Cam profile mania

[The Artful Bodger]

Hopefully the exhaust duration is a lot more than 180 degrees extending over several revolutions of the miss period.

Which brings up another point, the cam turns once for 720 degrees of crank rotation and depending upon such variables as load, temperature and the phases of the moon the engine must coast up to two revolutions between the governor tripping and the beginning of the hit cycle. This variable delay may explain why the engine must hit twice sometimes which tends to spoil the effect of a H&M on display. Maybe there is some practical means of stopping the cam during the miss cycle and reengaging at the appropriate point on the next revolution rather than having to wait for up to two revolutions for the next cycle to begin.

[BCRider]

Bodger, there are so many different designs for H&M engines out there that I have no doubt that at least one of them used a method that interrupted the cam in the max opened position for coasting then caught the cam when a firing cycle was needed. I'm sure someone will be along shortly with a name of such an engine.

The hit and miss times are not a precise sort of thing because the action of the flyweights are not tied to the rotational timing of the engine. So yeah, often the engine will fire twice in a row and that alters the coasting times. But that is just part of the enjoyment. If they sounded too much like a metronome they would not be as much fun. They are just way more fun when they have a more jazz like syncopation to their exhaust.

[Cuttings]

My memory is not too clear on this but I do remember the old double-ended fish boats with an Easthope hit and miss engine. They would take a few chugs then it seemed like quit a long time before they cut in again and gave a few more chugs. There must have been a fair RPM difference between cut out and cut in. On those old slow turning engines that may not have been a lot.

[BCRider]

You got me looking up the Easthope engine company. I had no idea that this was a local Vancouver BC company. All that history right in my own backyard!

[Paul Alciatore]

Brian, I confess that I did not read all the responses, but I do not see any drawings of the follower(s) that you are using with these cams. Unless the follower has a sharp point that rides on the cam, the shape and width of the follower will have an effect on the overall action. You can not analyze the action of a cam with just the shape of the cam. For instance, your first drawing shows a flat surface for the rise and drop of the follower and they each occupy a 37 degree angle. But you can not assume that the follower just follows this profile as the edge of the dwell area will hold it open for some time into that 37 degrees on both the rise and fall. And there could be other effects due to the shape and width of the follower.

Perhaps you are discounting them, but they are there.