Re: Gasoline grade BTUs per gallon?

On Fri, 26 May 2006 13:48:08 -0400, "K. Jones"
<shadetree1999@xxxxxxxxxxxxxxxxxx> wrote:


"Bill Ward" <bwardREMOVE@xxxxxxxxxxxxx> wrote in message
news:445d50c8.5748823@xxxxxxxxxxxx
On 6 May 2006 17:15:40 -0700, "BobG" <bobgardner@xxxxxxx>
wrote:

BW:
Higher grade gasoline has about the same energy content, it just
allows a bit more efficiency from the engine.
=======================================
Can you quantify this somehow? To me efficiency is power out/power in.
If the power in is the same, how does it increase the power out?

Engine efficiency is a function of compression ratio (peak
temperature). Advancing the spark to ignite the fuel
earlier before top dead center raises the effective
compression ratio, but is limited by octane rating.

Lower octane fuel "pings" or detonates instead of burning
smoothly. The engine control unit detects the pinging and
retards the spark, preventing detonation, slightly reducing
the effective compression ratio and efficiency, but greatly
prolonging engine life.

When you use higher octane fuel, the engine has slightly
greater efficiency, because the effective compression ratio
is higher. The trade off is fuel cost. It's usually about a
wash in $/mile, but you do get higher peak performance (max
power) with higher octane, if that's important.

Bill, would it be more accurate to say an engines can "*be built* with
slightly greater efficiency by utilizing a higher effective compression
ratio"?
AFAIK, on a typical, un-modified, car engine, as delivered by the factory,
built to use say, 87 octane gasoline, you will see no "preformance
improvement" by using "premium" (say 91 octane).....more likely less
performance. It's only on engines that have a higher effective compression
ratio, that performance gains will be found.

I was using "effective compression ratio" to include the
effect of spark timing. The earlier the mixture is ignited
(to a point), the higher the eventual flame temperature, and
thus Carnot efficiency. If the octane is higher, _and_ the
ECU advances the spark, then you should in theory get a bit
more efficiency and peak HP.

I must admit I have never actually tested this theory
myself, but it seems plausible and is accepted by many
e.g.:

http://www.jandssafeguard.com/tech.html

If my engine is designed for "regular" gasoline, I'll get slightly more
miles/gallon, and a higher "max power" from regular gasoline, than I will
from premium.

The following link agrees with you:
http://www.automedia.com/High/Octane/Fuel/ccr20050501ok/1

It says:
"Unless you're driving one of the very small percentages of
performance vehicles with an engine designed specifically to
utilize the properties of higher-octane premium gasoline,
there is no performance gain in using premium."

I drive a little 2000 Civic Si, which says in the operators
manual:

"Use a premium unleaded gasoline with a pump octane rating
of 91 or higher.
If you are unable to find premium unleaded gasoline, you may
substitute an unleaded regular gasoline. The engine will
compensate for the lower octane, but you may notice a slight
decrease in power as a result."

Being cheap, I've used 87 octane only, with no problems at
all, but I never thought of the engine as "high performance"
per the above link.

So I guess it all depends on how you look at it - designed
for 91 octane with a degradation on 87, or designed for 87
with a boost on 91.

Thanks for the comments - I learn a lot in this NG.


Regards,

Bill Ward



.

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