<VV> Valve Rotators--no Corvair

[J R Read_HML]

Oh - But you MIGHT find this interesting... Most especially the first 
sentence in paragraph 4.

4-stroke cycle engine valves
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Four-stroke engines, of either spark ignition or compression ignition 
varieties, use poppet valves to allow air (or an air/fuel mixture) into the 
cylinder and exhaust gases out. Very early engines used alternative valve 
types such as slide valves, but these proved less satisfactory, especially 
for high-speed engines.

The poppet valve is pushed into the cylinder by a camshaft, usually acting 
on some kind of cam follower (tappet) in order to open it. It is 
self-closing under spring pressure, with the notable exception of 
desmodromic valves, mostly used on Ducati motorcycles, which are closed 
positively by a linkage. F1 racing car engines use compressed air rather 
than metal springs to close their valves, for the same reason as desmodromic 
valves; at very high engine speeds a simple spring cannot close the valve 
quickly enough and the valve "floats" or loses contact with its camshaft, 
spoiling valve timing and reducing power. The valve seals against a seat cut 
into the cylinder head, which may be a simple 45 degree angle or, in modern 
engines, may have compound angles in order to achieve less fouling and 
better gas flow. New valves are generally lapped (ground) into the head 
using a fine abrasive in order to achieve a perfect seal.

Modern automobile engines often use more than one inlet and/or exhaust 
valve. The reasoning behind this is to increase the total valve area, which 
is a major determinant of the power the engine can be made to produce. The 
circular shape of the combustion chamber limits the size of two single large 
valves, and it is easier to fit four smaller valves into the space while 
still achieving greater valve area.

Valves are almost always round since they have the advantage that they can 
rotate harmlessly in use and, as described above, can be lapped in. However, 
it is easier to achieve very large valve areas with non-round valves, and 
heat transfer to the cylinder head is more effective due to the larger 
circumference per unit area.

The exhaust valve is usually the hottest part of any engine. It is exposed 
to the full heat of the exhaust gases and can only lose heat to the cylinder 
head (and hence the cooling system) when it is closed, heat transfer to the 
valve guide being rather slow. Special steels are required, and some valves 
are filled with liquid sodium in order to conduct the heat away from the 
face. Aluminium cylinder heads always require hard steel valve seats, but 
cast iron cylinder heads often had the valve seats machined directly into 
the head until the 1980s. With the move to unleaded gasoline (petrol), a 
phenomenon known as valve seat recession occurs with these engines. The hot 
exhaust valve micro-welds to the seat under hard running conditions and 
tears material away as the valve reopens. Leaded fuel prevented this action, 
and the remedy is either to use a suitable fuel additive with the same 
protective function as tetraethyl lead, or to have an engine shop insert 
hard steel seats.

Since the valve has its head in the cylinder and its tip is exposed in the 
cylinder head, the valve guide must be sealed to maintain cylinder pressure. 
A rubber lip-type seal ensures that excessive amounts of oil are not drawn 
in from the crankcase on the induction stroke and that exhaust gas does not 
enter the crankcase on the exhaust stroke. Worn valve seals are 
characterised by a puff of blue smoke from the exhaust when pressing back 
down on the accelerator pedal after allowing the engine to over-run, such as 
when changing gear.

Retrieved from "http://en.wikipedia.org/wiki/4-stroke_cycle_engine_valves"

Later, JR

'61 Rampside Standard 4/110
'65 Monza Convertible 4/140 (times 2 for now)
'66 beater Coupe - icemobile 4/140


----- Original Message ----- 
From: "Chuck Kubin" <dreamwoodck at yahoo.com>
To: <virtualvairs at corvair.org>
Sent: Friday, June 02, 2006 11:42 PM
Subject: <VV> Valve Rotators--no Corvair


> This thread is going the way of bolt stretch of grade
> 8 bolts while dropping Corvairs from space shuttles or
> discussing technology on 1948 Italian cars. Regardless
> of what they are and how they work, they WEREN"T ON
> CORVAIRS!!!!!!!!!!!!
>
> Chuck Kubin
> LIFCCXGP
>