2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello J.A.W.

You write:
“Did you peruse the 'motorcycle analyzer' program I linked? ( see post 2nd from last, two pages back)”


Now, yes. Thanks.


For comparison, I used in the RoadLoad program “your” dyno graph of Yamaha RD350LC at page 112:



The rest data are from your link (motorcycle analyzer): http://www.motorcycleperformanceanalyze ... d-350-1987

(the maximum speed is taken as 113mph)


Here is the “output” of the RoadLoad program:



At a glance, I see a significant difference in the time required for the first 400m (1/4 of a mile); the RoadLoad gives 13.6sec, the “motorcycle analyzer” only 12sec.

Thanks
Manolis Pattakos

[Santozini]

'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

Just shimming in but if we consider approximately 750hp from the last generation of 2400cc V8s, we would be looking at 321.5 hp/litre

Additionally, the latest 125cc 2T GP bikes were putting out approximately 50hp, that is 400 hp/litre

I apologies if i missed the point, but if we are talking purely about hp/litre, 2t have the upper end when comparing to 4t NA (from what has been developed so far, bearing in mind the development of the V8s was frozen due to regulations)

[Tommy Cookers]

if we scaled the 211 cc cyl to 300 cc we would have ......
26.5% more port area per cylinder (but 70.3 % as many cylinders per litre)
ie about 89% of the gasflow per litre - so 'only' about 305 hp/litre
(allowing the same rpm despite the cylinder enlargement increasing the stroke by 12.4%)


wall-ported engines have more attainable port area per litre with smaller cylinders
and head-ported engines usually so

but in such relative port area wall-ported engines do not benefit at all from increased bore:stroke ratio .....
though head ported engines benefit greatly in port area from increased bore:stroke ratio

increased bore:stroke ratio allowing rpm increase proportionate to the square root of the b:s ratio .....
is why the 21st century F1 NA 4 stroke, profiting from a 39 mm stroke that allowed c. 20000 rpm, would not be beaten in hp/litre ......
by a conventional NA 2 stroke of the same 300 cc cylinder size whose port area would suffer at a stroke less than 67 mm and corresponding 10000 ? rpm
imo

[manolis]

Hello Henry.

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



Regarding the proximity between transfer and exhaust:

I think we must talk about spatial and temporal proximity.

In the PatATE when the blowdown is almost finished, the hybrid port at its one end starts operating as a transfer port, while the rest hybrid port operates as exhaust port.
The spatial distance (around the periphery of the cylinder) between the transfer “port” and the exhaust “port” can be as large as desirable (with the respective compromise on the port timing; for instance the transfer and exhaust “ports” can be arranged at a distance of, say, 45 degrees around the periphery of the cylinder, which is extreme; more likely, an angular distance of, say, 20 degrees around the periphery of the cylinder (i.e. about the angular distance of two successive hybrid port bridges, as in the animations) is more than necessary).

Nothing to do with the “proximity” used in the following conventional 2-stroke:



In the PatATE the large height of both “ports” (exhaust and transfer) enables not only faster blowdown but also larger port areas, which in turn allow mild port timing without spoiling the breathing efficiency or the revving capacity of the 2-stroke engine.

The better “temporal proximity” between the exhaust and transfer “ports” of the PatATE is another characteristic of its architecture:





because this way the exhaust port closes together with, or earlier than, or substantially earlier than the transfer port (it depends on the design), trapping the fresh fuel inside the cylinder.

From the above graph it appears the PatATE breathing is more like a 4-stroke with big overlap than like a conventional 2-stroke.

From the two proximities, spatial and temporal, the temporal seems having the dominant role on the changes the PatATE brings to the 2-strokes.



You also write:
“Also it seems to present less flexibility in the placement of flow of the transfer than a conventional design.”

I think not.

With the rotary valve spinning at half crankshaft speed, if desirable, two hybrid ports can be arranged anti-diametrically on the cylinder.

With faster blowdown the designer needs not to worry about the port-time-area because there is plenty of it.

Thanks
Manolis Pattakos

[manolis]

Hello Henry.

You write:
“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.”


Thanks Henry.

The US patent for the PatBox CVT is to be granted in a few weeks (as agreed with the Examiner).

The GB2,526,675 UK patent for the PatBox has been granted a year, or so, ago.



An important characteristic of this CVT is that when the driver / rider likes so, he can turn it to a “manual” gearbox having infinite, if desirable, gear ratios.


As the guy who built the first PatBox prototype (video: https://youtu.be/Ou3dBx-O-V4 ) for a 50cc moped wrote:

“Tonight I went to the store.
Coming back I had to gun it uphill to catch the yellow.
The bike just went uphill so fast compared to my other tune I had on it.
It is nice to let the gearing do the work sometimes, and let the revs work other times.”


This is the idea: to let the gearing do the work sometimes, and let the revs work other times.




You also write:
“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.”


As it is now, to shift to another gear ratio in a conventional bicycle is not so simple or easy for the average bicyclist.
Maybe this is the reason for using the same gear at all conditions.

With the PatBox CVT on a bicycle, all it takes in order to up-shift or to down-shift is a push or pull of the lever for, say, a couple of centimetres. And you have infinite gear ratios.



Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis..

I'm pleased you are getting results with your CVT work..

& thanks for posting the findings comparison of the 2 maths model programs.

I'll add this actual test result, obtained by a skilled rider, who got into the mid 12 sec range on 1/4 mile acceleration,
& tucked in well for the top-end run.. (albeit it is an earlier, lesser-powered, if evidently well-tuned 1983 RZ 350).

[J.A.W.]

'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

T-C, I posted the video as an example of 2T performance potential - still being untapped by 2T enthusiasts..

That such results are gained by 'backyard' engine tuners using converted MX machines is perhaps a tad startling..
..surely by comparison to what the pinnacle of N/A 4T race-tech achieved anyhow, with their massive resources..

The 345 hp/litre certainly beats, & by a serious margin, what Moto G.P. manages..
& if given the development facilities enjoyed by F1 engine makers, I'd wager your predictions would prove pessimistic..

[Tim.Wright]

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

Nice program. Obviously this is the best way to compare powertrains if you have the complete gearing information. How do you deal with the initial launch phase where you are grip and/or wheelie limited?

I have written a similar program which I use to calculate shift points on both real vehicles and simulation models. It doesn't model the launch and traction limited phase but for the power limited regions it will tell you what setup is faster.

I plugged in the numbers of the 4T 903 and the 2T 750 Mach IV and it explains a lot with regards to the quarter mile time. Fundamentally, the reason that these two bikes clocked a practically identical 1/4 time regardless of the 33kg weight advantage of the 2T 750 is a simple question of gearing which is obvious when you look at the tractive power diagram created using the dyno plots and gear ratio data kindly provided J.A.W. A bit of reading also uncovered the fact that the 2T was running 1/4 mile specific gear ratios and still set effectively the same ET the 4T with it's stock ratios.

In fact, looking at the data sheet on http://www.motorcyclespecs.co.za for these 2 bikes you can see that their nomiated top speed are very different due to the gearing and this is what limits the 903 4T in the low speed range as you can see below. The 2T is geared favourably for low speeds but falls short at the top end by quite some margin purely by choice of gear ratios:


If you were to regear the 903 to have the same top speed (18% shorter gearing accross the board) you can see very clearly that the superior torque curve of the 4T engine creates a wider power band and therefore is going to be simply faster and able to accelerate harder at practically any point in its rev range save for a tiny part where the 2T is briefely in its powerband:


It's pretty clear, in this case, that the weak point in the 903 concept was not at all the engine but rather the gearing for a higher top speed.

[J.A.W.]

The limitations of a purely mathematical exercise have surely been made "pretty clear", anyhow..
Back to the old text book, which discusses the Kawasaki factory data 'running performance curves' for the 2T 750/3..

"Returning to the real world, the peaked curves show available driving forces in each gear.
The lines marked in degrees show the driving force required to climb the indicated gradient at the indicated road speed,
read off the bottom axis.
These required-force curves slope upward to show the effects of wind resistance.
This shows that 1st gear can easily cope with a 40 degree slope & 5th gear will barely pull on a 10 degree hill.

The peak of each force-curve should occur at an RPM near the torque peak of the engine,
& would be exactly at the torque peak, if there were no gear-losses.

To check this, project vertically from the peak of a force curve to the RPM curve for the same gear.
Then read RPM off the RPM scale...

...The maximum speed in 5th gear on a level road, is shown to be about 130 MPH at an RPM of about 7,800.
This is well above the power peak of the engine & shows that, at 100 MPH, on a level road the machine has excess power.
We mentioned above that the bike will barely pull a 10 degree hill in 5th gear.
Look again.
The road speed at which the bike has enough push to run up a 10 degree hill is about 100 MPH."


As for the practical outcome of this 2T engine's torque advantage.. here's what Brit road tester Dave Minton found..

"The speed advantage of this machine lying solely in its immense torque spread,
allowing big throttle openings from as low as 2,000 rpm in top gear.

As this is only around 40 mph, the usefulness of such a wider power spread can be appreciated fully by
riders with experience of long, fast rides when the continual effort of gearchanging becomes a wearying
& sometimes painful chore.

In other words, the 750, once free of town can be held in top gear almost indefinitely without losing ground
during low speed acceleration.

I know full well that Vincents used to do this, & Squariels, & other oldies.
What they would not do - & never did, so forget the legends - was to cruise at the tremendously high speed of this one.

It has no limit. Top speed, which on test came out at around 125 mph, was also cruising speed."

[gruntguru]

Imagine how impressed he must have been with the torque spread of the 903.

[J.A.W.]

Imagine how impressed he must have been with the torque spread of the 903.

No need to "imagine" at all gg, since - as the dyno chart clearly shows..
& road testing confirms.. as stock standard, the 2T 750 has the 4T 900 well covered, torque-wise.. for real-world use..

[gruntguru]

Tim's first chart shows the 903 clearly has a superior torque spread and power spread. (Peak torque is an irrelevant number - think of the gas turbine vs diesel example)

[J.A.W.]

Tim's first chart shows the 903 clearly has a superior torque spread and power spread. (Peak torque is an irrelevant number - think of the gas turbine vs diesel example)

Such maths exercise confections are contrivances - that actually bear little relevance in 'real world' usage gg..

.. if you refer back to the linked test, the 4T 900/4's relative lack of useful on-road torque below 7,000 rpm is noted..
& duly compared unfavourably with the Ducati 4T 750/2, & its initially steeply rising torque curve - at useful rpm for top gear..

[gruntguru]

Tim's first chart shows the 903 clearly has a superior torque spread and power spread. (Peak torque is an irrelevant number - think of the gas turbine vs diesel example)

Such maths exercise confections are contrivances - that actually bear little relevance in 'real world' usage gg..

.. if you refer back to the linked test, the 4T 900/4's relative lack of useful on-road torque below 7,000 rpm is noted..
& duly compared unfavourably with the Ducati 4T 750/2, & its initially steeply rising torque curve - at useful rpm for top gear..

The "maths exercise" is actually a physics exercise and presents data in a way that allows the engine's contribution to performance to be evaluated at a glance.

I am not saying the 900 was faster or torquier or anything else - only that the data represented in those graphs indicates that - all else being equal - the 900 engine would produce better acceleration and ride-ability than the 750.

"All else being equal" is a useful get-out clause. The lower mass of the Mach 3, its lower gearing and lower rotating inertia of its engine would also contribute to its acceleration.

I think the main take-away is that a normalised power curve comparison is a far better metric than any other (especially peak torque).

[J.A.W.]

No gg, the "main takeaway" is that the 2T 750/3 bested the 4T 900/4 in the actual comparison.. inc' on the dyno..

Obviously - the fact that the 4T 900/4 engine is such a big heavy lump - will negatively affect its on road performance, too..