What is an Engine?
An engine is a machine designed to convert one form of energy into mechanical
energy. Heat engines like internal combustion engines burn their fuel inside the
engine cylinder.
On the other side, external combustion engines are those heat engines that burn their
fuel outside of the engine cylinder.
These are steam engines. The energy developed during the combustion of fuel is
transmitted to steam which acts on the piston inside the cylinder. In internal
combustion engines, the chemical energy stored in their operation.
The heat energy is converted into mechanical energy by the expansion of the gases
against the piston attached to the crankshaft that can rotate.
Types of Engine
Following are the different types of engine:
- Types of Fuel Used
- Petrol engine
- Diesel engine
- Gas engine
2. According to Cycle of Operation
- Otto cycle engine
- Diesel cycle engine
- The dual combustion cycle engine
3. Number of Strokes per Cycle
- Four-stroke engine
- Two-stroke engine
4. Classified by type of ignition
- Hot spot ignition engine
- Spark ignition (S.I.) engine
- Compression ignition (C.I.) engine
5. Number of Cylinders
- Single-cylinder engine
- Two-cylinder engine
- Three-cylinder engine
- Four-cylinder engine
- Six-cylinder engine
- Eight-cylinder engine
- Twelve-cylinder engine
- Sixteen cylinder engine
6. Classified by Valve arrangement
- L-head engine
- I-head engine
- F-head engine
- T-head engine
7. Classified by Cooling System
- Air-cooled engine
- Water-cooled engine
- Evaporation cooling engine
In addition to the above classifications, the internal combustion engines are
classified on the following bases also :
1. Speed
- Low-speed engine
- High-speed engine
- Medium-speed engine
2. Method of fuel injection
- Carburettor engine
- Air injection engine
- Airless or solid injection engine
3. Method of governing :
- Hit and miss governed engine
- Qualitatively governed engine
- Quantitatively governed engine
4. Application:
- Stationary engine
- Automotive engine
- Locomotive engine
- Marine engine
- Aircraft engine
Several other types of internal combustion engines are also being proposed for use as
automotive power plants. These include
- The free-piston engine
- Wankel engine
- Sterling engin
1. Classification by Types of Fuel used
According to the type of fuel used the engines are classified into three categories
- Petrol engine (or Gasoline engine)
- Diesel engine
- Gas engine
Petrol Engine
The petrol engine uses petrol for its running. Petrol or gasoline is a hydrocarbon,
made up of hydrogen and carbon compounds. Air-petrol mixture is sucked into the
cylinder during the suction stroke of the piston. The correct air-petrol mixture is
obtained from the carburettor.
The mixture is compressed during the compression stroke, ignited during the power
stroke and the exhaust gases pushed out during the exhaust stroke. A spark plug is
fitted at the top of the cylinder which gives a spark to ignite the mixture.
Diesel Engine
In these types of engines, diesel oil is used for its running. Diesel oil is light, with low
viscosity and high cetane number. In the diesel engine, only air is sucked into the
cylinder during the suction stroke and compressed to high pressure, the compression
ratio is as high as 22:1. Its temperature also rises by about 1,000°F.
The diesel oil is injected by an injector at the end of the compression stroke which
catches fire and burns due to the high temperature of the compressed air. No separate
ignition system is required. The burnt gases expand pushing the piston down during
the power stroke and finally, the gases are pushed out during the exhaust stroke.
Gas Turbine
A gas turbine essentially consists of two sections-a gasifier section and a power
section. The fuel used in a gas turbine can be gasoline, kerosene, or oil. The gasifier
section burns fuel in a burner and delivers the resulting gas to the power section,
where it spins the power turbine. The power turbine then turns the vehicle wheels
through a series of gears.
The gasifier consists of a compressor which has a rotor with a series of blades around
its outer edge. As the rotor rotates, the air between the blades is carried around and
thrown out by centrifugal force into the burner. Thus air pressure rises in the burner.
The fuel is injected in the burner where it burns and further raises the pressure.
2. Classification by Cycle of Operations
According to the cycle of operations, the automobile engines may be of the following
three types:
1. Otto cycle engine.
2. Diesel cycle engine.
3. Dual cycle engine.
Otto Cycle or Constant Volume Cycle
Otto Cycle or Constant Volume Cycle. This cycle was introduced in practical form by
a German scientist Otto, in 1876, although it was described by French scientist Beaude
Roches in 1862. The engines operating on this cycle are known as Otto-cycle engines.
The petrol engines operate on this cycle.
An I.C. engine does not undergo a cyclic change but it is assumed here that the
working medium is pure air which does not undergo any chemical change. The air is
simply heated and cooled to undergo a cycle. It is also assumed that the ideal
indicator diagram is strictly followed.
The ideal Otto cycle consists of the following operations
1-2 Adiabatic compression.
2-3 Heat addition at constant volume.
3-4 Adiabatic expansion.
4 1 Heat rejection at constant volume.
Diesel Cycle or Constant Pressure Cycle
Diesel cycle was introduced by Dr. Rudolph Diesel in 1897. The engines operating on
this cycle are known as Diesel engines. The figure shows the p-v diagram for a Diesel
cycle.
It consists of the following operations:
1-2 Adiabatic compression.
2-3 Heat addition at constant pressure
3-4 Adiabatic expansion.
4-1 Heat rejection at constant volume
The Diesel cycle differs from the Otto cycle in one respect. In the Diesel cycle, the heat
is added at constant pressure instead of a constant volume.
The air is compressed in the cylinder during the compression stroke from point 1 to 2.
Now the heat is added at constant pressure from point 2 to 3, and then the air is
expanded adiabatically from point 3 to 4. Finally, the heat is rejected at constant
volume from point 4 to 1. The air returns to its original condition and the cycle is
complete.
Dual Cycle (or Dual Combustion Cycle)
In these types of engines, more time is allowed for the combustion of fuel in the
Diesel engine without adversely affecting the efficiency.
The fuel is injected in the cylinder before the end of the compression stroke so that
combustion proceeds partly at constant volume and partly at constant pressure. Such a cycle is known as Dual cycle. In fact, all the Diesel engines actually operate on this
cycle. The figure shows the dual cycle on the p-v diagram.
It consists of the following operations.
1-2. Adiabatic compression
2-3. Heat addition at constant volume
3-4. Heat addition at constant pressure
4-5. Adiabatic expansion
5-1. Heat rejection at constant volume.
Because the fuel is injected in the cylinder before the end of the compression stroke in
the dual cycle, it takes care of ignition lag characteristic of fuel.
3. Classification by No. of Strokes per Cycle
According to the number of strokes per cycle, the automobile engines are classified as
Types of Engines
- Four-stroke engine.
- Two-stroke engine.
Four-stroke engine
The four-stroke completes a cycle of operations during the four-piston strokes,
namely suction, compression, power and exhaust. These four strokes require two
revolutions of the crankshaft. Thus, during every two revolutions of the crankshaft,
there is only one power stroke of the piston.
Two-stroke engine
The two-stroke engine completes a cycle of operations during the two-piston strokes. These two strokes require one revolution of the crankshaft. Thus, during every one
revolution of the crankshaft, there is one power stroke of the piston. Therefore, a two-stroke engine produces twice as much horsepower as a four-stroke engine of the
same size, running at the same speed.
In the two-stroke engine, the intake and compression strokes, and the power and
exhaust strokes are in a sense combined. Two-stroke engines are used in motorcycles,
Scooters. Four-stroke engines are used in cars, trucks, buses.
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