ABSTRACT
Automobiles, light duty trucks, off-road vehicles, and small utility engines have internal combustion engines that use volatile liquid fuels, especially gasoline. The fuel rapidly vaporizes and mixes with air prior to combustion in the engine, hence the term “premixed-charge engine.” The discussion here is limited to automotive spark ignition engines, and the emphasis is on the combustion aspects of the engine rather than overall performance or mechanical design of the vehicle. A compilation of combustion engine terminology is given at the end of this chapter to aid the reader.
From a combustion perspective an internal combustion engine consists of multiple piston-cylinders, each with separate intake and exhaust valves and a spark plug. After the spark plug –res, a turbulent ¢ame front propagates outward from the spark plug. This ¢ame front then creates elevated pressure against the piston and connecting rod, thus creating torque on the drive shaft. The piston travels in a repetitive cycle between being fully inserted at top dead center (TDC) and being fully withdrawn at bottom dead center (BDC). The four-stroke engine sequence for a cylinder (Figure 7.1) is as follows: (1) with the intake valve(s) open and exhaust valve(s) closed, a mixture of air and fuel is drawn into the cylinder as the piston moves downward toward BDC; (2) with all valves closed, the fuel-air charge is compressed as the piston moves upward toward TDC; (3) the spark discharges when the fuelair mixture is nearly fully compressed, and the power stroke occurs as the charge combusts rapidly forcing the piston downward toward BDC due to the high pressure in the cylinder; and (4) as the piston nears BDC the exhaust valve opens and the piston then moves upward toward TDC while expelling the combustion products. The cycle is repeated continuously with the power stroke occurring during every other revolution of the drive shaft.
