Re: Bonzo caught lying again



Cliff wrote:
On Sat, 02 Aug 2008 16:40:07 -0700, BottleBob <bottlbob@xxxxxxxxxxxxx> wrote:


Cliff wrote:

<snip>

Higher octane does not get better MPG either AFAIK.

The correct response to that would be, "it depends". This being a timely subject these days, I'll make a few comments.
Lower combustion pressure engines (normally aspirated low compression ratio engines) that do not experience detonation with low octane gasoline will not benefit from using a higher octane fuel. This is NOT one of those cases where you will automatically get what you pay for. The octane rating of a gasoline relates to its ability to resist detonation (knock), it does not relate to its energy content OR its flame front speed. In other words, the common misconception that higher octane gasolines reduce detonation because they are "slower burning" is a myth.

Is it?

Cliff:

So it would appear:

==================================================================
http://en.wikipedia.org/wiki/Octane_rating

Octane rating has no direct impact on the deflagration (burn) of the air/fuel mixture in the combustion chamber. Other properties of gasoline and engine design account for the manner at which deflagration takes place. In other words, the flame speed of a normally ignited mixture is not directly connected to octane rating. Deflagration is the type of combustion that constitutes the normal burn. Detonation is a different type of combustion and this is to be avoided in spark ignited gasoline engines. Octane rating is a measure of detonation resistance, not deflagration characteristics.
==================================================================

==================================================================
http://www.faqs.org/faqs/autos/gasoline-faq/part3/preamble.html

The antiknock ability is related to the "autoignition temperature" of the hydrocarbons. Antiknock ability is _not_ substantially related to:

1. The energy content of fuel, this should be obvious, as oxygenates have lower energy contents, but high octanes.

2. The flame speed of the conventionally ignited mixture, this should be evident from the similarities of the two reference hydrocarbons.
Although flame speed does play a minor part, there are many other factors that are far more important. ( such as compression ratio, stoichiometry, combustion chamber shape, chemical structure of the fuel, presence of antiknock additives, number and position of spark plugs, turbulence etc.)
Flame speed does not correlate with octane.
==================================================================

The Octane has a higher molecular weight IIRC. This may then
take longer to fully combust.

You seem to be confusing molecular Octane with octane ratings in modern gasoline formulations.


BUT, and this is a BIG but; *IF* you are running a high combustion pressure engine (either a high static compression ratio OR low compression ratio with significant boost from a turbo or supercharger),

Such as a diesel?

I was talking about gasoline engines. Diesels use a different fuel rating system called cetane number, which measures the time between injection of diesel fuel and the beginning of combustion.


which IS prone to detonation, then using a lower octane fuel that causes the engine management system to retard the spark and/or richen the mixture... you'll be running the engine inefficiently and no-doubt hurting your power AND mileage. You may never hear any pinging/knocking due to the speed of the electronic knock-sensing and retardation system, so might be led to believe that lower octane fuel is more than adequate, when it may not be.

Nobody said that lower octane was better for high compression.

Someone claimed that using a higher octane gasoline doesn't give an increase in MPG. I just showed a case in which switching to the use of a higher octane fuel, in a high combustion pressure engine, DOES give better power & MPG.

--
BottleBob
http://home.earthlink.net/~bottlbob

.

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