What is the meaning of engine?

The term "engine" indicates a mechanical device/machine used to convert input energy to a specific mechanical output. The input energy may be in the form of chemical energy, thermal energy, or so on and mechanical output is in the form of mechanical energy (work done). Generally, the engines classify into two groups which are internal combustion type and external combustion type, and further, these two types can be classified into two types that are reciprocating and rotary engines.

What is an IC engine?

The term "IC engine" signifies its meaning as an internal combustion engine. The combustion process in an IC engine takes place in the internal part of the engine that is known as the combustion chamber. There is a major difference in the location where the combustion process of an IC engine and an external combustion (EC) engine takes place. The scope of the IC engine is very wide due to its better efficiency than the efficiency of an external combustion engine. Example- diesel engine, petrol engine, etc.

Types of IC engines

IC engines can be classified into several categories like the nature of the thermodynamic cycle, the number of strokes per cycle, on the basis of type of ignition, fuel used, method of ignition, number of cylinders, arrangement of cylinders and cooling system, etc.

On the basis of the nature of thermodynamic cycle

IC engines are divided into three types based on the nature of the thermodynamic cycle: diesel engine, petrol engine, and dual engine. The details about these engines are given below.

Diesel engine/cycle

It is an engine in which diesel is used as fuel, and the ignition of diesel takes place with the help of high compression of air in the combustion chamber. The other name of the diesel engine is compression ignition (CI) engine because the ignition of fuel takes place due to compressed air. The operation of a diesel engine is complete through four different processes: suction, compression, combustion of fuel, and exhaust of burnt gasses. The cycle of a diesel engine is given below:

Suction

In the operation of a diesel engine, first (0 to 1) the suction of air inside the combustion chamber occurs, and during the suction, the movement of the piston from the top-dead-center (TDC) to bottom-dead-center (BDC) takes place. The exhaust valve in the engine remains closed during the complete suction of air inside the combustion chamber.

Isentropic compression

After the suction process, the second process (1 to 2) of operation is compression inside the combustion chamber. During this process, the movement of the piston from BDC to TDC takes place, and the exhaust valve remains closed. The temperature and pressure of air after compression reaches the limit where self-ignition takes place.

Constant pressure heat addition

After the compression of air inside the combustion chamber, the third process (2 to 3) of operation is the ignition of air-fuel mixture by injecting diesel fuel inside the combustion chamber. When the ignition of air-fuel mixture takes place then heat is generated. The exhaust valve during this operation remains closed.

Isentropic expansion

Due to the generation of heat inside the combustion chamber, the expansion of the air-fuel mixture takes place, and due to this, the movement of the piston from TDC to BDC takes place, which results in the mechanical work output through the crank rotation. In the expansion process, the exhaust valve remains closed.

Constant volume heat rejection

After the isentropic expansion of the burnt air-fuel mixture, the heat rejection (4 to 1) from the combustion chamber takes place through the exhaust valve. The heat rejection takes place at a constant volume process.

Exhaust

After the heat rejection from the combustion chamber, the exhaust of burnt air-fuel mixture outside the combustion chamber takes place, and after this process, the next cycle starts.

The expression of thermal efficiency of diesel cycle can be represented as:

$\eta =1-\frac{1}{r^{\gamma -1}}\left[\frac{{r_c}^{\gamma }-1}{\gamma \left(r_c-1\right)}\right]$

Here, $\eta$ represents the thermal efficiency of diesel cycle, $r$ represents the compression ratio, $r_c$ represents the cut-off ratio and $\gamma$ represents the ratio of specific heats.

Otto cycle

It is a cycle in which petrol is used as fuel, and the ignition of air-fuel takes place with the help of a spark plug in the combustion chamber. The other name of the petrol engine is spark ignition (SI) engine because the ignition of fuel takes place due to spark plug. The operation of a petrol engine is completed through different processes: suction of air-fuel mixture, isentropic compression, constant volume heat addition, isentropic expansion, constant volume heat rejection, and exhaust of burnt gasses. The air standard Otto cycle is given below.

The expression of thermal efficiency of air standard Otto cycle can be represented as,

$\eta =\left[1-\frac{1}{r^{\gamma -1}}\right]$

Here, $\eta$ represents the thermal efficiency of Otto cycle, $r$ represents the compression ratio and $\gamma$ represents the ratio of specific heats.

Dual cycle

It is a cycle in which diesel is used as fuel, and the ignition of air-fuel takes place with the help of compressed air in the combustion chamber. The dual cycle is the same as the diesel cycle, but the heat addition process is different here. The addition of heat partially takes place at constant volume and constant pressure. The dual cycle is most useful in the field of the automobile because the latest diesel engine design is based on this cycle. The air standard dual cycle is given below:

On the basis of number of strokes per cycle

Based on the number of strokes per cycle, IC engines can be classified into four-stroke engines, two-stroke engines, and six-stroke engines. The basic details of these types of engines are given below:

Two-stroke engine

The name "two-stroke" indicates an engine that requires two strokes of the piston to complete an operating cycle of the IC engine. The two strokes of pistons are compression strokes and expansion strokes. In the two strokes of the piston, only one crankshaft rotation is required.

Four-stroke engines

The name "four-stroke" indicates an engine that requires four piston strokes, suction, compression, power, and exhaust to complete an operating cycle of the IC engine. The four strokes of pistons require two revolutions of crankshaft rotation.

Six-stroke engines

A new type of engine refers to a six-stroke engine to make some advancements in two-stroke and four-stroke. In this engine, one of the cylinders makes two strokes, and the other makes four strokes, making six strokes per cycle.

On the basis of fuel used

Based on the fuel used, generally, IC engines can be classified into diesel engines, petrol engines, and bio-fuel engines. The petrol engine performs on the basis of the otto cycle in which petrol is used as a fuel, and a spark plug helps ignite the air-fuel mixture. The diesel engine performs on the basis of the diesel cycle in which diesel is used as a fuel, and an injector is used to inject diesel inside the combustion chamber with compressed air. The bio-fuel engine runs with the help of biofuels like natural gas. It works the same as the otto cycle.

On the basis of number of cylinders

IC engines divide into two groups, single-cylinder and multicylinder, based on the number of cylinders. In the single-cylinder engine, only one cylinder is used, whereas, in the multicylinder engine, more than one cylinder is used to perform the thermodynamic cycle.

On the basis of arrangement of cylinders

IC engines can also be classified into different types based on the arrangement of cylinders. There are horizontal cylinder engines, vertical cylinder engines, V-type engines, radial engines, inline engines, and W shape engines. The basic details of these types of engines are given below:

Horizontal cylinder engine

In these types of engines, cylinders are arranged horizontally on both sides of a crankshaft. Only one crankshaft connects with both cylinders and performs the operations. The other name of this engine is a flat engine.

Vertical cylinder engine

It is a type of engine in which the movement of the piston is vertical, viz. vertically up and down, and the placement of the crankshaft is below the cylinder. A schematic of a vertical cylinder engine is given below.

V-shape cylinder engine

Whenever an engine's cylinders are placed at a specific angle whichvaries approximately from 60 to 90 degrees, then it forms a V-shape. These types of engines are used in sports bikes and high-speed vehicles.

Radial cylinder engine

The radial engine is a type in which all cylinders are arranged in the form of a wheel and spokes towards outwards direction from the crankcase. The common crankcase lies in the center of the wheel. A schematic of a radial engine is given below:

In-line cylinder engine

It is a type of engine in which all cylinders are arranged in a straight line. Due to the arrangement of the cylinder of this engine, it is referred to as the straight engine. Generally, the number of cylinders in this engine is an even number.

Common Mistakes

• Students sometimes get confused about the difference between the efficiency of diesel engines and petrol engines. However, for a constant compression ratio, the petrol engine is more efficient than the diesel engine but other than this condition, the diesel engine is more efficient than a petrol engine.
• Sometimes, students also get confused about the difference between the load-carrying capacity of a diesel engine and a petrol engine. However, the diesel engine can carry heavy loads with low speed, whereas the petrol engine is used in lightweight vehicles that run with high speed.
• The student also gets confused about the difference between the dual engine and diesel engine. However, the dual-engine also uses diesel as fuel like a diesel engine, but the significant difference is the heat addition process. In a diesel engine, heat addition occurs only at constant pressure, whereas heat addition occurs at constant volume first and then at constant pressure in the dual cycle.

Contexts and Applications

IC engine is very significant in the several professional exams and courses for undergraduate, Diploma level, graduate, postgraduate. For example:

• Bachelor of Technology in Mechanical Engineering
• Bachelor of Technology in Automobile Engineering
• Master of Technology in Mechanical and Automobile Engineering
• Doctor of Philosophy in Mechanical Engineering

Related Concepts

• External combustion engines
• Efficiency of IC engines
• Efficiency of external combustion engines
• Piston and cylinder mechanism
• Effects of different types of load
• Work done by IC engines
• Functioning of spark plug and carburetor
• Nomenclature of IC engines parts
• Thermodynamic cycles

Practice Problems

Q1. Which of the following option is correct regarding the ignition quality of petrol?

1. Octane number
2. Cetane number
3. Calorific value
4. None of these

Correct option: (a)

Explanation: Petrol is a combination of various hydrocarbons, and the ignition quality of petrol is identified with the help of octane number. Higher octane number represents high-quality petrol.

Q2. Which of the following option is correct regarding the ignition quality of diesel?

1. Octane number
2. Cetane number
3. Calorific value
4. None of these

Correct option: (b)

Explanation: Diesel is a fuel used in compression-ignition (CI) engines. The ignition quality of diesel is identified with the cetane number of fuel. The high cetane number of diesel represents good quality fuel.

Q3. Which of the following option is correct regarding the air standard efficiency of an IC engine?

1. Specific heat of air varies with temperature
2. No intake or exhaust processes
3. Both (a) and (b)
4.  None of these

Correct option: (b)

Explanation: In the air standard efficiency of the IC engine, it is assumed that the working fluid throughout the cycle is air, and it is treated as an ideal gas. Also, there are no intake and exhaust processes because they are replaced by heat addition and heat rejection processes.

Q4. The thermal efficiency of a petrol engine as compared to a diesel engine is,

1. High
2. Low
3. Equal
4. None of these

Correct option: (b)

Explanation: The thermal energy of the engine is a function of its compression ratio. Therefore, the thermal efficiency of a petrol engine is lower than a diesel engine because the diesel engine has a high compression ratio compared to a petrol engine.

Q5. What is the function of carburetor in a petrol engine?

1. To supply petrol only.
2. To supply air only.
3. To supply air and diesel fuel mixture.
4. To supply petrol and air mixture.

Correct option: (d)

Explanation: The main function of a carburetor in a petrol engine is to supply the correct air-fuel mixture to the combustion chamber. It also controls the engine's speed by supplying fuel to the combustion chamber.