ESE's works engine tuner

[Frits Overmars]

The trend has been for a long time to reduce the volume of all the ducts in modern engines. Aprilias Ex duct had the bottom filled in such that it was higher than BDC and the corners filled to reduce short circuiting from the A ports. And the cylinder duct vol was CNC machined to be smaller and smaller in total vol,but in the process heavily promoting flow from the Aux ports to increase effective blowdown flow.

True, true, Wob. Except, how do you machine a duct smaller ?

... for sure the ratio between the A and B port duct entry areas was tested to death within the limitation of the case available area between the studs

True again.

The idea here is to reduce the inertia of the initial volume available to the cylinder,that has to be accelerated out of the duct by the negative pressure ratio across the port.

Here our opinions seem to differ. The mass of a gas column in a duct equals specific mass * duct length * duct cross area (let's assume for now that the area is constant over the duct's length). You want to get that mass into the cylinder in a given amount of time and that requires a certain flow velocity. This velocity is inversely proportional to the cross area. And the required pressure difference is proportional to the square of the velocity. So, though a longer duct will slow down the mass transfer, a larger cross area will help.

Where the charge goes, is another matter. It would be quite easy to enhance transfer flow by aiming the leading edges of the A-ports more towards the exhaust port, but that would cause massive short-circuiting. When looking at the inner duct curves of TZ350's cylinder, that thought crossed my mind...

[TZ350]

So, though a longer duct will slow down the mass transfer, a larger cross area will help. Where the charge goes, is another matter. It would be quite easy to enhance transfer flow by aiming the leading edges of the A-ports more towards the exhaust port, but that would cause massive short-circuiting.



When looking at the inner duct curves of TZ350's cylinder, that thought crossed my mind...

Yes, actually hadn't realy noticed that before.



The idea was to have a better curve with a bigger radius from inside the cylinder to the port window with a 1.3 entry area ratio. The area around the port window itself does not have any glue near it, so I just assumed that the port would still discharge in the same direction as before, but maybe not.

The single port cylinder on the left made 31hp so I am tempted to remove most of the glue from the transfers or at least heavily re shape it.

[wobbly]

I am sure that changing the Exhaust duct core size in the casting was an easy task at the factory,so that subsequently the nicely finished CNC machined area was able to be made progressively smaller and smaller
to good effect.

The second point is explained by your small add on comment - "mostly" the ducts are not constant cross section,the CSA gets progressively larger toward the entry point.
My small observation was that the trend has been to reduce that area at the entry to the duct, thus reducing the total volume, and thus the inertia of the volume total within that duct.
As always im gun shy of arguing with you Frits as you are easily capable of helping me shoot myself in the foot, even if I cant feel it.

[Haufen]

So the reduction in cross sectional area at the bottom of the RSA exhaust port works like an integrated oval to round transition?

Do you remember the percentage of the reduction in cross sectional area, Frits?

[speedpro]

I wonder if "inertia" is the right word to use when referring to the fluid in the transfer ports. For the same density the inertia of a fluid will be constant. With a larger volume port there will be a larger volume of fluid which if it was all to be accelerated at the same rate as the smaller volume would require more energy due to the greater mass being accelerated. However with a larger volume would all the fluid be required to accelerate at the same rate as the smaller volume? If the port window is unchanged at any moment in comparison to the larger and smaller volume ports there should be a similar amount of energy available to accellerate the fluid in the port as it is the differential in pressure at the port window that determines the energy available to accelerate the fluid.

For any given duct volume or cross-sectional area the energy level will be similar. Whether that results in a lot of fluid moving slowly or a smaller amount moving at a higher velocity will be dependant on the port volume or cross-section. Given the constantly varying everything there won't be one perfect configuration for "every" scenario. There will surely be an optimum configuration for a particular operating condition and desired outcome. A smaller port would result in a higher velocity which could be useful as the port closes to jam a bit more into the cylinder, due to inertia.

sits back and waits to be shot down

[Frits Overmars]

...As always im gun shy of arguing with you Frits as you are easily capable of helping me shoot myself in the foot, even if I cant feel it.

I'm glad to have you as a sparring partner, Wob. It forces me to think a bit harder before hitting my keyboard. And feel free to aim at my feet anytime .

So the reduction in cross sectional area at the bottom of the RSA exhaust port works like an integrated oval to round transition?

Verstehe nur Bahnhof, Haufen.

Do you remember the percentage of the reduction in cross sectional area, Frits?

I am still in a place far, far away (Dresden) and I have no access to my home archive; I only remember that the entry area of the B-ports is a trifle under their exit area. That is not something you should aim at (on the contrary); it evolved historically because Aprilia wished to keep the cylinder stud positions in their original place, so new cylinders would fit old engine cases. The RSA125 should have been free from that restriction because it was designed at Derbi, not at Aprilia. But in the development phase it was decided to fit an existing well-proven Aprilia cylinder, so the funny entry/exit relation remained.

I wonder if "inertia" is the right word to use when referring to the fluid in the transfer ports. For the same density the inertia of a fluid will be constant. With a larger volume port there will be a larger volume of fluid which if it was all to be accelerated at the same rate as the smaller volume would require more energy due to the greater mass being accelerated. However with a larger volume would all the fluid be required to accelerate at the same rate as the smaller volume? If the port window is unchanged at any moment in comparison to the larger and smaller volume ports there should be a similar amount of energy available to accellerate the fluid in the port as it is the differential in pressure at the port window that determines the energy available to accelerate the fluid.

For any given duct volume or cross-sectional area the energy level will be similar. Whether that results in a lot of fluid moving slowly or a smaller amount moving at a higher velocity will be dependant on the port volume or cross-section. Given the constantly varying everything there won't be one perfect configuration for "every" scenario. There will surely be an optimum configuration for a particular operating condition and desired outcome. A smaller port would result in a higher velocity which could be useful as the port closes to jam a bit more into the cylinder, due to inertia.
sits back and waits to be shot down

I'm not aiming at your heart, Speedpro, only at your foot (learned that from Wobbly). It makes a difference whether all of the fresh charge has to come from the crankcase, or whether it is already waiting just outside the transfer windows, as is the case with large-volume ducts. These ducs themselves can then be refilled at leisure between transfer phases.

I will counter your final remark "A smaller port would result in a higher velocity which could be useful as the port closes to jam a bit more into the cylinder, due to inertia" with a quote of your own: "it is the differential in pressure at the port window that determines the energy available to accelerate the fluid".
The flow velocity is pressure-dependent, not port-area dependent (otherwise a closed port would give an infinite velocity). This velocity will only rise somewhat higher in a narrow duct because the smaller mass transfer through it will result in a slower pressure drop, therefore in a prolonged time during which the mass is accelerated.
Using the jamming effect, or RAM effect as the Mericans call it, is just an attempt to correct in the end phase of transfer (or intake) what went wrong at the beginning.

[crazy man]

l feel the love in here

[Frits Overmars]

l feel the love in here

I hope Speedpro does too. It is never my intention to cut someone down to size; that would not make me look any taller.

[TZ350]

Having managed 29hp with the taper bored 24mm OKO I wanted to see what difference a bigger carb might make. The taper bored OKO is 31mm at the engine flange so I bored another OKO out to 30mm at the slide and 31 at the flange.

Set the 30mm carb up with a main jet one size bigger than the 24 and ran it up. It started and warmed up ok but when I tried to move up through the gears it wouldn’t accelerate.

Anything over half throttle and it just sat there going Blarrrr, sounded rich but I was caught out thinking that the other night. If it was lean I would have expected it to just cut out, it kept going but no matter what I did with the throttle it just would not pickup, sounded flat and rich.

The taper bored 24 had a 145 main, the 30 a 150 so it’s hard to imagine the 30 needing a main smaller than the 24. ???? ran out of time to look into it. I didn't think of it at the time but I should have pulled the power jet feed line off. Killing the power jet would have quickly told me if it was rich or lean.

Kel and I now think the flat sound to the exhaust is due to the top exhaust port chamfer blunting the exhaust note as the port is no longer cracking open suddenly, is that a good thing ???.

[speedpro]

I will counter your final remark "A smaller port would result in a higher velocity which could be useful as the port closes to jam a bit more into the cylinder, due to inertia" with a quote of your own: "it is the differential in pressure at the port window that determines the energy available to accelerate the fluid".

As the time the port closes though the fluid has already been accelerated and is now being decellerated by a port window that is reducing in size and thus restricting potential flow. That energy in the moving mass doesn't disappear but could in certain situations result in a pressure differential rise at the port window resulting in flow continuing.

I see your point about the duct holding the volume ready to flow through the port, a sort of mini plenum. However I also envisage an optimum size duct providing the volume of fluid to move through the port with little restriction at the start of the transfer while also providing a column of fluid which is moving with some velocity to facilitate flow towards the end of the transfer phase.

I don't have the knowledge though to determine if a large slow moving volume would be better in that situation or a smaller faster moving volume.

I compare it to velocity stacks on a dirty 4-stroke engine though I think it's a bit easier to visualise on a 4-stroke. Once that air is moving into the velocity stack it wants to keep moving even though the valve is closing or the piston has started up on the compression stroke. As in our lovely 2-stroke engines transfer ports, there is an optimum length and diameter(volume) for the application and tune of the engine.

[Frits Overmars]

...Kel and I now think the flat sound to the exhaust is due to the top exhaust port chamfer blunting the exhaust note as the port is no longer cracking open suddenly, is that a good thing ???.

If you only added a chamfer to the existing port, outflow will improve, waisting less energy in turbulence and sending more energy the pipe. It will also have raised the exhaust timing, opening the port when cylinder pressure is higher; another reason for the pipe to perform better.
I have met people who claimed they could tell just by listening whether an engine had auxiliary exhaust ports or a T-port. All I can hear is whether the silencer is still silencing or needs to be repacked.

Funny sounds can turn up if the ignition is borderline. Improving the cylinder filling can be enough to prevent the spark from sparking at the right time every time.
A simple test is to close the plug gap to 0.2 to 0.3 mm. If that improves things, you need a stronger ignition.

[RAW]

I am still in a place far, far away (Dresden) and I have no access to my home archive; I only remember that the entry area of the B-ports is a trifle under their exit area. That is not something you should aim at (on the contrary); it evolved historically because Aprilia wished to keep the cylinder stud positions in their original place, so new cylinders would fit old engine cases. The RSA125 should have been free from that restriction because it was designed at Derbi, not at Aprilia. But in the development phase it was decided to fit an existing well-proven Aprilia cylinder, so the funny entry/exit relation remained.


Frits I can't quite see what your meaning here, do you have any photos of the Derbi cylinder or something along these lines we could all view

Cheers

[Frits Overmars]

As the time the port closes though the fluid has already been accelerated and is now being decellerated by a port window that is reducing in size and thus restricting potential flow. That energy in the moving mass doesn't disappear but could in certain situations result in a pressure differential rise at the port window resulting in flow continuing.

True. But that energy first has to be put into that mass. The longer the gas column and the higher it has to speed up, the slower will be the initial cylinder (or crankcase) filling. That was what I meant by 'correcting at the end what went wrong at the beginning'.
Utilizing ram effects requires long timings, and vice versa. But the shorter you can keep your timings, the more user-friendly the engine will be (exhaust timing partly excluded because you need a certain minimum timing for resonance).

[Frits Overmars]

Frits I can't quite see what your meaning here, do you have any photos of the Derbi cylinder or something along these lines we could all view

There is no Derbi cylinder, RAW. That is, before Jan Thiel moved to Derbi to build the RSA engine, there had been Derbi reed valve racing engines, but their cylinders (and pipes) were exact Honda copies. The cylinder on the RSA has been an Aprilia pot from the beginning. I posted several zip-files here on the forum with lots of Aprilia engine details. You'll just have to do some searching.

[Neal]

[/QUOTE]


Hi Haufen ,

where did you get this picture from ? Are you casting your own barrel ?