Cost of gas is beginning to hurt

In article . com,
wrote:

On Apr 27, 6:34 pm, "Morgans" wrote:
"JGalban via AviationKB.com" wrote

That's what I figured. Turbocharged engines are a different kettle of
fish.
The main reason is the higher temperatures in the induction system lowers
the
detonation margin considerably.

Not just the temp, but the pressures involved.

In a turbo, not producing boost, a low compression ratio is fine with low
octane gas. When the boost is putting all of that extra air and gas into
the combustion chamber, it is still compressing at the same ratio. You end
up with the normal internal pressure, plus the extra pressure the turbo
boost shoved in there. Then detonation becomes a big problem, without the
extra octane.

But you knew all of that, already. g
--
Jim in NC

If the engine is turbo "normalized", it never increases the boost
above what the engine would see at sea level power, right? That's why
turbo aircarft engines are rated at the same max power as non turbo
engines. If the engine doesn't need high octane gas at sea level, why
would it need it at altitude where the cylinder pressures are no
higher (merely boosted back to sea level MP). Am I confused?

Regards,
Bud

Even turbo normalizing increases the temperature of the fuel/air mix
entering the cylinders, due to compression of the air to achieve
"normal" pressures. The increased temperature increases the octane
requirements, all by itself.

On Apr 28, 9:09 am, Orval Fairbairn wrote:
In article . com,





wrote:
On Apr 27, 6:34 pm, "Morgans" wrote:
"JGalban via AviationKB.com" wrote

That's what I figured. Turbocharged engines are a different kettle of
fish.
The main reason is the higher temperatures in the induction system lowers
the
detonation margin considerably.

Not just the temp, but the pressures involved.

In a turbo, not producing boost, a low compression ratio is fine with low
octane gas. When the boost is putting all of that extra air and gas into
the combustion chamber, it is still compressing at the same ratio. You end
up with the normal internal pressure, plus the extra pressure the turbo
boost shoved in there. Then detonation becomes a big problem, without the
extra octane.

But you knew all of that, already. g
--
Jim in NC

If the engine is turbo "normalized", it never increases the boost
above what the engine would see at sea level power, right? That's why
turbo aircarft engines are rated at the same max power as non turbo
engines. If the engine doesn't need high octane gas at sea level, why
would it need it at altitude where the cylinder pressures are no
higher (merely boosted back to sea level MP). Am I confused?

Regards,
Bud

Even turbo normalizing increases the temperature of the fuel/air mix
entering the cylinders, due to compression of the air to achieve
"normal" pressures. The increased temperature increases the octane
requirements, all by itself.- Hide quoted text -

- Show quoted text -

Never disagreed with this. It is the assertion that the turbo "is
putting all of that extra air and gas into
the combustion chamber" that is of contention. The total mass of fuel
(air + gas) the cylinder injests is the same as a normally aspirated
engine at sea level. That is why having a good fuel flow guage is such
an important factor in running lean of peak, and is a major part of
TAT turbo installations. Heck, when doing the lean test they
recommend, monitoring fuel flow is the primary issue. The poster said
it is "not just the temp". It is. It is also why intercoolers are so
nice. They reduce the work the turbo has to do, and increase the
detonation margin without lowering power output.

Regards,
Bud

Orval Fairbairn wrote:

Even turbo normalizing increases the temperature of the fuel/air mix
entering the cylinders, due to compression of the air to achieve
"normal" pressures. The increased temperature increases the octane
requirements, all by itself.

Correct. That's why I only mentioned inlet temps. I've lived in the
desert most of my life and I get a demonstration of this effect every summer.
I have two vehicles that are designed to run on regular grade fuel. In the
middle of summer, when the weather man says it's 115F, temperatures above a
hot layer of asphalt hover in the 130F - 140F range. Both my truck and
motorcycle will experience detonation under load if I use 87 octane in the
summer. The motorcycle being worse, as it is air-cooled and has less control
over cylinder temps. I have to run at least 89 octane to avoid detonation.
In the winter time, I can run 87 octane without a hint of detonation.

John Galban=====N4BQ (PA28-180)

--
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