[Vwdiesel] Why gasss cars mpg sucks (fwd)

[Val Christian]

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.

I have not established that the IDI pumps also shut off fuel flow 
in a coast, but from an observed behavior, it seems they do.  If 
anyone knows for certain, I would really like to know.

With respect to fuel economy going up and down hills, there are 
several factors.  Included is the efficiency of the powerplant
at low power levels.  In the case of the TDI, it is conceivable
that operation going uphill may be in a more efficiency portion 
of the curve, than level driving.  If we assume that the down hill
portion of the trip is without fuel consumption, it is certainly 
conceivable that the total fuel consumed for a level trip 
COULD be more than for a uphill-downhill trip of similar
distance.

This same kind of issue happens in flight dynamics.  The powerplant 
behavior plays a big role, in that it may be more efficient at 
some settings and not others.  Consider a turbine aircraft...low
power settings may be VERY fuel inefficient.  If the vehicle running
up the hill and then down the hill were powered with a turbine,
indeed the fuel efficiency could also be poorer in the level scenario.
Further differentiating between a turbine and a TDI, all the turbines
I've flown do not shutoff fuel in an idle thrust scenario.  But a 
TDI does.

Yes, there is rolling resistance, and there is aerodynamic resistance,
but both of these are small compared to powerplant efficiency.

Going back to old tests I did in a Rabbit (IDI), the lowest cost 
per mile happened about 40 to 45 MPH.  Slower than that, and the 
fuel cost per mile went up, due to powerplant inefficiency.  Faster
than that, and the aerodynamic penality went up.  The trades 
for uphill-downhill are different, but they are still there.
Aerodynamics are a big factor, powerplant performance is a bigger
factor, but rolling resistance is not.  (Rolling resistance is not
the trade of kinetic energy for potential energy that happens when
you go uphill.)

Val





Forwarded message:
> In a message dated 5/11/2005 1:30:43 PM Pacific Standard Time, 
> MarkRShirley at eaton.com writes:
> 
> > Hills will cost you energy to go up, but when you
> > >go back (coast) down, you save fuel (no throttle plate and your
> > > fuel injection shuts itself off, at least in TDI it does!).
> 
> 
>   Not quite.  It never shuts off, it drops to idle volume but the rpm 
> is high unless you drop into neutral.  So you're at minimum fueling 
> but at higher rpms.  It's injecting fuel at every power stroke the 
> engine turns over.
> 
> > >
> > >If you end up at the same altitude at the end of your trip, it
> > >(weight) should not make a HUGE difference in MPGs... right? 
> > 
> 
>   Simple way to test a thought or theory is to take it to the extremes. 
> Imagine the fuel/power/etc it takes to roll a moped, Honda 600, Mini, 
> Rabbit, et al down the road and compare that to rolling a boxcar, semi
> tractor/trailer etc down the road.  The coefficient of friction (rolling 
> resistance) is higher for the higher weight.  Sure Newtons laws pertain 
> about staying in motion but it's not in a vacuum, there's gravity so there 
> are several outside forces acting on it.  Weight DOES matter and it 
> CAN matter a lot.  Just like tire pressures can.
>     Loren
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