Why Diesel Engines are Heavier ?

You might have noticed that petrol engines are less noisy and vibrate less compared to diesel engines. This is because the combustion process in a pre-mixed mixture is smooth and propagates well (first part Fig.5). But in a diesel engine, the combustion could begin anywhere in the combustion chamber, and it turns out to be an uncontrolled process.

power flow in automobile

Fig.5 Combustion is smooth and well propagating in petrol engine, but in diesel it is highly unpredictable

For this reason, to reduce the excessive vibration and noise problem, diesel engines require a more rugged structural design than petrol engines. To normalize the heavy unbalanced power production of diesel engines a heavy fly wheel is often required. This is why petrol engines are always preferred for light-weight applications, such as in 2-wheeler or portable devices.

Why Diesel Engines are More Fuel Economical ?

Since the diesel engine is compressing only the air, it can achieve a good compression ratio without risk of self-ignition. But, in a pre-mixed petrol engine, such a high compression ratio is not possible. As we increase compression ratio of petrol engine the mixture becomes more prone to self ignition. This is known as knocking. Over the period of operation knocking badly damages the engine.

power flow in automobile

Fig.6 Variation of mechanical efficiency of the engine with compression ratio

The reason why your diesel car gives more mileage than a petrol car is due to the difference in the compression ratio. The higher the compression ratio, the better is the efficiency of the cycle. A qualitative efficiency variation is shown in the graph of Fig.6. This is the reason why diesel engines have better fuel economy as compared to petrol engines.

Petrol in Diesel Engine or Vice Versa ?

An interesting question many people wonder is: What if I put petrol into a diesel engine or vice versa?. From what we have learned so far, we will get a logical and practical answer for this intriguing question in this session

Diesel in Petrol Engine

Diesel in a petrol engine will not even cause firing. The reason is simple. Diesel is less volatile and will not mix with the air properly. In fact you will find it is impossible to make a good diesel-air mixture using carburetor or direct injection technology. This means if you apply spark to such a poor quality mixture, it will not initiate any combustion.

Petrol in Diesel Engine

On the other hand, if you put petrol in a diesel engine, you are spraying a highly volatile fuel into a chamber of highly compressed and hot air. This will lead to detonations rather than smooth combustion. Eventually, the engine components will get damaged. Moreover diesel generally acts a good lubricant for the fuel pump and the injection system. When you put petrol (which does not have any lubrication property) into a diesel car your are actually making the intricate components to wear down over the time. So that’s a big no for petrol in a diesel engine.


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