It is standard practice in one-dimensional gasdynamic simulations of high performance two-stroke engines to model the exhaust tail pipe entry as an area change using an algorithm similar to the area change of the reverse cone. In the reverse cone the area continually steps down while at the tail pipe entry it changes from stepping down to constant area. At this point a vena contracta can form that effects the flow resistance of the tail pipe. In an effort to improve the accuracy of the gasdynamic simulations the area change algorithm at the tail pipe entry was replaced with a restriction algorithm that incorporates a coefficient of discharge and allows an increase in entropy on the expansion side. The coefficient of discharge is defined as the actual measured mass flow divided by the mass flow predicted by the restriction algorithm. An experimental set up was designed and constructed to measure mass flows for a variety of tail pipe entry geometries at a range of pressures covering the pressure ratios encountered in a real engine. From the mass flow results the coefficients of discharge for a range of pressure and area ratios and reverse cone angles could be calculated and arranged into matrix form to define Cd-maps. The Cd-maps were incorporated into the simulation software and tested to ensure that it functioned correctly. <p<Finally, the simulation results with and without the Cd-maps were compared to measured results and it was shown that incorporating this refinement improves the accuracy of the simulation results on the “over run” part of the power curve. This is the part of the power curve after maximum power and very important in the development of high performance two-stroke engines. These maps can be used for all future simulations on any engine size that uses the same tail pipe geometry. / Dissertation (MEng (Mechanical Engineering))--University of Pretoria, 2006. / Mechanical and Aeronautical Engineering / unrestricted

This work concerns a systematic study of IC engine operation with 100% biogas as fuel (as opposed to the dual-fuel mode) with particular emphasis on operational issues and the quest for high efficiency strategies. As a first step, a commercially available 1.2 kW genset engine is modified for biogas operation. The conventional premixing of air and biogas is compared with a new manifold injection strategy. The effect of biogas composition on engine performance is also studied. Results from the genset engine study indicate a very low overall efficiency of the system. This is mainly due to the very low compression ratio (4.5) of the engine. To gain further insight into factors that contribute to this low efficiency, thermodynamic engine simulations are conducted. Reasonable agreement with experiments is obtained after incorporating estimated combustion durations. Subsequently, the model is used as a tool to predict effect of different parameters such as compression ratio, spark timing and combustion durations on engine performance and efficiency. Simulations show that significant improvement in performance can be obtained at high compression ratios. As a step towards developing a more efficient system and based on insight obtained from simulations, a high compression ratio (9.2) engine is selected. This engine is coupled to a 3 kW alternator and operated on 100% biogas. Both strategies, i.e., premixing and manifold injection are implemented. The results show very high overall (chemical to electrical) efficiencies with a maximum value of 22% at 1.4 kW with the manifold injection strategy. The new manifold injection strategy proposed here is found to be clearly superior to the conventional premixing method. The main reasons are the higher volumetric efficiency (25% higher than that for the premixing mode of supply) and overall lean operation of the engine across the entire load range. Predictions show excellent agreement with measurements, enabling the model to be used as a tool for further study. Simulations suggest that a higher compression ratio (up to 13) and appropriate spark advance can lead to higher engine power output and efficiency.