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Particulate matter (PM) emission of non-road diesel engines is more and more stringently restricted by US, EU, Japan, etc. In order to achieve these emission regulations, diesel particulate filter (DPF) system is applied. However DPF system requires extra fuel consumption in order to burn accumulated particles. Furthermore, since it is difficult to install large DPF systems in limited packaging space of non-road applications, compact DPF system is desirable. Reducing soot emission with engine technology is effective for reducing PM emission, which results in reducing extra fuel consumption and downsizing or removing of DPF system. Soot emission level mainly depends on excess air ratio (EAR), and can be reduced by keeping EAR high (lean combustion). However, lean combustion under the limited amount of air and maximum in-cylinder pressure requires decrease in fuel injection quantity, and yields decrease in engine power.

In a DI diesel engine, THC emissions increase significantly with lower compression ratios, a low coolant temperature, or during the transient state. During the transient after a load increase, THC emissions are increased significantly to very high concentrations from just after the start of the load increase until around the 10th cycle, then rapidly decreased until the 20th cycle, before gradually decreasing to a steady state value after 1000 cycles. In the fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperatures or during the transient period just after increasing the load. This THC increase is due to the formation of over-lean mixture with the longer ignition delay and also due to the fuel adhering to the combustion chamber walls. A low distillation temperature fuel such as normal heptane can eliminate the THC increase.