RFWhat is the difference between 2 stroke and 4 stroke
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The machine that converts heat produced by burning fuel into useful work is known as a heat engine. In simpler terms, a heat engine is equipment that generates thermal energy and transforms it into mechanical energy. Heat engines can be classified in various ways, and one such classification is based on the working cycle: 2-stroke and 4-stroke.
2-stroke and 4-stroke engines are two distinct types of internal combustion engines widely used in applications like automobiles, motorcycles, small engines (lawnmowers, chainsaws), and some marine and industrial equipment. They differ significantly in their operation, design, and cycle of operation.
Two-Stroke Engine
In 2-stroke engine, the whole sequence of events i.e. suction, compression, power and exhaust are completed in two strokes of piston i.e. one revolution of crankshaft. A 2-stroke engine completes its cycle of operation in two strokes of the piston: one upward stroke and one downward stroke.
During the upward stroke, the engine compresses a mixture of air and fuel in the cylinder. Near the top of the stroke, a spark plug ignites the compressed mixture, causing an explosion that drives the piston down (the power stroke). As the piston reaches the bottom of the stroke, it uncovers exhaust ports, allowing burnt gases to be expelled from the cylinder. Simultaneously, fresh air/fuel mixture is drawn into the cylinder through intake ports. The cycle repeats, with each stroke being a power stroke or an exhaust/intake stroke.
Four Stroke Engine
In 4-stroke engine, all the four events (suction, compression, power and exhaust) take place inside the engine cylinder. The 4 events are completed in 4 strokes of the piston i.e. two revolutions of the crankshaft.
A 4-stroke engine completes its cycle of operation in four strokes of the piston: intake, compression, power, and exhaust. During the intake stroke, the piston moves downward, and the intake valve opens, allowing air to be drawn into the cylinder. In the compression stroke, both intake and exhaust valves are closed, and the piston moves upward, compressing the air. At the top of the compression stroke, a spark plug ignites the compressed air/fuel mixture, causing an explosion that drives the piston down (the power stroke). During the exhaust stroke, the exhaust valve opens, and the piston moves upward again, expelling the burnt gases from the cylinder. The cycle then repeats, with each stroke having a specific function.
Figure 1: Two-stroke engine (petrol type) and four-stroke engine (petrol/diesel type).
The figure-1 depicts two stroke engine (petrol type) and four stroke engine (petrol/diesel type). In 4 stroke engine all the four events viz. suction, compression, power and exhaust occur in four different strokes of piston. In 2 stroke engine, suction and compression occur in upward stroke of the piston while power and exhaust occur in downward stroke of the piston.
Difference Between 2-Stroke and 4-Stroke Engines
Following table mentions comparison between these engine types with respect to various features. The table provides a concise overview of the main differences, there may be variations and exceptions based on specific engine designs and applications.
| Parameters | 2-Stroke Engine | 4-Stroke Engine |
|---|---|---|
| Number of strokes | When the cycle is completed in one revolution of the crankshaft, it is called a 2-stroke cycle engine. | When the cycle is completed in two revolutions of the crankshaft, it is called a 4-stroke cycle engine. |
| Power stroke frequency | Every revolution | Every two revolutions |
| Fuel/Oil mixing | Typically required | Separate fuel and Oil |
| Lubrication system | Mixed with fuel/oil | Separate oil sump |
| Emissions | Typically higher | Typically lower |
| Noise | Often noisier | Quieter |
| Weight | Lighter, Weight of engine per hp is comparatively low. | Heavier, Weight of engine per hp is high. |
| Thermal efficiency | comparatively low | high |
| Removal of exhaust gases | comparatively difficult | easy |
| Power output | For a given weight, a 2-stroke engine gives twice the power compared to a 4-stroke engine. | For the same weight, a 4-stroke engine would give only half the power of its 2-stroke counterpart. |
| Engine speed | Mostly high-speed engines are of this type. | All speed types are possible. |
| Rotating direction | This engine can be operated in both directions, i.e., clockwise and anti-clockwise. | This engine can be operated in one direction only. |
Let’s summarize the key differences below.
- 2-stroke engines have a simpler design and produce power in every revolution of the crankshaft. This makes them more lightweight and gives them a higher power-to-weight ratio compared to 4-stroke engines.
- 4-stroke engines are generally more fuel-efficient and produce less pollution. They have a dedicated exhaust stroke and better control over the fuel mixture and combustion process.
- 2-stroke engines tend to be noisier and produce more smoke due to their less efficient combustion process.
- 4-stroke engines are commonly used in most automobiles and larger machines, while 2-stroke engines are often found in smaller applications and some specialty vehicles (e.g., dirt bikes).
Conclusion: Both types of engines have their advantages and disadvantages, making them suitable for different purposes based on factors such as power output, emissions, fuel efficiency and application requirements.
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