Automotive Systems, Quiz Yourself 4

The compression ratio refers to the ratio of the volume of the air-fuel mixture in the cylinder at the beginning of the compression stroke to the volume at the end of the compression stroke. It indicates how much the mixture is compressed during this stroke. In this case, the air-fuel mixture is compressed to about one-eighth of its original volume, which is a measure of the compression ratio.

The top dead center refers to the position of the piston at the instant its motion changes from up to down. This is the highest point that the piston reaches in its upward motion before it starts moving downward. At this point, the piston is at its farthest position from the crankshaft and the cylinder head.

For every two strokes made by the piston, the crankshaft makes one complete revolution. This means that the piston moves up and down twice, corresponding to the intake, compression, power, and exhaust strokes in a four-stroke engine. Each time the piston completes these two strokes, the crankshaft rotates 360 degrees, completing one full revolution. This is a fundamental principle in the operation of internal combustion engines, where the reciprocating motion of the piston is converted into rotary motion by the crankshaft.

The action of a four-stroke cycle engine is comprised of four parts. Each stroke - intake, compression, combustion, and exhaust - is necessary for the engine to complete a full cycle.

During the intake stroke, the intake valve needs to be open in order for the air-fuel mixture to enter the combustion chamber. This allows the piston to move downward, creating a vacuum that draws in the mixture from the intake manifold. The open intake valve ensures a smooth flow of the mixture into the cylinder, optimizing the combustion process.

The exhaust valve opens just as the power stroke is finished. During the power stroke, the ignited fuel-air mixture expands and pushes the piston down, creating power. Once this stroke is completed, the exhaust valve opens to release the exhaust fumes produced during combustion. This timing allows for efficient expulsion of the exhaust gases and prepares the cylinder for the intake stroke, where fresh fuel-air mixture enters.

Ignition takes place as the piston reaches TDC (Top Dead Center). At this point, the piston is at its highest position in the cylinder, and the spark plug ignites the air-fuel mixture, initiating the combustion process. This position allows for maximum compression of the mixture, resulting in efficient and powerful combustion.

When the piston moves downward inside the cylinder, it creates a partial vacuum. This is because the downward movement of the piston increases the volume inside the cylinder, causing a decrease in pressure. As a result, the air or fuel mixture in the cylinder is drawn in to fill the space, creating a partial vacuum. This partial vacuum is important for the efficient operation of the engine as it allows the intake of fresh air or fuel mixture for combustion.

When fuel burns inside a cylinder, it generates heat and expands, creating pressure. This pressure causes the pressure inside the cylinder to rise. The correct answer states that the pressure rises to between 600 - 700 pounds per square inch. This means that the pressure inside the cylinder reaches a range of 600 to 700 pounds per square inch due to the burning fuel.

The intake valve closes when the piston reaches BDC (Bottom Dead Center) during the compression stroke. This is because, at this point, the piston is at its lowest position and is about to start moving back up towards the cylinder head. Closing the intake valve at this point prevents any more air or fuel mixture from entering the cylinder, allowing the compression process to begin.

The correct answer is bottom dead center. This is the position of the piston at the moment when its motion changes from moving downward to moving upward. At this point, the piston is at the lowest point in its stroke, closest to the crankshaft.