RE:Theoretical HP

["John Fielding" <johnf@xxxxxxxxxxxxxxx>]

Hi all Digestibles,

Many folks have alluded to the old fashioned method of establishing the HP rating.  The
British system was based purely on total piston area, that is, only taking the bore
diametre into account.  This explains the peculiarly odd design of British engines during
the period where long stroke engines were the norm.  The French also had a similar formula
for taxation purposes.  The thinking behind this was that large HP vehicles did more
damage to the roads and so should be taxed at a higher rate.  As there was little
possibility of establishing the true HP a formula was derived based on engine type and
number of cylinders.  The common factor was the bore size x no of cylinders = total piston
area.  This was accepted by the motor industry as a policable means to an end.  Hence to
get into a lower taxation bracket the piston diametres were reduced and the stroke
increased to achieve the original swept volume. This set back British engine design by two
or more decades. (I once owned a 1938 Morris 12 (HP) car which had a 1500cc ohv engine
which was actually rated at about 45 HP).

Some describe this as "design by the exchequer" as the Minisiter of Finance (in British
terms the Chancellor of the Exchequer) laid down the formula for taxation purposes.

Reading through Colin Campbell's book "The Design of Sports Cars" I came across an
explanation of this.

Titled the "Ghost of Dr. Lanchester", Campbell goes on to explain the thinking behind this
odd design proceedure.  In 1906 Dr. Lanchester, who was a brilliant automotive engineer
and was also a very accomplished theoretical mathematician, read a paper to the Institute
of Mechanical Engineers which detailed the theory behind the limiting power aspects.  In
this paper he demonstrated that the specific power of an engine with a constant
bore/stroke ratio varies directly as the function of piston area.  Obviously some one in
the finance department appreciated what Dr. Lanchester was alluding to and used
Lanchester's formula to devise the taxation scheme based on piston area.

There are two ways we can design an engine to have a large piston area.  The recommened
way by Dr. Lanchester was to increase the bore size and reduce the stroke, at the time the
taxation system did not support this move as the general stroke to bore ration was around
2:1.  So Dr. Lanchester in 1906 mathematically proved what all racing engine designers now
know today to be the way to produce power, square or under-square bore/stroke ratios
produce the maximum power, because they allow higher rpm at the limiting piston speed.
Vittoria Jano in 1954 with the Lancia GP engine managed to obtain a power output that was
almost three times that of Ernst Henry 1912 Peugot design for a similar sized engine by
being capable of rotating at three times the speed of the Henry design by using a square
bore/stroke ratio.  Henry used a 110mm bore and a stroke of 200mm and the maximum
safe speed was only 2200rpm.  Long stoke engines however provide higher torque at
lower rpm.  Power is torque x rpm.

Laurence Pomeroy (senior), the designer of the Vauxhall GP cars, expressed it in another
way "Whoever can make his engine rotate faster than his competitors, and also arrange for
it to remain in one piece, will succeed in obtaining the highest power, always accepting a
condition of satisfactory aspiration".

John
Durban
South Africa
Alfetta 1.8L turbo
--
to be removed from alfa, see /bin/digest-subs.cgi
or email "unsubscribe alfa" to majordomo@domain.elided