Otto cycle diagram

Four Stroke Engines

The engines with are generally used in our car or bike is a four stroke engines. Earlier two stroke engines were also used in scooters and motorcycles but now they are almost obsolete.

For successful operation, an Engine has to complete a sequence of operations in a cycle. In case of four stroke engines, the operations which form a cycle are as follows

  1. Intake
  2. Compression
  3. Combustion
  4. Expansion
  5. Exhaust

Four stroke engine working animation

Four Stroke Engine Working Animation
Four Stroke Engine Working Animation

This animation shows all the five operations mentioned above in the four strokes.

In this animation blue color represents fresh charge and brown color represents exhaust gases.

four stroke engines require two rotations of crank shaft to complete one cycle of operation, unlike two stroke engine which requires only one.

Both four stroke spark ignition (SI) and compression ignition (CI) engines has many things in common but they are fundamentally different, for e.g. the type of fuel used and the method used to ignite the fuel.

Generally Gasoline is used as a fuel in SI engines and Diesel is used as a fuel in CI engines.

The credit of invention of SI engines goes to Nicolaus A. Otto whereas CI engines were invented by Rudolf Diesel.

Here we will discuss both four stroke SI & CI engines in detail.

Four stroke Spark Ignition (SI) Engine

In four stroke SI engine the combustion process starts with the help of an externally produced spark (generated by spark plug), hence it is called a spark ignition engine.

SI engine works on Otto cycle, below is the diagram of Otto cycle

Otto cycle diagram
Otto Cycle

Explanation of Otto cycle

  1. Suction or Intake stroke

As shown in diagram the suction stroke starts when the piston is at top dead center and about to move downward the inlet valve is opened and the exhaust valve is closed simultaneously (refer animation above). Due to vacuum created by downward movement of piston fuel-air mixture (charge) is sucked inside the cylinder. When the piston reaches bottom dead center the inlet valve gets closed.

  1. Compression stroke

The charge inside the cylinder is compressed when piston starts to move from bottom dead center to top dead center (process 1-2). At this time both inlet and exhaust valves are in closed position. When the piston reaches top dead center, a spark is ignited to initiate the combustion process.

In ideal engine it is supposed that the combustion takes place instantly and it is a constant volume (process 2-3).

In the combustion process the chemical energy of the fuel gets converted into the thermal energy of fuel (approx. 2000-degree Celsius temperature is generated). It produces considerable amount of pressure.

  1. Expansion or power stroke

Because of the high amount of pressure generated due to combustion, piston again starts moving from top to bottom dead center (both inlet and exhaust valves are closed). This is called power stroke (from 3-4).

Both temperature and pressure decrease during the expansion stroke.

  1. Exhaust Stroke

After the completion of power stroke, the piston starts moving from bottom dead center to top dead center. At this time the exhaust valve is open (process 4-1) which results in the evacuation of all burnt gases from the cylinder and a sudden drop of pressure inside the cylinder.

Four stroke Compression Ignition (CI) Engine

In four stroke CI engine the combustion process starts because of the pressure developed inside the cylinder, hence it is called a compression ignition engine.

SI engine works on Diesel cycle, below is the diagram of Diesel cycle

diesel cycle diagram
Diesel Cycle

Explanation of Diesel cycle

  1. Suction Stroke

As shown in diagram the suction stroke starts when the piston is at top dead center and about to move downward the inlet valve is opened and the exhaust valve is closed simultaneously (refer animation above).

Due to vacuum created by downward movement of piston air is sucked inside the cylinder.

When the piston reaches bottom dead center the inlet valve gets closed.

  1. Compression stroke

The air inside the cylinder is compressed when piston starts to move from bottom dead center to top dead center (process 1-2). At this time both inlet and exhaust valves are in closed position which results in pressure buildup inside the cylinder.

When the piston reaches top dead center, fuel is injected which initiates the combustion process. In ideal engine it is supposed that the combustion takes place instantly and it is a constant pressure (process 2-3).

In the combustion process the chemical energy of the fuel gets converted into the thermal energy of fuel (approx. 2000-degree Celsius temperature is generated). It produces considerable amount of pressure.

  1. Expansion or power stroke

Because of the high amount of pressure generated due to combustion, piston again starts moving from top to bottom dead center (both inlet and exhaust valves are closed). This is called power stroke (from 3-4).

Both temperature and pressure decrease during the expansion stroke.

  1. Exhaust Stroke

After the completion of power stroke, the piston starts moving from bottom dead center to top dead center. At this time the exhaust valve is open (process 4-1) which results in the evacuation of all burnt gases from the cylinder and a sudden drop of pressure inside the cylinder.

I hope now you are able to understand four stroke engines. If you still have some doubts please do connect with me through the comments section.

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