What is a 4-stroke engine?

Types, Advantages, Disadvantages, Need

What is a 4-stroke engine?

A 4-stroke engine is an Internal combustion engine, where four successive strokes (i.e. Suction-Compression-Power-Exhaust) completes in two revolutions of the crankshaft. In recent days the majority of automobile runs on a four-stroke cycle.

Let us first understand some basic terms associated with a 4-stroke engine.

Top dead center (TDC): It is the extreme position of the piston at the head end of the cylinder. At TDC the crank angle is 0°
Bottom dead center (BDC): The extreme position of the piston at the crank end side. At BDC, the crank angle is a 180°
Cylinder bore: It is the internal diameter of the cylinder.
Stroke: It is the distance through which the piston moves in the cylinder during 1 stroke.

Parts of 4-Stroke engine

A Four-stroke engine 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 used to allow 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 controls 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 charge or fresh 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 spray the fuel inside the engine cylinder.
Carburetor: It is used in a Petrol engine to mix the air and fuel properly.
Flywheel: It is made of cast iron and mounted on a crankshaft. It stores energy in the form of inertia.

Classification of 4-stroke engine

The four-stroke engine is generally classified into 2 types:

Four-stroke petrol engine
Four-stroke diesel engine

Let us understand each one in detail.

Four-stroke Petrol engine

This type of engine is based on the Otto cycle. Let us have a look at individual parts of a 4-stroke petrol engine.

Construction of Four-stroke Petrol engine

A four-stroke petrol engine consists of the following:

A cylinder
Cylinder head attached with spark plug
Piston attached with piston ring
Connecting rod
Crank
Crankshaft
Valves

In four-stroke engines, Valves are used instead of Ports. Following are the two valves used:

Suction valve
Exhaust valve

And these valves are operated by means of Camshaft. It is rotated at half the speed of a crankshaft.

Working principle of the Four-stroke petrol engine

The travel of the piston from one dead center to another is called piston stroke and a four-stroke cycle consists of four strokes which are as follows :

Suction Stroke
Compression Stroke
Power or Expansion Stroke
Exhaust Stroke

Suction stroke
In this stroke the suction valve opens while the exhaust valves remain in close condition. The piston moves from the top dead center to the bottom dead center and the charge (Air+Fuel) enters the engine cylinder due to the partial vacuum developed inside the cylinder. When the piston reaches the BDC, the inlet valve closes and the flywheel makes a half revolution.

Compression Stroke
In this stroke, Piston moves from BDC to TDC. The air inside the cylinder is compressed and heated. In this stroke both the valves are in closed condition. The mixture, which was sucked into the cylinder during the suction stroke, is compressed into the combustion chamber. The heat produced during the compression stroke vaporizes the mixture. As the piston approaches the top dead center, the compression stroke completes, and then the spark plug gives the spark.

Power Stroke or Expansion stroke
Again both the inlet and exhaust valves are closed in this stroke. The compressed charge is ignited by the high intensity of the spark and expands adiabatically. Therefore, it pushes the piston downward i.e. towards BDC to complete the stroke.

Exhaust Stroke
During the exhaust stroke, the inlet valve is in a closed condition and the exhaust valve is opened. The piston rises up from BDC to TDC. The stroke is completed when the piston reaches the Top Dead Center. In the exhaust stroke in one situation both the valves are in an open condition for some seconds so that fresh charge enters the engine cylinder and pushes the burnt gases out of the cylinder. This process is called Scavenging.

Valve-timing diagram for four-stroke Petrol engine

Theoretical valve-timing:

The Inlet valve opens when the piston is at TDC
The Inlet valve closes when the piston reaches BDC
Spark produces when the piston reaches TDC
The exhaust valve opens when the piston is at BDC
Exhaust valve close when the piston is at TDC

Actual valve-timing:

The Inlet valve opens about 10°-20° before TDC
Inlet valve close 30°-40° after BDC to take advantage of rapidly moving gas
The spark occurs 20°-35° before TDC
The exhaust valve opens about 30°-50° before BDC
Exhaust valve close about 10°-15° after TDC

Applications of Four-stroke Petrol engine:

Automobiles
Motorcycles
Cars
Buses
Trucks
Airplanes
Small pumping sets
Mobile electric generator

Four-stroke Diesel engine:

Construction:

A fuel injector
Fuel pump
Cylinder
Cylinder head
Inlet and exhaust valves
Piston
Piston ring
Connecting rod
Camshaft
Crankshaft
Cam

Working principle of Four-stroke Diesel engine:

The cycles of the Four-stroke diesel engine are the same as that of Petrol engine.

Suction Stroke
Compression Stroke
Power or Expansion Stroke
Exhaust Stroke

Suction Stroke:
In a suction stroke, the inlet valve is in the opened condition and the exhaust valve remains closed. The atmospheric air is drawn into the cylinder as the piston moves from TDC to the BDC due to the partial vacuum air sucked inside the cylinder.

Compression Stroke:
In this stroke, Piston moves from BDC to TDC. As a result, the air inside the cylinder is compressed and heated. In this stroke both the valves are in closed condition. The temperature of air is around 800°C. At the end of the compression stroke, the fuel injector injects the fuel at high pressure to the compressed hot air, thus the ignition takes place inside the cylinder.

Power or Expansion Stroke:
During this stroke, both the valves are in closed condition. The piston at the TDC is pushed downward by the expansion of burned gas. Therefore a Power stroke is produced.

Exhaust stroke:
In this stroke, the exhaust valve is opened and the inlet valve is in the closed condition. The piston travels from BDC to TDC and pushes the burned gas out of the cylinder. And here also the scavenging process is used to remove the burnt gas completely.

Valve-timing diagram for four-stroke Diesel engine:

Actual valve-timing diagram:

Inlet valve open at 10°-30° before TDC
The Inlet valve closes at 20°-50° after BDC
The exhaust valve opens approximately 40° before BDC
Exhaust valve close 15° after TDC
Fuel injection starts 5°-15° before TDC
Fuel injection stops 15°-25° after TDC

Applications of Four-stroke Diesel engine:

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

Difference between 2-Stroke engine & 4-Stroke engine

Four-stroke engine	Two-stroke engine
Power is developed every two revolutions of the crankshaft.	Power is developed every one revolution of crankshaft.
Consists only of valves.	Consists only of Ports.
Engine is water cooled.	Engine is air cooled.
Weight of the engine is more because of heavier flywheel construction.	Weight of the engine is less because of light flywheel construction.
Maintenance cost is high.	Maintenance cost is low.
The exhaust gases are fully burnt and leave as the exhaust so engine output is high.	Some fresh charge mixes with the exhaust gas and leave as the exhaust so engine output is less.

Difference between Petrol Engine (SI) and Diesel Engine (CI)

Otto-cycle/S.I Engine	Diesel-cycle/C.I Engine
Mixture of air fuel is admitted during suction stroke.	Only air is admitted during suction stroke.
Low compression ratio	High compression ratio.
charge is ignited by spark plug	Ignited by high temperature of compressed air.
Expansion takes place at constant volume process.	Explosion takes place at constant pressure process.
Quality method of governing is used.	Quantity method of governing is used.
Light in weight.	Heavier in weight.
Low thermal efficiency.	High thermal efficiency.

Advantages and Disadvantages of Four-Stroke engine

Advantages of 4-Stroke engine are as follows:

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 for longer engine life
Less noisy
Scavenging happens really well, therefore Thermal efficiency and HP of the engine is high

Disadvantages of 4-Stroke engine are as follows:

There are many moving parts and hence there is more friction. Due to this, 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, the maintenance is required quite often.

Fuel supply system in Four-stroke engine

The petrol engine fuel supply system consists of the following:

Fuel feed system
- Gravity system
- Vacuum system
- Pressure system
- Pump system
- Fuel injection system
Fuel tank
Fuel lines
Fuel filter
Fuel pump
Carburetor

At first, an electrical petrol pump or mechanical petrol pump sucks the fuel from the fuel tank via a fuel filter as shown in figure. Then the fuel through the fuel lines goes to the carburetor where air and fuel is mixed with proper ratio which then enters into the engine cylinder for combustion.

To know more, checkout the Fuel supply system in petrol engines

Parts of Diesel Engine Fuel Supply System

Diesel engines fuel supply system consists of the following:

A Diesel tank
A feed pump
A filter
An injection pump
An injector
Connecting lines

Types of Fuel Injection System

There are two types of the fuel injection system in a Diesel engine which are as follows:

Air injection system or Indirect injection
Solid injection system or Direct injection

Solid injection system is further classified into the following two types:

Common rail fuel injection system
Individual pump fuel injection system.

The lubrication system in Four-stroke engine

The supply of lubricating oil between two moving parts of the vehicle is called lubrication.

Why we need a lubrication system?

The primary objectives of lubrication are as follows:

To reduce friction between moving parts so that power loss is minimum
To reduce wearing out of moving parts
To provide a cooling effect
To provide the cleaning action

Components of 4-stroke engine lubrication system

Following are the components of a 4-stroke engine lubrication system:

Oil pump
Oil cooler
Oil strainer
Oil filter
Oil pressure gauge
Crankcase dilution

Parts to be lubricated in an Engine

The following parts in the engine requires lubrication:

Crankshaft bearings
Crankpins
The big end of connecting rod bearing
The small end of the connecting rod bearing
Bushes of gudgeon pin or Wrist pin
The inner wall of the cylinder
Valve operating mechanism
Timing gears
Camshaft bearing

The cooling system in the Four-Stroke engine

All the heat produced by the combustion of fuel is not entirely 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 cause pre-ignition of the charge and also lubricant is burned away due to the heat. Therefore the engine may seize. Keeping above factor in mind we need to install a proper cooling system. In a Four-stroke engine there are two types cooling method.

Cooling by air
Cooling by water

FAQ's

What is Scavenging?

In the exhaust stroke, at a particular instance both the valves are in open condition for some seconds so that fresh charge can enter the engine cylinder and push the burnt gases out of the cylinder. This process is called Scavenging.

What is the Four-Stroke Engine?

A four-stroke engine is an Internal combustion engine, where four successive strokes (i.e. Suction-Compression-Power-Exhaust) completes in two revolutions of the crankshaft. Therefore, the engine is called a Four-stroke engine.

What are the Parts of Four-stroke Engine?

A four-stroke petrol engine consists of the following:

A cylinder
Cylinder head attaches with spark plug
Piston attach with piston ring
Connecting rod
Crankshaft
Valves