I was just browsing the galleries and I have to say the work on the gears is awesome. Hope to learn to make gears some day. Sorry about the error on the cam.
There are lot of parts of building an engine I really enjoy and the gears are one. The cutters are about $35 each and you need one for the big gear and another for the small one. Machinery’s Handbook has formulas for tooth depth. Then you make a blank and put it in a rotary table. If you pick easy numbers of teeth that divide into 360 no fancy index plate is needed. I must admit there is some faith needed, the teeth look wrong, and I always know I missed an angle by a degree. But at the end is a perfect gear. After a few sets the $70 for cutters seems like a gift.
Cams are another, I spent most of lunch trying to figure it out. A bunch of CAD drawing proved what happened is what should have happened. When you cut a cam as a offset on the lathe the center of the face radius is not where I expected it to be. It moves around with that radius and the radiuses of the cam. Anyway I also made an error on the fixture that made the face radius too big. When I fix it all the gap to cut a 90 degree lift cam turns out to be 128 degrees. It is not intuitive. Tomorrow I will continue on the quest to figure out where the extra degrees come from.
I'm a bit confused. Are you hoping to get the valve to open for 90 degrees? In crankshaft degrees that would be a 180 duration. Ass/u/me ing this is a gas engine, I would think you would want a duration of at least 240-280 degrees and on the cam that would be 120-140 degrees on the cam. Ass/u/me ing the lobes are about 110 degrees apart, the cam mistake seems to be a useable piece that will work well.
David... since you like gear cutting I was woundering if you have ever tryied to cut gears with a hob type cutter. They can be bought ready to use but can also be made in the shop... which should be the ultimate satisfaction. Strictly IC has a great Hob making article that walks you through it. The one hob can be used to make both cam and crank gears.
I haven't tried it yet but I have the tool steel balnks ready
Steve, I look at parts as individuals so as you noted my 90 degrees on the cam is 180 on the crankshaft. I have never built for speed or power and I am happy when it runs. It has no overlap and valve are closed at TDC and BDC. This cam is easy to make and I understand how to change the angles to get what I draw. It might be time to look into more performance. I looked into cam angles and see that most real engines have exhaust open before BDC, have intake and exhaust overlap for a few degrees, and keep the intake open a few degrees past BDC. SO I am thinking of making a second cam. 5 degrees (10 degrees crank) overlap and possibly 10 degrees (20 crank) early exhaust and 5 (10 crank) late intake. Is that enough to notice? The plan would be start with my original. Get the engine going, then try the new cam and see what happens.
Jan, I recall that article. I can’t see the difference between the cutters I have. I suspect these gears don’t need to be that precise.
Just for reference, the peewee has 60 degrees overlap. Try to ignore all that stuff and think about lift, duration and cam angle.
An every day use, good idle, good milage cam is usually around 260 duration and 110 degrees seperation. When you factor in the valve lash, duration falls to 240 or even lower. Your 90 degrees will be less depending on the shape of your lobe.
The exhaust lobe centerline (max open) should lead the intake lobe centerline by 105-115 degrees. I like to use 110 degrees.
Make sure you have a 110 degree seperation on the lobes. Put extra duration in for lash. I would go at least 240 degrees. Make the lift at least 1/4 the size of the valve. More does not hurt, unless the valve hits the piston.
Want me to make you a cutting chart? Haahaahaa
I had a guy comment on the Peewee. "Finally someone built a model engine that sounds like a real engine". I think that it sounds good because the camshaft is close to what is in a real engine. I think most people under cam there engines.
So how does it work? With my dead center no overlap cam it is simple to understand. I can see opening exhaust a few degrees early to get it all out. I can see a few degrees over overlap when not much is happening flow wise to maximize the flow. Again keeping the intake open past BDC would seem to pump mixture out. I can see how all this could significantly change the sound but not how it alone improves operation.
Some overlap can help the cylinder purge itself of exhaust gasses. At the end of the exhaust stroke the gasses are pushed down the exhaust pipe. That can cause a negitive pressure in the cylinder. If the intake valve opens when the pressure is negitive, the cylinder draws in some fuel mixture and is able to purge the spent fuel that would otherwise be trapped. The piston can't clear the gasses in the combustion chamber. Kind of like a hypodermic needle. It can only push out what the piston can displace and the piston doesn't go into the combustion chamber.
Think about how far the valve is open in the last 5-10 degrees. Chances are the lift in that range is less than the valve lash and the valve wont be open at all.
In theory Peewee's overlap is 60 degrees. In reality the valves are only open together for maybe 30-40 degrees because the valve lash has to be taken up before the valve opens. Also, in that few degrees of time, the valves are only open a few thousands of an inch. On paper it sounds like alot but in real life it is'nt.
With the 240 duration and 110 lobe seperation you would have 20 degrees overlap and maybe 10 after lash. Basically what you are trying to get to anyway.
240 Duration 110 seperation Set the exhaust lobe to be fully open 109 degrees before TDC.
Sounds crazy but this is actually a very conservitive camshaft. I'm not trying to change your mind David.
Ok here is my 2 cents beside the engine sound and performance, one other factor is because of the speed the engine runs, if we open and close the valves at TDC and BTC probably in time it will damages the valve train parts and won't last that long. So is better to open them gently over a few degrees of crankshaft rotation. As we know, the piston barely moves at all for about some degrees before and after BCD and TDC. We can use this time to gradually open and close the valves instead of slamming the valves open and closed at TDC and BDC.
i just wanted to say steve you hit the nail right on the head... im an auto technician and specialize in engine performance the way i explain to people is that opening and closeing the valves past tdc or bdc is that it gives those fast moving gases time to "settle" a few milliseconds in real time.
Steve, I understand on engines with 4 or more cylinders there is a more continuous flow of intake mixture and of exhaust and the inertia of that flow can pull some exhaust out or force extra mixture in, like a siphon or ram. I believe this would have an effect in a multi cylinder design. My car has tuned intake and exhaust and it is obvious when the engine speed hits a happy speed. I am building a 1 cylinder vertical. No tuned pipes no other flows. The plan today is to make 2 cams, one 180 degree(crank) lobe with 180 degree(crank) apart, and 200 degree(crank) lobe with 8 degree(crank) overlap. Then see how they run at the end. It is about half the extra you are describing, but if it is important is should be noticeable in RPM.
Kamran, when I talk about open I mean not closed. My turned cams have a S curve lift the valve off the seat for 180 degrees(crank) total with about 20 degrees(crank) 100% open mid stroke.
Everything that I have explained is theory for a single cylinder. Tuning intake and exhaust is a subject that can further muddy the water.
How about making one to your 90 degree spec and one to my 110 degree spec. I suspect that the 110 degree cam will "free up" the engine due to less losses pumping gasses at specific times in the cycle. I suggest this because i'm not sure that a difference between your 2 cams can be noticed. I suspect the Difference between yours and mine will be night and day.
Jason, I'm sure you would agree that the 240 spec I am suggesting is a very mild cam profile. Specs for a performance cam start at 280 and run thru the mid 300's.
I'm not trying to rev up the little engine, just suggesting a cam profile that I believe will give you a smooth and free running engine. I also think you will need less flywheel with the 110 to achieve a good idle.
What ever you decide to do, I am very interested in your findings.
I guess the design of the cams geometry as their placement in relation to each other will be dictated by the engine purpose, if you are looking for a nice torque producing engine like for pulling loads or a high RPM and quick engine, or an engine to be used off road for sand or desert. Or do you want an all around engine that can do well in all conditions ? If the engine is for display, short runs, then you can be more aggressive in your design. Some basic design ideas: low RPM Overlap 5 deg BTDC Intake, 20 deg ATDC exhaust duration of 150 intake, 145 exhaust. Lobes at 110 deg of separation.
High speed Bentley style (1960's) Overlap 20 deg BTDC INtake, 30 deg ATDC exhaust Duration of 260 deg intake, 280 exhaust. Lobes at 110 deg separtion.
Here is one I designed for high torque applications ( been thoroughly proven ) Overlap: Intake opens 26 deg BTDC, Intake closes 54 deg After BDC Exhaust opens 60 deg before BDC Exhaust closes 32 deg after TDC Valve lash 0.006" Lobe centers at 104 deg Duration of Intake 300 deg Duration of exhaust 310 deg
This are the theoretical dimensions, the net effect is I get a 260 deg duration intake and 272 exhaust due to lash, friction, timing end end play in components. Whatever you decide, make sure the other components in the train can deal with the cam, another critical component is tappet follower and rocking arm geometry they change dramatically the end result in the valve.
This discussion is what this site is intended for. Sharing ideas and explaining what you believe about engines.
My plan is to make 3 cams. The original basic cam, a half way version, and one that Steve and Jaime have recommended. I made 3 blanks today and I have 20 o the 3 drawings done.
The engine has tappets in bronze guides and short push rods. And that is locked in metal now. The rocker arms have adjusting screws and I normally set a 0.003 inch gap. These will move the valve by the cam lift. The lift is 0.060 peak. The largest diameter is 5/16.
My goal on this engine is to get it running so I can reduce the size again. So I am interested in conservative design and solid operation.
The cam errors are continuing. I cut the 3 cams this weekend. I was very careful on cam1, Cam 2 I just dialed in the color coded numbers from my drawings. I did it again for cam 3 but the color codes were revered so the lobe phase backwards. I guess if I want to test that one, the half way version, I can run the engine in the opposite direction. All in all, they look wonderful I can see the different size lobes and the different spacing. The valves, rocker arms, and head are next.
I didn’t think about that option but with those symmetrical push rods it would be so easy. I have been struggling with the push rods on the 3D cad drawing. One of the designers at work took on the challenge and figured out how to get it positioned. I will get it posted in the next few days. I will have some new photos later today in work in progress.
It has been about month since I worked on the engine. The nice weather ended this week and the yard is looking reasonable. I got some silver solder flux which interrupted the project. The head is drawn and the carburetor from the last engine should be easy to duplicate. I seem to have not designed an ignition. I recall thinking about a Hall sensor in the gear housing but I will need some way to do timing adjustment.
I got some time to look at the .700 engine. I found the new tools for cutting the slots in the head. I think I found a spot for the hall sensor that was forgotten. I am looking to make a new fixture for heat treating rings by scaling down the old 1 inch version that has worked so well for so long. These rings I drew are 0.700 OD and .031 square, Oil control is .031 X .049. I am concerned since they are so small. Has anyone made rings this small or smaller? Any tips?
I have found some time and worked on the head. When I went to press the valve cages in the went in too easy. More than a finger push but not enough to seal or be secure. Since the head is aluminum and the valve cages are stainless I doubt LocTite would work. Any ideas before I start remaking parts? David
I just thought of something. If you can wait a month, the loc-tite rep will be at our next club meeting. If you can wait until the second wed of next month I might be able to give you some good news. No garantee
I used Loctite 620 for slip fit in one of Bob Shores, Silver Bullet, valve cages of stainless steel and head in Aluminium, fit was like you describe, a bit in the loose side, cleaned the parts in Acetone, then coated them nicely with 620, both parts, pushed them in place and placed a heavy piece of steel in the cages to keep some pressure while curing, ( 24 hours), the weight is to counteract the hydraulic pressure created by loctite while curing.
So far engine has been running with out any leaks, temperature was a concern but so far really good. You can also use Loctite 635 or 680, not sure which one has more temperature upper limit. There is a similar product from Permabond HL138. In case you have a big gap, use of activator 7471 from loctite helps in filling the gap, but you have to be speedy in placing the cages once you treat them. Also surface finish, you can make some groves in the cage to make a retaining groove, but I've found that machine finish is great, do not polish them..
I thought about LocTite 620. And it has worked in other areas. I thought about a knurl but it would not be a leak free fit, just tighter. These parts are too small to try a bushing or fix. I took the parts to work for a good measure and one cage is 0.0001 under and the other is0.0003. I am thinking about copper plating a few tenths on the valve cage. I might supplement that with the 620. These are the best parts I have ever made and they are so close. David
I guess the best advice maybe the one we don't want to hear "make a new set", in Mexico we have a saying, lazy works twice, sometimes we spend more time, effort and frustration finding ways to avoid the unavoidable reality, give them another go and try fit them to the bore as much as possible.
Similar thing happened not too long ago when I broke a tap in a complex piece and I tried everything (out of EDM) and ended up biting the bullet and starting all over.
It has been a while since I had any time for engines. I found the parts in a box the week of July 4th and poked and fitted some. I ended up getting a ball bearing, one of those ¼ OD X 1/8 ID ones stuck in a hole that was too small. I needed to make a puller and lost interest. I will get back to it all someday. Presently I am involved with electric cars ->link<- on several levels. Combine that with regular work and time gets tight. David
I know I have bothered you in the past about this but can I get a copy of your sketches for this engine. I still think about making one every once in a while.