Four-Stroke Engine: Parts, Working, and Otto Cycle

What is a 4-stroke engine?

The four-stroke engine is the most common type of internal combustion engine and is used in various automobiles that specifically use gasoline as fuel like cars, trucks, and some motorbikes.

A four-stroke engine delivers one power stroke for every two cycles of the piston (or four-piston strokes). There is an animation of a four-stroke engine and a further explanation of the process below.

  • Intake stroke: The piston moves downward to the bottom; this increases the volume to allow a fuel-air mixture to enter the chamber.
  • Compression stroke: The intake valve is closed, and the piston moves up the chamber to the top. This compresses the fuel-air mixture. At the end of this stroke, a spark plug provides the compressed fuel with the activation energy required to begin combustion.
  • Power Stroke: As the fuel reaches the end of its combustion, the heat released from combusting hydrocarbons increases the pressure which causes the gas to push down on the piston and create the power output.
  • Exhaust stroke: As the piston reaches the bottom, the exhaust valve opens. The remaining exhaust gas is pushed out by the piston as it moves back upwards.
four-stroke

The thermal efficiency of these gasoline engines will vary depending on the model and design of the vehicle. However, in general, gasoline engines convert 20% of the fuel (chemical energy) to mechanical energy in which only 15% will be used to move the wheels.

One way thermodynamic efficiency can improve in engines is through a higher compression ratio. This ratio is the difference between the minimum and maximum volume in the engine chamber A higher ratio will allow a larger fuel-air mixture to enter, causing higher pressure, leading to a hotter chamber, which increases thermal efficiency.

Four-Stroke engine’s parts:

Four-stroke engines consist of the following parts:

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  • Cylinder: It is the heart of the engine. The piston reciprocates in the cylinder.
  • Cylinder head: It is the top cover of the cylinder, towards TDC, which is called the cylinder head.
  • Piston: It is the reciprocating member of the engine. It reciprocates in the cylinder.
  • Piston rings: Two or three piston rings are provided on the piston. It seals the gap between the cylinder liner and piston.
  • Crank: It is a rotating member. It makes a circular motion inside the crankcase.
  • Crankcase: It is the housing of crank and other engine parts. It is also used as the sump of lubricating oil.
  • Connecting rod: It is used to convert the reciprocating motion of the piston to rotary motion of the crankshaft.
  • Crankshaft: It is the rotating member, which connects the crank.
  • Cooling fins or Water jackets: It is used for cooling purposes.
  • Cam and Camshaft: It is provided to operate the opening and closing of the Inlet and Exhaust valve and also operate the fuel injection pump in the Diesel engine.
  • Inlet valve: This valve controlled the admission of charge or air inside the engine cylinder.
  • Exhaust valve: This valve controls the removal of burnt gas after combustion.
  • Intake manifold: This is a passage that carries the fresh charge or air.
  • Exhaust manifold: This a passage through which the exhaust gas goes out of the engine cylinder.
  • Spark plug: It is used in a Petrol engine or SI engine to ignite the fuel.
  • Fuel injector: It is used in a Diesel engine or CI engine to sprayed the fuel inside the engine cylinder.
  • Carburetor: It is used in a Petrol engine to mix the air-fuel properly.
  • Flywheel: It is mounted on the crankshaft and is made of cast iron. It stores energy in the form of inertia.

The Otto Cycle

The pressure-volume diagram (PV diagram) that models the changes the fuel-air mixture undergoes in pressure and volume in a four-stroke engine is called the Otto cycle. The changes in these will create heat, and use this heat to move the vehicle or machine (hence why it’s a type of heat engine).

The component in any engine that uses this cycle will have a piston to change the volume and pressure of the fuel-air mixture. The piston gains motion from combusting the fuel (where this happens is explained below), and an electric boost at the startup of the engine.

The following describes what occurs during each step on the PV diagram, in which the combustion of the working fluid gasoline and air (oxygen), and sometimes electricity, change the motion in the piston:

  • Real cycle-step 0 to 1 (ideal cycle-green line): Referred to as the intake phase, the piston is drawn down to the bottom to allow the volume in the chamber to increase so it can “intake” a fuel-air mixture. In terms of thermodynamics, this is referred to as an isobaric process.
  • Process 1 to 2: During this phase, the piston will be drawn up, so it can compress the fuel-air mixture that entered the chamber. The compression causes the mixture to increase slightly in pressure and temperature however, no heat is exchanged. In terms of thermodynamics, this is referred to as an adiabatic process. When the cycle reaches point 2, it is when the fuel is met by the spark plug to be ignited.
  • Process 2 to 3: This is where combustion occurs due to the ignition of fuel by the spark plug. The combustion of the gas is complete at point 3, which results in a highly pressurized chamber that has lots of heat (thermal energy). In terms of thermodynamics, this is referred to as an isochoric process.
  • Process 3 to 4: The thermal energy in the chamber as a result of combustion is used to do work on the piston which pushes the piston down increasing the volume of the chamber. This is also known as the power stoke because it is when the thermal energy is turned into motion to power the machine or vehicle.
  • Purple line (Process 4 to 1 and exhaust phase): From process 4 to 1 the exhaust valve opens and all the waste heat is expelled from the engine chamber. As the heat leaves the gas, the molecules lose kinetic energy causing a decrease in pressure. Then the exhaust phase (step 0 to 1) occurs when the remaining mixture in the chamber is compressed by the piston to be “exhausted” out, without changing the pressure.

Advantages and Disadvantages of Four-Stroke engine:

Advantages of 4-Stroke engine:

  • More torque
  • More fuel economy than a 2-stroke engine
  • Less consumption of lubrication oil
  • Less pollution because of proper burning of fuel
  • More durability
  • Well managed cooling system, so that longer engine life
  • Less noisy
  • Scavenging happens really well, therefore Thermal efficiency and HP of the engine is high

Disadvantages of 4-Stroke engine:

  • There are many moving parts and hence there is more friction, therefore the engine has less mechanical efficiency.
  • The design is complicated
  • The weight of the engine is more because of heavier flywheel construction.
  • Due to more parts, maintenance required quite often.

Applications of Four-stroke Diesel engine:

The four-stroke Diesel engine is one of the most popular prime overs. It has various types of applications like:

  • Small pumping sets
  • In the automobile industry
  • Air compressor
  • Drilling jigs
  • Boats and Ships
  • Power saw
  • Bulldozers
  • Tanks
  • Diesel-electric locomotives ETC.

The cooling system in the Four-Stroke engine:

All the heat produced by the combustion of fuel is not totally converted into power at the crankshaft.

Only about 20 percent of the heat is used as power at the crankshaft. And 35 percent of the heat is transferred to the cylinder wall which causes pre-ignition of the charge and also lubricant burned away due to the heat, therefore the engine may be seized.

Keeping the above factor in mind we need to install a proper cooling system.

In a Four-stroke engine, there are two types of cooling methods.

  • Cooling by air
  • Cooling by water