Author Topic: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine  (Read 85667 times)

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #285 on: June 01, 2018, 12:13:26 PM »
Thank you again Jim.

The tool showed looks like you use it as a broach ..?.. and if I read the drawing correctly, you have bigger transfers / boost channels in the block than in the liner - I always made sure I didn't have any ridges / edges in the flow - or am I reading them wrong ?

Best wishes

Per

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #286 on: June 04, 2018, 08:40:07 AM »
Per,

Yes, the tool is used as a broach to cut the 3 boost & 2 main transfer passageways in the crankcase. Yes, the width of any transfer passageway in the crankcase is slightly larger in the radial direction than the width of any matching liner window. However, the top of any transfer passageway will match exactly the top of any liner window. The machining of the crankcase passageways is done on a rotary table with the fixture shown & the windows in the liner are cut with a dividing head. All dimensions concerning the engine timings are calculated mathematically. No degree wheel is ever used! I posted some prints to show the types of calculations required to modify or build an engine. Since this type of engine does not have a piston ring, the taper used in the cylinder & the top taper used on the piston can be determined only by trial & error. As a general rule, the shorter the stroke, the higher the taper on both & vice versa. I machined tapers in .001" increments to find out what was best for the stroke used.

Jim Allen

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #287 on: June 04, 2018, 08:43:03 AM »
More photos.

JA

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #288 on: June 04, 2018, 08:43:52 AM »
last photo.

JA

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #289 on: June 04, 2018, 11:57:02 AM »
Thank you very much for clarifying everything Jim  :praise2:

Quote
Yes, the width of any transfer passageway in the crankcase is slightly larger in the radial direction than the width of any matching liner window. However, the top of any transfer passageway will match exactly the top of any liner window.

So essentially you have a tapered transfers - that makes quite good sense and that I didn't get all the details out of the drawing first time.

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #290 on: June 05, 2018, 01:01:15 AM »
Per,

Yes. However transfers shaped with a radius inside & outside have proved themselves to be better than anything used thus far. These type transfers have proved themselves to be better than the standard "tea cup shaped transfers". The posted photos show what a transfer with a radius inside & outside looks like. Transfers with a radius inside & outside cannot be machined but must be cast into the crankcase.

JA

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #291 on: June 05, 2018, 02:06:09 PM »
Per,

One very important function of any main transfer design is in its ability to help attach the incoming charge to the piston's crown. In my engine the main transfers were flat on the top & the piston's crown was also flat on top. The engine with the radius type transfers uses a domed piston crown to help attach the incoming charge to the piston's crown. Cooling the piston crown in this way, combined with a toroidal head design, allows for very high compression to be used, without detonation. More on this subject latter.

Jim Allen

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #292 on: June 05, 2018, 10:17:43 PM »
Jim,

Your last tree cylinder pictures has transfers and ports that are shaped as I would expect a modern racing two-stroke (or very close to) and I'm guessing this is a 26cc gasoline engine. Your cylinder does not look very different to my nephews Yamaha YZ 80 with 28Hp except for cooling and size.

When I did a lot of tuning in my youth, I only cared about the gas flow (+ inlet and pipe) and I had a very low compression on most of my engines but still a lot more power down the straight than a Yamaha 80cc MotoX racer of the time from a 50cc and time and knowledge has increased quite a bit since then (the Yamaha had 8-10Hp back then and 28Hp today).
I used a water faucet as a source of a visible "gas" and corrected my ports until I was satisfied with the angles and lack of disrupting turbulence, but I never changed the transfer port angles in the cylinder wall (had no proper tools to do so) and did not encounter any problems - most likely down to my low compression (7-8:1) and about 12-13Hp => not nearly as highly thermal stressed.

I can see why your attention to angles are important in relation to not only flow, but also to cooling and gas boundary layers (insulation) in order to keep the thermal loads / stresses in check at the most important places like piston crown and combustion chamber + plug ..... and I never got that far, before money ran out back then and I stopped racing .... but the subject still fascinates me quite a bit.

Per

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #293 on: June 05, 2018, 10:31:50 PM »
Per,

Actually the last photos are from the MV .45 cu in FAI racing engine. These engines operate in the 35,000 to 38,000 RPM range. The transfers are copies of the type of transfers that are used in the 125 cc Aprilia racing engines. They make use of the Coanda gas flow principle.

JA

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #294 on: June 18, 2018, 07:03:23 PM »
Per,

The photos are of the 50 cc Bidalot engine cylinder. They are examples of what needs to be done in 26 cc to 35 cc high performance engines. Notice how thick the cylinder wall is, the number & shape of the ports, their location & the radial, axial angles involved.

JA

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #295 on: June 18, 2018, 11:37:44 PM »
Hi Jim

I'm rather surprised, or should I say that the wall thickness is even greater than the biggest I seen so far - but I can certainly see that it will provide a smooth and fast flow when changing direction. The port shape on the inside of the cylinder wall is very similar to the Aprilia 125 and the only difference to my nephews YZR80 is the bottom of the exhaust.

Thank you very much for the pictures.

Per

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #296 on: June 19, 2018, 01:24:02 AM »
Hi Per,

Yes & the head shape is similar to the Toroidal shape used on the 125 cc Aprillia. I use a Toroidal head on my 27 cc gas engine. One of the main features of the Toroidal head design is a very high resistance to detonation when high compression is used.

Jim

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #297 on: June 19, 2018, 10:16:03 PM »
Hi Jim

Quote
One of the main features of the Toroidal head design is a very high resistance to detonation when high compression is used.

I thought that it as almost mandatory for any serious high performance two-stroke design for the last few decades. You have posted your drawings here earlier in this thread, but I must admit that though I seem to remember seeing somebody posting the formula for the shape somewhere, but I can not remember where that was - do you have any suggestions for somebody reading this thread in the future (or me)?

Per

Offline strictlybusiness1

  • Full Member
  • ****
  • Posts: 237
  • ME., Tool Maker
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #298 on: June 23, 2018, 10:56:30 PM »
High Per,

The starting point for my engines was to machine a toroidal head shape that had a volume which was much less than a typical hemi head. I continued to lower the toroidal head shape until it could not be lowered any more. The volume in this head is less than 1 cc. I could not lower this head anymore because the ground strap of the spark plug was .016" away from the piston crown at TDC. There was no evidence of detonation, death ash or high engine heat when running at WOT.

JA

Offline Admiral_dk

  • Full Member
  • ****
  • Posts: 1400
  • Søften - Denmark
Re: .90 cu in, 30,000 RPM, 7.2 HP custom built nitro engine
« Reply #299 on: June 24, 2018, 09:33:22 PM »
Hi Jim

That is extremely compact and a very small volume as you mention  :o

Very impressive that it burns so clean too  :ThumbsUp:

Per