Radio Controlled Engine Flow Bench

[Mike Rappold]

Greetings!

I have a slightly "different" application for a flow bench than the more common automotive application. Mine is for a flow bench of for a 0.67 cu inch 2 stroke RC engine - in this case for a boat.

I have been reading the forum posts and I am getting closer to understanding some of the details of the orifice type benches. I still have a long way to go though. I have several questions that I am hoping others can point me in the right direction.

Background on the engines:
- 2 stroke, burn a mixture of methanol, nitromethane, and oil
- 0.67 cubic inch (11 CC) engine is what I currently run.
- Want to build a bench to handle at least up to 1 cu inch.
- The 0.67 engine turns 23,000 rpm today but I hope to increase 2-3000 rpm. Max would be about 30,000 rpm
- Carb bore is 0.550 inch diameter

Current Thoughts on Design:
- The cfm requirment of the engine is theoretically ~5.8 cfm at 30,000 rpm based upon the displacement - if I calculated correctly.
- I was planning on using a single shop vacuum for the air source. Bleed off valve to start but looking into motor speed control via a router speed control box.
- The flow bench would have a settling box before and after the orifice plate with baffles on both sides. Approximate size of chambers will be two 16 inch cubes.
- Manometers would be home built - more questions on these to come.


Questions:
1. For such a small air flow bench, any general advise or items to watch out for? My main concern is getting the resolution on the manometers small enough to see a change.

2. Using the spread sheet for calculating orifice size (in the forum spreadsheet section), for 5.8 cfm, this is a 0.8 inch diameter orifice. The 5.8 cfm is what the engine pulls at full rpm. Should the orifice that is used be based upon the engine displacement and its needs or the carburator bore (0.55" - 2.9 cfm)? I am a little confused on getting the size.

3. For test pressure, any recommendations? Does the small size of the bench require some thing less than an automotive application?


Thanks in advance
Mike Rappold

[blaktopr]

Hey Mike, Based on theoretical cfm needed and to see increases on the inclined more closely, I threw in some numbers myself. I am sure that others have a better idea of sensitivity on the inclined, but in the spreadsheet, the inclined was set at 5" of WC. Figured if at 5" and something like 16-24" long, would spread out the % steps to see smaller gains. Then using a .5 orifice would give a 7.6 cfm at 100% on the inclined. That would make your target cfm at 80% on the inclined, putting it at the better measurement "range". I have to check on the size of the tubing, but I have medical air tubes with a small ID which also may help. This is where someone else can help in regards to sensitivity. If that is better, I can send you some for you manometers.

Chris.

[bruce]

I actually machined some plates in the past few weeks for a similar application. When you machine your plates make sure they are dead-on for sizing. A hole that small being +/-.001" will make a difference in your reading.

The plates I machined were 1.5, 3.5 & 7cfm @ 1", I used the PTS Digital Manometer for reading my flow numbers @28" and 1" to verify them.

[Mike Rappold]

[49-1183904562]

Mike;

Since your application is self compairison only and such small sizes you may be better served to use a low depression like say 3 to 5 inches.

You are a perfact canditate for light oils in the monometers for sensativity. Or I also might make my depression monometer an incline also, I/E say 45 deg using water 1" of rise would equal .5 inch depression. Twice as sensative. 22.5 Deg would give you 1/4 = 1" simple scales.

Just a thought.

Rick

[bruce]

The 1" was specified by the customer's application. I tested the plates at various flow depressions to compare to the calculated numbers to "check my work" since it was such a small hole.

I wasn't sure if my bench would even work at 1" of static . . . which I can say it did



Edited By bruce on 1229811520

[bruce]

Your plate diameter will be driven by your max cfm and differential pressure you want to use. A higher differential will have a smaller hole. As Rick stated for the changes you want to see I'd look at a short rise but long incline.

[Mike Rappold]

[49-1183904562]

[color=#000000]Mike,

I do not want to confuse you but different fluids have different Specific Gravities. H2O is theoretically 1.0 Dot 5 brake fluid is like .985 +- but MMO is like .8?? and then Dwyer makes an oil even lower down near .6??.

What does this mean to you?

On a vertical monometer as and example if the SG of the fluid was say .50 than the scale would read 2 inches of column to equal 1 inch of Water column.

Since in this type of testing (most) everything is referenced to inches of water column as the scale we use.

So, in your case where you might be measuring at a very low pressure/depression 3

[Mike Rappold]

Thanks for the help and answers. I am slowly starting to understand better. This weekend I hope to begin building the cabinet. The manometers and orfice plate will be after that. Attached is a picture of the plans for the flowbench so far. Not all the details are done yet but hopefully you can see what I have in mind. The front cover is hidden inorder to see the inside.

The carb will mount on "top" and the vacuum source will be from the side. The main building material will be white melamine coated shelving material.

Any recommendations or changes are welcome.

Mike

[49-1183904562]

Mike;

For what you are doing, melamine will be good you will just need to use a good Adhesive caulk on all the joints as a glue, then finish all inside corners with another good bead. At your low depression and flow numbers even a little leak will throw off your calibration.

Plans are taking shape.

Rick

[106-1194218389]

This is exactly what mine is except turned on it's side. I have 2 shop vac's plumbed into it and it will pull an honest 260+ cfm at 10". I also used 3/4" melamine. Works great!



John

[blaktopr]

[Mike Rappold]

[106-1194218389]

You can do tongue and groove with a table saw, but you have to be a lot better than I am

John