2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all.

Thank you Grunguru.



Quote from https://www.kiwibiker.co.nz/forums/show ... r/page1663 /

Frits Overmars:

“Seeing that the in-cylinder pressure of the RSA (i.e. Aprilia 125) is about 12 bar at exhaust opening.”


Quote from https://www.kiwibiker.co.nz/forums/show ... r/page1665 :

Frits Overmars:

“That would be a perfect example of history repeating itself. Suzuki used this idea in 1962; they made the third transfer port of their 50 cc works racer as high as the exhaust port, with the intention of pressurizing the crankcase. But in 1963 this third transfer port was no longer any higher than the others...”


Jamathi:

“All 'normal' 2-stroke engines are rev-limited by the blowdown.
When the blowdown becomes insufficient, at high rpm, exhaust gases enter the crankcase.
They usually cause backfiring in the carburettor, sometimes destroying even the inlet valve or reeds.
At Aprilia we could see this very clearly: the transfer passages and crankcase inside became black.”



A faster blowdown is the key for increasing the rev limit of a 2-stroke in order to provide proportionally higher power output.


The PatATE 2-stroke-design not only enables a substantially faster blowdown, but, at the same time, it can close the exhaust substantially earlier then the transfer.




Hello J.A.W.

A flat torque curve may be boring, however it is way safer for the driver / passengers on the road.

A faster overtake may prove a matter of “life-and-death”, literally speaking.

The original Honda V-TEC B16A2 1600cc engine has at 5,000rpm a deep hole in its torque curve (there is where the engine shifts between the low rpm and the high rpm cam-lobes / rocker arms). It is not surprising that after the hole, the driver feels as having more power than what he really has.



The modified pattakon roller VVA Honda Civic has a higher flat (and boring) torque curve.

The original Honda Civic can operate either with the green curves or with the red curves:



The modified to pattakon roller VVA has infinite available pairs of curves (valve lift profiles):





https://www.youtube.com/watch?v=-zzW8YkReLU

With infinite valve lift profiles you can fill all holes in the torque curve.


From another viewpoint,:
having an engine with a flat torque curve, it is quite easy to replicate an engine with holes in its torque curve,
while the opposite is not possible (unless the maximum torque required is that at the bottom of the holes).


Thanks
Manolis Pattakos

[J.A.W.]

Hello J.A.W.

A flat torque curve may be boring, however it is way safer for the driver / passengers on the road.

A faster overtake may prove a matter of “life-and-death”, literally speaking...

Thanks
Manolis Pattakos

Hi Manolis..

A rising torque curve does of course provide a shorter (safer) overtaking time, since the hp/acceleration force is also rising at a quicker rate - than a flat torque curve can possibly provide..

"Holes", dips & other tuning problems as may show up in the powerband/dyno chart - are quite another issue..

[manolis]

Hello J.A.W.

The meaning of the flat torque mentioned, is not to cut the peaks of a typical torque curve in order to make it flat, but to raise the entire curve at its maximum.

Any other (non flat) torque curve cannot achieve the same acceleration.

When Toyota tried to replicate / to copy the VTEC of Honda (with their Corolla VVT-i, I think) the hole in the torque curve (at the transition point) was so deep that, according the testers-journalists, the car was dangerous for the typical driver, because for the average driver it takes a lot of crucial time to get out of a deep torque hole.

The lack of torque at specific conditions (i.e. the deep holes in an, otherwise, strong torque curve) was a lethal characteristic of the Bimota V-Due 500 2-stroke, that, after killing several motorcycle drivers, caused the bankruptcy of the famous Italian company.



Here is the port map of the famous Aprilia RS250 of 1999 (two cylinders in V90, oversquare design with 56mm bore and 50.6mm stroke):



The porting is focused on the faster blowdown: the “peak” (?) exhaust duration is 209.3 degrees and the “typical” (?) exhaust duration is 193.4 degrees, while the duration for the side transfer ports is only 129.6 degrees and the duration for the boost port is only 127.3 degrees.

The transfer is not only 64 degrees narrower than the exhaust, but its maximum (at the BDC) is not bigger than the maximum of the exhaust.


The above way of presenting the porting of a 2-stroke does not fit with the asymmetrical porting of the PatATE.


Here is a different presentation of the porting of a, say, modified to PatATE Aprilia RS 250:



The ports of the Aprilia RS250 are shown by black line.

The red triangular is the exhaust of the PatATE RS250, the blue triangular is the transfer of the PatATE RS250.


Going from top to bottom:

The exhaust of the PatATE starts opening substantially later (at, say, 87 degrees before the BDC) than the exhaust of the original RS250 (which opens at 105 / 97 degrees before the BDC).

The (red) exhaust opens at a higher rate than in the RS250.
Soon the exhaust area of the PatATE gets larger than the exhaust area of the RS250 (and as mentioned before, the Aprilia RS250 is focused on a larger exhaust area for the sake of a faster blowdown).

Some 20 degrees later the transfer (blue) of the PatATE starts opening at a very slow rate, giving time to the blowdown to complete. At the angle wherein all the transfer ports of the original RS250 open, the area of the transfer of the PatATE is still quite small, with a substantially smaller rate of increase.

At the BDC the rate of exhaust closing and the rate of transfer opening of the “PatATE” are about the same.
At the BDC the area of the exhaust port is still larger than the area of the original RS250 exhaust port.

As the piston moves towards the next (lower in the diagram, see the beige arrows) TDC, the transfer of the PatATE strengthens and the exhaust weakens / reduces until it closes completely.

For another 20, or so, degrees the transfer of the PatATE remains open to complete the transfer, while the exhaust is closed.

According the plot, the transfer remains open several degrees later than the transfer of the original RS250; also the exhaust of the RS250 remains open several degrees after the closing of the transfer of the PatATE.



On the same diagram, going from left to right is like moving along the periphery of the cylinder, with the zero angle being at the center / middle of the exhaust ports.

The exhaust of the RS250 covers, at maximum, about 120 degrees on the periphery of the cylinder, the transfer of the RS250 covers the rest cylinder but has, necessarily, substantially shorter height.

The exhaust and the transfer of the PatATE extend, each, for some 180 degrees along the periphery of the cylinder, but they are substantially asymmetrical relative to the BDC: the exhaust is significant before the BDC (blowdown, upped half of the plot) while the transfer is significant after the BDC (filling of the cylinder by the compressed gas in the crankcase, lower half of the plot).
The height of the transfer and the height of the exhaust are equal.



It is a long post.

However this is a different and useful way to present the porting (and not only) of a 2-stroke engine.

Even for the conventional 2-strokes, in a diagram like this one can put the relative pressures (or temperatures, or gas velocities etc) without the need to explain whether they refer to the downwards motion of the piston (expansion stroke) or to the upwards motion of the piston (compression stroke).

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis..

A rising torque curve enables an exponential increase in acceleration, & that's handy for overtaking..

Here a couple of 'in metal' tests - showing how well even a sans-valves, basic piston-port 2T (thus 'symmetrical')
is amenable to fairly simple, inexpensive tuning development - based on flow improvement..

The harmonic pulse characteristics utilizing well designed exhaust systems are, clearly - of value..

http://www.kawatriple.com/performance/p ... ompare.htm
http://www.kawatriple.com/performance/s ... ckvs39.htm

Per your view on the reasons for the failure of the Bimota Vdue, that's the 1st I have heard of crash liability issues..
..my understanding is that it was an design engineering/manufacturing problem, stemming from the fact
that Bimota had no previous experience in engine building..

Today the faults which caused running problems - are well known & have been solved, a bit too late, sadly..

[manolis]

Hello J.A.W.

You write:
“A rising torque curve enables an exponential increase in acceleration, & that's handy for overtaking..”


A rising torque curve enables a linearly proportional acceleration.

More correctly: the ratio of the force pushing forwards a car or motorcycle, to the torque provided by the engine, is constant and depends on the selected gear ratio in the gearbox.

In the RoadLoad DOS program at http://www.pattakon.com/pattakonEduc.htm the basis for the calculations is the torque curve and the overall gear ratios:



The torque curve at the bottom of the above image is the same with the "engine force" pushing forwards the motorcycle (top of the image).

The acceleration results by subtructing from the "engine force" the overall resistance force, and then by dividing with the total mass of the motorcycle.


Aren't we say the same thing with diferent words?

Thanks
Manolis Pattakos

[Tommy Cookers]

the so-called 'flat' torque curve allows a car, once moving, to be driven essentially by use of the 'accelerator' alone and so is attractive
http://www.automobile-catalog.com/curve ... _axle.html
RETRO-EDIT - the above plot seems synthetic and actually pessimistic at the bottom end and the top end !
real data for XK150S (various mph 3rd and top gear) has 176 lbft at 617 rpm, 186 @ 1234, 200 @ 2468, 207 @ 3085, 210/1 @ 4080/5610)
this XK150S engine is exactly the same as the XK 'E' type's
here we can see available torque after mechanical and aerodynamic losses (and so acceleration) is nearly constant over the speed range
('Motors' 3.8 litre E type test car ran from 6 mph in top gear (NB not 150 mph o/drive top) taking c.6 sec for each 20 mph increment to c.120 mph)

of course gear shifting is available if even greater acceleration in the lower speed range is desired
but (with the same max power) a rising torque curve will demand gear shifting for adequate acceleration and/or climbing a slope

presumably the 2 stroke could give a 'flat' torque curve - what exactly is the compelling reason against this ?

granted today, CVT or the CVT-emulating 8 or 9 speed automatic makes acceptable otherwise inferior torque characteristics

[Tim.Wright]

presumably the 2 stroke could give a 'flat' torque curve - what exactly is the compelling reason against this ?

I think that for any engine/motor (electric or combustion) you can flatten the torque curve by sacrificing peak power. This is essentially the trade-off between straight line performance (which depends mainly on peak power) and driveability (which depends largely on the 'flatness' of the torque curve).

Sacrificing peak power on an engine which is 'peaky' (by architecture) will by definition leave you with a flat curve of little useable torque. I.e. the performance loss is too great for the driveability gain.

Obviously, given 2 engines with identical peak power but one with a flat torque curve and the other with a rising torque curve, you are going to accelerate much faster with the constant torque motor.

[godlameroso]

I can easily tune out the VTEC hump. I do it regularly. All I need is just a stand alone ECU.

[J.A.W.]

Obviously, given 2 engines with identical peak power but one with a flat torque curve and the other with a rising torque curve, you are going to accelerate much faster with the constant torque motor.

Not necessarily T.W., see the dyno charts linked below.. & compare the two Kawasaki machines..
http://www.kawtriple.com/mraxl/articles ... bikes2.htm

The 4T 900/4 develops nearly identical peak hp to the 2T 750/3, via flat torque, spinning @ higher rpm..

As for actual on road effects: https://www.youtube.com/watch?v=1N5rm0glYwY

[J.A.W.]

...presumably the 2 stroke could give a 'flat' torque curve - what exactly is the compelling reason against this ?

Indeed T-C, here is a dyno plot of a Hirth engine, another 2T 3cyl, & comparatively softly tuned, for flight..



So, no "compelling" technical reason, rather its a matter of marketing/target application..

For motorcycles, the Suzuki 2T 750/3 was offered with a similar soft tune, as a 'grand tourer'..

Whereas the peppier Kawasaki 2T 750/3 was marketed - on its sporty/exciting - performance delivery..
..even though it was still fairly quietly tuned, as stock, & well able to cruise via a wide rpm range in top gear..

[gruntguru]

Hi Manolis..
A rising torque curve does of course provide a shorter (safer) overtaking time, since the hp/acceleration force is also rising at a quicker rate - than a flat torque curve can possibly provide...

Sorry JAW, I must take issue with that. Although a rising torque curve will result in higher power (same engine, same average torque) . . .

For single gear acceleration, the flat torque curve will get the overtaking done sooner since acceleration at the beginning of the manouvre is more important than acceleration in the middle or at the end. (Again this assumes average torque is the same for both cases.)

The rising-torque engine will provide better acceleration through the gears provided the ratios are close enough to keep the revs in the region where it has more torque than the other engine.

[J.A.W.]

Hi Manolis..
A rising torque curve does of course provide a shorter (safer) overtaking time, since the hp/acceleration force is also rising at a quicker rate - than a flat torque curve can possibly provide...

Sorry JAW, I must take issue with that. Although a rising torque curve will result in higher power (same engine, same average torque) . . .

For single gear acceleration, the flat torque curve will get the overtaking done sooner since acceleration at the beginning of the manouvre is more important than acceleration in the middle or at the end. (Again this assumes average torque is the same for both cases.)

The rising-torque engine will provide better acceleration through the gears provided the ratios are close enough to keep the revs in the region where it has more torque than the other engine.

Your final paragraph gg, hits the nail on the head - naturally, sporty engines get sporty gear sets..

As for acceleration being "more important" at the beginning for gaining distance "sooner",
..many a race has been won by that exponential top-end rush - as gifted by a rising torque curve..

[J.A.W.]

Hello J.A.W.

You write:
“A rising torque curve enables an exponential increase in acceleration, & that's handy for overtaking..”


A rising torque curve enables a linearly proportional acceleration.

More correctly: the ratio of the force pushing forwards a car or motorcycle, to the torque provided by the engine, is constant and depends on the selected gear ratio in the gearbox.

In the RoadLoad DOS program at http://www.pattakon.com/pattakonEduc.htm the basis for the calculations is the torque curve and the overall gear ratios:

http://www.pattakon.com/tempman/roadload1.gif

The torque curve at the bottom of the above image is the same with the "engine force" pushing forwards the motorcycle (top of the image).

The acceleration results by subtructing from the "engine force" the overall resistance force, and then by dividing with the total mass of the motorcycle.


Aren't we say the same thing with diferent words?
Thanks
Manolis Pattakos

Hi Manolis..

Your provided data is misleading/invalid - since the machine measured is only capable of 105 km/h...

Note the time listed as taken to accelerate from 100-to-105 km/h - clearly the wee bike has run 'over-the-top'..

..gearing-wise..& the subsequent time-to-distance numbers are thus flat..

Try that exercise again with a 125cc G.P. bike, & you'll get proper/valid results..

[manolis]

Hello J.A.W.

You write:
“Your provided data is misleading/invalid - since the machine measured is only capable of 105 km/h...”

The data used for the graph were provided, some 20 years ago, in a technical analysis of the KA125KX in a magazine.

The maximum speed is not the theoretical maximum speed of the motorcycle; it is restricted by the rev limit of the engine in combination with the short transmission ratios.

So, give me the maximum speed of this motorcycle (26PS peak power, case wherein the final transmission gets as long as necessary) in order the RoadLoad program to recalculate everything and compare the differences.



You also write:
“Try that exercise again with a 125cc G.P. bike, & you'll get proper/valid results..”

The required data:

1. the dyno graph of your preferable 125cc G.P. motorcycle.
2. the primary transmission ratio.
3. the gear ratios in the gearbox
4. the final transmission ratio (or the number of teeth of the sprockets)
5. the working diameter of the rear tire
6. the top velocity of the motorcycle on a flat road.

Give me the above data, and the RoadLoad program will make the graphs and calculate the maximum possible accelerations, the points wherein the best gear-shifts should happen, etc,etc.

Thanks
Manolis Pattakos

[gruntguru]

If your graph were to show the constant-max-power thrust curve (the curve followed by an ideal CVT at WOT - a hyperbola touching the tops of the individual gear thrust curves) it would be easy to answer the "top speed" and "top speed gear ratio" questions.