Expert Chat Posted Sunday, January 6, 2002 by MartenDavis
We have several people who have committed to giving some of their vast knowlege this Winter on the Guest Chat of the RCBoat.com Listbot. http://rcboat.com/echat36/guest/index.html
Some of these are Mike Betke, Rod Geraghty, Stu Barr, Jerry Dunlap, Ron Zaker Jr., Brian Callahan, Andy Lofton, Terry Keeley, Al Hobbs, John Ackerman, Phil Thomas, etc. I amy even do one myself. Look for the times and dates for these chats.
Marty Davis

presentation rescheduled Posted Thursday, May 31, 2001 by Brian Callahan
My presentation about Drivetrain Dynamic Effects on Performance of a Single Cylinder Two-Stroke Engine has been rescheduled. Instead of presenting it in Pisa, Italy this November, I will present it in Detroit, MI, USA at the SAE World Congress on March 4-7, 2002.

Again, the conference is open to the public, with reduced entry fee for students and SAE members. Details can be found at:
http://www.sae.org/congress/index.htm

See you there!

one pulse per revolution! Posted Tuesday, May 29, 2001 by Brian Callahan
Anyone trying to design a dynamometer or telemetry system for our little boats and engines needs to know something I learned during the course of my PhD research here at QUB concerning rpm measurement.

That is, if you are using a crank trigger to infer rpm from the time between electrical pulses, make sure you have only ONE pulse per revolution.

The reason is that the crankshaft speed of a single cylinder engine fluctuates wildly within the cycle. Four-strokes will be worse than two-strokes, but still, I measured up to 300 rpm fluctuations on my Yamaha YZ250 test engine at 3000 rpm mean speed. That's 10%!

How did I get 300 rpm? I measured the time between each pulse on a 360 pulse/revolution encoder to the nearest 50 ns (yes, nanoseconds). At 3000 rpm, I could calculate the "instantaneous" rpm at each degree to the nearest 3 rpm.

For instance, say your .21 fluctuated similarly. From 0 to 30 degrees (e.g.) the crank was going 28500 rpm, and from 120 to 150 degrees it was going 31500 rpm. If you had a 6 pulse per revolution crank trigger, your rpm calculation could be up to +-5% in error depending on which period you measured. If you measured a random period from your 6 ppr system, you would get a 10% noise amplitude on your signal. Not good!

However, there is a saving grace. It turns out that the speed fluctuation is due to simple drivetrain dynamics. So, the profile of instantaneous speed versus crank angle stays very much the same cycle-to-cycle as long as the engine is 2-stroking. So, with one period measurement over the whole revolution, the time will stay quite constant and representative of the mean crank period.

Note that if you are instead measuring pulses per time, the more revolutions per measurement period, the less this will be a problem. For good telemetry systems, however, this is not an option, because the time between samples needs to be prohibitively long at low rpm to get good resolution.

Happy testing!

results from 3.5cc cylinder pressure study Posted Saturday, April 28, 2001 by Brian Callahan
The results from the high speed cylinder pressure test are in. On April 4th, we ran the Nova Rossi .21 at 30,000 rpm against a Kavan #12 test air propeller and measured firing cylinder pressure with a high speed, high temperature pressure transducer mounted in the combustion chamber. Relevant engine specs were as follows:
Fuel: 60% nitro, 18% oil, and 22% methanol
Ignition: McCoy 9 glow plug
Trapped Compression Ratio: 9.9:1
Squish: .0085"
Exhaust Open Duration: 186 degrees, "1/3" eyebrow top
Transfer Open Duration: 125 degrees
Pipe: Irwin "rpm" nitro pipe

At Marty's request, I am posting below a discussion I had with Dan Cousin on the ListBot technical forum. Using data from the test, I was able to answer Dan's question quantitatively.

> From: Dan C. [mailto:hornet_hydro45@hotmail.com]
>
> I read a while back that there was an effective maximum nitro limit for
> different sized engines. The numbers they gave were something like
> .21 = 65%
> .45 = 55%
> .67 = 40%
> .90 = 30%
>
> The reason they gave was that on ignition, the flame moves slower
> in a high
> nitro fuel than in a low nitro one. Since the distance that a the flame
> must travel to completely burn the fuel (from the plug to the cylinder
> walls) is shorter in a 21 than in a 90, the 21 can completely
> burn a higher
> % of nitro than a 90. I would guess this situation would be agrivated by
> the higher exhaust timings used on a modified 90s compared to 21s
> (according
> to Martys old tech notes). But it would be offset a little by the higher
> RPM range the 21s run in.
>
> Any personal experiences with high (50-60%) vs low (25-35%) nitro in 90
> sized engines? How true is the theory above with respect ot
> other motors?
>
> Marty? Brian? Andy?

Dan,

Yes, experience shows that the larger engines can't take as much nitro and compression as the smaller engines. However, based on the recent cylinder pressure study we did here at QUB, I don't think flame propagation is the limitation.

We measured total burn durations on the order of 55 degrees in the .21 on 60%, and the middle 80 per cent of the burn took a mere 30 degrees. These are not very long. Gasoline 125 and 250cc GP motorcycle engines tested here exhibited similar burn durations. The nitro engine we used had .0085" squish clearance, which is not particularly tight for a .21, hence the engine did not have particularly high squish velocities. So, other .21s would probably exhibit faster burning yet.

The next question is what would be the expected difference in flame speed needed to propagate half the bore in 55 crankshaft degrees in .91 versus a .21. Experience here at QUB shows that flame speed ends up staying about constant with engine size. So, comparing a Nova Rossi .21 to an OS .91 (actually an .86):
.21: bore/2 = 8.2 mm. At 30,000 rpm, 55 deg = 26.8 m/s
.91: bore/2 = 13.5 mm. At 22,000 rpm, 26.8 m/s = 66 deg

That's only a 20% difference in burn duration due to the increased bore albeit lower rpm.

Therefore, I would be surprised if the .90 engines exhibited any sort of limit on nitro content due to flame propagation speed. The extra joules/kg air available from the higher nitro with it's low stoichiometric air/fuel ratio would more than make up for the loss in thermodynamic efficiency. Also, notice that fuel would finish burning more than 40 degrees before the exhaust opened (see my previous post). [Note: in that previous post, I had reported that the End of Burn occurred about 55 degrees before exhaust port opening.] So, the assumption that the fuel is not finished burning by the time the exhaust opens is incorrect for both engines.

I currently believe the reason the little engines can stand more nitro and higher compression ratios is far more basic. That is, they have a higher surface area to volume ratio in the cylinder. Assuming:
.21: bore = 16.4mm, stroke = 16.4mm, surface area halfway along the stroke = 845 mm^2, volume = 1730 mm^3, surface area / volume ratio = 0.488 /mm.
.91: bore = 27.05mm, stroke = 24.5mm, surface area halfway along stroke = 2190 mm^2, volume = 7040 mm^3, surface area / volume ratio = 0.311 /mm.

The .21 has almost 60% more surface area per volume. This means it dissipates more heat to the cylinder walls, head, and piston per joule of heat introduced from the combustion. Therefore, for the same % nitro, compression ratio, air/fuel mixture ratio, and charging efficiency, peak flame temperatures will be lower, and detonation will be abated. So, you can run the .21s with higher c.r. and higher nitro and still lean the needle down for good power before blowing the plug.

Prop Heat Treating Posted Sunday, March 25, 2001 by MartenDavis
If you have access to a temperature controlled oven, you can perfectly heat treat your props. McMaster-Carr has .002" soft stainless bags that are part number 3438K11 price $17.17 (10 qty) which you place your props in, drop in a wooden kitchen match (to burn up the oxygen when it get hot enough)and place in the heat treat furnace. Temperatures are: 1100 degrees for 1 hour, quench (if you have the nerve, urine is best, if not then icewater), place back in the furnace for 1 hour at 600 degrees, turn off oven and allow to air cool. The prop should NOT be tarnished at all and will be HARD. This makes props hold their edge very well. McMaster-Carr has a website http://www.mcmaster.com/

The finishing touch is to spray your prop with Dykem, to see where the prop bites, and were there are areas that do nothing (you can remove them). After running your props for a while with the Dykem (red) on them you can see all these areas (Dykem will be rubbed off by the water, in these areas). Part number for the Dykem is 2131A64 price $7.72

Outboard Lower Units Posted Thursday, March 22, 2001 by RodGeraghty
When looking for speed in either the K&B or O.S. outboards it is important to remember that everything under the water is a source of drag.
A sharpened skeg and an aligned skeg to the propeller thrust line can make the difference of 2-3 mph and a much better boat handling package.
This is a very important consideration in the IMPBA 3.5SuperStock class.
Take a 12" piece of 1x2 and gently fasten it in a vice with the skeg of your outboard so the 1x2 extends out behind the motor.
Then take a 12" piece of 3/16" I.D. brass tubing and slip it over the prop shaft and extend it out over the 1x2.
Sight down the 1x2 and tubing and if the tubing and 1x2 do not line up,you are losing speed and freed up boat handling package.
Do not try to bend the skeg to align it.It will not work.You will break the skeg.
The skeg can be filed and shaped to facilitate the alignment.
You may find some of the K&B lower units that will require a re-tapping of the prop bearing threads to get them to align.

High Speed Cylinder Pressure Test on a 3.5cc Nitro Engine Posted Wednesday, March 14, 2001 by Brian Callahan
Peter Smyth, a final year undergraduate student at Queen's University Belfast, is nearing completion of a new test rig to measure the in-cylinder pressure versus crank angle of a 3.5cc Nova Rossi firing at full load at 30,000 rpm. It will use an Irwin "rpm" pipe, D&J carburetor, McCoy 9 plug, and 60% nitro.

Using these data, we hope to learn the ignition timing, IMEP, fuel heat release characteristic, and combustion duration for high nitro fuel and this particular engine/pipe/plug combination. To date, the literature contains no such data for engines of such small displacement, burning nitromethane fuel mixtures, or at such high speeds.

Peter will use a 5mm Kistler 6053 piezoelectric pressure transducer mounted directly in the combustion chamber and a 360 pulse per revolution crankshaft angle indication system designed here at QUB. Data will be taken using a Nicolet Odyssey acquisition system and processed with OPTIMUM Power Technology's PTrAn software.

The engine, pipe, and carburetor were donated by Marty Davis, and the project is co-supervised by Dr. Robert Kee and myself. Stay tuned for further developments...

Drivetrain Dynamics Presentation Posted Wednesday, March 14, 2001 by Brian Callahan
I will be giving a talk at the 2001 Society of Automotive Engineers (SAE) Small Engine Technology Conference in Pisa, Italy on November 28-30.

The topic of discussion will be Drivetrain Dynamic Effects on Performance of a Single Cylinder Two-Stroke Engine.

The general public is invited, and SAE members and students receive a discount on admission fee to the conference.

See

http://www.sae.org/calendar/set/index.htm

for further conference details.

New Gas Engines Tested!! Posted Thursday, March 8, 2001 by MikeHoffmeister
First Dyno and On-Water Test Results for the New QD 25 Engine.
We were fortunate enough to get some of the first parts that Mitch completed for the new Quickdraw 25 cc engine kit that he's been working on for quite some time. The parts look great, and we put the engine together with no extra mods, and ran it.
We were a bit skeptical of the claim that was in the RC Boat Modeler magazine of 5 HP, especially since the engine never even ran before last week! Well, the 5 HP number turned out to be false, as expected - but in opposite direction of what we expected! After running the engine in with an airplane prop, and setting the needles, it pulled over 6.5 HP peak on it's first dyno pull, and it showed the ability to run well above 17,000 RPM on the water.
We tested it last Saturday in a Stryker Cat, and it pulled 17,500 RPM with a standard QD pipe, and relatively mild exhaust timing. With our SAW pipe, it lost some low-end, but pulled 18,000 RPM on the water.
For the non-believers, I have a video clip the I'd be happy to e-mail to you (it's a big one, at 1.6 Mb). I'll be out of town for 10 days starting Saturday AM - apologies if it takes me a while to reply. More to follow.
Dyno and On-Water Test Results of the New J&G 35
To be fair, and to report impartially on both manufacturer's recent progress, we would like to share some info on the new J&G 35 cc billet engine, which we tested over the past few weekends.
Jack has been working hard to get his new 35 cc engine to market, and the key features include light weight, a compact package, good RPM capability, and great looks!
We dyno'ed the engine, and found that it had some un-usual characteristics for a two-cycle engine (in a good way). The engine has very strong bottom-end power, but it also revs like there's no tomorrow.
The engine weighs just over a pound more than a Zenoah G23, and fits standard 5" mount rails with no problem. I tested it in my Insane 47 mono (a fairly big heat-race boat, which normally runs about 50-51 MPH in heat-race trim), and it ran quite well. With a Prather 280 prop, the engine turned over 17,000 RPM with no problem. With an Andy Brown modified 1475, it turned about the same RPM, but was a bit faster, at 58-59 MPH. The engine was also recently tested in a Sport Hydro by Larry Ingelson, and was running about 64 MPH in heat-race trim, in a 20 lb boat!
This is a very exciting time for Gas Boating. Jack and Mitch may be competing with each other more than ever before, but in my opinion, it's bringing out the best in both of them, which we all stand to benefit from! It's no longer acceptable to keep on churning out more of the same parts - now it's a matter of continuously improving the performance, and also focusing on more efficient production so that prices can come down to some degree.
I think the performance of these two new engines will take high-performance Gas Boating to the next level.

Nitro and cold weather Posted Thursday, March 8, 2001 by RodGeraghty
Andy is absolutly right.The hotter plug will help but here is another hint.
Being from the cold and wet Northwest we faced this problem quite often,especially in the winter at record trials.
A drop or two of lighter fluid[naptha] as a prime will light off that motor on the the first roll of the starter.This approach will also light 60% in below freezing temperatures.

Squish Band Posted Thursday, March 8, 2001 by MartenDavis
In all the testing that I have done with the dyno, backed up with a huge amount of on water testing, I have arrived at a conclusion about the configuration of the squish band. To make maximum power, make the squish band FLAT. You will get some shake (like engine being out of balance with the flat squish band). Many engines come with 3 degree across the squish band. I believe that you are giving up a lot of low end power that way (launchability). A good compromise is to make the outer 1/2 of the squish band flat and the inner 1/2 with a 1/2 degree angle. These results are using the currently available pipes for the various engines (Irwin for the 21, Big Belly CMB for the 67) All other pipes that I have tried have also produced the same characteriestics, but with a very well optimized pipe there may be some change in this result.

Nitro and cold weather Posted Thursday, March 8, 2001 by AndyBrown
As the new season is just begining in some areas the air and water will be relativly cold. Remember that nitro is very difficult to ignite compared to methanol. If your engine was running perfectly on
65% nitro last summer, that may not be the case on your first weekend out when the air is 50 dergees F and the water is 38 F. In these conditions your engine will run much better on about 40% nitro.
Also, on cool damp days it can be very beneficial to use a hotter glow plug.

Past Technical Notes Posted Wednesday, March 7, 2001 by MartenDavis
Be sure to make use of the numerous Technical Notes located at http://rcboat.com/past.htm There is a great deal of valuable information located here!

New News Section of RCBoat.com Posted Wednesday, March 7, 2001 by MartenDavis
We have installed a New "News CGI Script" that will provide timely information to members of the RC Boating Family. Hopefully, we will post information which is helpful and provides a service to all model boaters. Look for technical tidbits and other items of interest.