[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 4)

the second phase of an explosion is a depression.
So, once the exhaust port opens, the burnt gas went off in the exhaust and a stage of low pressure
is left inside the cylinder.
When the ABC ports open, the fresh charge is sucked from the crankcase.

It's the way I understand the events but not sure of it

Haufen a écrit:

would you say opening the A ports last, so that they actually flow first, because they have the longest path to travel would be beneficial in your concept?

Frits Overmars a écrit:

I've seen this remark before and I can't understand why anybody would say that. The port that opens first, flows first.

Makr a écrit:

Frits, You have glossed over this before which makes me think that it is questionable thinking. The idea is that there is positive pressure in the combustion chamber when the first transfer port opens pushing the incoming charge back into the transfer port.

Let's split some hairs first, Mark, so we get it out of the way: negative pressure does not exist, at least not in our universe.
OK, with positive pressure you mean a pressure in the cylinder that is higher than the pressure in a transfer port.
That is undesirable when it happens while that transfer port is opening; it means insufficient exhaust blowndown time.area, but so be it. Then yes, the spent gases will enter the transfer port.
Like I said, the first transfer port to open is the first one to flow, although not always in the desired direction  .

I understand.

jfn2, I don't yet understand the reasoning for drawing air directly into the transfers. How can it get mixed well with fuel at that point? Is it because this is a quite small stream of air? Would you get a similar result if you passed the same amount of extra air through a bigger carburetor? Evidently that small amount of added air was not correcting an over-rich condition since you had to come up three jet sizes in the carb. Could something else be going on? How about the possibility of that little airstream maybe breaking up a layer of fuel-rich and oil-rich air running around the outside turn of the transfer (put there by inertia slinging some of the fuel out of the air/fuel mixture wherever the mixture takes a turn)? (4-stroke guys worry a lot about fuel/air separation). You certainly seem to have found something that works. I just don't get why. It just seems to me that those two little jets of air must be correcting a tuning or design issue, and plainly you already were good at tuning. The fact that it's effective in low rpm and starting into mid-range but not at peak power/rpm should be a clue .  .  .  is all of the performance improvement coming before the engine gets up into the tuned range of the pipe?

fjn2, when he started describing his experiment, referred us back to P. 14 and some ideas from Ken Seeber, to which Frits responded:

Frits Overmars a écrit:

"Been there done that. Well, sort of. I once attached carburetters to the sides of a two-stroke cylinder, bypassing the crankcase. It could be made to run but the con rod bearing didn't like it." 

EDIT - - Somehow almost all of this post was deleted, so that it now makes no sense. Maybe I did it inadvertently, but I thought it was all up and good for a day or so. Maybe I'll try to re-compose it.
Sorry,
--Smitty

Smitty,
The thrust of my suggestion is to introduce combustible mixture into the upper cylinder by bypassing the crankcase. This has the potential to overcome all the pressure drop, crankcase volume and inlet system issues related to the normal crankcase charged system. In its simplest form it cannot be used to start the engine so a hybrid system is probably necessary whereby a conventional crankcase scavenged system is used, but is supplemented with a direct from atmosphere inlet (DAS sounds nice, doesn’t it).  This hybrid system offers starting, part load/speed operation, crankcase & piston cooling and crank & rod lubrication.
What jfn2 did represents a small version of the suggestion and it seems to offer some gains, hopefully with some more progress reports as the application is developed.  As they say in Oz, “good onya mate!”
As I originally suggested, if such a system were to be applied to a generic RSA type layout, the DAS inlets would replace the B ports. Obviously the DAS inlets would necessarily be throttled to provide opening at the appropriate engine load & speed conditions, primarily when the exhaust is into its “tuned speed” status.
Applied to a “contra axial flow” scavenging system, with say 4 exhausts and 6 transfers, perhaps two of the transfers could be replace with the DAS inlets. So, with no immediate access to Solidworks, I have made a rough sketch of how this might look.
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Assuming the exhausts were above the crankshaft centreline (ie exiting on either side of a conventional bike layout) the two DAS inlets would be fore and aft (ie on the piston thrust axis). From what Frits has noted, by blocking off one of the transfer passages, the main central upwards flowing column of incoming mixture fortunately just tilts, rather than breaks up. So, with the two opposing DAS inlets, I would presume the flow column might lozenge into a slight elliptical shape.
Obviously Frits could make a much more informed comment on this. As to the sizing of the DAS inlets and exhaust design, I would be completely guided by Frits and Wobbly (kiwibiker). The DAS inlets might in fact be carburettors, otherwise they could be injected or just supply air, with the extra fuel being provided via the crankcase system.
Merry Xmas

That seems to be essentially similar to what I was suggesting. As I tried to describe it, my suggestion was to apply Frits' 24/7 intake to the side opposite the exhaust on a conventional 2-stroke cylinder, say the Aprilia we refer to frequently. By modifying the lower end of the boost port and maybe the B transfers as well to be fed from a no-valve carburetor intake passage near the bottom flange of the cylinder block you'd have your intake source that doesn't come through the crankcase.

One added advantage of this is that if you for some reason wanted water-injection, here's the place to put it into the cylinder above the piston without getting emulsified water on your various bearings . . . .

The only solution is a total solution. That entails much added complexities, both mechanically & digitally, with variable rotary valve and port tunnels / openings utilizing apparatus to control their shape / flow direction. Ditto for the exhaust pipe as it too should be variable.

Come up with an elastic material that can be morphed electronically for both motor, inlet & pipe, plug in the engine management system and forget slides, butterflies, and related valving do-dads that cost money, cause trouble, & carbon-up.

Or, if it's pure speed being desired, simply realize that advanced aircraft uses a different, non-reciprocating system for an obvious reason.


Merry Christmas to you Mad Men / Thinkers / Tinkerers.

ambike a écrit:

The only solution is a total solution.
. . . if it's pure speed being desired, simply realize that advanced aircraft uses a different, non-reciprocating system for an obvious reason.

But that is NOT what's desired, ambike.

From the late 1940s to the early 1980s, the sport of Unlimited Hydroplane racing used WW2 fighter plane engines, displacement from about 27 to about 37 liters, huge centrifugal 2-stage superchargers producing 130" of manifold pressure (whereas the "war emergency" maximum was only about 60" for five minutes), and short open exhaust stacks. The resulting sound, the SOUND!!!!!!, of seven boats making their run for the starting line with seven of these technologically-obsolete engines at full-boost on the nitrous, was simply glorious! Entire cities where the races were run fell in love with that sound, which resounded for miles into surrounding neighborhoods.

Then somebody had to put one of your thoroughly advanced gas-turbine engines in his raceboat, and others followed. These first turbine boats took a while to start winning races, but the owners discovered that they needed a lot less labor from crews and crew chiefs, some of whom expected to be paid for their labor. Since the majority of owners were not racers themselves, but businessmen using racing to promote a product or service, they had little awareness of the aesthetics of the sport, and mostly cared about cutting overhead. So the Unlimited hydros entirely switched to turbine power.

And the result is that hardly anyone cares about that sport anymore. The boats are very fast, but the motors are very quiet; old-timers refer to them as "vacuum cleaner motors." I fell in love with the Unlimiteds as a boy in the '50s and '60s, but I haven't seen a race since the vacuum cleaners took over, and couldn't care less.

By contrast, USAC banned the turbines from Indy-car racing in the Sixties. Smartest thing they ever did.

Ambike, I think I speak for many here when I say that an essential reason for our deep interest in 2-stroke engines, as we have known them so far, is that in contrast with 4-strokes or Wankel rotarys, a 2-stroke is something an amateur mechanic can work on in his home shop. I can't grind a camshaft. But I can open up ports, roll and weld a new pipe, turn a new combustion chamber shape on my little lathe. I can get creative, carve a new 24/7 intake tract into a cylinder and maybe screw things up, and it won't cost terribly much to repair. Four-stroke owners are pretty much limited to bolting together a collection of store-bought parts. If 2-stroke technology ever "advances" to a configuration that is no longer home-mechanic-friendly, it will be a sad day for many of us.

Finally, an un-muffled 2-stroke SOUNDS like a real racing engine! I would not cross the street to watch a motor racing sport of any kind, anywhere, ever, that uses electric motors.

(If this interruption in the tech-talk is unwelcome, please delete it. Meanwhile, best wishes for the holidays.)

Touche', I would not even go to that street!

OK, you asked for it :

First of all, I didn't say anything about any four-stroke motors let alone hydroplanes.

But, that was a fine report on the old wet-bottomed gals. You know the deal !

My old man flew WW II aircraft. Make that before & during the war.

Stuff like P-38's & earlier, lesser wings.

I'll spare you guys his full background, except to say a fair amount of his knowledge, and many others' was conveyed. Isn't that common for all of us ? I'd say it is.

I also have all of his books / hard copies pertaining to those Army Air Corp days. ( wasn't called USAF then )

Yep, genuine Allison V-12, etc.

As for the boats, I've been to early races.

More than likely, if he'd NOT gotten a gut full of the hands-on involvement , something similarly " surplus " just might have been in my life. Sadly, that was not to be.

But, I raced dirt & pavement for 30 solid years. And the dirt was mainly on clay ovals ( the bigger, the better ) using everything from ' 69 AT1-M's to XR-750 Harleys. IOW, not much Muddo-Crossing and dirt-donking.

A pack of 12 - 18 big bore four-strokes lined-up is nothing compared to your hydros, but go do it and see what's it like. It's pretty intense, and if your motor is powerful, it'll burn rubber on the dirt.

I also have a great assortment of 650 cc - 750 cc dirt trackers. Brits, Japs, and about every H-D comp bike from WR 45 inchers & up. My ' 48 WR came from Floyd Emde.

Each and every bike was purchased by me, and then built, modified & massaged , and maintained with little outside help.

We have a full-on machine tool shop.

I grew up around metal working equipment, old-timers, & hard-nuts.

The grinding shop has Swiss machinery ( Kellenberger & others ) and I'm looking to upgrade with same.

We're talking a few hundred thou USD for that. Serious stuff for serious high-quality production.

I was truing junk-looking H-D & Jap-crap crank wheels on the Kellenbergers back in the 70's. Finished parts looked like something mfg'd today. Almost. ( in a highly controlled atmosphere, those machines can work to 40 millionths of an inch )

As for lathe & mill work....

Name your poison.

We have manual to full tilt CNC's capable of performing the finest work that you've seen posted here.

But, don't get me wrong. We build oil tools and machine tools, not engine parts.

OK, enough of that and let's get back to terrorizing with two-strokes.

Since I began riding two-cycles in 1968, I've had a few thousand motorcycles.

That's thousands, not hundreds.

Privateer work to independent shop owner to owning modest-sized dealerships.

I've raced against every common brand of two cycle known. Built & modified all of them myself.

I've seen hack work that shouldn't have lasted five minutes, and didn't.

That's why I quit racing.

The racing that interested me was with the earlier machines, and too many are now fatigued-junk and should have been retired, or trashed. Only takes one bad start, and then running into a blown-up shit crate in Turn 3, to break a neck. Sometimes there's no where to go but up & over a downed machine and/or rider. Obamacare / commie-care won't fix being paralyzed, so I had my fun & called it a day.

But you know, when it comes to looking for horsepower, it's hard to beat a well-tuned factory machine.

The transmission ratios are engineered to match the " stock " power band, and with a decent band, a rider can enjoy working the throttle more & the shifter less. And with a pinch more torque, the tough corners are easier to master, while the faster corner can be handled even better after dropping a tooth on the rear.

I'm going to make myself more popular by saying that KTM mini-bike with the extra plumbing is a waste.

Whatever is actually happening is unknown without BEFORE & AFTER data .

Just the way it IS.

If a two-popper suddenly needs three sizes larger on the low-speed jet, then the air speed thru the carb during that throttle position has been REDUCED. Otherwise, the fatter jet would not be needed. IF the air speed had accelerated, it's quite possible that a slight jetting reduction would tune-out. Ditto for the needle.

But hey, like I said, without empirical data PROVING a horsepower gain at such RPM, I'm going on general tendencies. Of course the other analysis would need to be known.

Backtracking to what I wrote...it's three options. One doable, the second maybe in a different solar system, and the third, just because it's true.

Concerning electric motors...damn...good point ! I didn't like racing the powerful SOUNDING Harley big-bores cause I got the Bar & Shield tattooed on my ass ! No sir, I dig the power and the sound. I don't have any tats, or nipple rings...a couple scars, but that's it. lol

And in the same connection, well Hell Yeah ! A sharp running two stroke is as good as any. The first Bul Pursang did it for me, and now into the 48th year of motorin' , it still does. Last summer, I snagged an OE Sherpa S. After the usual handi-work, its bark was as good as ever.

Thanks for your write-up. Enjoyable info, no apology needed, but let's go easy on the head mods / CR !

a general question to anybody who has some experience on the subject :
tuning a KR1S almost always ends with some day cylinders cracking arround the base where they are bolted on the crankcase.
they always crack first at the back, so people reinfocre the back like this :

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but then they possibly crack at the front :

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and at the front there is no space to weld something on.

I don't want to do any welding as it's expensive (entire cylindre will distored, needs to be trued and newly nicasiled. I want another solution that I can use cylinders as they are, so I came up with something like this :

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original conrods are 106mm, change them for 115mm rods from RD400, have a 9mm spacer under the cylinders, make brackets that bolt on top of the head with the cylinder head bolts and like that bolt them onto the spacer.

will a 9mm aluminium spacer be strong enough ?

[quote="ambike"]OK, you asked for it :

Touche, ambike! I knew a Texan wouldn't let it go! Happy new year!

Why only 9mm for the spacer ? Play it safe and use 12mm 7075. For the studs I recommend highly a nickel bronze material with a tensile strength of 800 N/mm2 and which is very important with a thermal epxansion rate of 16.8 x minus 10/degree C. Your cylinder expands at around 19-20 x minus10/degree C. By doing this you'll have absolute minimal distortion of your cylinder . With steel studs that expands with around 11 x minus 10/degree C your cylinder woul distort even at lets say 90 degrees C. I've got the nickel bronze in stock ad can also roll the threads for you if required. Send me a pm if you want. Tada

PS

Even better would be making studs of Dilavar. But pity enough hardly available.

Bob, what about stainless steel as a stud material

Will the studs be the same temp as the cylinders?Although they would be in contact with the head and crankcase it would be through the plate at the crankcase and whatever is used at the head the retain it the studs would not have a direct path of conduction and to a degree be air cooled.Or so it seems.

Need a new topic?

Other solution:you can use studs like Honda design in head/cylinder block assembly, the studs have a smaller diameter along their body, for exemple dia 6mm and extended to M8 at the ends for better élongation. You can calculate an élongation under different temperature loads between cylinder and studs materials.
A+ Dan

Bob van der Zijden a écrit:

Why only 9mm for the spacer ? Play it safe and use 12mm

because than I wouldn't have to change much more. 9mm spacer compensates 9mm longer rod. squish and everything else stays the same. I know timings will change a little, but it still is a roadbike, not meant to squeeze the last HP out of it.
I could make the spacer 9mm under the cylinders and the parts that stick out thicker, but don't think that will add much strenght ?

I'll pm you about the studs, but it won't be for a while. it's winter and it's coold in the garage , so it's mostly thinking/designing time ;-)

I guess I better explain a little about what my modification was for. In an ideal loop scavenge
engine the mixture exits from the transfer ports following the sides and angles of these ports and
loop around behind the burnt exhaust gases and push them out the exhaust port. But in the real
world it's not so cut and dry. In the real world when the transfers open some of the mixture
makes a sharp turn and heads straight out the exhaust port. This phenomenon is shortcircuting.
This part of the mixture is being robbed from the main stream and does not contribute to any work
that will be done. Also this shortcircuting continues the whole time the transfers are open and the
pressure is lower in the pipe than in the cylinder. But you say, the pipe is designed to return this
mixture back into the cylinder. NO! The pipe is designed to return the part of the mixture that did
do it's work but followed the burnt gases down the pipe and only in a certain rpm range. So,
actually, we have two parts of the mixture headed down the pipe. Not good for power and very
bad for emmisions. What I have done is substituted plain air to be shortcircuted instead of some of
the fuel mixture. You read above that I mounted the air inlet as close to the top and front of the
transfer port as I could so that this air would be the first into the chamber and the first to be
shortcircuted. This creates a very lean mixture in this area. Very good for emmissions and I'm
waisting very little of the fuel mixture. Now lets add two more factors in the process. The first is
TIME. Lets associate time with rpm. More rpm, less time. The second is VELOCITY. Lets associate
velocity with throttle opening. More throttle opening, more velocity. At full throttle and top rpm (rpm
the pipe was disigned for), everything is happening very fast. We have a large amount of mixture
moving through the ports at high velocity. The air ports I installed are still flowing but in contrast
with the total amount of mixture it's small. Therefore no rejetting of the main jet is necessary (or
very little). Now lets slow things down. Part throttle opening=less velocity, rpm's are lower=more
time for events to happen. Now the air ports are creating a leaner over-all mixture situation
requiring a jetting adjustment in the low end and lower mid range. This rejetting makes more power , under the curve, so to speak.
I know I've left out a lot of things but I hope you understand the total concept.

Very clear, now I've got it, thanks jfn2.

Simple test without comparative dyno or exhaust gas analysis :

Don't change even your socks, but plug the jet hole and see how it runs.

Then we can talk about bringing the mountain to Mohammed.

jfn2 a écrit:

I guess I better explain a little about what my modification was for. In an ideal loop scavenge
engine the mixture exits from the transfer ports following the sides and angles of these ports and
loop around behind the burnt exhaust gases and push them out the exhaust port. But in the real
world it's not so cut and dry. In the real world when the transfers open some of the mixture
makes a sharp turn and heads straight out the exhaust port. This phenomenon is shortcircuting.
This part of the mixture is being robbed from the main stream and does not contribute to any work
that will be done. Also this shortcircuting continues the whole time the transfers are open and the
pressure is lower in the pipe than in the cylinder. But you say, the pipe is designed to return this
mixture back into the cylinder. NO! The pipe is designed to return the part of the mixture that did
do it's work but followed the burnt gases down the pipe and only in a certain rpm range. So,
actually, we have two parts of the mixture headed down the pipe. Not good for power and very
bad for emmisions. What I have done is substituted plain air to be shortcircuted instead of some of
the fuel mixture. You read above that I mounted the air inlet as close to the top and front of the
transfer port as I could so that this air would be the first into the chamber and the first to be
shortcircuted. This creates a very lean mixture in this area. Very good for emmissions and I'm
waisting very little of the fuel mixture. Now lets add two more factors in the process. The first is
TIME. Lets associate time with rpm. More rpm, less time. The second is VELOCITY. Lets associate
velocity with throttle opening. More throttle opening, more velocity. At full throttle and top rpm (rpm
the pipe was disigned for), everything is happening very fast. We have a large amount of mixture
moving through the ports at high velocity. The air ports I installed are still flowing but in contrast
with the total amount of mixture it's small. Therefore no rejetting of the main jet is necessary (or
very little). Now lets slow things down. Part throttle opening=less velocity, rpm's are lower=more
time for events to happen. Now the air ports are creating a leaner over-all mixture situation
requiring a jetting adjustment in the low end and lower mid range.  This rejetting makes more power , under the curve, so to speak.                                                                
I know I've left out a lot of things but I hope you understand the total concept.

You ad air. And compensate it with more fuel.
It works.

Bonne annee et bonne sante, y'all!!