DOHC valve angles and centerline

[Lambent32]

I have been trying to find information on whether DOHC heads have different angles for intake and exhaust and if not how the intake valves seem to go past the centerline of the cylinder. Are the sides at different angles? Is the intake side set deeper into the head? Trying to figure this out has been very frustrating because i can't find any information on this subject at all. Any help will be greatly appreciated.

 

[patprimmer]

They can be at whatever reasonable angle and size and lateral position the designer decides he wants to use.

All I have seen seem to be equal angle but definitely not the same size and often not the same distance laterally of the edge of the bore, although I never measured any, I can only say by casual observation.

Regards
Pat
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[evelrod]

My experience only extends to a few of the older DOHC designs. Mostly Cosworth, Lotus, Fiat, Alfa, Kawasaki and, Toyota...NONE of which have the same valve "angles". Two valve, four valve...some are better than others but all have been used successfully.
The Japanese seem to like to copy (oh well, It's true) the British and the British like to do things opposite whatever the French and Germans do. The Italians? I dunno. They have their own ideas.
My most personal experience is with British engineering so the others seem oddly done, oft times. Not bad necessarily, just different.

Rod

 

[BrianPetersen]

It varies from one design to the next, depending on the choices that each designer has made. There is nothing "legislating" where anything needs to be placed, other than the fundamental law that two chunks of metal can't share the same space at the same time.

The intake valves are bigger, and the outer periphery is dictated by clearance to the cylinder wall, so almost always, the intake valve centerlines will be closer to the centerline of the cylinder. This could result in the edge of the intake valves extending a little on the "other" side of the centerline.

It's not unusual for the included angles to be slightly different between the intake and the exhaust sides.

Since two chunks of metal can't share the same space at the same time, and since most engines like having some overlap (the intake starts opening before the exhaust closes), all hemi-style engines (I include standard 4-valve designs in this description) need clearance between the intake and exhaust valves so that they can't hit each other when both are partly open. And caution related to choosing the cam timing so that collisions can't happen.

Since positioning valves too close together, or positioning valves too close to the cylinder wall, causes shrouding of the flow, there needs to be a little space between them.

Since the exhaust valves are smaller, theoretically they could be spaced a little closer, and some engines are like this, but often simplicity of design and machining and dimensioning is chosen by spacing them the same distance apart.

"It all depends"

"Splitting hairs" on a mass-production design usually isn't a worthwhile exercise. If anything, "split hairs" towards the side of being cheaper.

 

[MikeHalloran]

See also "BB Chevy".


Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

Or the old Chevrolet "stovebolt" six, if you want to talk about wacky valve positioning and angles!

 

[BrianGar]

Sometimes the choice depends on if the cams above them will hinder the fitting/position of the head studs. If the designer is hell bent on a certain position that does effect the fitting/access to head studs, then a separate cam carrier/box can be made and fitted once the main head containing ports and valves is bolted to block.

I have seen and thought about many designs, and the narrow valve angle leading to a compact chamber seems to be the sensible way to go. Piston crown mass resulting in chamber shape cannot be ignored either if your going for high compression.

Cam wheels may clash also if your running tight valve angles. Meaning smaller sprockets are needed, which can lead to a tiny sprocket on the crank below.
You can get around this of course by just using one cam sprocket up top, and driving the other cam off it via a chain and two smaller chain sprockets within the head, and in a slight oil bath.
Chains wear and noise though, and can be less accurate than using two cam belt sprockets. You also have to crack the head open to adjust cam relations should you choose to fit vernier chain sprockets. Not a problem if you go with two belt sprockets, whereby you can adjust these easily, and externally.

You could also fit gears internally instead of the chain for more exact cam relation timing/reliability than that of the chain, but I guess they will be noisy too and possibly more costly to make.

Another driving solution is a mini cam belt as with the chain setup. This is a quieter option, but will need to be changed everytime the main cam belt is changed. This can be fitted externally, behind the main pulley to keep it easy to access and away from the oil.

Hope this adds to your thoughts some more, theres many more reasons, but they are the ones often forgotten when concentrating on the finer details.

BG

 

[Lambent32]

Thank you guys you have help me a lot and answered my questions in more detail than i could have ever asked.

 

[BrianPetersen]

BrianGar makes a good point about the head studs. It's common on the motorcycle engines that I deal with, for the camshafts to be centered on the bolt pattern for the head studs/bolts. Obviously the head is bolted down before the cams are installed, then the cams are installed overtop of the holes for the head fasteners. If you are using bucket-type followers, this pretty much dictates what the angles are going to be. It will be slightly different on intake and exhaust sides because of the different valve sizes. If you want to use tighter valve angles than that, some engines use finger-followers with the pivot shaft outboard of each cam and the tip of the follower (that presses against the top of the valve) inboard of each cam.

 

[BrianGar]

Brian, something else just occurred to me lastnight too,

You could also get around small cam sprocket issues resulting in the need for small crank sprockets by running(timing belt) the cam sprockets off an intermediate shaft, where the shaft is driven off the crank via drop down gears. Since The intermediate shaft is then running half speed, timing belt pulleys the same size as the ones on the cams, can be fitted to the intermediate shaft for belt drive.
This gets you over the problem of the timing belt having to conform around a very small toothed crank pulley, as mentioned in my post above.

So a recap to be clear,


Crank with gear sprocket fitted drives gear sprocket located just above it(mesh distance)at half crank speed.
This gear sprocket/intermediate shaft transmit out through engine case, where it is fitted with a timing belt pulley, the same size as the cam pulleys above them.
The belt then has a much easier life, not then having to conform to a tiny crank pulley.

I remember yrs ago seeing the smallest crank pulley I ever did see, it was from memory approx just 40mm in diameter(total dia)
It was shocking to see the belt around it, as the outside belt profile resembled a dodecagon instead of a circle. But I guess it passed everything, but I couldn't help feeling sorry for the belt, and it had a bit of an eekkkk factor looking at it, and thinking about it, at 7000rpm!

Obviously the gear drop down method I mentioned above costs way more...

It all 'depends' really I guess..

BG

 

[dgallup]

Brian

I believe the last generation of the Honda VF800F used that arrangement. It was after the all gear drive versions so it was actually cheaper & quieter. They also added Vtec at that time but I'm not sure why, it just put a nasty bump in the power curve.

 

[BrianGar]

I didnt know that, Interesting, thanks,

BG

 

[Peter7307]

Lambent 32,
have a search for books by Ian Falloon on the Ducati DesmoQuattro and other Ducati engines and also for a book by Alan Cathcart and Marc Cook on the Ducati 999.

Both discuss the valve layouts , angles , packaging requirements etc in detail and Falloon makes comparisons with offerings from other bike makers in the same field.

Just as an aside the new 999 engine has the cams inserted from the side through a small opening so the head is a one piece casting... and it is a desmodromic actuation as well.

I bet the assembly crew loved the designer for that one!

Peter.