Characteristics of Unburned Hydrocarbons in a Low Compression Ratio Diesel Engine (Characteristics under Transient Operation and Reduction with Low Distillation Temperature Fuels)

In a DI diesel engine, reducing the compression ratio is effective to reduce smoke emissions without deterioration of NO., and thermal efficiency. However, with lower compression ratios, a low coolant temperature, or during the transient state, THC emissions significantly increase when using ordinary diesel fuel. The THC emissions during increasing loads significantly increased to very high concentrations from just after the start of the load increase until around the 10th cycle, rapidly decreased until the 20th cycle, and then gradually decreased to a steady state value after 1 000 cycles. The dominant components present in the THC transient spike were lower hydrocarbons with carbon numbers around or below eight. In particular, ethylene showed a much higher concentration than the other components. In fully-warmed steady state operation with a compression ratio of 16 and diesel fuel, THC is reasonably low, but THC increases with lower coolant temperature or during the transient period just after increasing the load. This problem can be eliminated with a low distillation temperature fuel such as normal heptane. Benzene, butadiene, and other unregulated harmful emissions also increase under the high THC conditions with diesel fuel.