Two Stroke Petrol Engine Working

Two Stroke Petrol Engine Working

The term ‘stroke’ refers to the maximum vertical movement of a piston within a cylinder, with one stroke being a full descent of the piston to the bottom dead center (BDC) and another being its movement all the way up to top dead center (TDC). This movement is prompted by the rotation of the crankshaft which the pistons are connected to via connecting rods.

Every engine has a cycle which is defined as the combination of suck, squeeze, bang and blow that results in the pistons completing the intake, compression, detonation and exhaust stroke. A four-stroke engine takes four full movements of the pistons to complete the engine’s cycle. Whereas in two stoke engines all processes are completed in 2 movements of the piston.

To perform the engine cycle in half the number of strokes to a conventional four-stroke engine, the orientation of the cylinder has to change. In a normal four-stroke cylinder, you have inlet and outlet valves which open and close with the rotation of the camshaft.

In a two-stroke setup, ports are machined into the cylinder itself and are opened or sealed by the vertical movement of the piston. There are only two – an inlet port and an exhaust port – along with a spark plug nestled in the cylinder head.


Piston – In an engine, piston is used to transfer the expanding force of gases to mechanical rotation of crankshaft via a connecting rod. The piston is able to do this because it is secured tightly within cylinder using piston rings to minimize the clearance between cylinder and piston !

Crankshaft – A crankshaft is a part which is able to convert the reciprocating motion to rotational motion.

Connecting rod – A connecting rod transfers motion from a piston to crankshaft.

Counterweight – Counterweight on crankshaft is used to reduce the vibrations due to imbalances in the rotating assembly.

Inlet & Outlet ports – It allows to enter fresh air with fuel & to exit the spent air-fuel mixture from the cylinder.

Spark Plug – A spark plug delivers electric current to the combustion chamber which ignites the air-fuel mixture leading to abrupt expansion of gas.

How Two Stroke Petrol Engine Works?

As the two-stroke’s piston rises on compression, its underside pulls a partial vacuum in the crankcase. An intake port of some kind (cylinder wall port, reed valve or rotary disc valve) opens, allowing air to rush into the crankcase through a carburetor.

As the piston nears Top Dead Center, a spark fires the compressed mixture. As in a four-stroke, the mixture burns and its chemical energy becomes heat energy, raising the pressure of the burned mixture to hundreds of psi. This pressure drives the piston down the bore, rotating the crankshaft.

As the piston continues down the bore, it begins to expose an exhaust port in the cylinder wall. As spent combustion gas rushes out through this port, the descending piston is simultaneously compressing the fuel-air mixture trapped beneath it in the crankcase.

As the piston descends more, it begins to expose two or more fresh-charge ports, which are connected to the crankcase by short ducts. As pressure in the cylinder is now low and pressure in the crankcase higher, fresh charge from the crankcase rushes into the cylinder through the fresh-charge (or “transfer”) ports. These ports and piston head are shaped and aimed to minimize direct loss of fresh charge to the exhaust port. Even in the best designs, there is some loss, but simplicity has its price! This process of filling the cylinder while also pushing leftover exhaust gas out the exhaust port is called “scavenging.”

While the piston is near Bottom Dead Center, mixture continues to move from the crankcase, up through the transfer ports, and into the cylinder.


Because fuel-air mixture is constantly being pumped by the crankcase, it is not practical to lubricate piston and crank by pumped circulating oil—it would be swept away by the mixture rushing in and out. Therefore, we must either mix a little oil with the fuel (2 to 4 percent) or inject it very sparingly into the bearings with a tiny metering pump. The fact that there is so little oil dictates that such simple two-stroke engines must employ rolling bearings, whose need for oil is very small.

Two-stroke diesel engines are scavenged with pure air, not a fuel-air mixture. Their fuel is injected only after all ports have closed, preventing any loss. Certain crankcase scavenged two-strokes do the same, and are called “DI,” or Direct Injection two-strokes. They can be made as fuel-efficient and low in exhaust emissions as four-strokes.

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