# Nitromethane two stroke glow engine (80cc) questions. . .



## nesikachad (Oct 20, 2010)

Hi Guys,

I joined this forum in hopes of getting some guidance by someone who's very knowledgeable about air cooled reed valve two stroke engines used for model aviation. Particularly with wringing them out for max performance.

I am building an 80cc nitromethane/methanol glow race engine for use in a 42% giant scale pylon racer and I need help with setting up the port timing and case geometry. I have the crank, rods, liners, and pistons done. (the easy part. . .)

If anyone loitering here is a genius with this stuff I'd be grateful for some help/guidance. 

Thanks.

C


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## GailInNM (Oct 20, 2010)

A wealth of information on two stroke design is available at
http://modelenginenews.org/

A place to start there is 
http://modelenginenews.org/design/index.html

Then go to the alpha site index and read the listings under
Model engine development

While most of this is for compression ignition engines and of a smaller size, the design process outlines may be useful to you.

Gail in NM


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## rklopp (Oct 20, 2010)

Years ago, Strictly IC magazine had a whole series of articles on two-stroke design, including port timing. I think you can still buy all the back issues. http://strictlyic.com/


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## kf2qd (Oct 20, 2010)

If you are going for 80cc why not go for a Gasoline/spark ignition and get better performance. There is a reason why they went to gasoline for the big planes.
Or are you also trying to make a scale appearing engine? A scale engine would be 4 stroke.


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## Jasonb (Oct 21, 2010)

Model Engine Builder Mag have had a number of articles about building for performance, such as valve design, timing, ignition, etc. Back issues are available

http://www.modelenginebuilder.com/default.htm

Jason


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## waynes world (Oct 21, 2010)

running an engine of that size on nitro would be wasting your time.
the available glow plugs will not give you a complete burn in a large combustion area like that with small glow plugs.

i would suggest to you to go to your nearesst karting club and either look at the yamaha kt 100 se or th comer 80 cc engines uses for the kids .
as far as port times exhaust no more than 186 deg ,intake 150-155 deg in engines tha big.

other than that zenoha engines or rcmk engines for marine application will be your best bet for pylon .6hp 18-19 k rpm comes easy from them with mods @ 25.4 cc , or you can look at the 2 cyl inline versions to get 100cc .

good luck with your quest.


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## Admiral_dk (Oct 21, 2010)

THE MOST IMPORTANT part in regard to high performance in a two stroke engine is the resonance exhaust chamber, without it it Will NEWER be a real high power unit.

I'll have to agree about using gasoline instead of nitro, unless you're into a multi cylinder unit, and ignition Will give you a higher output too, especially with higher compression than can be used on a nitro model.

I've had great success with 50cc tuning in my youth (I raced 50cc Moto-x) and a 10-12 HP 50cc engine would only give you some 2-3 HP without the pipe !!!!

There's a few individuals that are selling two-stroke software on the net, that will give you the right pipe dimensions to start with.


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## nesikachad (Oct 21, 2010)

Thanks for the replies.

To clarify:

This is a racing engine. I need 3.5hp per cubic inch to be competitive. Gasoline in an aspirated engine that's air cooled will never deliver that.

I have 85, 100, and 170 cc gassers for my giant scale aerobatic planes that use spark ignition, etc.

This plane needs to do one thing: Go really fast and turn left even faster.

The defacto practice right now is to use multiple glow plugs per cylinder. I plan on running two. The heat ranges used are typically the same as the ultra high rpm ducted fan and racer engines. (Jett, Nelson, Rossi, etc)

I have three Moki 2.10 cubic inch engines (roughly 35cc's) that I've fiddled with. These engines were designed for 10% nitromethane. After a long learning curve I've successfully modified one to tolerate 40% nitro and swing a 16X14 pitch APC prop to a static ground RPM of almost 9000 rpm (at 3600ft elevation) It unloads about another 800-900 rpm in the air and gets my Hangar 9 80" sundowner going somewhere around 150+mph.

Simple logic suggests that I just duplicate my port timing figures for this twin since the displacement is very similar. I plan to use the exhaust and transfer port locations from the Moki as a starting point for the twin. What's giving me fits is the intake timing for the crankcase The moki is a front rotary valve design where's this engine will likely end up as a reed valve. (unless I figure out how to make a rotary valve out of this thing without having manufacture a new crank)

I'm assuming that a reed valve doesn't really get "timed" like a rotary since the individual reeds just work off the pressure differential. From what I've read the way these are tuned is by the stiffness of the reed itself. A "soft reed" acts like a long duration camshaft for better high rpm efficiency where's a stiff one picks up the bottom end and midrange. I want to spin a 20x17 pitch prop at around 9000 rpm on the ground. It should unload about another 1000 in the air.

I'm very aware of how tuned exhausts work. Unfortunately the classes in USRA don't allow for tuned pipes. Open header only. I do plan to experiment a bit and see if timing up the exhaust header exit with the leading edge of the wing's root (slightly in front of) helps with any scavenging to help cram some of the intake charge that's trying to come out the exhaust back when the piston is on it's way back up into the cylinder. Obviously the wing will need a little extra work to keep it from getting cooked.

Thanks again guys.

C


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## Admiral_dk (Oct 22, 2010)

Reed valves are wonderfull at low to medium high revs, where they are "self-adjusting" timing wise, but for absolut max power you're better of with a rotary valve, because they flow a lot better at very high revs. Reed valves adjust their timing according to the pressure difference in the crankcase and the intake + resonance in the intake, so exhaust and transfer port timing matters here, when in the upper range of the revs.

I'm not sure that I would call 9000 rpm up in the high range if there's a pipe in the system, but without it sure is. I'm sure that you get a lot more control and better flow if you use a rotary valve or a drum intake - but problably a worse idle and low speed pickup.

Must the header be the same diameter for the whole run or are you allowed a slight taper ?

17.5 HP is quite a bit for a 80cc without a pipe. I would use some rather conservative porting times and some very wide ports. Two exhaust ports and five or six transfer ports sounds about right to me. Starting with the times of a comercial engine equals a good reference point.

Good luck with your project.


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## waynes world (Oct 22, 2010)

the best way around all of the mentioned thing on this subject would be to explore what others are doing towards performance gains in that specific area of pylon/ aero application.

I myself personally run/race boats so we make great use of pipes raising port times ,combustion area, shape squish area etc it all gets very involved nitro or gas/petrol engines.

as mentioned previously there is computer programs books with more information than you will soak up in a life time .

I will tell you that tuned pipes wont work with timings less than 180 deg on exhaust on any given motor.

If it was me doing pylon i would be looking at what you can and cant do with current rules in the particular class.without knowing whats allowable i would investigate the use of an overdrive gearbox setup to make use of the torque of the engine,as for rpm Conservative around 8500-10000 for most production 2 stroke gas engines.A tuned pipe is worth around 5000 + rpm extra to most engines.


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## nesikachad (Oct 22, 2010)

Wayne I'd love to take advantage of the "monkey see monkey do" principle.

The challenge I'm faced with is that there is very, very little information floating around about this stuff. This side of things represents a microscopic % of the model aviation hobby. What complicates things further is the bulk of the races are hosted in the SW part of the US. I live in the Black Hills of SD. No one around here has or wants anything to do with it because of how expensive it is and the complications with finding a place to fly them. It takes almost 1,000 feet of runway just to get a plane back on the ground due to the landing speeds and the "ground effect" that takes place when the wing gets close to the tarmac.

These are essentially streamlined gliders with big blocks shoehorned in the cowls. Luckily I have a good relationship with the owner of the local municipal airport here in Sturgis so I have a venue for testing/practice. I've been working with the USRA and the city of Sturgis to possibly someday host a race here. Time will tell. . .

I'd like to thank everyone again for the contributions. The port timing figures offered are pretty close to what I estimated. Now the chore of CAD'ing up the cases and writing code begins. (I own a machine shop for building custom rifles and vintage motorcycle parts)

- just recently finished up a half dozen 1909 H/D atmospheric cylinders

Thanks again.

C


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