10/21/2003

Diesel Knock

Think Gas Engines.  Pilot Ignition.  

A methane/natural gas engine is a diesel with a "carb".  Pilot fuel is fired
into the cylinder ala diesel pump to ignite a lean homogeneous mix of methane
and air.

Works equally as well if not better with CO Carbon Monoxide than with
methane/cng.

Diesels knock exactly the same way are SI for the same reason.  Diesel fuel -
shot in a plume into the cylinder is too rich to burn, but the heat is
furiously working away at combustion.  When enough time has passed, the
mixture finally lets go in a detonation aka constant volume combustion.  The
continued flow of fuel burns the rest at constant pressure.

Cetane improvers work primarily by releasing oxygen in the fuel stream, thus
allowing the fuel to start burning before detonating.

Emulsified diesel with water also allowed a much smoother burn - because the
water is about 83% oxygen by weight.

Propane works ok, but will a lower amount of fuel.  Lower limit of flamibilty
thing working here and its "octane" is not as high as methane.

Nasty thought - pure benzene, toluene and zyline all have a 15 to 1 or higher
supercharge critical compression ratio - higher than propane and approaching
methane.  If you were to inject a lean amount of them into the intake valve,
you could get about the same effect as propane.  Nobody would want to do that
right?



On Tue, 21 Oct 2003 14:03:52 -0700 (PDT), you wrote:

> >Date: Tue, 21 Oct 2003 17:06:09 -0400
> >From: "Kevin 
> >
> >Has there ever been a diesel (CI) engine that mixed the fuel and air
> >before entering the combustion chamber?  (besides my buddies old
> >Citation that thought it WAS a diesel when the ignition was turned
> >off!)  I was thinking that excess air is required for diesels because
> >of the short time available to mix and burn the fuel.  It is a
> >heterogeneous mixture, as it isn't mixed well.  If there wasn't excess
> >air the mixture wouldn't burn completely.
> >
> >But, if it could be mixed thoroughly such as a nice carbed SI engine
> >does, then the mixture would be homogeneous and could be burned at the
> >correct 14.5:1 ratio - or in effect for a given amount of air pumping
> >ability the diesel could burn more fuel.
> >
> >But how could it be mixed better?  Well, fancy smancy piston and head
> >designs, prechambers, high port velocities, tumble, swirl....  These
> >and more have been tried and they still only get so far.  It's a time
> >thing.  There needs to be more time.  So....
> >
> >Let's assume a simple diesel engine.  Fuel injection occurs at a
> >certain time BTDC on the compression stroke.  If fuel was added to the
> >intake air in addition to the injected fuel, this added mixture would
> >be homogeneous, or nearly so, before entering the chamber.
> >
> >    But, how much could be added to the intake air, before the mixture
> >could pre-ignite, due to the high compression ratio of the CI engine?
> >
> >    Would there even be any advantage?  Would the extra fuel
> >completely burn or would it be no more effective than adding it
> >through the normal injection?
> >
> >    It seems part of the reason why propane would function in a diesel
> >is that it is homogeneous when it enters the chamber, and helps
> >promote better combustion - using the excess air that was there to
> >burn more fuel - in that case propane.
> >
> >    Am I missing something important?  Ah, it was just a thought
> >exercise after I picked up my Schweitzer 2 stroke diesel scavenging
> >book and skimmed a few parts for some light reading.....    It matters
> >not as I will soon be off to Flood's to bask in the joys of habaneras,
> >chicken wings, Guinness & Bass, and good friends....There are very few
> >joys in life that can compete with hot wings and black & tans...
> >

-------------------------------

Propane works because the mixture is lean, requiring more energy to ignite
than there is energy during the compression stroke.  It has a critical
compression ratio of about 12 to one, which means that it must be compressed
at high temperature to at least this number in a GM supercharged compression
test engine to knock.  

Diesel generally has excess air and scavenged charge and chamber cooling and
although the compression is higher than the critical compression, the charge
is cooler and it does not ignite the propane.  

The critical compression is determined at stoic, and leaning it greatly
increases the amount of heat in a given time needed to ignite it.

The plume of burning diesel fuel acts as a monster spark plug and ignites the
mixture.  The fuel could also be Methane/CNG or Carbon Monoxide, both of which
take a considerably higher compression to ignite.

The other part is lower limit of flammability.  Propane can burn effectively
at approach 100% excess air, so modest quantities will still ignite and burn.

--------------------------------

Two actions going on.  A fuel, below the limits of flammability will not
sustain combustion.  You need the lower limit of flammability present to
sustain combustion - either from a Spark or Compression Ignition.

Then you deal with a homogeneous charge compression ignition.  You blend a
homogeneous and small amounts of fuel with the air, then you raise the
compression until the charge self ignites in a constant volume combustion.
With gasoline, the upper limit of HCCI is about 30 to 1 air fuel, and lower
limit about 70 to 1 air fuel.  This is caused by the uniform heating of the
charge over time to the point where a self sustaining hydrogen chain reaction
occurs.

Propane in a diesel at very low levels burns homogeneously in HCCI because the
pilot injection causes a sudden serious increase in pressure which will force
the HCCI to occur.

Butane, with a lower critical combustion ratio, and lower octane, would not
survive standard diesel compression and would start burning prior to pilot
injection and will cause havoc.

Above the level of flammability, the charge burns in a spark ignition manner
where the ignition source is the compression ignited diesel plume.