ESE's works engine tuner

[husaberg]

Injectionguys!

I have had a though for a longer while.
I know when you got injection the demands of a carburetor disappears.(having to big causes function of carb to be disturbed)
The atomisation of fuel is held by the injectors and fuelpressure etc etc.

And, a supercharged 4stroke with throttleblade on atmospheric side cannot be to big.

So my thought:
REALLY oversize the reedcase(or slide if possible) and then run a huge inlettract, say 50mm on a 125cc engine, this to make sure there are no restrictions for air to travel into crankcase, well as little as a reed engine can provide that is

Anyone thought the same?

I thought differently until i figured out KTM might pay for it......

[TZ350]

It doesn't look very unbridged.

Yes,, and IF you can run it with no bridge, you can certanly run it with a small bridge also.

80% and I would very much like to go to 85% or 95% with a bridge if I have too. But I very much prefer the single exhaust port.



I have just realized that the damage on the back of the cylinder opposite the exhaust port took some time to develop. The cylinder has seen several race meetings and a number of hours on the dyno.

So life expectancy at 85% may be acceptable.

[F5 Dave]

Nice work Rob. What are the ports at the barrel like? Did you flush them? Would have been interesting with those closed off first, and then with block removed.

[Grumph]

Injectionguys!

I have had a though for a longer while.
I know when you got injection the demands of a carburetor disappears.(having to big causes function of carb to be disturbed)
The atomisation of fuel is held by the injectors and fuelpressure etc etc.

And, a supercharged 4stroke with throttleblade on atmospheric side cannot be to big.

So my thought:
REALLY oversize the reedcase(or slide if possible) and then run a huge inlettract, say 50mm on a 125cc engine, this to make sure there are no restrictions for air to travel into crankcase, well as little as a reed engine can provide that is

Anyone thought the same?

You're working along the lines I've observed in fourstrokes. I've ported heads for literally decades but the game changed when injection became common.
Previously you had to maximise flow without dropping gas speed to the point where a carb wouldn't be in it's effective gas speed band. Now it's huge ports and simply go for maximum flow at the required valve lift - because the injection atomises the fuel for you. Too easy.

So given that gas speed is no longer a factor in the choice of diameter or length of the inlet tract, do you then have to recalculate on the basis of resonance frequencies for the inlet tract and case volume ? Would a bigger inlet with less restriction mean a reduction in case volume ?

[TZ350]

What are the ports at the barrel like? Did you flush them? Would have been interesting with those closed off first, and then with block removed.

Inlet ports were filled flush with the bore. Yes it would have been interesting to do it more step wise but it all takes time and effort. EngMod tells me that there will be some power advantage if I can go extra wide on the exhaust port (85%) and reduce the duration from 200 to 194 deg.

[TZ350]

Injectionguys! I know when you got injection the demands of a carburetor disappears.(having to big causes function of carb to be disturbed) The atomisation of fuel is held by the injectors and fuelpressure etc etc.

So my thought: REALLY oversize the reedcase(or slide if possible) and then run a huge inlettract, say 50mm on a 125cc engine. Anyone thought the same?

Good idea. A lot of TPI fuel is in the transfers but a fair proportion finds itself in the crankcase too so you would have to allow for that.

I know its not exactly the same idea as yours but we tried this big inlet short inlet tract.



Turns out, with the bigger inlet we needed the extra kinetic energy of a longer inlet tracts ramming effect to stop excessive blow back.



We needed to go to the longer inlet tract to avoid fuel being ejected out nearly a meter.

On a two stroke you will definitely need to balance the inlet diameter with the inlet length, fuel injected or not.

More of the story can be found here:-

Every one knows there is a relation ship between Tuned-Length and RPM but there is also a relationship between Inlet-Tract-Length and Diameter and Inlet-Closing-Point, all three things have to work together with the crankcase volume.

[SwePatrick]

You're working along the lines I've observed in fourstrokes. I've ported heads for literally decades but the game changed when injection became common.
Previously you had to maximise flow without dropping gas speed to the point where a carb wouldn't be in it's effective gas speed band. Now it's huge ports and simply go for maximum flow at the required valve lift - because the injection atomises the fuel for you. Too easy.

So given that gas speed is no longer a factor in the choice of diameter or length of the inlet tract, do you then have to recalculate on the basis of resonance frequencies for the inlet tract and case volume ? Would a bigger inlet with less restriction mean a reduction in case volume ?

Yes, i´ve been tuning and porting a lot of faulstrokes earler in the years.
And i see similarities in a two stroke and a supercharged 4t engine.
Underside of the piston is the supercharger in the 2t case.

And yes, some sort of resonance tuning is probably a demand, as one need some sort of ram charging after piston is in TDC.
Basically a bigger inlet needs longer throat to maintain resonance.
Coming from speakerbuildin there is less restriction in bassports when going up in diam even though length is greater.(it minimizing blow sounds when going up in diam)

[JanBros]

Underside of the piston is the supercharger in the 2t case.

a lot of people will disagree including me. a 24/7 open intake wouldn't work if you were right.

[Frits Overmars]

Basically a bigger inlet needs longer throat to maintain resonance.

Or a shorter inlet timing, which I prefer.

[wobbly]

The underside of the piston is only working as a supercharger in the case of a 2T lawnmower engine.
Around BDC in a race engine the pipe diffuser sucking on the transfers creates a huge depression in the case , and it is this that begins to open the reeds.
If the inlet length is tuned correctly for the rpm , it then produces a positive pressure ratio at the same time on the opposite side of the petals.
Near TPC , the reeds are open , and the piston rising has had almost nothing nothing to do with this state in the 2T cycle.
This scenario is the naturally occurring " supercharging " effect in a 2T race engine.

This gives the lie to some peoples idea that there is " flow " thru the intake , into the case , and out thru the transfer ports - doesn't happen.
The total amount of mixture stored in the transfer ducts is greater than the swept volume of the cylinder , so it is this that is drawn upon by the pipe action - even accounting for a Volumetric Efficiency above 100%
It is only the pressure differential across the ports, at the bore , that is transmitted to the case.

[F5 Dave]

So I'm home yesterday packing and stacking boxes for the dreaded carpet next week, when the old fella who does our lawns (my father in law) shows up.

To my shame of marrying into this family, I realise in horror that he is using a 4 stroke lawnmower.

I'm going to have to have a word with him aren't I?

[OopsClunkThud]

I've been kicking this exhaust port shape around for months now and I'm finally going to give it a try.

Thinking is that by keeping the top of the main port width to ~50% of bore the roof can be flatter and the top corner radius smaller. This gives more blowdown area before the aux ports open and the width that is lost from the main is picked up by the aux ports so more blowdown after the aux are open as well. Should be nicer on the rings too.

70% follows FOS port shape concept:


Guides at 0.5, 1.0, 1.5, and 2.0 of bore assure consistent taper


Main port angled down 20° at cylinder wall, Aux ports are 0° for maximum flow in blowdown.

[Vannik]

I've been kicking this exhaust port shape around for months now and I'm finally going to give it a try.

Thinking is that by keeping the top of the main port width to ~50% of bore the roof can be flatter and the top corner radius smaller. This gives more blowdown area before the aux ports open and the width that is lost from the main is picked up by the aux ports so more blowdown after the aux are open as well. Should be nicer on the rings too.

70% follows FOS port shape concept:

This is what the RC guys run:

[wobbly]

Two issues I can see with the concept.
1st - the smaller blowdown initial wave front exiting the main port will be dumping into a relatively greater duct volume , reducing its amplitude.
And thus the diffusers efficiency
This is the very reason the Aux need to have sufficient height stagger, to allow a steep wave front of high amplitude to initially exit down the duct.
The Aux simply allow greater blowdown flow to occur before TPO.

2nd - Constant taper is not what is needed. Best initial flow is gained with a 25* down angle that then turns into a convex radius, this "downward facing
hump ",is combined with a ski jump floor , where the initial exit is perp to the bore. This creates the smallest main duct area at the point where the Aux ducts enter from the side. Thus reducing a sudden huge total cross sectional area change.

I will run it thru the midnight 3D mind calculator , before dreams of Jennifer Annistons arse and Mick Doohans NSR take over.

[Frits Overmars]

I've been kicking this exhaust port shape around for months now and I'm finally going to give it a try. Thinking is that by keeping the top of the main port width to ~50% of bore the roof can be flatter and the top corner radius smaller. This gives more blowdown area before the aux ports open and the width that is lost from the main is picked up by the aux ports so more blowdown after the aux are open as well. Should be nicer on the rings too.
Main port angled down 20° at cylinder wall, Aux ports are 0° for maximum flow in blowdown.

Because of the necessary corner radii a port width of 70% of the bore is the optimum (not the maximum) value if there is just one single exhaust port.
But when auxiliary exhaust ports come into play, things change and narrowing the main port in favour of the auxiliaries makes sense.

Will your oblique bridges between the main and the auxiliaries be nicer to the ring? I need to think about that.

I wonder why you maintained the main port width below the blowdown area and why you put the port floor at BDC, by the looks of it.
It will induce more turbulence in the exhaust duct during the blowdown phase and it increases the risk of mixture short-circuiting from the A-transfers into the exhaust.
Raising the exhaust floor will also reduce the duct volume which helps concentrating the exhaust pipe energy at raising the cylinder pressure.
And finally a raised exhaust floor helps the return flow of washed-through mixture over the piston. Remember, as the transfer ports are about to close, the exhausts ports become transfer ports themselves.

A consistent 2,5° taper in the exhaust duct is fine, provided you use the blowdown area as the initial area.

A 0° downward port angle will give the maximum cross flow area but it won't give maximum flow. 25° is better.
Incidentally that's also the maximum angle you can give the auxiliary duct floors because of the A-transfer duct roofs directly underneath that also have a 25° axial angle.