Information on cam timing in small engines

[Jo]

  The only gentleman that works with gives me the books, like my mint copy of Elmer's engines 

No this one was either on the shelf of that wonderful second hand bookshop down by the crofton beam engines , which accounts for the 90p inside the cover or was part of that bundle that I brought for £1 which included the mint copy of "Chapelon, Genius of French Steam" 

Jo

[Brian Rupnow]

Jo---I think that you and I got off to a bad start, and I feel bad about that. I am a technology nut, and I have been for 50 years or more. My profession has always worked hand in hand with my hobbies, and now I am ready to take this small engine thing to the next level. I said to my wife this afternoon "I am capable of designing and building a working engine, but some of the things I do, I don't fully understand, other than that they work." I am hoping that the books I have ordered will help take me to that next level---even if I do have to pay more than one pound for the books.---Brian

[Brian Rupnow]

I purchased the book "Miniature internal Combustion Engines" by Malcolm Stride published by Crowood Press in Wiltshire U.K.    It is an excellent book on small engine design, with easy to understand articles covering all aspects of small internal combustion engines, including a great section on cam design. The book is a hardcover, of 175 pages, with many colour pictures and technical drawings. I highly recommend it to anyone interested in model i.c. engines. I believe I paid about $40 Canadian for it, including the shipping charges. You can contact Crowood Press at enquiries@crowood.com   I purchased my copy through the Canadian book store Chapters-Indigo.---Brian

[Brian Rupnow]

I have just received and read through the second book I had ordered, "Model Four Cycle Gasoline Engines" by L.C. Mason. Although it is quite a good book, it doesn't give any more information than the book by Malcolm Stride, and doesn't have as many pages, illustrations, nor technical drawings, and was written in 1976, quite a lot earlier than Malcolm Stride's book, (although very little has changed in the world of small model engines since then.)---I really don't think it is worth anywhere's near the $200 Canadian that is asked for a new copy of it on Amazon.com   So--a summary of what I have read/learned. I am quite sure I knew about 90% of the information contained in these two books. They have cleared up some of the mystery surrounding cam design, and a bit of the size relationships between various engine components. The rest of the stuff, I knew already----but then I should. I have worked as a mechanical design engineer for the past 49 years, been involved in hot-rod building and drag racing for almost as long, and have built 10 steam engines and seven i.c. engines. They will certainly be valuable reference books to add to my ever growing stack of "resource literature" about this marvelous hobby.---Brian

[Brian Rupnow]

So--After multiple readings of both books, I have began to comprehend the basics of cam design.I hope you are able to read the drawing posted here, because it will explain some of the theory/mystery behind the actual design of a cam itself. The timing of cams is a whole different issue, and all I will say about it here is that a relatively mild cam for a 4 cycle i.c. engine, running at relatively low rpm (500 to 2500 RPM) would begin to open the intake valve approximately 10 degrees before the piston reached top dead center on the exhaust stroke, stay open while the piston travels from top dead center to bottom dead center on the intake stroke , and remain open until it closes at about 50 degrees after the piston has started upwards on the compression stroke. I know that sounds like a lot , 240 degrees of total cam influence, but remember that when talking about piston movement in relationship to degrees of crankshaft rotation, the piston travels relatively short distances during the 30 degrees either side of top or bottom dead center, and travels most of its total distance to be covered during the 120 remaining degrees in half a crankshaft rotation. In the drawing, I picked the 3/8" shaft diameter. That is a somewhat arbitrary number, but must remain smaller than the "base circle diameter". The .080" of valve lift is a dimension that is calculated by determining that the volume of space between the face of the valve and the seat when the valve is fully open should be equal to the volume of a cross section taken through the intake runner leading to the valve. If anyone takes exception to the information I have given, or wants to ask a question, I will be glad to discuss it. Remember--Although I have been designing mechanical devices for 49 years now, this entire cam design thing is relatively new to me too.---Brian Rupnow.

[tangler]

Brian,

I think you've summed it up pretty well, at least from my understanding which is also based mostly on the 2 books.

A slightly different way of looking at the design is to consider the displacement, velocity and acceleration curves of the valve.  Ron's modelenginenews website  http://modelenginenews.org/design/index.html  has the necessary calculater or you could use my Excel spreadshet appended below.  These calculaters assume that the main components driving the behaviour of the valve are the flank radius and the action angle - the nose radius falls out from these 2 parameters.  Knowing the weight of the valve train you can use the acceleration figure to calculate the size of spring required to keep the follower in contact with the cam.  The main purpose of the program was to calculate the co-ordinates required to mill the cam which is the same as the displacement.

My Excel version calculated the nose radius from the other parameters, in this case I've adjusted the flank radius to give the same nose radius as in your example shown in these screenshots:












The acceleration figure maxing out at 18g suggests that you will be able to use very weak springs which will help to conserve power on a slow revving engine.  Alternatively you could increase the flank radius to give a longer open period which would be shown by a flatter top to the displacement curve, here shown at 1", and a consequently larger nose radius (0.129"). 



and an increased acceleration maximum



Incidently, Model Engineer magazine has a series running at present on exactly these topics from a retired lecturer in the subject.  It's very good so far and, I suspect, rather more authoratitive than either Mason or Stride who, for all their qualities, were, like me, just amateurs.

Cheers,

Rod

[lakc]

My Excel version calculated the nose radius from the other parameters, in this case I've adjusted the flank radius to give the same nose radius as in your example shown in these screenshots:

Rod

 

So thats your spreadsheet. I have had it for so long, and I searched and searched but could not find where I first got it. 

I have made quite a few changes and additions along the way, but since I dont understand the base calculations well enough to rewrite it, I have had to refrain from reposting it in several of these camshaft discussions over the years. Its kind of hard to ask permission or give attribution when you cant remember where it came from in the first place. Some day I hope to have the time to sit down with Ron's .js and the Strictly IC article and straighten it all out.
Thank you for your efforts, I have gotten a lot of use out of it over the years. Here is what it looks like now.

[tangler]

I'm so glad you've found it useful.  One thing I never managed to do in Excel (but managed in my Qbasic version) was to actually draw out the shape of the cam.  I feel this that this is quite useful on the premise that "If it looks right it is right"

Cheers,

Rod