SOME ASPECTS OF THE HEAT RELEASE RATE IN THE DUAL FUEL ENGINE FUELLED CNG WITH PARTICULARLY CONSIDERATION THE GAS COMBUSTION

The test results reported in this paper concern the single cylinder compression ignition direct injection engine fed with compressed natural gas (CNG). The variable parameters adopted in the research include: the engine speed of 2000-3000 rpm, the engine load from minimum to maximum, regulated by the knock combustion effect, variable gas concentration in the mixture λo=1.8–10.5, and the initial dose quantity 8.2–25.5 mm3/cycle. The combustion parameters, calculated from the registered indicator diagrams, were analysed in the tests. The tests proved that the combustion of gas and liquid fuel in the dual fuel engine differs as far as the time and speed of the process are concerned. For small initial doses the liquid fuel burns faster than the gas fuel. Two distinct heat release rate maximums can be observed then: of the liquid and gas fuel respectively. For bigger initial doses and higher engine speeds the liquid fuel combustion process is delayed, while it is accelerated in the case of the gas fuel. This results in mutual overlapping of the two processes. For small initial doses and high engine load the heat release rate dQ is lower by about 40÷50 % than in the conventionally fuelled engine. At part load running the dual fuel engine efficiency was lower by about 1.2-2.8%. The efficiency loss increased as the knock combustion limit was approached, and amounted then to 6-9%. The engine knock effect can be reduced by means of increasing the initial dose, but at high load it results in lower engine efficiency. 1