Engines > Your Own Design
Dave's twin
metalmad:
Hi Dave
I always enjoy your posts as they show me how good I wish I was ;D ;D
Pete
Dean W:
--- Quote from: Dave G on July 22, 2012, 02:20:55 PM ---The crankshafts will rotate in opposite directions but as long as no one tells it, it shouldn't know the difference. There is only one cam lobe to operate both cylinders as it will have an even firing order.
--- End quote ---
I just realized that about the opposite rotation crankshafts, after looking at the gear case for a minute or two. I was
going to ask about that! :)
The "even firing order" you mention; Do you mean the cylinders will fire at the same time, and is that how you get
away with the single cam lobe for the exhaust valves?
Is the carb the one you plan to use, or do you figure you'll need a larger one?
It's a very interesting engine, Dave. Thanks for your comments and explanations. :ThumbsUp:
Dave G:
The arrangement I will use is called a waist fire system. The single coil has 2 high tension leads which will attach to the 2 sparkplugs. Most 2 and 4 cyl motorcycle engines use this system for simplicity. Both plugs fire at the same time but because only one cylinder is at the point of combustion only that cyl will make power. The other plug will fire but because the cam is in overlap mode and the exhaust valve is open no power will be made. I plan to use one of Roy Scholl's fine ignition systems, he does make them in waist fire.
The cam runs at one half speed of the crankshafts like a normal 4 stroke engine . It will take 720 degrees of crankshaft rotation to complete all 4 cycles also. The cylinders are out of phase by 360 degrees of crankshaft rotation giving an even firing order. Only one exhaust cam lobe is needed to operate both cylinders. If you were to grab the flywheel by hand and rotate it until the pistons are at TDC you would see that on one cylinder the exhaust valve is just about closed. At the same time the other cylinder has compressed the mixture and is waiting to be fired. Rotate the flywheel 180 degrees by hand from this point and both cyls are at TDC once again but the cam events have switched cyls. The flywheel is directly coupled to the cam and runs at half speed also.
The carb is the one I plan to use. I normally make my carbs but I had this one on hand and figured I'd give a try. For Hit-Miss style engines I try to make my throttle dias about 10 - 15 % of bore size. For a throttle governed engine such as this one I usually will try to make the throttle dias around 15 - 20% of bore size. This carb was close to what I figured was needed. If I can get this engine to run I want it to be a slow runner and not worry about taking too much throttle. I want it to just chug along as slow as I can get it to. When designing an engine from scratch like this one I will use the dimensions from a full size engine like a V-8 for cam lift and throttle dias and scale them accordingly. This engine has a bore of 1.25" so it is just under 1/3 scale of a 4" bore engine. The valve opening is .125" as a stock V-8 engine lift is around .400". The throttle bore is a little smaller than this scale so I get good velocity through the carb. The smaller the throttle bore the greater the velocity. This helps to draw the fuel from the tank easier. I put a SS check ball in the foot valve at the base of the fuel tank to keep the fuel up to the carb. Very important in Hit-Miss engines.
Hopefully this clarifies things a little. I spent alot of time thinking about this engine before starting to build it and I think it will run. My concerns are the amount of power loss due to friction of all the moving parts( gears and bearings). Engines always seem to run better with a load on them and I hope the load of turning the gears and bearings will help in the tuning. I have tried to foresee any challenges in the design but one never knows how good the soup will be until he takes his first sip. So far I haven't burnt my tongue yet. Who knows, I might have to throw out the whole pot. :shrug: Dave
chuck foster:
holy :censored: now thats a neat engine 8)
i don't know how i missed this on the other forum :facepalm:
i will be watching this one :happyreader:
chuck
cfellows:
Very interesting engine, Dave. The workmanship is also first rate.
Chuck
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