[Vwdiesel] Why gasss cars mpg sucks

[Shirley, Mark R]

> -----Original Message-----
> From: Arnie Grubbs [mailto:vhuntertdi at yahoo.com]

> 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?)

Actually, I got that info out of my engines reference text, I can't 
remember the title, but it's John Heywoods' book.  He's been the
reference standard in engineering schools for a while now.  
Your big lever on economy is weight, not due solely to acceleration,
but loss of potential energy, IE, change in altitude.  You have 
to work against gravity to go uphill, and you normally don't get
a whole lot of that energy back going downhill.
But, you are right, at higher speeds, aero takes over as top dog.
The graphs cross at the higher speeds, as energy wasted is a 
square function of the velocity, and delta Z changes are not.

> 
> 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? 

Yes, because energy conversions are never 100% efficient.  Best economy
on a long flat stretch.  If you do the same trip starting at 600ft
and ending at 600ft through the mountains, your fuel economy will suck.

> 
> 
> =-=-
> 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.. 

Pumping losses are not insignificant.  It's one of the major factors
that makes a diesel efficiency higher.  The other major one is 
compression ratio.  Each point increase in CR makes about a 4% 
economy impact.  Frictional losses in gas/diesel are about the same,
and diesel BTU content is slightly higher than gas.  So the big reasons
are 
1.  Low pumping losses
2.  CR increase
3.  BTU content higher. 

These together combine to give you the 20% or so efficiency increase.

> 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.

Correct.

> 
> 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. 

Not quite.  Diesel efficiency is higher at lower rpms because
frictional losses are much lower.  Pumping losses don't change
that much with rpm, it's still unthrottled at all rpms.
You have some changes in pumping losses due to the changes in VE
at different rpms.  This is a factor of your intake plumbing, exhaust
plumbing, cam timing, etc.

Mark