Four Stroke Cycle

The four-stroke cycle engine, commonly called 4 cycle engine, is one of the two common types of engine cycles used for both spark and compression ignition engines. In 4 cycle engines, the eight events occur during four strokes of the piston or two revolutions of the crankshaft. The four strokes are called:

1. Intake

2. Compression

3. Power

4. Exhaust

Study Figure 5.2 and the following description of the events. During the intake stroke, the intake valve opens, and the piston travels from the top of the cylinder (TDC) to the bottom (BDC). In both the spark and the compression ignition engines, during the intake stroke, the movement of the piston reduces the pressure inside the cylinder. Higher atmospheric pressure outside the engine causes air to flow through the intake system and into the cylinder as the intake valve opens. In the carbureted engines fuel is introduced into the air stream as it flows through the carburetor. Shortly after the piston reaches the bottom of the intake stroke, the intake valve closes, thus trapping the air-fuel mixture inside the cylinder. Any restrictions in the flow of air into the engine reduces its volumetric efficiency. Volumetric efficiency is the comparison of how much air flows into the cylinder compared to the amount of air the cylinder can hold when it is totally full at atmospheric pressure. Reduction in volumetric efficiency reduces the horsepower produced by the engine. One common cause is a dirty air filter.

The compression stroke follows. Because the air-fuel mixture is trapped in the cylinder, as the piston returns to the top of the cylinder during this stroke, the air is compressed, and as it is compressed, the temperature rises. As the piston nears the top of the compression stroke, ignition occurs. Pressure is the key for converting the energy of the fuel to power. If any of the combustion pressure escapes from the combustion chamber, less power will be produced. On engines with a lot of hours and/or poor maintenance, the rings become less capable of

Intake Exhaust open closed

M-Jq

Intake Exhaust open closed

Intake

Intake

Intake Exhaust closed closed

Intake Exhaust closed closed

Power

Power

Intake Exhaust closed closed

Intake Exhaust closed closed

Exhaust

Exhaust

FIGURE 5.2. The four strokes of the four-stroke cycle.

sealing the combustion chamber and some of the compressed gases blow by the rings and into the crankcase.

The rapid expansion of the burning mixture causes the pressure to rise very quickly. The rapid rise in pressure (lb/in2) causes a force (lb) on the face of the piston,

This pressure forces the piston away from the cylinder head. This is called the power stroke. It is during this stroke that the chemical energy of the fuel is converted to power.

Before the piston reaches the bottom of the cylinder on the power stroke, the exhaust valve opens, and the exhaust gases start to flow out of the engine. As the piston returns toward the top of the cylinder, the remaining byproducts of combustion are expelled from the cylinder through the open exhaust valve. This is called the exhaust stroke.

At this point, the intake valve opens, and the process repeats. Four strokes of the piston and two revolutions of the crankshaft have been completed.

In a two-stroke cycle engine (commonly called the 2 cycle engine) the eight requirements for operation occur during two strokes of the piston and one revolution of the crankshaft. Two very noticeable differences exist between a 2 and a 4 cycle engine. Some variability exists in the ways 2 cycle engines are constructed, but Figure 5.4 will be used to illustrate the primary functions of the common designs. In the 2 cycle, the carburetor is attached to the crankcase, and there are no intake and exhaust valves. Instead gases flow into and out of the cylinder through ports in the cylinder wall as they are exposed and covered by the movement of the piston, Figure 5.3.

Study Figure 5.4 and the following description of events to understand the 2 cycle engine. In a 2 cycle engine, intake and exhaust occur at almost the same time. The description of the cycle starts after combustion. As the piston moves away from the cylinder head, it first exposes the exhaust port. The combustion pressure starts the process of expelling the gases. As soon as the piston moves away from the cylinder head, the reed valve closes. This causes the crankcase to become pressurized. As the piston continues to travel it exposes the intake port. The pressurized air-fuel-oil mixture in the crankcase flows into the cylinder. This flow delivers the next fuel-air charge for combustion and helps expel the exhaust gases.

As the piston continues toward the cylinder head, the intake port is closed and then the exhaust port is closed. As soon as the intake port is covered, the continuing movement of the piston lowers the pressure in the crankcase, and the air-fuel-oil mixture flows from the carburetor, through the reed valve, and into the crankcase. As soon as the exhaust port is closed and the piston continues to move toward the cylinder head, compression occurs.

0 0

Post a comment