The compression ratio is a determining factor in the behavior of any engine. It is the evaluation of the engine cylinder’s ability to compress fuel and air. Different engines, such as gasoline engines and diesel engines, have different compression ratios. However, the diesel engine’s ratio is comparatively higher. Why are diesel engine compression ratios higher than those of other engines? This question must be haunting you, and you will find the answer right now.
So let’s find out together!
Definition of Compression Ratio
For an engine to function properly under highly variable conditions, the following conditions must be met:
- Air and fuel mixture
- Good compression ratio
- Ignition process
Thus, the compression ratio is calculated by the maximum volume of the cylinder (when the piston is at the bottom of its stroke) divided by its minimum volume (when the piston is at the top of its stroke).
- Gasoline engine: The compression ratio will need to increase to help boost engine power, but the risk of knocking will be high.
- Diesel engine: The compression ratio would need to be reduced to make the engine “quieter,” but in return, lower pressure and temperature conditions would reduce the auto-ignition capability, making combustion more difficult.
Therefore, the automotive engineer will need to limit the compression ratio of the engine using each type of fuel to a certain range.
Why Are Diesel Engine Compression Ratios Higher? – Main Reasons to Check!
Generally, the compression ratio of diesel engines is higher than that of gasoline engines (with the exception of some special gasoline engines like Mazda’s).
A typical gasoline engine has a compression ratio of about 8:1 to 12:1. However, in diesel engines, the compression ratio is typically around 15:1, and there are “special” diesel engines with compression ratios that can reach up to 25:1.
The main difference between gasoline and diesel engines lies in the ignition mechanism. While gasoline engines need spark plugs to carry out the combustion process of the compressed air-gasoline mixture, diesel engines operate on the principle of auto-ignition. When the air-diesel mixture is compressed at high pressure (high compression ratio), the heat generated creates the spontaneous combustion process. This characteristic gives diesel engines high energy efficiency.
To learn more about the reasons, we have listed below the main reasons why a diesel engine has a higher compression ratio than any other engine. So, take a look!
1. According to Application
Diesel engines are ideal for heavy vehicles such as locomotives, ships, trucks, and other gigantic automobiles. Therefore, they have a higher torque application. To start such a large vehicle, a large amount of energy is needed, which ultimately leads to an increased combustion rate. The compression of air is the process that occurs in a large cylinder inside an engine. Therefore, fuel is also burned rapidly. It produces more power inside the engine, which ultimately leads to a higher compression ratio. Therefore, the engine cylinder size and, of course, the application are the reason why diesel engines have a higher compression ratio.
2. Knock Avoidance
Knocking is a serious problem in engines. It occurs when there is abnormal combustion inside the engine cylinder. Knocking problems are generally more frequent in gasoline engines. When the compression ratio increases, the gasoline engine knocks, which is why it is not considered an ideal compression engine by many people. Coming back to the diesel engine, there is no knocking. The larger cylinders provide enough space for your fuel to flow and burn inside the engine.
The main reasons why an engine can produce knocking are:
- Ignition delay inside the injectors
- Increase in temperature inside the engine
- When the fuel injector fails to completely ignite the fuel
Thus, to avoid engine knocking, the diesel engine has a maximum compression ratio.
3. Spark Plug
Gasoline or petrol engines require a spark plug to burn the fuel. The diesel engine does not need a spark plug to generate power, but the high compression ratio is enough to ignite the fuel. For the engine to auto-ignite, a higher compression ratio is needed. Furthermore, diesel is very volatile, and only air compression is enough to ignite the fuel. Thus, to prevent the engine from burning or presenting other hazards, a diesel engine is built to acquire a high compression ratio. Additionally, to handle any unexpected issues with the diesel engine, you can learn some useful maintenance tips from the experts.
Conclusion
So, whenever you wonder why diesel engines have a higher compression ratio, remember this useful information and choose the engine of your choice.