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Why petrol is so bad for a diesel engine

By James Mills 3 February 2014

Putting the wrong fuel in your car could be disastrous for your engine as well as your pocket. Dr Geraint Owen from the University of Bath explains exactly what happens when you misfuel a car. He should know. As a senior lecturer in automotive engineering he helps nurture future generations of mechanical engineers and knows everything there is to know about engines.

Critical component 1: The fuel pump

There is a simple reason why petrol is not good for a diesel-powered car. Petrol acts as a solvent which means it prevents the lubricating action that diesel fuel delivers to the precious components of the engine, most significantly the fuel pump. Fill a diesel-powered vehicle with petrol and drive away until it shudders to a halt and you can do serious damage to the fuel pump. That sets off a chain reaction that is very bad news.

Think of it like running an engine without oil: there is a high level of friction, the high-pressure fuel pump’s high tolerance components are running without lubrication and grind together producing ‘swarf’. These are tiny metal fragments, almost like shavings. They enter the fuel system and travel towards the fuel injectors with disastrous consequences.

Critical component 2: The fuel lines

You’ve filled your diesel car with petrol, started the engine, driven down the road and suddenly the car conks out. In order to travel from the fuel tank (usually at the rear) to the engine (usually at the front), the petrol will use the fuel lines. These are now contaminated, as are their rubber seals and those of the fuel injectors. Petrol can eat away at some seals in a diesel engine so it needs to be flushed through with a cleaning agent and then assessed for potential damage. In a worst-case scenario, all contaminated parts must be replaced, which is a time-consuming and costly job.

Critical component 3: The fuel filter

A diesel fuel filter’s job is to prevent any contaminants coming up from the fuel tank and getting into the engine. Like a racing driver wearing fireproof overalls and a crash helmet, it’s a necessary preventative measure but once contaminated by petrol it has to be replaced. It’s the simplest and cheapest component of the chain to fix, perhaps costing less than £100, but that’s small consolation if the garage finds that serious damage has been done to other parts of the fuel system.

Critical component 4: The fuel injectors

Modern diesel engines operate common rail direct fuel injection systems. These inject diesel directly into the combustion chamber, and are incredibly efficient because they operate at high pressure and under precisely controlled timings. If there’s a drawback to these high-tech systems, it’s that they have incredibly fine tolerances, injecting fuel through very small holes. The production of metal swarf fragments from the failed fuel pump can have catastrophic consequences. The swarf blocks the holes in the injectors, preventing fuel from entering the engine to be burned which causes the engine to misfire and eventually stop running. Replacement of the fuel injectors and the ‘common rail’ – the pressurised storage chamber for the diesel fuel – together with the fuel pump is extremely expensive, as in thousands and thousands of pounds.