How dangerous is it to refuel with the engine running?


Alright, we now know the danger of static electricity charge (which could cause a fire) in the petrol station, how about leaving your vehicle engine running during the fueling process? There is instruction in all petrol stations to turn off the engine before refuelling, but how dangerous is it really?

Is it dangerous to leave your engine running while you refuel your car? Probably not, in the overwhelming majority of cases.

Think back to the traditional fire triangle. It has three components—first, fuel; second, an oxidiser (usually the oxygen in the air); and third, heat, or a source of ignition. You need all three to start a fire. By the way, if you want to get philosophical, think of fire as being more of an ‘event’ that needs a continuous supply of all three factors, rather than a stable ‘thing’ or ‘entity’.

The correct proportion of petrol vapour for ignition

First, the fuel from our fire triangle. Yes, you can definitely smell fuel (petrol vapour) when you refuel at a petrol station. But petrol vapour usually will not burn.

Petrol vapour burns only in a very narrow range of concentrations—between 2 and 8 percent of the volume of the air. When the concentration is less than 2 percent, there’s not enough fuel to sustain a flame. And when it’s over 8 percent, there’s not enough oxygen available to combine with the petrol vapour—so again, no burning. The chance of having the right concentration of petrol vapour immediately next to both heat and the oxidiser is very low.

The Oxygen

The second part of the fuel triangle is the oxidiser. The air surrounding us is about 20 percent oxygen. While you are filling your tank with petrol, you are also fully surrounded by one of the three parts of the fire triangle.

The Heat or Igniter

So what about the third part of the fuel triangle, the heat, or the igniter? Yes, indeed, there are several potential sources of ignition present in a car.

The starter motor on the engine draws a lot of currents and can create hot electrical sparks. But it’s well and truly buried in the guts of the engine, where the outside air (with or without petrol vapour) has no access.

The spark plugs and their leads have the job of igniting the petrol/air mixture inside your engine. They carry tens of thousands of volts of electricity, but they are normally well insulated. However, if the insulation in the spark plug leads breaks down, it could ignite petrol vapour (because of electrical sparks along the leads). But you would have difficulty in starting and running the engine, especially in wet weather.

So definitely replace bad spark plug leads—but, bad spark leads would have great difficulty in igniting petrol vapour.

Another potential source of ignition is the catalytic converter—the magic box that cleans up the engine exhaust. It can get very hot. The older ones had a safe limit of 750°C, while some newer ones are rated at 900°C. But they cool down quickly, and are buried deep inside the exhaust system.

What about other sources of ignition? It is also possible, but very unlikely, that there could be random sparks from a faulty relay, or loose battery terminals, or any poorly installed electrical accessory.

But in the real-life situation of refuelling at a petrol station, consider this logical inconsistency.

Like a law-abiding citizen, you pull up at the petrol pump, turn off the engine and then start refuelling. Meanwhile, people drive past you with their engines running!

Nobody expects you to switch off your car’s engine while still on the road (well away from the petrol pump) and then push the car to the petrol pump. That’s because there’s no regulation ruling that running engines are not allowed near somebody refuelling. The only rule is that your engine can’t be running.

Does that make sense?

Yes, it is definitely dangerous to leave the engine running with children inside the car. They might want to copy their parents, and shift the gearbox into drive. Or, while you’re inside paying for petrol, a bad person could hop into the driver’s seat and drive away.

In the United Arab Emirates, the local regulations are that the engine should be switched off while refuelling. Most of the locals ignore this regulation. This is because it gets so hot in summer that many drivers leave the engines running with the air conditioning on full blast—for the comfort of the passengers.

In Abu Dhabi, in most cases, the cars are refuelled by petrol station staff. The drivers and passengers often smoke cigarettes, with the windows slightly down to remove the smoke. A car did catch fire in Abu Dhabi in 2015, while refuelling. It was blamed on ‘a poorly maintained car’.

Overall, if you’re worried about your car catching fire if you leave the engine running while refuelling, I guess that it’s better to be safe than sorry, and not be ‘fuellish’.

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