[Vwdiesel] Need help getting parts from Germany

[James Hansen]

> There's other things involved there.  Ignoring the difference in engine
> sizes, most of their injection pumps are have very much more fuel capacity
> per injection than the Bosch pump used on VWs.  This means their time
> (in degrees of engine rotation) to inject the required fuel is much
> shorter.  So even if they are turned up to put more fuel in, the total
> time to get it injected is still short.
>
> The VW injection pump doesn't have the "capacity per injection" to do this
> in just a few degrees of rotation.  I've personally observed the problem
> I talked about.  On a VW the solution would be to install a larger
> capacity injection pump, if such an otherwise compatible pump were
> available.  I'm sure it would be expensive.  Injection pumps for the
> Cummins engine as used in my Dodge truck are well over $2000.  Lacking
> a larger capacity pump on a VW, if you are going to put a lot more fuel
> in per injection then you need to watch the exhaust temperature.

Excuse please, but the Bosch pump on your engine right now can inject enough
fuel to melt your pistons to slag. Turn your max fuel screw in as far as you
can, and drive up a big hill if you don't believe me.  With increasing the
fueling over factory settings, you are injecting the fuel over a long period
of time, and the latter phase of injection happens too late in the cycle to
do much other than be exhausted as unburnt carbon and fire/heat just as you
stated earlier.  Larger capacity injectors (ie- bigger nozzle) address this-
the duration of the injection pulse is varied by nozzle size, not by the
pump.  The pump has a fixed amount of time to rid itself of the fuel charge,
and being a positive dislacement pump, it will. The injector has a break
pressure as set by the person adjusting it, and that is just the pressure
that the fuel must reach before it will open, to get proper spray pattern
and all. But the nozzle size dictates how much fuel can get in over a
specific time, or the rate of injection. Running small injectors and as big
a pump as you like will only get in so much fuel, to the point of PUMP
failure- the fuel being relatively incompressible, can't go out the
injectors fast enough, and the pump is positive displacement, then something
has to give, usually ending in premature pump failure. Remember the engine
is turning at an rpm fixed to the pump, it has a fixed amount of time in
which to accomplish the injection.  The injection cycle amounts to the pump
having to put a given amount of fuel into the engine within a set number of
degrees of rotation, and it will get it there, as it is all postive
displacement stuff.  Problem being, it can reach the point at which the
mechanical stresses on the internal pump components are unacceptably high,
hence bigger injectors needed to lower the peak injector pressure. Bigger
injectors will lower your egt's as well, but only to a point. It's more a
factor of the entire system- the static timing, rate of advance, fueling
amounts and duration, air charge temps, all play a role in max sustainable
power.


> Using higher capacity injectors isn't really an option, since it's
> the injection pump that governs how much fuel gets put in per degree
> of crank rotation.

Not really. You can install whatever size injector nozzles you like. Larger
nozzles shorten the duration of injection pulse, smaller lengthen the pulse,
but only to a point. They must be matched.  Um, fluid dynamics.... the rate
of flow through a nozzle is proportional to the square root of the pressure?
That is to say, doubling the pressure gives only 25% more flow. You can see
how mechanical limitations could be reached quickly...

  Common rail systems that use electric injectors
> can have fueling increased by either longer injection duration or
> by using larger capacity injectors, or both.

Usually only duration.  The nozzle size is fixed, and matched to the
application. Cummins uses the same injector to make from 400 to 600 hp in
the same engine, just changing the fuel map. In many cases, there are a
number of small short injection pulses per power stroke, rather than one
long one.

>
>
> >   You can also claim you're changing the timing every time you step
> > on the throttle.
>
> Not true.  Stepping on the throttle only changes the max amount of
> fuel injected per injection - which means the "time to inject" gets
> longer, it takes more degrees of pump (or engine) rotation to get all
> the fuel put in.  The point of BEGINNING injection stays the same.
> (Understanding that moving the throttle is done with the engine
> at the same speed).
>

You are both actually right, depending on the point of view you want to look
at the fuel from.  When fuel is increased by either screw turning or
throttle application, the injector sees the break pressure necessary to open
reached sooner, hence a slight increase in advance.  Same for changing the
break pressure of your injectors- higher break pressures mean a delay in
opening, but in the grand scheme of things, not a heck of a lot in terms of
degrees.

Stepping on the throttle, incidentally tells the pump governor that it isn't
going fast enough, and has to increase fuel to get the engine rpm's to the
point that your foot is asking for.

>
> > The timing advances with relation to speed and pump pressure.  I think
> > it's mostly dependent on internal pump pressure.
>
> True.  The advance (of the point where injection begins) is a factor
> of internal pump pressure pushing against a spring loaded piston that
> adjusts start of injection time when it moves.  This advance is adjustable
> by changing shims on the spring, which you generally don't want to do.

You are referring to the RATE of advance or by changing the spring pressure,
you change how soon the increase in advance for a given rpm will occur (the
advance curve).  You do not want to change this unless you REALLY know what
you are doing, say, to change some EGT's at a range of rpm's you derive from
dyno results. This is about the only application in a diesel where this is a
useful adjustment to make I can think of off the top of my head. Lots of
Bosch skull sweat went in to figuring this out the first time, difficult to
duplicate on your own, unless you have something empirical, specific to your
engine, like dyno data to go from.


-James
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