Working Mechanism of a Four-Stroke Engine

Internal combustion engines are very important to prime movers in automobiles, power plants, and standalone applications. These are heat engines that convert fossil fuel energy into mechanical energy by thermodynamic cycles. The most important thermodynamic cycles that these engines are working on are the Otto and Diesel cycles. Otto cycle is used in petrol or gasoline engines, and the 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 the slider crank mechanism as shown in the image below.

Stroke is defined as the motion of the piston from the topmost point of its motion(TDC) to the bottommost point of its motion(BDC). Both the cycles referred to before are four-stroke cycles. That means to complete one thermodynamic cycle, the piston has to complete four strokes inside the cylinder.

The Otto Cycle

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

Also, the different positions of the piston for the four strokes are 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 has a spark plug which implies it uses gasoline or petrol as fuel. So it works on the Otto cycle, which uses a 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 carburettor through a nozzle, and the fuel is mixed with the air. This mixture of air and fuel is known as a charge. This intake charge is then drawn into the engine.

Compression stroke(1-2): In this stroke, the piston moves from 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 starts 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 from the cylinder. The exhaust valve remains open in this stroke. The piston moves from TDC to BDC, and the piston pushes on the burnt gas out of the cylinder.

Processes 2-3 and 4-1 are heat addition and rejection, respectively. Heat addition is the combustion, and heat rejection is the blowdown process.

The valves are operated by using a camshaft which rotates at half the speed of the crankshaft. The camshaft has cam lobes that push 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 occurs 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, we will have an injector at the top of the cylinder head.

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