A study of combustion in a dual-fuel compression ignition engine was conducted to determine the effects of the gaseous fuel addition on the several properties of the combustion process. In particular, the presence and intensity of both end gas knock and diesel knock were measured.
A CFR research engine was equipped to run as a dual-fuel engine. Natural gas blends were used for the gaseous fuel fraction, and diesel pilot injection was used as the ignition source. The engine was run at an overall equivalence ratio of 0.7, with premixed equivalence ratios ranging from 0.2 to 0.5. The intake temperature was also varied from 66-110°C. Cylinder pressure data was collected at each point. Three separate methods were used to measure the knock behavior of the engine. Two of these methods were used to quantify the amount of end gas knock which was occurring. Cylinder pressure records were used to calculate a non-dimensional knock factor. In addition, an FFT analysis was done on the pressure records. The third method measured the extent of diesel knock in each cycle. Diesel knock was measured by calculating the maximum rate of pressure rise during the cycle.
The power of a cylinder pressure FFT spectrum in the frequency range of 2000-2500 Hz was found to be an accurate indicator of end gas knock intensity for this particular engine. As the amount of gas substitution or intake temperature increased, the FFT power also tended to increase as expected. Diesel knock was also detected at points which had high amounts of gas substitution. In particular, runs with a premixed equivalence ratio of 0.5 had cycles which possessed both end gas and diesel knock.