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

Thu Mar 17 07:38:30 EST 2005

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
> > _______________________________________________
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> >
>
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