Measurement And Analysis Of Some CNG Engine Indicator Diagram

In order to utilize global source effectively and reduce engine exhaust emission, CNG (Compressed Natural Gas)engine is widely used. The accurate measurement and valid analysis of the indicator diagram are crucial since it can not only reflects the working condition and combustion process of the CNG engine but also be used to research dynamics characteristic.In this paper, the cylinder pressure signal, fluted disc impulse signal and top dead center impulse signal are sampled synchronously. The top dead center is confirmed using cylinder compressing line method and magnetoelectric method and the position of top dead center is validated with variety index method. The crank angle is determined by both top dead center reference method and angle impulse interpolation method proposed in this paper which are compared in the capture of indicator diagram using cylinder pressure, top dead center impulse signal and fluted disc impulse signal. The indicator diagram is quite different from the pressure figure at combustion expansion stroke and almost coincides at intake-exhaust and compression stroke when the engine cycle change is large.According to this, the composite method of local multiple mean cycle method and five point second order fairing method is proposed for the pre-treatment of pressure figure which keeps the characteristic of cycle indicator diagram at combustion expansion stroke and reduce the error of the intake-exhaust and compression figure as well.The cycle variability analysis of main parameters of CNG engine indicator diagram such as pressure shows that the indicated mean effective pressure of the engine changes a little while the maximum combustion pressure and the maximum pressure rising rate change obviously, and the crank angle corresponding to maximum combustion pressure is getting far away from the top dead center as the pressure decreases. Then the condition is stable, the effect to power under the change of cycle combustion is unconspicuous although the maximum combustion pressure of the indicator diagram has a big variation. The altitude of lower harmonic gas pressure at infrasound level rises only a little as the maximum combustion pressure and the rising rate of maximum pressure rise for the indicated mean effective pressure changes unobviously. But altitude of gas pressure at hearable level rises as more obviously as the harmonic is higher with the maximum combustion pressure and the rising rate of maximum pressure rise. The maximum of the altitude of the pressure is two times more than the minimum which results in accretion of structural vibration and combustion noisy.The available harmonic of the CNG engine in this experiment is much less than that of diesel engine. To estimate the dynamics of CNG engine with the restoration of the indicator diagram, the restoration of the preceding 12 harmonic gas power is adequate. The analysis of the CNG engine suggests the measurement error of crank angle vibration is due to pitch error and gear surface abrasion. The merge gear method proposed in this paper can promote the measurement precision of the crank shaft angular velocity and meet the need of restoration of the indicator diagram. The resonant frequency of lowest order of the shafting is 71Hz by analyzing crank shaft angular vibration of the CNG engine. So the shafting can not act as pseudo-rigid model when inversing indicator diagram using crankshaft angular vibration. But the restoration of the indicator diagram using the transitive relation of crank shaft angular velocity and crank shaft tangential torque under given velocity is feasible. The restoration precision is affected mainly by the measurement error of angular vibration and the precision of transfer function of resonant frequency.