ESE's works engine tuner

[TZ350]

Just a scrape from the net so take it with a pinch of salt ........... interesting though.

The carburation of a disk-valve engine is less sensitive to inlet resonances than that of a reed-valver. And it is far less sensitive to exhaust resonances. I have seen that on the Cagiva 500-4. First it had piston induction, then it got disk valves, and finally reed valves. The disk-valve engine was fairly rideable but its successor was not a pleasant bike, especially in the wet.
On a reed-valve engine the reeds are opened by the suction from the exhaust pipe. When you close the throttle, the energy flow to the exhaust stops, and thus the suction stops.
When you open the throttle again, nothing happens because the exhaust suction is absent; the reeds stay closed and there is nothing in the cylinder to combust. So the rider opens the throttle a bit more, and a bit more, and a bit... and then there is enough mixture in the cylinder to start combustion again. But now the throttle is wide open: Whamm, high-sider!
In a disk-valve engine the disk always opens, combustion or no combustion. So, contrary to popular belief, it is much easier to ride than a reed-valver.
(The Cagiva was cured by inserting 1 mm strips between the reed casing and the reeds, so the reeds did not really close anymore. It cost more than 10 hp, but the laptimes improved...
Because of those 'leaking' reeds setting the carburation was a nightmare. So Cagiva concentrated on injection, and got fairly good results.)

Frits Overmars.

[kel]

I think that is the real problem with the side rotary valve - the last pic shows the "ultimate" development of the side port, whereas if you look at the RSA it has an almost identical shape,but turned 90*.
This in effect gives a much narrower port width - the narrower the better, as this means that the blade spends less time actually covering it up ( assuming the same open/close timing), but they have got the area back by making the port very high,with a big valve diameter.

If Ive got this right the original image I posted is the latest RSA

Heres the earlier incarnation

shows they are always learning/developing

[TZ350]

Ok some more interesting stuff................

You must be able to rely on the ignition, even when the electrode gap has gotten on the big side, when the mixture is so weak, rich or diluted that you can hardly ignite it, or when the compression end pressure is high because of good cylinder filling or a high compression ratio.
All parts of the ignition system are important: the spark plug of course, but also the plug lead (never use carbon core leads), the plug cap (never use resistor caps; if there has to be a resistor in the high tension line, it sould be inside the plug, not in the cap, because caps fail), the coil (with Jamathi, Bultaco and Garelli the fat, blue, oil-filled Bosch coils were used despite the fact that they were almost as heavy as the engine. Replacing them with slim Japanese coils cost several hp...)

Let's take a look at the spark plug. What exactly happens there when the coil sends its high tension voltage? The gas molecules between the electrodes are ionized by the high potential. Starting at one of the electrodes, the ions begin to form a conductive channel through the otherwise non-conductive gas and as soon as this channel reaches the other electrode, a connection is established and a current starts to flow: the spark.
The ionisation needs a certain voltage per mm electrode gap, dependent on temperature and pressure of the gas. The squish turbulence can blow the ions away before they have had a chance to form a channel from one electrode to the other. Then, when the channel is finally completed, it is no longer straight but arched, also longer. Because of the required voltage per mm you then need a higher ignition tension.
Some time ago Honda RS125s would not run with more than 0,4 mm electrode gap, whereas Aprilia used 0,7 mm gap. Then Honda offered a kit with a different (better!) ignition system...

Talking about better ignition systems: our fellow forum member Emotracing offers something nice:
an ignition extension. The normal ignition takes care of the ionisation channel; then as soon as the current starts to flow, the Emot-set sends a big extra current through this channel. It looks like this: http://www.youtube.com/watch?v=7BfQuxyWN90

And if you really want to know all about sparks, look here:
http://www.madsci.org/posts/archives...3778.Ph.r.html

Frits Overmars

[TZ350]

Clearly and unashamably mining the net for interesting ideas................

There is one other possible objection against effective squish. When a gas comes in contact with a solid surface, it tries to cling onto it: it forms a boundary layer. This layer of stationary gas is a fairly good heat insulator. The piston crown enjoys this insulation, as the boundary layer protects it from the combustion heat. In theory the squish flow can blow the boundary layer away, exposing the piston directly to the combustion.
But there is always a far greater danger: detonation. The shock waves in a detonating engine are a thousandfold more effective in shaking the boundary layer loose. So I prefer a good squish that speeds up combustion.

If detonation occurs, it happens in the end phase of combustion, when the pressure in the combustion chamber is high and the temperature of the unburnt mixture in the edges of the chamber is raised by the heat radiation of the burning mixture.
Effective squish flings burning parts of the mixture to the edges of the combustion chamber, igniting all mixture before detonation factors there can reach a dangerous level.

I do believe in these
micro-eddies at the inner edge of the squish band though; it is mr. Coanda making himself noticed.

At Aprilia Jan Thiel performed a series of tests with different radii at this edge. The outcome was crystal-clear: R=0 was best and the bigger the radius, the worse the result. It is all the more surprising that Honda has always stuck to a radius (sometimes as large as 5 mm).

Finally a simple advise for junior tuners: make the squish gap so tight that there is no more room for mixture; if it is not there, it cannot detonate.

Frits Overmars

Squish gap so tight that there is no more room for mixture; if it is not there, it cannot detonate, now where have we seen that before on this thread....... :laugh:





Piston just kisses the copper squish band.

[Buckets4Me]

Squish gap so tight that there is no more room for mixture; if it is not there, it cannot detonate, now where have we seen that before on this thread..........

about the same place you should have seen somthing about not letting water get sprayed up off the back tyre and into the carb

[TZ350]

Ok now we have a stepper motor controled power jet........

The 'high compression brickwall' is no longer as hard as it used to be. With leaded fuel the compression ratio was 19,5; nowadays it is about 15. The 'brickwall' arose from the high expansion ratio (which is identical to the compression ratio). The higher this ratio, the more the burnt gases in the cylinder cool down before they enter the exhaust, thus lowering the resonance frequency of the exhaust system.
Riders complained that the engine would not rev, especially not in the lower gears, where the revs rose so quickly that the rising wall temperature of the exhaust pipes could not keep up with them. So you had cold exhaust gas and cold pipes.

The problem was to a large extent solved when the solenoid-controlled on/off-power jets were replaced with stepper motor-controlled jets that could continuously adjust mixture strenght, allowing the engine to rev more freely.



This is it. The stepper motor itself originates from a Fiat Uno where it regulates the idling rpm

Regarding the Aprilia RSW/RSA125 single, Jan Thiel told me that in seven years of testing the racing department had not been able to establish which was best: low or high inertia. I would choose low.....

Frits Overmars

[TZ350]

about the same place you should have seen somthing about not letting water get sprayed up off the back tyre and into the carb

Yep, at Taupo in the rain, hydrauliced the piston, jammed the ring and ruined Av's ride ..........

[speedpro]

squish velocity needs to be taken into account as well as there is an optimum amount of turbulence required in the combustion chamber to control flame propogation rate etc.
Interesting comment about the radius from the squish area into the bowl. The original MB100 chamber has a quite pronounced edge and I've had good results on the dyno by simply making it a curve with about a 3mm radius. As much as possible all other measureable variables stayed the same.

[TZ350]

Someone on this thread was talking about plugs in the piston pin instead of clips, well here is Frits doing exactly that........

Avoiding short-circuiting between transfer and exhaust ports is the most obvious goal of the plugs. But it is not the only one. Do you remember Honda's TRAC system where at low revs a resonator volume was coupled to the exhaust duct to lower the exhaust system's Helmholtz frequency (Suzuki and Kawasaki used similar systems)?
The volume inside a piston pin does more or less the same: during every revolution it is momentarily connected to the auxiliary exhaust ducts and it dampens the system's resonance, even if you don't want it to, i.e. when the engine is running in the power band.

Below left you see the latest version of the original Aprilia piston pin with domed end caps at 136 euro a piece, and at the right my re-usable click-on plugs (that also make the usual steel-wire clips superfluous) at a fraction of the price

[wobbly]

The two pics of the RSA shows where and why the later version was "better".The RV port has been turn 90* and is very high but narrow, thus reducing the time the blade covers the port ( increasing the STA with the same timings).

Frits isnt quite right re Honda and the squish corner radius - the A kit engines had a sharp corner for ages - here is a pic of the chamber CNC shape I did 4 years ago for a RGV100

Re the piston pin plugs, having cutouts above the pin in the skirt is just as bad, as these also can connect the Aux Ex to the transfers.

And re crank inertia, it has to be taken in context - all the top engines run very high inertia ( but low mass),a RS125 is a pig with NO overev if you take off the flywheel and run total loss - that is why the factory and VHM had "high inertia" options.When Frits refers to "low" this still means very high in relation to a "normal" bike crank.

[TZ350]

Thanks Wob..........

[F5 Dave]

Jan Thiel has retired and the two stroke era is over so the designers dont mind shareing info, all kinds of really good info!
KTM injection well thats a strange one even though their GP two stroke program is long gone that still dont talk or release photos (press releases aside). But yes I do know how it works, supplementary injection that feds into the crankcase to richen the mixture, just another approach to a power jet.

Yeah I had heard a number of years ago of some crazy arrangement where people were injecting into the air scoops. It sounded queer & well kind of a fire hazzard & I didn't see why you would do it. But (unless it was total bolocks) maybe they were attempting a similar anti seize measure & hoping to get some atomisation of the fuel/oil in the airstream before it went under the closed slide. I'd forgotten about it till mentioned here.

[gatch]

And re crank inertia, it has to be taken in context - all the top engines run very high inertia ( but low mass),a RS125 is a pig with NO overev if you take off the flywheel and run total loss - that is why the factory and VHM had "high inertia" options.When Frits refers to "low" this still means very high in relation to a "normal" bike crank.

What type of steel do they use for cranks ? And what for the balancing weights ?

[jasonu]

What type of steel do they use for cranks ? And what for the balancing weights ?

1. Crank steel
2. Balance weight steel

Sorry, had a couple and couldn't resist...

[gatch]

1. Crank steel
2. Balance weight steel

Sorry, had a couple and couldn't resist...

It might not look like I'm laughing, but I am, really..