Tham khảo tài liệu 'internal combustion engines fundamentals episode 2 part 9', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | nữ INTERNAL COMBUSTION ENGINE FUNDAMENTALS FIGURE 14-10 Predicted and measured indicated specific fuel consumption and exhaust temperature as a function of the fuel air equivalence ratio for a spark-ignition engine operated at 1250 rev min and imep of 379 kPa. MBT maximum brake torque timing. MBT 10 combustion timing retarded to give 10 percent fuel consumption the fuel air equivalence ratio at fixed load and speed. The isfc predictions and data agree well except for very lean mixtures with retarded timing where cycle-by-cycle combustion variations are sufficiently large so predictions based on the average cycle lose accuracy the predicted curves for exhaust temperature show the same trends as the experimental data. However they are higher due to underestimation of the heat losses during the exhaust The output from such thermodynamic-based cycle simulations has replaced the fiiel-air cycle as a predictor of effects of major variables on engine performance and efficiency. An instructive example of the value of such predictions is shown in Fig. 14-11 where fuel consumption at constant equivalence ratio load and speed has been computed as a function of total bum duration and heat loss to the chamber walls increasing burn duration and heat loss both worsen fuel Such data can be used to assess the efficiency improvements that should result from reduced heat transfer . reduced chamber surface area and increased bum rate. Obviously the dependence of bum rate on engine design and operating parameters has not been modeled the bum rate profile was a calculation input. Such models are most useful either 1 when the burn rate profile is not critical to the problem under study or 2 when predictions for a range of assumed bum rate profiles provide the required information. So far we have discussed engine cycle simulations where details of the combustion process have been specified as input. The same thermodynamic-based simulation structure
