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Difference Between 2Stroke and 4Stroke

Found this article a while back, it's good to know what exactly the difference is between the two for working on them!

Difference between 2 Stroke and 4 Stroke Engines
"Engines" are the power producing units of the vehicle. From the cheapest to the costliest vehicle, every vehicle has one thing in common, that is an "Engine". There are many types of engines to meet the requirements of say airplanes, rockets, cars, ships, trains etc. The kind used in automobiles is an "Internal Combustion" Engine. The term means that the combustion of the fuel takes place inside the engine in cylindrical compartments called "Combustion Chambers". It is inside these combustion chambers that the conversion of energy from the chemical form to mechanical form takes place. A small amount of fuel and air mixture is compressed by pistons and exploded in certain time periods using spark plugs or by self-ignition (for diesel engines) causing the mixture to expand pushing down the pistons again. This causes an up and down movement of the pistons. The vertical motion of the pistons is converted into rotational motion by "crankshafts" which are connected to the pistons using "connecting rods". The vertical motion of the piston in one direction (either up or down) is called a "stroke".
The difference between 2stroke and 4stroke engine arises from the number of strokes piston makes while completing one cycle. This one cycle consists of the compression and combustion of the air-fuel mixture. Basically there are four parts to one cycle:

  • Intake of the fuel-air (and oil) mixture
  • Compression & Ignition
  • Expansion due to explosion
  • Exhaust release

In a 4-stroke engine, each quarter of the cycle causes one strokeof the piston. The cycle of the 4-stroke engine is called the "Otto Cycle" named after the inventor of the internal combustion cycle "Nikolaus Otto".In the 2-stroke engine, the cycle gets completed in two strokes of the piston (one upward compression stroke and one downward exhaust and intake stroke). Almost all cars we see today use the 4-Stroke engines including most motor bikes, chainsaws, lawnmowers, leaf blowers etc. 2-Stroke engines are used in lower frequencies in smaller lawnmowers, mopeds etc. 2-Stroke engines were never really developed to its full potential but were rather used as a means to cut costs and lower the complexity of construction. We shall discuss in detail how each of the engines carry out the combustion cycle.

Note: The position where the spark plug is made to ignite so as to produce maximum mechanical efficiency (or the top most position that the piston reaches in the chamber) is called the "Top Dead Centre (TDC)" and the bottom most point is called "Bottom Dead Centre (BDC)"

2 Stroke Engines
The main differences in 2 stroke engine from their counterparts are the simplicity in construction as the two stroke engine does not need valves for fuel and air intake and exhaust outlet unlike the four stroke engine and the firing frequency (ignition of the fuel) of the two stroke is greater than the four stoke (once per revolution of the crankshaft as compared to once per two revolutions in a four stroke). This enables the two stroke engine to produce more power. The lack of valve systems in a two stroke enables it to be lighter and operate in any orientation as compared to the 4 stroke which can operate efficiently in the upright position (countering this problem would require design complexities). Thus the 2-Stroke engine has a higher power to weight ratio. Let us look at the cycle from the ignition. At this point, the fuel and air mixture is compressed and ignited using the spark plug (or self-ignition) which causes the gas to expand. When this happens, the burnt fuel (exhaust) is sent into the exhaust outlet which is uncovered just as the piston moves down below it. Next the piston uncovers the air, fuel and oil intake which is covered by a "reed valve" which opens due to the low pressure in the combustion chamber and shuts when there is enough pressure (an ingenious mechanism using simple mechanical principles). The gas intake is then compressed again using the momentum created by the combustion stroke (expansion) and ignited repeating the entire cycle once again. Since the piston acts as both to pressurize the crankcase and as a valve to the outlet and fuel intake, a lot of friction is generated by it with the walls of the combustion chamber. For this reason a significant amount of oil is mixed with the fuel which is poured in which acts as a lubricant. If no oil is mixed, then the engine's life will not last very long. The oil requirement introduces an increased pollution factor by the two stroke engine. Also, as the spark plug has to fire many times, it wears out quite quickly. The faster fuel intake leads to decreased fuel efficiency in the two stroke engine.

4 Stroke Engines
The 4-Stroke engine is much more complex and more "civilized" as most automobile engineers would like to put it. It has been given more attention and development unlike its brother, the two stroke engine. It consists of a complex valve system which is operated by time shafts or cam shafts that open and close the valves at the required time. The 4-stroke engines have many different cylinder layouts such as Inline 4 (used as a default in most of the mid-range cars), V5, V6, V8 (used is higher end car engines). All the pistons follow a different firing order to produce maximum possible efficiency and the sparks are provided as per this firing order by a distributor. A car that is "tuned" well will fire that spark exactly as the piston reaches the "Top Dead Centre" position during the ignition stroke. Let us study the cycle of the four-stroke, one stroke at a time. Firstly, the inlet valve opens pushing in a mixture of air and fuel from (created from the carburettor) this pushes the piston down. In the second stroke the fuel-air mixture is compressed and ignited when it reaches the Top Dead centre (TDC) position. Generally compression ratio of engines is 9:1 (i.e., 9 litres of the gas is compressed to 1 litre, although it varies according to the manufacturer). In the third stroke the combusted gas expands pushing the piston down. In the 4th stroke, the exhaust valve is opened by the cam shaft and therefore the pressure difference created pushes out the exhaust gases out which moves the piston up. Again a new cycle starts from here. The valves are controlled by cam shafts which are either controlled by the crank or by other mechanisms. A 4-stroke Engine has more advantages when a large scale use is required. It has a longer life period than the 2-Stroke. There is a lot of room for designing in the 4-stroke engines.It has a better fuel efficiency than the 2-stroke. The pollution is also controlled; it is lowered further as the exhaust gases are sent into catalytic converters to convert the environment-hazardous gases to clean gases.

The original article can be viewed here. Hope this helps someone!  :D