[Vwdiesel] Why gasss cars mpg sucks

[Arnie Grubbs]

I think your close, but I don't think that the vehicle
weight has much to do with the MPG unless you are doing
a lot of in town driving in traffic. Accelerating a heavey
load will cost you more fuel, and Accel faster, it will cost more.
Inertia is wasted in city driving, mainly as heat in your brakes,
unless you have a way to reclaim the engergy (hybrid electric car
with regenerative brakeing?)

Once on the road at a constant speed, you have friction losses
in the running gear, and wind losses depending on the shape of
the vehicle. 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!).

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? 


=-=-
Anyway, imagine how much power it takes to draw a vacuume on
a pump with the displacement of the engine you are dealing with.
That takes a fair amount of power to draw 10-12lbs vaccum on
how ever many cubic inches your engine displaces.. 

 Ol' timers will remember the vaccuum gauges you could get to
check your cars with. When you mash the loud peddle, the vaccuum
disapeared. I remember an old '62 IH scout we had. It used the engine
vac to run two vacuum powered motors that were attached to
the wiper blades to clean the wind screens.  If you were lugging
the engine or accelerating hard, the throttle plate was not restricting
your intake, so no vaccuum, and your wipers would slow or stop. FUN!

Up to a certain point depending on engine speed, a gasser engine gets more
 efficent at turning fuel to HP the wider the throttle is open. This is because
there is less work involved in generating the vaccuum so more BTU's in the fuel
can be used to turn the load instead of sucking wind.  The faster the engine
goes though, you are going to see more pumping loss due to other restrictions,
such as exhaust and intake pipeing. So best efficency in a gasser is when it
is under a load that keeps the RPMs down to a reasonable level but has the
throttle plate opened up wide so the engine doesn't have to run against a vaccum.

The diesel engine gets best fuel efficency (KW/unit of fuel) when it is running at low
 RPMs (less air pumping loss) at a load where all the fuel that can be injected is burned.
Any more fuel injected would not burn and would cause smoke and waste fuel due to not enough O2. 

Being able to run a diesel lean means that you don't have to work against a vaccuum.
At idle you still have air pumping losses, but they are small. Only the amount of fuel
needed to keep the engine turning over against engine friction, air pumping losses, and
what ever assorted load is on the idleing engine (water pump, injection pump, pumping air
past the turbine of any turbo charger, and the alternator) is needed to keep it spinning.
 
 If you have a TDI, attach a diagnostic computer (VAG-COM) and look at the
amount of fuel the ECU is calling for at each injection event.  If you figure out how many 
Liters/hour it is, you will see why you can let your TDI run at idle for DAYS on a tank
of fuel.  I don't think it is a whole week, but it is close!

Ah well, my pain pill is taking effect.. my spelling is going to hell, and my fingers
and brain are running away unchecked.. 
 I dunno if what I typed makes much sense the way I typed it, but
if not, please excuse me.. I will be in lurker mode (really this time!) for a while.


Arnie


=============
MPG is primarily related to vehicle weight and to a lesser
extent, aerodynamic performance.  Not HP.  Since diesels are
unthrottled, unless you are actually USING the 150hp all the
time, you will get the same MPG (over very close) to the
90HP version of the same engine.  Not being tied to a specific
air / fuel ratio is one of the great benefits of the diesel engine,
you use only enough fuel to keep the vehicle moving and no more.
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