2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all.

Tim.Wright writes:
“An even better way to represent the problem would be to use the gear ratios and plot the tractive force in function of vehicle speed for each gear.”


This is exactly what the RoadLoad program does (it was written several years ago in Quick Basic and requires Windows/DOS environment to run).


It can manage the data for two vehicles and present in a single graph the available force vs the speed, for all speeds, giving the opportunity to the viewer for direct comparisons of various kinds.

Here is the (combined) graph for the Kawasaki 125KX and the Honda 125CR:



Please don’t focus on the specific models or curves.
The important is that by such a graph one can directly compare what two vehicles can do (like running on the road the two vehicles side-by-side).
All we need for such a graph is some basic data (a dyno graph, the total transmission ratios and the aerodynamic and rolling resistance at some point).



Another more general way is what Gruntguru summarises by:
“the engines should be compared using a dimensionless speed”.

The problem is that only few people can get this way of comparison, while this way does not take in account the available gear ratios.

Thanks
Manolis Pattakos

[gruntguru]

No gg, actually - its quite the opposite..
..since in the real world, of everyday road use -very little time indeed is spent in the last 1,000 rpm up near 'redline'..
..& even in racing, that last 1,000 rpm will not compensate for the time/ground lost in the previous several thousand..

My post did not speak of operating near redline. Read this bit again "the 4-stroke has more power at every road-speed"

You cannot rate the 20,000 rpm gas turbine in the same gear as the 1,000 rpm diesel. You must put both in an equivalent gear - probably one that produces the same road-speed at peak power. (I originally said "red-line" which is what Tim's graph does but peak-power-rpm is probably more appropriate.)

Tim's graph tells us at a glance, that the four stroke Kawasaki will produce superior acceleration (assuming identical bikes except with final drive adjusted to suit the rpm of peak power)

[manolis]

Hello J.A.W.

You write:
“Here below - is a 4T rotary sleeve valve design which was made commercially, but failed..
..due to, as the makers note - problems with heat flows, even on the small scale..
http://www.rcvengines.com/technology_ro ... valve.html “




In comparison to the PatRoVa rotary valve wherein the bearings run unloaded, the big diameter thrust bearing in the bottom of the cylinder of the RCV engine of the link takes a huge force, while the piston rings slide not only along the cylinder but also around the cylinder.

As for the cooling, it is not easy because the combustion chamber is not fixed, but spins inside an external cylinder whereon the cooling fins are.



A similar problem (having to do with “heat flows”, is met in decent 2-stroke engines.

And as the PatRoVa rotary valve (more at http://www.pattakon.com/pattakonPatRoVa.htm )







solves several problems of the rotary valves in the 4-strokes (among which is the "thermal flow": the exhaust gas exits directly out of the combustion chamber and of the rotary valve without touching most of the walls),

similarly the PatATE (more at http://www.pattakon.com/pattakonPatATE.htm ) solves some crucial problems (“heat flows”, lubrication, emissions, scuffing resistance etc) of the 2-strokes.



Here is what I posted yesterday in another forum:


QUOTE from Exhaust bridge - Honda 2-Stroke - ThumperTalk


UtahTrailRide:
"Can someone explain to me what the deal is with drilling pistons on the exhaust side. I've heard the CR125 needs the piston drilled but I sure didn't in my last rebuild. Now I'm all paranoid I'm gonna shred something on the hot side."


ChrisLPD:

"OK, this is an old-school 2-stroke engine builder trick Utah,...but that one that works VERY Well! The idea is that the exhaust port bridge in the 2-stroke is a necessary evil. It is the one fault of the efficiency of a tuned 2-stroke engine. Its necessary because it acts as a land for the rings to run against as the slipper piston passes the exhaust port on its stroke to keep the ring compressed. It’s the evil because it’s the absolute hottest part of the motor and the absolute MOST suseptable to cylinder failure or nikasil plating damage for 3 reasons…

1. It is the thinnest section in the combustion chamber and can not dissipate heat. (Imagine taking a torch and heating the entire surface of a sewing needle. If you heat the needle’s eye, the shaft, and the point evenly, which part glows red first? The needle point does because it is the thinnest surface area that can not dissipate the heat).

2. All of the expanding burning hot gasses must pass by it. (Physics of Convection Dynamics)... (Imagine taking that same blow torch and blowing the flame evenly thru the center of your cylinder. It heats up in 360 degree radiation of the heat source. Now focus the flame onto one part of the cylinder. In addition to the heat from thermal radiance, the surface contact point heats up dramatically hotter because of the convection (movement) of the hot expanding gas molecules are bombarding and moving around it. Same thermal convection effect as the hot expanding combustion chamber gasses bombarding and having to move around the exhaust bridge.)

3. In lieu of both of the above critical faults, the exhaust bridge is NOT encompassed by the water jacket to get cooled


Most cylinder failures in a properly jetted and sealed 2-stroke with good fuel and lubrication are on the exhaust bridge because it gets so damned friekin hot. This is especially true of plated cylinders where you first start to see the plating flake off around the exhaust bridge. Drilling the the piston is MASSIVE insurance against this failure from the bridge warping, cracking, or flaking its plated coating. This was done by numerous factory teams and engine builders in the good'ole days before MX had cams, valves, and timing chains. This was when the manufacturers wanted to sale thumpers and pressured the AMA to establish the rules that a thumper was allowed twice the displacement to be "Fair" against the oil burners.


This is how its done...When I set up a 2-stroke, I use set-up dye. An easily removable (wipe-off) acrylic-like paint that you can spray on a part to visually inspect the wear and contact areas during motor building. I spray the dye on the front of the piston (exhaust side) and allow it to dry. I then install the piston on the wrist pin (without the rings) and lightly seat the cylinder down on it.


Once I have the cylinder lined up and seated, I look thru the exhaust port (open exhaust port with no pipe), I cycle the crank (via kick starter) until the bottom ring land passes approx ½ inch above the top of the exhaust port into the compression/combustion chamber. Using an etching tool I lightly trace both the outer left and right sides of the exhaust bridge into the set-up dye on the piston, and then disassemble.


Now with the piston removed I can see where the exhaust port bridge is in the etchings of the dye and I find the center vertical axis between these two etchings. This is the center of the exhaust port bridge. Starting ¾ of an inch below the ring land, I drill two .050 holes vertically (one on top of the other) about ½ inch apart down the center axis between the two vertical left and right marks.


The idea concept is to allow the positive pressure in the crank case (on the pistons downward stroke) to mist just a fine little amount of the cool, raw fuel/air mix onto the exhaust bridge thru the piston holes each time the piston passes the bridge. The cool, raw fuel mix acts as a cooling agent and will surprise you by how much this little trick will drop the temp of the thin little exhaust port. At 6000 RPM’s the critical heat temp on that thin little exhaust bridge is getting misted and cooled with fuel and intake air 100x a second!


Some will say this looses horsepower, I don’t believe so as long as you line the holes correctly and mist the center axis of the bridge and not vent your positive crankcase pressure out of the open exhaust port. Even if does, the loss is very, very, very minimal…but IT WILL SAVE YOUR $400 plated cylinder.


As frdbtr pointed out… This is a commonality of Wiseco recomendation in SOME of their piston kits that most people don’t read. (Some applications do not use a single port exhaust bridge). I am not familiar with the other Wossner brand he named, but wiseco recommends it because IT DOES WORK and their pistons are FORGED instead of cast and do not lose integrity by drilling the holes in the piston wall.


A Note about OEM and cast pistons… The same cooling effect would be benefited from an OEM or any other piston. It’s not often recommended for cast pistons because the “sand-cast” piston will weaken at the metallurgical level when you drill holes in it unless you stress relieve and re-harden it.


Please, please, consider all of the above My Opinion Only,... but as an educated opinion I have been building cast-iron and nikasil 2-stroke motors for 25 years and I have yet to have a cylinder failure of the exhaust bridge since I learned this from World Champion Melvin Cooper (Cooper Company Machine and Racing) building 2-stroke Hydroplane racing engines back in my twenties."


END OF QUOTE




QUOTE from http://www.millennium-tech.net/pistonTrouble.php




EXHAUST BRIDGE SEIZURE
Piston seizures can occur at the center exhaust-bridge on a delta-shaped port, or the sub-exhaust bridges on a triple port design. The exhaust bridge is the hottest are of a 2-stroke cylinder and the ring pressure is at its peak because the bridge’s surface area is so small. Seizures happen for many reasons but the most common are 1) Lack of lubrication 2) Too lean jetting or air leak 3) Coolant over-heating 4) Not enough bridge relief 5) No oiling holes in the piston over-lapping the exhaust bridge.


END OF QUOTE




In the PatATE each bridge has not just two tiny holes to cool down it; it has all the fresh cool air / mixture to pass around the bridges to cool down them.




Reasonably, the temperature of the bridges of the PatATE can be about half of the temperature of a conventional 2-stroke exhaust port bridge.


Thanks
Manolis Pattakos

[J.A.W.]

No gg, actually - its quite the opposite..
..since in the real world, of everyday road use -very little time indeed is spent in the last 1,000 rpm up near 'redline'..
..& even in racing, that last 1,000 rpm will not compensate for the time/ground lost in the previous several thousand..

My post did not speak of operating near redline. Read this bit again "the 4-stroke has more power at every road-speed"

You cannot rate the 20,000 rpm gas turbine in the same gear as the 1,000 rpm diesel. You must put both in an equivalent gear - probably one that produces the same road-speed at peak power. (I originally said "red-line" which is what Tim's graph does but peak-power-rpm is probably more appropriate.)

Tim's graph tells us at a glance, that the four stroke Kawasaki will produce superior acceleration (assuming identical bikes except with final drive adjusted to suit the rpm of peak power)

Well gg, what it actually "tells us" - is that t.w.'s graph - is all very well, as a maths exercise 'on paper',
..but is in fact - sunk - by actual empirical/quantified - 'real-world' testing.. ( & FYI, the 2T pulls 'taller' gearing, too)

Funnily enough, as such - that very magazine test proved to be a major embarrassment - for Kawasaki too..
..since their 'theoretical' values - like t.w.'s - had assured them - of 'victory' for their new 4T 'flagship'..

& this was after Kawasaki's senior level motorcycle engineering team, which had invested major 'face' on a 4T win..
..had both lifted the 4T's capacity from 750cc to 900+, & had ensured the 'soft tune' for the 2T 750/3 had included..
..a reduction in carburettor throat from 32mm to 30mm.. as a late change, from pre-production to showroom..
..these measures were done in order 'to ensure the proper finishing order' as 'mana' required.. 'oh dear,so solly!'

[J.A.W.]

Hello J.A.W.

You write:
“Here below - is a 4T rotary sleeve valve design which was made commercially, but failed..
..due to, as the makers note - problems with heat flows, even on the small scale..
http://www.rcvengines.com/technology_ro ... valve.html “


http://www.rcvengines.com/images/sp-eng ... ection.jpg

In comparison to the PatRoVa rotary valve wherein the bearings run unloaded, the big diameter thrust bearing in the bottom of the cylinder of the RCV engine of the link takes a huge force, while the piston rings slide not only along the cylinder but also around the cylinder.

As for the cooling, it is not easy because the combustion chamber is not fixed, but spins inside an external cylinder whereon the cooling fins are...

Here is what I posted yesterday in another forum...
In the PatATE each bridge has not just two tiny holes to cool down it; it has all the fresh cool air / mixture to pass around the bridges to cool down them.

http://www.pattakon.com/PatATE/PatATE_R ... g_Slow.gif
Reasonably, the temperature of the bridges of the PatATE can be about half of the temperature of a conventional 2-stroke exhaust port bridge.

Thanks
Manolis Pattakos

Hi Manolis..
I do not doubt - the matter will draw much attentive problem solving, theoretical & empirical, just as it did for
the likes of Harry Ricardo & Roy Fedden when they were working on making their sleeve valve really functional..
..at the design level specific output/TBO..

Did you peruse the other ' rotary valve' designs - which that linked model engine maker - markets?
Your designs certainly make theirs look, ah.. IMO, somewhat.. untidy..

[J.A.W.]

Hello all.

Tim.Wright writes:
“An even better way to represent the problem would be to use the gear ratios and plot the tractive force in function of vehicle speed for each gear.”

...Another more general way is what Gruntguru summarises by:
“the engines should be compared using a dimensionless speed”.

The problem is that only few people can get this way of comparison, while this way does not take in account the available gear ratios.

Thanks
Manolis Pattakos

Hi Manolis..
Did you peruse the 'motorcycle analyzer' program I linked? ( see post 2nd from last, two pages back)

Of course you are quite correct to remark..
..that failing to take into account practical operational usage - does in reality, render such pretty maths, moot..

[henry]

...


Another more general way is what Gruntguru summarises by:
“the engines should be compared using a dimensionless speed”.

The problem is that only few people can get this way of comparison, while this way does not take in account the available gear ratios.

Thanks
Manolis Pattakos

Hello Manolis,

Going back to your point on safe overtaking.

Just as many people don't have the knowledge to cope with dimensionless representations, out in the real world many drivers/riders have no real concept of what gears do for them. For every skilled exploiter there are tens if not hundreds of thousands who don't select the best gear for maximum acceleration. In fact many don't change gear selection at all. For those people a flat torque curve with more power across a wider range is very appropriate.

I base this suggestion on many years of observation during cycle commuting. A significant proportion of those about me rode in a single gear, up hill and down dale, even when they had 3,5,10, or even 20, to choose from. Their lack of physics knowledge extended to leverage, with a significant number pressing with their heels or insteps effectively treating their lower legs as sticks.

So I would suggest that in the real, functional travel, world, as opposed to the esoteric, movement for fun, world, flat torque curves are a good thing.

Automatic gearboxes probably change that position somewhat, and from a safety perspective are probably a good thing.

And to return to the overtaking point I suspect that electric vehicles will be much superior to IC, at least in the hands of the average driver. I haven't driven one yet but road tests are riddled with comments from testers about how responsive and zippy they are.

But I guess that's enough heresy for a 2 stroke thread.

[henry]

Manolis.

in your shared port design the exhaust and transfer are inevitably in close proximity. Is this potentially a problem?

Also it seems to present less flexibility in the placement of flow of the transfer than a conventional design.

[J.A.W.]

...


Another more general way is what Gruntguru summarises by:
“the engines should be compared using a dimensionless speed”.

The problem is that only few people can get this way of comparison, while this way does not take in account the available gear ratios.

Thanks
Manolis Pattakos

Hello Manolis,

Going back to your point on safe overtaking.
Just as many people don't have the knowledge to cope with dimensionless representations, out in the real world many drivers/riders have no real concept of what gears do for them. For every skilled exploiter there are tens if not
hundreds of thousands who don't select the best gear for maximum acceleration. In fact many don't change gear selection at all. For those people a flat torque curve with more power across a wider range is very appropriate.

I base this suggestion on many years of observation during cycle commuting. A significant proportion of those about me rode in a single gear, up hill and down dale, even when they had 3,5,10, or even 20, to choose from. Their lack of physics knowledge extended to leverage, with a significant number pressing with their heels or insteps effectively treating their lower legs as sticks.

So I would suggest that in the real, functional travel, world, as opposed to the esoteric, movement for fun, world, flat torque curves are a good thing.

Automatic gearboxes probably change that position somewhat, and from a safety perspective are probably a good thing.

And to return to the overtaking point I suspect that electric vehicles will be much superior to IC, at least in the hands of the average driver. I haven't driven one yet but road tests are riddled with comments from testers about how responsive and zippy they are.

But I guess that's enough heresy for a 2 stroke thread.

Funny you mention a couple of things Henry.. & I'll add - at risk of going off topic..

Not only about poor driving technique - such as chronically too tentative pedal pressing..
( due to irrational fear of ' kickdown' response on acceleration, & of 'anti-lock' modulation response - on braking).

But I have raised the push-bike 'best torque/rpm' automatic efficiency factor with Manolis..
& he has devised a typically clever CVT for bicycles too, - as it happens..

[henry]

....

But I have raised the push-bike 'best torque/rpm' automatic efficiency factor with Manolis..
& he has devised a typically clever CVT for bicycles too, - as it happens..

I'm familiar with Manolis' CVT. typically clever.

I owned a couple of CVT vehicles and really liked them. But the general public didn't. They liked engine revs to rise as they got faster. So more recent CVTs mimic umpteen speed gearboxes, such as in the Honda Jazz. This sort of cultural inertia would, I suspect, be a significant handicap if anyone wanted to sell two strokes for road use to a general audience.

[J.A.W.]

...
This sort of cultural inertia would, I suspect, be a significant handicap if anyone wanted to sell two strokes for road use to a general audience.

I'm not as certain as you seem to be about that, H..

I doubt the typical young person who gets a small,cheap scooter.. even knows/cares about the technical difference..
..but does surely appreciate the 'peppy' specific output character of the 2T, which is very evident at such tiny scales..

& many of those - would be - 2T sports road-bike buyers, likely know & enjoy the hi-po 'rising torque curve' that their
2T off-road bikes produce, & will want a similarly light, but 'snappily' responsive machine, for on-road fun, too..

I know for sure, I do..

[Tommy Cookers]

'your' 72.9 hp 211 cc 2 stroke (345 hp/litre) is not (if scaled up to a 300 cc cylinder size) going to beat the 300 cc NA F1 cylinder
ie the power is in line with the port limiting effect of cylinder size eg relative to the 125 cc Aprilia cylinder

unlike the 2 strokes, NA F1 was not allowed VVT
their bit of throttle-by-wire software to 'dumb down'/smooth the power curve is surely legitimate for us, though banned in Moto GP

and (it has been said) unusual 'mouth organ' port aspect ratio and manifolding can degrade flow coefficients in novel engine layouts

[henry]

...
This sort of cultural inertia would, I suspect, be a significant handicap if anyone wanted to sell two strokes for road use to a general audience.

I'm not as certain as you seem to be about that, H..

I doubt the typical young person who gets a small,cheap scooter.. even knows/cares about the technical difference..
..but does surely appreciate the 'peppy' specific output character of the 2T, which is very evident at such tiny scales..

& many of those - would be - 2T sports road-bike buyers, likely know & enjoy the hi-po 'rising torque curve' that their
2T off-road bikes produce, & will want a similarly light, but 'snappily' responsive machine, for on-road fun, too..

I know for sure, I do..

I was thinking of larger vehicles.

I'm guessing 2T with CVT is probably the cultural norm for the scooter, moped users. I don't know, I'm far removed from them.

Do any of these have step gearboxes nowadays?

[Tommy Cookers]

presumably the stepthrough Honda type genus does (though of course the shift-linked clutch doesn't allow smooth shifts)

because the CVT likes low torque and high velocity/rpm not high torque and low velocity
the 2 wheel CVT is generally on small wheeled machines ie needing high transmission rpm and low transmission torque
(and there may be speed reducer gearing at the output (hub) end of the trans)
they are limited in torque multiplication (ratio) so the engines actually need rather flat torque curves
limited ratio means not well suited to 0 - 60 mph in 3 seconds requirement

some car CVTs now are in series with eg a 2 speed gear to match CVT torque to bigger engines

talking of shifting cultural expectations, the smaller 'proper' motorcycles now seem to have ABS as standard
and there's a lot of complaints about engine vibration (unsurprising with 13000 rpm now required for performance with green credentials)
counterbalance shafts are a compromise and cannot work perfectly, and valve motion also produces vibration
even a conventional-layout 2 stroke should be better in this respect, the lower rpm giving maybe half the vibration

[Pinger]

presumably the stepthrough Honda type genus does (though of course the shift-linked clutch doesn't allow smooth shifts)

because the CVT likes low torque and high velocity/rpm not high torque and low velocity
the 2 wheel CVT is generally on small wheeled machines ie needing high transmission rpm and low transmission torque
(and there may be speed reducer gearing at the output (hub) end of the trans)
they are limited in torque multiplication (ratio) so the engines actually need rather flat torque curves
limited ratio means not well suited to 0 - 60 mph in 3 seconds requirement

some car CVTs now are in series with eg a 2 speed gear to match CVT torque to bigger engines

CVTs are compromised. The simple self regulating (belt) type by the contrary sensitivities required to upshift and backshift, the more complex car type (steel 'belt') by the parasitic losses incurred through hydraulic control.
In the former case I believe more could be done, in the latter, probably not. Suspect DCTs are on their way out, so back to stick shifting manuals (the H gate is an anachronism I'd be happy to never again encounter) or TC autos. Pity the OEMs devoted so much energy to DCTs when a bit of time spent on a sequential manual (ie, robotised conventional manual box) would by now have delivered the lightest, simplest, most efficient transmission of them all. As usual, they chased the showroom glitz to maximise the sticker price.

talking of shifting cultural expectations, the smaller 'proper' motorcycles now seem to have ABS as standard
and there's a lot of complaints about engine vibration (unsurprising with 13000 rpm now required for performance with green credentials)
counterbalance shafts are a compromise and cannot work perfectly, and valve motion also produces vibration
even a conventional-layout 2 stroke should be better in this respect, the lower rpm giving maybe half the vibration

Perhaps - if the bore spacing is kept reasonable to avoid rocking couples. But, in the garden, 4T strimmers are replacing 2T precisely due to its improved vibration characteristic. I've been curious as to why this is so and can only guess that the 4T gains an advantage from a longer con rod (though I'm not convinced this is so). No matter, where hours of use are limited by legislation, the 4T is permitted a longer cycle before resting than is a 2T.
Does anyone know exactly why the 4T is 'smoother'?