Yes, you could be wrecking your engine if the compression ratio can't
take the faster burning fuel. In simple terms, it goes bang faster
than lower octane ratings, so the engine must be flexible enough to
take it.

Quite the opposite. Higher octane fuels are able to take higher compression
ratios without exploding.

Basically, a diesel engine works by compressing the mixture enough so that
it could explode without a spark (it doesn't stick the fuel in until the
compression's done for that reason). Diesel engines use high compression
ratios (i.e. 20:1) to ensure that the fuel will definitely spontaneously
burn when injected. They also prefer low octane fuel that will burn without
encouragement, although with sufficient compression and a suitably designed
fuel system, they will burn any old filth, including high octane petrols.

A petrol engine doesn't want the fuel will go off early. It wants the burn
to start when the spark fires. To do this, you either have to reduce the
compression ratio (8:1 or thereabouts) or use higher octane fuel that
doesn't explode under pressure.

If you use a high octane fuel in a low compression engine, you're just
wasting money, as neither the high octane nor the cheaper fuel would have
exploded anyway, so you get no benefit whatsoever from the increased octane
rating.

The octane rating of a fuel gives little indication of its energy content,
except that most higher octane substances have lower energy content. This is
especially true when oxygen containing compounds are introduced, as the
oxygen can't be burnt, so is "wasted" mass. However, such compounds have
truly excellent octane rating. Ethanol, for example, is 130 octane, but only
contains something like 60% of the energy.

Christian.

Christian McArdle wrote:
The octane rating of a fuel gives little indication of its energy content,
except that most higher octane substances have lower energy content.

Heptane has an octane rating of 0 IIRC, so where does that
leave the above peculiar statement?

The octane rating of a fuel gives little indication of its energy
content,
except that most higher octane substances have lower energy content.

Heptane has an octane rating of 0 IIRC, so where does that
leave the above peculiar statement?

Well, for a start, n-heptane with a RON of 0 has more energy content than
iso-octane with a RON100, so it fits into the general pattern. However,
there are genuine counter-examples out there, although most just have a
lower octane than you would expect for the energy. Finding a high octane,
high energy fuel is more difficult!

The common octane improvers, such as benzene and toluene, which have octanes
in excess of 100, have lower energy content. Toluene, for example, has
40.5MJ/kg, compared to 44.4MJ/kg for iso-octane.

When you start adding oxygen (i.e. alcohols, rather than pure hydrocarbons),
you get much lower energy content for a massive increase in octane.

Christian.