Understanding Four-Stroke Engines – Their Construction And Working

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Engines working with gasoline (petrol) or diesel are the prime movers of automotives all over the world. Most of 4-wheelers are driven by 4-stroke engines. To understand how how four-stroke engines work, one must know the two major classifications of Engines.

Two major classification of engines

Engines powered by petroleum based fuels can be classified into two major groups (a) Spark-Ignition Engines (i.e. Gasoline driven engines) and (b) Compression Ignition Engines (i.e. diesel driven engines).

All the engines used in cars and and other four-wheelers are essentially “4-stroke” engines. To understand these “strokes”, we must first get more idea about the construction of a 4-stroke engine.

Construction of an engine

Every engine contains one or more Cylinders (2,3,4,6,8 or 12 cylinders depending on the size and power of the engine). These cylinders are housed in a Cylinder Block (made of cast Iron or Aluminum). Inside each cylinder there is a Piston which can slide up and down (similar to the plunger inside an injection syringe).

The top of the Cylinder Block is covered tight with a Cylinder head. Above each Cylinder in the Cylinder block the Cylinder Head contains combustion chambers to burn the fuel and 2 Valves. One valve is to let in air (or air/ fuel mixture) known as Intake Valve and the other one to exhaust the burnt gases (known as Exhaust Valve). The Cylinder head will also contain a Cam Shaft to actuate opening and closing of valves in proper sequence. Depending on whether the engine is gasoline driven (spark ignition) or Diesel driven (Compression Ignition), you will have spark plugs or fuel Injectors too in the Cylinder Head against each cylinder.

The Cylinder Head is connected with Inlet Manifold (to bring in Air/ Air-fuel mixture) and Exhaust Manifold (to take away the burnt gases).

At the bottom of the Cylinder Block, is the Crank Case, which houses a Crank Shaft. Each piston inside each cylinder is connected to the crank shaft through a Connecting Rod (by a pin, known as Gudgeon pin that is fitted to each piston).

Basic Engine Function

When air mixed with fuel is compressed and ignited at the combustion chamber of the engine, the explosion produced in the restricted space pushes the piston forcefully downwards inside the cylinder; This linear movement of the piston is converted to rotary movement by the Connecting Rod and the Crank Shaft. This rotary movement of the crank shaft is then transmitted to the power-output shaft suitably through a clutch / coupling / speed changing gear drives, depending on the application.

This is the simplistic explanation of the engine’s working. But to understand the step by step activity behind this process, we must understand “4-stroke”.

Understanding 4-stroke Engine

One full rotation of the crank shaft is obtained by one upward plus one downward STROKE of the piston. In 2 rotations, we thus have 4 strokes. In a 4-stroke engine, there is one “power stroke” or expansion stroke, caused by the explosion (burning) of the fuel once in every 2 rotations (4 strokes). What are those 4 individual strokes? They have to be understood separately for Gasoline (Petrol) engines and Diesel Engines.

4-strokes in a spark ignition engine

In Car Engines using Gasoline (Petrol) as fuel, the following activities take place in each stroke:

(1) Intake stroke

The piston moves from its top most position to bottom. When it happens, the Intake Valve opens and lets in Air-Gasoline mixture into the cylinder. The CARBURETOR (in olden design cars)or FUEL INJECTORS (in latest cars) control the air fuel ratio to ensure that it is correctly proportioned for optimal performance. The fuel air mixture is now drawn down through the INTAKE MANIFOLD that connects to the cylinder head. At that point of time, the exhaust valve will remain closed.

(2) Compression stroke

Now the intake valve closes. The piston moves up and compresses the Air-gasoline mixture. When the maximum compression is obtained (i.e. when the piston reaches the top), the spark plug lets out an electrically produced spark and it ignites the air-fuel mixture.

(3) Power stroke

The burning of the compressed air-fuel mixture causes an expansion, building up tremendous pressure, which pushes the piston forcefully downwards. This is known as the power stroke or expansion stroke. It is precisely the stroke that gives the motive power for the vehicle.

(4) The Exhaust stroke

Now the Exhaust valve opens and the burnt gases get exhausted from the cylinder by the upward stroke of the piston. The gases leave through the Exhaust Manifold. At the end, the exhaust valve closes again.

4-STROKES IN COMPRESSION-IGNITION ENGINE

In Car Engines using Diesel as fuel, the following activities take place in each stroke:

(1) Intake stroke

The piston moves from its top most position to bottom. When it happens, the Intake Valve opens and lets in filtered air into the cylinder through the Intake Manifold that connects to the cylinder head. At that point of time, the exhaust valve will remain closed.

(2) Compression stroke

Now the intake valve closes. The piston moves up and compresses the air inside the cylinder. The compression ratio (original intake volume to compressed volume) is much higher in diesel engines when compared to gasoline engines. The temperature of the air shoots up by this compression (which is a thermo-dynamic phenomenon). When the maximum compression is obtained (i.e. when the piston reaches the top), the Fuel Injector injects highly atomized diesel at high pressure inside the combustion chamber. This fuel gets ignited on its own at the high temperature of the compressed air.

(3) Power stroke

The burning of the compressed air-fuel mixture causes an expansion, building up tremendous pressure, which pushes the piston forcefully downwards. This is the power stroke or expansion stroke. It is precisely the stroke that gives the motive power for the vehicle.

(4) The Exhaust stroke

Now the Exhaust valve opens and the burnt gases get exhausted from the cylinder by the upward stroke of the piston. The gases leave through the Exhaust Manifold. At the end, the exhaust valve closes again.

When all the above activities take place continuously, cycle after cycle at high speed, the crank shaft rotates continuously, powered by one power stroke per cylinder at every two revolutions.

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