Preliminary Heat Release Analysis in a Single-Cylinder Two-Stroke Production Engine 930431

The net heat release rate and net heat release fraction for a spark-ignited (SI) single-cylinder two-stroke, production engine were analyzed using the one-zone model. Three different throttle positions and four engine speeds for each position were considered for this study. The method used required cylinder pressure and crank-angle data which were obtained from a pressure transducer mounted in the cylinder head and a shaft encoder connected to the crankshaft. An effective routine referred to as the overlap method was used to smooth undesirable oscillations on the heat release rate curves after investigation of several different approaches. Parameters such as maximum heat release rate, rapid burn angle, combustion duration, maximum heat release rate angle, imep, bmep, etc. were calculated and discussed. It is shown that the trends for the results obtained by this simple model can be adequately explained using independent information regarding the effects of parameters such as residual burned mass fraction, equivalence ratio, and turbulence on flame burning speed. For not-too-rich mixture, a parameter is proposed to provide information regarding the short-circuiting in two-stroke engine. On this basis, “simple” one-zone heat release calculations can be used for rapid computations of important combustion-related parameters for engine analysis and diagnostics in two-stroke engines.

Two-stroke engines are being considered by auto-makers today as a possible alternative to the four-stroke engine because of their superior power-to-weight ratio, less number of parts, and smaller engine size. In the past, these engines were typically known to produce high levels of pollutants due to the scavenging of burned gases with the fresh incoming charge. But recent developments in electronic fuel injection have made it possible to overcome this problem, and to improve performance in other areas such as fuel consumption as well.

The more that is known about the combustion process, the easier it is to control it. One approach for examining the combustion process in an internal combustion engine is the heat release analysis. This provides valuable information on combustion behavior which is an important factor in fuel economy, engine performance, and emissions of harmful pollutants. Different models are used to analyze the heat release rate in engines. For example, Deshpande et al. (1) used a two-zone heat release model to calculate mass of the gases in the burned zone in a two-stroke engine. This was done by performing an energy balance on both the burned and unburned regions in the combustion chamber. The resulting equations were algebraic in nature requiring very little computer time. Consequently, a computer program using these equations could be employed for real time monitoring of the mass fraction burned behavior in a running engine. They also used this model to investigate the effects of the heat transfer term in the heat release rate equation on the mass fraction burned profile and found that it was negligible. Although it is generally understood that the two-zone model is more accurate, it has been shown by Chun and Heywood (2) that the fraction of fuel chemical energy released by combustion and the mass fraction of the air-fuel mixture burned can be estimated by a simpler one-zone model if an appropriate value for the “average ratio of specific heats”, γ (T), for the contents of the cylinder is chosen.

Based on the work of Rassweiler and Withrow (3), Abraham and Prakash (4) used a beat release analysis on their experimentally-obtained pressure-time histories to help them investigate causes of cyclic variations in a two-stroke engine. Their results suggested that cyclic variations depend primarily on the flame initiation period (the crank-angle duration from the spark to the point where 10% of the fuel mass is burned) due to variations of the mixture composition in the region where the spark occurs. They also concluded that the gross mixture quality, due to air/fuel ratio and incomplete mixing of fresh charge with residual gases, is not an important factor during the early phases of combustion. These conclusions were then tested analytically and confirmed using a thermodynamic model together with a model for turbulent flame propagation.

The amount of previous work in which a simple one-zone heat release model is used in a two-stroke SI engine is very limited. Hence, the purpose of this work is to investigate the usefulness of this “simple” model in a two-stroke engine by examining if the trends can be adequately explained using independent information regarding the effects of parameters such as residual burned mass fraction, equivalence ratio, and turbulence on flame burning speed. Other objective is to establish a basis from which further work can be done to improve the accuracy of the one-zone model for use with two-stroke engines.