Historically the high speed diesel engine for commercial vehicles has been developed along with its combustion system in compliance with political and economical changes. After the 1970's, stricter exhaust emission regulations and fuel economy requirements induced combustion developments and application of turbocharged and inter cooled engines. From the late 1980's, high pressure fuel injection has been investigated and recognized as an essential tool for lowering emissions especially of particulate matter. Although turbulence effects on both in-cylinder air motion and during the combustion process are quite effective, they show different phenomena in conventional and advanced high pressure fuel injection systems.
In the 1990's, multiple injection with high pressure has been attempted for further reduction of NOx and particulate matter. Also, though the idea was not so new, homogeneous diesel combustion based on bulk combustion has been investigated by several researchers due to a very low figure of emission, but it has a very limited operable region. Recently, a research based on the concept of combination of multiple fuel injection and homogeneous diesel combustion has begun for the purpose of widening the operable region.
Today EGR is considered to be widely applicable to future heavy duty engines because the wear problems have been improved by the reduction of sulphur content in diesel fuel, and several remaining problems are under discussion.
Future fuel must meet environmental requirements. The ultimate ideal fuel would be hydrogen. Research work on a hydrogen diesel is proceeding together with new methods of hydrogen production.