Question

Consider the design of an engine for a high-efficiency automobile engine design that is to work in conjunction with an electric motor in a hybrid drive. Management wants to know what would be the displacement and efficiency of an engine producing 50 hp (net). Since engines are always rated at ISO (sea level) conditions where their performance is best, take the inlet air to be at 14.7 psia and 59°F. Consider two different directions the company could take:

a) Otto Cycle (four-stroke): In order to convince customers that fuel costs will be low, the sales department recommends that a gasoline engine should use regular-octane fuel, which will limit the maximum CR to 9. The manufacturing engineers have suggested an aluminum alloy that they would like to use in building the engine block and heads, and the strength of this alloy will limit maximum cylinder pressures to 1000 psia. A peak engine speed of 8000 RPM is feasible for this small gasoline engine.

b) Diesel Cycle: Assume a compression ratio of 20 is feasible. Lower cutoff ratios are preferable, and you should design for a value of 1.3 based on the company’s experience with other Diesel engine designs. Maximum engine speeds are typically lower for Diesel engines – assume that 3500 RPM is the upper limit here. For simplicity, assume specific heats can be assumed constant, and take cp=0.24 Btu/lbm-R, cv=0.17 Btu/lbm-R, and k=1.4 throughout both calculations.

After modeling the two cycles, generate a table that compares the two cycles – the table should include (in the following order please) the maximum cycle pressure (in psia), maximum cycle temperature (in °F), volumetric flow rate into the engine (in CFM = cubic feet/min), engine displacement (in liters), and thermal efficiency. Finally, summarize in one or two sentences the primary trade-offs (from your tabular summary) that will dictate the final design choice.

Answer 1