ESE's works engine tuner

[reefmuncher]

Good Day all, just recently joined the forum was wanting to discuss design considerations around cylinder design with "C" ports with side boost ports incorporated into the design so the "C" port becomes in effect a "D" port. I have been working on a cyl design project for a street bike and due to cyl stud spacing and existing duct layout and sizes decided to adopt this approach. I have noticed some firms such as Bidalot keep the axial angle shallow likely similar to the B port axial angles and other firms, VOCO, DEA and a few others pitch them at close or the same as the typical C port angle. Ultimately I want to achieve a wide powerband with a large sweetspot as the bikes in question are 3 speed. I've attached an image of the current design layout.

[lodgernz]

As usual, I'm seeking advice.
My water-cooled Honda 50, with a 26mm carb, has EO 192, the ex port extended to twice the bore and water-cooled, with the Wobbly reduction to 90% at the flange. The CR is 14.0:1, running on 98 pump fuel with Motul 800 at 30:1.

I designed and built a pipe using a planned peak power RPM of 13000, using guessed EGT of 470⁰C. This gave SoS of 546m/s, and Lt = 672mm.

After much racing with this pipe, a dyno session showed the peak power RPM to be at 11800.
Of course, with a single ex port, blowdown is compromised, but there is reasonable over-rev (see graph), so I don't think the low RPM is due to that.

Calculating back, with known Lt and peak Power RPM, the actual average SoS is only 496m/s, suggesting an average EGT of only 340⁰C.

If I now make a new pipe, using this EGT and SoS, and still aiming for 13000RPM peak, Lt will be 610mm.

But maybe the higher peak power RPM would produce a higher EGT, so the pipe would then be too short.

So should I compromise somewhere between the two values of Lt for the new pipe?

[wobbly]

Jan , thats sad to hear of Rumi - he obviously ripped you off personally.
I always thought his 125 engine design looked really well thought out, with a dead straight inlet above cylinder.
Unlike Vankerveens 45* bent intake on his horribly uncompetitive cylinder down later design.
He probably had no water cooling around the Exhaust Duct as well, as you well know he was adamant that was " better ".

[husaberg]

Jan , thats sad to hear of Rumi - he obviously ripped you off personally.
I always thought his 125 engine design looked really well thought out, with a dead straight inlet above cylinder.
Unlike Vankerveens 45* bent intake on his horribly uncompetitive cylinder down later design.
He probably had no water cooling around the Exhaust Duct as well, as you well know he was adamant that was " better ".

years ago i posted a pic of the jan wittenturd engine.
i cant be assed looking for it but he just got a kart engine and flipped it even the gearbox some plug was left on an upside down position.

[husaberg]

Well i could be assed


TBH i had forgotten about the disc valve rumi


Am i the only one that gets excited about chromate finish on engines?

Yep, here is the Rumi "upside down " reed engine with dead straight inlet ie done right, unlike Mr Ws half arsed attempt.
Then there is the Rumi rear valve - not alot done wrong there, but I dont know the detail.

Who knows the RUMI details ?
Frits your turn

Here is another shot I found on EMOT

This was the engine i was meaning it seems by the simple expedient of turning an existing kart upside down a "new Engine" was thus created.
If you look at the oil Breather/Filler housing and surround on the Kart engine (right pics in compliation), and compare then it to the Maxtra Haojue engine Drain plug, you will see what i mean.

The Rumi engines have there own thread on pitlane.

Here's some more photos of the Rumi GP

http://2stroker.createforumhosting.c...=0&&view=print

Here's a relevant quote too from Jan Thiel: (auto translation from Dutch)

Rumi The engine was not a success but there were still some nice things, and also
some big drawbacks. The gearbox was designed by an artist who also
has signed a number of F1 engines. This tank had no seeger rings and washers and you
could him after removal of the cover without tools apart. There were also 3
different primary ratios, you could cover the primary disassembly without the ignition
rotor disassembly. The crankcase halves and the lids were sealed with O-rings. The engine is the cylinder downwards designed for 2 reasons. Rumi wanted an engine that was different from all others (no good reason) I myself had to Garelli once did a test with a kilo of lead in the tip of the keel of the fairing. Gresini found that the motor thus better sent, so I thought the cylinder down like a good idea. The carburetor was thus rightly at the place where normally the radiator sits. I thought this to fix the radiator in a different place to put behind us, under the tank. Something like the British did. The frame was made ​​by Nico Bakker, with even a monoarm swingarm for more space for the exhaust. The frame is also made ​​but were not collected nor paid. Rumi had a frame in Italy, with the radiator in the traditional place. Thus came the carburetor between 2 radiator little to worry about as a result poor cooling, hot air inlet and no place for a good airbox. Also, it was very difficult to place the outlet to find the thick part came up right where the footrest Sat We solved through a hole in the frame to make the exhaust was then in an s-curve before the rear wheel backwards along with the muffler under the seat. This went well but was very difficult to make. On the test bench to ensure no power cost. There were also problems with the mechanical Niese water pump 3.5 HP cost. A elektriese pump made ​​the ignition after a while too little power and the engine was badly began to walk. Rumi began increasingly to technology to interfere where I think he totally did not know, was also the organization of the team unimaginably bad. then also pay my salary a growing backlog was I decided only to disband, after eighteen months work.

Last but not least, another set of photos through the Rumi pits:
http://www.2t-special.it/forum/viewt...=3333&start=10

Finally, an upside down pavesi :P
http://www.motoripavesi.it/img/motore_moto_6_big.jpg



ps. I have mentioned it before, but it is possible to search any pic through google images. it usually leads to more relevant photos and info.

Sure. Much has already been said in Jan's Google-translated story above. But I'm not sure I would have understood one word of that Googletalk if I hadn't been there myself at the time and could reconstruct ill-translated Dutch words and expressions.
But I've got a problem: I can neither see nor post PNG-pictures on Kiwibiker. I run Windows 7 with Internet Exporer 8. On a friend's computer with Windows 7 and Google Chrome I can at least see the PNGs. Suggestions anyone?

There you go Frits!

http://www.winhelponline.com/article...-Explorer.html

Win Vista > same kernel as Win7, Vista~=7.
Why not also try the IE 9, which is a much better and faster browser than IE8?

Thanks dinamik, but I get this: "Cannot import C:\Users\FOS\Desktop\pngasso_vista.reg: Not all data was successfully written to the registry."
The problem with PNGs refusing to show only occurs when I'm visiting Kiwibiker; all other programs function OK.

Why not IE9? Because Microsoft won't let me install it, saying I should first substitute my beta-version of Windows7 with the final version.
You don't happen to know a way around that, do you?

[aljaxon]

hi everyone, ive been awol for a long time. good..... i hear some of you saying.

for years i was of the understanding that it was the crankcase and crankcase compression that delivered fuel up through the transfers into the cylinder.
then a couple of years ago i read on here or on pit lane biz that on modern designs the crankcase was only of use for starting the motor and once running it was down to the exhaust pipe to draw fresh charge up.

i just searched on google to find this and cant see it anywhere. is this belief still so? has it been shot down? or did i imagine reading this? old age and memory etc
cheers

[wobbly]

Crankcase compression is only a direct factor for transfer charging flow in a lawnmower.
In modern engines with a " proper " expansion chamber it is the diffuser that creates a depression around BDC that pulls mixture from the case, thru the transfer ducts
and into the cylinder. If the axial and radial angles of the transfers are correct, this negative pressure ratio at the port creates a coherent intake charge column that is angled back toward the rear wall,
and this then loops across the cylinder head and down toward the Exhaust port.
The larger the depression, and the better the transfer design, the higher the Scavenging and Charging Efficiency within the cylinder.

[aljaxon]

Crankcase compression is only a direct factor for transfer charging flow in a lawnmower.
In modern engines with a " proper " expansion chamber it is the diffuser that creates a depression around BDC that pulls mixture from the case, thru the transfer ducts
and into the cylinder. If the axial and radial angles of the transfers are correct, this negative pressure ratio at the port creates a coherent intake charge column that is angled back toward the rear wall,
and this then loops across the cylinder head and down toward the Exhaust port.
The larger the depression, and the better the transfer design, the higher the Scavenging and Charging Efficiency within the cylinder.

excrellent thanks wobbly. glad i'm not losing my marbles after all.

[ApolloMotoMoto]

As usual, I'm seeking advice.
My water-cooled Honda 50, with a 26mm carb, has EO 192, the ex port extended to twice the bore and water-cooled, with the Wobbly reduction to 90% at the flange. The CR is 14.0:1, running on 98 pump fuel with Motul 800 at 30:1.

I designed and built a pipe using a planned peak power RPM of 13000, using guessed EGT of 470�C. This gave SoS of 546m/s, and Lt = 672mm.

After much racing with this pipe, a dyno session showed the peak power RPM to be at 11800.
Of course, with a single ex port, blowdown is compromised, but there is reasonable over-rev (see graph), so I don't think the low RPM is due to that.

Calculating back, with known Lt and peak Power RPM, the actual average SoS is only 496m/s, suggesting an average EGT of only 340�C.

If I now make a new pipe, using this EGT and SoS, and still aiming for 13000RPM peak, Lt will be 610mm.

But maybe the higher peak power RPM would produce a higher EGT, so the pipe would then be too short.

So should I compromise somewhere between the two values of Lt for the new pipe?

If I were to channel my inner "wobbly" and attempt to throw down a hand at 'long distance troubleshooting roullette'


My first question is, why so cold, Batman?

Targetting 470c with the pipe design???

That should be ~600c if you want to make real power.

Only achieving a derived (not directly measured with high speed EGT probe....) 340c?

Re-running the pipe design for 340c would NOT be my first advice.

I would first wonder why your pipe is so cold to begin with, assuming it truly is.

The most likely culprit from a general theory perspective would be the tailpipt outlet diameter. Often termed "D_restrictor" in some pipe formulas....

If you have the exact diameter in your pipe on the bike from the formula you used, and you are that far under the formulas target EGT, I would have questions about the engine actually meeting the input parameters that went into the pipe calculator, or the pipe calculator itself, is it the FOS formula?

You should be closer to 600c EGT for an incredibly general ideal.

Being cold either means you are not delivering the heat energy that the calculator assumes you would have been delivering to the pipe or the tailpipe diameter on your actual pipe is larger than the calculator actually spit out.

Making the tailpipe restrictor diameter smaller WILL increase the pipes EGT, and if you are currently "too large", making it smaller WILL bring with it an increase in power up until the point where you go "too small" and you burn holes in your piston, so there is a balance point to be found, and the pipe formula wont give you the EXACT perfect number for your engine. It SHOULD give you a generally safe place to START, and you tune it from there.

Generally speaking shrinking the tailpipe diameter is going to increase the EGT.

Inreasing the EGT will increase the net temperature of the pipe.

Increasing the temperature of the pipe will make the speed of sound within it FASTER.

Faster speed of sound = faster wave travel.

Faster wave travel = pipes natural "resonant frequency" goes up.

Well look at that:


1. You pipe is too cold (not metting the formula number).

2. Your pipe is peaking at too low an RPM (resoanant frequency LOWER than formula number).

So, logic says:

Make the pipe hotter (decrease tailpipe restrictor diameter) and you will see an attendent increase in pipe resonant frequency, maybe even landing exactly where the formula said you would be once you actually achieve the 470c EGT you entered into the formula.

[husaberg]

Frits, Wob, could you give us an insight, into how one can use the big volume of the crankcase effectively, in a high performance design? Would there be any rules connecting crank volume with ducts volume or pipe volume?

I simulated two different ratios in EngMod, and the higher comp/smaller volume was up by 1+hp. Both with the same tfr duct volumes (~78cc).
I did a hasty overlay of the pressure waves of the intake and exhaust:

Attachment 256943

Attachment 256944

The higher comp case caused the reeds to open less. Tfr pressure was higher from Ex open until just-before Tfr closed. Crankcase pressure was higher too.
On the exhaust part, difference was mainly betweet Tfr open and close, both for cylinder and exhaust pressure - higher comp produced the higher pressure.
Also, delivery ratio was much bigger in the high-comp, while TubMax was slighly less.

edit: I don't know whether I am missing something here, but EngMod kind of shows what you told us about 'pipe sucking from tfr ducts and that vol being important'.
I run the same as above, with 1.45 crank comp ratio, to see a number of 30.4kw. Then, with longer TFR passages (new vol about 100cc), with 1.3 CCR it showed 33.2kw, while with 1.45 it rised to 33.6kw.
I think it favorises a little the high primary comp ratios.

A layman's (mine) view of the effect case volume has is that a larger volume has a smaller drop in pressure given any particular volume transfer up the transfer ducts. As it is the pressure differential between the cylinder and the case that causes the transfer flow having the largest possible pressure differential for as long as possible will result in the maximum volume flow possible. As with anything in pesky 2-strokes there is a trade-off and therefore an optimum value of crankcase volume given any particular application.

You could go for smaller volume, higher compression, crankcases but that would only benefit initial transfer flow before the crankcase pressure dropped. However there may be some advantage in this setup with smaller transfer ducts if the energy in the initially rapidly moving mass in the transfer ducts could be used.

Maybe.

I have tried all kinds of crazy things but I never had the guts to try 32° straight line ignition advance in the powerband of any engine with a decent BMEP.
Flat lining at 15° would be a safe way to start your ignition quest. Subsequently you can run flat lines with even less advance, en finally carefully try more advance at the rpm points that suffered from retarding.

Now that is something you can safely try. You may be generating ignition and fueling numbers that are of no use whatever in a properly setup engine, as Wob says, but I would not call it a complete waste of time. It will be educational either way and you won't have to get rid of broken engine parts afterwards.

I've long been a proponent of low compression ratios. In my experience, a low-compressed engine reacts much more clearly to changes. But to take advantage, you'll need an exhaust pipe that deals extremely efficiently with the energy contained in the exhaust gases of such an engine. And the engine will need a transfer layout that can handle that amount of pipe suction.


It was considered large compared to what had been done before.
The Aprilia RSW had a TDC crankcase volume of 650 cc and a Crankcase Compression Ratio of 1,238 .
The Aprilia RSA had a TDC crankcase volume of 675 cc and a CCR of 1,227, which gave a power improvement over its predecessor.
But is 1,227 the optimum? I don't know and Jan Thiel doesn't know either. You won't discover a border until you cross it, and Jan retired before it came to that.

No, balancing the pressures above and below the piston is not a point of consideration. You want a downward-moving piston to push on the con rod as much as possible, so you do not want the crankcase pressure to slow down a downward-moving piston. But after BDC the piston must be accelerated upwards, which requires negative work via the con rod; now you want the crankcase pressure to help accelerate the piston as much as possible.
In short: when the piston is moving down, you want a low crankcase pressure, i.e. a large crankcase volume, and when the piston starts moving upwards, you want a high crankcase pressure. But when the crankcase volume is small, the crankcase pressure will drop fast because of the rising piston, so that is one more argument in favour of a large crankcase volume.
However, all the above factors are unimportant against the requirement for mixture transfer: a large crankcase volume from which the exhaust pipe can suck up as much mixture as possible.

Above the piston, things are much simpler. Let's just look at an engine without ignition and combustion. On its way towards TDC the piston is slowed down by the compression pressure, so this pressure is applying negative energy. But after TDC that same compression pressure is accelerating the piston downward, applying just as much positive energy. In short: compression pressure above the piston is energy-neutral.

Of course, with combustion, things become more biased: a low compression ratio equals a low expansion ratio, leaving more energy in the exhaust gas. And this energy is used by the exhaust pipe in the same way that a turbo would: the more the better.

That depends on how you define the transfers. They are not only the ducts in the cylinder but also the sweeping curves in the crankcase. In any case it is not a matter of mixture drawn into the cylinder, but of mixture drawn through the cylinder. Some of it stays there, some of it exits and then comes back, some of it may be lost, especially at revs outside the optimum range. By the way, I like the distinction you make between mixture mass and mixture volume.

Correct. Blowdown should be finished by the time the transfers open, but it takes a certain time.area and if this time.area has not been reached by the time the transfers open, the exhaust gas just abuses the transfers as extra exhaust ports.

Maybe we should be more aware of the ambiguity of blowdown. It starts at the angle where the exhaust begins to open, but where does it end? It should end at the opening point of the transfers, but at low revs that is more blowdown time.area than we need, and at high revs the blowdown may carry on past the opening point of the transfers. Usually we take the opening point of the transfers as the end point of the blowdown because we have no simple way of knowing at what crank angle the cylinder and transfer pressures are equal.

You're thinking in the right direction but I'd like to rephrase your thoughts. Blowdown time.area is the product of port timings and port areas, divided by revs. If you don't alter the cylinder, the blowdown angle.area and time.area stay the same, irrespective of crankcase pressure. Blowdown should allow the cylinder pressure to drop to the level of the transfer pressure just before the transfers open. So a higher crankcase pressure means that the cylinder pressure does not have to drop quite so far.
In other words, a higher crankcase pressure requires less blowdown time.area.

That's right, but for real power you cannot vary the exhaust timing too much; it has to be about 180° effective or you'll get no true resonance.

Yes; that is the main reason that the exhaust timing has to be about 180° effective for true resonance; we want superposition of the old and the new +pulses.

They cover both; the variation of the crankcase volume during one crankshaft revolution should not exceed the permissible cylinder capacity, so both suction and primary compression are coupled to piston displacement. In addition, any mechanical device that creates positive boost, is forbidden (an exhaust pipe does that, but it is not considered a mechanical device).

hi everyone, ive been awol for a long time. good..... i hear some of you saying.

for years i was of the understanding that it was the crankcase and crankcase compression that delivered fuel up through the transfers into the cylinder.
then a couple of years ago i read on here or on pit lane biz that on modern designs the crankcase was only of use for starting the motor and once running it was down to the exhaust pipe to draw fresh charge up.

i just searched on google to find this and cant see it anywhere. is this belief still so? has it been shot down? or did i imagine reading this? old age and memory etc
cheers

....................................

Frits overmars

ujet: Re: [GP125] All that you wanted to know on Aprilia RSA 125, and more, by Mr Jan Thiel and Mr Frits Overmars (PART 3) (Locked) Jeu 13 Juin 2013 - 17:12
By '2 mm clearance' I assume you mean 1 mm either side between crank and case, and 1 mm between outer crank radius and inner case radius. Viscous drag increases strongly below 1 mm clearance; enlarging the clearance beyond 1 mm has only a limited effect on viscous drag. So if increasing the clearances beyond 1 mm improves power, I would assume that this improvement was caused by the increased crankcase volume. It could also have been caused by improved inlet flow, but I cannot comment on that without knowing your engine so I will leave this aside for now. The most striking example of crankshaft clearance I saw in the old Rotax-124 kart engine that I worked on from 1978 onward. It was really a 250 cc motocross engine with a 125 cc crank in it. If I remember correctly the case diameter was about 12 mm larger than the crank diameter! But it was easily the fastest engine of that era, and the most susceptible to tuning modifications. It was also the grandfather of the Aprilia RSW125 Grand Prix engine. How did you arrive at this value, Speedslut? The Aprilia RSA125 has a TDC crankcase volume of 675 cc. That gives a primary compression ratio of 675 / (675 - 125) = 1,23. Rather different from your value.

ot only, Graham. I've said lots of things that seem hard to swallow. Here is an anthology:


'The part of the exhaust port area beneath the transfer port level is a waste of real estate'.


'A high secondary compression ratio improves power in a four-stroke;
in a two-stroke with an efficient exhaust system it is the other way around'.


'Two-stroke rpm is limited by blowdown and scavenging angle.areas, not by crankshaft reliability issues'.


'Comparisons of differently sized engines should be based on mean piston speed, not on rpm'.


'Piston clearance, coolant flow rate, radiator size and ambient temperature are practical limits to engine cooling,
but in theory there is no such thing as too much cooling'.


'Top speed is the most unimportant thing in racing. Keeping your minimum speed high is the most important'.


The above collection should be sufficient to earn me a reserved place in an asylum, don't you think, Graham?
I'm not sure. Too many forums... The idea is that a large expansion ratio takes away exhaust gas energy that could otherwise be more useful supercharging the cylinder, so the next bang will be bigger, and the bang after that bigger still, and... you'll get the picture.
A low secondary compression/expansion ratio provides for more exhaust gas energy; it also provides for a larger cylinder volume above the exhaust port so the exhaust pipe has an easier time shoving washed-through fresh charge back into the cylinder because the resulting cylinder pressure rise will be smaller.
In theory it would be best to connect the transfer ports directly to the outside world. Then the engine could breathe cool mixture through short tracts with low dynamic flow resistance. Don't worry about the 'swing'; transfer tracts, cylinder volume and exhaust pipe form a compound Helmholtz resonator that will swing just fine without the crankcase attached.


When the crankcase volume gets really big, fuel may separate from the air. Direct injection would solve this, but for now the time available for direct injection in a competition engine is too short to form a homogeneous mixture.


I once made an experimental cylinder with carburetors bolted left and right against it. But starting was a problem and lubricating the big end was an even bigger problem, so for practical reasons we still use the crankcase.

en Seeber a écrit:
I would think that the pressure in the transfer passages would mainly be a function of the exhaust system, port timing and the rpm. I would suggest that if a well balanced (not crankshaft) engine was running at its optimum "tuned"speed that there would be no blowdown into the transfers and that no carbon would form. Outside the "tuned window" then it could be well possible, as evidence the formation of carbon. As always, I could be wrong though.
Not this time Ken; you are quite right Wink.
But rev any engine high enough and the blowdown time.area will become too small. Or use 'normal' revs with the throttle almost closed and the crankcase pressure will be so low that even with ample blowdown the cylinder pressure will not drop below the crankcase pressure before the transfers open. The latter situation was notorious on the racing Aprilias: they never detonated under full power but they could detonate like hell at 10% throttle.

I was musing something less mechanical more pneumatic as a side note. More in the reed cavity area, more a bypass.
One of the guys that used to work with Helmet was Ferry Brouwer (not sure of the spelling) he allegedly used to remove so much of the rear skirt of the piston on the (I think TR2) that the inlet closing became simultaneous with the transfer opening, this was pre reed days of course. Not sure what would make of the inlet duration or the primary compression on piston ports.

My old mate Ferry spells his name the same way you do, but I doubt if the late Helmut Fath would trade his first name for a headgear description.

Ferry and I did a lot of silly things. With about 134° transfer timing his piston shortening would give an inlet timing of 226°.
It was not uncommon in those days; Bultaco did it too on their TSS250. Remember: back then the carburettors were tiny by current standards.
A more drastic mod by Ferry was raising the inlet ports until they opened at BDC. That's right: C-transfer ports directly connected to the carbs; no reed interference (but quite a lot of Read interference; dear Phil MBE was not the easiest person to get along with).

atributed to Frits

Everything has to fitWhat makes a two-stroke engine a high-performance engine? The exhaust.
The vacuum from Exh Opening first empties the cylinder. When the exhaust gases are out, the transfers open
and fresh gas flows out of the crankcase via the scavenging channels in the cylinder.
The exhaust continues to suck, and part of that fresh gas passes through the cylinder to the exhaust manifold.
If speed, exhaust length and speed of sound all match, then before the port closes again and at the same time
changes the flow in the exhaust manifold, the direction of flow and the escaped fresh gas is pushed back in the
cylinder.
Finally the piston closes the exhaust port again, so that the pushed back fresh gas in the cylinder is trapped.
There are two cases where speed, exhaust length and sound velocity do not all fit together: If the engine speed is
too high (or the exhaust is too long, or the sound speed too low). Although still fresh gas sucked in the mani-
fold, the return flow begins too late for this speed: the outlet slot closes again before all this fresh gas is pushed
back.
That’s what causes overrev performance to fall.
On the other hand, if the engine speed is too low (or the exhaust too short, or the speed of sound too high), the
engine behaves even grumpier.
Although the cylinder is sucked empty and rinsed, and sufficient fresh gas comes in the manifold to be able to
charge the cylinder afterwards, the return flow starts much too early for this speed, if the transfers are still open.
The overpressure which is generated by the return flow into the cylinder (via the exhaust port), escapes immedi-
ately through the scavenging channels back into the crankcase. When the exhaust port finally closes, there is no
overpressure (“supercharging”) in the cylinder. There is overpressure in the crankcase, which is not helpful for
the next intake cycle.
And the backflow not only starts too early, but comes to a standstill too early and then changes the flow direction
again (Helmholtz). The little fresh gas that still remained in the cylinder is subsequently sucked out again. And
then finally, but far too late, the piston closes the exhaust port.
No wonder that there is then a huge torque hole. In addition, the engine is now drinking extra fuel: per horse-
power, it consumes a lot more gasoline and a significant part of it disappears unburned through the tailpipe.
Burning speed and expansion
There are two ways to adjust the exhaust to high or low speeds: change the exhaust length or change the speed of
sound. Exhausts with sliding manifolds like a trombone have been used, and also exhausts where the end cone
was able to slide. That may work, but it requires a lot expenditure.
The speed of sound is easier; this works over the exhaust gas temperature. The maximum temperature in the
combustion chamber can be up to 2300 ° C. But due to the expansion on the downstroke, this cools the exhaust
largely again before the outlet opens.
And this Expansion can vary. It starts namely, when the combustion is just completed and the cylinder pressure
is maximum, and it lasts until Eo. The sooner after TDC the combustion is finished, the greater is the subsequent
expansion and the cooler the exhaust gas when it flows into the exhaust pipe.
When combustion is completed depends on two factors: the ignition timing and the combustion speed.
The latter in turn depends on the quantity (much or little fresh gas), the quality (clean fresh gas or much mixing
with exhaust gas), the mixing ratio air / gasoline (rich, lean or just right), and from the turbulence that is caused
by the pinch edge.
If you really want to have hot exhaust gas, so set the ignition timing to late so that the combustion starts late, use
a small main jet because lean mixture burns slower and therefore longer, and install a handful of head gaskets so
that the there is very low compression.
You may have already experienced the reverse: pre-ignition, rich mixture and high compression prevent the
engine fmor revving.
But you should not play with all the above factors. For power and a healthy engine too it is important that the
combustion takes place as quickly as possible. So really, one uses a compact combustion chamber and squeezes
the mixture effectively.
To influence the exhaust gas temperature, this leaves us with ignition timing. Now we are at the heart of the
matter: at low speeds either the exhaust is too short or the speed of sound too high. Variable exhaust pipe lengths
require too much effort, so we want to reduce the exhaust gas temperature and thus the speed of sound.
We achieve this with advanced ignition timing.
And for high speeds, the exhaust is actually too long, so we compensate for that with late ignition timing.
Intersections
Any engine that is even close to healthy can withstand 16 ° of ignition advance. With this fixed value we do a
baseline test and thereby comes out a performance curve.
Then we set the ignition to 12 ° and measure again. When we compare that to the 16 ° power curve , the 16 de-
gree is best is up to 10,000 rpm, and the 12 ° in turn is better after 10,000 rpm.
At 10,000 rpm, both curves intersect; so they have the same performance at that point.
You could say: at 10,000 rpm 16 ° is just as early as the 12 ° is too late. 14 ° could be the optimal value for 10,000
rpm.
Then we set the ignition to 14 ° fix and make again a power curve. For example, this 14 ° curve crosses the 16 °
curve at 8000 rpm and the 12 ° curve at 11000 rpm.
Then we can conclude that 15 ° is optimal at 8000 rpm, 14 ° at 10,000 rpm, and 13 ° at 11000 rpm.
To dispose
At the top RPM you can experiment without worries. After the peak power speed there is very little detonation
risk, and it makes no sense anyway to give it a lot of ignition advance. On the other hand, you have to be care-
ful about the peak torque RPM; there, too much ignition timing can be expensive.
Even further down, where the engine has little torque, and thus poor cylinder filling, the danger is smaller again.
Below the torque peak, even GP engines can easily handle 30 ° ignition advance, and that works even up to 8000
rpm. But someone who tries a 30 ° flat advance for a complete power curve up to the maximum RPM, can actu-
ally avoid the effort and just dispose of the engine immediately. With so much ignition advance, you can only
test this where the engine has poor cylinder filling. So stop before the torque rises steeply.
From then on to the peak power speed you have to be very careful and After each partial measurement, spark
plug and piston must be checked for signs of detonation. A warning: do not look at four-stroke advance val-
ues; they operate with much more ignition timing. Formula 1 engines eg : With their huge bore and ultra short
stroke, have a combustion chamber like a pancake. There is little squish area, because valves are everywhere.
The things are therefore even at full throttle operating with more than 50 ° ignition advance because otherwise
the flame does not reach all corners in time.
GP-curve For example, I show the full-throttle ignition curve of a 125cc GP engine at 12750 rpm maximum
torque and at 13000 rpm has its peak performance:
Dynamic
The whole ignition story is a temperature game. It only serves to lower the exhaust gas temperature to optimize
for each situation.
It is important that the circumstances at the test bench are exactly the same as on the race track. The accelera-
tion time, ie the time in which the exhaust is heated, must be practical. That’s why you can only determine these
ignition curves on a dynamic test bench; in a static power measurement, the exhaust is much too hot.
Incidentally, this also applies to the design of exhaust systems: Pipes developed on a static test bench are far too
long. Then you turn the engine up on the track you either have to retard the timing (costs performance) or too
meager (cost piston, Cylinder and possibly driver).
Another advantage of a dynamic test bench: because the engine only measures about ten seconds during a mea-
surement instead of five minutes, in this case it can also survive a bit too much ignition advance, which at one
static measurement would end in tears.
Incidentally, even with an optimal exhaust and a matching ignition curve of the engine will not over-rev un-
restricted, because the time cross sections (blowdown time-area, I am guessing- RN) get too small at overrev.
Because of the insufficient blowdown time-area after peak, the cylinder pressure is still above the case pressure
and exhaust gas will back-flow into the transfer ducts.
When the cylinder scavenging finally starts, you will first be flushed with exhaust gas. (because exhaust gas has
been pushed down into the transfers) Next comes partially-contaminated charge up from the transfers and final-
ly clean purge gas enters the cylinder. That’s why overrev performance drops so steeply .
Balance and residual energy
Dips in the power curve, I already explained: the resonances are no longer fit for speed and disturb the scaveng-
ing instead of promoting it.
Fortunately, with low cylinder filling, the combustion temperature and thus also the exhaust gas temperature are
low, so that the speed of sound drops. At the next working stroke, there is somewhat less scavenging and a little
more filling. This is how a balance is established.
That works too without ignition adjustment. Automatic ignition advance is still a positive effect. With more ad-
vance, the expansion due to burning before Eö is greater. This reduces not only the exhaust gas temperature but
also the residual energy that is available for exhaust resonance.
At very low speeds then come the resonances totally in the wrong moment, but at least they are not so strong and
have less effect.
Last note
Finally, a practical note: if you are dealing with an unknown engine, you should always first start with a much
larger main jet than ideal, and then reduce until the mixture is right. The danger lies in installing a slightly
larger main jet, especially if the engine was originally too much was lean. Much too lean means: totally no per-
formance and therefore no heat development. But if you give this very lean engine a slightly larger jet, it becomes
only just a little too lean; then the power comes, and thus the heat, which can then be terminal. By the way, here
too the difference between dynamic and static testing can decide how to measure life or death for the engine
https://www.zweitaktforum.de/wbb/att...nce-curve-pdf/

Pretty sure Frits did a post in the Pitlane thread were where said something along the lines of ...... so ending the myth of low crankcase volume. i can't find it
Does that sound like you Frits? Or was it Wob saying ending a fairy tale about crankcase compression here?

[Frits Overmars]

Boy, I wrote quite a lot over the years, didn't I?
I've got a slight headache now, feeling a bit dizzy after reading that last long text. What does it say there? What should it have said? What did this more or less English text look like in the German in which I originally wrote this piece for some friends?
As it happens I'm in Germany again, getting snowed in, so I'll have plenty time to read and drink coffee (or Glühwein).

To answer your last question: both Wobbly and yours truly have been trying for decades to eradicate the persistent misconceptions about the desirability of small crankcase volumes and high crankcase compression ratios. What we still encounter on the internet every day can make you want to cry.

[Wos]

As usual, I'm seeking advice.
My water-cooled Honda 50, with a 26mm carb, has EO 192, the ex port extended to twice the bore and water-cooled, with the Wobbly reduction to 90% at the flange. The CR is 14.0:1, running on 98 pump fuel with Motul 800 at 30:1.

I designed and built a pipe using a planned peak power RPM of 13000, using guessed EGT of 470⁰C. This gave SoS of 546m/s, and Lt = 672mm.

After much racing with this pipe, a dyno session showed the peak power RPM to be at 11800.
Of course, with a single ex port, blowdown is compromised, but there is reasonable over-rev (see graph), so I don't think the low RPM is due to that.

Calculating back, with known Lt and peak Power RPM, the actual average SoS is only 496m/s, suggesting an average EGT of only 340⁰C.

If I now make a new pipe, using this EGT and SoS, and still aiming for 13000RPM peak, Lt will be 610mm.

But maybe the higher peak power RPM would produce a higher EGT, so the pipe would then be too short.

So should I compromise somewhere between the two values of Lt for the new pipe?

Maybe your ex timing / blowdown area is a little to low for 13000



Typical for advanced 50 cc with peak power at about 13000 are 195 - 197 degree and there is a need for boost ports.

Earlier ex opening is rising egt too

[Flettner]

54 x 54 twin port cylinder underway. Rear disc valve this one, this is for Vinduro so is 125cc.
This same setup ( bottom end) with a smaller cylinder could well be my next Bucket engine, after what, thirty odd years.

[husaberg]

Boy, I wrote quite a lot over the years, didn't I?
I've got a slight headache now, feeling a bit dizzy after reading that last long text. What does it say there? What should it have said? What did this more or less English text look like in the German in which I originally wrote this piece for some friends?
As it happens I'm in Germany again, getting snowed in, so I'll have plenty time to read and drink coffee (or Glühwein).

To answer your last question: both Wobbly and yours truly have been trying for decades to eradicate the persistent misconceptions about the desirability of small crankcase volumes and high crankcase compression ratios. What we still encounter on the internet every day can make you want to cry.

You have wrote far more and we are greatfull for everpart thats just little what a quick search turned up.
That last translated part i had never seen before? I never had a chance to read it. My own german is limited to what was in comics.
looks like it was indeed translated from something your wrote in 2009 translated using google and posted by RN? 2019?

Ignition curves
Copyright © 2009 Frits Overmars - (translated via google translate and some manual editing RN Jan-2019Ignition curves

here is another verson
looks closer to english
https://opensimspark.org/_media/fos-...curves-eng.pdf


hint for kb users

when looking back at reposts
Where the arrow is after the poster name of you click there it takes you to the original post when posts are quoted it looks requoted bits.

[F5 Dave]

54 x 54 twin port cylinder underway. Rear disc valve this one, this is for Vinduro so is 125cc.
This same setup ( bottom end) with a smaller cylinder could well be my next Bucket engine, after what, thirty odd years.

Wooden Powervalve technology. Sustainable. I like it.