A comparison of two stoke and four-stroke engines on various aspects are given below: -

 Performance Parameters 

(i) Indicated power: -

The total power developed by the combustion of fuel in the combustion chamber is called indicated power (1.P.)

Where, 

Pim = Indicated mean effective pressure, bar

L= Length of stroke, m

A = Area of piston, m²

n = Number of working strokes per minute,

=N, for two-stroke engine

N = Engine speed, r.p.m,

K = Number of cylinders,

(ii) Brake power: - 

Brake Power The power developed by an engine at the output of the shaft is called as brake power (B.P.)

Where,

T = Engine torque, N-m.

How to Measurement of Brake Power: - 

The brake power of an engine is measured by measuring the torque and angular speed of the engine output shaft. The device used for torque measurement is called a dynamometer. A dynamometer is essentially a brake, in which an additional device has been incorporated for measuring the frictional resistance.

Dynamometers can be classified into the following two types -

(i) Absorption dynamometers.

(ii) Transmission dynamometers.

(iii) Mean effective pressure: - 

Mean effective pressure (M.E.P) is the hypothetical pressure that is thought to be acting on the piston throughout the power stroke.

Since power generated by an engine depends upon its size and speed, therefore engines cannot be compared on the basis of power. While m.e.p. can be considered as a true indicator of the relative performance of different engines.

When m.e.p. is calculated on the basis of indicated power, it is called indicated mean effective pressure (Pim) and when it is calculated on the basis of brake power it is called brake mean effective pressure (Pbm).

(iv) Indicated thermal efficiency: - 

(v) Brake thermal efficiency: - 

(vi) Mechanical efficiency: -

(vii) Volumetric efficiency: - 

(viii) Relative efficiency: -

Valve Timing Diagram for Two-stroke Cycle

Firstly, the exhaust port opens as the piston reaches BDC(Bottom Dead Center ) during the expansion and the burnt gases start leaving the cylinder. After a small fraction of crank revolution, the transfer port opens and the fresh air enters into the cylinder. This is done to push the burnt gases by fresh air. Now the piston reaches BDC and then starts moving upwards. As the crank moves a little beyond BDC, first the transfer port closes and then the exhaust port also closes. This is done to suck the fresh air and exhaust the burnt gases simultaneously. 

Now the charge is compressed and the fuel valve is opened before the piston reaches TDC (Top Dead Center). Fuel is injected in the form of spray and ignited due to the temperature of compressed air. The fuel valve closes after the piston has come down a little from the TDC. Now the burnt gases push the piston downward with full force and expansion of gases takes place. And again the above-said cycle continues. In this cycle, the exhaust and transfer ports open and close at equal angles on either side of the BDC position.

Valve Timing Diagram for Four-stroke Cycle

 engine. We see that the inlet valve opens before the beginning of the suction stroke. After reaching TDC the suction stroke starts. The piston reaches BDC and then starts moving up. When the crank moves a little beyond the BDC, the inlet valve closes. This is done as the incoming air continues to flow into the cylinder although the piston is moving upward from BDC.

Now the air is compressed with both valves closed. The fuel valve opens before the piston reaches TDC. Fuel is injected in the form of fine spray and gets ignited due to the high temperature of compressed air. The fuel valve closes after the piston has come down a little from TDC. Burnt gases push the piston downward and expansion takes place. Now exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. The inlet valve opens before TDC to start a suction stroke. This is done as the fresh air helps in removing the burnt gases, and again the process continues. 

 In a two-stroke cycle CI engine, there is one working stroke after every revolution of the crankshaft. All the stages of the two-stroke cycle are discussed.

(i) Suction Stage - In the suction stage, the transfer port and the exhaust port are open while the piston moves toward BDC. The fresh air flows into the cylinder from the crankcase.

(ii) Compression Stage - In this stage, the piston moves toward the top dead center (TDC) while moving up, first covers the transfer port and then the exhaust port. After that, the air is compressed as the piston moves upward. In the compression stage, the inlet port opens and the fresh air enters into the crankcase.

(iii) Expansion Stage - Before the piston reaches the TDC during compression stroke, the fuel oil is injected in the form of fine spray into the engine cylinder through fuel injection valve. At this moment, temperature of the compressed air is sufficiently high to ignite the fuel. It suddenly increases the pressure and temperature of the products of combustion. Due to increased pressure, the piston is pushed with a great force. The hot burnt gases expand due to high speed of the piston. During the expansion, some of the heat energy produced is transformed into mechanical work. 

(iv) Exhaust Stage- In exhaust stage, the exhaust port is opened and the piston moves downwards. The products of combustion from the engine cylinder are exhausted through the exhaust port into the atmosphere . This completes the cycle, and the engine cylinder is ready to suck the air again.

 In diesel or CI engines, the ignition takes place due to the heat produced in the engine cylinder at the end of the compression stroke.

The four-stroke cycle of a CI engine are discussed below -

(iv) Exhaust Stroke- In exhaust stroke, the exhaust valve is open as the piston moves from BDC to TDC. This movement of the piston pushes out the products of combustion from the engine cylinder through the exhaust valve into the atmosphere. This completes the cycle and the engine cylinder is ready to suck the fresh air again.

 The real and ideal p-V diagrams for the 4-stroke diesel cycle are shown in the upper figure. 

The difference between them is due to the following reasons - 

(i)  The suction of air inside the cylinder is possible only if the pressure inside the cylinder is below atmospheric.

(ii) The exhaust of gases is possible only if the pressure of the exhaust gases is above atmospheric pressure.

(iii) Compression and expansion are not isentropic, as there are some heat and pressure losses. 

(iv) The combustion is not possible at constant pressure as the fuel will not burn as it is introduced into the cylinder. 

(v) The sudden pressure release after the opening of the expansion valve is not possible and it takes through some degrees of crank rotation.

 working principle of 4 stroke cycle SI engine

 Basic Components of Internal Combustion Engine

The cross-section of a single-cylinder spark-ignition engine with its basic components. Some of its main components are discussed below --

                                     Fig. - Basic Components of Internal Combustion Engine

i). Cylinder - It is a cylindrical vessel in which the piston slides. It contains gases under pressure and guides the piston. The cylinder is usually made of hard-grade cast iron, cast in one piece.

ii) Piston - A piston is also a cylindrical component fitted to each cylinder as a face to receive gas pressure and transmit the thrust to the connecting rod. Pistons are made of cast iron or aluminum alloy.

iii) Connecting Rod - Connecting rod is made up of steel and it interconnects the piston and the crankshaft and transmits the forces from the piston to the crankshaft.

 (iv) Crank - Crank along with connecting rod converts the reciprocating motion of the piston into useful rotary motion of the output shaft. 

(v) Crankshaft- The crank is a part of the crankshaft. Crankshaft receives the efforts supplied by the pistons to the connecting rods. All the engine auxiliary mechanisms are connected with the crankshaft in one or another way. It is usually a steel forging but can also be made from spheroidal graphitic or nickel alloy.

 (vi) Gudgeon Pin- These are hardened steel parallel spindles fitted through the piston bosses and the small eye of the connecting rod. 

(vii) Valves- Valves used in engines can be inlet and exhaust valves. Inlet valves are used for taking the charge in the cylinder and exhaust valves are used for discharging the products of combustion. Valves are fitted either on the cylinder head or on the sides of the cylinder.