Tips from Used Car Advisor

Car Advisor Tamotsu Todoroki

Hi, this is Tamotsu Todoroki. I am a car advisor of PicknBuy24.com.
I write an online column every week to take care of your vehicle. My column is all about something useful and practical for your vehicle. Please have a look once to keep your car in good condition.

Posted:

What Happens if Diesel is Put in Petrol Engines? - Vol.348

This write-up will attempt to answer the consequences of mistakenly putting diesel fuel into the gas tank of a car that runs on petrol. First of all, let's be clear that this is nowhere as bad as mistakenly putting petrol into a car that runs on diesel.

If for some reason you missed the flashy signs that proclaim a pump or nozzle to be for diesel and accidentally pumped diesel into your car, do not panic. If you have only added a little bit of diesel, top the tank off with petrol and your car's petrol engine will still run, albeit not as smoothly and perhaps with minor knocking.

Effect on Octane Value of Petrol
This is because #2 diesels (the type used in diesel engines) have an equivalent octane rating of 40 or less.Since diesel fuels are measured by cetane rating (C16H34), it would not contain much octane (C8H18).

For example, having 5% diesel in your petrol tank would lower the octane rating of your fuel by about 3%. This may not cause any problem at all if you're using higher-octane petrol than your car engine requires.

On the other hand, this would be enough to cause moderately bad knocking in sports cars with high compression engines. The lower-octane fuel would self-ignite while it's still being compressed. The upward moving piston is knocked back violently, thus the knocking noise. The engine would deliver much less power and you may not be able to shift above the third gear (auto or manual), though you should still be able to drive to some place safe and get assistance.

The octane value of petrol is an indication of how nonflammable the petrol is. Higher octane petrol is less flammable, which is why it can be compressed more. In a petrol engine, the fuel is compressed to less than its self-ignition level (usually 8:1 to 12:1) and then ignited by spark plugs. The cetane rating of diesel is an indication of how flammable the diesel is. There are no spark plugs in diesel engines ? the fuel is compressed at much higher ratios (14:1 to 25:1) until it self-ignites. (The flash points of liquids become lower at higher pressure).

The Big Trouble is?c
The trouble becomes bigger when you have too much or mostly diesel in the fuel tank of your petrol-running car. At the lower compression ratio of a petrol engine, diesel fuel with much heavier hydrocarbons will not ignite reliably even with sparks. The result is the car would not start at all. Even if by chance it managed to turn over, the engine would choke and shut down in no time.

The fact is you should not try to start the car at all. The more dieselyou put through a car's petrol fuel system, the higher the chances of damaging parts in the fuel system. Whatever happens, remember that it will not ruin the whole car so remain calm and don't panic.

While you could always try to siphon out the whole fuel tank yourself and refill it with petrol, it's better to bring the car to a service garage to have the fuel system flushed and inspected.

You'll feel better when you consider how lucky you are that you didn't accidentally put petrol in a diesel car. The presence of petrol would lower the flash point of the fuel so much that it would ignite prematurely and violently under the much higher compression ratio of a diesel engine. The poor lubricating property of petrol would almost certainly destroy diesel injectors and regulators as well.