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

Kelly McMullen kellym at aviating.com
Thu Mar 17 00:14:45 EST 2005

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.
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

>   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.


>   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
> load.  You'll still be using 1.2 gph at 60mph.  The variations come in
> 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

More information about the Vwdiesel mailing list