Date: Sun, 13 Oct 2002 16:04:22 +0100
Reply-To: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Clive Smith <clive.harman-smith@NTLWORLD.COM>
Subject: Re: High Octane was: what's the best kind of gasoline?
Content-Type: text/plain; charset="iso-8859-1"
>Most detonation can be heard if you have the engine cover off and race the
engine while you're standing over it.
Sorry Stan, totally disagree!
a) Impossible to load the engine up sufficiently, although accelerating it
loads it up a bit.
b) Usually impossible to regulate rpm range under these circumstances, the
transient conditions might detect an incipient pinking, but not high speed,
high pressure detonation per se (what causes real damage in an instant)
c) Engine unlikely to be hot throughout, I mean hot throughout, including
exhaust valves and plug electrodes 'almost' incandescent.
d) The engine will never accelerate as fast as this under driving
conditions, so might even lead to backing off ignition off too much
(unlikely but true).
Pinking, whilst undesirable, is normally caused by (a whole multitude of
things but...) overly advanced ignition (maybe just for a small range of
throttle/rpm/load), carbon deposits glowing, too low octane petrol, poor
combustion chamber design, plug heat range way too hot and can occur at low
to medium speds and throttle settings (usually vacuum advance (if any) is
the culprit at low throttle if all else to spec) and ... probably forget
something else.
High-speed-detonation usually results at high engine speed!, high throttle
settings, and although slightly high heat range plugs can be the culprit, as
can too low octane fuel - (the danger when full throttle for long periods),
static advance settings combined with centrifugal advance might also set it
off. Whilst carbon deposits going incandescent are normally cleaned off at
these pressures and temperatures (why old granny's engine usually pinks when
first blasted out on the highway), other chamber excrescences and
irregularities including poorly seating or damaged exhaust valves do start
becoming liabilities under these conditions;
OR purely the exceedingly high pre-spark cylinder pressures (eg.
supercharged, tuned or racing engines) can exceed the octane tolerance of
the fuels;
AND once ignited (the truly dangerous detonation) the flamefront develops
unnaturally, through poor chamber design, lack of swirl and consequent
stagnation, and thus local rich spots in the gas. The result is an explosive
flamefront with colossal pressures generated, enough to test the engines
strength thoroughly, and certainly enough to shake up the top rings and
lands, loosen up any deposits which might stick under an exhaust valve,
exarcebating the problem in fractions of a second - in extremis rods can
bend or break (a Reno air-racing RR Merlin at boost pressures of circa
120-140 psi - if it detonates, will self destruct in a second, rods, crank
bent or broken even).
.. now this is the real difference between light pinking and high
speed/pressure detonation. The former is usually the result of early
ignition for whatever reason (with a flamefront that is still not at truly
explosive velocities); the latter the consequences of an EXPLOSION, where
the flamefront is not burning and advancing at typical speeds for the
mixture, but totally out of control, at least in small pockets. Local
richness might cause small detonation pockets that can do damage, but under
high speed if the majority of the advancing flamefront decides to explode -
you've got engine destroying detonation.
Thus the latter is unlikely to occur in our VW boxer engines, being mildly
cammed, untuned, low speed engines with plenty of safeguards (i.e. the plug
heat range isn't that critical, cylinder filling won't be good at high
speeds). Small scale detonation, and low speed, moderate load pre-ignition
(pinking) are likely when everything isn't just so and should be avoided if
possible, but don't usually damage the engine in a few seconds. Knock
sensors are primarily for supercharged engines with mapped ignition systems
that can be rapidly retarded (within a cycle or two) and those with a
particularly sensitive ignition timing point.
Note that one effect of a rich mixture is to effectively advance the
ignition, due to its increased flamefront speed and on say, motocross
2-strokes, not an effect one can ignore.
I bet you didn't want to know all that but I'm well aware that this is an
incredibly complex area with plenty of room for contention - this is why Sir
Harry Ricardo spent a lifetime reseaching such things, almost every engine
produced in the 30, 40's and 50's using data that came out of the Ricardo
labs at some time or other.
Rather than Research Octane Number, RON ought really to stand for Ricardo's
Octane Number, to salute his lifetimes work... a bit of a Brunel or Barnes
Wallis - but not so famous!
Clive
'88 Syncro Transporter
----- Original Message -----
From: "Stan Wilder" <wilden1@juno.com>
To: <clive.harman-smith@NTLWORLD.COM>
Cc: <vanagon@GERRY.VANAGON.COM>
Sent: Sunday, October 13, 2002 3:00 PM
Subject: Re: High Octane was: what's the best kind of gasoline?
> If you want to hear how that motor sounds and you don't have anybody to
> drive slow and lug the engine for you.
> You can find a tall brick wall or back of a building, get as close as you
> can and still open your drivers door.
> Drive along the wall at the speed required to reproduce the noise you are
> faintly hearing before .............. that noise will resound back at you
> from that wall like its amplified. Works the same for noises on passenger
> side you just need to leave the sliding door open to get the full audio.
> Most detonation can be heard if you have the engine cover off and race
> the engine while you're standing over it.
> A good detonation test is simply to get your front wheels against a
> parking stop and accelerate the van as if you were going to drive over
> the parking stop. This lugs the engine pretty good and you can detect any
> number of stress related noises from your van.
> Knock sensors are a common topic on this list but most vanagons with
> standard engines don't have them. I believe that they are on some in line
> 4 conversion engines.
> Be wary of any knock or ping your engine develops, it can be caused by
> overheating, overworking the engine or something showing its wear or that
> its going to soon fail.
> Then to negate everything I just wrote: There are ticking, clicking,
> taping, knocking sounds that are common to VW engines, I've never heard
> some of these noises from other automobile engines. A carbuerated engine
> will produce a complete different range of sounds that you don't get with
> an injected engine. It takes an educated ear with VW experience to
> identify many of these sounds and relate them to the proper engine part
> or function.
> If you suspect detonation, it can be caused by carbon build up on your
> piston tops and combustion chamber. Run 16 oz of Marvel mystery oil in
> your next tank of gas and possibly in the second tank as well. This
> should reduce the carbon deposits. There are other products that can
> perform the de carbonizing and I wouldn't discard them, I just prefer the
> Mystery Oil.
>
> Stan Wilder
> 83 Air Cooled Westfalia
>
>
> On Sun, 13 Oct 2002 13:08:33 +0100 Clive Smith
> <clive.harman-smith@NTLWORLD.COM> writes:
> > High speed detonation isn't pinking. You'll not hear it at full power
> > on
> > many engines, but it'll damage the engine in seconds flat, the
> > spark plug's
> > earth electrode often being the first to go (with inevitable damage
> > to
> > cylinder walls/rings) , then bits off the top ring land. Looking
> > for
> > microscopic ally spatter on the plug's ceramic insulator will
> > reveal
> > 'incipient' high-speed detonation - always cast a cursory eye over
> > your
> > plugs whenever they are out.
> >
> > Pinking now and again is a different set of cylinder conditions and
> > doesn't
> > in the short term cause a lot of damage.
> >
> > Clive
> >
> > ----- Original Message -----
> > From: "Andrew Grebneff" <andrew.grebneff@STONEBOW.OTAGO.AC.NZ>
> > To: <vanagon@GERRY.VANAGON.COM>
> > Sent: Saturday, October 05, 2002 8:31 AM
> > Subject: Re: High Octane was: what's the best kind of gasoline?
> >
> >
> > > >
> > > >The key is to listen for pinging. If you can't hear your motor
> > very
> > > >well, try warming the motor up thoroughly, then take the motor
> > cover
> > > >off, and drive up a fat hill in 4th. Ping? Retard the advance
> > slightly.
> > > >No ping. No worries.
> > >
> > > Be careful. Just because you can't HEAR pinging, it doesn't mean
> > that
> > > it itsn't happening (ie subaudibly).
> > > --
> > > Andrew Grebneff
> > > 165 Evans St, Dunedin 9001, New Zealand
> > > <andrew.grebneff@stonebow.otago.ac.nz>
> > > Seashell, Macintosh, VW/Toyota van nut
> >
> >
>
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