Spark Ignition Engine Model for Heat Pump System Simulation

摘要

A mathematical model has been developed for the simulation of a natural gas powered spark ignition engine to be used in powering a heat pump. The model is based on a thermodynamic analysis of the combustion chamber gases, with empirical models for combustion rate, convective heat transfer from combustion chamber gases, and engine friction. Comparisons of the predictions of the model with experimental data show that the model accurately predicts trends in engine performance and energy distribution with respect to changes in speed and load. The magnitudes of predicted engine efficiency are about 10 percent low, predicted exhaust gas temperatures are about 70 degrees high and heat losses to the coolant are within experimental error. The model has the potential to simulate changes in engine parameters such as bore, stroke, number of cylinders, compression ratio and fuel type, although the accuracy of this capability has not yet been established through a comparison with experimental data. A sample calculation for a heat pump application using natural gas with a typical carburetor is given and indicates that at low heat pump COP, surface heat losses from an uninsulated engine and the unavailable portion of the exhaust gas energy are the prime sources of losses. At high loads, losses due to imcomplete combustion products in the exhaust gases may be present, depending on carburetor calibration. High exhaust gas temperatures reduce sensitivity of heat pump performance to load variations at low heat pump COP as compared to a diesel engine. At high heat pump COP, the brake thermal efficiency of the engine becomes the prime factor in determining the primary energy utilization efficiency of a natural gas engine driven heat pump system. The effects of engine load factor are much more important in determining system efficiency than those of engine speed. (ERA citation 10:018598)