The principals are surprising simple. Different fuels burn at different rates. Diesel burns a lot slower than petrol and is more difficult to ignite. This is the same for paraffin, kerosene, heating oil etc. For engines to get around this different burn rate, compression ratio is lowered to a level where basically you don't get any pre-detonation problems. The lower the compression ratio, the less oxygen there is mixed with the fuel, and so the bang is less violent. However, this has its own problem. By lowering the pressure and with a smaller bang you have less power output. To get around this, the engine has a long stroke crankshaft, so the power stroke lasts considerable longer, making up for the power loss.
Think of a playground 'see saw', the closer you are to the centre, the more effort is required to lift the other end. But the further away you are from the centre, considerable less effort is required. You could theotically continue until away from the centre until a feather is enough to lift a one tonne block.
There is a down side. The longer you make the stroke, the further the piston has to travel. This is known as 'piston speed aspect', this aspect cannot increase beyond a certain speed. To get round this, multi-fuel engines rev less, probably around 4000 rpm is the norm. Try and push them beyond this for a long period, and your engine will go upstairs, to see the master mechanic with the big white beard! In other words things will start working loose. But don't worry, all engines are fitted with a throttle butterfly pull off valve. Connected via the camshaft, the carb butterfly valves will shut off at a certain speed.
Why aren't all engines like this? Perhaps 20 years ago before emision laws, you could have done this. Now, not a chance. Emission levels are such now days that nearly clean air and water are coming out exhausts. But back in the eighties this wasn't the case, and fuel was cheap. The other problem is multi-fuel engines tend to be physically bigger, and this its self would caused major problems. For example the six litre Zil engine is 75mm longer and 100mm deeper than an 8 litre cadilac engine.
What exactly is multi-fuel?
It is exactly what it says, a mixture of different fuels, but no less than 50% petrol.
Can I just use diesel?
No, the engine must have petrol, to ignite fuel. If you diesel it won't start.
I'm concerned by using the wrong fuel, I might damage the engine.
Don't worry, you can't. The worse scenario is the engine just won't start or will be under powered. This will cause no harm.
Will the engine smoke running on mult-fuel?
It could, depending on how much diesel you put in the fuel.
You mention about free fuel in your adverts. How can you get free fuel fuel?
Ok, you need to go to a reasonable sized garage, they should have a waste policy for disposing of cross fuel mix. This fuel is where somebody has filled their vehicle with the wrong fuel. This fuel is now useless unless your engine is multi-fuel. The garage has to pay to have this fuel disposed of. If you ask whether you can have this fuel. Most will only be to happy to oblige. If you are good with you negociating skills, you could even get paid for removing the fuel.
The good thing is, petrol vehicles have a smaller size filler. And so all multi-fuel will contain a high percentage of Petrol. The diesel pump nozzle is to big to fit in a petrol filler car.