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

[LBaird119 at aol.com]

  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.  
  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