Working Principles Of The 2 Stroke And 4 stoke Engines And Their Differences

Operation of 4 stroke Engines

 Almost every car sold today has a 4 stroke engine. So do a lot of motorbikes, lawnmowers, snowblowers and other mechanical equipment. But there are still a lot of 2 stroke engines about in smaller motorbikes, smaller lawnmowers, leaf-blowers, snow blowers and such.

The difference between the two stroke and four stroke engine types is the number of times the piston moves up and down in the cylinder for a single combustion cycle. A combustion cycle is the entire process of the suck, squeeze, bang and blow (sucking fuel and air into the piston, pressurizing it, igniting it and expelling the exhaust) 

Working principle of a 4 stroke engine

4 stroke engines are typically much larger capacity than 2 stroke ones, and have a lot more complexity to them. Rather than relying on the simple mechanical concept of reed valves, 4 stroke engines typically have valves at the top of the combustion chamber. The simplest type has one intake and one exhaust valve. More complex engines have two of one and one of the other, or two of each. So when you see "16v" on the badge on the back of a car, it means it's a 4-cylinder engine with 4 valves per cylinder - two intake and two exhaust - thus 16 valves, or "16v". The valves are opened and closed by a rotating camshaft at the top of the engine. The camshaft is driven by either gears directly from the crank, or more commonly by a timing belt.

The following animation shows a 4 stroke combustion cycle. As the piston (red) retreats on the first stroke, the intake valve (left green valve) is opened and the fuel-air mixture is sucked into the combustion chamber. The valve closes as the piston bottoms out. As the piston begins to advance, it compresses the fuel-air mix. As it reaches the top of its stroke, the spark plug ignites the fuel-air mix and it burns. The expanding gasses force the piston back down on its second stroke. At the bottom of this stroke, the exhaust valve (right green valve) opens, and as the piston advances for a second time, it forces the spent gasses out of the exhaust port. As the piston begins to retreat again, the cycle starts over, sucking a fresh charge of fuel-air mix into the combustion chamber.

Because of the nature of 4 stroke engines, you won't often find a single-cylinder 4 stroke engine. They do exist in some off-road motorbikes but they have such a thump-thump-thump motion to them that they require some large balancing shafts or counterweights on the crank to try to make the ride smoother. They also take a little longer to start from cold because you need to crank the single piston at least twice before a combustion cycle can start. Any more than one piston and the engine gets a lot smoother, starts better, and is nowhere near as thumpy. That's one of the advantages of V-6 and V-8 engines. Apart from the increased capacity, more cylinders typically means a smoother engine because it will be more in balance.

Operation of 2 stroke Engines

Working principle of a 2 stroke engine

The 2 stroke engine is different from a 4 stroke engine in two basic ways. First, the combustion cycle is completed within a single piston stroke as oppose to two piston strokes, and second, the lubricating oil for the engine is mixed in with the petrol or fuel. In some cases, such as lawnmowers, you are expected to pre-mix the oil and petrol yourself in a container, then pour it into the fuel tank. In other cases, such as small motorbikes, the bike has a secondary oil tank that you fill with 2 stroke oil and then the engine has a small pump which mixes the oil and petrol together for you.

The simplicity of a 2 stroke engine lies in the reed valve and the design of the piston itself. The picture on the right shows a 4 stroke piston (left) and a 2 stroke piston (right). The 2 stroke piston is generally taller than the 4 stroke version, and it has two slots cut into one side of it. These slots, combined with the reed valve, are what make a 2 stroke engine work the way it does. The following animation shows a 2 stroke combustion cycle. As the piston (red) reaches the top of its stroke, the spark plug ignites the fuel-air-oil mixture. The piston begins to retreat. As it does, the slots cut into the piston on the right begin to align with the bypass port in the cylinder wall (the green oblong on the right). The receding piston pressurises the crank case which forces the reed or flapper valve (purple in this animation) to close, and at the same time forces the fuel-air-oil mixture already in the crankcase out through the piston slots and into the bypass port. This effectively routes the mixture up the side of the cylinder and squirts it into the combustion chamber above the piston, forcing the exhaust gas to expel through the green exhaust port on the left. Once the piston begins to advance again, it generates a vacuum in the crank case. The reed or flapper valve is sucked open and a fresh charge of fuel-air-oil mix is sucked into the crank case. When the piston reaches the top of its travel, the spark plug ignites the mixture and the cycle begins again.

For the same cylinder capacity, 2 stroke engines are typically more powerful than 4 stroke versions. The downside is the pollutants in the exhaust; because oil is mixed with the petrol, every 2 stroke engine expels burned oil with the exhaust. 2 stroke oils are typically designed to burn cleaner than their 4 stroke counterparts, but nevertheless, the 2 stroke engine can be a smoky beast. If, like me, you grew up somewhere in Europe where scooters were all the rage for teenagers, then the mere smell of 2 stroke exhaust can bring back fond memories. The other disadvantage of 2 stroke engines is that they are noisy compared to 4 stroke engines. Typically the noise is described as "buzzy". 

Picture credit: mechanicalbooster.com

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