Single cam working both inlet and exhaust

Hi

I have been trying to find information on the cam setup where 1cam operates both valves vi a cam follower

I have seen it on old blackburne and jap engines but can't find any info on this

Can anyone tell me what this system is called so I can try to model it

Thanks

Dougie

Don't know whether it had a particular name. IIRC, the 16 valve Triumph Dolomite used this arrangement. I remember them going like stink, and very noisily, at Silverstone - in the 70s? Might be able to find info about them...

I believe therenarena couple of model ic engines use this idea where one valve is operated above the cam and the other valve below the cam with a pivoted bellcrank

Ariel motorcycle single cylinder 350 & 500 ohv engines had 1 cam with 2 followers

NH & VH iirc.

D.

Edited By Dave Daniels on 24/01/2023 09:18:29

That must be done for economy of manufacture I would think as performance must be compromised to make this work. Not quite sure how the Dolomite Sprint engine achieved its performance if limited by this arrangement.

Mike

Dougie, Re the Dolomite Sprint valve gear Here I think the term is Uni cam.

MichaelR

With strange bell crank cam followers that pivoted above the camshaft, one running on each side of the cam lobe. Some (earlier) models had two lobes, others (later I think) had just one lobe. Famously Sammy Miller's trials bike GOV had the single lobe cam.

Edited By Hopper on 24/01/2023 09:52:00

Edited By Hopper on 24/01/2023 09:52:15

Edited By Hopper on 24/01/2023 09:53:46

As Bernard mentions Find Hansen has a nice model vertical engine where a single cam drives the two pushrods by a simple bell crank arrangement

The Sprint engine is limited by this arrangement and by other questionable design decisions. While it is poor compared to its theoretical potential, it does make more power than 8valve versions of the same engine. Merlin V12s use a similar design, which is probably where they borrowed it from.

Beautiful model engine there. And shows perfectly how the single lobe cam works. Looks like by having the cam followers contacting the cam below the centre line you get less than 180 degrees of cam rotation between when the inlet valve is fully open and when the exhaust valve is fully open. So you could vary the cam timing by raising or lowering the pivot points of the bell crank cam followers. The same effect as varying the lobe separation angle on a normal multi-lobe cam.

Edited By Hopper on 24/01/2023 11:17:08

That must be done for economy of manufacture I would think as performance must be compromised to make this work.

Doesn't appear to affect performance too much, as this is a method used by Honda on current bikes - taken from a description of the latest Hornet 750 :

It’s instantly apparent that the new Hornet’s engine takes a leaf from the Africa Twin’s book, sharing a similar Unicam setup – where a single overhead camshaft acts directly on the four intake valves and opens the four exhaust valves via a set of rockers – to give similar performance to a DOHC design but with fewer components and a more compact layout. Honda’s CRF450R motocross bikes have long adopted a similar setup.

Doubtless having to make fewer parts will reduce costs as well.

Nigel B.

Chuck the Muddle Engineer described a Glow Plug horizontal engine in ME some time back. This used a single cam, but other than a scale drawing no build description was give,

Regards

Gray,

But don't the Hondas have separate intake and exhaust lobes, on one camshaft?

Some pictures here of a ner a car engine ( blackburne) showing the arrangement

Thanks for all the replies

Some interesting reading there

The motorcycle style shown in the post is what I was thinking of

I'm thinking of building a jap or blackburne v twin and I'm just trying to understand the single cam setup

Any ideas on how to work out the angles

Dougie

CAD

What do you mean Jason

Dougie

The two valves need to open at a given rotation, one after the other, x degrees apart. Once your timing is selected, mark out on the cam gear the two opening positions, and arrange the two bell-cranks so that the followers start to rise on the cam, one for the first angle, and then x degrees later the other follower starts.

If I was designing from scratch I would make a cardboard (etc) disc to represent the half-speed gear, centred on a drawing pin, and mark on it the required opening and closing positions. Then I would play with various options (eg one pivot or two, etc) until I arrived at a pair of followers which give the desired timing. It would help, as you are starting from scratch, to make the gear with a keyway, and a spindle with a matchning longer keyway to fit, and then the two cams to the same profile to fit the spindle, but no keyway yet. Then you can position each cam where you think it should go, and turn the engine slowly to check where the valves lift and close. You can do each cam separately to avoid confusion, but be sure to mark each cam so that you always get them in the desired position and the desired way round. When you are sure you have it right, mark each keyway position and cut the slots.

As regards the 'best' timing for your engine, it depends a lot on what you want to use it for. Look at the timing details for a range of full-size engines, and you will find a big difference between lawn mowers, designed to slog all day, and racing machines, designed to go flat out for short periods. And the further back in time you go, the slower and steadier the engines are, with smaller angles of 'overlap'.

Hope this gets you going

Cheers, Tim

Dougie, I would no doubt use my CAD drawing package to work it out on the computer. But you can also use Tim's suggestion of old school Cardboard And Drawingpins

As well as positioning the two followers the required angle apart you may well need to make allowances for the fact the bellcranks move in an arc rather than pushing directly on a pushrod. The lengths of the bellcrank arms could also be tweaked if you wanted more "lift" on one valve than the other simply by having different lengths to each arm to reduce or increase the amount of movement. Diameter of roller followers may also allow some difference in duration for the same cam profile

I always wanted a Dolomite... but they were British Leyland...

valve pic here

https://www.documentosdelmotor.com/noticia/26-triumph-dolomite-sprint

They only one I ever went to see had a swimming pool in the footwell so you'd have to wear wellies to drive it

So we bailed out the integrated foot bath and I had a look, I even got it started for him and he was really chuffed with me

I didn't take it home