With the aid of sketches, describe and contrast the 4 stroke petrol engine cycle and Dieselcycle in relation to their P-v diagrams. Label each stroke clearly and show where heat issupplied and where work output occurs
Question
With the aid of sketches, describe and contrast the 4 stroke petrol engine cycle and Dieselcycle in relation to their P-v diagrams. Label each stroke clearly and show where heat issupplied and where work output occurs
Solution
The 4-stroke petrol engine cycle and the Diesel cycle are both internal combustion engine cycles, but they operate differently. Here's a step-by-step comparison of the two:
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Intake Stroke:
- Petrol Engine: The intake valve opens, and the piston moves down, drawing a mixture of fuel and air into the cylinder.
- Diesel Engine: Only air is drawn into the cylinder as the piston moves down. The intake valve opens, but no fuel is injected at this stage.
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Compression Stroke:
- Petrol Engine: The intake valve closes, and the piston moves up, compressing the fuel-air mixture. This is a low compression ratio compared to a diesel engine.
- Diesel Engine: The piston moves up, compressing the air and increasing its temperature. This is a high compression ratio, and no fuel is present during this compression.
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Power/Combustion Stroke:
- Petrol Engine: The spark plug ignites the compressed fuel-air mixture, causing an explosion that pushes the piston down. This is where work output occurs.
- Diesel Engine: At the top of the compression stroke, fuel is injected into the cylinder. The heat of the compressed air ignites the fuel, causing an explosion that pushes the piston down. This is where work output occurs.
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Exhaust Stroke:
- Petrol Engine: The exhaust valve opens, and the piston moves up, expelling the combustion gases.
- Diesel Engine: The exhaust valve opens, and the piston moves up, expelling the combustion gases.
In a P-V (Pressure-Volume) diagram:
- The Petrol engine cycle would show a lower compression ratio, meaning the 'loop' of the cycle in the diagram would be smaller.
- The Diesel cycle would show a higher compression ratio, so the 'loop' of the cycle in the diagram would be larger.
Heat is supplied in the combustion stroke for both engines, but at different points: at the beginning of the stroke in the petrol engine (due to the spark plug) and at the end of the compression stroke in the diesel engine (due to the fuel injection). Work output occurs during the power/combustion stroke when the explosion pushes the piston down, turning the crankshaft.
Unfortunately, I can't provide sketches here, but you can easily find P-V diagrams for both cycles with a quick internet search.
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