[GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 5)

To this I think you may add that the foul stroke is firing only once every 2 rotations, and has a lot more moving parts.

Charly a écrit:

Why two stroke have a so small engine braking compare to the four stroke ?

When a piston moves up, the pressure above it rises and creates a braking force, which can make it difficult to rotate the crankshaft. Try to kickstart or push-start a big single and you will know what I mean. The higher the compression ratio, the steeper the pressure rise and the braking force on the piston.
The compression of a two-stroke can only start when the piston has closed the exhaust port; therefore its effective compression ratio is usually lower than that of a four-stroke, and starting a two-stroke is usually easier than starting a four-stroke.

But engine braking is not the same thing as starting. Compression ratio and compression pressure in itself do not act as an engine brake.  Once you manage to rotate the crankshaft through its Top Dead Center position, the same pressure that was acting against the piston during its upward movement, pushes the piston down after TDC and generates a positive force. Apart from some friction and leakage past the piston rings, the nett effect is zero: no braking.

A four-stroke has more mechanical losses than a two-stroke. Try to turn a camshaft with a torque wrench and you'll be surprised how much torque you need to move the valves against their springs. The exception is the desmodromic Ducati system with very weak valve springs. Today Ducati has the most powerful and also the most fuel-efficient motorcycle engine.

The biggest factor in engine braking is pumping. As I said, the pressure above the piston, that generates a braking force from BDC to TDC, works in a positive way from TDC to BDC. But that is only true as long as the volume above the piston is closed. If you open a valve just before TDC, the pressure that was acting against the piston movement, can escape and there will be no positive force on the piston from TDC to BDC.

On the exhaust stroke a four-stroke pushes its cylinder contents out through the exhaust valve, and then the exhaust valve closes: no positive pressure to push the piston down after TDC. Instead, the inlet valve opens and the piston has to suck mixture in, which requires negative force on the piston. If the throttle is closed, the mixture has trouble coming in and the negative suction force will be even bigger. In this respect a diesel is more efficient because it has no throttle; its inlet tract is always fully open.

I once used the braking effect of engine pumping on an ice-speedway bike. The regulations do not allow any brakes at all on these machines, but I used a Honda push-pull throttle handle with the opening cable attached to the carburettor in the usual way, and the closing cable attached to an exhaust valve lifter. If the rider closed the throttle and then turned it still a bit further shut, the exhaust valve was slightly opened and the compression pressure could leak away. The engine braking with this system was so powerful that the rider was afraid to use it in front of competitors with hundreds of long sharp spikes, but with no brakes at all...

In a normal engine, engine braking is mainly caused by pumping losses. A four-stroke cylinder is a powerful pump; a two-stroke cylinder is not pumping at all because of the symmetric exhaust timing: if there is no combustion, the pressure on the upward piston stroke is equal to the pressure on the downward stroke, so the nett result is zero. And the crankcase of a two-stroke is a very poor pump with a compression ratio of less than 2.

Thank you very much for this very interesting explanation, Frits, I would not have thought about the pumping losses.

Explanation clear and complete, thank you very much Frits !

Frits Overmars a écrit:

....the exhaust valve was slightly opened and the compression pressure could leak away. The engine braking with this system was so powerful....

I knew this system used on trucks engines, but not on ice-speedway bike ! Great idea.

Thanks, Frits. Now, we are on the same page! I should have referenced the Aprilia design earlier. So the outlet of the duct is the same area and basic shape as the exhaust ports at the bore? The inlet of the flange is the same shape as the outlet of the duct, and transitions to a circle of the same area over the length of the flange?

From reading the whole thread, the port floor was raised 3mm above BDC at Aprilia. Have you or anyone else tried raising the floor to the effective transfer opening? It seems to me that the idea is similar to the "Wobbly" exhaust duct but it is easier to weld and then machine, a flat floor. Then after a baseline run on the dyno, the floor can be lowered 1mm at a time and the outlet of the flange opened up to the new area.

polcat88 a écrit:

From reading the whole thread, the port floor was raised 3mm above BDC at Aprilia. Have you or anyone else tried raising the floor to the effective transfer opening? It seems to me that the idea is similar to the "Wobbly" exhaust duct but it is easier to weld and then machine, a flat floor. Then after a baseline run on the dyno, the floor can be lowered 1mm at a time and the outlet of the flange opened up to the new area.

That's right Steve. Now why do I have this deja-vu impression about a certain Polcat88 on another forum?
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As a matter of fact I did raise the exhaust floor, even to 3 mm above the effective transfer opening. Normally this would not have left sufficient exhaust port area, but I put a second exhaust port opposite the first port, which gave me plenty port area, even with a mild 190° exhaust timing:
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That's me, Frits! Before starting on the exhaust duct modification and testing, I am weighing all my options and learning as much as I can.

The exhaust duct 'floor' was raised by filing the sand core, in 1995, never by welding!
The inner height was then determined by the 'trial and error' method, never calculated.
Even now I don't know the port, inner and exhaust flange surfaces...…
I only looked for what gave best power!
Later the port underside was raised 3mm, 1mm at a time, which gave more power.
Some cylinders were cast with the floor as high as the transfer opening, in 2007.
But they were never tried, and CNC machined to normal size, because I retired...

Thanks, Jan. I understand your cylinders raised floor was cast. I will have to weld my cylinder to raise the floor. I plan to raise the floor to the effective transfer opening, then machine down 1mm at a time, testing each change.  I am glad that you got to retire and enjoy life. And sad that your development work did not continue.

Take care to have good blowdown through your auxiliary ports!!!

I would like to ask a question regarding crankshaft ball bearings,
basically any info would be welcome.
Specifically yamaha uses in one motor 6305 ball bearings
i have used KOYO of the same dimensions (but bought of the shelf)and the crankshaft rocks like
a baby's cradle within a few hundred kilometers.
The original ones seem to last forever and the crankshaft is very tight-no up and down play once installed.
If I was to make a conversion to a crankcase and use
different size bearings witch could only be found in industrial sizes (or even  needle roller bearings)
what should I look out for?
should they be c3 or c4?or are they to "loose"?
I'm not looking for something highly specialized but of the shelf products,suitable for a 2stroke build/conversion.
Any advise is welcome.

Hi Panas,
I think your question is not precise enough to be answered by anyone.

"If I was to make a conversion to a crankcase": please explain what you want to do.

My own experience: if the original ones work well and last forever, use them for any replacement.
C3 or C4? It depends on how precisely your engine cases are machined. If it is not so concentric, you may use C4, so you are not wearing your bearings too quickly because of geometric defects.

carlovitch1 a écrit:

Hi Panas,
I think your question is not precise enough to be answered by anyone.

"If I was to make a conversion to a crankcase": please explain what you want to do.

My own experience: if the original ones work well and last forever, use them for any replacement.
C3 or C4? It depends on how precisely your engine cases are machined. If it is not so concentric, you may use C4, so you are not wearing your bearings too quickly because of geometric defects.  

hello carlovitch,
lets say for some reason i want to replace the original size ball bearings with another bearing witch has
a smaller outer diameter,(hence convert the crankcase)that bearing cannot be found as a original part but must be sourced from the aftermarket lets assume the machining is "perfect".
What would the appropriate  bearing type  be?
I'm digging in here but cant seem to find any  info on crank bearings or the  rsa/w bearings.
please re-direct me if you are aware of any  the part and page.

RSA used C5 roller bearings

Jan Thiel a écrit:

RSA used C5 roller bearings

Thanks once again Jan!
I guess the C5 is for thermal reasons in this case.

Possibly the quality of the bearings were
not up to the task and not the specs.

I've only seen these roller bearings  in four strokes and needle roller bearings in old "utility"2 stroke engines.
Since i m not going to repair any race spec engine,a good quality ball (SKF,? ?) C3 to C5 wil do?
edit.   25mm inner diameter of that skf...
....outer 52mm....

Happy birthday!!
Thank you for sharing your knowledge and giving inspiration.
ps. I would pay ''top dollar''for a book on 2 stroke development and/or biography,just saying...

Happy Birthday, Jan Thiel!

Happy birthday Jan !

Jan,would you describe your dream 2 stroke motor or what
the next generation 2 stroke race bike would be like for you?
We already know somethings...
double rotary,
rear facing exhaust,
belt primary drive,
ceramic exhaust coating on the inside,
an architecture would NOT be like the rumi/maxtra...
anything else?
lets say you were given unlimited resources what would have tried?
even bigger crankcase volume?
if im not bothering you to much...

It would be 50cc of course as I always loved 50cc engines the most!
The most difficult choice would be between double rotary or RSA-type inlet.
With unlimited recources I would try both!
Gear primary drive, and a balance shaft
I would choose a belt primary drive if I had to make a kart engine.
4 crankshaft bearings.
The RSA-type with a well-guided flow from inlet to transfers.
5 transfers, and a triple port exhaust of course.
It would be most interesting to see if a 50cc can reach the same level as a 125!

Jan Thiel a écrit:

It would be 50cc of course as I always loved 50cc engines the most!
The most difficult choice would be between double rotary or RSA-type inlet.
With unlimited recources I would try both!
Gear primary drive.
I would choose a belt primary drive if I had to make a kart engine.
4 crankshaft bearings.
The RSA-type with a well-guided flow from inlet to transfers.
5 transfers, and a triple port exhaust of course.
It would be most interesting to see if a 50cc can reach the same level as a 125!

Why 50?first love i guess...  
50cc power level of a 125?!!!!
4 crankshaft bearings?!
3 I get it -one after the  primary gears but 4 ?or you mean stacked next to each other?
So the objective would be to "saturate" the transfers with mixture asap,"ignoring"
the center crankcase.
edit.
almost horizontal configuration?

I suppose 4 bearrings to stop the crank from bending, being able to run even tighter squish (or in an ideal world "no" squish).