Spark Ignition (S.I) engine

Let's study the movement of piston and how it defines the four strokes of an S.I engine. As you know that there are two valves provided at the top of the cylinder head, viz. the inlet valve and the exhaust valve. Both the valves are operated by the camshaft. The camshaft either opens the valves directly or via a rocker arm. There are four strokes in a S.I engine. They are:-

1. Inlet stroke


2. compression stroke


3. power stroke


4. Exhaust stroke

INLET STROKE:

                                           

During the inlet stroke, the piston moves from T.D.C to B.D.C and the inlet valves are opened. Air-fuel mixture with the stoichiometric air-fuel ratio of 14.7:1 is passed into the combustion chamber because of the low pressure created inside it . The air and fuel was mixed traditionally with the help of carburetors, but fuel injection pumps are replacing the carburetors. The inlet valve remains open for a few angles after B.D.C depending on the design of an engine. The intake valve then closes and the air-fuel mixture is trapped inside the combustion chamber. During this entire process crank rotates 180 degree.

COMPRESSION STROKE:

                                                 

During the compression stroke, both inlet and the exhaust valves remain closed. Now the piston starts moving up from B.D.C to T.D.C making the crankshaft to rotate another 180 degree. The air-fuel mixture inside the cylinder is compressed until the piston reaches T.D.C. The entire volume to which the mixture is compressed is the clearance volume of the cylinder.

Clearance volume = Total volume of the cylinder - Volume swept by piston in the cylinder (swept volume)

A high amount of pressure and heat is generated while compressing the mixture. This atomizes the fuel droplets. The higher the compression ratio, the higher the fuel atomization. Finer droplets results in increased surface area of the fuel droplets and thus helps the fuel burn completely when the spark ignites. Compression ratio of an S.I engine varies from 6:1 to 10:1. The air-fuel mixture is compressed in the range of 8 bar to 14 bar depending on the throttle position and the load.

Compression ratio = volume of the combustion chamber when piston is at B.D.C/ volume of the combustion chamber when piston is at T.D.C

IGNITION:

                         
Just before the piston reaches the T.D.C, the spark plug fires and ignites the compressed air-fuel mixture inside the combustion chamber. The spark plug fires at 15 degree before T.D.C and it varies for different engines.

POWER STROKE:

                                           
Both the inlet and the exhaust valves remain closed. The spark given by the spark plug ignites the mixture and generates a lot of heat and pressure up to 60 bar. It also generates a lot of force that pushes the piston from T.D.C to B.D.C and this rotates the crankshaft another 180 degree. The power generated during the power stroke is stored in the flywheel and given to the transmission. Combustion is the result of a chemical reaction that produces several exhaust gases such as carbon monoxide (CO), Hydrocarbons (HC), particulate matters and several others. All these gases expand within the combustion chamber and thus reduce the pressure inside it.

EXHAUST STROKE:

                                             

The exhaust valve opens and now the piston moves from B.D.D to T.D.C, completing the last stroke of the combustion process. The exhaust gases are pushed outside through the exhaust valve due to piston movement. The pressure drops to 2 to 4 bar inside the cylinder.