Piston speeds

[JHertzman@xxxxxxx]

In AD7-125 Reuben Erickson asked "Does anyone know the basis of the 2500 fpm
piston speed index used by R&T in the 50s and 60s to establish max cruising
speed of a car? 

In AD7-126 Keith Hyndes offered some interesting comparisons and comments, but
said "I do not know origin of the R&T 2500 fpm - -"

The question was frequently asked and answered, or answered preemptively in
"How we road test" discussions in R & T and undoubtedly many other places.
This one is from the Tech Correspondence column in R & T January 1962, the
first one I hit in a three-minute search:

(question)

"Piston Speeds

"Your road test data panel has listed rpm at the piston speed of 2500 ft/min
and also equivalent miles per hour. Is this construed to be the maximum
theoretical cruising speed? Would you please clarify your reason for setting
2500 ft/min as a theoretical limit?
                       Edwin McKinney
(response:)

"Generally speaking, approximately 2500 feet per minute is the maximum mean
piston speed the average engine will tolerate for prolonged running.
Naturally, this is only an average, and specific engines may thrive on even
higher speeds, or explode well below that arbitrary limit.

"Generalizing further, we might say that long-stroke engines tend to accept
higher piston speeds than those with short-strokes. This is due to the fact
that the actual rate of piston acceleration is usually lower, for any given
mean piston speed, in the long-stroke design. A great many now-outdated long-
stroke engines proved quite reliable at 3500 ft/min piston speed; the type
35-B Bugatti is one example and the outstanding 1927 Delage GP car, which was
considered safe (by its designer, who occasionally was proven an optimist) up
to a staggering 4000 ft/min. The only engine that comes to mind that is now
being built with a safe maximum near 4000 ft/min piston speed is the
"Indianapolis-Offy," but it has had the benefit of an unnaturally prolonged
development time.

"These racing engines are, of course, extreme examples, and experience with
production car engines (not our experience, but that of the working
automobile-engineering profession) has shown that the 2500 f t/min limit is
generally a good guide. Beyond that, there is likely to be an early failure of
the piston and/or its rings. And too, the inertia loads encountered at high
piston speeds tend to work something of a hardship on the parts that comprise
the rest of the crank train."
- -------------------------

The usual answer they gave was that it was just a convenient peg (among
others) on which to base comparative assumptions about probable longevity.

I would add parenthetically that pre- WW1 voiturette racing contributed much
to the state of early knowledge about piston speeds, just as other classes
contributed other engine-limit knowledge for wartime aircraft. The voiturette
(or 'small car') classes had successive limits on piston-head area, and
sometimes on numbers of cylinders, but not on strokes, leading to appreciably
undersquare designs like the narrow-angle V-twin Lion-Peugeot, which in its
largest version had a bore and stroke of 80 mm x 280 mm and developed forty
horsepower at 2,200 rpm, which corresponds to a piston speed of 4,000 ft/min,
in 1910. (The record bore/stroke ratio in this period was a V-4 with 65 mm x
260 mm) The typical big racers of the period had appreciably shorter strokes
and lower speeds, a fair example being Fiat's successful 1905 GP car which had
a 180 mm bore x 160 mm stroke, sixteen liters, producing 120 hp at 1,100 rpm
with a piston speed of 1,040 ft/min.

The slow big-engine vs fast small-engine dichotomy had been around from the
very start, with a relatively small, high speed engine producing appreciably
more than its larger, lower rpm counterpart. The first Benz had a 90 x 150 mm
bxs, for 954 cc, producing .75 hp at 400 rpm, while the competing Daimler had
a 70 x 120 mm bxs, 462 cc, producing 1.1 hp at a vertiginous 600 rpm. De Dion
Bouton's first engine, with a 50 mm bore and 70 mm stroke, 137 cc
displacement, was intended to produce a half a horsepower at 900 rpm but had
bearing problems until Bouton found that it smoothed appreciably at about
1,200 rpm and ran well up to 3,000 rpm, and consequently it was redesigned as
a 250 cc, 1,500 rpm 1.75 hp fireball. Sources say that the early De Dions ran
as high as 4000 rpm in tests, and it was a 5 hp De Dion which powered Santos-
Dumont's first successful dirigible baloon. (Trepardoux, who had designed the
De Dion axle, had nothing to do with this; he was a steam guy, and had left
the company because he didn't want to ruin his reputation by being associated
with explosion engines.)

Enough- 

John H.

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