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Accidentally Fueling a Diesel with Gasoline

Can gasoline harm a diesel engine?


2013 Honda Civic diesel
NRMA Motoring and Services/Flckr

What happens if you inadvertently fill your diesel car with gasoline? Whether you're new to diesel ownership, or might have both diesel and gasoline powered vehicles in your own personal fleet, it can be oh-so-easy to accidentally mis-fuel your diesel tank with gasoline. Filling your fuel tank is such an ordinary and mundane task, that just a moment's inattention can cause you to grab the wrong nozzle and pump away. Bad enough if you realize the mistake right away and can get the car towed to a shop to have the tank drained (an awfully expensive nuisance), but what if you don't even realize the mistake and end up driving away with a tank full of gasoline? Chances are you won't get very far (maybe just a mile or so until the diesel in the fuel line gives way to the fresh batch of gasoline on the way from the tank) and the engine starts to run "funny."

Of course, it all depends on how much diesel remained in the tank before the gasoline was added, and how new and sophisticated the diesel engine is. In a 2007 or newer clean diesel engine, any amount of gasoline will probably damage the sensitive emissions control components (DPF, OxyCat and SCR) and system. In older engines with much less sophisticated and "touchy" emissions systems, a lightly diluted (say 90 percent diesel/10 percent gasoline) mix would probably pass through with little or no detriment. It might simply cause reduced engine power, perhaps a bit more noise, and possibly a sharp warning from the emissions sensors that detect something other than pure diesel exhaust. It's high concentrations of gasoline that spell real trouble. Whether a modern clean CRD diesel or an old indirect injection unit, burning straight gasoline or highly diluted diesel fuel will almost certainly result in catastrophic damage to the mighty diesel engine.

Diesel Fuel and Compression Ignition

Diesel engines rely on compression ignition and run at much higher compression ratios and cylinder pressures than their gasoline counterparts. It is this higher compression ratio that creates the necessary heat to auto-ignite and burn low volatility oil-type fuels such as diesel fuel and biodiesel.

Gasoline and Spark Ignition

Gasoline engines are built much less stoutly than diesels and generate far lower combustion chamber compression ratios. Because these engines don't build sufficient pressure and heat to ignite dense oil-type fuels, they rely on timed electric spark ignition to initiate gasoline's highly volatile burn sequence.

So What Does Gasoline do to a Diesel Engine?

The problem is multi-faceted. It is a function of the completely different burn characteristics of the fuels (volatile and explosive gasoline versus high flash point diesel fuel), and the peculiarities of engine design in regards to how fuel is ignited (spark ignition versus compression ignition). Gasoline is formulated to resist auto-ignition in a spark engine (see octane), so this fuel introduced into a diesel engine either won't ignite or will (much more likely) ignite at the wrong time causing severe detonation. Though diesel engine reciprocating components (pistons, wrist pins and connecting rods) are built to withstand enormous explosive force, the shock wave effects of uncontrolled detonation can easily destroy them.

In addition to the specific burn characteristics that diesel fuel and biodiesel afford to compression ignition diesel engines, the fuel itself acts as a lubricant for the fuel pump and delivery system as well as the valve train (recall that diesel fuel is actually a very light oil). Running thin, low viscosity gasoline through a diesel fuel system would starve it for lubrication and cause those sensitive components to rub together (metal-to-metal) eventually destroying them.

But What About the Opposite: Putting Diesel Fuel in a Gasoline Engine?

Fortunately, this one is almost impossible (notice we said almost), since the filler nozzles on diesel fuel dispensers are larger than those on gasoline pumps. The typical slow-fill diesel pump nozzle is 15/16" (24 cm)--fast-fill truck nozzles are even larger, well over 1-inch (25 cm), while the nozzle for a gasoline pump is about 13/16" (21 cm). Simply put, a large diesel spout won't fit in a narrow gasoline filler neck, while a smaller diameter gasoline snout will slip right in a diesel tank opening.

But if you do manage to get diesel fuel in your gasoline tank, (and this also is dilution dependent) the engine will probably not even start--and if it does, it'll run terribly and probably smoke like a chimney. Engine damage will probably be minimal to none, but a thorough (and expensive) fuel system flush will certainly be in order.

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