Engine observation and question

[LohringMiller]

The secret, especially in larger engines, is scavenging and breathing.  The secret at the model level is friction reduction, especially in the piston and liner fit.  As you scale down an engine the power producing displacement  decreases as the cube of the scale.  Surface areas decrease as the square of the scale, a much slower reduction.  That scaling helps a lot with port and heat dissipating areas, but not with friction.  Friction losses depend to some degree on the surface areas of the rubbing parts, a larger fraction of the displacement in model engines.

Larger two strokes' rpm is usually limited by breathing, but model engine rpm is often limited by inertia and the rod big end bearing.  Scavenging plays a part, but isn't as critical.  The best scavenging comparison I have is the difference between the M&D modified Zenoah cylinder and their cast cylinder.  Both are nearly identical except in the transfer passages.  The cast cylinder doesn't have an ideal shape, but does have an inner wall.  Quickdraws have even better scavenging but may be limited by their piston port intake.  Tuned pipes have the biggest influence on scavenging compared to all these other factors.  For that reason, I bet relatively poor transfer shapes can be overcome with carefully fitted ABC or AAC piston and liner construction and the right pipe for the application.

Lohring Miller

 

[PaulHail]

I believe in the larger two strokes, specifically the 100 and 125cc's it's the strength of the pistons that was the limiting rpm more than anything else.  Before karting killed the 100cc class they were spinning 20,000 rpm but pistons would have to be replaced after each race.  125's have been spinning over 17,000 rpm.

 

[Jim Allen]

"Tuned pipes have the biggest influence on scavenging compared to all these other factors.  For that reason, I bet relatively poor transfer shapes can be overcome with carefully fitted ABC or AAC piston and liner construction and the right pipe for the application." After designing & "building", all the parts required for thousands of miniature two cycle engines, I believe that both of these statements are absolutely true! The biggest problem with making those miniature ABC or AAC parts, or any part that would be necessary, is the initial investment cost of the machines & tools required plus the cost of exotic materials. How many engine manufacturers have machines capable of the tolerances required or the tools required to be able to measure? "NO ONE IS ABLE TO WORK BEYOND THE CAPABILITY OF THE MACHINES OR TOOLS  BEING USED!!

Jim Allen

 

[PaulHail]

(sic) "NO ONE IS ABLE TO WORK BEYOND THE CAPABILITY OF THE MACHINES OR TOOLS  BEING USED!! and the skill level of the operators

Jim Allen

There is a world of difference between manufacturing to +/- 0.001" and +/- 0.0001"  not to mention the additional q/c, hand fitting and part sorting.

 

[Jim Allen]

There is a world of difference between manufacturing to +/- 0.001" and +/- 0.0001"  not to mention the additional q/c, hand fitting and part sorting.

Actually Paul, many of the parts shown in the posted photos are made to a tolerance of +/- 0.00005". This would include pistons & cylinders with a roundness of that amount. Notice the micrometer mounted in the grinding vise to prevent its anvil from being affected by the heat of your hand.

Jim Allen

 

[ACTION JACKSON]

This is awesome information that we all appreciate. But right now I think we should all work on getting motor prices down. Price is what's hurting this hobby. Not to many people are going to drop 500 on a motor if you think about it. Maybe it would be better to enjoy the hobby more than to be first all the time. Not taking away from the speed junkies I just think the hobby is in a transition. All this tooling & design comes at a price. Hope I didn't go to far off this discussion but price was mentioned. Thanks again for this very informative post. I like his thinking. 

 

[David Ashcroft]

I am waiting for the day Novarossi build a 101+ engine with steel rod..

 

[PaulHail]

When I think back of what engines cost when I got into the hobby in the 80's and account for inflation, the costs really haven't changed on engines.  Holding the tight tollerances mentioned takes high quality machinery and machinists.  Loosening up the tollerances to cut costs will result in engines that don't perform reliably.  Then there is the economy of scale, building a few hundred or a thousand or so engines of a given displacement per year makes our engines much more expensive then if they were manufactured by the 10's of thousands.

 

[RaceMechaniX]

Several years ago I started designing my own 26cc gas motor based on the Kreidler 50cc GP motors.  A colleague who designed GP engines at Yamaha tutored me on the design and we went through the exercise of building up the BoM and tooling costs to do it properly.   This was a no expense spared approach as if we were building the next GP engine.   In small batches of 50 engines, the tooling and NRE was a little over $250k.  Each engine would cost around $1450 excluding a pipe and ignition system with no profit.

The exercise stopped as there was no way to finance this type of effort for a personal endeavor.  Zenoah's and their derivatives are excellent value for the performance.

I would estimate a nitro engine would be even higher investment due to the precision required for non-ringed engine.  

 

[Jim Allen]

This is exactly what I'm talking about Tyler. Many of the CNC machines including lathes, mills, surface grinders, OD - ID cylindrical grinders, etc. at Aero Precision Machine are in the $500k range. They were purchased to produce money making parts which does not include many of the required parts for high quality miniature toy engines. Many of the necessary materials & parts  required cannot be purchased inside the USA. What many people do not know is that many of the parts used in Nova Rossi engines are NOT made on their site any more. This also applies to CMB engines which are assembled in a garage. A un-named person, who is planning to bring an American made engine to market in the near future, recently visited both of these facilities & this is where this information comes from. This person also visited Aero Precision Machine for two days & was totally impressed with the engine manufacturing facility. He was accompanied by a well known model boater who is on this site.

The future for a high quality American made product isn't very promising! This includes both gas & nitro miniature engines for hobby use! There is also a lack of interest, by people in the hobby, to learn how to design & make their own stuff. I guess this thing is a dying art!!!

Jim Allen

 

[LohringMiller]

The American model engine makers often built engines as a sideline to their main business.  Also, after World War II there was a huge interest in model airplanes.  That made a much larger market for engines than exists today.  If you think of the nitro engine as one of several ways to power a vehicle, more modern power plants have taken over.  Small model boats and airplanes are being increasingly powered with electric motors.  Larger models use industrial style gasoline engines.  Both are mass produced for many other uses, keeping costs down.  The change I see is increasing use of electric power as battery and ESC costs keep coming down.  Already 7.5 cc size boats can be powered with a 6S battery power plant at a cost similar to a nitro engine and its fuel.  Smaller electric models are already less expensive.  This is only going to continue. 

Lohring Miller

 

[Dennis Somers]

Well ... as I said in post one, I'm an ol fart, and to me the sound ( or lack there-of ) of a buzz and a splash along the water does nothing to get my blood pumping. ANY speed without the scream of an IC engine trying to blow itself to bits, is dull ( sorry FE guys ). Can anyone imagine a NASCAR high bank race with full size electric slot cars buzzing by? Or hydros on the Detroit River buzzing down the front straight? I know, electrics are the future. And in the future if wifey wants me to go get a loaf of bread, I'm not going to mind buzzing out to the store, or going to a restaurant in my new battery eater. But my son and I will never get over the adrenaline rush and pure excitement of experiencing an NHRA Top Fuel fire up and literally shake the air and earth, for the first time. Electric can be interesting and shocking ( LOL ), but that's about it. Speed is nothing without the sounds and smells. Speaking of which, did I say I was an ol fart ... turn on the fan!!

 

[dwilfong]

The Russian eng had great port designs.

Cyclon engines had a very thick sleeve to make the inside radius of the port.

IR just cast the whole shooting match as a jug and then ceramic coated it.

I have a IR .21 boat eng and it is a work of art to say the least.

All Hobes has a few I am told.

Will see if I can get them from him.......Hint Hint........

Making a extra thick sleeve will solve the port design problem or the perceived problem.

Then you can make a good port cross section with half in the sleeve and half in the case.

Not that hard just have to do it.......LOL

 

[Rudy Formanek]

A little bit off topic, but very entertaining reading. /www.cycleworld.com/tags/ask-kevin   look for the narrative of two cycle scavenging---- 6 parts (so far)

 

[Jim Allen]

A little bit off topic, but very entertaining reading. /www.cycleworld.com/tags/ask-kevin   look for the narrative of two cycle scavenging---- 6 parts (so far)

Interesting stuff Rudy. I believe one problem in attempting to control axial & radial scavenging directions in our miniature engines is the result of using thin cylinder wall thickness. I have looked at this by building the same engine with progressively thicker cylinder walls. For example a .90 cu in engine needs cylinder walls approximately .125" thick. 

JA

 

[LohringMiller]

I'm also an old fart who loves the sound of IC piston  engines.  I remember this and this from my youth and the sound of 1200 hp a few years ago.  However I think the future of engines like these is around the same as piston steam engines in the 20th century.  Steam still uses turbines and many large piston IC engines have also been replaced by turbines.  Smaller piston engines are being replaced by electric motors.  My lawn mower, leaf blower, and weedeater are now electric.  The next generation will be battery powered.  Inside the house there haven't been IC engines for a long time. 

The king of modern two stroke scavengine is the Aprilia 125.  Most engine developers use variations on that 5 transfer design.  It's possible to shape the power curve with variations on the port angles using the right pipe.  A newer scheme, I call the FOS system, uses 360 degree exhaust porting over 360 degree transfers.  This should allow much better breathing at high rpm.  Earlier attempts were the Rolls Royce Crecy that used a sleeve valve to get a similar result.  The difference is the scavenging pattern.  The FOS design forces a loop pattern that results in a rising central column that loops at the head to flow down the cylinder wall and out the exhaust.  The Crecy system may leave some of the cylinder unscavenged without careful intake port design.  The limitations were mechanical in the Crecy and might also be mechanical in smaller engines.  A motorcycle size sleeve valve two stroke has been built, but I haven't heard how well it worked.  I believe it's waiting on a pipe design.  He has also built a FOS style cylinder for the same crankcase.  Below are some drawings of the FOS system.

Lohring Miller

 

[Jim Allen]

Excellent illustrations Lohring that show the radiused transfers, not tea cupped shaped, which could use the Coanda effect & the downward direction of the twin, water cooled, exhaust tracks.  Is there any thing in our market that comes close? Notice the very important inside radius of the transfers which is different from the outside radius!

Jim Allen

 

[JimAllen]

Maybe there are new guys out there that are able to visualize this stuff and have access to some modern R&D type equipment that could build some different design crankcases with an FMD 3d printing type system? I have seen online examples of firearms printed in stainless steel, and also people building rocket engines that couldn't be built with conventional means. You guys shouldn't give up on young guys pushing the envelope, they have heard and felt the impact of Top Fuel engines too. I'm not a young guy, but not a really old guy yet either. I read Gordon Jenning's book about two stroke design when I was in my early 20's and followed the motorcycle GP engine development as an interested gear head. I was deeply disappointed when most 2 stroke development went away, they're inherently better for racers. There are lots of people in this world who love high performance engines, and most of us cannot afford full scale racing. Modeling gives us a creative outlet we can afford. We should cultivate the interest in others to push the technology ahead. Lots of people in the world would love to help, if they have access to some trick machines, maybe some will find a way to slip in a few "government" jobs on occasion. I work in an optics lab, and I've noticed how some of our coatings like to repel water. I am going to apply some to a few props and see what happens. Maybe the effect is too small to make a difference, but who knows? you got to try stuff to find out... It's the desire and drive to try that matters. BTW, I'm a different Jim Allen, not the national racer on here. Although I read a lot of his stuff!

 

[Dennis Somers]

WOW!! Welcome Jim, and your comments are refreshing. I had to pick my jaw up off the floor there for a couple of minutes. LOL

 

[dwilfong]

You know that with the new 3D printers you can do any thing you want.

Thy are getting very affordable at this point.

The hard part is learning the 3D modeling to input into it.

The only limit is your imagination.