I said same displacement not same BMEP. The BMEP is calculated from the torque, horsepower, and rpm that the reading was taken at, so the BMEP will be different. If you add different timing or scavenging in to the model you start talking apples and oranges.

Interesting that the newest Nelson racing 40 has changed from the ST X-40 bore stoke of 21.4 x 17.8 cm that has been used for the last 25 years by ST, Nelson and Jett to 20.4 x 20cm.

DESAXE, Thank you. I knew it was something like that. The old HP still runs great and is hand numbered A782 there aren't any plastic parts on it either....smiles

Quote:

ORIGINAL: downunder

....

So in the real world long, or short stroke makes no difference as far as usable rev range goes.

Brian,


In a previous thread thread, this fact was discussed more elaborately...

All our engines; long-stroke and short-stroke, as someone else wrote here, are practically 'square', so I did not put more emphasis on this here, but this theoretical rev-limitation imposed by the long-stroke design really is a very minor limitation... The smaller, lighter piston makes it even more so.

The lighter (again - in theory) piston can endure higher acceleration forces, but it will wear faster, with all other things being equal.


Did you calculate the maximum piston acceleration numbers at TDC, or at BDC?

Moki made many design changes when they went long stroke, that accounts for the greater power over their short stroke.

Those were about my thoughts, in addition to the rogue value with the 11x7 prop.
Two totally different engines. Stroke influence can only be determined if angles, Time/area, and shapes are the same, and then still. It needs good basic lab equipment and a test engine that can have the basic parameters set up as required. Not even Blair had this, and used a Yamaha cylinder to compare numerical approach with actual measurements.

All the reasoning and speculating we're doing here is of little import to the end user, he is only concerned that the LS one is stronger than the shorter stroke version. I waited and watched a long time for the MOKI .61 LS and found one in NC for $200.00 still sealed in its vacuum bag. They sold new for $184.00.

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So, how did it run? Come on, Dave. Don't be stingy with the analysis! <G>


Ed Cregger

Why do you ask? There's enough info in my post for you to work it out and know for sure .

Ed, I'll just have to run it again, I do remember that a Graupner 12x5 was too small, I ran it on Fox 5%/20% castor. I'll take pics. and re-run it for some numbers. It flew my Sig FourStar 60 really well, it now wears a Saito .82.

Here it is sans its carb, someone had a problem with their LS adjustment so I showed them how it re-assembled. The other pic. is of an adapter I made to adapt a Bisson .60 sized muffler to it. Also it now sports socket head head bolts in lew of the cheese heads.

Maximum acceleration/deceleration will always take place on the TDC side of the equation. As the rod length approaches infinity, the forces at BDC start to equalise with those at TDC.

In general engine designs a square is the best all round design. Any up or down from square produces plus and minus to the extreme. Longer and shorter rods and the piston pin has to shift up or down in the piston. the perfect combustion design would be a Full Hemi. What is a full hemi you ask. That would be a dished piston and dished head design. The trouble with this is low compression. Flat pistons are cheaper and give better compression. The perfect design would be a clam shell hemi with a shallow dished piston and a shallow dished hemi head to get the compression up. the object is to keep the ignition and force as close to center and exsert the most even pressure in a 360 degree radius and not have the glow plug hit the piston.

[color=#000099]
Of course there is, but I am sometimes too lazy to do it myself...

I ask because unlike what most people think, piston acceleration is not the same at TDC and at BDC...

At BDC, the piston shift and acceleration is reduced by the con-rod big-end moving sideways.
At TDC, the piston shift and acceleration is increased by the con-rod big-end moving sideways.

The angular velocity of the con-rod big end is the same as that of the crankshaft (around a radius equal to half the stroke) and a different value, around a radius the full length of the con-rod.

Supposing a stroke of 24 mm, the radius 'r' is .012 meters and the speed 'v' is, at 19,000 RPM, 23.876 m/s.
Acceleration of the rod's big-end is in m/s squared is 'v' squared divided by 'r', or 570/.012 = 47,506 m/s squared, which are 4842 G. Supposing the con-rod is 1.8 times the stroke, so the radius is increased times 3.6 and is now .0342.

Piston acceleration at BDC is reduced by 16,667 m/s squared and is 30,839 m/s squared, or 3,143 G.
At TCD Piston acceleration is increased by this number and is 6,540 G.

...This is the very reason an inline four-cylinder engine, which has two pistons going down and two going up, or two at TDC and two at BDC, seemingly balancing each other out, has such strong secondary order vibrations at twice crankshaft speed.

Rod angularity is the cause for this vibration and if an engine has cam-driven pistons, that have neither con-rods, nor rod-angularity, this type of vibration can be averted.

But this belongs in another discussion...

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Thanks for the comments and pics. That is a handsome engine.


Ed Cregger

Rod length can play tricks with the designer. Just try a rod length of half the stroke, and see what happens

Ed, I ran it just a few minutes ago, I used, as always Fox 5%/20% castor, it's 25 degrees here, it started right up but had a little trouble idling for more than just a couple of seconds. I changed from a Fox idle bar plug to a Fox Miracle plug and that took care of the shaky idle. I took a shot of the baffles in the Bisson, as you can see they are open on opposite sides giving it a deep, very cool sound.
Temperature 25 degrees MVVS .61 Long Stroke
Fuel Fox 5%/20% castor
Prop==Bolly 12.5x8
Max rpm with Fox Idlebar plug 10,775
Max rpm with Fox Miracle plug 10,825 with steady 2,100 rpm idle

That sounds like a really strong engine, of the OS .61 RF/SF type of strong.

A friend of mine owns an older OPS long-stroke .61 and a Moki similar to yours, if not the same model. He used them in an Omen pattern ship and they both provided very good power for that type of a pattern ship.

I have a shoe box full of used OS .61 SF's I have to go through and dismantle, clean and rebuild. I haven't checked as yet. I hope they are all large rear bearing versions.

I wonder what kind of bearing life one can expect with the smaller, original rear bearing on a piped engine?


Ed Cregger

I ran an early small bearing RF for a season without problems. There were some pattern flyers who had problems, I never got to examine a failed bearing. I would not worry about it too much considering old Saito 120s used the same rear bearing as an OS 40.

Thanks, Kweasel, for the input. Now I feel better. <G>


Ed Cregger

I am trying to make picture more clear to understand how short and long stroke, long and short connecting rod make difference in piston velocity.

The short stroke engine has two obvious effects. The distance traveled by the piston per revolution is reduced and the load on the crankpin are increased for a given shaft torque (due to the reduced "throw" ). Also the resulting engine is squatter, enabling its external dimensions to be reduced, with the possibility of an appreciable saving the weight. And for the very high revving engines the reduction in friction and wear resulting from a lower piston speed makes the short stroke design more to be favoured than the long stroke counterpart. This advantage is gained at the expense of higher loads on the crankpin and main bearing for the same torque and a greater leakage patch around the piston (due to the increased circumference).

The long stroke engine was best for high compression ratios and a essential for model diesels, in general terms, the improved perfomance of model diesels has largely been due to "tailoring" them for high speed operation by increasing the bore-stroke ratio. In the long stroke engine the piston has to be accelerated from zero at BDC up to a maximum one-quarter of a revolution later, then decelerated to zero again at TDC. The corresponding velocity gradient for a short stroke engine is appreciably flatter. This means that appart from the mid position and BDC and TDC, the piston is sweeping any other point on the cylinder faster with a long stroke than with a short stroke at a given RPM. If, therefore, the port depth is limited the gas flow will have to be correspondingly faster, a feature which may not be clear from a study of a timing diagram alone. For the same opening period, compared with a short stroke engine, depth would have to be increased to correspond to the same percentage length of stroke in each engine. thus the only way to compare port timing without taking the bore/stroke ratio into account is to express it in terms of percentage stroke.

The timing period is also modified by the length of the connecting rod, relative to the stroke. Lengthening the connecting rod (for a given stroke) will modify the "velocity gradient" of the piston so that it tends to dwell about TDC and accelerate more rapidly through BDC. Similary, shortening the connecting rod will have the opposite effect - the piston tending to dwell about the BDC and accelerate rapidly through TDC.

In the Desaxe engine will piston accelerate faster from BDC., promoting quicker opening and slower closing, giving in effect a larger opening for a size of port. Actually the timing feature of a Desaxe cylinder is not necessary the reason for its adoption. It may be employed for mechanical reasons in that it greatly reduces the side thrust of the piston during power stroke.

Invariably, however all high speed engines are of short stroke design: and long stroke engines where still made, designed for generating high torque at low or moderate speeds. The true "general purpose" engine, it has been suggested, should have a stroke slightly greater than the bore, this being what we would classify as a "sports" type engine with a maximum life.

Obviously, however, many other factors come into account in commercial productions - following proven practice established by earlier designs - designing for "reworking" to a different capacity later for a new model in a different class; and so on...

Thankyou Jens, well done and easy to see and understand.

I have learned alot by reading this!
You all have some very impressive knowledge.
Thank you for the information.

This subject is getting too much attention. There is very little difference in power between a standard and long stroke .61 with similar porting specs. A tuned pipe with porting to match is the key to making power. Few modelers take advantage of a good exhaust system.

....yep....like I said....

...but you gotta love all the fancy footwork in this thread.

FBD.