If you drive a gasoline-powered vehicle, its engine is either a 2-stroke engine or a 4-stroke engine. These two engine types deliver different performance levels. Knowing how each type operates will give you a better idea of how your vehicle’s engine runs, helping with its maintenance and care.
The names of these two engine types give a clue about the most distinct feature that sets 2-stroke and 4-stroke engines apart: the number of power cycles their pistons make during a combustion revolution.
The 2-stroke engine needs 2 strokes, which is one revolution of the crankshaft to complete a combustion process, while the 4-stroke engine needs 2 revolutions of the crankshaft to complete the process.
Power stroke refers to the single transition of the piston and its connecting rod between the top dead center (TDC) and the bottom dead center (BDC) positions inside the engine cylinder. The TDC is the closest position that the piston can get near the valve that allows gas to enter or exit the cylinder. Conversely, the BDC is the furthest it can move away from the valve.
When the piston moves from the TDC to the BDC or vice versa, it completes one stroke. The direction it takes doesn’t matter as long as it completes one trip from one position to another.
As for the combustion revolution, it’s the process where air and gasoline enter the cylinder for burning and the resulting exhaust gases are expelled from the engine. The process generates motion that the crankshaft turns into rotational movement, which the transmission brings to the wheels.
Going back to the engines, a car 2-stroke engine performs two power strokes per combustion revolution. In comparison, a car 4-stroke engine does four strokes. Lots of things happen during these strokes.
In the 2-stroke engine, the first power stroke in the revolution is the upstroke. It’s also called the ignition/compression stroke. The piston starts at its BDC and moves up to its TDC.
During the intake stroke, the cylinder compresses and ignites the air-fuel mixture inside it. The fuel injector sprays fuel into the cylinder. At the same time, air from the air intake system flows into the cylinder. The air and fuel mix together, undergo compression, and are ignited by the spark plug and then compressed by the piston.
The second stroke of the 2-stroke engine is the downstroke, also called the power/exhaust stroke. Once the air-fuel mixture combusts, the piston travels from the TDC down to the BDC. As it descends, the piston pushes the exhaust gases out of the cylinder. The gases go to the exhaust manifold and then to the exhaust system for cleaning and eventual expulsion.
This type of engine offers several advantages:
Not everything goes smoothly for this engine type. Here are some of the cons of a 2-stroke engine:
The 4-stroke engine performs four strokes during a single combustion cycle. It spreads the steps covered by its 2-stroke counterpart over the course of its quartet of strokes.
The piston moves downward in the first stroke ,called the intake stroke. The intake valve opens, and the fuel enters the cylinder to mix with air from the air intake.
The second stroke is the compression stroke. Here, the piston goes upward. The air-fuel mixture gets compressed.
The third stroke is also called the power stroke. The spark plug ignites the pressurized air-fuel mixture. The ensuing combustion produces the power required to propel the vehicle.
Last but not least is the exhaust stroke. As its name implies, the exhaust valve draws open during this step. The exhaust gases produced by the combustion process can leave the engine and go to the exhaust system.
There are some distinct benefits to this engine type:
Some of the cons of a 4-stroke engine are:
The advantages and disadvantages of each engine type makes them suitable for different roles. 2-stroke engines are lightweight, small, and simple. They’re ideal for jobs that require small engines, such as dirt bikes, the outboard motors of boats, chainsaws, and smaller gasoline-powered machines.
2-stroke engines are ideal for jobs that require small engines, such as dirt bikes, the outboard motors of boats, chainsaws, and smaller gasoline-powered machines.
– Anthony Harlin, ASE Certified Master Automobile Technician
4-stroke engines produce more torque and fewer emissions. Most internal combustion engines in gasoline vehicles are 4-stroke engines.
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