[Diy_efi] water injection

Klaus Allmendinger klaus
Fri Sep 28 23:55:41 UTC 2007


Which brings back the original question: Does WI help on a diesel?
I did some more research on that and also talked to some buddies at
diesel injection development at Bosch (after all, these guys should know
:) ). All sources agree, WI on a small diesel, especially if its
introduced in the intake, LOWERS output power (but can also lower
emissions under some circumstances). In a race diesel engine that also
uses NOX, it can lower combustion temps enough to keep the pistons from
melting. But STILL costs power. The reason WI works on a gas engine (to
reiterate again) is that it lowers the in-cylinder temps pre-spark, so
the fuel does not auto-ignite. In a diesel you WANT the fuel to
auto-ignite. Raising the effective Octane number (and therefore lowering
the effective Cetane number) increases the ignition delay of the diesel
fuel. This means that the injection timing is late relative to the
optimum peak cylinder pressure. Like retarding ignition from MBT in a
gas engine. This cost power. The second reason is that diesels already
run with excess air. If you dilute that air further with water that
turned into steam during the compression cycle, you slow down the burn
time further. This lowers the effective work the engine can extract from
the fuel. Any increase in effective burn time lowers the thermodynamic
efficiency of an engine (diesel or gas). Lowering the efficiency means
you are lowering the output power on a diesel because the amoung of fuel
that can be sprayed during the small crank angle window where it can be
introduced is already limited. The effects of charge density increase
from WI on a diesel do not make up for the power loss from the other
effects. On a heavy duty, large, low rev diesel engine, WI is sometimes
used to reduce NOx production for emissions reasons, as the effects of
lowering efficiency or from increased burn time or increased ignition
delay are small at low revs.
Af course, all the above applies to pure water injection. Not for water
mixed with relatively high proportions of methanol or ethanol. There you
are also introducing additional fuel, which can be beneficial in a
diesel at higher RPMs.

- Klaus

-----Original Message-----
From: diy_efi-bounces at diy-efi.org [mailto:diy_efi-bounces at diy-efi.org]
On Behalf Of Adam Wade
Sent: Friday, September 28, 2007 4:11 PM
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] water injection

----- Original Message ----- 
From: "Buck Williams" <buckwill33 at hotmail.com>
To: <diy_efi at diy-efi.org>
Sent: Friday, September 28, 2007 2:26 PM
Subject: RE: [Diy_efi] water injection

> diesellsss doant jsut make more poweeer when all the avaalaiable air 
> is used,,,it makes more ower when any moere fuel is injectedd,, if 
> there isa enough available air,,, enoaugh evailable oxygaen to burn 
> the extra fuel, thats why the urbo is used, add more air for the 
> extra fuel,,,

That's another way of saying what I said.  :)  I left out the "extra 
fuel" bit as implied, and of course it is always harder to get more 
oxygen into the combustion chamber than fuel.  Our point is the same; 
that you have a limited amount of power based on oxygen content (and 
of course adding enough fuel to make good use of the oxygen); a turbo 
gives you more air, so if you then provide more fuel, you make more 
power.  With a naturally aspirated diesel, adding more fuel will cause 
power to FALL when under 100% load.

|    Kawasaki Zephyr 615 (Daphne)     1985 RX-7 GSL-SE (Rex)    |
| "It was like an emergency ward after a great catastrophe; it  |
|    didn't matter what race or class the victims belonged to.  |
|  They were all given the same miracle drug, which was coffee. |
|   The catastrophe in this case, of course, was that the sun   |
|               had come up again." -Kurt Vonnegut              |
|  Motorcycle Fuel Injection Hndbk. - http://tinyurl.com/2v5o97 | 

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