HCOS1550 | Introduction: auxiliary engines
From HM Revenue & Customs · Hydrocarbon Oils Strategy
Gas oil can be used to power auxiliary engines of vehicles and can therefore be found on the vehicle and at the premises of these businesses:
refrigerated transport - fridge unit
cleansing lorries - donkey engine
fairgrounds - showmen’s’ generators.
They must not however be connected, or be capable of being connected, in any way to the propelling engine and / or its fuel supply.
Note - dual fuel systems, where one engine can be switched from a tank containing gas oil to carry out a stationary application (eg as a gulley emptier) to one containing DERV for road propulsion are not permitted.