----- Original Message -----
From: "DaveCole" <davidcole@tk7.net>
To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
Sent: Monday, April 02, 2001 7:48 PM
Subject: Re: [MV] runaways
> This may sound a bit extreme, but I can't imagine how less extreme a
runaway
> diesel might be, but what if someone was to dump some water (hopefully in
a
> controlled manner into a diesel engine that was running away. Has that
ever been
> done??
>
Very tricky indeed.
> It is common (at least to me) to run a small amount of water into a
running gas
> engine to clean out carbon deposits. It actually works quite well. The
trick is
> to dump it in slowly so the engine can pass the water through it, however
if you
> dump it in too fast, the effect is that the engine will slow down
substantially
> until it stalls. The water cools off the charge sufficiently to shut down
the
> engine. Obviously sticking a running water hose into the air intake would
> probably blow the engine, but I'm not talking about doing that. Having a
can of
> water near the engine upon startup might be a lot more available than a
C02
> extinguisher or something that can effectively be used to cut off the
airflow.
>
Firstly remember there is a very fundamental difference between the petrol
(gas) engine and a CI diesel.
At idle the spark ignition petrol (gas) engine is taking in very little gas
charge as the throttle butterfly is barely cracked open by a few thou and so
it is running at a CR of very much less than 1, its often overlooked but the
spark ignited engine does not realise its CR until full open and then it is
less than the calculated static figure owing to imperfect cylinder filling.
The diesel has its intake wide open at all times and is controlled by the
quantity of fuel injected into an always "full" cylinder of compressed air.
The amount of water needed to stop a spark ignited engine at idle or a bit
above is quite small and in any case virtually sticking a water hose in the
intake will only take on a minimal amount of water that can get past the few
thou of the butterfly especially as water is a lot thicker than air, very
little will kill the engine purely by removing the air supply and pose
little or no danger of a hydraulic lock.
To stop the diesel, even at idle (where you don't need to anyway), you would
have to replace most of the wide open inlet air charge with water and this
quantity would invariably cause a hydraulic lock with disastrous
consequences. An inert gas like CO2 would be effective and safe but I
wonder if the average extinguisher bottle actually has sufficient with the
inevitable wastage to inhibit firing whilst the engine runs down and stops.
Sure, you can clean a spark engine with a drop of water, preferably gingerly
sprayed into the carb mouth with a fine mist hand sprayer, the effect is
like a partial steam clean as the already finely atomised water flashes into
steam when the cylinder fires. It has long been known that water injection
to petrol SI engines keeps them clean and realises some extra power by
keeping the inlet charge cool, any drag racer will be able to recite rhyme
and verse about water injection.
Where the inlet charge has excess unwanted heat entrained in it thus
reducing volumetric efficiency and leading to detonation, controlled water
injection is an old technique to alleviate the effect, many WWII military
aircraft used in PR duties were fitted with almost "drag" engines
(Mosquitoes especially) and had facilities to allow higher boost
(supercharge) pressures with methanol and auto water injection to cool the
mixture owing to the effect of supercharger compression. (Some even had
nitrous-oxide. . . . .)
I just happen to have a manual for the P&W R4360, a 4 bank radial of 7
cylinders per bank displacing some 70 litres, there are many references to
the water injection which is actually a 50/50 mix of water and methanol,
broadly, it activates automatically at some 50 to 54 inches of mercury
(that'd be about 25/28psi I guess) inlet boost pressure to cool the now
compressed and heated inlet charge.
Richard
Southampton - England
This archive was generated by hypermail 2b29 : Tue May 01 2001 - 07:42:39 PDT