Other posters have already noted that there's no advantage to using
higher octane fuel than necessary. Here's a quick introduction to
octane ratings to explain why:
Gasoline is a mixture of various hydrocarbons, each with somewhat
different characteristics, including different ignition points. Since
the ignition point of a type of gasoline is important to an internal-
combustion engine (as I'll explain later), gasoline is rated as if it
were a mixture of just two hydrocarbons, hexane and octane. Hexane
has a lower ignition point than octane; octane's is close to what
most car engines would want, but a little high. The octane number
represents (in theory) the percentage of octane in a pure octane/
hexane mix that would have the same ignition point as that gasoline.
So "87 octane" gasoline ignites at about the same point as a mix of
87% octane and 13% hexane. The actual octane number, in the US, is
the average of the Motor Octane Number and the Research Octane Number
(which is why you may see the notation "(R+M)/2" on the octane
sticker), which are two standard ways of determining the octane
Why does the octane rating matter? In the cylinder, the fuel vapor
should ignite when the spark is emitted from the plug; and the burn
should propagate evenly out from the spark in a smooth wave. If the
vapor ignites prematurely (before the spark, or elsewhere in the
cylinder at the same time as the spark), you'll get less than ideal
performance - the piston may still be on the compression stroke, for
example, so the burn is forcing it the wrong way - and "knock", which
is the sound of an engine that's firing at the wrong time.
In an older engine, if you get premature ignition, you hear knocking,
get reduced performance, and risk engine damage. Newer engines have
knock sensors, and if they detect knock they'll retard the ignition
timing to compensate. That will generally eliminate the knock but
reduce performance. So using a fuel with the right octane number
will increase performance over one with too low a rating, but going
any higher won't improve things any further.
The higher the compression in the cylinder, the more likely the fuel
is to ignite, because of higher density and temperature (by Boyle's
Law). So high-compression engines need fuel with a higher ignition
point, which means a higher octane rating.
High-compression engines get more output per unit volume, but they're
more expensive (because they have to withstand higher stresses), so
they're generally found on more expensive models. Thus it's common
for more-expensive cars to take more-expensive gasoline.
Michael Wojcik email@example.com
Unfortunately, as a software professional, tradition requires me to spend New
ok, so if that's true, how come you can't run a diesel engine on
gasoline? the compression is much higher and the adiabatic heating is
much greater in diesel engine....
rubbish. the compression on even the most high compression si engine is
low compared to diesels. the price differential comes down to paying a
premium for "performance", but that's nothing to do with having to build
an engine that can cope with compression stress.
You seem to be assuming that, simply because air is compressed in a diesel,
with diesel fuel (ordinarily) then injected "at the right instant," that the
timing could not be messed up via the use of a fuel with a much higher
resistance to ignition and/or different ignition properties.
I wouldn't assume this.
The ignition properties of the two fuels are too different.
(This of course contrasts with a gasoline engine, where fuel/air mixture is
compressed together, and the spark causes ignition. The timing of the spark
is key. Whereas with a diesel engine, the timing of fuel injection is key.)
You're saying any old fuel you inject into a diesel engine will ignite at
the same instant as diesel fuel?
First, ignition is not "instant." Ignition may start, but full ignition of
all the fuel in the cylinder at any instant takes a certain amount of time,
occurring over a certain number of degrees of the diesel cycle.
There is a certain "rate of burning" (or "rate of ignition") that will vary
with the fuel used.
The difference in "ignition rate" (or "rate of burning") is significant for
gasoline vs. diesel fuel. Using gasoline in a diesel engine will mess up the
diesel engine's timing, with a potentially (highly likely?) catastrophic
outcome (massive engine damage).
As usual, this is not rocket science and is amply discussed in texts
(including the net) on diesel vs. gasoline engine design.
WHY did DIESEL come up in the first place,if you're talking about an RSX?
AFAIK,there are NO diesels offered for that model.
Thus,discussion of them is NOT RELEVANT.
Does Acura/Honda even offer any diesels??
(and who's going to run gasoline in a DIESEL????)
no, that's you putting words in my mouth for the sake of picking a fight.
eh? ignition /is/ instant. combustion takes time. you're confusing
the two terms, seemingly because it suits your pitch.
dude, you can't use gas in a diesel because the ignition temperature of
gas at those pressures exceeds the adiabatic heating temperature.
unlike with diesel fuel. which is of course why "diesel" fuel is used.
yeah. i own several of those strange "text" thingies. and have
bothered to read them.
I am not yet ready to agree with you that putting gasoline into a diesel
engine is likely to result in premature ignition. Jim is right insofar as
the fuel enters the diesel cylinder at the same time whether it be gasoline
or diesel fuel. (I imagine you're aware of this, too, if you have any
familiarity with the diesel cycle.) Gasoline, however, is designed to be
more "auto-ignition resistant under pressure." Octane is a measure of its
resistant to auto-ignition. Higher octane = less knock, as I think most
people here know. Diesel fuel doesn't have an octane number but instead has
a cetane number, which measures something different... google yada.
The timing will be off because the gasoline requires a spark or, if it does
ignite, it will do so at the wrong time and burn at the wrong rate. You said
Michael's wording was a little off. He didn't deserve to be jumped all over
and told his comments were rubbish, etc. His post needed a bit of tweaking,
though I think he would have served the group better if he simply cited a
well-written web site or two, preferably one that was a dotcom or maybe
dotedu, so reputations were at stake.
I've not seen propogation fronts of the different fuels in the same
engines. and i've not seen, or driven a diesel now since i moved to
the US, so i'm a bit rusty. Never realy been into diesel engines
myselfso my basis is mainly theoretical, but my gut feeling is, were
we to see the flame wave propigation in slow motion, we'd find the
petrol (gasolene) irniting before, probably at the injector tip, and
would continue throughout the pulse of the injector. This, to my mind,
would also produce a very smokey combustion, due to the limited
combustion area, and thus poor oxygen availability.
I would also be interested to see how the new highly refined, high
performance diesels that are becomming not so much commonplace, as the
norm (something like 33% of new cars in the UK are diesels) in
comparison with the older, noisier smokier designs more commonplace in
the US, would handle gasolene injection. I have a good video somewhere
of a kid who's running a late 70s golf GTi (rabbit to americans) and
has just put a couple of gallons of diesel in the tank, and is driving
home. Lots of white smoke everywhere.
Perhaps so. In the fifteen years or so that I've been posting to
Usenet, I've certainly written messages I've later regretted, and
many others that could have been better. I don't feel this was
one of the former, but it probably falls into the latter category.
Michael Wojcik firstname.lastname@example.org
Sure we're tossing out fluff, but tell me, where does anyone deal in words
Would someone please define "adiabatic heating temperature"? Please provide
your source for this definition. What is the approximate number, in degrees
F, for the "adiabatic heating temperature" (AHT) of gasoline at the
pressures with which we are concerned here?
Or is Jim just being sloppy?
I know what "adiabatic heating" is, but the statement above seems to be
implying the AHT phrase altogether is part of engineering or scientific
Rather than impetuously call this statement "rubbish," he or someone gets a
chance to tweak it.
Motorsforum.com is a website by car enthusiasts for car enthusiasts. It is not affiliated with any of the car or spare part manufacturers or car dealers discussed here.
All logos and trade names are the property of their respective owners.