Which statement best describes emissions from diesel engines compared with gasoline engines?

• A. Diesel engines typically emit more NOx and PM than gasoline engines, and diesel particulate filters can mitigate PM.
• B. Gasoline engines emit more NOx and PM than diesel engines.
• C. Diesel engines emit less NOx but more CO than gasoline engines.
• D. Gasoline engines emit more PM than diesel engines, with no mitigation possible.

Answer: (A)

Diesel engines tend to produce more NOx and more particulate matter (PM) than gasoline engines. This comes from how diesel operates: high compression and lean, high-temperature combustion promote the formation of nitrogen oxides, and the combustion tends to leave behind soot particles. To control PM, diesel designs commonly use a diesel particulate filter (DPF) that captures the soot and can be regenerated to burn it off, dramatically reducing PM emissions. Gasoline engines burn a mixture with lower peak temperatures and are generally better at oxidizing soot, so they emit less PM and NOx, though catalytic converters are used to cut CO and hydrocarbons. However, the main point for this comparison is that diesel engines have higher NOx and PM, and PM can be mitigated with a DPF, which is exactly what option describes.