Internal Combustion Engines

Introduction to Internal Combustion Engine:

As the name implies or suggests, internal combustion engines (briefly written as I.C. engines) are those engines in which the combustion of fuel takes place inside the engine cylinder. The internal combustion (IC) engine is a heat engine that converts chemical energy stored in a fuel into mechanical energy, usually made available on a rotating output shaft. These are petrol, diesel, and gas engines.

Classification of Internal Combustion Engine or IC Engine:

Internal Combustion Engine can be classified in several different ways:

a. Method of ignition.

b. Working cycle.

c. Valve or port design and location.

d. Basic engine design.

e. Combustion chamber design.

f. Air Intake Process.

g. Application.

h. Fuel Used.

i. Method of Mixture preparation.

j. Type of Cooling.

Main Components of Internal Combustion Engines or I.C. Engines:

An I.C. engine consists of hundreds of different parts, which are important for its proper working.

However, the main components, which are important from academic point of view is shown below:

The sequence of Operations in a Cycle:

When an engine is working continuously, we may consider a cycle starting from any stroke. We know that when the engine returns to the stroke where it started, we say that one cycle has been completed.

The following sequence of operations in a cycle occurs when an engine is working.

1. Suction stroke: In this stroke, the fuel vapor in correct proportion, is supplied to the engine cylinder.

2. Compression stroke: In this stroke, the fuel vapor is compressed in the engine cylinder.

3. Expansion or working stroke: In this stroke, the fuel vapor is fired just before the compression is complete. It results in the sudden rise of pressure, due to the expansion of the combustion products in the engine cylinder. This sudden rise of pressure pushes the piston with great force and rotates the crankshaft. The crankshaft, in turn, drives the machine connected to it.

4. Exhaust stroke: In this stroke, the burnt gases (or combustion products) are exhausted from the engine cylinder, to make space available for the fresh fuel vapor.

Two-stroke and Four-stroke Cycle Engines:

In a two-stroke engine, the working cycle is completed in two strokes of the piston or one revolution of the crankshaft. This is achieved by carrying out the suction and compression processes in one stroke (or more precisely in inward stroke), expansion and exhaust processes in the second stroke (or more precisely in outward stroke). In a four-stroke engine, the working cycle is completed in four strokes of the piston or two revolutions of the crankshaft. This is achieved by carrying out suction, compression, expansion, and exhaust processes in each stroke.

Advantages and Disadvantages of Two-stroke over Four-stroke Cycle Engines:

Following are the advantages and disadvantages of two-stroke cycle engines over four-stroke cycle engines:

Advantages:

1. A two-stroke cycle engine gives twice the number of power strokes than a four-stroke cycle engine at the same engine speed. Theoretically, a two-stroke cycle engine should develop twice the power as that of a four-stroke cycle engine.

But in actual practice, a two-stroke cycle engine develops 1.7 to 1.8 times (greater value for slow-speed engines) the power developed by a four-stroke cycle engine of the same dimensions and speed.

This is due to the lower compression ratio and effective stroke being less than the theoretical stroke.

2. For the same power developed, a two-stroke cycle engine is lighter, less bulky, and occupies less floor area. Thus, it makes, a two-stroke cycle engine suitable for marine engines and other light vehicles.

3. As the number of working strokes in a two-stroke cycle engine is twice that of a four-stroke cycle engine, the turning moment of a two-stroke cycle engine is more uniform.

Thus, it makes a two-stroke cycle the engine has a lighter flywheel and foundations. This also leads to higher mechanical efficiency of a two-stroke cycle engine.

4. The initial cost of a two-stroke cycle engine is considerably less than a four-stroke cycle engine.

5. The mechanism of a two-stroke cycle engine is much simpler than a four-stroke cycle engine.

6. The two-stroke cycle engines are much easier to start.

Disadvantages:

1. Thermal efficiency of a two-stroke cycle engine is less than that of a four-stroke cycle engine, because a two-stroke cycle engine has less compression ratio than that of a four-stroke cycle engine.

2. Overall efficiency of a two-stroke cycle engine is also less than that of a four-stroke cycle engine because, in a two-stroke cycle, the inlet and exhaust ports remain open simultaneously for some time. Despite careful design, a small quantity of charge is lost from the engine cylinder.

3. In the case of a two-stroke cycle engine, the number of power strokes is twice those of a four-stroke cycle engine. Thus the capacity of the cooling system must be higher. Beyond a certain limit, the cooling capacity offers considerable difficulty. Moreover, there is greater wear and tear in a two-stroke cycle engine.

4. The consumption of lubricating oil is large in a two-stroke cycle engine because of the high operating temperature.

5. The exhaust gases in a two-stroke cycle engine create noise, because of the short time available for their exhaust.

Four-stroke Cycle Petrol Engine:

It is also known as the Otto cycle. It requires four strokes of the piston to complete one cycle of operation in the engine cylinder. The four strokes of a petrol engine sucking fuel-air mixture (petrol mixed with a proportionate quantity of air in the carburetor known as charge) are described below:

1. Suction or charging stroke: In this stroke, the inlet valve opens and the charge is sucked into the cylinder as the piston moves down from the top dead center (T.D.C.). It continues till the piston reaches its bottom dead center (B.D.C.).

2. Compression stroke: In this stroke, both the inlet and exhaust valves are closed and the charge is compressed as the piston moves upwards from B.D.C. to T.D.C. As a result of compression, the pressure, and temperature of the charge increase considerably (the actual values depend upon the compression ratio). This completes one revolution of the crankshaft.

Fig: Sequence of operations in a Four-stroke cycle petrol engine.

3. Expansion or working stroke: Shortly before the piston reaches T.D.C. (during compression stroke), the charge 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. It may be noted that during this working stroke, as shown in Figure, both the valves are closed and the piston moves from T.D.C. to B.D.C.

4. Exhaust stroke: In this stroke, the exhaust valve is open as the piston moves from B.D.C. to T.D.C. This movement of the piston pushes out the products of combustion, from the engine cylinder and is exhausted through the exhaust valve into the atmosphere, as shown in Figure. This completes the cycle, and the engine cylinder is ready to suck the charge again.

Valve Timing Diagram:

A valve timing diagram is a graphical representation of the exact moments, in the sequence of operations, at which the two valves (i.e., inlet and exhaust valves) open and close as well as the firing of the fuel. It is, generally, expressed in terms of angular positions of the crankshaft.

Valve Timing Diagram for a Four-stroke Cycle Petrol Engine:

In the valve timing diagram, as shown in the figure below we see that the inlet valve opens before the piston reaches TDC; or in other words, while the piston is still moving up before the beginning of the suction stroke. Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC.

This is done as the incoming charge continues to flow into the cylinder although the piston is moving upwards from BDC. Now the charge is compressed (with both valves closed) and then ignited with the help of a spark plug before the end of the compression stroke. 

This is done as the charge requires some time to ignite. By the time, the piston reaches TDC, the burnt gases (under high pressure and temperature) push the piston downwards with full force and the expansion or working stroke takes place.

Here,

TDC: Top dead center

BDC: Bottom dead center

IVO: Inlet valve opens (10°-20° before TDC)

IVC: Inlet valve closes (30°- 40° after BDC)

IGN: Ignition (200-30° before TDC)

EVO: Exit valve opens (30°-50° before BDC)

EVC: Exit valve closes (10°-15° after TDC)

Figure: Valve Timing Diagram for a Four-stroke Cycle Petrol Engine.

Now the exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. Now the piston reaches BDC and then starts moving up, thus performing the exhaust stroke. The inlet valve opens before the piston reaches TDC to start the suction stroke. This is done as the fresh incoming charge helps push out the burnt gases.

Now the piston again reaches TDC, and the suction stroke starts. The exit valve closes after the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards. It may be noted that for a small fraction of a crank revolution, both the inlet and outlet valves are open. This is known as valve overlap.

Four-stroke Cycle Diesel Engine:

It is also known as a compression ignition engine because the ignition takes place due to the heat produced in the engine cylinder at the end of the compression stroke. The four strokes of a diesel engine sucking pure air are described below:

1. Suction or charging stroke: In this stroke, the inlet valve opens and pure air is sucked into the cylinder as the piston moves downwards from the top dead center (TDC). It continues till the piston reaches its bottom dead center (BDC).

2. Compression stroke: In this stroke, both the valves are closed and the air is compressed as the piston moves upwards from BDC to TDC. As a result of compression, the pressure, and temperature of the air increase considerably (the actual value depends upon the compression ratio). This completes one revolution of the crankshaft.

Figure: Sequence of operations in a Four-stroke cycle diesel engine.

3. Expansion or working stroke: Shortly before the piston reaches the TDC (during the compression stroke), fuel oil is injected in the form of very fine spray into the engine cylinder, through the nozzle, known as 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.

 The fuel oil is continuously injected for a fraction of the revolution. The fuel oil is assumed to be burnt at constant pressure. 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.

4. Exhaust stroke: In this 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.

Valve Timing Diagram for a Four-stroke Cycle Diesel Engine:

In the valve timing diagram as shown in the Figure below, we see that the inlet valve opens before the piston reaches TDC; or in other words, while the piston is still moving up before the beginning of the Suction stroke. Now the piston reaches the TDC and the suction stroke starts. The piston reaches the BDC and then starts moving up. The inlet valve closes, when the crank has moved a little beyond the BDC. This is done as the incoming air continues to flow into the cylinder although the piston is moving upwards from BDC.

Now the air is compressed with both valves closed. The fuel valve opens a little before the piston reaches the TDC. Now the fuel is injected in the form of very fine spray, into the engine cylinder, which gets ignited due to the high temperature of the compressed air. The fuel valve closes after the piston has come down a little from the TDC. This is done as the required quantity of fuel is injected into the engine cylinder.

The burnt gases (under high pressure and temperature) push the piston downwards, and the expansion or working stroke takes place. Now the exhaust valve opens before the piston again reaches BDC and the burnt gases start leaving the engine cylinder. 

Heare,

TDC: Top dead center

BDC: Bottom dead center

IVO: Inlet valve opens (10° -20° before TDC)

IVC: Inlet valve closes (25° - 40° after BDC)

FVO: Fuel valve opens (10° -15° before TDC)

FVC: Fuel valve closes (15° -20° after TDC)

EVO: Exhaust valve opens (39°-50° before BDC)

EVC: Exhaust valve closes (10° -15° after TDC)

Figure: Valve Timing Diagram for a Four-stroke Cycle Diesel Engine.

Now the piston reaches BDC and then starts moving up thus performing the exhaust stroke. The inlet valve opens before the piston reaches TDC to start the suction stroke. This is done as the fresh air helps in pushing out the burnt gases. Now the piston again reaches TDC, and the suction starts.

The exhaust valve closes when the crank has moved a little beyond the TDC. This is done as the burnt gases continue to leave the engine cylinder although the piston is moving downwards.

Comparison of Petrol and Diesel Engines:

The following points are important for the comparison of petrol engines and diesel engines:

Scavenging of Internal Combustion Engine or I.C. Engines:

The process of removing burnt gases, from the combustion chamber of the engine cylinder, is known as scavenging. Now we shall discuss the scavenging in four-stroke and two-stroke cycle engines.

1. Four-stroke cycle engines: In a four-stroke cycle engine, the scavenging is very effective, as the piston during the exhaust stroke, pushes out the burnt gases from the engine cylinder. It may be noted that a the small number of burnt gases remain in the engine cylinder in the clearance space.

2. Two-stroke cycle engines: In a two-stroke cycle engine, the scavenging is less effective as the exhaust port is open for a small fraction of the crank revolution. Moreover, as the transfer and exhaust port are open simultaneously during a part of the crank revolution, therefore fresh charge also escapes out along with the burnt gases. This difficulty is overcome by designing the piston crown of a particular shape.

Types of Scavenging:

Though there are many types of scavenging, the following are important from the subject's point of view:

1. Crossflow scavenging: In this method, the transfer port (or inlet port for the engine cylinder) and exhaust port are situated on the opposite sides of the engine cylinder (as is done in the case of two-stroke cycle engines). The piston crown is designed into a particular shape so that the fresh charge moves upwards and pushes out the burnt gases in the form of cross-flow as shown in Figure (a)

Figure: Types of scavenging.

2. Backflow or loop scavenging: In this method, the inlet and outlet ports are situated on the same side of the engine cylinder. The fresh charge, while entering into the engine cylinder, forms a loop and pushes out the burnt gases as shown in Figure (b).

3. Uniflow scavenging: In this method, the fresh charge, while entering from one side (or sometimes two sides) of the engine cylinder pushes out the gases through the exit valve situated on the top of the cylinder. In uniflow scavenging, both the fresh charge and burnt gases move in the same upward direction as shown in Figure (c).

Detonation in Internal Combustion Engine or I.C. Engines:

The loud pulsating noise heard within the engine cylinder is known as detonation (also called knocking or pinking). It is caused due to the propagation of a high-speed pressure wave created by the auto-ignition of the end portion of unburnt fuel. The blow of this pressure wave may be of sufficient strength to break the piston. Thus, the detonation is harmful to the engine and must be avoided.

The following are certain factors that cause detonation:

1. The shape of the combustion chamber.

2. The relative position of the sparking plugs in case of petrol engine.

3. The chemical nature of the fuel.

4. The initial temperature and pressure of the fuel.

5. The rate of combustion of that portion of the fuel which is the first to ignite. This portion of the fuel in heating up, compresses the remaining unburnt fuel, thus producing the conditions for auto-ignition to occur.

The detonation in petrol engines can be suppressed or reduced by the addition of a small amount of lead ethide or ethyl fluid to the fuel. This is called doping.

The following are the chief effects due to detonation:

1. A loud pulsating noise that may be accompanied by a vibration of the engine.

2. An increase in the heat lost to the surface of the combustion chamber.

3. An increase in carbon deposits.

Cooling of Internal Combustion Engine or I.C. Engines:

We have already discussed that due to the combustion of fuel inside the engine cylinder of I.C. engines, intense heat is generated. It has been experimentally found that about 30% of the heat generated is converted into mechanical work.

Out of the remaining heat (about 70%), about 40% is carried away by the exhaust gases into the atmosphere. The remaining part of the heat (about 30%), if left unattended, will be absorbed by the engine cylinder, cylinder head piston, engine valves, etc.

It has also been found that the overheating of these parts cause the following effects:

1. Overheating causes thermal stresses in the engine parts, which may lead to their distortion.

2. Overheating reduces the strength of the piston. The overheating may cause even a seizure of the piston.

3. Overheating causes decomposition of the lubricating oil, which may cause carbon deposits on the engine and piston head.

4. The overheating causes burning of valves and valve seats.

5. Overheating reduces the volumetric efficiency of the engine.

6. Overheating increases the tendency of detonation. 

In order to avoid the adverse effects of overheating, it is very essential to provide a cooling system for an I.C. engine.

In general, the cooling system provided should have the following two characteristics for its efficient working:

1. It should be capable of removing about 30% of the total heat generated in the combustion chamber. It has been experienced that the removal of more than 30% of heat generated reduces the thermal efficiency of the engine.

Similarly, removal of less than 30% of the heat generated will have some adverse effects as mentioned above.

2. It should be capable of removing heat at a fast rate when the engine is hot. But at the time of starting the engine, the cooling should be comparatively slow, so that the various components of the engine attain their the working temperature in a short time.

Comparison of Air-Cooling and Water-Cooling Systems:

The following points are important for the comparison of air-Cooling and water-cooling systems:

Supercharging of Internal Combustion Engine or I.C. Engines:

It is the process of increasing the mass, or in other words density, of the air-fuel mixture in a spark ignition engine) or air (in compression ignition i.e., diesel engines) induced into the engine cylinder. This is, usually, done with the help of a compressor or blower known as a supercharger. It has been experimentally found that supercharging increases the power developed by the engine.

It is widely used in aircraft engines, as the mass of air, sucked into the engine cylinder, decreases at very high altitudes. This happens because atmospheric pressure decreases with the increase in altitude. Nowadays, supercharging is also used in two-stroke and four-stroke petrol and diesel engines.

It will be interesting to know that a supercharged engine is lighter, requires smaller foundations, and consumes less lubricating oil as compared to an ordinary engine.

Following are the objects of supercharging the engines:

1. To reduce the mass of the engine per brake power (as required in aircraft engines).

2. To maintain the power of aircraft engines at high altitudes where less oxygen is available for combustion.

3. To reduce space occupied by the engine (as required in marine engines).

4. To reduce the consumption of lubricating oil (as required in all types of engines).

5. To increase the power output of an engine when greater power is required (as required in racing cars and other engines).

Lubrication of Internal Combustion Engine or I.C. Engines:

The moving parts of an I.C. engine are likely to wear off due to the continuous rubbing action of one part with another. To avoid an early wearing of the engine parts, a proper lubrication arrangement is provided in I.C. engines.

In general, the following are the main advantages of lubrication of I.C. engines:

1. It reduces the wear and tear of the moving parts.

2. It damps down the vibrations of the engine.

3. It dissipates the heat generated from the moving parts due to friction.

4. It cleans the moving parts.

5. It makes the piston gas-tight

Difference Between Petrol Engine and Diesel Engine