Multi valve engines

Multi-valve Engines

3-valve engines

The earliest mass production multi-valve engines were 3-valves because of its simple construction - it needs only a single camshaft to drive both intake valves and the exhaust valve of each cylinder. Today, there are still a few car cars using this cheap but inefficient design, such as Fiat Palio and Mercedes V6 and V8 engines. Mercedes claim to use them because of emissions rather than cost reasons.

4-valve engines

A typical 2-valve engine has just 1/3 combustion chamber head area covered by the valves, but a 4-valve head increases that to more than 50%, hence smoother and quicker breathing. 4-valve design also benefits from a clean and effective combustion, because the spark plug can be placed in the middle.

4 valves are better to be driven by twin-cam, one for intake valves and one for exhaust valves. Some Honda and Mitsubishi models prefer to use sohc, driving the valves via rocker arms like the aforementioned Triumph. This could be a bit cheaper, but introduces more friction and hurts high speed power. Therefore the sportiest Honda and Mitsubishis still use dohc.

5-valve engines

It is arguable that whether 5 valves per cylinder helps raising engine efficiency. Audi claimed it does, but fail to provide evidence to support. In reality its 5V engines are no more powerful and torquey than its German rivals with 4 valves per cylinder.

5-valve design doesn’t guarantee covering more head area than 4-valver. Nevertheless, if the combustion chamber is in irregular shape like the picture shown, the valves may cover a larger area. The Ferrari F355 makes use of this to enhance high-speed breathing. Is there any disadvantage? Yes, faster breathing also harms low-speed torque if no counter measure is taken. Therefore it is really more suitable to sports cars.

All existing 5-valve engines have 3 intake valves and 2 exhaust valves per cylinder, still arranged as cross-flow. The exhaust valves are larger, but in terms of total area the intake valves are larger. In the F355, by arranging the outer intake valves opening 10° earlier than the center valve, it gets the swirl needed for better air / fuel mixture, hence more efficient burning and cleaner emission.

The advantage of 5-valve engine is still under question. Only a few car makers used it (VW group, Ferrari and the bankrupted Bugatti), but Formula One cars also no longer favour it. Even the Ferrari F1 cars which was once famous for its 5V engine has switched back to 4-valve design a few years ago.

Multi-valve Engines
In the F355, by arranging the outer intake valves opening 10° earlier than the center valve, it gets the swirl needed for better air / fuel mixture, hence more efficient burning and cleaner emission.

Toad wrote:<SNIP>but the extra benefits of FSI outweighed the loss of the valve area?

kevhaywire wrote:And we all like curtains

I'm not sure VAG's move back to 16V for FSI was valve space related, cause if you look at this image, you can see the injector is actually underneath the intake valves, so 20V could have worked also.

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I think as DT's post said, VAG just couldn't justify the extra cost of machining 20V heads when there is minimal gain. My cynical side still thinks the whole 20V thing was a marketing gimmick. And it also still strikes me as being a little odd that the first 20V heads had big inlet ports, but later went down to small ports. Very strange. Perhaps they had gas speed issues with the big ports and needed to speed it up for improved bottom end torque? So a 16V to begin with would have been better