Gasoline has a much higher antiknock index than diesel, so it is much, much harder to ignite from the heat of compression than diesel fuel. The higher the octane of the gasoline, the less prone to ignition due to heat or pressure. In short, to start on gasoline you'll need a carburator and spark ignition, or a compression ratio much higher than a regular diesel, which isn't the point really...
I'm building a diesel special soon, not VW but very similar in many respects, AX 1.4 idi diesel (excellent engine, bad alloy block). I'm using a petrol GTI cast iron block and thinking about putting the later 1527cc head on the 1360cc block, which will drop compression to 20:!, via a larger pre combustion chamber.I also am thinking about full ceramic treatment. I have read a research paper which showed that fuel economy of 5% better was achieved by dropping compression down to 15:1, of course starting in the cold may be hard but I'm not too concerned about that especially not at 20:!.Wanted to start a theory & pratical discussion on lowering compression ratio, enlargin the pre chamber and *possibly* reduce pumping frictional losses by increasing the bore of the entry hole to the pre chamber cup, anyone messed with this stuff?Oh by the way 1st stage is to see what mpg I can get from this motor, it did 71mpg in stock form before, I have several improvements to make and a couple on the vehicle as well (narrower taller tyres, better fluids, aero engine guard, electric brake vacuum pump if I can find one). 2nd stage may be a turbo to see how quick I can make it, so a low compression and coated engine would be good prep for that.I don't want to confuse with leaking rings/valves/worn out low compression, I'm talking a 100% fit and fettled engine, but working on bigger pre-combustion chamber size which is a different thing to an engine that won't start because all the compression is coming out the breather, obviously that's a big waste of power!Greg.
Are you in the UK?My father's AX does 86+ MPG imp on a run and abot 68mpg on the village farting about. It has a funny problem with the I/P where it surges like a turbo. [May have rcently stopped actually] Bosch or Citroen don't know what it is but I suspect something stretched inside. Strangely a couple of months ago someone gave two AX's away because the better one had developed this same fault and failed the MOT and she didn't have the time to r & r ... Missed them :cry: Dad's is running a CAV setup and these run with lower injection pressures than Bosch. I suspect that tuned up it will be more like a Renault 5 turbo Gordino (showing my age) and leave most Golfs behind. IMO good injectors running less pressure are better mpg wise.
well gasoline might not be the greatest option, but i'm sure there is something out there (other than quickstart...)i thought that the higher compression ratio requires more work to achieve, therefore lower compression ratio relieves that stress... so it's more efficient to have higher compression ratio's?? why do race gas engine's use higher CR's then???
Gasoline engines have an average compression ratio of 10:1, much lower than the 23:1 of a VW IDI diesel. The higher the compression on a gas car, the more prone it is to knocking, thus requiring high octane fuel or race fuel when a certain ratio is reached (12.5:1 and higher). Gasoline is much easier to ignite on compression alone, which is why gassers fear pre-detonation.
Yes, higher CRs are more efficient, which is why race cars use them. More hp and torque for the same displacement. However race cars have to use very high octane to prevent damage from the high CR. That fuel is much more expensive, and may still use lead to achieve those numbers. When I raced, the pump at the track said something like 4 grams, or maybe milligrams of lead per gallon, with 104 octane. It also cost $3.75/gallon back in 1994. That is one of the reasons diesels are more efficient, along with running lean, and the main reason they have more torque for the same displacement.