An engine is a machine that produces heat energy from the fuel’s chemical energy and converts it into mechanical work. It is a landmark invention in the history of human scientific endeavor.
Broadly speaking, all engines are classified into two types:
- External Combustion (EC) Engine: In the external combustion engine, the combustion of the fuel takes place outside the cylinder of the typical piston-cylinder arrangement. Examples include steam engines, steam turbines, closed-cycle gas turbines, and so forth.
- Internal Combustion (IC) Engine: In the internal combustion engine, the combustion of the fuel takes place within the cylinder of the piston-cylinder configuration of the engine. Examples include petrol engines, diesel engines, homogeneous charge compression ignition engines (HCCI) engines, and others.
There are certain benefits of IC engines over its EC counterpart that are described below:
- Higher mechanical efficiency
- Higher volumetric efficiency
- Overall simple engine design
- Low footprint
- Higher weight-to-power ratio
- Lower initial capital investment
- Easy and quick start
- Compact in construction
On the other side, there are quite a few advantages of external combustion engines over internal combustion as well:
- Higher torque at the engine start
- Adaptability with multiple fuels
- Flexible design arrangement for combustion
- Self-starting compared with a comprehensive ignition mechanism in the IC engine (petrol/gasoline engine)
Classification of the Internal Combustion Engine
Internal combustion engine are classified as follows:
Based on Cycle of Operation
According to the cycle of operation, there are generally two types of internal combustion engines as follows.
- Otto Cycle Engines: It is also known as a petrol engine in which the mixture of air and fuel is prepared in the engine carburetor and then a metric amount of this mixture is pumped into the engine cylinder for combustion. The sequence of events is as follows: intake, compression, power, and exhaust. Four strokes of the piston make up two crank rotations which in turn give one power cycle of the engine.
- Diesel Cycle Engines: As its name implies, it takes diesel as fuel for firing. Combustion in such engines does not take place through a proper ignition system. Rather air-diesel mixture is allowed to auto-ignite during the compression process. The sequence of events is the same as for the petrol engine as cited above. Here, also, the two rotations of the crank give one power cycle of the diesel engine.
Apart from the Otto cycle and diesel cycle engines, there are other forms of engine as well which are the extension of these. They include:
- Atkinson Cycle Engine: It is the modified form of the Otto cycle engine with a longer expansion stroke and shorter compression stroke in order to give higher thermal efficiency of the engine in hybrid vehicles.
- Miller Cycle Engine: It is also the design extension of the Otto cycle engine with the use of superchargers and turbochargers to reduce the effective compression ratio of the engine yet give higher expansion ratios.
Based on the Number of Strokes
According to the number of strokes that make a full operating cycle of the engine, the IC engines are classified as follows:
- Two-stroke Engine: In these engines, the two strokes of the piston complete one operational engine cycle. These two strokes are called the compression stroke and power/exhaust stroke. In such engines, the intake and the exhaust processes take place during a single. They produce more power per engine resolution compared with their four-stroke counterpart.
- Four-stroke Engine: In these engines, the four strokes of the piston complete one operational engine cycle. Each stroke is associated with a single process and includes intake stroke, compression stroke, power stroke, and exhaust stroke (as cited above). It produces less power per engine revolution because it requires two engine revolutions for the completion of one operating cycle.
- Six-stroke Engine: These are the experimental engine design with two additional strokes to the already existing four-stroke design configuration for the petrol engine. These two new strokes include air-injection stroke and water-injection stroke. They are best at recovering the waste heat of the engine. Ample research is being done to make it commercially available on account of its improved cycle operations.
Based on Arrangement of Cylinder:
This class of engine is governed by the arrangement of engine cylinders within the engine case. It includes the following:
- Single Cylinder: It consists of one piston-cylinder setting attached to a crankshaft.
- In-light or straight: The piston-cylinder couplets are mounted in line one behind the other along the whole length of the crankshaft.
- V-engine: Two piston-cylinder banks are mounted at an angle in V-shape geometry on the same crankshaft. Usually, V-engines are available in eight-cylinder designs.
- Opposed Cylinder: It consists of two pistons one in each cylinder with the fuel combustion in the middle.
- W-engine: Three piston-cylinder banks are mounted in a W-shape setting on the same crankshaft. Usually, W-engines are designed for the racing cars.
- Opposed piston: It consists of two pistons in each cylinder with the combustion chamber in between the two pistons.
- Radial engine: Made in an odd number of engine cylinders from 3 to 13, a radial engine consists of a piston-cylinder arrangement in a radial manner. The connecting rod of each piston-cylinder couplet is connected with the master rod which in turn is joined with the crankshaft of the engine. The operating cycle is a four-stroke petrol cycle. Each cylinder has its own power stroke that runs the engine operation smoothly.
- Boxer Engine: It consists of two piston-cylinder sets facing each other and are mounted on the same crankshaft. It is also called a pancake engine.
- H-engines: The engine cylinders are arranged in a fashion that resembles H-shape geometry.
Based on Applications
Engines are also classified based on their applications. This class includes:
- Stationary engine
- Portable engine
- Marine engine
- Automobile engine
Based on Engine Speed
This class categorizes engines based on engine rpm and includes the following:
- High-speed engine
- Medium speed engine
- Low-speed engine
Based on Method of ignition
According to the method that is used to ignite the charge, the engines are classified as:
- Spark ignition engine
- Compression ignition engine
Based on Engine Cooling Mechanism
It is an engine’s class in which the taxonomy is based on the coolant that is used to cool off the engine’s hot body.
- Air-cooled engine
- Water-cooled engine
Based on Method of Governing
According to the method of governing, the engine classification is as follows:
- Hit and miss governed engine
- Quality governed engine
- Quantity governed engine
Based on Engine Valve Arrangement
The engine valve arrangement also defines a new engine class as follows:
- Overhead valve engine
- L-head type engine
- T-head type engine
- F-head type engine
Based on Number of Cylinders
The number of engine cylinders makes a class of engines as defined under:
- Single cylinder engine
- Multi-cylinder engine
Based on air-intake process
How the air is taken into the engine cylinder constitutes another engine class as follows:
- Naturally aspirated: It is provided with no air-intake boost pressure mechanism. Rather, the air is aspirated naturally into the engine cylinder without a draft or external fan.
- Supercharged: A compressor that is driven off by the engine crankshaft sends compressed air with increased pressure to the engine cylinder.
- Turbo-charged: The engine exhaust gases run a turbine that in turn drives a compressor to send compressed intake air to the engine cylinder.
- Crank-compressed: It is a two-stroke engine with a crankcase that acts as an air-intake compressor.
Based on Fuel
Premised on fuel type, the engines are classified as follows:
- Petrol engine
- Diesel engine
- Oil engine
- Kerosine engine
- LPG engine
- Dual-fuel engine
- Alcohol ethyl, methyl engine
Based on Fuel in-put in the Petrol Engine
In this class, the engines are classified thus:
- Carburated: The air-fuel mixture is prepared in the carburetor prior to reaching the cylinder for combustion.
- Multi-point fuel injection: More than one fuel injector is provided in each engine cylinder. A comparative illustration of both single-point and multi-point fuel injection arrangements is shown below.
- Throttle body fuel injection: The throttle body consists of a throttle plate that regulates the quantity of airflow to the cylinder and is controlled by its connection with the accelerator pedal. Fuel is injected into the passing stream of air via fuel injectors yet before reaching the cylinder, a mixture of air and fuel is now drawn into the cylinder for firing.
I am the author of Mechanical Mentor. Graduated in mechanical engineering from University of Engineering and Technology (UET), I currently hold a senior position in one of the largest manufacturers of home appliances in the country: Pak Elektron Limited (PEL).