2, 3, 4 or 5 valves per cylinder?

What are the advantages and disadvantages of having any of these valve configurations:

1 inlet-1 outlet
2 inlet-1 outlet
2 inlet-2 outlet
3 inlet-2 outlet

2 in - 2 out seems to be the standard today. Why?

      • It has to do with how much of the area of the cylinder head you can cover with valve space, while still leaving room for the spark plug (the spark plug neds to be on or near the center of the cylinder axis as well to work best). More valves cost more money, and if you use 1- or 2- valve arrangements, then their actuators can be lined up along the camshaft, and so are easier to engineer than 3-valve-in types. …5-valves was the best last time I read, and the intakes are smaller than the exhausts.

They say that engines with 2 valves tend to be more torquey in the low-mid RPM range, while 4 valve engines have less torque there, but more horsepower at high RPMs. Is that true?

Some Honda engines are designed to only use 2 valve below a certain engine speed which improves torque at lower revs, and that 2 more valve will be actuated as soon as the engine speed increases above the set rate, and this allows the engine to rev more freely and make more power.

I have seen 3 valve enigines on Honda motorcycles, specifically the Superdream engine, but it does not seem to have been carried through to other machines so I’d guess thre was not enogh of an advantage to it.

Remember these three points:[ul][li]It’s easier for an engine to push air out of a cylinder than it is to suck it in.[]If you can move more air into and out of an engine, you can produce more power.[]Air has mass, so it has momentum. A column of air will continue to move once it starts moving.[/ul][/li]
Taking them in order,
It’s easier for an engine to push air out of a cylinder than it is to suck it in. While the piston rising in a cylinder can push exhaust gases out at very high pressure, the Earth’s atmosphere is at about 14.7psi at sea level, which means that there is no way that the piston dropping in the bore will produce more than 14.7psi of pressure differential to pull the intake mixture into a cylinder. Since you need to move the same amount of matter into the engine as you will move out, the intake path needs to be bigger in cross-section than the exhaust path. Typically, either the intake valves in an engine are larger than the exhaust valves, or there are more intake than exhaust valves.

If you can move more air into and out of an engine, you can produce more power, so it would make sense to make the intake and exhaust paths as big as you can. It would be great if they could make an engine where most of the head area would open up as the intake valve and the rest opened as an exhaust valve. It isn’t practical, but increasing the number of valves helps work towards this ideal.

The smaller valves can also be operated at higher RPMs, since their lighter weight makes them easier to control at high speed.

Air has mass, so it has momentum. A column of air will continue to move once it starts moving. Fewer or smaller valves may help when running an engine at lower RPMs, since the higher port velocities may help pack air and fuel into the engine and draw exhaust out.
I think that having four valves per cylinder on a dual overhead cam engine is popular because it is easy to maximize valve area at a minimal cost. Typically, each valve is directly operated by a cam.
Casdave, I’m not familiar with Honda’s V-TEC engines, but I think that they use a similar principle to what Yamaha used for the XT-600, where there are separate intake tracts for the primary and secondary ports of the carburetor. The primary (which is used for all speeds) has a short duration camshaft lobe; the secondary (for high speeds and high load) has a longer duration lobe. Each path is optimized for its RPM range.

      • Audi had a rally car that was using 5-valves per cylinder, and furthermore, the like valves were not even adjacent to each other: moving clockwise around the axis of the cylinder, you had one (smaller) intake, one (bigger) exhaust, two (smaller) intakes and then the other (bigger) exhaust. And the cylinder head was domed as usual, so the valve stems stuck off in five different directions. The engine was like two liters, made up around 1200 HP or some huge amount.
  • And I am pretty certain that there have been Japanese GTP-style motorcycles as well that used 5-valves also, but I can’t remember any instances right off.

Just a side note to the discussion: I’m inordinately annoyed by car ads in the U.S. that solemnly proclaim “32-valve engine”. I guess it’s supposed to sound more impressive than “8-cylinder engine”. Madison Avenue apparently believes we are all morons.

The Ford V8 that MG Rover uses for their range topping MG ZT is supposedly pretty low powered for it size but quite torquey, is that because of the two valves per cylinder?

The VTEC engines I was referring to are injected, so there will be throttle bodies but no carbs, at least on Honda’s bikes

AFAIK increased oil pressure operates an actuator which in turn causes the valve to be operated by the cam.

It is more impressive, if the “other” V8’s are only 16-valve engines. The alternative to these six syllables takes away commercial time: “V8 engine with four valves per cylinder and dual overhead cams.”

Surely there are still 16-valve V8’s in production? I think the Big 2.5 still even have some pushrod engines, don’t they?

Slight hijack for how the vtec works

Peace - DESK

Another aproach to increasing the amount of head area available to the valves is to change the shape of the head. I remember Honda developed an oval-pistoned engine in the eighties, but the idea clearly never caught on. Does anyone know why?

It is much more expensive to machine ovals than it is to machine circles.

The new(ish) Hemi in the Chrysler 300C is a 16 valve pushrod V-8.
There was an article in a recent car magazine (Road & Track I think) which explained why 2 valves per cylinder was more efficient for the hemi.
I think all the engines offered by Audi in the USA are 5 valves per cylinder.
That includes the 1.8T which is also offered by VW.

As Cornflakes mentioned it is all about flow, more air = more HP.
Do a little simple math do the area of a circle, the double the radius and look at what hapens to the area. It goes up by four times. So by putting is two small valves, the engine will flow more than one large valve.
As far as torque goes, and midrange vs top end, that is more a function of cam profile and lift then it is valve configuration.

Honda had to run the bikes above 20,000rpm to get the performance they already had with their standard four cylinder bikes.

I seem to recall that the project came about because V-8 engines worked for road racing, well enough that they had been banned. Honda’s concept was more to make a V-8 engine with siamesed cylinders than it was to increase the valve area in a four. Personally, I’d assume that it would be harder to compress the mixture in an oval cylinder to a small area than in a round cylinder without getting detonation.

Oh yeah, gazpacho (respectfully), the engines were prototypes for road racing, hence for marketing and for prestige. Machining costs weren’t that big of an issue.

Personally, I was surprised that they could get the rings to seal on an oval piston, but according to what Honda told the press it wasn’t that hard to do.