ESE's works engine tuner

[Muhr]

@Muhr, sorry I didn't understood your calculation, are you refering to carb size? I think that is not a bottleneck in engine like that.
But you got me thinking, maybe your theory that mixture mass doesn't have time to accelerate is more applicable to transfers and mixture escaping into exhaust port and shoving it back to cylinder.

It was just as vague as usual when I write ...
21 * 21 * π / 13000 * 16800/125 * 50 / π = √ * 2≈30mm
21 * 21 * 3.14 / 13000 * 16800/125 * 50 / 3.14 square root * 2 = 30.196mm

I intended to refer to the opening area of the carburetor and rotor valve inlet, but it was as unclear as usual😁. What I was trying to say was that where you have pressure gradient force you will have an acceleration every time it opens. (Which is controlled by the pressure difference, turbulence and pressure waves but not RPM at least not for the better)
The only place where you will not have more favorable conditions for higher rpm due to lower piston speed is in the rotary valve rather the opposite. That's my view of it when I tore my hair in desperate attempts to improve the time area for 15500k on the latest 50cc engine I fiddled with. On the engine I am building now, I have the same hopeless hopes of peak rpm

[Frits Overmars]

For every time the port opens, an acceleration time is needed and it is controlled by the pressure difference. which does not increase with RPM. !

The pressure difference acts upon each mm² of the cross flow area of the gas column in a duct, so the cross flow area has no effect on the inertia of the column; only the column length plays a role here.
And luckily higher revving engines tend to have shorter ducts, so we need not worry too much about engine rpm either. In fact I hardly occupy myself with rpm; I prefer to look at the mean piston speed.

[katinas]

Peljhan Muhr, amazing little engines, so nice to see this work. Thanks for pictures.

[Muhr]

The pressure difference acts upon each mm² of the cross flow area of the gas column in a duct, so the cross flow area has no effect on the inertia of the column; only the column length plays a role here.
And luckily higher revving engines tend to have shorter ducts, so we need not worry too much about engine rpm either. In fact I hardly occupy myself with rpm; I prefer to look at the mean piston speed.

Maybe it's something I do not understand here but my point is the shorter the opening time (higher rpm) the more percentage of time will consist of acceleration of the flow, the lower the efficiency per unit time. Is that time insignificantly small?

[Frits Overmars]

Maybe it's something I do not understand here but my point is the shorter the opening time (higher rpm) the more percentage of time will consist of acceleration of the flow, the lower the efficiency per unit time. Is that time insignificantly small?

No, that time is anything but insignificantly small. But a given pressure difference will give a short gas column a higher rate of acceleration than a long gas column, that is what I was trying to say.
So in the shorter time that is available for accelerating the gas column in a high-revving engine, that column may still reach the same flow speed that it would reach in a lower-revving engine because of the shorter ducts that we are likely to find in the higher-revving engine.

[Muhr]

No, that time is anything but insignificantly small. But a given pressure difference will give a short gas column a higher rate of acceleration than a long gas column, that is what I was trying to say.
So in the shorter time that is available for accelerating the gas column in a high-revving engine, that column may still reach the same flow speed that it would reach in a lower-revving engine because of the shorter ducts that we are likely to find in the higher-revving engine.

Ok now I understand better (sometimes I'm a little slow) what you mean, you are referring to friction loss will decrease with a shorter intake on a high revving engine. I can only agree! Thank you Frits for the more detailed answer

[Peljhan]

So here is EngMod result.

Disclaimer:
I did not yet correct exhaust power valve action (that's why top is strange).
I took 140°/85° as baseline, as this was best so far.
Let's leave impossible carb setting with odd timings aside for this test...

First results 17.X: reducing opening time and kept 85° closing as constant.. At 115/85 power was best, and all gain was down low at 12000rpm.

Results with 18.X: reducing closing time and kept 140° opening as constant. At 140/65 power was best and again, all gain at 12000rpm (and probably lower).

If I went up over 145 or over 90, power remained the same.

Then I tried some combinations, but none of them gained nothing, so best result overall was 140/60 (no variating at all)


Now, as EngMod doesn't simulate influence of odd timings to carb being nigthmare to setup..

I am wondering, Wobbly, do you know where in rev range odd timings make carb impossible to setup? You said, if going over 145°/90° is nightmare.. Is it maybe one of those only hard to set at low rpm?
Only for example: at low rpm-you can't start a bike with 160/100 timing, but it works really good at high rpm.

[teriks]

Ok now I understand better (sometimes I'm a little slow) what you mean, you are referring to friction loss will decrease with a shorter intake on a high revving engine. I can only agree! Thank you Frits for the more detailed answer

I'm more inclined to think in terms of lower inertia of the shorter gas column though...

[Frits Overmars]

Ok now I understand better (sometimes I'm a little slow) what you mean, you are referring to friction loss will decrease with a shorter intake on a high revving engine. I can only agree!
Thank you Frits for the more detailed answer

And thank you for the S, Muhr .
As a matter of fact I was not refering to friction losses, but to the flow's resistance to acceleration. If you are an electronics man, you may find it helpful to compare friction losses to Ohm's resistance, and inertial resistance to impedance, where there is no resistance as long as the flow is constant.

I'm more inclined to think in terms of lower inertia of the shorter gas column though...

While I was pondering about how to explain things without creating more confusion, you nailed it Teriks

[Muhr]

And thank you for the S, Muhr .
As a matter of fact I was not refering to friction losses, but to the flow's resistance to acceleration. If you are an electronics man, you may find it helpful to compare friction losses to Ohm's resistance, and inertial resistance to impedance, where there is no resistance as long as the flow is constant.

While I was pondering about how to explain things without creating more confusion, you nailed it Teriks

I'm trying my best Frits! I just feel stupider for every attempt to understand. I assumed you were referring to friction loss (or skin friction) when you mentioned length as a factor. I thought Newton's second law does not care about the length of the intake as the area towards the low pressure zone does not change?
I think inertia is a consequence of Friction loss in this context?

[Frits Overmars]

Wobbly, do you know where in rev range odd timings make carb impossible to setup? You said, if going over 145°/90° is nightmare.. Is it maybe one of those only hard to set at low rpm? Only for example: at low rpm-you can't start a bike with 160/100 timing, but it works really good at high rpm.

There are practical limits at both sides of the inlet timing. You can open the inlet as soon as the pressure in the crankcase has dropped to the pressure level upstream of the inlet disc.
Open it any earlier and you will lose crankcase pressure to the outside world; open it any later and the crankcase pressure will drop more than it needs to, slowing down the transfer flow.
The Helmholtz resonance of the blowdown+scavenging cycle determines how many milliseconds after exhaust opening the pressures upstream and downstream of the inlet disc are equal.
This fixed amount of time means that at low revs you could open the inlet even before BDC. But this is impractical for two reasons (unless your name is Neil Hintz).
A: The inlet needs to open later as the revs increase.
B: Opening the inlet exactly when the pressures upstream and downstream of the inlet disc are equal means that there wil be a zero-pressure opening signal sent to the carburetor, and carburetors don't respond well to something they don't notice.

Summary: opening the inlet too early or too late will both cost power, but opening it too early will cause more problems than opening it too late.

When should we close the inlet? When the inlet flow is slowed down to zero by the ever-rising crankcase pressure. Close it any earlier and you will miss out on the last bit of crankcase filling; close it too late and you will lose mixture flowing back from the crankcase towards the outside world, passing the carburetor on its way out and taking some more fuel with it. This same amount of now overly-rich mixture will be sucked in again at the next inlet cycle, passing through the carburetor for the third time, taking some fuel with it for the third time and trying to drown the engine.

Summary: don't close the inlet too late. For a well-developed racing engine 85° after TDC will be about the limit. If you are really power-hungry, you can try a couple of degrees more, but it may harm your lap times.

If you do find more power past 85°, you may instead consider returning to the 85° closure timing and fitting a bigger carburetor.
By the way, 85° after TDC is also considered the maximum foolproof timing for a piston-ported inlet system.

In case you are interested in the Aprilia RSA125 inlet system: cylinder capacity=124,8 cc; crankcase volume @ TDC=675 cc; inlet timing= 142,5°/88°; carburetor diameter= 42 mm.

So this is it? Not quite yet, because closing the inlet too early can also give strange effects, as I discovered when I put an experimental distance piece between the carburetor and the inlet disc.
This increased the inertia of the inlet column, so this column came up to speed slower, and was subsequently cut off by the closing inlet disc while it was still flowing towards the crankcase at a respectable speed. The sudden closure caused a high collision pressure upstream of the inlet disc and the mixture column bounced back like you wouldn't believe. It filled the entire dyno room with a mixture mist within seconds, and the tiniest spark might have blown the roof off. Ever since this experience I distinguish between blow-back, which is a nuisance, and bounce-back, which is outright dangerous.

[Frits Overmars]

I just feel stupider for every attempt to understand. I assumed you were referring to friction loss (or skin friction) when you mentioned length as a factor. I thought Newton's second law does not care about the length of the intake as the area towards the low pressure zone does not change? I think inertia is a consequence of Friction loss in this context?

Just forget all about friction for now; Newton won't mind.
And don't feel stupid. It took me a lifetime to get where I am now and I'm sure that I will have reasons to feel stupid if I look back on today in a couple of years.That's life.

.

[Muhr]

Just forget all about friction for now; Newton won't mind.
And don't feel stupid. It took me a lifetime to get where I am now and I'm sure that I will have reasons to feel stupid if I look back on today in a couple of years.That's life.

.

What should I say! Something that is affected by length but not by surface. I guess Isaac did not leave the building based on how you write. the only thing I can think of is sound but have a hard time applying it to something meaningful (could be something about Isaccs third). You have me by the balls on this one! What is the point of doing something if you do not understand why. Thanks for the thought exercise Frits.

[wobbly]

I made a small error in my statement about the inlet timing.
140 opening is the practical limit , not 145 as I wrote , as is 90 on the closing.
I did a huge project on RV timing when at Zipkart dynoing the Rotax tanden twin.
The standard valve had staight edges and was timed at 135/85.This setup was easy to tune and gave a nice flat torque curve.

But to my ( evil ) mind it made way too much power , where we never used it on the track.
I quickly found that 140* was the limit on opening , as carburation became a real issue in the midrange area we were using on track - starting at around 9800.
Going in steps up from 85 closing , the engine made more and more peak and overev power , all the way to 90*.
Past this nothing was gained.

So the next mod was to make the cylinder port sides straight , instead of the slightly squashed oval.
With the same timings a bunch of power appeared in the overev.
So next was a bigger carb bore - I could go to 39.4 from 38 in the magnesium round slide Dellortos.
This also added to the peak and overev power.

Now the thing would rev , whereas before it dropped off a cliff at 12200 , now 13000 + was easy.
So I went back to 88* closing and this gave the best off corner power , but still allowed overev to 12800.

Much , much later with real good JL pipes , and better ( flatslide ) carbs and a servo PV etc the Superkart engine eventually gave 94 Hp at 12750 and would rev to 13400.
This was achieved with 41mm carbs and the 140/90 timing. - going back to 88* timing it simply would not rev past 13,000.

As we now know , the Aprilia RSW ended up with 42 carbs and virtually the same timings, the RSA worked best with slightly more opening and slightly less closing.
Learning from this I would say that to make high rpm power the closing is critical , and the limit is a crossover point where the carburation in the midrange becomes impossible Vs overev power capability.
Thus EngMod in this case is of limited value as the sim will still run , whereas the engine struggles to accelerate at all on the dyno with too much opening/closing timing.

EDIT - the other last point is that the RV engine loves a solenoid Power Jet , the very short inlet seems to respond way more than a reed setup to shuting off ( PWM is better ) a quite big PJ over the top.

[Flettner]

Bring on the sliding Gibs then.