Combustion and Performance Characteristics of a DI Diesel Engine Operating from Low to High Natural Gas Supplement Ratios at Various Operating Conditions 2008-01-1392

The compression ignition engine of the dual fuel type has been employed in a wide range of applications utilizing various gaseous fuel resources, while minimizing soot and nitric oxide emissions without excessive increase in cost against that of the conventional direct injection diesel engine. Fumigated dual fuel compression ignition engines are divided into two main groups: the conventional dual fuel engines where part of the liquid fuel is replaced by gaseous one and the pilot ignited ones where a pilot amount of the liquid fuel is used as an ignition source. Due to the high auto-ignition temperature of the natural gas, it can be used as a supplement for the liquid diesel fuel in conventional diesel engines operating under dual fuel mode. Moreover, the use of natural gas as a supplement for the liquid diesel fuel could be a solution towards the efforts of an economical and clean burning operation. In the present work, experimental results are provided concerning the combustion of natural gas in a compression ignition environment, where the liquid diesel fuel is partially replaced by natural gas fumigated into the intake air. The experimental investigation has been conducted on a single cylinder, naturally aspirated, high speed direct injection (HSDI) diesel engine, located at the author's laboratory, properly modified to operate under dual fuel mode conditions. Results are given revealing the effect of liquid fuel percentage replacement by natural gas on the engine performance and emissions. By comparing the results, an important effect of the presence of natural gas on the exhaust emissions and main combustion characteristics is observed. Specifically, the heat release rate is affected remarkably by the liquid fuel percentage replacement by natural gas. The presence of natural gas in the combustion chamber affects the heat released during the premixed combustion period and also the duration of combustion. As far as the engine efficiency is concerned, it is revealed that the dual fuel diesel - natural gas operating mode results in a deterioration of engine efficiency compared to normal diesel operation, which is more intense at low and intermediate loads. As far as the exhaust emissions are concerned, it is revealed that the dual fuel operation has a positive effect (reduction) on nitric oxide (NO) and soot emissions while it has a negative effect (increase) on carbon monoxide (CO) and unburned hydrocarbon (HC) emissions. Taking into account that the experimental measurements cover a very wide range of natural gas supplement ratios (0% up to 90%) without any appearance of knocking phenomena, the belief is strengthened that the findings of the present work are extremely valuable if it is opted to apply this technology on existing DI diesel engines.