[Vwdiesel] Diesel octane or why can you run a diesel engine lean?

Thu Mar 17 12:51:41 EST 2005

While we may be varying in semantics, and yes, absolute EGT will vary with
specific installations, the curves for mixture, EGT, CHT do not change
their relationship one iota between different gasoline powered engines.
Normally aspirated engines in aircraft burning 100 octane avgas have been
demonstrated by highly instrumented testing to not detonate at all. GAMI
and related companies in Ada, OK have thoroughly investigated this realm
and detonation simply does not occur. The area between about peak and 75
rich of peak is very hard on the engine at power settings above 75%
continuous rated power because the CHT will run very high, as will the
peak cylinder pressures. I think you are splitting hairs as to whether
fuel or air cools the combustion by limiting the peak combustion
temperature or by other mechanisms. The facts are that the cylinder head
runs cooler with mixtures at some distance either side of peak EGT, and
the peak CHT occurs at about 40 rich of peak EGT, which is also where peak
cylinder pressures occur and you are closest to detonation.
Detonation in ground based vehicles is very different because they are
operated very differently, mostly with partial throttle, and often with
significant wheel loadings, so that a full throttle application while
going up a hill generates very high peak cylinder pressures in a way that
simply does not occur in aircraft. Ground vehicles have to limit those
peak pressures by mixture, better (liquid) cooling, EGR, spark timing,
etc.
Fortunately for the vehicle owner, ground vehicles produce very audible
noises when detonating, something that is not audible by humans in an
aircraft.
Val Christian said:
> Kelly,
>
> Before the general statements...about 50 degrees rich of Peak EGT is
> where the most detonation in most aircraft engines occurs.
>
> The general statement:  Peak EGT is not assured to be stoichiometric.
> Stoichiometric burns can occur rich or lean of peak, although usually
> lean of peak.  Again, it depends on your engine.
>
> Another general statement: Cooling with fuel...is a misnomer.
> The physics doesn't support it.  However, there is clearly a different
> absorbed tempterature (CHT) and expelled heat (EGT) with different
> F/A ratios.
>
> Kelly, it may be semantics, and we could be close to saying the same
> things, but I have a problem with the "cooling with air" or "cooling
> with fuel" statements.  [Why not just inject water for cooling?
> Yes, water injectors exist, and are used for a different purpose.
> But heating fuel alone is not an effective way to cool.  Besides air
> usually costs less.]
>
> The real point is that the instrumentation is just one parameter, and
> while it is correlated to F/A ratios, the mapping varies situationally.
> Peak EGT might be one mixture in one engine, and in another production
> engine installation, that same mixture may occur at a different EGT point.
>
> Val
>
>
>
>>
>> Val,
>> I wasn't referring to the garbage the FAA puts out.Their stuff comes
>> from
>> pre-WWII myths. Peak EGT is stoichiometric. About 70-100 degrees lean of
>> peak is about as lean as you can get an engine to sustain combustion
>> without
>> extraordinary measures. The hottest place(CHT) to run an engine is
>> approximately 40 rich of peak. Rich of there you are cooling with excess
>> fuel, lean of there you are cooling with excess air. You are right that
>> the
>> mixture does affect combustion speed and peak pressures in the cylinder.
>> The
>> above information comes from the old Wright performance manuals for the
>> R3350 engines that delivered somewhere around 3700 hp with a fuel
>> efficiency
>> that hasn't been seen since, diesel or gas.
>>
>> ----- Original Message -----
>> From: "Val Christian" <val at swamps.roc.ny.us>
>> To: <vwdiesel at audifans.com>
>> Sent: Wednesday, March 16, 2005 10:28 PM
>> Subject: Re: [Vwdiesel] Diesel octane or why can you run a diesel engine
>> lean?
>>
>>
>> Kelly,
>>
>> My discussion was relative, and when I wrote I was assuming the rich
>> side of stoichiometric.  After all, the lean side of peak on a
>> aircraft engine (at a normal setting) is still richer than
>> stoichiometric.
>> Peak EGT is near the higher speed burn of a lean(er) mixture, and
>> because the flame propagation is faster, the cylinder pressure peaks
>> higher.
>>
>> I know we call it "extra fuel" to cool the engine, but primarily, it's
>> running cooler because of a different burn characteristic of the richer
>> mixture.
>>
>> The aviation community, including the FAA, have propagated myths, as
>> a mechanism to have pilots remember things.  A good example of this
>> is the old myth that there is less oxygen at altitude at night, so
>> you need to get on oxygen sooner at night, lest you get dizzy and
>> can't fly right.  The scientific reality is that the O2 levels are
>> nominally the same at any given altitude, day or night.  The exception
>> is close to the vegatation level, and we're not flying there often.
>> At 5000 feet the difference between day and night is less than 1%.
>> The myth that oxygen depletes at night was a way of encouraging pilots
>> to use O2 earlier at night.  Since O2 sat levels correlate with
>> visual sensitivity, it makes sense to get on O2 earlier, especially
>> when flying pilotage, is desolate areas.
>>
>> The myth of extra fuel cooling the engine, is simply a method of helping
>> pilots remember that enrichening a mixture lowers temps.  An extra
>> gallon of fuel per hour vaporized, doesn't make a cooler running engine.
>> But burn the fuel going into the engine differently, and there
>> is a BIG change.
>>
>> I'll get off the soapbox.
>>
>> Val
>>
>>
>> >
>> > Actually lean mixtures burn slower than stoichiometric mixtures. There
>> is
>> no
>> > problem with lean mixtures done right. The  problem is with mixtures
>> to
>> > close to peak EGT at high power causing peak cylinder pressure to go
>> very
>> > high and cylinder head temps to rise above safe temps. Fuel injected
>> > aircraft today run lean of peak all day long, except at takeoff  power
>> when
>> > they need the extra cooling of a mixture more than 200 degrees rich of
>> peak.
>> > KM
>> > Com pilot and A&P mechanic
>> > ----- Original Message -----
>> > From: "Val Christian" <val at swamps.roc.ny.us>
>> > To: Sent: Wednesday, March 16, 2005 9:42 PM
>> > Subject: Re: [Vwdiesel] Diesel octane or why can you run a diesel
>> engine
>> > lean?
>> >
>> >
>> > >   Not quite.  :-)
>> > >   You can't run a gasser lean except under certain circumstances
>> that
>> > > have to do with air density, which the pilots can much better
>> 'splain,
>> > > because
>> > > the fuel is used as a coolant.  Gas evaporating as it comes in, is
>> used
>> > > to cool the valves and even the intake charge.  Helps with
>> detonation
>> > > and preventing burned valves.  A gasser also has to run within a
>> certain
>> > > A/F ratio in order to burn with reasonable efficiency.
>> >
>> > The problem with lean mixtures on a gasser, is that they
>> > burn fast (explosively), and somewhat longer (hence higher
>> > EGTs).  A richer mixture burns slower, and obviously
>> > incompletely.  It's true that the hot fuel leaving the
>> > engine absorbs thermal energy, but mostly, it just
>> > doesn't create as much, and for as long.  So the valves are
>> > running cooler, and the pistons don't melt their faces.
>> >
>> > If you adjust the ratings on gassers, you can run very lean,
>> > but at a reduced power setting.  This permits better cooling.
>> > Not often done.  Some ceramic-lined engines did it, but the
>> > NO levels are higher.
>> >
>> > Thinking of it differently, the whole charge in a gasser needs
>> > to burn, so the entire load needs to have a A/F ratio which
>> > will support combustion.  In a diesel, only the boundry layer
>> > at the injection region needs to have a A/F ratio which supports
>> > combustion.  This happens over a smaller area, and over a longer
>> > time period, which is why the diesel can inject over most of the
>> > power stroke.
>> >
>> > Injector patterns, and injection timing, probably influence
>> > overall efficiency of the diesels we try to tune.  We can set
>> > IP timing, but I'm not sure that we can play much with how
>> > long during the stroke some fuel is injected.  My guess is that
>> > the fuel injected over a power stroke is fixed, for a given
>> > fuel flow.
>> >
>> > Run a diesel on propane, where the propane injection system
>> > works like a poor man's carburator, and you will have detonation.
>> > The propane will light off upon compression, and the timing will
>> > be "advanced" and you'll hear knock.  Just like when the car
>> > suffers from runaway.  The oil is being burned in a runaway,
>> > without metering (obviously) and without any precision in timing
>> > (essentially igniting on the compression stroke, once the
>> > temp gets to the flash point for the mixture).
>> >
>> > The propane boost folks don't see the timing issue, because they're
>> > creating much more power with oil, than with propane.  But run just
>> > on propane, and at some higher (cruise) power level, and there
>> > will be noticable problems.
>> >
>> > Large diesels burning propane, have injection systems which can
>> > introduce propane at high pressures.  LPG would inject similar to
>> > oil, and requires only a small "head" of about 150 PSI.  I haven't
>> > seen a propane diesel up close, but the LNG (natural gas) systems
>> > inject the fuel as a liquid.  These exist on city busses, and some
>> > industrial powerplants.
>> >
>> > For those who prefer gross simplification, an Otto cycle engine
>> > pops the fuel, and a Diesel cycle does a slow burn.  And then
>> > gas turbines, like on airplanes, and large NG powered generators,
>> > simply continuously burn.  Life is good.
>> >
>> > Val
>> >
>> >
>> >
>> >
>> > >   A diesel doesn't bring the fuel in with the air.  It's combusted
>> AT
>> TDC,
>> > > give or take a little.  A gasser ignites the fuel quite a bit BTDC
>> which
>> > > gives optimum power at the downstroke but also makes longer burn
>> > > time, which makes more heat inside the engine.
>> > >   A diesel always runs basically atmospheric pressure into the
>> cylinders
>> > > whereas a gas is almost always pulling against a vacuum, so there's
>> > > less air to cool things off.
>> > >   A diesel really doesn't run "lean" since the speed of the engine
>> will
>> > > increase if the fuel injected is more than is needed at the given
>> load
>> > > and speed.  You can run one "rich" if you have it set up to put in
>> > > more fuel than can be burned under maximum load, maximum
>> > > rpm, more or less.  Also if things are set up so that it will inject
>> > > too much fuel in too short of a span of time so as to increase the
>> > > rpm under a given load.  In other words, it allows you to "put your
>> > > foot into it and smoke, before the rpm's are able to catch up to
>> > > what your foot is demanding.
>> > >   Make any sense?
>> > >   Lean really doesn't happen, let's say you're cruising along at
>> 60mph,
>> > > using 1.2 gph, on level ground and then stop and turn the pump down.
>> > > It'll run "leaner" right?  No.  You'll now have to push the throttle
>> lever
>> > > down a little farther in order to get up to 60 and maintain because
>> you've
>> > > set the pump to inject less fuel at a given throttle setting under a
>> > certain
>> > > load.  You'll still be using 1.2 gph at 60mph.  The variations come
>> in
>> > with
>> > > how much you mash the throttle, how much change in load you get
>> > > from hills, wind, etc.  If you're a throttle masher to increase a
>> small
>> > > amount of speed, then turning things down may increase your overall
>> > > mileage.  If you're soft as a feather all the time, you're not using
>> the
>> > > maximum fueling capabilities and likely nothing will change.
>> > >      Loren
>> > > _______________________________________________
>> > > Vwdiesel mailing list
>> > > Vwdiesel at vwfans.com
>> > > http://www.audifans.com/mailman/listinfo/vwdiesel
>> > >
>> >
>> > _______________________________________________
>> > Vwdiesel mailing list
>> > Vwdiesel at vwfans.com
>> > http://www.audifans.com/mailman/listinfo/vwdiesel
>> >
>> > _______________________________________________
>> > Vwdiesel mailing list
>> > Vwdiesel at vwfans.com
>> > http://www.audifans.com/mailman/listinfo/vwdiesel
>> >
>>
>> _______________________________________________
>> Vwdiesel mailing list
>> Vwdiesel at vwfans.com
>> http://www.audifans.com/mailman/listinfo/vwdiesel
>>
>> _______________________________________________
>> Vwdiesel mailing list
>> Vwdiesel at vwfans.com
>> http://www.audifans.com/mailman/listinfo/vwdiesel
>>
>
> _______________________________________________
> Vwdiesel mailing list
> Vwdiesel at vwfans.com
> http://www.audifans.com/mailman/listinfo/vwdiesel
>