(How many) Valves per cylinder...?

[Podie]

Right, please excuse my ignorance here... but this has been bugging me.

Cars are often advertised as being 8 or 16 valve... and I understand what valves do, how they work and their placement on 2 and 4 valve per cylinder engines..

BUT, on a 20 valve four-pot (like the current Audi 1.8 turbo lump), WHERE does the extra valve reside, and what does it do? (inlet or exhaust?). Someone has suggested the extra valve does both jobs... but I'm not convinced.

Thoughts?

[pdv6]

Not sure, but don't they have 3 small-ish inlet valves (to improve fuel/air mixing?) and 2 large exhaust valves (don't care about the content of the exhaust - just get it out fast?)

PS How on earth can you have 1 valve that handles fuel and exhaust?

>> Edited by pdv6 on Monday 5th August 09:35

[Nick M (nmilton)]

Yep, spot on.

[Podie]

quote:
PS How on earth can you have 1 valve that handles fuel and exhaust?

No idea... hence as I said in my post "I'm not convinced" - but you know what pub talk can be like.

Thanks for the info though guys...

[V8Bert]

The extra valve is an inlet valve.

Given the shape of the cylinder is round, it turns out that by having 5 valves in one cylinder instead of 4 allows you to use more of the available area in the cylinder head.

Thus, the total area of valves is greater than in a 4 valve/cyl engine.

[sjm]

Definately inlet valve. Saw a cutaway of this engine at the motorshow last year and it's pretty impressive - almost all of the top of the cylinder is a valve of one sort or another.

[funkihamsta]

More smaller valves are more efficient from a gas flow perspective than fewer larger valves even if the area of the valve inlets sums up the same.
This is primarily because at the start or end of a valve lift cycle gases are only passing through a tiny crack on the perimeter of the valve inlet. Two smaller inlet valves (for example on a 16v) will almost always have a greater total perimeter/region of gas flow than the single larger inlet valve on its 8v counterpart.

got it?

[funkihamsta]

And no a valve is either exhaust or inlet never both.

[gnomesmith]

Unless it is a sleeve valve ( look under Daimler ).

[funkihamsta]

I stand enlightened.
Bit before my time though!

[JonVickers]

Try http://autozine.kyul.net/technical_school/engine/tech_engine_2.htm#Multi-valve

The guy is an enlightened amateur, but knows his stuff.

JonV

[Podie]

Top banana - much appreciated.