Subject: Re: 2009 A4 TSI
Armanmik at earthlink.net
Thu Jun 5 09:50:45 PDT 2014
> Subject: Re: 2009 A4 TSI
> As far as I know, the energy content (BTU) is the same regardless of the
> grade of gasoline used. The difference comes in the octane rating which
> measures the gasoline's resistance to ignition. The reason premium is
> almost required for turbo engines is because the cylinder pressures
> generated by the forced induction increase the likelihood of pre-ignition,
> where the fuel ignites before the piston is in the power stroke. This is
> also known as knocking and is very bad for the engine. When knocking is
> detected, ignition timing is retarded (pulled) so the spark doesn't occur
> until later in the combustion cycle. This leads to more wasted fuel, poorer
> fuel economy, and less power because the gasoline doesn't have a chance to
> burn completely. When using good quality fuel, the ignition timing can be
> advanced a little, where the spark occurs sooner in the combustion cycle.
> The fuel can then be burnt more completely, allowing for more power and
> increased fuel economy. Most modern engines are sensitive enough that they
> can detect a higher-octane fuel and adjust ignition timing accordingly.
> I may have this completely backwards.... feel free to correct me if I do.
Actually, you have it almost completely forwards ;-)
The difference between regular and premium (high octane) gas is that high octane gas has additives
which suppress pre-ignition. One of them is Tetra-Ethyl-Lead or TEL, a/k/a lead based chemical
When the fuel-air mixture is compressed on the compression stroke, it gets hot. If it gets hot
enough, it will fire off by itself without waiting for the spark. This is pre-ignition, and it
breaks engines, sometimes spectacularly.
The higher the compression ratio and/or the hotter the incoming fuel-air mixture the greater the
tendency towards destructive pre-ignition.
High compression ratios are used to get more horsepower out of a given displacement engine. High
octane gasolines let us use high compression ratios (to get more horsepower) without risk of engine
damage. There is NO more "power" in high octane gasoline than in regular - I can fill my riding
mower with 100 octane avgas and it won't mow my lawn so much as one second faster.
Turbocharged engines are worse, and need high octane gasoline even more. When the turbo compresses
the fuel-air mixture (to jam more into the cylinder) it gets hot. This is a double whammy - the
incoming fuel-air mixture is already hot, and now we compress it again and even more in the cylinder
- instant pre-ignition. That's why we use an intercooler to lower the temperature of the fuel-air
mixture as much as possible before we stuff it into the cylinders, and that's also why turbo engines
usually run LOWER compression than the equivalent non-turbo engines (as well as using different cam
profiles, usually with less overlap to prevent the compressed fuel-air mixture from blowing right
out through the still-open exhaust valve). They have to, just to survive.
Engines are equipped with knock sensors to to tell the CPU to retard the ignition timing slightly.
They run right on the ragged edge of pre-ignition, and retarding the timing even a few degrees helps
a lot. You will notice that knock tends to occur when you are going fast or open the throttle under
load, such as going up a hill. The engine is now working harder (to make more HP) and making more
HEAT, which is what causes the pre-ignition (knocking). Remember that a significant portion of the
energy in the fuel is wasted as heat, and the more fuel you use to make horsepower, the more heat
you'll also make.
There are a number of ways electronic trickery can be used to run regular gasoline with very high
compressions. One of them was the "stratified charge" system, which could run regular gas at
compressions as high as 11 to 1 (and I think I heard it worked up to 14 to 1). This was done for
fuel economy, not horsepower, and allowed the engine to use very little fuel at small throttle
openings. The newer Direct Fuel Injection systems can also do this.
Does an engine run better on high test than regular? Maybe. Maybe not. Cheap regular is sometimes
claimed not to have various engine cleaning additives, but this may be marketing hype. I do know
that at Port Everglades, the fuel for everyone's brand (including the no-names) comes right out of
the same giant tank . . .
On engines that require high test, sometimes the timing gets retarded to stop the pre-ignition, and
the CPU isn't smart enough to try again later and re-advance the timing to see if the knocking has
stopped. As a result, the engine is running with the timing retarded somewhat, and won't make
There's also the proven "it cost more so it must be better" syndrome at work here plus a good dose
of FUD factor. Nobody likes to admit they're wasting money, and it is not likely that many people
have done deliberate A/B tests under properly controlled conditions.
Basically, if you have a high compression engine which requires high octane (92 octane or more)
fuel, that's what you should be running. If your engine is rated to run on 87 octane regular,
running high test is a waste of money.
90V8Q, and almost 50 years of fussing with gasoline engines and collecting and reading engineering
literature on them as well.
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