Small IC engine camshaft design

[Moper361]

Hi all ,I have just completed a small steam engine and am in the throws of setting a new challenge for myself .

I fancy having a go at building a small IC engine from scratch similar basic design to my steam engine I have just finished .

Basic specs
26mm bore
30mm stroke
Rod length approx 112 mm long
2 x flywheel at 115 mm diameter

I am unsure if to try with atmospheric inlet valve or mechanical on both inlet and exhaust .

I have never built an IC engine before and would like to gather basic information for thoughts on a camshaft that would work for either types of valve arrangement

I don't want to get to brain washed with the since of cams etc but want something basic that would be workable .

I would be working along the lines of a mushroom follower design lifter so a cam with quite steep ramp could be used as I can make a camshaft like this on my set up here without to much issue .and think this is more inline with what older engine designers used .

Any help or feed back appreciated

Regards Nat

 

[Moper361]

Here is a test camshaft I made up just to see how it looks I have a sketch also of Specs but not sure if it's any good really I'm no engine designer

 

[Moper361]

This is how it turned out and looks ,do any of you seasoned engine builders think that this would work okay with mushroom followers and using this profile as both inlet and exhaust if I was to go with both mechanical actuation on inlet and exhaust ?

 

[Cogsy]

That looks to be a lot of lift, which in turn means you need a lot of piston clearance at TDC, which means a low compression ratio. You can design around these things of course, but it might just be easier to reduce the lift somewhat.

If you want to build more of a round flanked cam, have a look at the method which uses a boring head in conjunction with a rotary table (assuming you've got a boring head).

 

[Mechanicboy]

The common rule for model engine: Valve lift = Valve diameter (valve in cylinder head) : 4.

Also the valve lift is 4 times less than the valve diameter.

The lift angle is divided by lift period on crank rotation. To example the lift period for intake is 240 degree angle : 2 (camshaft is rotating 1/2 by crankshaft) = 120 degree angle lift from open to close .

Shorten the valve lift diameter after you has calculated the valve lift when the valve diameter is known.

 

[Moper361]

That looks to be a lot of lift, which in turn means you need a lot of piston clearance at TDC, which means a low compression ratio. You can design around these things of course, but it might just be easier to reduce the lift somewhat.

If you want to build more of a round flanked cam, have a look at the method which uses a boring head in conjunction with a rotary table (assuming you've got a boring head).

Gday Cogsy it has about 4.1 mm lift as to turned out .as per Jens reply a rule of thumb would be take a bit of lift of once I know the valve diameter ,I have been thinking I can most likely squeeze a valve diameter of approx 12 mm in there and still have room for a spark plug .

With what Jens has brought to light I could reduce the lift by approx 1 mm giving me 3 mm lift using a 1 to 1 rocker ratio .this would give the valve diameter divided by 4 to give lift .reducucing the lift and leaving the a longer dwell time for valve to stay open at max lift .I could reduce the valve opening degrees a touch also this would give a bit more piston to valve clearance at critical times ,a bit of trial and error with what I have played around with and I think it's doable .😬 I'm just at the scratching head stages first then that's it I'll have a go at it

 

[Moper361]

Cogsy As you say if I had a Boaring head which I am in the throws of looking at just now for the purpose you have described .It gives a few more options to muck around with .

 

[Cogsy]

Your valves overall sound a bit on the large side. If you go 12mm x 2 in a 26mm bore then I'm guessing you wouldn't use valve cages (which is a mistake in my opinion - far easier to remake and refit a valve cage than a head if you have sealing issues) and there will be a very thin wall between them. For that size engine 8mm valves with around 2.5mm lift will work just fine. Take a look at some of Brian Rupnow's engine plans for good reference dimensions. He has designed and built some very nice running engines around the same bore size you're contemplating. Here's a video of the last one of his I built:
[ame]https://www.youtube.com/watch?v=dRmc6s-I2Co[/ame]

 

[Moper361]

Your valves overall sound a bit on the large side. If you go 12mm x 2 in a 26mm bore then I'm guessing you wouldn't use valve cages (which is a mistake in my opinion - far easier to remake and refit a valve cage than a head if you have sealing issues) and there will be a very thin wall between them. For that size engine 8mm valves with around 2.5mm lift will work just fine. Take a look at some of Brian Rupnow's engine plans for good reference dimensions. He has designed and built some very nice running engines around the same bore size you're contemplating. Here's a video of the last one of his I built:
https://www.youtube.com/watch?v=dRmc6s-I2Co

That runs as sweet as a nut mate I did see some of Brian's engines before they look good and run well ,you are correct and I'll take your advice on valve diameters .that would give more room to make a more rounded camshaft if lift is only 2.5 mm