Here is something I wrote to a radio personality I know about the
subject of overuse of premium gas. With fuel costs rising it’s
important not to overpay. I just bought a new Ford Ranger
pickup. It’s a two-wheel drive four cylinder standard
transmission (air conditioning the one option.) This 3,000 pound
curb weight vehicle gets an EPA of 24/29, just a hair below compact car
average, using regular unleaded gas. Here is the piece I wrote.
————————————————————–
I was just arriving at an appointment when on Tuesday your discussion
of premium gas started. On gas pumps you will find an “octane number”
with regular having maybe 89 and premium having maybe 94 or 95. You
might recall in the old days “Clark Super 100″ gas stations. These had
100 octane fuel.
Originally a compound that included lead was used to raise the octane.
This was phased out in the early 1970′s with unleaded since the lead
additive was considered a significant source of lead pollution.
“Unleaded” uses sophisticated refining teams to boost the octane.
To oversimplify, the higher the octane, the higher the “flash point” or
temperature of spontaneous combustion of the vaporized version of the
fuel. A higher octane fuel in vapor form also burns slower. We
basically want the fuel to ignite with the electric arc from the “spark
plug” not on it’s own. If the fuel ignites too late we have less time to
use the “power stroke” of each ignition cycle. At 7200 RPM, near the
safe maximum from most car engines, each cylinder had 60 power strokes
per second. The effective time to use the the power from each
individual combustion can be one or two thousandth of a second. To use
an analogy, think of pedaling a bicycle. Basically, you only get power
when the foot is pushing the bicycle pedal down.
Since the “power stroke” is of such short time car engines tend to use
ignition advance. If you get too much you get “ping” or “knock”. As
the sounds of the words implies “ping” is a minor “pre-ignition” and
“knock” is a major pre-ignition. With “knock” the engine is facing the
force of ignition before the piston reaches the “power stroke”. This
wastes energy and puts a lot of stress on the engine parts.
To further complicate things, If you raise the compression of an engine
the power goes up but it increases temperatures and pre-ignition risks.
Air heats when it compresses. Almost all cars today, except for a few
very high performance ones have far lower compression than the
higher-performance cars of the past. 9 to 1 or lower is normal today.
That is in the basic specs for the engine in your owner’s manual. My new
Ford Ranger pickup has an 8.9 to 1 compression ratio.
In the old days you used to have a “mechanical advance” ignition
distributor. You might recall someone hooking up a strobe light to
“time” the engine. For the past 15 years or so there is a microphone
that detects the very early “pings” and a computer that instantly
adjusts the spark advance to it’s optimal point. It’s rather like a
mechanic in the driver’s seat with a very keen ear who can constantly
adjust the ignition timing but electronics is much faster and more
accurate. By optimizing the ignition timing you get more power, better
fuel economy, less pollution and you avoid “knock” which is hard on the
engine. With premium the engine might run vary slightly better but you
would need a test bed to detect the difference, I’d guess well under 1%
which is well under the extra cost of premium fuel.
When is premium needed? It is needed for the older “high compression”
cars, mostly the vintage “muscle cars” and a few very high end
“performance sports” cars, think a Ferrari or something like that. It
is also helpful for cars that are “blown” meaning they have
turbochargers or superchargers. These use a fan to force air into the
engine rather than the normal vacuum air intake. This effectively
raises compression. Another reason for premium is “carbonization” in
engines. Essentially carbon “soot” can accumulate in combustion
chambers, less today with very lean fuel mixtures but it still can.
It’s more a problem here in the North because very cold starting
temperatures cause more carbon accumulation in the engine combustion
chambers. (Think of the balky lawn mower that had a spark plug covered
by a black soot). This raises the compression in the combustion chamber
and the edges of the carbon can retain heat causing pre-ignition. Thus,
ironically, when new a car might run fine on low octane but need higher
octane as it gets older. A good way to check this is to look at the
tailpipe. Is the inside grayish (good) or is there black soot on it?
Anyway, A good way to do a do-it yourself test is try using regular and
drive a car with a fully warmed up engine into an enclosed area. An
underground garage, late at night is good since it has up ramps so you
have a bit of a load. Turn off the radio, fans and any other background
sounds. Have windows open, accelerate as much as possible on the up
ramp and listen for “knock” (where it has a diesel engine” sound. If
you don’t hear it, lower grade gas should work fine. )
.
