At 10:55 PM 6/26/00 -0400, Daniel Wallance asked:
>I have heard from a friend that if you have a car which takes super unleaded,
>that you can also put regular unleaded gas in the car. I have a jeep
>Cherokee and was wondering if this is bad to do for my car and for any car.
That's generally _not_ a good idea. It could cause terrible knocking (also
known as pinging).
To understand this, you need to know the difference between _ignition_ and
_detonation_. Non-experts tend to use the terms quite sloppily. If the
flame-front in the cylinder moves subsonically, it's called ignition. If
the flame-front goes supersonic, it's called detonation. Once it goes
supersonic, it tends to go VERY supersonic. Imagine your friend is sitting
on a playground swing and wants a push. You could give a gentle push
(ignition) or you could haul off and punch him with your fist
(detonation). If the total energy is the same in the two cases, the
detonation might be tremendously damaging.
>Also what is the difference between Regular unleaded, Super, Super + and
>Premium.
The whole point of high-octane gas is that it is resistant to
detonation. Detonation is more likely to occur at high pressures, high
temperatures, low engine RPM, and low-octane gas.
There is a chemical called iso-octane (more properly called 2,2,4 trimethyl
pentane; iso-octane is not the proper term in the modern chemical
nomenclature). By definition, this stuff is rated 100 octane. Heptane,
which produces a nightmarish knock in an ordinary engine, is rated zero
octane. Linear combinations (and extrapolations thereof) define the
"octane" scale.
Aside from octane, there can be various bad impurities and/or good
additives. Most of the advertised additives are just fairy dust; you
won't notice the difference.
Of course, if there is gross contamination, all bets are off. You might
think that this is a reason to avoid off-brand fuel. Think again. Last
December it was Mobil who sold a contaminated batch of aviation fuel in
Australia. It sure is a good thing Mobil merged with Exxon; we wouldn't
want to have _two_ incompetent and irresponsible companies, would we?
At 03:03 AM 6/27/00 -0400, Alex Azima wrote:
>I believe all you'd have to do is to reset the ignition timing, if you
>change from super to regular gas!
That's partly true under some conditions, but that's not all there is to it.
1) Depending on details of the design, a car might not run on low-octane
gas no matter how you set the timing. For instance, if the compression
ratio is high enough, it might Diesel even with no spark at all.
2) To accommodate a lower octane you would need to retard the timing, which
would
a) lower your mileage
b) lower your performance (acceleration and hill-climbing power)
c) cause the engine to run hotter, which may or not be a big problem
3) Most likely, you would also need to be careful never to fully open the
throttle at low RPM. This would
a) lower your performance, and
b) require some combination of MAP (manifold absolute pressure) gauge,
tachometer, electronic ping detector, and/or superhuman driving skill.
========================
Note that low-octane gas is cheaper but contains essentially the same
amount of raw energy, so that if you can make an engine that runs _with
comparable efficiency_ on low-octane gas, it's a good thing.
Saab makes some engines which can adapt to a wide range of octanes. Here's
roughly how it works:
*) It's a turbocharged engine. The turbocharger is under computer
control, so effectively they've got control of the compression ratio of the
engine.
*) It has no distributor. It has separate high-voltage electronics
attached to each spark plug all the time.
*) Following the spark, during the combustion phase, they continue to
run a little bit of current through the spark gap. The conductivity
depends on pressure, and they are measuring the pressure. This allows them
to detect the threshold of knocking.
*) Using this information, they continually monkey with the timing, the
mixture, and the boost limits, to give the best power and/or performance
safely achievable under the circumstances.
Result: If you buy high-octane gas for this car, it goes faster; if you
buy low-octane gas it's more economical.