Yes, the diesel engine is a more fuel-efficient engine than petrol engines.
They have been since their introduction in the early 1900s. They were not as popular as they are now. In the past, diesels were less appealing to the average passenger car owner due to their unattractive body design, noisy engine, and their performance was not everyone's cup of tea. Diesels were designed to carry heavy loads, not the school drop-off.
In this article, I am referring to fuel efficiency, not economy. The economy depends on where you fill up your tank, and how you drive and service your diesel.
Maintaining emission systems such as the Diesel Particulate Filter (DPF) is essential to preserve efficiency. If you’re unsure whether a DPF fault requires replacement, read our guide on DPF warning lights and replacement considerations.
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 system. However, a petrol engine still relies on spark ignition to ignite the fuel, and this continues to be less efficient.
One of the key factors of a fuel-efficient diesel engine is that it does not require a spark. They are designed to produce 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 ‘thermal’ engine. The same combustion temperature can be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and thus reducing the exhaust temperature.
HOW DOES COMPRESSION IGNITION WORK?
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 chamber via diesel injectors to create a fine spray.
Increasing fuel injection pressure improves atomisation and combustion efficiency, which is where the common rail diesel fuel system performs particularly well.
In comparative testing between similar modern petrol and diesel vehicles, diesel variants typically demonstrate lower fuel consumption and higher torque output at lower RPM.
In one comparison, the diesel variant consumed 1.4 litres per 100km less than the petrol equivalent, while producing higher torque (400Nm vs 290Nm). The diesel also achieved peak torque at lower engine speeds, improving towing performance and drivability.
Torque refers to an engine’s pulling power, and diesel engines are specifically designed to deliver strong torque at lower RPM, making them well suited for towing and heavy loads.
While comparing petrol and diesel efficiency, it’s also worth considering servicing costs. Is it cheaper to service a diesel vehicle?
Diesel Help Australia works exclusively with professional workshops.
If you require diagnosis or repair, we can refer you to an independent workshop that is part of the Diesel Help technical support network.
Return to the Diesel Vehicle Owner’s Guide for more topics and maintenance information.
