How does a 4 stroke petrol engine works?

How does a 4 stroke petrol engine works?

An engine, or motor, is a machine designed to convert one form of energy into mechanical energy. Heat engines, including internal combustion engines and external combustion engines (such as steam engines) burn a fuel to create heat, which then creates a force. In simpler words an engine is a machine with moving parts that convert power into motion. It can be broadly classified:

According to the type of cycle

i.)                  Otto Cycle (Air Cycle)

ii.)                 Diesel Cycle

iii.)               Dual Cycle

According to the alignment of cylinders

i.)                  In Line type

ii.)                 V type

iii.)               Rotary type

According to the speed

i.)                  Low speed

ii.)                 Medium Speed

iii.)               High Speed

According to the type of ignition

i.)                  Spark ignition

ii.)                 Combustion ignition

According to the type of fuel used

i.)                  Petrol

ii.)                 Diesel

According to the number of strokes

i.)                  2 Stroke

ii.)                 4 Stroke

There are few more types.

In this post we will understand about how a petrol engine works. A petrol engine is based on the principle of Otto cycle (Air Cycle). It can be either 2 stroke or 4 stroke. In this articlewe will see how the 4 stroke petrol engine works.

1.)    Suction Stroke

2.)    Compression Stroke

3.)    Power Stroke

4.)    Exhaust Stroke

The main important parts of the engine are the piston, the crankshaft, the connecting rod and the cylinder block which includes the cylinder head. The cylinder head comprises of the rocker arm and spring mechanism which with the help of a cam controls the opening and closing of the inlet valve and exhaust valve. The cylinder block is surrounded by water jackets which helps for cooling of the cylinder block. The parts of the cylinder are shown below.

Suction Stroke

The first stroke is suction stroke. In this stroke, the piston moves from TDC (Top Dead Centre) to the BDC( Bottom Dead Centre), which are the two extreme most positions of the piston. When the piston moves from TDC to BDC the inlet valve opens through which fresh charge (atmospheric air + petrol) is sucked in.

Compression Stroke

The second stroke is the compression stroke in which the piston moves from BDC to TDC. When the piston moves from BDC to TDC the inlet valve closes and the charge is compressed thereby increasing its temperature.  

Power Stroke

The third stroke is the power stroke which is the most important stroke in which the piston moves from TDC to BDC. When the piston is at TDC the spark plug makes a spark inside the cylinder block thereby igniting the charge hence combustion takes place inside the cylinder block and displaces the piston from TDC to BDC. The main power of the engine is generated from this stroke. Some amount of power is stored the flywheel to execute the other three strokes and the rest of the power is used to drive the wheels of the car. The inlet and the exhaust valves remain closed.

Exhaust Stroke

The fourth stroke is the exhaust stroke in which the piston moves from BDC to TDC. The hot gases and the fumes which are generated from combustion of the charge are pushed out through the exhaust valve which opens when the piston moves from BDC to TDC.   

The Otto Cycle graph is shown as follows

Constant Volume Process: Heat is rejected and received at constant volume.