brian
Joined: 02 Jan 2006
Posts: 643
Location: Sacramento, California, USA
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 Posted: Wed Jan 11, 2006 6:56 am Post subject: starting on nitrogen (was: Air bottles) |
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Walter Lannon wrote:
| Quote: | Either engine will start using N2 provided that it starts in the first
one or two attempts. After that there is no hope. The reason is that the
starting charge (air or N2) is injected on the power stroke. Ignition
takes place in a different cylinder which is picking up AIR from the
intake system. If the start fails and all cylinders become contaminated
with an excess of N2 then you might as well go and have a drink cause it
ain't going to start no how.
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The engine is an air pump.
The exhaust stroke pushes the N2 out of the cylinder into the exhaust
manifold thus leaving the cylinder to draw in a fresh charge of fuel/air
on the next intake stroke. There is no excess of N2 unless the engine is
really badly broken.
If what you said were true, the engine would never run because, "the
cylinders would become contaminated with an excess of..." exhaust.
There is something that came to me last night which puts the lie to
something I said yesterday. I said that the N2 would dilute the O2 in
the air until the mixture wouldn't fire and that turns out to not be the
case. The mass of O2 needed to combust with an appropriate mass of fuel
remains the same (partial pressure of O2 is fixed). We just inject an
excess of N2 but the stochastic mixture of O2 and fuel is already fixed.
The engine should fire just fine if a charge of N2 is injected into an
already stochastic mixture of air and fuel. Only the initial pressure
will be different. (We are back to PV=nRT but as applied to gas partial
pressure.)
So, the engine *should* start regardless of what gas is in the pneumatic
starting system. So why won't some engines start on N2? (I am not
arguing that some engines won't fire on N2 because enough people I trust
have clearly observed that some engines won't fire if the air system is
charged with N2.)
If an engine has a proper fuel/air mixture and a properly timed spark,
it *WILL* fire. There is no question about that. If it fires when the
system is charged with air then we know that the spark timing is OK. So
by my reasoning the introduction of N2 somehow changes the mixture. I
have just reasoned that N2 injected into the cylinder will change the
cylinder pressure but will not change the fuel/O2 mixture. Based on that
I am thinking that we are looking in the wrong place. It probably is not
a function the gas being injected into the cylinder at all but somehow
the gas, N2 in this case, displacing the air in the induction system. If
there were a leak path between the air distributor and the rest of the
induction system (say into the supercharger housing) the N2 would
displace the air in the induction system, forcing it back out through
the carb. This would purge the induction system of air leaving only N2
and fuel from the primer. Now the engine draws in the fuel/N2 mix which
will never fire.
Engines that don't suffer from this kind of leak will start on N2. Those
that have this leak will never start on N2.
--
Brian Lloyd 2243 Cattle Dr.
brian-yak at lloyd dot com Folsom, CA 95630
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery
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_________________
Brian Lloyd
brian-yak at lloyd dot com
+1.916.367.2131 (voice) +1.270.912.0788 (fax)
I fly because it releases my mind from the tyranny of petty things . . .
- Antoine de Saint-Exupery |
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