DOHC valve angles and centerline
DOHC valve angles and centerline
(OP)
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.





RE: DOHC valve angles and centerline
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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RE: DOHC valve angles and centerline
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
RE: DOHC valve angles and centerline
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.
RE: DOHC valve angles and centerline
Mike Halloran
Pembroke Pines, FL, USA
RE: DOHC valve angles and centerline
RE: DOHC valve angles and centerline
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
RE: DOHC valve angles and centerline
RE: DOHC valve angles and centerline
RE: DOHC valve angles and centerline
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
RE: DOHC valve angles and centerline
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.
RE: DOHC valve angles and centerline
BG
RE: DOHC valve angles and centerline
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.