Pumping loss, as I understand it, has nothing to do with expended fuel
exiting the exhaust port. Most of the fuel should be burnt, except with
lots of valve over lap. Pumping loss can be calculated by multipling
the MEP of the pumping loop by displacement by engine speed. It can be
approximated by driving the engine by an electric motor with spark plug
removed (friction loss) and then with spark plug in (pumping and
friction loss). PL does increase with increased displacement and or
increased speed. It also varies with throttle opening (less under full
throttle) for obvious reasons.
In an earlier post, BSFC was referred to. For a stationary engine, as
far as I remember, BSFC curve is generally the inverse of the torque
curve. Max torque is the most fuel efficient speed. This is not taking
drag into account! and normally there should be lower fuel consumption
for lower engine speed at a given road speed. Whether OD saves a
significant amount of fuel is a moot point but I'd rather have am MG
with OD than with out. 'nuff said!
Cheers
MJV
Barney Gaylord wrote :
<<< CUT
Now consider the other component of the exhaust gas, the amount of gas
that
is required to exactly fill the cylinders at one atmosphere of pressure.
This amount of gas stores and returns energy like the spring in an
accumulater, with the intake charge being compressed, and then the
exhaust
charge being expanded to recover the energy. This net energy flow is
zero.
There is a certain amount of fuel being burned to create this amount of
exhaust gas, and that amount of expended fuel going out the exhaust port
is
the pumping loss.
The pumping loss varies directly with engine displacement and engine
speed
(and atmospheric air pressure and fuel mixture ratio being minor
contributors), and is totally independent of all other operating
parameters
(as best I understand it). So first, if you cut the engine displacement
in
half for a given engine speed and load, you cut the pumping loss in
half.
Secondly, if you cut the engine speed in half with a given engine
displacement and load, you also cut the pumping loss in half.
Most of our LBC's are generally good at the first example, because they
have relatively small engines, so they get pretty good fuel economy
inspite
of running rather fast. Still, if you can slow the engine down some
with
an overdrive gear you can reduce the pumping loss even more. This works
as
long as the engine is not lugging under load at speeds far lower that
the
peak of its efficiency curve.
At fairly high engine speed and moderate travel speed, the pumping loss
can
be about equal to the energy used to overcome mechanical friction and
wind
resistance. In this case, when you reduce the engine speed by 30% with
an
overdrive gear you may reduce the fuel consumption by 15%. This is the
key
reason that so many new cars are equipped with such gear ratios to make
the
engines run slower, as the manufacturers need to meet the mandated CAFE
numbers in the US, and the economy demands of (some) customers in all
parts
of the world. As someone stated, the results do depend on the engine
speed
being fairly well matched to the peak of the efficiency curve, but
there's
a wide window of opportunity there.
Now aside from helping to meet the mandated requirements, in the real
world
OD may well not make good economic sense. When you figure the added
cost
of manufacture of the OD unit (especially an OD unit separate from the
main
gearbox), and the added maintainance cost of the OD unit in the long
term,
these may well out weigh the value of the fuel savings. These costs are
quite obvious to those of us dealing with these older cars now in need
of
repairing or replacing the OD units, or wanting to install OD in a car
that
did not previously have it.
And there are many new car buyers who have no idea what OD is or what it
does and who would be very happy to pay a little less for a car without
OD.
And there are many who do know the value of OD, but are on a tight
short
term budget and would rather pay less for a new car regardless of the
fuel
economy numbers.
So in summary, if it wasn't for our government mandates, there would be
far
fewer OD cars on the road, and many more satisfied car owners. You see,
not everyone wants one (including me), in spite of the obvious benefits.
For our beloved old LBCs, I believe OD is a loosing economic
proposition,
so if you like and/or want OD it should be founded on other reasons,
like
reduced noise. I for one happen to get off on the neat noises my MG
makes,
and couldn't bear to neuter it with OD.
$.02,
Barney Gaylord
1958 MGA with an attitude (and a happy 4400 RPM @ 75 MPH)
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