Combustion - was Re: Propane EFI

[Greg Hermann]

>Uh folks, as far as energy per gallon is concerned, gas is gas, it is
>all the same.

Not at all true--each individual hydrocarbon has its own heat of
combustion. Generally, higher octane "rated" fuels tend to have slightly
lower heats of combustion than lower octane rated fuels. Diesel, for
instance, has a somewhat higher heat of combustion than a typical gasoline
blend. With mixtures to the rich side of lambda = 1, some hydrocarbons will
burn "hydrogen first" selectively more readily than others will, thus
giving  a higher heat of combustion per unit of oxygen present.

What octane measures is how fast it burns and to a lesser
>extent ignition pressure. Higher octane gives slower burning & more
>resistance to "spontaneous combustion". The secret to a SI engine is
>controlled combustion. Since the pressure rise is not instantaneous
>mechanical spark advance is needed. Since the burn rate varies with
>chamber pressure, you also need vaccuum advance. The goal is for peak
>cylinder pressure by about 10 degrees ATDC (varies a bit with engine
>geometry). This will vary for any fuel used.

All true.
>
>Might I suggest review of chapter 1 "Combustion" in Ricardo's "The High
>Speed Internal Combustion Engine" ? Is better than Obert

Personally, I feel that Obert is a bit more up to date than Ricardo,
especially his more recent edition.

Regards, Greg