Working of Four Stroke and Two Stroke IC Engine | I.C. Engine | Thermal Engineering

In this article we will discuss about the working of four stroke and two stroke IC engines.

Four Stroke Cycle Engine:

Principle:

In IC Engines the following events take place in the cyclic order:

(1) The quantity of air or mixture of air and fuel must be admitted in the engine cylinder. It is generally known as the charge.

(2) The charge in the engine cylinder must be compressed to the desired pressure.

(3) If the charge contains only air, the fuel must be injected at the end of compression.

(4) The charge must be ignited at the proper time.

(5) The combustion of the fuel should take place completely.

(6) The energy developed in the engine cylinder must be transferred to the crankshaft.

(7) After the energy is transferred, all the burnt gases should be removed from the engine cylinder so that it is ready to receive a fresh charge.

Fig. 13-21 illustrates in a simple diagrammatic way the four strokes of the piston or two revolutions of the crank which make up the cycle of operation of a four stroke cycle I.C. Engine.

The working of the four stroke cycle is as follows:

During the suction stroke, only air in case of C.I. engines or air with gas or air with atomised fuel in case of S.I. engines is drawn into the cylinder by the moving piston. The charge enters the engine cylinder through the inlet valve which is open. During this stroke the exhaust valve is closed.

During the compression stroke, the charge taken in during the suction stroke is compressed in the clearance space. On the completion of compression, if only air is taken in during the suction stroke, the fuel is injected into the engine cylinder at the end of compression. The mixture is ignited and the heat generated, while the piston is nearly stationary, sets up a high pressure.

During the power stroke, the high pressure developed as a result of combustion of fuel, causes the piston to be forced forward or downward and we have the working stroke. This is the important stroke of the cycle.

During the exhaust stroke the products of combustion are swept out through the open exhaust valve while the piston returns. This is the scavenging stroke. All the burnt gases are completely removed from the engine cylinder and the cylinder is ready to receive the fresh charge for the new cycle.

Thus we see that in four stroke cycle engine; there is one power stroke and three idle or non-working strokes. The power stroke always supplies the necessary energy to carry out this internal and external work, so that the engine is kept running.

Fig. 13-22 shows the nature of the resulting indicator diagram traced during the working of the four stroke cycle petrol engine.

The diagram is traced in the following manner P Q R S T P. UV is the atmospheric line. PQ denotes the suction stroke. This line is below the atmospheric line which shows that during the suction stroke the pressure in the engine cylinder is less than atmospheric pressure.

This is only true in case of naturally aspirated engines. In case of suer charged engines the pressure during the suction stroke is higher than the existing atmospheric pressure. QR is the compression curve. ST is the working or expansion stroke of the engine and TP is the exhaust or scavenge stroke. It will be noticed that this diagram consists of two loops.

The lower one from P to Q, represents negative work while upper one which is larger one represents the positive work. The effective work done per cycle on the piston is represented by the difference of the two. It should be clearly understood that both these diagrams are taken with the different springs. The negative loop is only traced by the weak spring.

Two Stroke Cycle Engine:

This cycle was invented by Duglas Clerk in 1830. The purpose of this cycle was to give one working stroke per revolution. The four stroke cycle engine gives one working stroke per two revolutions.

There are two types of two stroke engines:

1. Port Type Engine:

In two stroke cycle engine, there are ports instead of valves. In case of two stroke cycle engine, ports are covered and uncovered by the moving piston. Through the inlet ports, the mixture of air and fuel is taken in the crankcase of the engine cylinder and through the transfer ports the mixture enters the engine cylinder from the crankcase. The exhaust ports serve the purpose of exhausting the gases from the engine cylinder. These ports are more than one in number and are arranged circumferentially.

Fig. 13-27 illustrates diagrammatically the two stroke cycle petrol engine, employing crankcase compression, of the type used in, say in an ordinary motor cycle.

Suppose the engine under consideration is a vertical one and the piston is at its bottom dead centre position; in this position the transfer ports and exhaust ports are both open. A mixture of air and fuel slightly compressed in the crankcase enters through the transfer ports and drives the burnt gases from the previous stroke before it.

The shape of the top of the piston is such that the incoming charge is deflected upward to aid in sweeping the burnt gases out of the cylinder through the exhaust ports an operation known as scavenging. As the piston begins to move upwards fresh charge passes into the engine cylinder for about 1/6th of the revolution.

For the remainder of the upward stroke the charge taken in the engine cylinder is compressed after the piston has covered the transfer and exhaust ports. During the same time a partial vacuum is created in the crankcase, as a result of which mixture of air and fuel or only air is taken in the crankcase.

The arrangement is such that the mixture can enter into the crankcase, but the reverse flow is prevented. When the piston reaches the end of its stroke, the entire charge is compressed in the clearance space and the ignition is timed to occur and the ignited charge exerts pressure on the top of the piston.

During the downward stroke the piston is forced downward as a result of the pressure generated. When the piston is moving downward, first of all exhaust ports are uncovered by the piston and so the gases leave the cylinder. The downward movement of the piston causes the compression of the charge taken into the readiness for passing through the transfer ports as soon as they are uncovered by the downward moving piston.

When the piston moves further down, the transfer ports are open and so are exhaust ports and the stroke is completed when the piston reaches the end of the downward stroke. Fig. 13-28 is a typical valve timing diagram for a two stroke cycle engine in which it is seen that critical points of respiration are symmetrical about the dead centre positions.

 

When the piston of the vertical I.C. engine is at the top of its stroke, it is said to be at its Top Dead Centre (TDC) position. When the piston is at the bottom of its stroke, it is said to be at its Bottom Dead Centre (BDC) position. For horizontal engines the similar dead centre positions are Inner Dead Centre (IDC) position and Outer Dead Centre (ODC) position.

It should be clearly seen that in two stroke cycle engine both the sides of the piston are effective, which is not the case with four stroke cycle engine. Two cycles are being performed and two indicator diagrams will be traced: one for the top of the piston the main cylinder and other for the bottom of the cylinder.

Fig. 13-29 and fig. 13-30 illustrates the actual indicator diagram taken for the two stroke cycle engine. The diagram is only for the top of the piston.

 

The indicator diagram for the crankcase is known as the negative loop. The effective work done per cycle on the piston is represented by the differences of the work done represented by the positive loop and the negative loop. It is easy to perceive that the cylinder acts as the cylinder of the engine while the crankcase acts as the cylinder of the compressor, the same piston working for the engine as well as for the compressor.

2. Reed Valve:

The reed valve engines are the recent development in the I.C. engine-technology. In the reed valve engine, there is no inlet port. But a reed valve is fitted in the crank case. The reed valve operates on the principal of pressure difference. It is a metallic strip which has spring action. The reed valve is a valve which opens when there is a pressure difference between the crank case and atmospheric pressure.

If the pressure in the crank case is lower than atmospheric pressure i.e. when vacuum exist in the crank case then the fresh charge is admitted in the engine cylinder because of the opening of the reed valve. The reed valve closes when the pressure in the crank case is more than atmospheric pressure. The charge which is admitted in the crank case passes to the engine cylinder through the transfer ports. Fig. 13-31 shows the diagram of reed valve engine.

 

The advantages of the reed valve engine are:

(1) It reduces the length of stroke of the engine.

(2) The size of the piston is reduced.

(3) The leakage of gases from the inlet port can be prevented.

(4) The opening and closing of the valve depends on pressure difference.