Valves Discussion..

[naderson]

Thereare different factors totakle into consideration while designing valves in the cylinder..
when it comes to valves, things get really complicated..
what i want to know is the following:

1- How does the area of the valve affect the engine (Performance,RPM, Efficincy, Fuel consumption)
2- How does the numberof valves -in intake & exhaust- affect the engine (Performance,RPM, Efficincy, Fuel consumption)
3- what about multiple valves but wit different timings? how to manage the timing?

+++
- i know that the intake valve is usually bigger thatn the exhaust, andi do know why..
- i know that the bigger the area the better, and the design plays the role inchosing the area..
but i want theeffects...
- regarding the multiple valves.. i know engines with 1 by 1, 2 by 2, 2 by 1..
what ould that affect..

thanks in advance..

[curtis73]

1- Larger valves have the potential for greater flow within certain parameters. If you go too large, one side of the valve will get closer to the cylinder walls and that part of the area becomes shrouded and can hurt flow. Greater flow potential has greater HP potential if the rest of the engine is tuned for it (cam, head flow, compression, rotating mass, exhaust, etc). The downside to greater flow is less flow velocity which can reduce torque production in the lower RPM range. More flow makes power numbers come at higher RPMs, hence the requirement for higher stall converters, and lower gearing.

2- Increasing the number of valves increases the potential area of valve surface. It also decreases the real estate in the chamber to dissipate heat into the coolant. Any time you increase flow potential, you have the ability to more more fuel and air and that tends to increase fuel consumption. The problem is compounded by the fact that the engine now has to rev faster to make its power, so not only are you drawing in more fuel, you're doing it more times per minute.

3- individual valve timing has been used. Its intensely complicated and its major benefit is inducing swirl. Sometimes its used with a variable volume intake and only one valve operates at low RPM and the other opens up at higher RPM. Varying results, but with today's technology it seems to add more complication than benefit.

[Moppie]

Originally Posted by curtis73

3- individual valve timing has been used. Its intensely complicated and its major benefit is inducing swirl. Sometimes its used with a variable volume intake and only one valve operates at low RPM and the other opens up at higher RPM. Varying results, but with today's technology it seems to add more complication than benefit.

Honda made, and I believe still does make good use of only opening one inlet valve per cylinder at low RPM. Then opening the second at mid RPM, then opening both a little further at high RPM. They it called VTEC-E and sold it in a number of low end Civic models in the 90s.
Now I think its used in a variety of forums in the Jazz/Logo/Fit, and maybe the new Civic. Altough most VTEC systems now either open the second valve at mid RPM, but do nothing else. Or, the more traditional open both valves further at high RPM.

[UncleBob]

Originally Posted by curtis73

2- Increasing the number of valves increases the potential area of valve surface.

just an added note on this: up to a point.

5 valve per cylinder engines have waxed and waned in the last couple decades (3 intake, 2 exhaust) because some believe that they flow better than 4 valve per cylinder designs.

Yamaha, one of the leaders in head designs, did some interesting studies in the mid 80's on number of valves per cylinder. They tested multiple configurations upto 7 valves per cylinder, and found that the highest bench flow was 5. So they started making their genesis motors with 5 valves per, and it was a big selling point (aka, brochure selling point) that they touted often.

But the real result was, they were not outperforming the competition. Nobody else was playing with 5 valves per, and yamaha was often beat by them as far as peak engine performance. In the early 90's they retracted a bit and limited their 5 valve per design to liter plus engines, they stated that engines smaller than that had no worthy benefits, then in just the last couple years, they finally did away with them all together, going back to the more traditional 4 valve per.

The details are complex. The flow patern over the valves as they open are hard to quantify even with our modern computer models. The belief is, although 5 valve per designs have the best "realistate" as far as valve surface vs combustion chamber, some believe that they are lacking because the flow over the intake valves create self counseling dead spots that 4 valve per head designs seem to never encounter.

Anyway, its a complex subject, but there's a bit of history and detail as far as the higher end spectrum, as far as number of valves go

[Steel]

Originally Posted by Moppie

Honda made, and I believe still does make good use of only opening one inlet valve per cylinder at low RPM. Then opening the second at mid RPM, then opening both a little further at high RPM. They it called VTEC-E and sold it in a number of low end Civic models in the 90s.
Now I think its used in a variety of forums in the Jazz/Logo/Fit, and maybe the new Civic. Altough most VTEC systems now either open the second valve at mid RPM, but do nothing else. Or, the more traditional open both valves further at high RPM.

Honda is still using it with their VTEC. My dad's civic si (that i got to work on while working at honda) one of the intake valve opens fully, while the other just breaks open, until the VTEC kicks in and then both valves open fully.

But cams and valves are just silliness anyway. Go with a self valving engine- GO ROTARY!