A few design questions

[Entropy455]

I am a new board member, looking to design and build my first engine (hit-and-miss) and I have a few questions.

What compression ratio do you guys typically incorporate within your designs? I understand that back in the day of 60 octane gasoline, engines were made with 5:1 compression. But with modern gasoline, I’m thinking 8:1 would be more appropriate?

My next question is on flywheel design. I recently picked up a nice used cast iron flywheel. It’s just under 9” in diameter, and weighs about 12 pounds. It was used in a relatively high RPM application (treadmill DC motor flywheel) so it’s got a very good factory balance.

I want to use this flywheel in my first attempt at building a hit-and-miss engine, and I’m trying to determine an appropriate bore and stroke for the engine.

Below are my design requirements:
Desired compression ratio = 8:1
Desired minimum operating speed = 120 RPM
Stroke = 1.5 times the bore
Fuel = gasoline
Ignition = spark

Design process:
The flywheel has a rotating inertia of 1.345 lbm-ft^2.
At 120 RPM, the flywheel kinetic energy is 3.301 ft-lbf
So the question is - what volume of air will the flywheel just be able to pump on the compression stroke?
Air at 70 degrees F has an enthalpy of 126.66 Btu/lbm.
Assuming a polytrophic compression (at 8:1), the compression stroke will increase the air’s enthalpy to 288.76 Btu/lbm. The air’s temperature will be 730 degrees F, and the pressure will be 263.6 psia.
Thus the energy input required on the compression stroke is 162.1 Btu/lbm, or 126,140 ft-lbf/lbm.
With this specific energy requirement, the flywheel’s 3.301 ft-lbf of energy would just be able to compress 2.617 x 10^-5 lbm of air.
Air at 70 degrees F, and 14.7 psia, has a density of 4.333 x 10^-5 lbm/in^3, and 2.617 x 10^-5 lbm of air has a volume of 0.6037 in^3.
Thus if stroke = 1.5 times the bore, the engine would have a 0.8” bore, and a 1.2” stroke.

Discussion: obviously this will not work, as the flywheel will come to a virtual stop under the compression stroke. With friction losses, it will likely fall just short of just completing the compression stroke. I neglected the rotating inertia of the crankshaft, because the crankshaft inertia contribution would be relatively small compared to that of the flywheel.

So I’ve got a few options for using this flywheel: I can either lower the compression ratio, which would reduce the specific power consumption on the compression stroke. Or reduce engine displacement, or increase the minimum operating engine rpm.

What would you guys do? Recommendations???

 

[Longboy]

....Your design requirements by the math will never get you to a running engine. As you get lost in the math your conclusions will make the design parameters come more into what a successful engine build needs as priority. Your first I/C engine should be a common design others have done and your scale is usually determined by the cyl. size. Your base RPM of 120 has killed your project. Recalc at 600 RPM and your 12 lbs. flywheel will overcome the high compression probally with a larger bore. The smallest bore with the largest dia. flywheel I've seen here would be the Henry Ford engines. When you find a hit/miss model doing 120 rpm let us know.

 

[Entropy455]

I’m not sure I understand your point Longboy.

The math shows that an 8:1 compression engine, with a 0.8” bore and a 1.2” stroke, will stall at 120 RPM, with the flywheel I’ve selected.

I’m not looking for smooth operation at 120 RPM. The intent was to establish the absolute minimum speed that the engine can turn, and still have enough “wind” to hit. I’d expect the operation of the engine at 120 RPM to be far from smooth, and quite erratic. Nonetheless, do you think that 120 RPM is an unrealistically low minimum hit velocity for a hit-and-miss engine?

What’s your opinion on the bore, stroke, and compression ratio that I’ve selected for this particular flywheel - assuming the engine will normally be operated above 120 RPM?

What is an acceptable percentage of kinetic energy consumption on the compression stroke, for “smooth” engine operation? Practically every reference I’ve read on the subject states that the more the inertia, the better. But that’s not exactly quantitative.

I’m also interested in designing a crankshaft with counterweights, to balance the engine for the smoothest possible operation. However I need to determine the bore and stroke before I can think about sizing the counterweights and linkage components.

 

[Niels Abildgaard]

Why not lower the compression ?
Fuel economy is not an issue.Fun is.

 

[steamer]

Entropy,

Your probably right considering the stall condition. Most hit and miss engines run in the 5-6:1 range. Once you increase that the minimum speed will go up as well as the difficulty in starting them.

120 rpm is much too slow for a hit and miss at full scale. My 8 HP 2400 pound Simplicity with twin 42" flywheels won't run below about 300 rpm

Would suggest a common design that's out there already.

Dave

 

[Ken I]

Without bird dogging your calculations, Longboy is correct - if you doubled your speed the energy would be four times greater.

120 rpm seems unrealistically low.

Other than that go with your design - I presume you are working comparatively from something ?

Start with a lower compression - you can always jack it up to improve performance later but who's interested in performance ? particularly with a hit & miss ?

No critisim is implied - merely my 2c worth.

Regards,
Ken

 

[Longboy]

I’m not sure I understand your point Longboy.

The math shows that an 8:1 compression engine, with a 0.8” bore and a 1.2” stroke, will stall at 120 RPM, with the flywheel I’ve selected.



What is an acceptable percentage of kinetic energy consumption on the compression stroke, for “smooth” engine operation? Practically every reference I’ve read on the subject states that the more the inertia, the better. But that’s not exactly quantitative.

I’m also interested in designing a crankshaft with counterweights, to balance the engine for the smoothest possible operation. However I need to determine the bore and stroke before I can think about sizing the counterweights and linkage components.

.......the point is using the math for theoreticals that don't exist for a pratical up and running I/C project. And when you tie yourself to your design requirments that don't exist in real life engines ....you are done. Unless you are building an air pump, I don't see anywhere in your design proccess any mention of combustion and the part it plays in raising energy levels to drive the mechanisms so the math is short of considerations here. I agree here about sizing your model, where I said that cyl. bore determines scale you will know the overall diameter of your crankshaft counterweights when you decide bore and stroke. A 12 lb. flywheel can suck up alot of imperfect balancing issues.

 

[bearcar1]

Entropy', first of all welcome. Look at it this way, according to all of the mathematical calculations a bumble bee should not be able to fly and yet it does quite well. Do not let yourself get so involved in the numbers that you lose the fun part of the equation. (no pun intended, well maybe just a bit). Start putting your ideas down on paper and go from there. Stop trying to over analyze the processes, they will work themselves out as you go along.

BC1
Jim

 

[Entropy455]

Two years ago at a tractor show, I observed a mini hit-and-miss engine running smooth as day at 120 rpm. The flywheel was about 6 inches in diameter. It ran slower and smoother than the engine linked below:

[ame]http://www.youtube.com/watch?v=28RCDPqYiBg&feature=related[/ame]

I remember talking with the guy about his design. He said the trick was using needle bearing construction to minimize friction, and obtaining the best achievable dynamic balance. If I ever see the engine again, I will take a video of it running and post it for you guys.

 

[Entropy455]

I’m not really sure why people are so hung up on the 120 RPM value? If 120 RPM is unrealistic, then I’ll double it. If 8:1 compression is too high, then I’ll drop it three points – it’s no big deal. Besides, I’d rather take steps to ensure success on my first engine build, rather than attempting to meet difficult design challenges.

I am not using someone’s plans, or building my engine from a kit. My goal is to design and build a working hit-and-miss engine from scratch.

And on that note – what material are you guys using for valves and seats? I was thinking about fabricating the valves from 310 stainless, and the exhaust seats from 410 stainless (condition-A). I’ve got access to beryllium-copper bar-stock for making valve seats, but I’m a bit afraid of the heavy metal hazards of machining beryllium. . .

 

[jpeter]

8:1 compression would likely be unmanageable in a hit and miss. On ignition that baby would jump right off the floor. 4 or 5 to 1 is more common, I think.

 

[Dave G]

Hi Entropy. I have successfully designed and built model hit-miss type engines. My engines were designed by the seat of the pants method, meaning I just started to build what I thought looked good and would work well. I use no more than 4-5:1 comp ratio, the higher the compression the more likely the engine will jump when it fires. I use 303 SS for my valves and will either use cast iron, mild steel or al-bronze for the valve seats. One thing I have found is to make the governor linkages as light as possible, this quickens the responce time of the governor. The governor should work freely in operation.
The fuel supply line for the carb should keep fuel at the jet so the engine will only need to take one gulp of mixture to fire. I do this with a 3/32 SS check ball at the bottom of the supply line in the fuel tank.

I have been building all my engines with just one piston ring with good success. Less friction and I use a Sunnen hone on my cylinders so I get a good round bore. A roller follower on the cam will help with friction. Pay close attention to the squareness and alignment in your setups so you can use proper clearances in the assembly of your engine. Just a few tips I could come up with and good luck on your build. Dave

 

[steamer]

I’m not really sure why people are so hung up on the 120 RPM value? If 120 RPM is unrealistic, then I’ll double it. If 8:1 compression is too high, then I’ll drop it three points – it’s no big deal. Besides, I’d rather take steps to ensure success on my first engine build, rather than attempting to meet difficult design challenges.

I am not using someone’s plans, or building my engine from a kit. My goal is to design and build a working hit-and-miss engine from scratch.

And on that note – what material are you guys using for valves and seats? I was thinking about fabricating the valves from 310 stainless, and the exhaust seats from 410 stainless (condition-A). I’ve got access to beryllium-copper bar-stock for making valve seats, but I’m a bit afraid of the heavy metal hazards of machining beryllium. . .

Hey Entropy,

Not really hung up...just answering your question. There is a lot of talent and experience on this board...and they love to help.

As for Beryllium-copper....keep it where it is...no place here in our shops that's for sure
300 series works great for valves.....machines better too.

Go slow...listen....and most of all...HAVE FUN....! ;D

Dave

 

[dalem9]

What is Beryllium-copper and why is it dangerous I hope this is not a dumb questions ,But I have never herald of it . Dale

 

[Entropy455]

Beryllium is an alloying metal that has excellent thermal stability over wide temperature ranges. It is commonly mixed with copper to form a tough, non-sparking alloy.

Beryllium is extremely hazardous when inhaled. It can kill you at best, or give you a very serious lung disease for several years, then kill you. Any type of welding and/or machine work on Beryllium alloys requires extensive air filtration. Not something you'd try to control with a shop-vac.

It’s the asbestos of metals. . .

 

[Ken I]

Beryllium Oxide is more toxic than cyanide - since it is always going to be present it represents a very real danger.

Some people develop blisters simply from touching the stuff.

That said it made great welding tooling and connectors (no fatigue mechanism I beleive) but for obvious reasons no longer used.

Typically replaced by Chrome Zirconium Copper.

Ken

 

[robcas631]

Entropy, well you did your math well. Make it.

 

[Entropy455]

The math is the easy part. Machining on the other hand. . . . . .

I’m going to establish a photobucket account so that I can keep you guys posted as to my progress.

 

[chuck foster]

hey steamer you might want to look at the video in post #7 on this website http://www.smokstak.com/forum/showthread.php?t=38746&highlight=slow+running+engine ................now that engine runs slooooow

chuck

 

[ShedBoy]

Someone beat that! That is amazing
Brock