Four-stroke cycle engine

Internal combustion engines, both diesel and gasoline, must have mechanisms for (1) getting the charge of air and fuel into the cylinder, (2) igniting the fuel, (3) permitting the hot gases to expand and move the piston, and finally (4) expelling the spent gases to prepare for the next charge. This sequence of events in the engine cylinder is called a cycle. Each directional movement in the cylinder is called a stroke. There are two basic types of internal combustion engines: four-stroke cycle and two-stroke cycle.

In the four-stroke cycle gasoline engine four events (or strokes) occur. They are the intake (admission), compression, power (expansion, firing) and exhaust (ejection) strokes.

On the intake stroke the air-fuel mixture from the carburetor enters the combustion chamber through the intake valve as the piston moves down. The exhaust valve is closed.

On the next stroke – or compression stroke – the piston moves up increasing the pressure on the fuel-air mixture. Both intake and exhaust valves are closed now.

As the piston completes its upward compression stroke, the compressed fuel-air mixture is ignited by the spark plug. The force of the combustion causes the piston to move downward with great force. Both intake and exhaust valves remain closed.

On the exhaust stroke the piston begins to move upward as the exhaust valve opens. Spent gases are forced out of the combustion chamber through the exhaust valve opening.

The four-stroke cycle diesel engine, like the gasoline engine, has four basic strokes in a single cycle, however, there are differences.

Intake stroke. The intake stroke in the diesel engine differs from that of the gasoline engine in only air is drawn into the cylinder – not an air-fuel mixture as in the gasoline engine. As the piston moves downward, the intake valve opens, allowing air to enter the cylinder. The exhaust valve is closed.

Compression stroke. The major difference between the compression stroke in the two engines (diesel versus gasoline) is the compression ratio in the cylinder. The compression ratio is described as the relationship between (1) the total volume inside the cylinder when the piston is at its greatest distance from the cylinder head and (2) the volume when the piston is closest to the cylinder head. This compression ratio is much greater in the diesel engine than in the gasoline engine (16:1 versus 8:1).

Another difference in the compression stroke of the diesel engine is that fuel as injected directly into the cylinder by a fuel injector and is then mixed with air already in the cylinder. The heat of compression ignites the injected fuel. Both intake and exhaust valves are closed.

Power stroke. The temperature rises in the cylinder as the heat of compression ignites the fuel, it starts burning, the tremendous pressure forces the piston downward and expansion begins. Both intake and exhaust valves remain closed.

Exhaust stroke. The exhaust valve opens shortly before the piston reaches bottom lower dead center. As the piston begins to move upward, it pushes the spent gases out of the exhaust valve port, clearing the cylinder for the next charge.

Answer these questions:

а)

1. Как происходит работа четырехтактного двигателя?

2. Каковы основные различия между четырьмя тактами бензинового и дизельного двигателей?

b)

1. What are the basic strokes in the four-stroke cycle diesel engine?

2. What are the main differences between the four strokes in a gasoline and a diesel engine?

Read Text 5 and translate it into Russian. Make up your own Vocabulary. Try to memorize the main terms.

Text 5