As usual this time of year, I am running a little behind on the really
important stuff in life so I am running a little behind on my MG reading.
Santa was good to the Eastman's in general but apparently does not get the
Moss catalog because, although my letter to Santa included part numbers,
page numbers, and prices, no MG parts made it from the sleigh down my
chimney and under the tree ;-( Oh well, Santa was still good to me and,
just as important, showed extremely good judgement in his delivery to
Nancy, she who must approve all LBC expenditures, (marriage is a
partnership, after all) so I foresee sufficient funds available early next
year to prepare the A for Summer.
A while back there was a discussion about when our LBC engines deliver the
best fuel economy and whether OD improves said economy. I believe that
this discussion was triggered by Dan Master's continued attempts to make a
decent driver out of a Tri**ph. Its a good thing the MG's are fun to drive
as is!
As most of you know, I was trained "a few" years back in internal
combustion engine theory (the text was written in WWII back when an engine
was an engine). You probably also remember that I have not looked at these
text books for "a few" years so, if this info is less than perfect, blame
the lapses on the unreliable firing of my Lucas OF synapses.
For a constant throttle setting for an engine in a specific state of tune,
the output can be defined by several curves. The two most common curves
are those of torque vs rpm and hp vs rpm but these are actually calculated
values and, furthermore, they are not independent. That is the hp curve is
actually derived from the torque curve by incorporating a time term (torque
per unit time) into the equation. The preferred curves for actual
scientific analysis of engine performance are the brake mean effective
pressure (bmep) and the brake specific fuel consumption (bsfc). The bmep
is the theoretical constant pressure that would have to be applied to the
piston surfaces to provide the torque output measured against the
dynamometer resistance or brake. By using the brake number instead of the
ideal mep you account for parasitic losses such as friction. The bsfc
measures how much fuel is used to develop the brake torque (or horsepower
but I believe that the time term cancels and you get ft-lbs/lb of fuel or
some other silly unit). If you were building an engine for constant load
you would design it to run at the peak bsfc. This would give the best
efficiency of fuel use.
To get the most bsfc out of an engine, you want to do two things. First,
you would try to keep as much heat inside the engine as possible in order
to reduce energy waste. The preferred engine material for this is cast
iron and our MG's are blessed with plenty of cast iron in their motors.
The second thing you would want to do is reduce parasitic losses. By
using conservative camshafts that encourage efficiency at low rpm and three
bearing cranks to reduce friction- at least in the technically superior
MGA/XPAG/XPEG engines- our LBC's again shine in this area.
So, to answer the original question the point at which an MG would deliver
the best economy at a given road condition (speed, incline, top down, etc.)
would be the rpm and throttle setting at which the engine develops the
greatest bsfc for the required hp. Bsfc is, in general, less sensitive to
throttle position than bmep so bsfc peaks at a lower rpm than bmep since
this reduces parasitic losses- friction in particular. So reducing rpm at
cruise is usually a good idea for fuel economy.
Three other popular method of measuring efficiency in automotive
powerplants are bmep/lb of engine weight (important since the car has to
haul the engine around) bmep/cu ft of engine size (important since space
for the engine is unavailable for other things but who cares in a sports
car) and bmep/engine displacement (important only in racing classes where
displacement is limited and also in people's minds where displacement can
also be limited at times). We will ignore these measures as unimportant
since LBC engines do not shine in these areas. As an aside, notice what
cars are doing well in the WSC races where the engine limitation is air
consumed (by restrictor plates) rather than displacement. Fairly large
V-12's dominate this because, for the air volume allowed, they are the
most efficient.
Regards,
Bill Eastman
61 MGA with scientifically proven superior engine to 5 bearing MGB's or,
heaven help us, 7 bearing C's
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