With an increasing shift by Indian car buyers to diesel cars, one would have also noticed that most diesel cars have a much higher claimed mileage than an equivalent petrol car. Typically a diesel car has about 25% more mileage than an equivalent capacity petrol car. Why is this so?

Take for instance the Volkswagen Vento petrol /diesel engines. Both are of 1.6 litre capacity. The petrol and diesel both develop 104 bhp of power. But the petrol only has 153 Nm torque and gives a claimed 15 kmpl, while the diesel has 250 Nm torque and claimed fuel efficiency of 20 kmpl.

diesel or petrol

CarToq looks at the reasons why diesel cars are more fuel efficient that petrol cars.

Also see: Trying to decide whether diesel or petrol will save you more money? Check out our Diesel Decider tool!

Diesel carries more energy

Diesel as a fuel has a higher energy rating than petrol. Each litre of diesel produces more energy than an equivalent litre of petrol. Diesel contains 38.6 Mega Joules per litre of energy while a litre of petrol contains only 34.8 MJ of energy. What this means is that diesel burns hotter and with more of an explosive force than petrol. This would mean you need to use less diesel to achieve a certain amount of power than you would with petrol. This amount is typically about 25% lesser diesel than equivalent petrol.

Also read: When should you sell your petrol car and buy a diesel?

Diesel does not need a spark

Diesel fuel is not as highly inflammable as petrol. It, however, can auto ignite at high temperature and this is the principle on which a diesel engine works. Air is compressed to a high ratio of about 18:1 or 21:1 in the cylinder and compressing air generates heat. When this temperature inside the cylinder in an engine goes higher than 210 degrees centigrade (the auto ignition temperature of diesel), a small spray of diesel is injected into the cylinder which then ignites spontaneously. This is why in cold temperatures it takes longer to start a diesel car because the pistons need to pump a few times before the air can get warm enough to ignite diesel. Some cars use pre-heaters or glow plugs also for this in cold weather.

Also see: Most value for money diesel sedans

Precise fuel injection

Because diesel is minutely directly sprayed into a cylinder, lesser fuel is used and since it has cetane (six-sided) rings in its makeup instead of octane (eight-sided hydrocarbon rings), diesel’s burning characteristics are better – it is more slow burning and complete burning, allowing for a longer burn time and better efficiency. This is also the reason why diesel engines don’t reach high rpm ranges. Now some new generation petrol engines are adopting similar technology like the Hyundai Sonata’s GDI (gasoline direct injection) petrol engine, which technically should give better mileage and torque.

Related: Upcoming cheap diesel cars for India in 2012

Diesels develop more torque

The high compression ratio in a diesel engine means that it needs heavier parts and longer pistons and connecting rods to compress air better. It has a higher thermal efficiency than a petrol engine. This increased length of pistons also means it has a better mechanical advantage than a petrol engine and develops higher torque. Torque is the turning force that the engine generates – and since more of this comes at a lower rpm in a diesel engine, it is easier to move away without revving the engine too much and this also saves fuel. Also see: Diesel cars with turbo lag – and is turbo lag a problem?

Therefore, even if fuel prices of diesel and petrol were to become equal at some point in the future, diesel cars will still be the cars of choice for those who travel longer distances, because of the higher mileage that they give.

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