Start TDC with the beginning of the POWER STROKE. Compression is at its peak when fuel injection has been completed and combustion is taking place. Power is delivered to the crankshaft as the piston is driven downward by the expanding gases in the cylinder. Power delivery ends when the exhaust valve opens.

After the exhaust valve opens, the piston continues downward to BDC and then upward in the EXHAUST STROKE. The exhaust gases are pushed out of the cylinder as the piston rises to TDC, and the exhaust valve closes a few degrees after TDC to ensure proper scavenging. The crankshaft has made a complete revolution during the power and exhaust strokes.

The intake valve opens a few degrees before TDC near the end of the upward exhaust stroke to aid in scavenging the cylinder. As the crankshaft continues to rotate past TDC, the INTAKE STROKE begins. The intake stroke continues for the whole downward stroke and part of the next upward stroke to take advantage of the inertia of the incoming charge of fresh air.

The rest of the upward stroke is the COM- PRESSION STROKE, which begins at the instant of intake valve closing and ends at TDC FUEL INJECTION may begin as much as 40° before TDC and continue to TDC, thus completing the power cycle and the second complete revolution of the engine.

By showing an approximate ignition point in place of fuel injection, figure 3-6 could easily represent a timing diagram for a typical gasoline engine. For additional information on diesel fuel injection system tests that can be made both in the shop and in the field, refer to the manufacturer's service manual.

Two-Stroke-Cycle Engine Timing

Figure 3-7 shows a timing diagram of a two- stroke-cycle diesel engine. This engine is typical of the General Motors series, which uses a blower to send fresh air into the cylinder and to clear out the exhaust gases. The movement of the piston itself does practically none of the work of intake and exhaust, as it does in a four-stroke-cycle engine. This fact is important to the mechanic in detecting two-stroke-cycle diesel engine power losses.

Beginning at TDC (fig. 3-7), the fuel has been injected, and combustion is taking place. The piston is driven down, and the power is delivered to the crankshaft until the piston is just a little more than halfway down. The exhaust valves (two in each cylinder) open 92 1/2° after TDC. The exhaust gases blow out through the manifold, and the cylinder pressure drops off rapidly.

At 132° after TDC (48° before BDC), the intake ports are uncovered by the downward movement of the piston. Scavenging air under blower pressure swirls upward through the cylinder and clears the cylinder of exhaust gases. This flow of cool air also helps to cool the cylinder and the exhaust valves. Scavenging continues until the piston reaches 44 1/2° after BDC. At this point, the exhaust valves are closed. The blower continues to send fresh air into the cylinder for just a short time (only 3 1/2° of rotation), but it is sufficient to give a slight supercharging effect.

The intake ports are closed at 48° after BDC, and compression takes place during the remainder of the upward stroke of the piston. Injection begins at about 22 1/2° before TDC and ends about 5° before TDC, depending on the engine speed and load.

The whole cycle is completed in one revolution of the crankshaft, and the piston is ready to deliver the next power stroke.

Multiple-Cylinder Engines

Theoretically, the power stroke of a piston continues for 180° of crankshaft rotation on a four-stroke-cycle engine. Best results can be obtained, however, if the exhaust valves are opened when the power stroke has completed about four-fifths of its travel. Therefore, the period that power is delivered during 720° of crankshaft rotation, or one four-stroke cycle, will be 145° multiplied by the number of cylinders in the engine. This may vary slightly according to the manufacturers’ specifications. If an engine has two cylinders, power will be transmitted for 290° of the 720° necessary to complete the four events of the cycle. The momentum of the flywheel rotates the crankshaft for the remaining 430° of travel.