IMPROVEMENTS TO SPARK IGNITION ENGINES
Overview
The spark ignition (SI) engine is the dominant passenger car and light truck powerplant 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 polytropic 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 stoichiometnri, 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.
Because of these losses, production spark ignition engines do not attain the theoretical values
of efficiency, even at their most efficient operating point. In general, the maximum efficiency point
occurs at an engine speed intermediate to idle and maximum rpm, and at a torque level that is 60
to 75 percent of maximum.“On-road” average efficiencies of engines used in cars and light trucks
are much lower than peak efficiency, since the engines generally operate at very light loads--when
pumping losses are highest--during city driving and steady state cruise on the highway. The high
power that these engines are capable of is utilized only during strong accelerations, at very high
speeds or when climbing steep grades. And during stop-and-go driving conditions typical of city
driving, a substantial amount of time is spent at idle, where efficiency is zero. Typical modem
spark ignition engines have an efficiency of about 18 to 20 percent on the city part of the
Environmental Protection Agency driving cycle, and about 26 to 28 percent on the highway
part of the cycle.
During the 1980s, most automotive engine manufacturers improved engine technology to
increase thermodynamic efficiency, reduce pumping loss and decrease mechanical fiction and
accessory drive losses. These improvements have resulted in fuel economy benefits of as much as
10 percent in most vehicles.
