How a Four stroke engine works

Internal combustion engines are a very important prime movers in automotives and also in power plant and standalone applications. These are heat engines that convert the fossil fuel energy into mechanical energy by some thermodynamic cycles. The most important thermodynamic cycles that theses engines are working on are Otto cycle and Diesel cycle. Otto cycle is used in petrol or gasoline engines and diesel cycle is used with diesel fuel and biodiesel fuel. The basic construction of an engine consists of a piston and a cylinder assembly. The whole assembly is an inversion of the four bar linkage and is known as slider crank mechanism as shown in the image below. 

Stroke is defined as the motion of the piston from the top most point of its motion(TDC) to the bottom most point of its motion(BDC).Both the cycles referred before are four stroke cycles. That means in order to complete one thermodynamic cycle the piston has to complete four strokes inside the cylinder. 

The Otto cycle 

In Otto cycle, There are four strokes to be required by the piston to complete on cycle. The P-V diagram of otto cycle is shown in the image below. 

Also the different position of the piston for the four strokes are also given in the image below. 

In the above figure we can see the four strokes clearly. The P-V plot also shows the four strokes of the piston. Also this engine is having a spark plug Which implies it uses gasoline or petrol as the fuel. So it works on the Otto cycle which uses spark to ignite the fuel. Now  let's look at the different strokes in the Otto cycle. 

Intake stroke(0-1): In this Stroke of the piston the Intake  valve is open and the piston moves from TDC to BTC. That will create a suction in the cylinder which will draw air from the atmosphere. This air will draw fuel from the carburetor through a nozzle and the fuel is mixed with the air. This mixture of air and fuel is known as charge. This intake charge is then drawn into the engine. 

Compression stroke(1-2):In this stroke the piston moves brom BDC to TDC and the intake and exhaust valves are closed. This will increase the pressure and temperature inside the cylinder. At the end of this stroke the spark plug ignites and start the combustion. 

Power stroke(3-4):The process from 3-4 is known as the power stroke or the expansion stroke. During this stroke the piston moves from TDC to BDC again. The high pressure generated from the combustion will push the piston downward and produce an output torque. 

Exhaust stroke(4-1): In this stroke the burnt charges will be expelled out of the cylinder. The exhaust valve is remaining open in this stroke. The piston moves from TDC to BDC and the piston pushes on the burnt gas out of the cylinder. 

The processes 2-3 and 4-1 are heat addition and heat rejection processes respectively. Heat addition is combustion and heat rejection is blowdown process. 

The valves are operated by using a camshaft which rotates at half the speed of crankshaft. The camshaft has cam lobes which push open the valves at the right moment. 

The Diesel Cycle 

The diesel cycle is very similar to the Otto cycle. The only difference is in how the combustion process is occurring inside the cylinder.Here the combustion is started by injecting fine fuel spray into the hot compressed air inside the cylinder.Instead of a spark plug here we will be having an injector at the top of the cylinder head. 

In the four stroke diesel cycle all the four strokes as mentioned for the Otto cycle are present. Only difference is in the fuel induction and fuel ignition.