Designing an engine

[Metal_slicer]

Hello guys,

I have been designing a two cylinder engine but I may need a little help with cam design. Right now the intake and exhaust center lines are 30 deg. apart I did a search on google and can't find what I am looking for. I did find a chart but was not able to make sense of all the angles and degrees.

Here is what I have so far. I have not yet made the cylinder block.

 

[BronxFigs]

Looks great. Very interesting to see the design process. Wish I could help you, but with no experience in engine design....I can't.

Is there a reason why you couldn't duplicate/borrow the timing events/cam profiles from a proven model engine design? Maybe that will at least get you close to a solution. I'm sure with some very slight modifications to the lobe design you'll have a working camshaft.

Just thinking out loud...can you make the camshaft by sliding on each individual cam-lobe/profile, so they can be rotated into different timing positions? Then, you wouldn't have to re-machine the complete camshaft if a specific cam didn't work as planned. I think in the "Gemini" 2-cylinder gas engine, the builder used add-on cams to a shaft.

I can't wait for the rest of this design to show up on the forum. It's nice to read about and see how designs evolve. Thanks for the interesting subject.


Frank

 

[petertha]

Perhaps this past post of mine will get you started. I had similar questions. Note the gas vs methanol comments regarding timing, not sure what you had in mind there but I suspect that will factor in too.

http://www.homemodelenginemachinist.com/f26/valve-timing-4-stroke-glow-17093/

Actually, I'm in the process of drawing up a single cylinder design for myself as a starter IC project. A while back I cooked up an Excel spreadsheet to allow input of Inlet/Exhaust open/close values typically referenced on designs Then it spits out resultant cam phasing values, overlap, visual plot etc. I'm posting some screen caps for now because I need to go back & double-check the (late night spare time) math. But I'm just about to cross this bridge for my own project so hopefully can offer a tool if everything pans out.

In the interim, if you go to this awesome site & type 'camcalc' in the search box, it will go to a cam profiling tool app. Running it was kinda hit & miss for me in the past, but probably a function of my PC or OS.

http://www.modelenginenews.org/

 

[stevehuckss396]

Hello guys,

I have been designing a two cylinder engine but I may need a little help with cam design.

It depends on the configuration of your crankshaft. If both pistons oppose each other things will be different than if both pistons go up and down together. Also the direction of rotation of the cam would be needed.

What info would you be looking for specifically?

I typically make mine with about 280 duration and 110 degrees lobe separation for ease of machining. 5 degree increments works good with my method of machining.

Tell us what you need and i'm sure you will get as much help as you will need.

 

[Metal_slicer]

Thanks Guys,

I am going to have the pistons opposing each other, one TDC and one BDC. The bore is going to be .9" and stroke 1". I have read an article on cam design and they discussed factors like fuel to be used, RPM ranges and even valve float all are taken in to consideration in the design of the cam. Since I'm not making a racing engine, all I would need is a simple cam design that would allow me to run the engine and rev it a few times. Fuel I'm not sure about, what ever is recommended for model engines that is cheap and available everywhere.

 

[BronxFigs]

Just throwing out some general question:

Since this engine is being designed from scratch, is there a way to design an engine for use on propane? Or, is it just a matter of converting a conventionally designed gas engine, to be run on propane, by using the correct mixing demand valves, etc? And, what about running an engine like this on Coleman's lantern fuel, instead of pump gasoline?

It would seem that barbeque propane is always available, cheap, less fumey, safer to carry, and easier on the engine parts. It seems that running an engine on propane is always either a conversion, or, an afterthought. At least, I do not remember ever seeing any model engines designed specifically, to be run on propane, but then again, I'm usually wrong.

I'm really curious about this design-from-scratch process, and the choices that are available.

Thanks,


Frank

 

[dman]

Just throwing out some general question:

Since this engine is being designed from scratch, is there a way to design an engine for use on propane? Or, is it just a matter of converting a conventionally designed gas engine, to be run on propane, by using the correct mixing demand valves, etc? And, what about running an engine like this on Coleman's lantern fuel, instead of pump gasoline?

It would seem that barbeque propane is always available, cheap, less fumey, safer to carry, and easier on the engine parts. It seems that running an engine on propane is always either a conversion, or, an afterthought. At least, I do not remember ever seeing any model engines designed specifically, to be run on propane, but then again, I'm usually wrong.

I'm really curious about this design-from-scratch process, and the choices that are available.

Thanks,


Frank

gasoline engines run fine on propane but there may be a loss of power. gas fuels are light and displace a lot of air so there is less air and fuel mass per stroke.

you can raise the compression with a propane engine and there is less need for velocity i nthe ports to keep the fuel atomized so propane engine sometimes have large ports and high compression, but if you want maximum torque there is an ideal velocity and valve timing. that also depends on how fast you want the engine to spin.

cam design has a lot to do with airflow and rpm goals. what are your goals for the engine?

also are you going for even firing order? or will the pistons try to counter ballance each other to some extent?

you can break things down into opening and closing events, or you can go with intake centerline, duration, and lobe separation which is more useful for manufacturing the cam.

every 180 degrees of the crank is a different stroke, the cam turns half the speed so the strokes are 90 cam degrees apart. because the valves need to open smooth and resembling sinusoidally to avoid stability problems, when the lobes are big enough there is too much overlap so they spread the lobes apart a bit more. lobe separation is usually between 100-120 degrees and the majority of them are somewhere close to the center of that range.

the intake center line is measured in crank degrees and the lobe centers are measured in cam degrees so the numbers are usually pretty close. you should be able to balance the cam in the overlap period with the springs of the intake and exhaust valves with the crank at tdc and the engine should run. remember to check for piston clearance. the valves often move faster than the piston and if you don't have the lobe separation and clearance right things can crash.

 

[BronxFigs]

dman:

Thanks for the great answer. Now, I have to think about all the important points that are outlined in your answer. I appreciate the time and thoughts, and for your help.


Frank

 

[Metal_slicer]

My plan is to go with some sort of liquid fuel. I don't mind even using r/c nitro fule.

 

[Metal_slicer]

Did some more work on the cad design. It's quite a challenge especially when my laptop crashes when I try to do to much. I have included a wire frame picture so that you can see the various parts inside. I created the crankshaft, rods, and pistons. I have started working on the lower portion of the block and I need to make several changes. This design will likely change several times, but here is what i have at the moment.

 

[Henry]

That looks really nice!!!. About your crashing problems, normally it will be the memory, if the crash got worse when the model become more complex, that will be the problem, It is really cheap now and you will be able to buy 4 Gb for like 40 or 50 $, cheaper look how many programs are using the memory now and you will be able to stop a few of them.

 

[Metal_slicer]

I had to start over with the design. I was able to re-use some of the parts from the previous design though. Here is a picture of it. I'm almost ready to start machining. Still need to figure out if I can improve the spark plug clearance which is very close to the valve spring. I'm afraid it may rub on the spring. There is not much room when you add the screws and intake/exhaust ports. I am going to try to experiment cutting an angle for the sparkplug.

 

[Metal_slicer]

The individual parts...
Still a work in progress, not all the parts are shown.

 

[gbritnell]

Don't take what I'm about to say as anything but helpful information. Others on this board know that I am one of the most helpful when it comes to information giving so let me start.
I would like to start off by asking what your modeling background is? By that I mean have you ever built an I.C. engine of any sort? Most of us here started by making a simple hit and miss engine or something like that so we could learn how all the pieces interact and what elements are required for a functioning engine.
My background is extensive. I grew up in an era when people worked on their cars and hot rods, everything from simple spark plug changing to complete engine and transmission rebuilds. Added to that is my background in machining. I started in my teens and then worked at it for 40 some odd years, not counting my home shop work. I have designed and built quite a few model engines, steam, aircraft, automotive and motorcycle to name a few. This was not an overnight process.
While I applaud your interest in designing and possibly building it's not just a matter of drawing pieces and parts. Your question about cam design prompted this reply. A couple of the fellows gave you information on cam design and characteristics which are essential for the proper operation of an engine. Knowing the difference between mild cam timing and radical timing and how they affect the performance of an engine are basic to understanding any engine's operation.
There are so many other questions that need to be answered when designing and building:
What type of material are the parts to be made from?
What compression ratio are you going to use?
How big should the valves and stems be?
What size porting and manifold should you use?
What are you going to use for a carburetor?
What type of ignition will it have, points or electronic?
How are you going to trigger the ignition, from the cam or through a distributor.
All the above and more have to be taken into consideration when designing and building and engine.

 

[BronxFigs]

Thought provoking comments.

To watch an engine being designed and be "built" on a computer screen is very interesting.

I'm staying around for the real build that I hope will follow. Good luck Metal Slicer.


Frank

 

[Metal_slicer]

What type of material are the parts to be made from?
What compression ratio are you going to use?
How big should the valves and stems be?
What size porting and manifold should you use?
What are you going to use for a carburetor?
What type of ignition will it have, points or electronic?
How are you going to trigger the ignition, from the cam or through a distributor.
All the above and more have to be taken into consideration when designing and building and engine.

My modeling experience is self taught. I am using Viacad 2d/3d which is a non professional software program. I have used it to design robot parts and have made several items with it. I realize most have started off with a hit-n-miss engine but I don't really have an interest in this engine.

I have graduated from a trade school with a diploma in automotive and diesel technology back in 1989 and worked in an automotive shop for 3 years before changing my profession to the electronics manufacturing arena. I understand learning to turn a wrench vs. designing an engine are two very different things, but my goal here is not to make a running engine although I would like it to be able to run for a few seconds, and even better a few minutes. I have an interest designing something and then making it. Because I am not an engineer, I don't expect it to actually run.

The materials I plan to use are free machining steel, stainless steel, and brass. The size of the valves and ports are based on tool sizes available, example, 1/8" for the valve stems because 1/8" is a common size for a drill bit. Any smaller than 1/8" and the strength becomes less and its harder to machine on the lathe. Ignition is going to be electronic triggered by the cam shaft. The cam design I may need help with the lobe design but I will worry about that when I cross that bridge. I'm not going to be over critical about Exact port size, the perfect size valve stem, or compression ratio.

I want to design this engine and learn as I build it. I will enjoy this project even if in the end the engine wont run.

 

[Metal_slicer]

Material question:

Should the cylinder and piston rings both use the same cast iron material?

 

[stevehuckss396]

Material question:

Should the cylinder and piston rings both use the same cast iron material?

Dont have to. You can use Steel liners with the cast rings.

Cast cylinders, cast pistons, cast rings
Steel cylinder, cast piston, cast rings
steel cylinder, aluminum piston, cast rings

I like the last one personally.

 

[Metal_slicer]

Dont have to. You can use Steel liners with the cast rings.

Cast cylinders, cast pistons, cast rings
Steel cylinder, cast piston, cast rings
steel cylinder, aluminum piston, cast rings

I like the last one personally.

Yes I like the last option also. Steel cylinders, 6061-t6 pistons, and cast iron rings.

 

[stevehuckss396]

Yes I like the last option also. Steel cylinders, 6061-t6 pistons, and cast iron rings.

I use 12L14 for my liners. It's a leaded steel so it machines like a dream. I have made liners for 4 engines now with no second thoughts.

If your feeling a little froggy, make the bottom of your cylinder smaller than the top by a few thou. Then size your liner to the top and bottom and undercut the center. This way even a 2+ inch long liner will only need to be pressed in the last 1/2 inch or so. Much easier than pressing the entire length of the liner. A little more work making liners but they go in easy and undamaged so i think it's worth it.

Liner drops right in to here and almost lines it self up for pressing.