CYCLE OF OPERATION

Most automotive engines work on the same principle. Whether the cycle of operation is that of a 2- or 4-stroke engine, the same events must occur. In this lesson we will discuss the theory of operation, the 2- and 4-stroke engines, and types of ignition. Let's start by discussing the theory of operation. 

1. Theory of Operation

For an engine to operate, certain events--intake, compression, power, and exhaust--must take place. In the gasoline engine, both fuel and air enter (intake) at the same time and are then compressed (compression). Next, the compressed gases are ignited by an electrical spark. The gases then burn and expand, creating the pressure that drives the piston down (power). Once all the power that the burning gases can deliver has been used, we must get rid of the burned gases. This takes place during the final event known as exhaust. In the diesel engine, only air enters (intake) the combustion chamber and is compressed (compression) by the upward motion of the piston. The compression of the air causes it to heat up to a very high temperature. Just before top-dead-center (TDC), fuel is injected into the hot compressed air causing the fuel to ignite. The ignited fuel expands, applying pressure to the piston which drives it down (power). As in the gasoline engine, after the burned gases have provided all the power they can, they are expelled (exhaust). 

To accomplish these events, the piston must travel up and down inside the cylinder. "Stroke" is the term used for this movement. When the piston travels from its highest point to its lowest point in the cylinder, this is referred to as a down-stroke. When the piston travels from its lowest point to its highest point, this is the up-stroke. For some engines, all the events take place during these two strokes. Others require four strokes to complete the cycle of events. In either case, an engine's cycle of operation must consist of all four events: intake, compression, power, and exhaust. We will cover the two cycles of operation and how they differ, but first, let's be sure you fully understand the strokes. Notice the large space created between the top of the piston and the top of the combustion chamber. When the stroke occurs, a suction or partial vacuum is created in this space because the pressure in the cylinder drops far below atmospheric pressure. When the piston travels up again, as illustrated in position "B", the pressure in the space rises far above atmospheric pressure. 

THE PISTON STROKE

2. Four-Stroke Cycle Engine

The cycle of operation for the four-stroke engine is easiest to understand because it is less complex. Each part plays a role during the cycle of operation. The piston is at TDC at the beginning of the intake stroke. The intake valve opens and the piston starts its down-stroke. This creates a low pressure area inside the cylinder. Atmospheric pressure then forces the fuel-air mixture in to enter the cylinder of the spark-ignition engine. In a compression engine, only air will enter the cylinder at this time. Even after the piston has reached bottom-dead-center (BDC), the cylinder pressure is still lower than atmospheric pressure, so the valve remains open and the gases or air (depending on engine type) continue to enter the cylinder. 

INTAKE STROKE

From the TDC position to the BDC position of the piston, the crankshaft rotates 1800. The intake valve closes and the exhaust valve remains closed just as it was during the intake stroke. Both valves are closed, the combustion chamber is sealed and the piston is traveling upward. What is going to happen to all the contents (air-fuel or air depending on engine type) in the combustion chamber? The cylinder contents are being compressed. This is the action or operation of the compression stroke. 

COMPRESSION STROKE

Let's see if you can answer this without looking back. What is the cycle of operation for a four-stroke engine? Did you say intake, compression, power, and exhaust? If you did, great; you remembered. How many degrees does the crankshaft rotate from TDC to BDC? Right again! 1800. The crankshaft has now completed one 3600 revolution while the piston has completed only two strokes of the four-stroke cycle. The piston is at the end of the compression event, which, depending upon the engine design, is just before or after TDC. To illustrate this, look at

figure.

 

END OF OMPRESSION

At this point, the fuel-air mixture ignites and begins to burn. The burning of these gases causes them to expand with tremendous force and drives the piston downward in the cylinder, creating the power stroke.

 

POWER STROKE

The valves remain closed for most of the power stroke but the exhaust valve opens just before the BDC on this stroke. The exhaust stroke begins here even though the piston has not yet started on its upward travel. Why do the exhaust valves open early? For one reason, the pressure inside the cylinder from the burning gases is still great enough to expel the burned gases out through the exhaust port. Another reason is to get as much of the hot exhaust gases out of the cylinder as soon as possible before the next cycle of operation begins with another intake stroke.The figure illustrates the exhaust stroke. When this stroke is completed, the crankshaft has completed two revolutions during the cycle of operation and the piston has completed four full strokes. 

EXHAUST STROKE

3. Two-Stroke Cycle Engine

The two-stroke cycle and the four-stroke cycle engine may be used as either a spark-ignition or compression-ignition engine. The basic difference between the two-stroke cycle engine and the four-stroke cycle engine is that in the two-stroke engine, all four events (intake, compression, power, and exhaust) must occur within two strokes of the piston instead of within four strokes. This means that every down-stroke is a power stroke and every up-stroke is a compression stroke. At some time during these two strokes, you must get raw gases into the cylinder and burned gases out of the cylinder. Look at figure to discover how this process is accomplished. 

 

EVENTS IN A TWO STROKE CYCLE

During the compression stroke, the gases are permitted to flow into the crankcase. The gases, now being compressed, ignite, driving the piston down on its power stroke. Near the end of the power stroke, the piston travels by two openings (the intake and exhaust ports) in the cylinder wall. As the piston uncovers these ports, the burned gases are exhausted into the atmosphere, and the raw gases are expelled from the crankcase via the cylinder. Therefore, intake and exhaust occur at the same time. The piston now begins to travel upward inside the cylinder, closing both the intake and exhaust ports in the cylinder wall. The fresh gases inside the cylinder are compressed to begin a new cycle. Therefore, two strokes of the piston (or one complete revolution of the crankshaft) completes the cycle of operation in the two-stroke engine. 

Can you explain the difference between the four-stroke and two-stroke engine? If you said the four-stroke performs one function per stroke while the two-stroke performs two functions per stroke, you're right. Now that you know the two- and four-stroke engine cycles of operation, we will discuss the two types of automotive ignitions: the spark-ignition and the compression-ignition. Let's start with the spark-ignition. 

4. Ignition

The spark-ignition engine has a spark plug installed in each cylinder of the cylinder block and at just the right time, a spark is emitted from the plug. This spark ignites the fuel surrounding it and the remaining fuel burns at a very rapid rate. The rate is so rapid, in fact, that it is similar to an explosion.

The combustion chamber of the compression-ignition engine is NOT located in the cylinder head but in the top of the piston. As the piston travels downward on the intake stroke, only fresh air is taken into the cylinder. During the compression stroke, this fresh air is compressed into such a small area that it becomes extremely hot due to the high pressure exerted upon it. Fuel must then be introduced into the cylinder at exactly the proper time. Just before the top of the compression stroke, the fuel injector sprays fuel into the combustion chamber. When the fuel is injected, it must first vaporize and then superheat until it finally reaches the spontaneous-ignition lag or ignition delay. At the same time, other portions of the fuel are being injected and are going through the same phases behind the igniting portion. As the flame spreads from the point of ignition, appreciable portions of the charge reach their spontaneous-ignition temperatures at practically the same instant. This rapid burning causes a very rapid increase in pressure, which in some engines is accompanied by a distinct and audible "knock." The pressure of the burning gases drives the piston on through its power stroke and on to the exhaust stroke just like the spark-ignition engine. This completes the cycle of operation.

Note: Increasing the compression ratio in the diesel engine will decrease the ignition lag and thereby decrease the tendency to knock.