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Why are diesel engines more fuel efficient?

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Updated 11 Oct 2019

Clinton Brett

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Yes, the diesel engine is a more fuel-efficient engine than a petrol. They always have been.

It’s just that in the past they were not that interesting to own either because of their ‘ugliness’ in body design, or the noise and performance was not everyone's cup of coffee (I’m not a tea drinker!).

So, I am referring to fuel efficiency, not economy.

Economy depends on where you fill up your tank, how you drive and how you service your diesel.

One of my recent articles concerned the Diesel Particulate Filter (DPF).

Amongst other important service items for diesels, the DPF does need attention and ignoring my advice about maintaining your DPF means your diesel will become very uneconomical and less efficient.

Recently, the modern petrol engine has almost matched the fuel efficiency of the common rail diesel due to the manufacturer replicating the common rail diesel (CRD) system.

However, a petrol engine still relies on spark ignition and continues to be less efficient.

One of the key factors of a fuel-efficient diesel engine is it does not require spark.

They are designed to produce much higher compression than a petrol engine which leads to compression ignition.

This higher compression allows an engine to extract more mechanical energy from a given mass of air–fuel mixture due to its higher thermal efficiency.

This occurs because the diesel is referred to as a ‘heat’ engine.

The same combustion temperature can be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature.


I use this example at my training courses for mechanics.

Using a bicycle pump, draw air in when extracting the plunger.

Hold your thumb over the outlet whilst compressing the plunger at the same time, then slightly release your thumb to reveal a tiny gap.

You should hear a squawk and feel the heat on your thumb.

There you have it! You have created heat without injecting any fuel.

To gain the most efficiency, fuel is atomised inside the combustion area via diesel injectors to create a fine spray.

Increasing the fuel injection pressures will increase the efficiency and this is where the common rail diesel fuel system works well.

I recently conducted a test drive of the BMW 5 series to provide information for our training sessions.

I was impressed by the performance of the petrol, yet the diesel still wins my heart.

The diesel was more fuel efficient by 1.4-litres per 100kms than the petrol.

And there was significant differences in performance too, particularly in torque.

The diesel posted figures of 140kW/400Nm and a 0-100km/h time of 7.5 seconds.

The petrol delivered 135kW/290Nm and 0-100km/h time of 7.8 seconds.

The torque on the diesel was achieved at a much lower RPM than the petrol.

Torque refers to an engine’s take-off or pulling power, and the diesel engine was designed to deliver that for sectors such as heavy haulage.

Improved torque is much better for towing and efficiency.

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Written By

Clinton Brett

Clinton Brett is a qualified Heavy Vehicle Mechanic and Diesel Fuel Injection Specialist.

In 2013, he established Diesel Help Australia, an innovative business providing cost effective diagnostic and repair solutions for the automotive industry.

DHA delivers on-vehicle common rail diesel diagnostic training across Australia to the light and heavy industries.

Clinton is a Technical Writer for The Automotive Technician, Australian Workshop Manager and Australian Diesel Mechanic Magazine, as well as the Diesel Guest Speaker for the AAAA Conventions for the last 3 years.