Designing a cam

[RoarkS]

Hey folks,
Would like an idiot check.
I'm working on designing the camshaft for a small 4 stroke generator engine that will run at 5000 rpm. I've spent about 3 weeks total on cam design thus far and am having an absolute panic attack. lol

the cam has a base circle of Ø1.125 and a roller follower Ø.5

I started with cycloid, then modified trap, 3-4-5, then higher order... started messing with Cubic Bézier splines... and more less ended back up with a 3-4-5.

Anyway... I've been trying to keep things under 100g of acceleration and finally made it.

Ended up with a 3-4-5 that has a 100deg rise, and 100 deg fall no dwell. The actual equation says I should be getting about 79g but the simulation came back with a peak of 96.7g. I think that has everything to do with roller contact angle/pressure angle...

That said I was like oh... I'll just plot the roller center then cut out as it rotates to get the lifter to do what I want... omg. that's not possible... at least my simple brain can deal with it.

Anywhoo... I'm already at 200 degrees seat to seat on the cam. It's boxer engine, so no offsetting the cam bore I think this is it.

Is there anything else I can do?

 

[Lou Scannon]

Along with the mechanics for 5000 rpm, you should be looking at the fluid dynamics...
Do you have a calibrated engine model telling you what volumetric efficiency you will be getting with a given cam profile, valve diameter, and port flow coefficient set? ASSuming you have a power target and aren't willing to just take what you get.

 

[BrianPetersen]

200 degrees seat-to-seat duration sounds shy. Intake valve opening will probably be 10 or 15 degrees BTDC and intake valve closing will probably be at least 30 degrees ABDC and probably more, unless there's something going on that the rest of us don't know about. Lobe center at 100 - 105 degrees ATDC is in the plausible range. That would give 110 - 120 degrees to open and the same to close (if you use a symmetrical lobe) which would help cut down your acceleration. Obviously the right way to pin the cam timing down would be with a proper simulation.

Nominal valve event timings are often given at 1mm or 0.050" lift depending on whether you live in a metric or imperial world - this will differ quite a bit from the seat-to-seat durations.

Complexities ... If you are dealing with manually-adjusted mechanical solid lifters (no hydraulic automatic lash mechanisms) you are going to need a lead-in and lead-out ramp at the beginning and end of the lift profile, to allow the valve clearance at the maximum clearance-specification limit to be taken up consistently before lifting the valve off its seat and to set it down against the seat in a consistent manner. Not an expert but I suspect these will be linear motion profiles, which also means your cam-lobe profile needs to start from a non-zero slope (the lead-in slope) and end in another one.

 

[RoarkS]

LouScannon: I'm doing a lot of ASSuming. lol... but I'm trying to be very conservative on the assumptions. I'm shooting for 22hp ISA sea level. Done a lot of comparative stuff. I don't have any good fluid dynamics but I'm essentially copying a well known design for head and port geometry. I wouldn't be surprised if I end up closer to 30hp. I just got started with openfoam last week... but there's a steep learning curve there.

BrianPetersen: Really... I was getting scared 200 was too much. I've not done something designed to live at this speed before but I guess. 120 degrees ought to drop it down A LOT more. (yes symmetrical lobe). I've got a pretty good dynamics simulation going right now and feel pretty good about things... but certainly not fluids... just a bunch of hand calcs for combustion speed and so on.

love child of metric stock parts, and inch design... lol. I had read about that .050" thing a long time ago thanks for reminding me that certainly makes me feel better about the 120 degree idea. I'll math it out tomorrow and run the simulation. probably gets me down into the 60-70 g calculated... probably high 70's in the dynamics sim.

Yeah going with solid lifters.... ugh that sounds gross. The 3-4-5 is VERY shallow on it's approach... I think it will be fine? :/

 

[RoarkS]

Okay. I'm freaking out a little here...
cam lift is 100 degrees on the cam. that means 200 degrees lift to lift on the cam. that's 400 degrees on crank. I'm mapping it out and that seems impossible.

What's considered an acceptable G load?