Unsteady flow in the induction system of a gas fuelled S.I. engine

Abstract

A one-dimensional computer program has been developed to simulate the fluid flows in the induction system of a single-cylinder four-stroke engine fuelled with natural gas. A step by step verification of the 1-D numerical program was conducted by comparing the calculated results with the available experimental results. The program was used to investigate the effects of both the engine operating condition and the engine system design on air flow rate, fuel flow rate and air/fuel ratio. The program uses the SIMPLER algorithm. This widely used finite-volume numerical solution procedure has proved a successful method to deal with unsteady, compressible and viscous fluid flow problems. When it is applied to solve the set of differential equations strongly linked by pressure, this solution procedure has shown its advantages in terms of computer CPU time saving and the accurancy of the results obtained. This study has revealed that the pressure variation curves in the induction system are actually the combined effects of the influence made by the current and multiple previous cycles. The influence contributed by the current cycle plays the most important role in affecting pressure, velocity, and both air and gas flow rates. The influence coming from one of the multiple previous cycles gets weaker and weaker as that previous cycle becomes further and further away from the current cycle. In addition, the study indicates that the combined pressure variations from both current cycle and many previous cycles are periodic curves at the- engine operating frequency. This occurs even though the individual contributions made by different cycles are originally formed by pulses which propagate in the pipes according to the resonant frequency of the induction system. This resonant frequency depends on the pipe lengths and the wave propagation speed (which is the speed of sound plus the velocity of the flow). Based on the theory of wave propagation in a pipe having one open and one closed end, the procedures of constructing the pressure variation curves in the induction system are introduced. The information needed for doing this includes only the engine operation conditions and the engine system itself. In the process of pressure construction, the decay of the pressure pulse due to the energy loss to friction has been included into the consideration. The possibility of using pressure-time history curves (either measured or constructed) to obtain the flow velocities for both air and gas flows, and air/fuel ratio are discussed. The study shows that: the flow velocities at the inlet of the air pipe can be integrated from the variation curves of the relative pressure (to atmospheric pressure P0); and the gas flow velocities at the T-junction end of the gas pipe can be obtained from the relative pressures of (P0 - P) at the T-junction by applying the velocity conversion coefficients on the pressure curves. Finally, an easily operated zero-dimensional computer program based upon the simplified correlations between the pressure and velocity variations at both air pipe inlet and the T-junction is presented. This program can be used to carry out some fast calculations about the possible air and gas flow rates for a proposed new Jngine system when operated at conditions of interest.