As such, they are also a substantial contributor to global air pollutants. They currently produce more than 5% of the total man-made hydrocarbon emissions that contribute to ozone formation. In some regions of the U.S. the lawn-mowers using small engines are responsible for 13% of unburned hydrocarbon emission. Yet very few standards exist to regulate the emissions from these small engines.
Based on the type of ignition, internal combustion engines are classified as either spark ignition engines (SI) or compression ignition engines (CI). A newer operating mode for internal combustion engines called homogeneous charge compression ignition (HCCI) can lead to an increase in fuel efficiency and a reduction of emissions (particularly NOx) when compared to engines under SI or CI operation. Until recently, however, most applications and research efforts in HCCI operation have been focused on passenger cars and light trucks with engines that are 2L or larger in displacement.
Very little research has been conducted on the potential of smaller scale HCCI engines. Thus, further study in HCCI operation in small scale engine may improve the fuel efficiency and emission in an area where regulation is not yet stringent.