Engine design

[quk@sievers.com (Phil Payne)]

In message <199610112215.WAA147793@smtp-gw01.ny.us.ibm.net> "Bill" writes:

> Can anyone give a clear explanation of what happens when the timing 
> belt breaks on an interference type engine.  Also what are the 
> advantages of this engine design?

There are several forms of "interference".  In the simple case, the inlet or 
exhaust valve opens so wide that it occupies the same piece of space sometimes 
filled with piston.

Neither of the valves is ever fully open when the piston is at the top.  At the 
end of the exhaust stroke, the exhaust valve closes as the piston approaches.  
At the end of the compression stroke, the valves are obviously closed anyway.

So you can make the combustion chamber flat, or put a large proportion of it 
in the top of the piston.  When the valves open fully, the piston isn't there 
-so they can open into space the piston sometimes occupies.
 
All this is fine as long as the timing belt holds the crankshaft and the 
camshaft(s) in the correct relative position.

Lose the timing belt, and the camshafts move out of sync.  Within milliseconds, 
a rising piston will find a valve that's still open, and hit it at some speed 
and with _considerable_ force, ramming the valve back up against the lifter and 
camshaft.  Actually, it's even possible to wreck a mis-set interference engine 
by hand turning it.
 
In the complex case, the inlet and exhaust valves are driven by two camshafts 
(20V engines) and are set at an angle - they're never _fully_ open at the same 
time, and so they can sweep the same space as each other (and/or the piston).

There's been a lot of discussion about timing belt change intervals.  VAG makes 
no recommendation (for legal reasons) but (IMO) 60k miles is the _absolute_ 
limit for a timing belt.  I personally believe in 30k changes - this opinion is 
partly coloured by the _horrendous_ cost of short MB engines.  $$$ divided by 
$$$$$ is simply not a big number.

--
 Phil Payne
 phil@sievers.com
 Committee Member, UK Audi [ur-]quattro Owners Club