Abstract
In the recent years there has been a lot of interest in the use of different gas fuels for feeding combustion engines. Pro-ecological properties of gas fuels and constant restrictions concerning the exhaust gas purity shall in the coming years increase the share of gas fuels in the total balance of fuels used around the world. The major gas fuel for engines (including medium size traction engines) will be the compressed natural gas CNG.
Positive properties of this gas are in particular revealed in dual fuel engines, which despite the gas fuelling retain most of the diesel engines positive properties. Due to the ignition that takes place in many points, dual fuel engines provide efficient combustion of gas-air mixtures in a wide range of composition, and secure steady and reliable engine operation, even for very lean mixtures.
The dual fuel engine parameters depend on the gas-air mixture composition, the initial dose quantity, the quality of spraying, the range of the liquid fuel stream, initial dose injection timing, and on the temperature of the inducted charge. Correct use of these parameters shall facilitate constructing gas engines with parameters comparable to those of the conventional diesel engines, while better than spark ignition gas engines.
The test results reported in this paper concern the single cylinder compression ignition direct injection engine fed with compressed natural gas (CNG).
The engine parameters include: the cylinder bore 90 mm, the stroke 90 mm, and the compression ratio 16.8. The variable parameters adopted in the research include: the engine speed of 2000-2750 rpm, the engine load from minimum to maximum, regulated by the knock combustion effect, variable gas concentration in the mixture lCNG=1.810.5, and the initial dose quantity 1025 mm3/cycle. The combustion parameters, calculated from the registered indicator diagrams, were analysed in the tests. The comparison included: the cylinder pressure p, the rate of pressure rise dp, the average temperature of the medium T, the heat release rate dQ, the ignition delay, the combustion time, and the indicated efficiency. The combustion process was represented by means of changes in the parameters of the crank angle function, made for the active burning in the TDC area. Also, during the tests the exhaust gas components concentration was measured: the CO, CH, NOx , and the exhaust smoke.
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 engine 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, with the rich gas-air mixture supply, there are higher maximum pressures than in the case of the diesel oil operation. The pressures development is moved further in time, compared to the conventionally fed engine. This results in lower rate of the pressure rise dp, within 0.2-0.3 MPa/deg.CA, as compared to the conventionally fed engine. This, however, leads the increase of the average temperatures of the medium, ca. 100-200 K.
For small initial doses and high engine load the heat release rate dQ is lower by about 40% than in the conventionally fuelled engine. The maximum cylinder pressures and charge temperatures can be decreased by means of increasing the initial dose. This requires, however, weakening of the gas-air mixture, which results in the lower engine efficiency, by ca. 3-4%.
The investigations of the effect of the gas concentration change in the gas-air mixture proved a slight influence of weakening of the mixture on the ignition delay (under 2 deg.CA with the change of lCNG=2.0-10.5), and considerable changes of dp and dQ for the rich range of lCNG=1.8-3.0 (change of dp by 0.15-0.20 MPa/deg.CA and dQ by 40%).
For bigger initial doses the effect of the mixture composition on the analysed parameters decreases.
At part load running the dual fuel engine efficiency was lower by about 3-4%. 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.
The dual fuel engine showed significant differences in the NOx emission and the exhaust smoke, as compared to the conventionally fed engine. The NOx emission and the exhaust smoke are highly dependent on the initial dose quantity. Because of the dose increase from 10 to 15 mm3/cycle for the same lCNG, the NOx concentration in the exhaust gases almost doubled, and the exhaust smoke amount tripled in the rich lCNG range.