Petrol to diesel

Hey guys, just heard the strangest thing. A friend wants to convert his petrol engined car to run on diesel. He told me a 'mechanic' friend has said there is NO need to change the engine, just the fuel tank and fuel pumps.

This must be a miracle, my understanding is that petrol and diesel engines are built in completely different ways.

Any thoughts.................................................

 

ask him to ask his mechanic friend to convert it to run on "water" very cheap to run and very green.
http://youtu.be/k9bzM0zqspk
I do know of people who have converted a petrol engine to run on wood,
Planet Mechanics -- Forget Horsepower-We're Talking Treepower - YouTube

 

I think he has every bit as much chance as with either of the above lol.

 

Ah but the bottom one does work i know of a guy with a landrover 101 converted and that runs ok looks more like a steam engine though!

 

Why not ask the mechanic to go the full hog and build a perpetual motion machine?

Jokes aside, lets look at some of the considerations:

A diesel engine is a 'compression ignition' engine. It ignites the fuel/air mix by relying on the little law of physics that things heat up when they are compressed. A petrol engine is a 'spark ignition' engine. It relies on a spark to ignite the mix. Because petrol has a lower flash point than diesel (the temperature at which it spontaneously ignites), petrol engines are built to allow some of the compression to escape down the sides of the pistons. So the first thing the mechanic would need to do is change the pistons in the engine, to ones that take more piston rings and allow less compression to escape, otherwise you wouldn't get the compression required to ignite the diesel reliably.

When petrol/air ignites, it might seem to us as an instant explosion, when in fact the combustion is relatively slow when compared to a diesel/air ignition. Because the petrol/air mix is ignited from a single point, it takes time (albeit very little time) for the flame front to reach all of the mix, so the combustion happens at a steady rate. Diesel on the other hand, because it is compression ignition, just explodes all at once, and goes with a hell of a bang. This is why diesel engines tend to be noisier and rattlier, and have more torque at the lower end.

Both of the above points bring their a whole set of new considerations. In a diesel engine, everything is built for that sudden 'bang', so the cylinder head gasket is stronger, tougher bearings are used on the piston rods and crank, the cylinder head is tougher. Even if you could get a petrol engine to run on diesel, it wouldn't live for very long because a petrol engine is not built strong enough.

So that's the engine block covered (in summary), what about fueling.

diesel is more viscous than petrol, so you'd need a stronger pump and a different filter (which the 'mechanic' has said, to his credit). Then the fuel pressure needs to be higher, so that's a change to the fuel pressure regulator. Then there's the injectors, which would probably need to be changed (at considerable expense), I'd expect petrol ones to clog if you ran diesel through them for too long.

The higher compression of a diesel engine introduces another problem, that is starting the engine. The starter motor on a petrol engine isn't beefy enough to work reliably with the higher compression required for a diesel engine.

If all that sounds complicated, what about the engine management computer, which is all programmed up to run the engine on petrol. The engine management computer is programmed to decide many times per second, how much petrol to inject, when to fire the spark plugs etc. It does this based on inputs from many, many sensors, including the 'knock' sensor that listens to the explosion in the cylinders, the temperature sensor, at least two (3 in newer cars) oxygen sensors etc. The computer knows what range all the readings should be in, and if anything is outside of the normal range, the computer decides there is a fault, and will either run badly (in 'limp home' mode) or not at all. I dread to think how one would even begin to convince the computer that the very different readings are actually ok.

All in all, I reckon your mate would be better off selling his car, and putting the money towards a diesel one of the same model, a diesel built as a diesel, by a manufacturer who knows how to build diesels.