Two Stroke Vs. Four Stroke Engine

Of the many inventions that drove the industrial revolution, one that deserves a place right up there with the best, is the internal combustion engine, which drives every vehicle around the world. Most people are familiar with the four stroke engine design which is the default type of engine installed in today's cars, but very few know about the two stroke engine design. Both designs are based on the same principle of internal combustion, but differ in design and efficiency.

Both designs consist of a closed engine cylinder with a piston, fuel injection and ignition systems fitted in. A fuel and air mixture is pumped into the compartment which is compressed with piston stroke and then ignited by a spark injection to produce the motive force that drives piston movement.

The oscillatory motion created by fuel combustion under compression is converted into a torque motion that ultimately drives the car forward. With that background, let us now see how 2 stroke and 4 stroke engines perform this operation differently.

Two Stroke Vs. Four Stroke Engine Comparison

Just like a beating heart, a car or motorbike engine must function incessantly to maintain motion. Though the end result of the functioning of a 2 stroke or 4 stroke engine is the same, they differ in the way every engine cycle is executed. Let us have a look at the difference in designing and functioning of two stroke and four stroke engines.

Design & Functioning
Let me first briefly describe four stroke engine operation. Picture a piston moving up and down in a cylinder. A four stroke engine consists of many engine cylinders with pistons fitted into each, along with intake and exhaust valves. At the beginning of the engine cycle, the piston travels down, opening the intake valve and allowing a fuel-air mixture to enter the cylinder (this is the intake stroke). Then the piston moves up to compress the mixture, which leads to an increase in density and pressure (this is called the compression stroke).

At this point the spark plug comes into action, igniting the highly dense mixture, with an explosive force, which sends the piston moving down again (this is known as a combustion stroke). This motion of the piston drives the crankshaft generating the torque required for forward motion. Then the piston moves up again in the cylinder to drive the left over exhaust gases out (this is known as the exhaust stroke). These four strokes complete one engine cycle, which goes on repeating itself, driving the car forward. In diesel engines, spark plug is not required as diesel ignites spontaneously during compression but it is an inseparable part of petrol engines.

Now let us have a look at the 2 stroke engine design and functioning. The 2 stroke engine is so designed and named, as every one of its engine cycles involves only two strokes. There are no intake and exhaust valves in its design. The intake and exhaust strokes happen during the compression and combustion strokes.

This is made possible by facility of two ports in the engine cylinder wall, which take in air-fuel mixture and send the exhaust gases out. During the combustion stroke, the piston travels downwards opening up the outlet port, through which the exhaust gases are ejected.

Consequently, during the compression stroke, the exhaust port is closed and inlet port exposed, through which air-fuel mixture is sucked in. The prime difference in these two designs is that while a 4 stroke engine is fired during every alternate revolution, the 2 stroke engine fires during every piston revolution; packing in more power in the process.

Efficiency
Though the simpler design and greater power provide the two stroke engine with more power and makes them less expensive, they score poorly when it comes to mileage and polluting emission levels. Two stroke engine wears out fast, due to lack of a proper lubrication system, which exists in 4 stroke engines. For two stroke engine tuning, engine oil needs to be added, along with fuel. It consumes fuel faster and pollutes a lot more due to its simpler design, besides being quite noisy.

So the added power of the two stroke engine comes at the price of efficiency and pollution. Two stroke engine troubleshooting is simpler compared to four stroke engine repair which can be complicated. Four stroke engines use up fuel much more efficiently and pollute less due to the valve mechanism. If you need mileage and efficiency, 4 stroke engines are the default choice.

As our discussion above revealed, four stroke engine design is more efficient and less polluting of the two alternatives, which is exactly the reason why it is the most preferred design by car manufacturers.