Author Topic: Polini Thor 250 Engine for the XL  (Read 12203 times)

Offline rvaught

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Polini Thor 250 Engine for the XL
« on: October 20, 2014, 12:31:12 PM »
I wonder how this engine would work on a Legal Eagle XL. Belite Aircraft is selling them now. They claim 36 hp -- water cooled -- dual ignition -- electric start -- low vibration -- 50 pounds weight without coolant .


http://jameswiebe.wordpress.com/2014/10/09/polini-thor-250-engine-review-the-best-ultralight-engine-ever-except/

Offline Dan_

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Re: Polini Thor 250 Engine for the XL
« Reply #1 on: October 21, 2014, 02:52:50 AM »
I'm sure it is a good engine.  It is expensive.  It looks to be a reed valve induction system.  (just glancing at it...) (could not find any info about that) 


Piston port followed by rotary valve would be more desirable in my opinion for aviation.  Reed valves were big in motocross but they only fly for seconds at a time...


“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline stevejahr

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Re: Polini Thor 250 Engine for the XL
« Reply #2 on: October 21, 2014, 05:57:09 PM »
From Polini's web site:

Polini Engine2 stroke monocylinder
CoolingLiquid cooled
Bore for stroke72 x 60
Displacement244 cm3
Power36,5 HP at 7500 R.P.M.
CylinderAluminum with Gilnisil coating
Compression ratio11,5:1
PistonTwo chromium plated rings mm 1
IntakeReed valve in the crankcase
CarburetorPolini PWK
Air filterAir box
Ignition 1Electronic
Ignition 2Electronic with battery
Battery chargerOutput power 80 W at 5500 RPM
Spark plug hood5k Ω resistance
Fuel typeLead free petrol with 2% synthetic oil
Gear reduction unitHelical teeth in oil bath with 2,8 reduction ratio
StartingElectric starter + pull start with self winding cable FLASH STARTER
ClutchCentrifugal in oil bath
MufflerExpansion with oval silencer
Engine weight18,5 Kg (19,5 kg with electric starter) without radiator
Propeller rotationClockwise

Definitely reed valve.  Piston porting is the dead simplest but has the drawback of intake timing being limited to equal duration on either side of TDC which is less than optimal.  The reed valves came into vogue added to piston porting as a means to control low RPM flow reversal with longer intake duration timing (bad things happen when you send the same air through the carb 3 times).  The downside of the read valves is another part to wear out and they do impede flow.  The rotary valve provides optimized timing at the expense of extra parts and intake runner design complexity.  It is worth noting that rotary valve 2 strokes have been some of the highest power output engines, Can-Am set land speed records in all their classes back in the day using rotary valve 2 strokes.

One other advantage of reed and rotary over piston porting is additional cylinder support for the piston and allowing more flexibility in transfer port locations.  And in a 2 stroke transfer port design is key to scavenging and good power.  Polini does use a third transfer port at the rear of the cylinder.  I would not be bothered by either reed or rotary for aircraft.

Most of high output 2 stroke engines have gone over to water cooling due to the ability to control operating temperatures more closely and evenly.  This would be a very good thing for aircraft use.  Note also the relatively high compression ratio made possible by a better cooling system (although the effective compression ratio is likely a lot lower).  Even so it is still possible to overwork, overheat, and under lubricate this engine like any other 2 stroke.

This engine uses a modern design to produce more horsepower than a Rotax 377 with 124cc less displacement and 30 lbs lower weight.  Very tightly integrated package (eg: tuned exhaust system with muffler).  Would I run it on an aircraft?  Well if you want the most power at the least weight then sure, then you are willing to accept the tradeoffs of noise (102 db), fuel burn, longevity (6000+ RPM does not lead to long lifespans), etc.

Offline Dan_

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Re: Polini Thor 250 Engine for the XL
« Reply #3 on: October 22, 2014, 02:43:22 AM »
Thanks for clearing that up Steve...
;D

The lower the parts count the better.  If it aint there it caint fail...


“A superior pilot uses his superior judgment to avoid situations which require the use of his superior skills.”

– Frank Borman, Apollo 8 Commander

Offline ray seals

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Re: Polini Thor 250 Engine for the XL
« Reply #4 on: July 05, 2018, 07:37:42 AM »
talked to james at sebring, he said they were 8000.00 plus

Offline Armilite

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Re: Polini Thor 250 Engine for the XL
« Reply #5 on: February 15, 2019, 02:45:55 AM »
A Good used Rotax 277UL with a Gear Drive goes for Avg $600. A complete rebuild kit Avg $160 on ebay (Gaskets & Seals, Piston & Rings.

277UL was rated 26hp@6250rpm with a Muffler!

277UL was rated 28hp@6400rpm with a Muffler, put on Scat Hover Craft! Made 1980 to 2008.

Have the Case & Cylinder Ported by a Pro for around $300 and you can add 10-15% more HP. I recommend this guy. https://2-stroke-porting.com/

He also does the different Engine Coatings. Like Ceramic Top Coated Pistons with the Moly sidewall Coating. There are Heat Dispersant Coatings and Ceramic Coatings for the Head Combustion Chamber, Cylinder Exhaust Port, Exhaust Wye Pipes.

Add a Good Tuned Pipe for the Rpm you want to use, 6250rpm, 6400rpm, 6500rpm, and you can add 20-45% more HP. On Avg it takes 7cc to make 1hp if using 11.5cr, a Good Tuned Pipe for Max rpm, and using 6500rpm.
277UL 268cc/7cc = 38.2hp@6500rpm. 268cc at 6500rpm at 94% = 39hp on the Calc!

or use a Calc like this. My Skidoo 670 Dynoed 115.7hp@7750rpm which is 94% Volumetric Efficient.
https://www.mk5cortinaestate.co.uk/calculator3.php

A 277UL can also be Big Bored to 82mm. (82mm x 66mm) 348.6cc with a Custom Sleeve. 348cc at 6500rpm can give you about 50hp. On Avg a Big Bore Cost around $350. A Blank 82mm Sleeve is around $40 on eBay. You can pop out your Old Sleeve Heating it in an Oven about 300F and then Copy Porting over from Old Sleeve with some Clear Transparency Paper and a Marker to get it close. Cut out undersize just a little. Then have Outside Aluminium Shell Bored for New Custom Sleeve. All Sleeves come undersize. So once you Install New Sleeve and have the Ports all Matched up, then have the New Sleeve Finish Bored & Honed for your 82mm Piston. You have Options with a Big Bore, make your  50HP at your Max 6500rpm, or turn lower rpm, but still make more HP than Stock 268cc.

If:
348cc at 94% at 6500rpm can make 50hp!
348cc at 94% at 6000rpm can make 46hp!
348cc at 94% at 5500rpm can make 42hp!
348cc at 94% at 5000rpm can make 39hp!
348cc at 94% at 4500rpm can make 35hp!

Offline Hercules222

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Re: Polini Thor 250 Engine for the XL
« Reply #6 on: August 19, 2019, 05:25:14 PM »
No point in starting a new topic, but where this thread left off the Polini motor was overpriced and maybe the same performance could be had for a lot less with a rotax 277 bored out.  As of right now, the polini 250 can be had for $3300, which puts it right in the same price range as the 1/2 vw, so my question is this: What kind of real world performance should be expected with this engine? Where I live, I have a 500' piece of pasture that could be leveled into a runway fairly easily, but I only have about 800' over a 50' obstacle (power lines). What engine would you recommend for my situation?

Offline Armilite

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Re: Polini Thor 250 Engine for the XL
« Reply #7 on: August 20, 2019, 10:02:15 AM »
A Good used 277UL can be had with a Gear drive for $250 to a $1000, Avg is $500. I got mine off a Scat Hover Craft, I traded a guy a pistol I had $250 in it. A Basic Rebuild is Cheap if needed. Upgrades, Porting Cylinder & Case $300 for 10-15% hp Increase. A Tuned Pipe can Cost Free if you can Build it yourself to $650 for 30-40% more Hp. The 277UL dynoed 25.4hp@6000rpm by R&D Aero. 277F/277UL is 268cc/7cc = 38.2hp is possible at 6500rpm. The Sled version came with a 34mm Mikuni, UL version had a Bing 54 36mm, I believe 38mm is largest you can go. With a Big Bore it can make more HP. With a Stroker Crank it can make more hp. The more Blades you use the more Thrust you have, but takes more HP.

How much HP do you really need?

Can you handle the Weight of a Twin, like a Rotax 377/447 or Kawasaki 340/440, which are lighter? A 380HO Stock was 48hp, 57.26hp with a better Tuned Pipe.

JLO & Sachs made a lot of Singles also, that would make nice UL Engines. They have the (4) holes around PTO like the 277. An ACE Redrive made for Honda/Clones could be adapted. With 11.5cr, proper carb, and a Good Wide Ban Tuned Pipe it takes on Avg 7cc to make 1hp. 11cc to make 1hp with a Muffler.

 

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