09/15/2003

Combustion and Water

In Pye, spent some time reading about the 5 valve ( two intake, 3 exhaust )
ricardo variable compression ratio machine with 4 spark plugs circa 1930's -
no possible clue about overheads from the inventor of the optimum flatheads.

Now using coal gas - non detonable, it was discovered the highest bmep was
made using both plugs nearest the inlet valves.

Not the exhaust as is taught by the holiest of the holy devotees.  The inlets.

Seems that the hot exhaust valves sped combustion and reduced combustion time,
meaning that less advance was needed for optimum power.  The end gasses burned
faster with the hot exhaust valves present than if the plugs were reversed.

Now, for a detonating fuel and "fixed" ignition, firing the plugs nearest the
exhaust valve produced the highest detonation limited bmep.

Now consider giving yourself the advantage of a knock sensor, variable spark
timing and other means of controlling detonation such as mixture control, egr
and water injection etc.

If we had the ability to relocate the plug, if we relocated it towards the
inlet, we could speed combustion and efficiency thru most of the operating
regimes and by sensing knock, could control knock by the appropriate means.

I think Greg and Dave will catch on - leave the exhaust as hot as possible
during normal firing.  On dual plug engines, with opposing plugs - one near
intake and one near exhaust, it might be interesting to kill the exhaust side
plug during light load and bring it on only at full load.

It definitely showed advantage with a weak mixture.  If stoic, and egr, it
should show the same advantage.

Previous knowledge engines work statically against knock.  With the ability to
control knock, this could be very interesting.

For the olde phartes, remember hit and miss operation?  Where instead of
throttling, fuel was dropped for a cycle as needed to stabilize rpm under
load.  Seems like this is the optimum method as the thermal efficiency DROPS
as you approach full load.  Formerly down only with low speed solenoid valve
gas engines.  You have to completely cut off the fuel per event with no wetted
manifolds, runners and ports.  Possibly with gasoline direct injection or high
pressure short duration port injection - maybe.

Normal firing, then dropped cylinder.  Next firing the cylinder has no
residual EGR and burns better.  "The reason is that on each cycle following a
missed or scavenging cycle, there is no exhaust gas present to heat the
incoming charge of gas and air, which is diluted instead with cool air.  This
has an appreciable effect, through a lowering of cylinder contents, in
improving both power and efficiency during the next cycle."

On the myth of fuel evaporating in the tract and reducing the air available,
at a 7.5 to one compression ratio, comparing pure benzol ( british for benzine
) with pure ethyl alcohol at the same air input shows an increase of 8 percent
in the power with the alcohol.  This was due to the exceptionally high latent
heat of vaporization.  "The evaporization process is completed during
compression, and the amount of heat absorbed is so large as to produce a
general lowering of temperatures throughout the cycle.  This is equivalent to
working with a weak mixture in so far as the lower temperatures of the cycle
reduce the detrimental effect of increasing volumetric heats upon the thermal
efficiency.  When running alcohol at full load it is noticeable that exhaust
valves remain cooler than on petrol.  They continue to look black under
conditions when with other fuel they would be glowing a good cherry red."

Nice little side note from the Aero Engine Vol II is that the injection of
petrol, either prior to the compressor or following the compressor, made no
difference in the final charge temperature, provided the fluid was fully
evaporated prior to induction to the cylinder.

What did make a difference, was the larger the amount of petrol vaporized
before the compressor, the higher the pressure ratio of the compressor at the
same air and rpm load.  More fuel before - more boost. Some cases up to a 20%
or more increase in pressure ratio.

Remember that there was an intense interest at reducing the temperature of the
burning charge at TDC.  The Carnotist say the largest temperature difference
is the most thermally efficient cycle.  Not a problem.  Except the air/fuel
mixture and the burning is not.  With a real charge, the heat is not released
instantiously - but over time and temperature.

First, a certain amount of energy contained within a particle will contain
both sensible ( measurable pressure ) and will contain specific ( energy
retained within the particle ).  The greatest sensible energy is at the
boiling point of the gas ( in percentage ) and the greatest specific is right
before it becomes a plasma.  Thus, the higher the temperature of the gas, the
less sensible energy available to move the piston.

The other is chemical equilibrium.  The temperature at which all of the fuel
is consumed and reduced to the lowest energy state molecules.  Above this
temperature, dissociation takes place and less particles release their energy.
Further, the chemical reactions stop at this point based on temperature and
the charge is chemically equalized.  Lower the temperature, more of the charge
completes production.  Keep in mind that every reaction has a counter reaction
at some temperature.  Water is classic.  Water is generated by the oxidation
of hydrogen, but water is also reduced to hydrogen and oxygen at a different
rate.  This rates balance at a certain temperature and the amount of O, H and
H2O is in equilibrium.  The dissociation rates require a hotter temperature
generally than the combination ( oxidation ) rates, so as the temperature
lowers but above the "freezing" point for the reaction, more hydrogen can be
oxidized than dissociated so their will be more released energy.

Pye emphasized that the higher the sensible energy and the greater the
completion of combustion near TDC, the greater the measured bhp.

Now lets toss some dihydrogen-monoxide into the reaction.  As it is fully
oxidized, its "mixture" is stoichemic.  The more dihydrogen-monoxide in the
mixture, the closer the mixture will come to a stoichemic ratio, and thus the
higher the temperature of combustion.

But, the only energy dihydrogen-monoxide has available is essentially the
latent heat of vaporization, which will be low with respect to the other fuel
components.

Thus, even thought the mixture is moved closer to stoichemic, the greatly
reduced energy will result in a lower temperature from the same fuel, and
since this reduction is near tdc, will result in an increase in bmep and bhp
from the reduced temperature.

On a supercharged engine, as Pye reported from Naca and British Air Ministry
tests, increasing the latent heat of combustion of a fluid and evaporating it
in the manifold prior to combustion lowers the charge temperature.  Note that
the power went up on approximately the same calories of alcohol - because of
its heat of vaporization.  

Plus, evaporating it ahead of the compressor will result in significant boost
( pressure ratio ) increases.

The immediate temptation is to conclude that cooling the charge with an
intercooler would result in the same gains.  Alas, me lads, tis not true.  The
gains were do to regeneration - the recovery of previous combustion cycles
heat, and absorbing it back by latent heat of vaporization and reconveying the
energy back to the chamber.  Intercooling only lowers the tendency to detonate
and is in fact, a loss of available energy to the engine by some percentage.

But enough, we all know that dihydrogen-monoxide can be lethal in certain
quantities and is implicated in virtually every health condition and while not
currently regulated, is suspect and regulations will certainly follow from
that Great Agency designed to protect us and Clean the environment for the
future, the EPA.

More useless antique facts.  Oh well.