Lyndon Tiu wrote:
> On Tue, 22 Nov 2005 09:05:10 -0700 mike@garfias.org wrote:
>
>> Hmm.
>>
>> Did you know that overhead cams are an OLDER technology than pushrods?
>>
>>
>
> Really?
>
> I wonder why most cars today use overhead cams?
>
> --
> Lyndon Tiu
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Reciprocating weight. Push-rods and rocker arms weigh more than the
thimble + shims that most ohc engines use. The heavier the reciprocating
weight, the heavier the valve springs have to be to prevent valve float
(which can lead to interference with the pistons, making very expensive
noises.) Many modern engines use twin cam, four valve per cylinder,
design (pioneered by Isotta-Fraschini in 1911, I believe. I used to have
some pictures of a marvelous old chain drive IF with a gigantic four
cylinder engine with just such a layout.)
At any rate, two valves of half the valve surface area weigh about half
of what one valve would weigh and would need lighter valve springs. Less
reciprocating weight equals higher rpm without valve float, and that
equals more power from a given displacement engine.
The upper end of the rpm scale is usually governed by a phenomenon
called valve spring surge. This occurs when the valve reciprocation
equals the natural vibration frequency of the spring, causing
catastrophic failure of the spring, resulting sometimes in an engine
devouring itself. Various schemes were tried to get around this
limitation: hairpin valve springs, torsion bar valve springs (both
difficult to contain, requiring huge valve covers.) Mercedes-Benz in the
fifties 300SLRs used desmodromic valve actuation, in which the valve is
forced open by a cam and forced closed by an "anti-cam." This won some
races for them, but was extremely expensive to manufacture and maintain.
Probably the most elegant answer to this problem was the one devised by
Ferrari a couple of years ago, when they set for themselves the task of
wringing 900 horsepower from a
V-10 twin cam multi-valved unblown three liter engine. They figured that
if they could get 300 hp out of a three liter engine at 6000 rpm then
they would have to spool up the engine to 18 grand to get 900 hp. No
known valve spring would survive that, so they used 150 lbs of air
pressure to close the valves. Air has no natural frequency, so the
valves could operate at this rate. The only thing limiting the rpm was
the reciprocating weight of the pistons and connecting rods. These they
had to fabricate from titanium, at a cost that probably made even
Ferrari blink. I never found out whether they ever raced that engine,
but what a marvelous tour de force!
Bob Eaton
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