What is a stroke?
Stroke is defined as the distance traveled by the piston from the top dead center to the bottom dead center.
But that is not the reason why engines are subdivided into two stroke or four stroke engines. They are named as such, depending on the number of cycles it requires to produce a power output.
There are 4 main cycles in the working of an engine
Intake is the induction or suction of fresh air or air-fuel mixture into the combustion chamber. It is facilitated by the vacuum produced by the piston during the downward motion.
Compression is the compressing up of the air or air-fuel mixture to get a higher pressure for combustion and energy development.
Combustion takes place when the compressed mixture is ignited either due to self-ignition or because of an external source like a spark.
Exhaust stroke is necessary to free up space for the fresh charge to come in. and another cycle to start.
The main differentiating fact of the two-stroke engine is the number of cycles it takes to generate power.
In a two-stroke engine, power is produced in each cycle or revolution of the piston. It means for every two strokes, i.e. one downward movement and one upward movement of the piston, one combustion cycle takes place which produces the power output.
A two-stroke engine used the Mist Lubrication Method. In this method, 3-4% of engine oil was added into the fuel, which formed a mist in the carburetor. This mist landed on the crankcase, piston walls et al, and lubricate them.
The cycle is as follows:
- In the upward motion of the piston, fresh air-fuel mixture enters the engine near the crankcase. Since the transfer port (the passage along the side) is covered, the mixture gets collected below the piston.
In the same cycle, we see that a spark is occurring above the piston. This is the mixture that is compressed during the upward motion of the piston and the spark ignites the mixture.
Hence, two cycles take place at the same time, namely:
- In the downward motion, the fresh charge that was accumulated below the piston gets pushed to the combustion chamber. Simultaneously, the exhaust gasses are pushed out. This is known as scavenging.
And how is this piston pushed down? That is where the combustion comes in. The spark that was ignited produces a sharp boom which gives out enough enthalpy to push the piston down. This is when the power is produced.
Hence the remaining two cycles occur:
2 Stroke engines have been widely used before and still used in small machinery. The stringent norms made them redundant as these engines produced a lot of emissions
They had various advantages:
- High power output
- Less complexity
- Less vibrations
However, their main disadvantages made them redundant:
- Smoke produced, as engine oil was added to the fuel for lubrication.
- High fuel consumption
- Short circuiting of charges
Two stroke engines are still used in Motocross competitions and are still superior in power to four stroke engine.
Just like in a two stroke engine all the cycles are completed in 2 strokes, four stroke completes it in, well, 4 strokes!
Those are namely:
The first step involves the induction of fresh air into the cylinder/s. As the piston moves downwards and the intake valve opens, a vacuum is generated.
The pressure difference generated causes the fresh air or air-fuel mixture to enter swiftly and fill up the void.
As the piston moves from the Top Dead Center (TDC) to the Bottom Dead Center (BDC), the whole chamber is filled and the inlet valve closes.
After the piston reaches the BDC, it turns around. All the valves are closed and this creates a closed chamber.
This starts the compression stroke. The piston starts compressing the air-fuel mixture according to the compression ratio until it reaches the TDC.
This produces a high amount of pressure, and an increase in temperature takes place.
As soon as the piston touches the TDC, the spark plug ignites the mixture. Thus, a flame front starts to travel through the cylinder and as it travels, it starts releasing a high amount of energy.
The piston that starts moving downwards with a high amount of energy.
This is the power stroke. Out of all the 4 strokes, this is where all the energy is generated and sent to the wheels.
After the piston again reaches the BDC, all the air-fuel mixture has been utilized to generate energy. All we are left with are the Carbon dioxides, monoxides, NOx and other combusted gasses.
All these gasses now need to leave to make way for fresh charges.
Thus, the exhaust valve opens up and provides a path for the exhaust to leave.
Since the piston is moving up due to its inertia, all the gasses are pushed out with high velocity.
The whole cycle again repeats itself.
This 4 stroke cycle thus produces the entire energy and is used in almost all of the modern day engines in automobiles ranging from the scooties to the lorries and trucks.
4 stroke engines have several advantages:
- Better emissions
- Better economy
- Higher balance and lower vibrations
- More control over combustion
- Efficient scavenging
However, they do come with some inevitable disadvantages:
- Lesser power
- More cost
- More complexity
However, all these are overshadowed by their innumerable advantages.
Yeup. There is such a thing.
In this system, the whole process till the 4 stroke engine is the same. However, in the fifth stroke, a non-detonating fluid is injected into the cylinders.
The leftover heat from the previous combustion pushes the piston down to produce power followed by another exhaust cycle!
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