Notices
Everything Diesel Discuss R/C Diesel engines here.

Lower Crankcase shape.

Old 12-22-2010, 02:13 PM
  #1  
Recycled Flyer
Senior Member
Thread Starter
 
Join Date: Oct 2009
Location: SydneyNew South wales, AUSTRALIA
Posts: 1,346
Likes: 0
Received 4 Likes on 4 Posts
Default Lower Crankcase shape.

Ok, we have a thread about upper combustion chamber shape but not one about the lower half of the engine and although this is not restricted to either glow or diesel two strokes Ifind this topic extremely interesting.

So to kick it off please view this link concerning the Cyclon PC6 combat engine and specifically the pic labeled "View from the rear," showing the most unusual crank web cut aways used to enhance gas flow at the expense of engine balancing. A lot of this can be done by someone handy with tools to a standard engine - if they dare!

http://www.clcombat.info/pcsix.html
Attached Thumbnails Click image for larger version

Name:	wu60485.jpg
Views:	17
Size:	28.6 KB
ID:	1537356  
Old 12-22-2010, 03:48 PM
  #2  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

It's not only the crank work.
Case compression has been raised to shift best pumping efficiency to high regions
Flow channels have been changed to obtain the same
The front bearing is fixed, so the rear bearing can float to allow heat expansion
The piston is adapted to allow good flow at high rpm.

This all works well in glow engines that rev up to 40,000. Diesels are a bit different in that they do not rev up all that high. I do however like the case stuffing and the front bearing fixing. For porting I would probably stick with 360° porting for least scavenge loss.
Old 12-22-2010, 04:25 PM
  #3  
Motorboy
 
Motorboy's Avatar
 
Join Date: Feb 2004
Location: Bergen, NORWAY
Posts: 2,234
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

Text from Edgar T. Westbury:

Crankcase compression
The limitation in volumetric effeciency of two-stroke engines is primarily due to the incomplete charging of the cylinder by using the crankcase as a pump. Apart from the loss of effective volume through imperfect port timing, the necessary clearance space to accommodate working parts imposes a limit on the pressure which can be produced in the crankcase. Many designers have taken staps, sometimes drastic, to reduce crankcase clearance to the bare minimum in order to improve both suction and pressure effects.
While these measures are often beneficial, they may in some cases defeat their own purpose by increasing the load involved in charging. The function of the crankcase is not so much that of a pressure pump as a displacer, and work done in producing pressure is a dead loss from the aspect of mechanical efficiency, which is one of the virtues of the simple two-stroke. Unfortunately, a certain amount of pressure is necessary to charge the cylinder in the very brief period allowed for transfer port timing, in a high speed engine; but, again, too high a transfer pressure may cause turbulence in the port and impair scavenging.

Attempts have often been made to improve the volumetric efficiency of two-stoke engines by using an oversize charging pump of some kind. This may take the form of a separate charging cylinder, as in the original engine by Dugold Clerk, or a rotary blower. Alternatively, crankcase displacement can be increased by adding a crank or or eccentric driven displacer piston, or using a stepped main piston. This was done in the Dunelt motor-cycle, which had a certain vogue in the 1920's, but did not prove the advantages of the method. Such engines have sometimes beeen described as "supercharged" but this term is more correctly applied to engines in which the cylinder is charged at more than atmospheric pressure.

This is clearly impossible in normal two-strokes in which exhaust and ports are open simultaneously. They can, however, be "super-scavenged" by an oversize charging pump, often with some advantage, but at the expense of economy, because wastage of fuel through the exhaust port is inevitably increased. This may be tolerated in racing engines, and is no disadvantage in diesel engines of the injection type, which are charged with air only. In my experiments, however, I have failed to obtain any substantial increase of performance in two-strokes by increasing the charging pump volume.

Edgar T. Westbury is not a fan of minimised crankcase volume for two-strokes, believing that any more than the bare minimum of primary compression needed to transfer the charge is energy wasted during the down-stroke. Duke Fox conducted where they fitted a telescoping backplate to an engine so they could vary the case volume as the engine ran. Goerge said it didn't make any significant difference at all!


Old 12-22-2010, 05:18 PM
  #4  
soarrich
My Feedback: (98)
 
soarrich's Avatar
 
Join Date: Feb 2002
Location: The Villages, Florida NJ
Posts: 4,677
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Lower Crankcase shape.


ORIGINAL: Motorboy

Duke Fox conducted where they fitted a telescoping backplate to an engine so they could vary the case volume as the engine ran. Goerge said it didn't make any significant difference at all!
I read this in a magazine in the early '80s also, but I remember it as "Pappy Debolt" doing the test.
Old 12-22-2010, 05:56 PM
  #5  
downunder
 
downunder's Avatar
 
Join Date: Oct 2002
Location: Adelaide, AUSTRALIA
Posts: 4,527
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

[link=http://www.novarossi.it/eng/products/engines/rex/plane/features.php?name=R50F&display=public]Rossi[/link] (Novarossi Rex) are now doing something similar with their crankshafts for plane engines but it's a common feature in car engines. The car guys are mad keen on doing things to their engines in the hopes of gaining more power and they'll cut away indiscriminately at the crankshafts grinding channels in the crank web and knife edging the leading edge of the counterweight (similar to that Rossi crank) but giving no thought to balance issues. I must admit I've always had my doubts that there's anything to gain from it because even at very high revs (30K or so) the diameter involved is very small. However if Rossi and Cyclon are doing it then there must be something to it.
Old 12-22-2010, 06:53 PM
  #6  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

This a PAW .60 crankcase, very straight forward and functional.
Attached Thumbnails Click image for larger version

Name:	Yw69073.jpg
Views:	20
Size:	40.2 KB
ID:	1537417  
Old 12-22-2010, 07:04 PM
  #7  
gkamysz
Senior Member
My Feedback: (19)
 
gkamysz's Avatar
 
Join Date: Jun 2003
Location: Crystal Lake, IL
Posts: 3,397
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

That Cyclon is what, 10 years old? Fora was doing this at that time as well. I think the notches do little more than allow the gas to flow out of the crank passage, which in these engine is a very large portion of total crankcase volume. I have a Stels where the crank passage is not drilled concentric to the journal. I think was done to assist balance though. Some of the older Rossi engines used sealed bearings and relieved crankwebs wrapped with a band to reduce crankcase volume.

It's all a big game of design compromise. You want the engine to breath well so you increase the crank passage diameter which increases crankcase volume. You shorten the rod to increase primary compression. Now you have piston skirt interference. So you cut off the skirt where it isn't needed and notch the backplate for the skirt that is. The rod can't be so short as to let the piston cover the transfer passages. And so on.
Old 12-22-2010, 09:21 PM
  #8  
Diesel Fan
Senior Member
 
Join Date: Aug 2010
Location: , AUSTRALIA
Posts: 108
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Lower Crankcase shape.

The lower crankcase compression ratio is what, about 1.6:1?
And stuffing the crankcase with a minimal backplate does little to raise thatcompressionratio so I am notsurprisedthat Duke Fox, George Aldritch or evenEdgar T. Westbury found any difference! It not like the primary compression ratio where a quarter of a turn can make all the difference.

This is all about gas flow and keeping the mixture in its gaseous form, and the least amount of non port surface area the better.

Just think of the lower crankcase as part of the transfer system and not a storage tank as this is what modern designs do.
Old 12-23-2010, 05:04 AM
  #9  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

Yamaha engineers have done a lot of research on the case volume issue. In all cases they achieved a volumetric efficiency of nearly 100%. The rpm at which that efficiency was obtained increased with the c'case compression ratio. So it is a matter of tuning the items together. For Example, it will not do to have a high CC compression ratio for a low rpm ported engine or vise versa.
Old 12-23-2010, 05:24 AM
  #10  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Mr. Reivers, please allow me to use another, "for example" and then you can tell me if I'm understanding this correctly. If one were to set up a slow turning torquey engine with a stuffed crankcase and high crankcase compression the result would be high speed flow velocities through the transfer ports which in a slower turning engine could result in much of the fuel/air mixture flowing right out the exhaust port. Does that big long sentence sound logical? Thanks, Dave
Old 12-23-2010, 07:52 AM
  #11  
Motorboy
 
Motorboy's Avatar
 
Join Date: Feb 2004
Location: Bergen, NORWAY
Posts: 2,234
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.


ORIGINAL: Hobbsy

Mr. Reivers, please allow me to use another, ''for example'' and then you can tell me if I'm understanding this correctly. If one were to set up a slow turning torquey engine with a stuffed crankcase and high crankcase compression the result would be high speed flow velocities through the transfer ports which in a slower turning engine could result in much of the fuel/air mixture flowing right out the exhaust port. Does that big long sentence sound logical? Thanks, Dave
The solution are tuned pipe and timing of the engine.
Old 12-24-2010, 07:57 AM
  #12  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.


ORIGINAL: Hobbsy

Mr. Reivers, please allow me to use another, "for example" and then you can tell me if I'm understanding this correctly. If one were to set up a slow turning torquey engine with a stuffed crankcase and high crankcase compression the result would be high speed flow velocities through the transfer ports which in a slower turning engine could result in much of the fuel/air mixture flowing right out the exhaust port. Does that big long sentence sound logical? Thanks, Dave
Yes Dave, that is very logical.
In engines, it is all about time-area for the porting.
A slow turning engine with highly stuffed crankcase would need very small transfer ports, and there probably would be too much time between transfer closing and exhaust closing. Such a setup is used for racey engines, not low rpm torquers.
Like Jens said, a very well tuned pipe system could make use of this long period. Because of the long period, the pipe should not be sharply tuned, but very mild in order to have a reflecting wave of long duration..
All in all a difficult setup. In your case it would bebetter to have less crankcase stuffing and larger transfer ports with quite short time between transfer closing and exhaust closing.

For those interested, you can download a copy of the Jennings handbook from my site.
www.prme.nl/download/Jennings_2stroke-guide.pdf
Old 12-24-2010, 08:25 AM
  #13  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Thanks Pe', here is another idea that I learned about when I first went to work at the phone company. I worked with a guy who had raced Go Carts professionaly in the 50s. He and his Dad who was his mechanic ran twin Mac 10s and at a certain rpm Frank would throw a switch that bypassed the ignition points and from that point on up the rpm band the iginition timing was only controlled by the flywheel magnets. I had never heard of that before and didn't think it would work but obviously it did. Is that a new one on you also. This thread called this to my mind because Frank was always talking about about crankcase stuffing and even changing it for certain track conditions.
Old 12-24-2010, 08:42 AM
  #14  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

In the Jennings book, reed from page 93 onward. Jennings also mentions the work of the yamaha engineers, and also, where the crankcase stuffing myth originated from.
Track demands, I guess so. Some tracks require lower rpm tuning that others, which prefer higher rpm due to how the straights, curves and climbs are configured.
Our model engines do not run on tracks, but have a propeller as very predictable load. So there is one varable we do not need to consider. Instead of adapting the engine to the prop, we can adapt the prop to the engine.
Old 12-24-2010, 04:55 PM
  #15  
Diesel Fan
Senior Member
 
Join Date: Aug 2010
Location: , AUSTRALIA
Posts: 108
Likes: 0
Received 0 Likes on 0 Posts
Default RE: Lower Crankcase shape.


ORIGINAL: pe reivers
Instead of adapting the engine to the prop, we can adapt the prop to the engine.
Now that, I like!

Nice quote mate.

Old 12-25-2010, 05:55 AM
  #16  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Instead of adapting the engine to the prop, we can adapt the prop to the engine.

That's why I liked those Bollys, they seem to have a prop size for every occasion, or engine size.

Read more: http://www.rcuniverse.com/forum/m_10...#ixzz198DvrJTP
Old 12-25-2010, 01:10 PM
  #17  
gkamysz
Senior Member
My Feedback: (19)
 
gkamysz's Avatar
 
Join Date: Jun 2003
Location: Crystal Lake, IL
Posts: 3,397
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

Pe, did you find the recommended time area figures to work with glow or diesel engines of small size? I'll be making a diesel from some OPS .45 marine engine parts I have and was going to borrow porting figures from the Irvine 40D. I haven't decided what I'm going to use for timing yet, my notes are on another computer, but I recall 140° Exhaust, and maybe 10° blowdown is what I came up with from other diesels. I planned on reducing port area little from the glow version. I may end up with a new rotor disc as it would be over aggressive for diesel RPM.

I always wonder what airplane guys mean when they say an engine has good "low end" power.

Greg
Old 12-25-2010, 01:30 PM
  #18  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

I found the recommended TA figures in the books very confusing, and deviating from them did not hurt at all in several experiments.
Blowdown time in diesels may be just as important as blowdown degrees. Since small glow engines rev higher than diesels, 10° may indeed be adequate, where normally I would prefer 15 - 20°
A good wellbehaved sportstiming could be:
120 - 140° exhaust
100° transfer
inlet opens at transfer close, closes at 35°-40° atdc

Don't look at others except for rough guidance. It never works their way in your own designs.
Old 12-26-2010, 05:31 AM
  #19  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Greg, my interpretation of, "good low end power" is when you're doing a slow flyby and it becomes a little too slow, you add one ratchet notch of throttle and the engine responds cleanly with no stumble, hesitation etc and the plane continues to fly merrily along with no discernible increase in the planes speed. A Telemaster practically demands that kind of engine. This is when you'll know for sure that the LS needle is set correctly.
Old 12-26-2010, 08:14 AM
  #20  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

Now Y wonder too.
To me low end power is the ability to pull large props without overheating, like the ST 2300 can easily be torqued down to 7000 rpm and probably a little bit lower, yet happily turns 12000 as well. That is a rpm range 1.7
The MVVS 160 is happy between 6,000 and 10,000, which is quite good, but not as good as the ST.
Old 12-26-2010, 08:48 AM
  #21  
gkamysz
Senior Member
My Feedback: (19)
 
gkamysz's Avatar
 
Join Date: Jun 2003
Location: Crystal Lake, IL
Posts: 3,397
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

Dave, to me that's good throttling. Any production car, motorcycle, etc, exhibits good throttling. Good low end power is like Pe says. If I'm driving my car and can pass somebody on a two lane without dropping a gear or two, that's good low end power. Propeller loaded engines don't have the low RPM, WOT scenario, at least not for more than a fraction of a second.

Pe, I was wrong about blow down, probably closer to 20° like you said. We'll see how it goes. I have two crankcases.
Old 12-26-2010, 02:33 PM
  #22  
pe reivers
Senior Member
 
Join Date: Jan 2002
Location: Arcen, , NETHERLANDS
Posts: 6,571
Likes: 0
Received 2 Likes on 2 Posts
Default RE: Lower Crankcase shape.

I have learned never to be sure of anything.
At one time, to get a 4.0cc engine, I mounted a short stroke 3.5cc crank in a large stroke 4.6ccengine, deviding the stroke difference top and bottom. This gave just 95° transfer, but with good port width. Exhaust was about 140°.
The engine had very good large prop (10x4) pulling properties from 12,000 upward, yet would rev up happily upto 19,000against all expectations. Not with all the power that one would expect from it's displacement, but there just seemed to be no upper rpm limit when the prop unloaded. Now this timing would not have been first choice, but it worked sooo well. The large blowdown surely helped rpm a lot.
Old 12-27-2010, 06:44 AM
  #23  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Greg, it is exactly the same thing, whether the engine is running slow because of slow flight or slow road speed, pulling away cleanly is the issue.
Old 12-27-2010, 07:16 AM
  #24  
gkamysz
Senior Member
My Feedback: (19)
 
gkamysz's Avatar
 
Join Date: Jun 2003
Location: Crystal Lake, IL
Posts: 3,397
Likes: 0
Received 1 Like on 1 Post
Default RE: Lower Crankcase shape.

That's transition. Going from one part throttle setting to another is not about power. Power is always measured at WOT. At low RPM it's clearly possible to be at part throttle and maximum manifold pressure (power) in a traction vehicle. That's the way I learned it in school and read about it in books and magazines. This is why I never understand the way airplane people use the term. Transition is typically not related to engine design but carb design and settings.
Old 12-27-2010, 07:44 AM
  #25  
Hobbsy
My Feedback: (102)
 
Hobbsy's Avatar
 
Join Date: Dec 2001
Location: Colonial Beach, VA
Posts: 20,370
Likes: 0
Received 24 Likes on 24 Posts
Default RE: Lower Crankcase shape.

Transition =change, my friends Kawalski 1400 Concours will transition from 15 mph in 6th gear to well above 150 with no hiccups, no bog. My Fox .74 conversion will transition from barely turning to 9,500 with a 12x8 three blade, the medium with which you hook up that transition whether air or asphalt doesn't enter into it.

Thread Tools
Search this Thread

Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service -

Copyright © 2024 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.