Both four-stroke petrol and diesel engines use the same fundamental four-stroke cycle: intake, compression, power, and exhaust. However, the details of each stage and the overall combustion process differ significantly due to the distinct characteristics of gasoline and diesel fuel.
Here's a breakdown of the differences:
1. Intake Stroke:
* Petrol: The intake valve opens, drawing in a mixture of air and fuel.
* Diesel: The intake valve opens, drawing in only air. Fuel is injected directly into the combustion chamber during the compression stroke.
2. Compression Stroke:
* Petrol: The air-fuel mixture is compressed, with a compression ratio typically ranging from 8:1 to 12:1.
* Diesel: Only air is compressed to a much higher compression ratio (typically 14:1 to 25:1), generating heat.
3. Power Stroke:
* Petrol: The spark plug ignites the compressed air-fuel mixture, causing a rapid expansion of gas, pushing the piston down.
* Diesel: When the compressed air reaches a sufficiently high temperature (auto-ignition temperature), the injected fuel ignites, producing a power stroke.
4. Exhaust Stroke:
* Both: The exhaust valve opens, expelling burnt gases from the cylinder.
Here's a table summarizing the key differences:
| Feature | Petrol Engine | Diesel Engine |
|---|---|---|
| Fuel Type | Gasoline | Diesel |
| Ignition | Spark Ignition | Compression Ignition |
| Compression Ratio | Lower (8:1-12:1) | Higher (14:1-25:1) |
| Intake Mixture | Air-fuel mixture | Air only |
| Fuel Injection | Before intake | During compression |
| Overall Efficiency | Lower | Higher |
| Emissions | Higher emissions of hydrocarbons and carbon monoxide | Lower emissions of hydrocarbons and carbon monoxide, but higher NOx emissions |
| Noise | Generally quieter | Louder |
| Maintenance | Typically requires more frequent spark plug changes | Requires less frequent spark plug changes but more frequent oil changes |
| Starting | Easier to start in cold weather | Harder to start in cold weather |
| Power/Torque | Higher horsepower at higher RPMs | Higher torque at lower RPMs |
Key points to remember:
* Diesel engines are more efficient due to the higher compression ratio and the absence of throttling.
* Diesel engines produce higher torque at lower RPMs, making them ideal for heavy-duty applications.
* Petrol engines are generally cheaper and easier to maintain.
* Diesel engines emit more NOx, while petrol engines emit more hydrocarbons and carbon monoxide.
Ultimately, the choice between a petrol and a diesel engine depends on the specific application and desired performance characteristics.
