On Thu, 21 Apr 2011 07:24:31 -0700 (PDT), "
wrote:
If it ran lean, it would be running too hot. If it ran lean the plug
temperature would have shown it burning white. Only once did I add 5%
alcohol by volume, the day it didnt go up a grade, its was always
normaly 1.25% or so, a Liter per 20 gallons, and that liter made a
dramatic difference in Europes crap gas. Knocking beat it up, and when
I blew it Â*the plugs-cilinders were running way under normal temp from
lower compression,
MAybe I'm confused here, but AFAIK, the compression ratio is
determine by the engine parameters and is fixed. It doesn't
vary by fuel used. Compression ratio is the volume of the
cylinder with the piston fully down divided by the volume
with it all the way up, no? What kind of fuel the engine
can safely run on then depends on the compression ratio
as well as fuel/air ratio, timing, etc.
You've got it pretty close.
"mechanical" or calculated compression ratio is the ratio of the
volume of the cyl with the piston at bottom dead center to the volume
of the cyl with the piston at top dead center (or swept volume plus
clearance volume devided by clearance volume)
ACTUAL or EFFECTIVE compression ratio is generally reduced somewhat by
2 factors. On old engines, cyl leakage reduces the effective ratio -
and more at low RPM than at high RPM.
Cam timing also reduces the effective compression ratio at low speeds
- the hotter the cam (the more overlap) the lower the effective
compression ratio, and the lower the compression pressure.
Since detonation is always more of a problem under high cyl pressures,
you usually experience it under load at lower RPM - where thankfully
the effective CR is lower, - so you can have an engine with a hot cam
and 15:1 CR that will run on 92 octane without a problem in a light
car, and an engine otherwise the same, but with a mild cam and 12:1
compression in a pickup truck, that pings on 97 octane.
Octane requirements also change with combustion chamber design. High
turbulence chambers, with lots of "squish" and "quench" (which
generally translates to higher CR as well, due to design restrictions)
can require lower octane at higher compression ratios than open
chambers.
Oil consumption can RAISE the octane requirement of an engine - partly
because the oil has a much lower octane, and partly because it SLOWS
DOWN combustion. SLOWER combustion, NOT faster combustion, is most
likely to cause detonation. The theory that faster burning fuels lower
octane, and slower burning fuels have higher octane is a total
missunderstanding of the detonatipon phenomenon, and octane
equivalency of fuels.
The longer the "end gasses" stay in the cyl, the more likely they are
to detonate, because they absorb more heat and are subjected to the
high pressures longer. If the fuel is fully burned, there are no "end
gasses" to disassociate and detonate .
The other reason older, worn out engines can require higher octane
fuel is engine deposits. If lead, carbon, etc have built up in the
engine combustion chamber, 2 things (can) happen. Compression ratio
can increase because the clearance volume is reduced by the
accumulation. This can cause, or at least contribute to, detonation.
The carbon can glow hot, causing "pre-ignition" which is similar to,
different than, often mistaken for, and can contribute to -
DETONATION.
Badly worn engines can also have narrow "valve margins", with almost
knife edges on the valves - which can also overheat, causing
pre-ignition.
Just remember - preignition is NOT detonation, but preignition can
contribute to detonation - and detonation can contribute to
pre-ignition. Pre-ignition is independent of ignition timing, and
happens BEFORE the spark.
Detonation happens AFTER the spark.
Pre-ignition is caused by (among other things) overheating, while
detonation CAUSES overheating.
If you have an engine instrumented with exhaust temperature guages and
cyl head temp guages, exhaust temperature will DROP when either too
rich or too lean, and the interesting thing is, when you have
detonation, the Cyl head temperature will climb, while the exhaust
temperature drops. This is a common way to determine if an aircraft
engine has reached the point of "incipient detonation" - which means
"get the throttle back NOW!!!!!"
and probably in need of timing, it was an old car
using oil. The amount of alcohol I added and the type of city driving
it was getting I cant see 1.25% of alcohol affecting it negatively,
but in performance it was great. .- Hide quoted text -
- Show quoted text -