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

Val Christian val at swamps.roc.ny.us
Thu Mar 17 00:06:48 EST 2005

>   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 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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