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So you’ve decided to buy a Toyota Fortuner, Toyota Revo, or a used 2000s BMW. Now you’re confused about whether you should go with a petrol variant or pay the extra money and choose the diesel variant.
The short answer is that diesel and petrol engines work differently, and one might suit some people while the other might not suit specific consumers. Here’s the detailed difference guide to clear the bushes in your head so you can decide if paying extra for diesel makes sense for you.
What is the Mechanical Difference in Both Engines?
In the broad sense, both are 4-stroke ICE engines that convert chemical energy into mechanical energy. Both have pistons attached to the crankshaft via connecting rods that burn the fuel and convert that into power.
The key difference is how the combustion occurs in both engines. In petrol engines, the air intake valve and fuel injector sprinkle the air and fuel mixture into the chamber. The piston compresses that mixture. Next, a spark plug ignites the mixture of fuel and air. As a result, a burst of immense energy is produced in the chamber that pushes the piston downwards, and hence, the tire of the vehicle rotates.
In diesel engines, the operating cycle starts with clean air being drawn into the cylinder. Note that it’s the clean air from the air filter, not the mixture of fuel like in a petrol engine. The piston is pushed upwards, compressing the air and heating it to an immense temperature. At the same time, diesel is injected into the chamber in the form of a fine mist. Each droplet of mist quickly begins to burn due to heated air, which produces energy and pushes the piston downwards, and hence, the tire of the vehicle rotates.
Here’s a table for a quick overlook:
| Petrol Engine | Diesel Engine |
| Air and fuel mixture is injected into the combustion chamber | Clean air is injected into the chamber, and fuel is injected afterward. |
| A spark plug is used to ignite the fuel. | No spark plug is required. Instead, the air is heated so much that it ignites the fuel. |
| Due to external spark plugs and lower compression ratio, petrol engines have higher RPMs, but lower torque. | The fuel is ignited by the piston’s compression, which means a higher compression ratio and, hence, lower RPMs but higher torque. |
Power and Torque | Which is Best for Who?
Both are good in a desired performance section.
Diesel engines have higher torque than petrol engines of the same displacement. However, a petrol engine would have higher power (horsepower) than a diesel engine.
Torque is the force that turns a car’s wheels and gets the vehicle moving. Higher torque means the engine can move the car more easily, especially from a stop.
Power (HP) is the engine’s ability to keep the car moving and speed it up. The more horsepower a car has, the faster it can achieve its top speed and accelerate.
For example, when you’re carrying a heavy load or climbing the hills of Naran, you need a car with higher torque. When you’re cruising on the M2 Motorway and drag racing with your friend, you need a car with higher power (HP).
Why are Diesel Engines Good at Torque and Petrol at Power?
Because their cylinder length is long and they have long pistons.
Diesel is a high-energy fuel compared to gasoline, meaning it produces more energy per unit volume when burned. However, diesel is harder to ignite than gasoline. Diesel engines have longer pistons, which means they have higher stroke lengths, which results in higher torque.
The petrol engine, on the other hand, has shorter cylinders because gasoline is easy to ignite, so it doesn’t require longer cylinders. Due to short cylinders, the piston completes the rotation faster than the diesel engine in a given amount of time; hence, their RPMs are higher. As a result, petrol engines have higher HPs at high RPMs.
Fuel Economy of Diesel is Good
We already told you that diesel engines can do the same work, but with lower RPMs than petrol engines; this somehow already explains why their economy is good. Suppose a petrol engine needs to move the vehicle from rest to 40 Km/h. The engine will work at 3,000 RPMs, while a diesel engine can do the same but at 1,200 RPMs. This is the only and main reason behind their superior economy. The difference in the economy depends on the car to car, but on average, you’ll see about a 3-4 Km/l difference in a petrol and diesel variant.
Conclusion | Which is Good for Who?
If you do long tours, especially if you regularly go to northern areas like Babusar Top, Naran, Kaghan, Swat, and Fairy Meadows, a diesel engine is best and highly recommended.
If you are into fun rides and high-speed driving and think, “I’ve paid for the whole speedometer, so I’m gonna use the whole speedometer,” then a petrol engine is best.
