Diesel Engines

We will be focussing on diesel vehicles, specifically, how they differ from a gasoline engine, the servicing requirements and common faults.

Key Differences

A petrol engine draws in a mixture of gas and air, which is compressed and then ignited via a spark plug. A diesel engine, on the other hand, draws in only air. The air is compressed, then fuel is injected and it is the heat from the compressed air that ignites the fuel spontaneously.

This difference in operation also leads to a difference in compression ratios. A gasoline engine compresses at a ratio of between 8:1 to 12:1, while a diesel engine compresses at a ratio of between 14:1 to as high as 25:1. The higher compression ratio of the diesel engine is required to enable the spontaneous ignition as described above, it also leads to better efficiency.


Rudolf Diesel designed the diesel engine in 1892. His intention was to develop a more efficient engine than the petrol engines used at the time. The diesel engine was initially run on peanut oil (the original biodiesel).

Diesel fuel, which was later produced by oil companies, was not introduced until well after the diesel engine was designed and the engine was subsequently modified to suit the fuel.


Gasoline engines generally use either carburetion in which the air and fuel are mixed long before the air enters the cylinder, or port fuel injection, in which the fuel is injected on to the back of the intake valve just prior to the intake stroke (outside the cylinder).

Diesel engines use direct fuel injection; the diesel fuel is injected directly into the cylinder or into a pre-combustion chamber joined to the cylinder.

Diesel Fuel

Diesel fuel is very close to kerosene in composition, it has more energy in it than petrol fuel, which is why it gets better fuel economy. Recently low sulphur fuel was introduced to the New Zealand market; this was mainly to reduce emissions. In some early diesel engines, this affected the seals in the injector pump and caused them to leak.

Injector Pumps

The injector pump is the heart of a diesel engine; it supplies the fuel at very high pressures, it also times and meters the injection of fuel to suit different engine load and speed conditions. Injector pumps can leak fuel, break down, or wear internally causing poor running due to incorrect timing or quantity or cause the engine to stop running altogether.


Diesels require servicing more often, some every 5,000km but there can be up to 15,000km intervals on newer vehicles. You can check your owner’s manual or ask your mechanic if you are unsure.

Diesels also require high-quality diesel engine oil; this is because of the higher loads on the engine, which leads to the engine oil becoming contaminated more rapidly due to a number of factors.

As tolerances are very fine in diesels, filters such as fuel, engine and air should be replaced regularly to keep the engine in good condition. Oil changes should increase in frequency as the engine gets older not decrease, as is often the case.

Glow Plugs

Because a diesel engine works on heat generated from compressing air when the engine is cold there can be insufficient heat to ignite the fuel. A glow plug is a heating element protruding into the cylinder; it is used to heat the air prior to cranking the engine. This ensures the engine will start easily. Glow plugs system faults result in hard starting, smoke on startup or not starting.


Unlike petrol, diesel fuel will not vaporise naturally so it must be sprayed in a fine mist into the cylinder to ensure complete combustion. Faulty injectors can cause black smoke, poor running, poor fuel economy and hard starting.


Due to its high compression ratio, a diesel engine is harder to turn over and requires a high capacity battery and starter motor to achieve the correct cranking speed.

Common Rail Injection

The latest technology in diesel engines is the use of common rail fuel injection. Common rail refers to the use of a single fuel rail consistently supplied with up to 1,600 bar (23,000 psi) of fuel pressure. The fuel is metered and timed electronically by the ECM (electronic control module), which precisely actuates the injectors releasing fuel directly into the cylinder.

The advantages are reduced emissions, less noise, more power, longer service intervals and better fuel economy. We are well trained and experienced with common rail diesel servicing and fault diagnosis.


General Enquiry

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