cutting off injection going downhill was [Vwdiesel] Why gasss cars mpgsucks

[Arnie Grubbs]

I think we are having a little misunderstanding here.. What is ment
by the engine shut off? It does not mean that you stop its rotation...

To most, in a TDI... it means that it is no longer producting power. That doesn't mean it
can't still be rotateing.  The inertia of the car in motion is causing
the engine to spin (thru the drive train).  The injection pump controls
the amount of fuel injected by varying the position of the spill valve, or fuel
control sleave. It can be set (by the computer) so that there is no pressure generated on
the fuel compression stroke. This means technicaly, the engine is shut off, as no
fuel is delivered. 

 When the engine needs to produce power, either at the request
of the foot peddle, or due to slowing down to far, the control sleave moves
ahead a little more and the fuel starts to be pressureized again.

The best picture showing this spill valve I have is of a mechanical pump, but the TDI
pump is about the same, except that the mechanical control levers are
now electrical servo motor controls. This includes the max speed gov. function
and the timeing advance.  The advance function is now controled by the 
ECU by a servo, so the ECU can adjust timeing according to engine
speed, engine loading, AND temperature. Temperature of both engine AND fuel.
Check out this link..

http://www.cs.rochester.edu:80/u/jag/vw/engine/fi/injpump.html

A good source of info on the TDI and its sensors is in this acrobat file.
It shows the fuel temp sensor in the pump. And several other things.

http://volkswagen.msk.ru/engine/tdi-tech.pdf

Its rather large, at 900+K, but it tells a lot, and shows the feedback
to the ECU of all the inputs the car uses to run.  Also it details what
the engine falls back to if one of the sensors goes bad, and the car goes
into limp home mode.


By the way.. 
Sometime if you have troubles starting a TDI, check that the fuel temp. reading
reported to the ECU is right.  I have seen the sensor go bad, which would cause the
car to be very hard to start.  From what I have been told it is not a hard 
job to replace the sensor, which is in the fuel pump. You don't have to get
compleatly inside, just remove a cover.

Anyhows, thats why a TDI engine can coast (some might call it engine brakeing)
and not use any fuel at all, until its rpm gets below the idle setpoint.
And thats also why there can sometimes be 'slow down shudder' when someone 
is slowing down in gear, then kicks out the clutch to stop.  If the adjustment
of the number in the ECU for the injection quantiy feedback calibration is off,
then the engine will shudder as it attempts to smoothly pick up the load to 
keep the engine at idle.  I have had that happen and had to adjust the IQ calibration
in my ECU at one time..


Free wheeling, probably would save fuel as you coast down hill, in SOME circumstances
as you would not have the engine drag slowing down the car...  But, if the hill
was steap enough, you would probably save fuel by letting the engine be in gear.

The reason being in free wheel, you would have to expend fuel to keep the engine
at idle rpm.  In gear, if your foot (or the cruse control) is not requesting
power, the fuel is shut off. Probably not much difference in consumption due
to the TDI not taking much fuel at idle speed, but there would be SOME difference
if you like to split hairs...

Anyway, I still am on pain pills for my busted leg, so if this does not make
sence, please forgive me.. My fingers are probably not typing exactly what my head
is tellin them to... :)

Laters!

Arnie
 









 Date: Sun, 15 May 2005 15:23:01 -0500
> From: dieseltdi at verizon.net
> Subject: cutting off injection going downhill was [Vwdiesel] Why gasss
> 	cars mpg	sucks
> To: VW diesel mail list <vwdiesel at www.audifans.com>
> Message-ID: <35A29CA6-9F3B-4794-B00F-87B010D8ABFC at verizon.net>
> Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
> 
> The injection amount commanded by the ECU may drop to 0 but that  
> doesn't mean injection stops.  If the TDI actually cut off the fuel  
> to the engine as you went down hill, it would have to know when to  
> "restart" the engine.  How would it know when to do that?  I spent 6  
> hours in my car yesterday going up and down hills in East Texas and  
> Louisiana.  The engine never stops running even if I let off the  
> pedal.  It simply returns to idle status, meaning only enough fuel  
> being injected to keep the car idling.  The ECU injection quantity as  
> I see it is a demand ABOVE AND BEYOND what is needed to keep the  
> engine running, not the total amount of the fuel injected.  In other  
> words, there is some base amount - X that is injected all the time,  
> but when the ECU senses a signal from the pedal, injection quantity  
> goes up.  Let up on the pedal, injection quantity goes down.  There  
> is simply no practical way for these engines to literally shut off  
> when going down hill.  Just my two sense worth but as I said, I spent  
> 6 hours in my car yesterday thinking about this and trying to  
> logically justify the concept.  It simply cannot work.  Hayden
> 
> On May 15, 2005, at 6:22 AM, Val Christian wrote:
> 
> >>> A couple of things...first, the TDI DOES shut off all injection
> >>> in a coast operation.  I've observed that, and verified it as
> >>> correct behavior for the TDI.
> >>>
> >>>
> >>>
> >>
> >>   So when you top over a hill, (and keep going of course)  the TDI
> >> engine actually shuts off?  Seems like something DOT wouldn't
> >> approve of regardless of ramifications or not.  Seems like a
> >> freewheeling (ala E-mode) would save even more fuel.  If that's
> >> the intent.
> >>     Loren
>


		
Discover Yahoo! 
Stay in touch with email, IM, photo sharing and more. Check it out! 
http://discover.yahoo.com/stayintouch.html