ESE's works engine tuner

[wobbly]

The red open/blue closed curves has reminded me about your question regarding pressure in the chamber TeeZee.
A residual pressure in the chamber is, part of the dynamic supercharge effect of the wave action,increasing its bandwidth - as from PV=nrT when the pressure increases, so does the
gas density and the temp.
This dynamically varies the wave speed with rpm.

This effect is used in reverse with big champ jetski engines.
They use a big stinger, to DECREASE the pressure bleed, and thus the effective pipe wave action, at lower rpm, then squirt water into the stinger to squeeze its area down at high rpm.
Essentially returning the chamber to "normal " conditions as the rpm increases.

As you have found - decreasing the pipes negative effect on the scavenging at lower rpm ( because its too short ) is achieved by reducing the residual pressure in the pipe, thus reducing the temp/density and wave speed - seems counter intuitive to many who think you would need a smaller stinger to make the pipe work " better ".

There may be some truth to the other comments I have heard about pipe back pressure reducing the loop speed and thus short circuiting into the header, but I prefer the physics equation - seems more elegant.

[TZ350]

Really interesting development Rob (should be easy to control the bleed with the ignitech).

I am not sure the bleed did much for controlling the deto but it does suggest that an ATAC valve could be very worthwhile.

is the copper sheet being lifted under compression (the photo eludes to it being out of shape); is there a possible limited life to the use of a thin copper sheet before it anneals (sp) itself to a point that it deforms under high temperature/pressure? again generating areas of heat pockets...

The copper forms the squish band, in the past its been very reliable and a good friend. But looking at the heat discoloration on the upper edge it looks like you may be right, the copper could have buckled and become a heat source.

But why is the deto only in the over rev zone and not peak torqe.

[TZ350]

The red open/blue closed curves has reminded me about your question regarding pressure in the chamber TeeZee.

Thanks Wob.

[Flettner]

Frits, The cooling of the exhaust piston in my uniflow engine was the most difficult problem. You can appciate that I wanted full cylinder exhaust ports all the way round the cylinder. In a standard 2T engine the piston at least gets cooling from the transfers around the rest of the piston away from the exhaust port, no such luxury on the uniflow. Under piston cooling is significant and is what is saving the uniflow piston. Because of the way the crankcases are both pumping into the same cylinder I was able to make one inlet ( under the transfer piston ) just induct air, the other intake ( under the exhaust piston ) induct air and twice the fuel, very rich mixture. It all mixed in the same cylinder anyway. This extra rich mixture under the exhaust piston made for a much safer engine. Also the exhaust piston is smaller than the inlet piston and flat top. This allowed for some squish in the combustion chamber. It would be nice to try also a different material for this exhaust piston, perhaps carbon / carbon.

[Haufen]

There may be some truth to the other comments I have heard about pipe back pressure reducing the loop speed and thus short circuiting into the header, but I prefer the physics equation - seems more elegant.

I don't know if the loop speed is reduced, but I know that a higher exhaust back pressure can influence the timing when the mass flow in the open transfers actually starts moving out of the ports.

Another effect of a higher back pressure is that the scavenging column has a higher resistance to overcome to short circuit out of the exhaust port.

In combination with exhaust pipes, higher back pressure reduces the suction on the transfers / crankcase, but increases the amplitude of the returning stuffing pulse.

[speedpro]

But why is the deto only in the over rev zone and not peak torqe.

Pretty sure there's been a few good theories about that from people who should know. My favourite is something along the line that at peak torque everything is being very efficient - exhaust out, fresh mixture in, squish, bang, repeat. There is minimal mixing of burnt gas and fresh mixture. In over rev, there isn't enough time for the burnt gas to exit and when the transfers open the pressure differential causes a momentary flow of hot burnt gases into the transfer ducts. The transfer ducts now have a combination of hot exhaust gas and fresh mixture in them which is then transferred into the cylinder. This mixture apart from being warmer than a nice clean fresh fuel/air mix also has a different chemical composition. In a methanol fueled engine this diluted fuel/air mixture has a significantly lower octane rating, so I've been told. I assume it's similar for petrol. Anyway, it starts out warmer and then gets compressed as usual and as a result of Boyles law gets to the point where it auto-ignites before the spark. This ignition event is early, advanced, which in conjunction with the lower octane rating of the diluted mixture also could result in detonation. No happy happy joy joy there I'm afraid.

That's my take on it.

A design that would help with this I would have thought would be a bridged ex port, or triple port, with lots of blowdown T/A and low and wide transfers. The low wide transfers will give the required T/A and the greatest blowdown T/A, for any exhaust T/A, which will reduce the mixing of exhaust and fresh mixture. This might be why proper race cylinders didn't seem to have such an issue with over rev deto. Riding styles and tracks contributed as well I think. Little power valves in the transfers might help by increasing blowdown T/A but would of course lower transfer T/A which may not be too much of a problem. They might be useful in the situation where revs are up and at smaller throttle openings where deto is also a problem.

[husaberg]

Pretty sure there's been a few good theories about that from people who should know. My favourite is something along the line that at peak torque everything is being very efficient - exhaust out, fresh mixture in, squish, bang, repeat. There is minimal mixing of burnt gas and fresh mixture. In over rev, there isn't enough time for the burnt gas to exit and when the transfers open the pressure differential causes a momentary flow of hot burnt gases into the transfer ducts. The transfer ducts now have a combination of hot exhaust gas and fresh mixture in them which is then transferred into the cylinder. This mixture apart from being warmer than a nice clean fresh fuel/air mix also has a different chemical composition. In a methanol fueled engine this diluted fuel/air mixture has a significantly lower octane rating, so I've been told. I assume it's similar for petrol. Anyway, it starts out warmer and then gets compressed as usual and as a result of Boyles law gets to the point where it auto-ignites before the spark. This ignition event is early, advanced, which in conjunction with the lower octane rating of the diluted mixture also could result in detonation. No happy happy joy joy there I'm afraid.

That's my take on it.

A design that would help with this I would have thought would be a bridged ex port, or triple port, with lots of blowdown T/A and low and wide transfers. The low wide transfers will give the required T/A and the greatest blowdown T/A, for any exhaust T/A, which will reduce the mixing of exhaust and fresh mixture. This might be why proper race cylinders didn't seem to have such an issue with over rev deto. Riding styles and tracks contributed as well I think. Little power valves in the transfers might help by increasing blowdown T/A but would of course lower transfer T/A which may not be too much of a problem. They might be useful in the situation where revs are up and at smaller throttle openings where deto is also a problem.

Just an observation

The devil is in the detail as it were.Shape EX Aux EX and A ports under AUX


I also have a suspicion the shape of your septums may be a little out of kilter ( referring to the to the direction)front and rear wall of the trensfers too.
but can't be arsed looking for the pics.(ok i could)
Ignore the pointer not what i am talking about

Note this detail is impossible to normally see in most pics. I will call it slant for lack of a better term.


Frits also alluded to lowering the comp for more over rev perhap's he or Wob could be kind enough to expand on it

[Frits Overmars]

Frits alluded to lowering the comp for more over rev perhap's he or Wob could be kind enough to expand on it

Judging from the way you wrote that word expand in italic, you have a pretty good idea yourself, Husa. A lower compression ratio before TDC equals a lower expansion ratio after TDC. That means a smaller temperature drop in the burnt gas during expansion, hence a higher exhaust gas temperature, a higher speed of sound in the gas and a higher resonance frequency in the pipe.

[husaberg]

Judging from the way you wrote that word expand in italic, you have a pretty good idea yourself, Husa. A lower compression ratio before TDC equals a lower expansion ratio after TDC. That means a smaller temperature drop in the burnt gas during expansion, hence a higher exhaust gas temperature, a higher speed of sound in the gas and a higher resonance frequency in the pipe.

Oh Frits But i would not have been so concise or eloquent though.
The Aprilia was still fairly high. I think either you or Jan wrote is was always generally 16:1 or similar is that right?
Did this vary say higher tight tracks lower open tracks, or was the comp just optimised for the best hp/acceration/over rev compromise at the circuit.

I remember the Poloni power head From Robinsons book did anyone ever use it in competition

Is there anything in the other bits i observed or have i drifted off on a tangent?

[Frits Overmars]

...The Aprilia was still fairly high. I think either you or Jan wrote is was always generally 16:1 or similar is that right? Did this vary say higher tight tracks lower open tracks, or was the comp just optimised for the best hp/acceration/over rev compromise at the circuit.

The compression ratio was always between 15.5 and 16, optimized on the test bench. No adaption to weather, tracks etc. At least not officially; what individual mechanics got up to, was beyond control. But such actions certainly were not recommended.

I remember the Poloni power head From Robinsons book did anyone ever use it in competition?

Not at Aprilia. I seem to remember that Team Roberts once tried variable comp heads on the factory Yamaha 500s, but the name Poloni does not ring a bell.

[husaberg]

The compression ratio was always between 15.5 and 16, optimized on the test bench. No adaption to weather, tracks etc. At least not officially; what individual mechanics got up to, was beyond control. But such actions certainly were not recommended.

Not at Aprilia. I seem to remember that Team Roberts once tried variable comp heads on the factory Yamaha 500s, but the name Poloni does not ring a bell.

I spelt it wrong too i guess "Polini" powerhead

[bucketracer]

I don't believe you. more than the 22 HP, Post the results. Oh, and I want to see a video of it on the dyno as well (just in case someone "pads" the results).....

Plenty of posts and video's on this thread, use "Thread Tools" then "View Thread Images" to find them.

You can also sort them 70 to a page from the begining or end, easy to troll through them and find what your looking for and other interesting posts.

[2T Institute]

The compression ratio was always between 15.5 and 16, optimized on the test bench. No adaption to weather, tracks etc. At least not officially; what individual mechanics got up to, was beyond control. But such actions certainly were not recommended.

Not at Aprilia. I seem to remember that Team Roberts once tried variable comp heads on the factory Yamaha 500s, but the name Poloni does not ring a bell.

Compression was varied with the track and weather..........well that's what I was told anyway Having now started to use 118mm rods in a 125 we are finding really odd things like the engine demands to be run rich with black wash patterns on the piston(like some GP pics I have seen) and ignition seems to be less of a influence. Good points are torque band is getting higher and wider.

[husaberg]

Having now started to use 118mm rods in a 125 we are finding really odd things like the engine demands to be run rich with black wash patterns on the piston(like some GP pics I have seen) and ignition seems to be less of a influence. Good points are torque band is getting higher and wider.

No time to dwelling on the Why?

[TZ350]

I have a suspicion the shape of your septums may be a little out of kilter ( referring to the to the direction)front and rear wall of the trensfers too.

Looking at the Rotax drawring and my piston, I think your right. I am in the process of making another cylinder and will be using better tools so hopefully a better result.



That old cylinder made 30hp, if I can fatten up the lower part of the torque curve with better direction from the transfer ports I will be very happy.