5 valves per cylinder?

[jitsukadave]

I've tried to search, but not sure what keywords would have been used, so can't find anything.

Anyways...

As a numpty, I've always assumed that you need an even number of valves per cylinder. At least two (one in, one out), sometimes four (two in, two out) like a 16V Clio, or a 24V V6 Probe.

However, I have seen an in-line 4 cylinder engine with 20 valves, and a V8 with 40 valves... Isn't that 5 per cylinder? ... 20 over 4 = 40 over 8 = 5?

How does that work then?

Am I missing something?

[matt_fp]

Three Inlets Two Exhausts Usualy.

Theres also three valve engines with two inlets and a single exhaust.

Idea is to get more valve area into the combustion chamber.

Matt

[Munter]

The Toyota 4AGE had a 5 valve per cylinder head in some configurations. And I think the 1st Proton's had 3 valves per cylinder.

[jitsukadave]

Fair enough, doesn't that make timing a little tricky, and leave more exhaust gass to remove through the lower number of vavles?

Still - it must work!

[esselte]

jitsukadave said:

Fair enough, doesn't that make timing a little tricky, and leave more exhaust gass to remove through the lower number of vavles? Still - it must work!

Won't the size of the valves sort this out?

[zaktoo]

Maseratis, Mercedes-Benzes and Mazdas have all had 3-valve heads in mass production. Ferrari, Mitsubishi, Toyota and some of the VAG products have had 5 valves per cylinder commonly. Having an even number of valves isn't necessary, as long as there is at least one inlet & one outlet.

HTH

Ciao

Zak

[catso]

zaktoo said:

Maseratis, Mercedes-Benzes and Mazdas have all had 3-valve heads in mass production. Ferrari, Mitsubishi, Toyota and some of the VAG products have had 5 valves per cylinder commonly. Having an even number of valves isn't necessary, as long as there is at least one inlet & one outlet. HTH Ciao Zak

3 & 5 valve configurations are used in some Motorcycle engines; Ducati make a V-twin engine with a 3 valve head and Desmodromic actuation (ST3) and Yamaha make a 4 Cyl with 5 valve heads (R1).



>> Edited by catso on Monday 5th September 23:12

[laurieb]

The reason behind it is that to get the gas into the cylinder you need big valve/port area. But if you have a single big valve the gas speed through the big 'ole will be low which is OK for max power maybe but not good for torque - esp so on the inlet side - so the ideal solution is to have "lots" of smaller valves so you get the best of both worlds - big valve area and good gas speed. The smaller valves also allow the designer to be more creative in getting a better combution chamber shape. That's the theory anyway ....

[annodomini2]

It's a requirement gas flow at low lift, with a multivalve engine compared to a single valve engine (in and out! ), lets say for example at fully open the the single inlet valve flows the same as a multi-inlet valve setup.

But with the multivalve setup at low lift it will flow more than the single valve setup, this improves gas flow performance at high rpm and why multivalve engines generally tend to rev better (on an ideal for ideal comparison.)

[pdV6]

laurieb said:

The reason behind it is that to get the gas into the cylinder you need big valve/port area. But if you have a single big valve the gas speed through the big 'ole will be low which is OK for max power maybe but not good for torque - esp so on the inlet side - so the ideal solution is to have "lots" of smaller valves so you get the best of both worlds - big valve area and good gas speed. The smaller valves also allow the designer to be more creative in getting a better combution chamber shape. That's the theory anyway ....

Actually, I thought its usually because as the valves open & close, the gasses are only entering/exiting through a relatively small gap at the lip. Having, say, 2 smaller valves with less overall area than one large one can give better breathing due to the fact that there is more "lip" area open at once. The length of time the valve is substatially open as opposed to closing or opening is relatively short, so it makes more sense to optimise the lip area than the total area.

[Andrew Noakes]

jitsukadave said:

doesn't that .. leave more exhaust gass to remove through the lower number of vavles?

Usually you need more inlet valve area than exhaust area. In two-valve or four-valve engines the inlets are often bigger than the exhausts.

At its simplest you can think of it like this: you have to suck gases in through the valve, and the harder that is the more power you waste doing it. But exhaust gas is under pressure when the valve opens, so even if the valve is small the exhaust gas fights its way out.

[jitsukadave]

Andrew Noakes said:

jitsukadave said: doesn't that .. leave more exhaust gass to remove through the lower number of vavles? Usually you need more inlet valve area than exhaust area. In two-valve or four-valve engines the inlets are often bigger than the exhausts. At its simplest you can think of it like this: you have to suck gases in through the valve, and the harder that is the more power you waste doing it. But exhaust gas is under pressure when the valve opens, so even if the valve is small the exhaust gas fights its way out.

Makes sense, just a matter of sorting out the timing for the valves opening.

Are the valves in the same group generally opened at the same time, or are opened over a spread of time? I'd have though opening at different times could give a better ignition, but would be quite risky as exploding fuel may make it's way back up the line?

[kevinday]

Another advantage of many small valves is that each has less mass, therefore less stress involved when changing directions.

[Andrew Noakes]

jitsukadave said:

Are the valves in the same group generally opened at the same time, or are opened over a spread of time?

Usually all together, though I think one of the three-valve Honda engines opened one inlet valve before the other to generate swirl in the combustion chamber.

[Tank Slapper]

There was an article by the tech bloke in EVO about this a while back.

If I remember correctly, the total circumfrence was the important thing, and not the total area due to most of the flow happening at the edge of the port.

I'll see if I can dig it out later.

[MR2Mike]

The main reason for multiple valves is simply increased valve area. With two valves, you can only go so big before they are touching in the middle. Going to more, smaller valves allows higher valve area and better flow.

Gas velocity will not really be improved, unless the multi-valve head is combined with a dual runner style inlet manifold.

[speedy_thrills]

Andrew Noakes said:

Usually all together, though I think one of the three-valve Honda engines opened one inlet valve before the other to generate swirl in the combustion chamber.

Hi Andrew, you are nearly on the money with that comment. Variable valve lift is at the heart of Honda’s VTEC system (and many other modern engine designs such as VVC), the reason they work so well is that Honda has developed (through earlier VTEC configurations) an elegant and reliable system that allows Variable lift not just in two stages but in fact three.

BMW had a system they where shouting about a few years ago that would allow continuous linear lift through variable length hydraulic tappets placed above a conventional valve (although this sounds like it should be easy its immensely complex and temperamental even compared to the VTEC system). I think they have since went very quiet about it due to technical difficulties, pity because other than electronic and hydraulically operated vales (which no car manufacturer uses due to cost and maintenance – real pie in the sky stuff) it seems to be the only system that would work better than Honda’s VTEC.

This explains the latest VTEC system which amalgamates variable valve timing, 3 stage valve lift, variable intake geometry and fly-by-wire throttle into the latest generation: http://asia.vtec.net/article/k20a/

As normally aspirated road engines go Honda still set the standard in engine technology (no engine delivers as well on all fronts), of course I cant exactly be counted as impartial “Says the man who drives a Honda Civic” – FrenchTVR (Which was originally supposed to be an insult I suppose , I’ll choose to take it as a compliment however given the above facts ).

[Andrew Noakes]

speedy_thrills said:

Lots of good stuff

But variable valve timing is a separate issue. The engine I was talking about doesn't have VTEC - the two inlet valves have different timing, but that timing never varies.

[fixedwheelnut]

Multi valve engines are to get more gas in but also keep up the speed of the gases, one large valve would cause slower gas and less swirl, by arranging more smaller valves the swirl is increased to get better atomisation of fuel and a more complete burn ergo more power.
The three inlet and two exhaust is as mentioned before inlet gas is drawn in at low pressure where as exhaust gas is under much higher pressure, sometimes the valves open at slightly different times to increase the swirl effect but this is designed in the shape of the camshaft not fancy timing equipment.

BMW use VANOS a form of variable valve timing where by the camshafts are advanced or retarded in relation to the crankshaft.
The later Valvetronic engines also use a variable valve lift to control engine speed/power, they still have a throttle valve but this is used for the warm up period and emergency running and controlling inlet manifold vacumn.

[speedy_thrills]

fixedwheelnut said:

The later Valvetronic engines also use a variable valve lift to control engine speed/power,

That’s cool, Is this a design on all BMW’s or just the top of the line models? Are all valves able to move independently as in other systems such that better swirl can be induced? Does the variable lift work on both inlet and exhaust valves? Can the system reduce lift to increase cylinder pressure under engine breaking?

fixedwheelnut said:

they still have a throttle valve but this is used for the warm up period and emergency running and controlling inlet manifold vacumn.

Forgive my ignorance but under what circumstances would you want to increase intake vacuum? Most intake designs seem to aim towards lowering the vacuum in the intake.