The spark ignition (SI) engine is the dominant passenger car and light truck power plant in the United States. The theoretical efficiency of the SI engine is: Efficiency = 1 – l/m-l where r is the compression ratio and “n” the polytrophic expansion coefficient, which is a measure of the way the mixture of air and fuel in the engine expands when heated.
For a compression ratio of 10:1, and an n value of 1.26 (which is correct for today’s engines, which require the air-fuel ratio to be stoichiometry, that is, with precisely enough air to allow complete burning of the fuel), the theoretical efficiency of the engine is 45 percent.
This value is not attained in practice, but represents a ceiling against which developments can be compared. Four major factors limit the efficiency of SI engines.
First, the ideal cycle cannot be replicated because combustion is not instantaneous, allowing some fuel to be burned at less than the highest possible pressure, and allowing heat to be lost through the cylinder walls before it can do work.
Second, mechanical friction associated with the motion of the piston, crankshaft, and valves consumes a significant fraction of total power. Friction is a stronger function of engine speed than of torque; therefore, efficiency is degraded considerably at light load and high rpm conditions.
Third, aerodynamic fictional and pressure losses associated with air flow through the air cleaner, intake manifold and valves, exhaust manifold, silencer, and catalyst are significant, especially at high air flow rates through the engine.
Fourth, SI engines reduce their power output by throttling the air flow, which causes additional aerodynamic losses called “pumping losses” that are very high at light loads.