Cam Profile Mania

[Brian Rupnow]

As many of you know, I have just spent a month revisiting my Kerzel engine, in hopes of getting a better "hit and miss" action. One of the things I did was to make a new exhaust cam lobe based on the original drawing be Kerzel. Prior to that, I had been using a cam which I made based on the original Webster cam. I am now trying to wrap my head around the differences in these two cam profiles. in truth, I don't see the engine behaving a lot differently with the new cam than I did with the old cam.
I am getting into the scientific end of cam design and operation, and some of it stretches me.
The area on the mainly round portion of the cams at the bottom, which is a true diameter, has no effect at all on the valves. When the lifter or cam follower is riding on these surfaces, there is actually a gap called the "valve lash" between the lifter and other components in the valve train. For purpose of simplicity, we are going to overlook the effect of that "valve lash" in this discussion. It does have some effect on valve timing, but the effect is very small and can be safely ignored on these small engines.
.......It is only the portion of cam that extends beyond the base circle profile that has any effect on the valve, and even then there is a trick. Since the camshaft revolves at only half the speed of the crankshaft, then the 117 degrees on the left hand valve actually has 234 degrees of effect on the valve as relates to the crankshaft, and the 84.22 degrees on the right hand cam has an actual effect of 168.44 degrees on the valve, as related to the crankshaft.
.....The kerzel cam shown on the right is for a slow running hit and miss engine with an atmospheric intake valve, so there is no benefit in having the exhaust valve stay open during a portion of the intake stroke. When the piston moves from bottom dead center thru to top dead center on the exhaust stroke, the crankshaft revolves thru 180 degrees. The Kerzel cam as originally designed by Kerzel exhibits only 168.4 degrees of influence on the valve as related to crankshaft revolution. So ---what does that mean to us? Well, it means that if the Kerzel cam just begins to influence the valve when the piston is at bottom dead center, then by the time the crankshaft has advanced 168. 4 degrees towards top dead center on the exhaust stroke, the valve will have opened, expelled any exhaust in the cylinder, and then fully closed 11.6 degrees before the piston reaches top dead center. Consequently, there is no benefit in having the valve begin to open before the piston reaches bottom dead center.
-----The cam on the left however, was designed for a faster running throttled engine (the Webster), still with an atmospheric intake valve. Again, since the intake valve is atmospheric, there is no advantage in having the exhaust valve stay open during any part of the intake cycle. However, that 117 of cam influence relates to 234 degrees of crankshaft rotation. Since the crankshaft rotates only 180 degrees getting the piston to move from bottom dead center to top dead center, and we want the exhaust valve to be closed by the time the piston gets to top dead center, then we have to begin opening the exhaust valve while it is still 234-180=54 degrees before the piston reaches bottom dead center on the power stroke.--and surprisingly enough, that is almost exactly what Webster asks for in his engine plans.
.....I am still not totally clear as to why the Webster cam has a much wider "dwell" area at 0.218" wide as compared to the 0.050" "dwell" area on the kerzel engine, but I'm sure if I keep plugging away at this cam business it will become clear to me. Notice that both valves go from fully closed to fully open in about 37 degrees of movement, and both close in about 37 degrees of movement. It could very well be that the longer dwell time on the Webster cam is only there to extend the amount of time that the exhaust valve remains open on the Webster, since it is a faster running throttled engine and needs the extended time period to allow time for a full charge of exhaust to leave the cylinder and have no pressure remaining in the cylinder when the piston reaches top dead center and begins it's descent on the intake stroke....Very, very interesting stuff indeed.---Brian

 

[GailInNM]

Brian,
To further muddy the waters a bit for you, consider the cam follower shape.
The Webster uses a flat cam follower.
The Kerzel uses a roller follower with an 1/8 inch radius.
Your analysis of valve motion appears to be based on using a point contact follower.

When doing a math analysis of valve motion is is common to use a general set of equations based on a roller follower and use a roller radius of 0 for a point contact and very large number to simulate infinity for a flat follower and of course the trlue roller radius for rollers.

This mostly affects the the valve acceleration profile with the lift and dwell being equal in all cases when using a flat flank on the cam. Where it makes a difference on models is in valve lash. With a flat follower the valve acceleration is very high at the beginning of the stroke so valve lash affects the open time of the valve the least. A point follower has the least acceleration at the beginning so the valve lash affects affects the valve open time he most.

From a practical standpoint, the smaller we make an engine the more difficult it is to keep the ratio of the valve lift to valve lash constant so it becomes difficult to scale so a flat follower is the easiest to adjust. And we get away with flat lifters and flat flanks on the cams because the masses of the valve components will tolerate the high accelerations on small low speed engines.

Gail in NM

 

[Brian Rupnow]

Hi Gail---Nice to hear from you. In this case my analysis holds true, because I had used the Webster profile on a cam in the Kerzel engine, so both cams had the same 1/8" radius follower bearing acting on them. I realize that the Webster cam in it's original setting has a flat lever style follower, but I'm not sure if that accounts for the wider dwell area or not. I find it interesting that both cams have almost identical rise and fall angles of 37 degrees. I don't really know if that is an acknowledged standard, or just serendipity.---Brian

 

[Mechanicboy]

Here is the picture who i can explain how it works..

 

[Mechanicboy]

Since the crankshaft rotates only 180 degrees getting the piston to move from bottom dead center to top dead center, and we want the exhaust valve to be closed by the time the piston gets to top dead center, then we have to begin opening the exhaust valve while it is still 234-180=54 degrees before the piston reaches bottom dead center on the power stroke.--and surprisingly enough, that is almost exactly what Webster asks for in his engine plans.
.....I am still not totally clear as to why the Webster cam has a much wider "dwell" area at 0.218" wide as compared to the 0.050" "dwell" area on the kerzel engine, but I'm sure if I keep plugging away at this cam business it will become clear to me.

This is to reduce the combustion pressure in the cylinder prior to the piston will not have too much resistance to expel the exhaust out of the cylinder. Thus the exhaust valve does not open at BDC/180 degree from TDC.

In the big engines, you can see there is still flame out of exhaust..

 

[Hopper]

Haha, cam design will tie your head in knots. Whole books have been written on it and there is some serious math involved in the design of modern lobes.

The reason for the larger "dwell" (aka nose) on the one cam in your example, leaving aside the issue of flat vs roller follwers for now, is that the cam with the larger "dwell" opens the valve faster, holds it fully open for longer, then closes it quicker. This means that for the same, or similar, duration from initial opening point to final closing point, the valve allows way more gas to flow in the same period of degrees because it is mostly in the fully open position. This is a more "performance" cam.

But with a flat follower, the Webster would have been faster opening again due to the geometry of the point of contact. This higher performance profile would allow more gas to flow in the same amount of degrees of crank revolution, so more power. The tamer profile with the less "dwell" would make less power but probably would run better at low speed and low load, and would have less wear and tear on the cam, pushrods and valve seats from the lower speeds of valve opening and closing.

Probably for a model engine running for demonstration use, the lower power profile might work best, ie with less "dwell".

There is a cheaply available reprint of a 1921 book available, called Cam Design and Manufacture by Frederic Burnham Jacobs that goes through the basics of cam geometry and design . It includes a good chapter on "Gas engine design" that would relate directly to hit and miss engines and others of that vintage.

If you want a good explanation of the current state of the art, V8 engine tuner extraordinaire David Vizard does a good job of laying it all out, duration vs lift, roller vs flat followers, different curves etc etc in his books, eg How to Build Horsepower. His stuff is the most understandable you will find on modern full-size cams, I reckon.

There are also several books on cam design that are university level engineering texts you can spend hundreds of dollars each on. I borrowed one from a college library and the math was way beyond what I can remember from school! (Double variable equations and calculus etc. Ouch.) So not much help out here in the greasy knuckled real world trying to make vintage motorbikes go faster, or models.

 

[Mechanicboy]

Here is this book you can download https://ia600300.us.archive.org/34/items/camdesignandman01jacogoog/camdesignandman01jacogoog.pdf

 

[Brian Rupnow]

No, I stand by what I said in my first post. Read the post slowly enough that the words "atmospheric intake valve" registers. An atmospheric intake valve (which has no cam to operate it) won't begin to open until the exhaust valve is fully closed and the descending piston begins to create a vacuum in the cylinder. That is when the intake valve opens. If the exhaust valve stays open after top dead center, the descending piston just pulls air thru the exhaust valve and the intake valve doesn't begin to open until AFTER the exhaust valve is fully closed. Normal 4 cycle engines with cam operated valves on both intake and exhaust definitely do have an overlap, on both valves both at top dead center and at bottom dead center. Engines with atmospheric intake valves gain no benefit at all from having the exhaust valve closing after top dead center.
---I am quite well aware of the "lead and Lap" involved with conventional 4 cycle engines where both valves are operated by a cam.

 

[Brian Rupnow]

Mechanicboy--thank you for the link. I have printed off the section dealing with cam design and will read it as soon as it is done printing.---Brian

 

[Foozer]

Just looked at the Webster - Long stroke with intake valve timing dependent upon the intake valve spring tension. Neat little Thumper, with the light spring values, they'll float long before the RPM gets high enough for the engine to KATO . . .

Cam design and valve geometry? you sure;y did pick an aspirin subject . . .

 

[gbritnell]

I know we're dealing with hit and miss type engines here so their performance numbers will be quite different from engines with mechanically operated valves (intake and exhaust). Their numbers will also be somewhat different than the full sized engines they were copied from but here's just a little bit of information that hasn't been considered.

With full sized working engines, no matter what the cylinder configurations may be, the only way to measure their performance is to put them on a dynamometer. Altering every component of an engine will affect it's operation, which includes, ignition timing, compression ratio, combustion chamber design, valve size, camshaft design, intake and exhaust port size and shape, exhaust manifold configuration and carburetion. By changing any one of these separately or in combination will produce numbers that will allow the builder to see what is going on with the engine.

As I presented in the thread about flywheels we as builders generally only get to the point of getting our engines to run. By that I don't mean once the engine fires a few times we call it quits. We generally play with the carb adjustment or the ignition timing but rarely do we make major changes to see if the engine will run better than where we started. How many builders have completed an engine then made another new head or piston to change the compression ratio. How many new heads have been made to change the valve sizes. As far as camshafts, especially on multi-cylinder engines who out there has made an entirely new cam just to see how the engine operation is affected, not I for sure.

Even if we were adventurous enough to do these things how could we measure the differences? It starts easier or harder, it revs higher or lower, it makes more noise or not.

As far as cam design for small engines goes probably the best we can do is copy the specs for a full sized engine and let it go at that.
gbritnell