[lightarrow (OP)]

Let's say you put some diesel fuel in the tank with already some petrol,  ~ 20 - 40% diesel fuel and the rest petrol. What happens?

[Pumblechook]

They did it deliberately on one motoring problem.. 100%  Diesel in a Petrol car and vice versa..  Both cars ran even if roughly and it didn't seem to do any damage. 

[chris]

This is not true. Diesel is much harder to ignite than petrol and relies on very high compression (5-10 times greater than a petrol engine) to superheat air in the cylinders and thus trigger ignition. Consequently the fuel is injected just prior to TDC (top dead centre). Petrol is injected on the downstroke, compressed on the upstroke and burns when ignited by the spark plug.

Diesel contains much longer chain hydrocarbons and is much "heavier" and less volatile. It doesn't burn completely in the petrol engine resulting in smoking (when part of a fuel mixture) to complete non-running as the mixture is enriched.

Petrol introduced into a diesel engine burns immediately it is blown in and with the wrong dynamics, making the engine run badly and unevenly, potentially causing damage.

My wife once inadvertently put unleaded in a diesel van she was using - but only at about a 20% mix. It ran okay.

Chris

[Pumblechook]

They both ran on the prog much to the surprise of the presenter.  Maybe they didn't run the cars for long before draining and filling with the correct fuel.   Was a few years ago. 

[Pumblechook]

From another forum...

I saw a test on fifth gear last year sometime.

They put petrol in a diesel engine and vise versa and neither car had any really adverse effects. However both these cars were old with basic engines. I expect a modern direct injection diesel may have more problems.

[turnipsock]

The old diesel engines will run with just about any hydrocarbon type stuff. Cooking oil is a fav as it's a lot cheaper than anything you get on a forecourt. New diesel engines wouldn't last long with cooking oil as is messes up the injection pumps.

I think wrong fuels mess up the cats these days.

I saw a van somewhere (M1 probably) that was running a business rescuing people that have put in the wrong fuel. He was probably looking for cars, smoking badly, coming down the slip roads from the services.

[lightarrow (OP)]

This is not true. Diesel is much harder to ignite than petrol and relies on very high compression (5-10 times greater than a petrol engine) to superheat air in the cylinders and thus trigger ignition. Consequently the fuel is injected just prior to TDC (top dead centre). Petrol is injected on the downstroke, compressed on the upstroke and burns when ignited by the spark plug.

Diesel contains much longer chain hydrocarbons and is much "heavier" and less volatile. It doesn't burn completely in the petrol engine resulting in smoking (when part of a fuel mixture) to complete non-running as the mixture is enriched.

Exactly for that reason, a mix diesel fuel/petrol should have a less O.N. and so should give pinking in a petrol engine, isnt'it?

[lyner]

A conventional petrol engine with a carburettor would be very difficult to start with diesel because the diesel wouldn't vaporise in the carburettor - it would hardly flow through the jets, even.  You can't even set light to a bucket of diesel fuel by dropping a lighted match in it - unlike petrol wooooomph!

It the engine were already hot (including the carburettor), then the diesel might vaporise enough to keep the engine going. I remember stories of people running petrol engines on paraffin, using a change over tap, once the engine was hot. Any heavy fraction like diesel, cooking oil or paraffin would burn badly and produce loads of smoke.

Running a diesel engine on petrol would be less of a problem - no reason why not, in principle, except for getting the fuel metering right.

[Andrew K Fletcher]

I have seen injector pipes shatter due to pre ignition of diesel, not once but several times in an old perkins engine. diesel in a petrol car will cause plumes of grey smoke and the engine runs like a bag of nails pinking and over heating. Petrol in a diesel engine again causes pre ignition and due to the high compression of a diesel engine this back pressure can cause damage to the diesel pump and the very high temperatures generated from burring petrol in a diesel engine will overheat the pistons, rings and valves and possibly warp the cylinder head given enough time.

I used to thaw out frozen diesel in truck when we had severe sub zero temperatures about 25 years ago. This was due mainly to water separating from the diesel and freezing. To prevent this we used to add a few gallons of petrol to the diesel, worked a treat and did not cause any damage due to the dilution of it in 40 gallons of diesel.

You can also run a diesel engine on old engine oil diluted with kerosene 3 to 1 part oil. I broke down in France once due to false reading on the fuel gauge. Didn't have any diesel but had a gallon of engine oil. Thought what the hell and tipped it in, ran a treat with minimal smoke but took it steady due to the viscosity of the oil. A cold engine would not have been able to run on neat engine oil but the hot engine did and got me to a fuel station.

A long time ago, I ran out of petrol in an old J4 pick up. Had oxy propane cutting gear on the back of the truck so put the burning torch into the inlet, turned on the propane and away it ran with no problem at all, even experimenting with adding a little oxygen to give it even more power, it accelerated and decelerated perfectly again getting me to a fuel station with no problems whatsoever.

Having a garage workshop for a few years we often purchased second hand engines from the breakers and frequently had them running by bolting on a starter motor and adding a wire to the coil and distributor on petrol engines. This proved easy with a propane bottle or in the case of a diesel engine with a small amount of diesel in a bottle. Rather noisy though without an exhaust. Fan of scrap yard challenge also

Andrew

[lyner]

Sounds like fun!