ABSTRACT A special SI engine designed to work with gasoline and compressed natural gas (CNG), in completely dedicated fashions, is tested while mounted in the vehicle using a chassis dynamometer and a complete set of measuring instruments. The main objective is to assess the cyclic and cylinder to cylinder variations of cylinder pressure under different operating conditions with both types of fuel. Four sets of pressure transducers and conditioning devices are installed to record the pressure-crank angle histories inside each of the four engine cylinders. Electronic data acquisition techniques and a specially developed software package are used to measure, record and analyze all the parameters at different test conditions. Observations show that Cyclic variations of pressure in each cylinder during the combustion process are 20 to 25% less with CNG. The variations in the maximum rate of pressure rise are also 15-20% less while the average values are slightly higher. The deviations (from average) of the cyclic values of Pmax and its timing CAPmax, show large inverse correlation to each other. The increasing or decreasing trends in Pmax and its timing do not continue for more than 3-4 successive cycles. This is more apparent with natural gas fuelling at all engine operating conditions. The cross correlation coefficient has a certain degree of repetition every 2-3 cycles and even less with gasoline. The single-sided cross power spectrum also shows the existence of some periodic dependency within 5 engine cycles or less. Cyclic deviations in Pmax and CAPmax from average cylinder values converge to almost the same patterns when averaged over 3 or more cylinders in firing sequence, with nearly complete negative correlation. The cross spectrum also shows the presence of a high peak always near the frequency of 0.3 per cycle.