Review of Vehicle Engine Efficiency and Emissions

This review covers advances in regulations and technologies in the past year in the field of vehicular emissions. We cover major developments towards reducing criteria pollutants and greenhouse gas emissions from both light- and heavy-duty vehicles and off-road machinery. To suggest that the transportation is transforming rapidly is an understatement, and many changes have happened already since our review last year [1]. Notably, the US and Europe revised the CO₂ standards for light-duty vehicles and electrification mandates were introduced in various regions of the world. These have accelerated plans to introduce electrified powertrains, which include hybrids and pure electric vehicles. However, a full transformation to electric vehicles and the required grid decarbonization will take time, and policy makers are accordingly also tightening criteria pollutant standards for internal combustion engines. California has published the Advanced Clean Cars II standards and Europe has held various workshops outlining the core elements of future Euro 7 regulations. These will likely be the last major regulations for criteria pollutants, and compliant vehicles will likely be zero-impact emitting, that is with tailpipe emissions at or lower than the ambient concentrations. Meeting these regulations will require adoption of several advanced engine and emission control technologies which we discuss here. Emphasis will be on reducing cold start emissions, likely requiring active thermal management strategies. The challenge will be to lower criteria pollutants while also reducing fuel consumption, and we review some approaches being considered. The story is similar for heavy-duty vehicles, where meeting California’s Low NOx regulations and Euro VII scenarios require significantly improved engine controls and after-treatment systems. New system solutions and hardware additions show a pathway to meeting the regulations, although we caution that much more work is needed ahead to achieve the reductions over extended durability limits and with healthy engineering margins. We also review the impact of alternative fuels on reducing well-to-wheels (WTW) greenhouse gas emissions, along with recommendations to continue improving market fuel quality to reduce negative impact on criteria pollutants. Finally, while this paper does not intend to provide a detailed review of battery electric or fuel cell vehicle technology, we touch upon a few studies which discuss the outlook of powertrain diversification from a total cost of ownership and greenhouse gas reduction perspective.