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What is an auxiliary engine used for on a lorry?

Some lorries have auxiliary equipment which requires power, but they don’t have a PTO or power take-off that draws power from idling the main engine. Instead, they have a small auxiliary engine which can provide the power.

Auxiliary engine used in a side-loader trailer

Common applications for this include chillers on refrigerated trailers, the cranes on side-loader trailers and air compressors on vehicle service trucks. For this reason, it’s important that the engines are reliable otherwise downtime causes lost revenue.

Auxiliary engines are serviced based on the number of hours they are run, compared to the lorry itself which will be serviced based on mileage.

The hours meter on the unit. This is located on the control panel

Instructions for operation are often shown on a decal

Auxiliary engines and connecting components need to be checked as part of the regular pre-start inspection. However, they can often be tucked away out of sight, so it’s important that if a new driver is using the lorry that they are shown where the engine is and how to stop and start it. Sometimes it’s right between the chassis rails, sometimes it’s behind a panel, sometimes you need to tip the cab, and sometimes it’s easily accessible near other components such as the toolbox.

Cover for the auxiliary engine of a transporter trailer
Showing the engine with the cover open.

There may be information about what to check before operating the engine, and what type of fluids to use.

This roadsweeper daily check and maintenance procedure information panel is located on the side of the vehicle. It shows what to check and what oil to use.

If there’s no instructions for the pre-start check for the auxiliary engine, it should include:

  • Ensuring the battery terminals are clean and tight, and that the battery is secured
  • Checking the fuel tank isn’t damaged or leaking
  • Checking that the fuel isn’t contaminated, there’s no sediment and that there’s sufficient for the requirements
  • Examining hydraulic hoses and other connections for leaks
  • Maintaining the engine oil and coolant at the correct level
  • Checking the fan belt tension and wear
  • Removing any obstructions from the radiator and checking there’s no damage
  • Logging a service request if the hours meter shows that it’s due.
The battery terminals should be secure and free from corrosion, and the battery itself should be tightly held in by its bracket

Using good quality fuel and oil will help keep the auxiliary engine running smoothly.

Check that you are putting the correct fuel into the auxiliary engine – in this case, petrol

Darren has owned several companies in the automotive, advertising and education industries. He has run driving theory educational websites since 2010.

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