and HC emissions increased with increasing throttling amounts.

Limitations still remain among vehicle emissions standards and regulations about high-altitude areas at altitude over 2400 m. In view of high vehicle-use intensity for buses, on-road measurements were performed on four heavy-duty diesel vehicles by employing a portable emissions measurement system (PEMS) in Lhasa, with an average elevation of approximately 3650 m. The result indicated that under real-world driving conditions of Lhasa, compared with China III buses, China IV buses did not show excellent emission performance consistently. For example, carbon monoxide (CO) emission factor of China III-5.2L buses is 2.4 times of China IV-5.2L buses, while the nitrogen oxides (NO_x) is only 36% of China IV-5.2L. Furthermore, an operating mode binning and a micro-trips method are used to link real-world emissions of each vehicle to driving conditions. For China III-5.2L and China IV-5.2L buses, we found strong correlations between relative emission factors of carbon dioxide (CO₂) and NO_x and average speed. However, these correlations for China IV-7.8L bus were less strong. This phenomenon proves that the role of traffic conditions in affecting road driving emissions would be mitigated when selective catalytic reduction (SCR) functioning properly. This study have potential implications for policymakers concerning vehicle emissions management and control strategies, such as promotion of alternative fuel with in-used buses, aimed at emissions reduction.

To determine whether liquefied petroleum gas (LPG)/gasoline dual-fuel vehicles reduce atmospheric pollution in high-altitude cities compared to gasoline vehicles, experiments were conducted on two LPG/gasoline and two gasoline vehicles in a high-altitude city using a portable emissions measurement system (PEMS), and real-world emissions from two gasoline vehicles in a low-altitude city were also analyzed. The emission factors were obtained and the effects of driving mode, speed, and acceleration on emission characteristics were examined. Results show that the mean emission factors of CO₂, CO, and NOx for LPG/gasoline-fueled vehicles in high-altitude cities are 159.22±11.81, 18.38±9.72 and 1.53±0.46 g/km, and those for gasoline-fueled vehicles are 226.64±27.80, 1.46±0.83 and 0.22±0.17 g/km. For gasoline vehicles in low-altitude cities, the emission factors are 188.01±4.82, 2.21±0.67 and 0.07±0.003 g/km. Unlike CO₂, CO and NOx emissions from LPG/gasoline vehicles are higher than those from gasoline vehicles. Further analysis indicates that idle and accelerating modes significantly affect CO and NOx emissions. In high-altitude tests, these two modes account for about 24.5% and 31.2% of total testing time, respectively, with higher emissions than low-altitude vehicles (22.5% and 27.7%). Generally, gasoline-fueled vehicles exhibit better gaseous pollutant emissions behavior than LPG/gasoline-fueled ones. These findings can serve as a reference for developing emission inventories and emission-control strategies in high-altitude cities