A 4-stroke engine is a type of small internal combustion engine that uses four different piston strokes to complete one operating cycle. During this cycle, the crankshaft rotates twice while the piston goes up and down twice to fire the spark plug.
What is a Four-stroke engine?
A four-stroke cycle engine is an internal combustion engine that utilizes four distinct piston strokes (intake, compression, power, and exhaust) to complete one operating cycle. The piston makes two complete passes in the cylinder to complete one operating cycle.
An operating cycle requires two revolutions (720°) of the crankshaft. The four-stroke cycle engine is the most common type of small engine. A four-stroke cycle engine completes five Strokes in one operating cycle, including intake, compression, ignition, power, and exhaust Strokes.
They have a slightly more complex design than 2-stroke engines that include a compartment for oil, meaning you won’t have to premix any fuel. This feature contributes to cleaner emissions, making 4-stroke engines the more environmentally friendly option.
Thanks to their larger design, 4-stroke diesel and petrol engines tend to be larger than 2-stroke engines and weigh more. A 4-cycle engine also has more parts, but these additional features all come with excellent advantages.
They help 4-cycle engines operate at much quieter noise levels, deliver better fuel efficiency, and achieve longer life spans. They also deliver higher torque levels at lower RPMs.
Four-stroke engines are the most common internal combustion engine design for motorized land transport, being used in automobiles, trucks, diesel trains, light aircraft, and motorcycles.
Four-Stroke Engine Cycle
The four strokes of the engine go by the following names:
#1. Intake Stroke.
Intake stroke occurs when the air-fuel mixture is introduced to the combustion chamber. In this stroke, the piston moves from TDC (Top Dead Center – the farthest position of the piston to the crankshaft) to BDC (Bottom Dead Center – the nearest piston position to the crankshaft.)
The movement of the piston towards the BDC creates a low-pressure area in the cylinder. The inlet valve remains to open a few degrees of crankshaft rotation after BDC. The intake valve then closes, and the air-fuel mixture is sealed in the cylinder
Key points
- Inlet Valve – Open
- Outlet Valve – Closed
- Crankshaft Rotation – 1800
#2. Compression Stroke.
In compression stroke, the trapped air-fuel mixture is compressed inside the cylinder. During the stroke, the piston moves from BDC to TDC, compressing the air-fuel mixture. The momentum of the flywheel helps the piston move forward.
Compressing the air-fuel mixture allows more energy to be released when the charge is ignited. The charge is the volume of compressed air-fuel mixture trapped inside the combustion chamber ready for ignition. The inlet and outlet valves must be closed to ensure that the cylinder is sealed, resulting in compression.
Key points
- Inlet Valve – Closed
- Outlet Valve – Closed
- Crankshaft Rotation – 1800 (Total 3600)
#3. Power Stroke.
The second rotation of the crankshaft begins when it completes a full rotation during the compression stroke. The power stroke occurs when the compressed air-fuel mixture is ignited with the help of a spark plug.
Ignition or Combustion is the rapid, oxidizing chemical reaction in which a fuel chemically combines with oxygen in the atmosphere and releases energy in the form of heat. The hot expanding gases force the piston head away from the cylinder head.
Key points
- Inlet Valve – Closed
- Outlet Valve – Closed
- Crankshaft Rotation – 1800 (Total 5400)
#4. Exhaust Stroke.
As the piston reaches BDC during the power stroke, combustion is complete, and the cylinder is filled with exhaust gases.
The exhaust valves open during this stroke, and the inertia of the flywheel and other moving parts push the piston back to TDC, forcing the exhaust gases through the open exhaust valve. At the end of the exhaust stroke, the piston is at TDC, and one operating cycle has been completed.
Key points
- Inlet Valve – Closed
- Outlet Valve – Open
- Crankshaft Rotation – 1800 (Total 7200)
Four-Stroke engine’s parts:
Four-stroke engines consist of the following parts:
- 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.
Common Applications for 4-Stroke Engines
Four-stroke engines are the most common combustion engines. They are used in a wide range of different applications across multiple industries, with the most common uses including:
- Watercraft
- Motorcycles
- Automobiles and trucks
- Riding and push lawnmowers
- Off-road vehicles and dirt bikes
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, the 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.
Differentiate Between 2 Stroke & 4 Stroke Engines
| Two Strokes Engine | Four Strokes Engine |
| It has one revolution of the crankshaft during one power stroke. | It has two revolutions of the crankshaft during one power stroke. |
| It generates high torque. | It generates less torque. |
| It uses a port for the fuel’s outlet and inlet. | It uses valves for the fuel’s outlet and inlet. |
| Its engines result in lesser thermal efficiency. | Its engines result in higher thermal efficiency. |
| It has a larger ratio in terms of power to weight. | It has a lesser ratio in terms of power to weight. |
| It generates more smoke and shows less efficiency. | It generates less smoke and shows more efficiency. |
| Requires more lubricating oil as some oil burns with the fuel. | Requires less lubricating oil. |
| Due to poor lubrication, more wear and tear occurs. | Less wear and tear occurs. |
| Engines are cheaper and are simple to manufacture. | Engines are expensive due to lubrication and valves and are tough to manufacture. |
| Engines are basically lighter and noisier. | Engines are basically heavier because their flywheel is heavy and less noisy. |
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.