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Why do diesel engines produce so much torque? Why or how can a diesel engine produce more torque than a petrol engine of the same capacity. If these are questions that intrigued you at least once, we have the answer for you.
The Maruti Suzuki Swift is available with both petrol and diesel engine options - a 1197cc four-cylinder petrol engine and a 1248cc four-cylinder diesel engine. The petrol engine produces 83bhp, which is higher than the 75bhp output of the diesel engine.
However, the diesel engine produces 190Nm of torque, 77Nm more than the 113Nm output of the petrol engine. The Toyota Fortuner is available in international markets with a petrol engine as well as a diesel engine.
The petrol engine is a 3,956cc V6 engine which produces 234bhp and 376Nm. The 2.8-Litre diesel engine in the new Toyota Fortuner on the other hand, has two cylinders and 1,251cc lesser than the petrol engine. Yet it produces 420Nm of torque, 44Nm more than the V6 petrol.
Why? What is it about diesel engines that make them so torquey? Here is the answer.
There are several reasons as to why a diesel engine produces more torque than a petrol engine. Here are a few:
The longer the stroke in the combustion cycle, the higher the torque output of the engine is. The logic behind this is very simple - the longer the piston travels to compress the air, the more potential energy it stores within itself.
This potential energy is converted into kinetic energy as soon as fuel is introduced into the cylinder as it combusts. Torque is defined as the ‘force that produces or tends to produce rotation'. Well, with a long stroke, the piston going back down in the powerstroke allows diesel engines to produce more torque.
Diesel engines unlike petrol engines, do not use a spark plug to ignite the fuel. Diesel engines compress air to a great extent in the compression stroke and then diesel is sprayed into the cylinder.
When the diesel comes into contact with the compressed air, it immediately ignites as the temperatures are extremely high. This, in combination with the long stroke generates high torque output.
Heavy Engine Components:
Since the compression ratio in a diesel engine is extremely high, the engine components must be strong enough to handle the extra pressure. The piston, crankshaft, connecting rods, etc are therefore stronger, and also heavier.
This is the reason diesel engines are not high speed engines. Heavy-duty industrial and commercial diesel engines operate up to 2,200rpm while those in small hatchbacks redline at around 4,500rpm. Since engine speed is less, diesel engines are inherently designed to produce a higher torque output at lower rpms.
Effective Cylinder Pressure:
A petrol engine is most effective at the time when the spark plug ignites the fuel-air mixture. This effectiveness slowly fades out by the time the piston reaches the bottom of the cylinder in the power stroke.
In a diesel engine though, it is only air inside the engine that is compressed and there is no fuel-air mixture. Since ignition happens when diesel is introduced into the cylinder, the combustion can be continued until the piston reaches the bottom of the cylinder.
High Turbocharger Boost:
The days of old DI engines are gone and all diesel engines in production today are turbocharged. Diesel engines being built strong, with tough and heavy internal components can handle high turbocharger boost pressure.
Running 15psi of turbo boost on petrol engines is considered high turbo boost. On the other hand, most production turbo-diesels run between 15-30psi. Running up to 50psi on high performance diesels is not very far fetched. This invariably leads to a high torque output.
Most performance enthusiasts consider the high-revving nature of petrol engines to be fun, and they are fun-to-drive. However, diesel engines are a different kind of fun given their torquey nature. Diesel engines with their unmistakable clatter, low-down torque and turbo boost kick-in are considered by many to be more fun-to-drive than petrol engines.