Senior Mechanical Engineer

Compare/Difference Between Two Stroke and Four Stroke Engine, Neat Sketch

2021-08-14T12:33:00.002+05:30

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

S. No

Aspect

Two Stroke Engine

Four Stroke Engine

(i)

(ii)

(iii)

(iv)

(v)

(vi)

(vii)

(viii)

(ix)

(x)

Completion of cycle/working

Flywheel required

Power produced Weight

Cooling requirement

Valve and valve mechanism

Wear and tear

Scavenging

Initial Cost

Efficiency

The thermodynamic cycle is completed in two strokes of the piston or in one revolution of the crankshaft.

One power stroke is available in each revolution of the crankshaft. Thus, turning the moment is more uniform, and a lighter flywheel is required.

More.

Since the power produced is more from the same size of the engine, therefore the engine is lighter and compact.

Greater cooling and lubrication are required because there is one power stroke in each revolution. There are no valves only ports are there.

There are no valves only ports are there.

Higher.

Scavenging (the process of clearing the cylinder after expansion stroke) is required, because there is no exhaust stroke at the end of the expansion stroke.

Because of lightweight and simplicity due to the absence of valve mechanism, initial cost of the engine is less.

The volumetric and thermal efficiencies are high. Part load efficiency is better compared to a two-stroke cycle engine.

The cycle is completed in four strokes of the piston or in two revolutions of the crankshaft.

One power stroke is available in every two revolutions of the crankshaft. Thus, the turning moment is not so uniform and a heavier flywheel is required.

Less.

Since power produced is less from the same size of the engine, therefore the engine is heavier and bulkier.

Lesser cooling and lubrication are required because there is only one power stroke in two revolutions. It contains valve and valve mechanisms.

It contains valve and valve mechanism.

Lower.

No need of scavenging because there is an exhaust stroke at the end of the expansion stroke.

Because of the heavyweight and complicated valve mechanism, the initial cost of the engine is more.

The volumetric and thermal efficiencies are low. Part load efficiency is poor than four-stroke cycle engine.

Performance Parameters of The I.C. Engine PPT, Definition, Working

2021-07-11T23:21:00.004+05:30

Performance Parameters

Engine performance is an indication of the degree of success with which it converts the chemical energy of fuel into useful mechanical work. To evaluate the performance of 1.C. engines certain basic parameters are chosen and the effect of various operating conditions, design concepts and modifications on these parameters are studied.

The Basic Performance Parameters Are Listed Below: -

(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: -

Indicated thermal efficiency is defined as the ratio of heat equivalent of power produced in the cylinder in-unit o time and heat supplied to the engine in unit time.

Where,

Mf = Fuel oil supplied in kg per sec,

CV = Calorific value of fuel in kJ/kg,

(v) Brake thermal efficiency: -

Brake thermal efficiency is defined as the ratio of heat equivalent of brake power in unit time and heat supplied to the engine in unit time.

(vi) Mechanical efficiency: -

Mechanical efficiency may be defined as the ratio of useful power available at the engine crankshaft and power developed in the engine cylinder.

(vii) Volumetric efficiency: -

(viii) Relative efficiency: -

Relative Efficiency is also called the efficiency ratio. The relative ere engine efficiency is defined as the ratio of indicated thermal efficiency to the air standard efficiency.

(ix) Specific fuel consumption,

(x) Specific power output.

Actual Valve Timing Diagram For 2 Stroke Cycle And 4 Stroke Cycle Cl Engine.

2021-07-10T10:44:00.002+05:30

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.

Define Working of A Two Stroke Cycle Diesel Engine, Principal, Applications

2021-07-10T01:06:00.000+05:30

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.

Working Of A Four Stroke Diesel Cycle Engine With Suitable PV Diagram

2021-07-07T16:04:00.004+05:30

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 -

(i) Suction Stroke - In this stroke, the suction valve opens and pure air is sucked into the cylinder as the piston moves downwards. This will continue till the piston reaches BDC (Bottom Dead Center).

(ii) Compression Stroke - In compression stroke, both the inlet valve and exhaust valve are closed and the air is compressed as the piston moves upwards toward TDC (Top Dead Center). As a result of compression, pressure, and temperature of the air increase considerably. This completes one revolution of the crankshaft.

(iii) Expansion or Working Stroke - Before the piston reaches the TDC during the compression stroke, fuel oil is injected in the form of fine spray into the engine cylinder through the fuel injection valve. At this moment, the 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 down with great force. The hot burnt gases expand due to the high speed of the piston. During this expansion, some of the heat energy is transformed into mechanical work. It may be noted that during this working stroke, both the valves are closed and the piston moves from TDC to BDC.

(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.

Define Working Principle of Four Stroke Cycle SI Engine, Diagram, Examples

2021-07-03T00:29:00.004+05:30

working principle of 4 stroke cycle SI engine

As the name suggests, in a four-stroke engine the cycle of operation is completed into four strokes of the piston.

There are 4 stocks in SI engines are: -

(i) Suction Stroke - During the suction stroke, the piston moves from Top Dead Center (TDC) to Bottom Dead Center (BDC). The inlet valve is open in this stroke and the fuel-air mixture in correct proportion is sucked inside the cylinder. The exhaust valve remains closed throughout the stroke.

(ii) Compression Stroke - In compression stroke both the valves are closed and the charge is compressed as the piston moves upwards from BDC to TDC. As a result compression pressure and temperature of the charge increases considerably. This completes one revolution of the crankshaft Compression stroke.

(iii) Expansion or Power Stroke - Just before the piston reaches the TDC during the compression stroke, the air-fuel mixture is ignited with the help of a spark plug. It suddenly increases the pressure and temperature of the products of combustion, but the volume practically remains constant. Due to the rise in pressure, the piston is pushed down with great force. The hot burnt gases expand due to the high speed of the piston. During this expansion, some of the heat energy produced is transformed into mechanical work. In this stroke both the valves are closed.

(iv) Exhaust Stroke - As the piston moves from BDC to TDC the exhaust valve opens and the piston pushes out the products of combustion, from the engine cylinder to the atmosphere. This completes the cycle and the engine cylinder is again ready to suck the charge.

Basic Components of Internal Combustion Engine(ICE), Inventor, Diagram, Definition

2021-06-30T15:39:00.002+05:30

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.