View A of figure 6-3 is based on the operation of a 4-stroke cycle engine that uses a centrifugal-type blower (turbocharger) to supply the cylinders with air under pressure. In a supercharged 4-stroke cycle engine, the duration of each event differs somewhat from the length of the same events in a nonsupercharged 4-stroke engine. The intake and exhaust valves are open much longer in a supercharged engine, and the compression and power events are shorter, permitting a longer period for scavenging. When the exhaust event is complete, the turbocharger fills the cylinder with fresh air under pressure before the compression event begins. In other words, the turbocharger supercharges the cylin-ders. To understand the relationship of scavenging and supercharging to the events of the cycle, look again at view A in figure 6-3 and follow through the complete cycle. Start your study of the cycle at TDC, the beginning of the power event. At this point, peak compression has been reached, fuel injection is nearly completed, and combustion is in progress. Power is delivered during the downstroke of the piston for 125 of crankshaft rotation. At this point in the downstroke (55 before BDC), the power event ends and the exhaust valves open.

The exhaust valves remain open throughout the rest of the downstroke (55), throughout all of the next upstroke (180), and throughout 85 of the next downstroke; a total of 320 of shaft rotation. At a point 75 before the piston reaches TDC, the intake valves open and the turbocharger begins forcing fresh air into the cylinder. For 160 of shaft rotation, the air passes through the cylinder and out of the exhaust valves, clearing the waste gases from the cylinder. The rapid flow of gases escaping through the exhaust manifold drives the turbocharger. The process of scavenging continues until the exhaust valves close at 85 past TDC.

The intake valves remain open, after the exhaust valves close, for an additional 140 of shaft rotation (45 past BDC). From the time the exhaust valves close until the piston reaches approximately BDC, the cylinder is being filled with air from the turbocharger. During this interval, the increase in pressure is too small to be considered because of the increasing volume of the cylinder space. (The piston is in the downstroke.) However, when the piston reaches BDC and starts the upstroke, the volume of the space begins to decrease as the turbocharger continues to force air into the cylinder. The result is a supercharging effect.

During the remainder of the upstroke (after the intake valves close), the supercharged air is compressed. Fuel injection begins several degrees before TDC and ends shortly after TDC. The actual length of the injection period in a specific engine depends on the speed and load of the engine. When the piston reaches TDC, a cycle (two complete crankshaft revolutions and four strokes of the piston) has taken place, and the engine is ready to repeat the cycle.