2 stroke thread (with occasional F1 relevance!)

[manolis]

Hello all.


The optimum bore to stroke ratio for the 2-strokes is 1.0 or very close to it (square design).

The four strokes (say, the Ducati Panigale 1299) use substantially bigger bore to stroke ratios (Panigale: 116mm bore, 60.8mm stroke, bore to stroke ratio 1.91).
Bigger bore means bigger valves and so freer breathing and more power at higher revs, it also means smaller inertia loads, lower friction, etc.
In Ducati they exploited their key advantage (the desmodromic valve train) and pushed farther the limits of the oversquare design.
20 years ago a sport motorcycle having more than 2.0 bore to stroke ratio would seem a bad choice, a mistake; today it seems quite reasonable (the expensive Panigale 1299 uses 1.9 bore to stroke ratio and is regarded one of the best engines).


How the previous are translated in specific power? (say, ratio of power to displacement, like PS per cc)


To make it more specific.

Take a 125cc 2-stroke. Square design means: bore and stroke are both equal to 54.2mm). Even in racing 2-strokes the rule is: square design.

A 125cc 4-stroke having the same bore to stroke ratio with the Panigale 1299, has 67.2mm bore and 35.2mm stroke.

A basic limitation for both engines is the mean piston speed of the piston.
25m/sec mean piston speed is the typical limit for both of them.

This means a rev limit of 13,800 rpm for the square 125cc 2-stroke, and a rev limit of 21,300rpm for the oversquare 125cc 4-stroke.


And how this translates into power?


The power equals to the torque times the revs.

A 2-stroke has one combustion per cylinder per crankshaft rotation, while the 4-stroke has one combustion per cylinder per two crankshaft rotations.

If the torque of the 2-stroke 125cc were double than the torque of the 4-stroke, the lower revving 2-stroke would make only 30% more power than the 4-stroke (2*13,800/21,300=1.3) at the selected rev limit (25m/sec mean piston speed).

However the torque of the 2-stroke is less than two times the torque of a an equal capacity 4-stroke.

For instance:

the peak torque of the Panigale 1299 (1285cc) is 144.6Nm at 8.750rpm (11.4mKp / lit of displacement). At peak power (205PS at 10,500rpm, 21.3m/sec mean piston speed) the torque is only 5% lower than the peak torque,

while the peak torque of the Rotax E-TEC 800cc 2-stroke is, according Rotax, 123Nm (15.7mKp / lit of displacement), i.e. it is not 200% but only 145% of the specific torque of the 4-stroke Panigale.

Supposing the 125cc 2-stroke has the same specific torque with the Rotax 800 E-TEC 2-stroke, supposing also that the oversquare 4-stroke has the same specific torque with the Panigale 4-stroke, the calculation becomes:
1.3*145/200=0.94=94%,
which means some 6% fewer peak power from the 125cc 2-stroke than from the high revving 125cc 4-stroke.


There are 2-strokes making more specific torque than the Rotax 800 E-TEC.

However, with an even bigger than Panigale’s bore to stroke ratio (for instance 2, 2.25, 2.5) the four stroke can have even freer breathing and can rev at even higher revs without exceeding the 25m/sec mean piston speed limit.

For instance, a 125cc 4-stroke with 2.43 bore to stroke ratio (73mm bore, 30mm stroke) and with a PatRoVa rotary valve (no rev limit cylinder head, freer breathing, compact combustion chamber) could run at 25,000rpm (25m/sec mean piston speed). If it makes the same specific torque with Panigale, it would make:
0.125*11.4*25*1.4 = 50PS.

With two pistons sharing the same crankpin:



(here the bore to stroke ratio is as in the Panigale: only 1.91;
spot on the single piece casing, on the single piece crankshaft and on the single piece connecting rods; even so, the assembly is possible and easy)

a 250cc V-2 at 90 degrees 4-stroke with 100PS could be made.


If a higher mean piston speed is allowed (Ryger claims 17,500 rpm at peak power, which means (for 54mm stroke) 31,5 m/sec mean piston speed), the peak power of the 125cc oversquare 4-stroke increases proportionally (from 50PS to 31.5/25*50=63PS).


Thoughts?

Objections?

Thanks
Manolis Pattakos

[gruntguru]

I would think the upper limit of B/S ratio for racing four stroke engines is dictated by compression ratio and combustion chamber geometry. With the pent-roof chamber, both become severely compromised as B/S approaches 2.0.

Valve operation also becomes difficult - increasing B/S means heavier valves and higher rpm - possibly one reason Ducati (with their desmodromic valve gear) is one of the few to successfully approach 2.0. Desmo valve gear also permits higher valve accelerations and velocities, which means similar breathing with less duration (particularly overlap) is possible. This takes us back to chamber geometry and CR which can both benefit by reducing the size of valve-pockets in the piston crown.

The best solution for extending B/S ratio beyond 2.0 is an alternative valve arrangement like the PAT rotary valve. This offers rotary-valve breathing, a clean piston crown (high CR) and a compact combustion chamber. It won't breathe as well as the Bishop rotary valve but wins on every other count. Manolis - it is worth thinking about how to maximise swirl in the design.

[J.A.W.]

Hi Manolis,

I doubt that many Ducati Panigale bikes spend too much time at those high rpm levels.
( other than in competition superbike racing, where they have not been very successful).
In road use, the large displacement means the relatively poor low speed specific torque output is masked.

As g-g pointed out, very high rpm 4Ts require expensive/short TBO components, & is one of the criticisms of
high rpm 4T bikes in 250cc MX competition, & also why F1 went to lower rpm, but high-pressure, turbo-compound mills.

Here is the last 2-stroke road bike equivalent to the Panigale, the Bimota Vdue 500cc V-twin from ~20 years ago.
It was ( AFAIR) over-square dimensionally, similar to the `70's era Kawasaki H2 2-stroke 750 triple at 71 X 63 mm,
yet even in road tune the Bimota would give 100+ hp at the rear wheel.

The other inherent advantages of 2-strokes, low mass/high power density/low inertia for good throttle modulation & etc, remain.

http://www.odd-bike.com/2012/11/bimota- ... imota.html

Curiously enough, the 250 MX 2Ts are undersquare, typically 66 X 72 mm, but they only make about 200 hp/ Ltr @ 8,500 rpm.


Bimota 500 dyno run, 103 PS/80NM.

https://www.youtube.com/watch?v=rK7GUJht8AM

[Brian Coat]

An under-developed bike which certainly could not claim "good throttle modulation" but what a lovely thing, none-the-less! Thanks for posting.

[manolis]

Hello Gruntguru.

The architecture of the Bishop rotary valve puts some limitations as the bore to stroke ratio increases.

With the same mean speed for the piston rings and for the rotary valve sealing means, the relation of the external diameter D of the Bishop rotary valve with the piston stroke S is:

D= S * 4/pi

Suppose that the “intake duration” T is 300 crank degrees (reasonably it is less even for extreme revs).

If f1 is the angle occupied on the periphery of the rotary valve by the “intake port” and f2 is the angle (about the rotation axis of the rotary valve) of the window in the roof of the combustion chamber (f2 is less than, or equal to, f1) then the duration T is:

T= f1 + f2

With f2=f1 the “window” area maximizes and the duration T gets 2*f1.

With T=300 crank degrees, the f1 is 150 crank degrees (i.e. 75 degrees around the rotary valve periphery, i.e. the width W of the intake port on the Bishop rotary valve is:

W = D * sin(75/2) = S * (4/pi) * sin(37.5) = S * 0.78

With a length of the intake port on the rotary valve equal to, say, 83% of the bore B
(and the same length for the window on the cylinder head), the window port area P is:

P = B * 0.83 * W = 0.65 * B * S = 0.65 * r * S^2 , wherein r is the bore to stroke ratio.

The P is the area of the rectangle hole at top right:



For constant cylinder capacity, the product S * (B^2) remains constant,

so the S * (S*r)^2 is constant,

so the r^2 * S^3 is constant, say C0,

so S = CubicRoot (C0 / r^2).

I.e. P = 0.65 * r * (CubicRoot (C0 / r^2))^2 = 0.65 * CubicRoot(C0/r) = C1*r^(-1/3), wherein C1 is a constant.

So, the P is inversely proportional to the CubicRoot of the Bore to stroke ratio.

For instance, for r=2.5 and for r=1.5 the ratio of the resulting port areas is: (2.5/1.5)^(-1/3)=0.84
This means that for constant cylinder capacity, the area of the window at the top of the combustion chamber of the Bishop rotary valve decreases substantially as the bore increases.


For instance:

with Bore 90mm, Stroke 60mm (i.e. r=1.5 and 382cc cylinder capacity) and 30m/sec mean piston speed and 30m/sec mean speed for the sealing means of the rotary valve (which means 15,000rpm), the external diameter of the Bishop rotary valve is D=76.5mm and the window area is P=34.9cm2 (46.8mm x 74.7mm).

while,

with Bore 106.7mm, Stroke 42.7mm (i.e. r=2.5 and, again, 382cc cylinder capacity) and 30m/sec mean piston speed and 30m/sec mean speed for the sealing means of the rotary valve (which means 21,000rpm), the external diameter of the Bishop rotary valve is D=54.4mm and the window area is P=29.4cm2 ( 33.3mm x 88.5mm), which is 16% lower than in the previous case wherein the bore to stroke ratio was 1.5.


Here there is another interesting thing revealed by the geometry:

with 54.4mm external diameter of the Bishop rotary valve, and, say, an inner diameter of 48mm (which means only 3.2mm width of the external walls of the Bishop rotary valve), the port area at the entrance of the rotary valve is only 18cm2, i.e. 39% less than the window on the combustion roof! (and 49% smaller than the 34.9cm2 window area in the case with the bore to stroke ratio 1.5)



With constant cylinder capacity, we decrease the stroke in order to increase the rev limit of the engine and so to get more power; the higher rev limit for a specific cylinder capacity requires freer breathing, i.e. bigger window area.

But in the case of the Bishop rotary valve what happens is the opposite: as the bore to stroke ratio increases, the window area reduces and the breathing worsens, which means the Bishop rotary valve is not good for engines having extreme bore to stroke ratio.


Worth to mention: during the combustion the piston rings move slowly while the rotary valve sealing means of the Bishop rotary valve move quickly.


Please do the calculations by your own and let me know if there is any mistake.

Thanks
Manolis Pattakos

[manolis]

Hello.

The V-4 motoGP Ducatis use crankshafts like:



(stereoscopic animation; more at http://www.pattakon.com/pattakonStereoscopy.htm )

The crankpins are arranged at 0, 0, 90 and 90 degrees (two double crankpins at 90 degrees angular distance from each other) about the crankshaft rotation axis.


In comparison with the two conventional crankshafts used in the V-4 engines (the one with crankpins arranged at 0, 0, 0 and 0 degrees around the crankshaft rotation axis (i.e. coaxial crankpin axes) and the other with crankpins arranged at 0, 0, 180 and 180 crank degrees (i.e. two double crankpins at 180 degrees angular distance from each other)), the "0, 0, 90 and 90" crankshaft of the stereoscopic animation eliminates the free inertia forces of the engine without significant side effects.


The unbalanced inertia force in a V-4 with conventional crankshaft is 70% of the unbalanced inertia force in the typical straight four (in-line-4 with plane crankshaft having crankpins at 0, 180, 180 and 0 degrees), i.e. the unbalanced inertia force in the typical V-4 is quite strong. With the “0, 0, 90 and 90” crankshaft, the unbalanced inertia force of the V-4 is more than 50 times smaller.

The previous are calculated supposing same pistons, same piston stroke, same connecting rods.

For instance:
if the conventional I-4 has at 10,000rpm an unbalanced inertia force of 1,400Kp (3,080lb),
at the same revs the unbalanced inertia force of the conventional V-4 is 1,000Kp (2,150lb) either it uses the “0, 0, 0 and 0” or the “0, 0, 180 and 180” crankshaft,
while at the same revs the unbalanced inertia force of the V-4 with the “0, 0, 90 and 90” crankshaft is less then 20Kp (44lb).

The in-line-4 with the cross-plane crankshaft of the Yamaha M1 (and R1) needs an external balance shaft rotating with the crankshaft speed at opposite direction, otherwise it suffers from an unbalanced strong inertia moment. An additional balance shaft adds weight and friction.


The yellow color on the piston crowns in the animation indicates combustion.
As shown in the animation, the minimum crank angle interval between the combustions in the two cylinders of each bank is 270 crankshaft degrees (allowing a common central exhaust passageway in each bank, as at http://www.pattakon.com/PatRoVa/PatRoVa ... _cover.gif ) .


For extreme revving it is required an extreme bore to stroke ratio (in the big Panigale this ratio is already 1.91; why not 2.5 or 3.0?).

A V-4 PatRoVa with “0, 0, 90 and 90” crankshaft (short and strong crankshaft, fewer crankshaft journals than in the I-4, elimination of the unbalance inertia force without adding external balance shaft, compact and lightweight cylinder heads without mechanical rev limit etc) seems interesting.

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis, as evidenced by both current F1 & recent undersquare B X S dimension/forced aspiration road cars,
- it has been shown that using extreme rpm as way of getting 4-strokes to do useful work is self-defeating.

The cost of the high quality components necessary for reasonable TBO, & poor low rpm torque mitigate against them.

For example, the high rpm 600/4 cylinder 4-stroke road bikes, ordinarily robust in everyday use, become very short-lived,
- when subjected to race-tune/high rpm stress, lasting only ~600 Km, so not even as long - as the oil in their crankcases.

2-strokes, by dint of the basic fact of making a power-stroke twice as often, can readily match the power of such high
priced/extreme rpm naturally aspirated 4-strokes at a lower tune level, be more compact, lighter, & less costly.

Honda found this out with their failed attempt to compete in 500cc G.P. racing, using such extreme 4T- tech.
They had to have the rules changed to double the 4T swept volume allowance & enable a softer tune, to succeed.
( Honda have also built motorcycle V4's in both 180` & 360` crankshaft types, but found inline 4's cheaper to do).

I would be interested, though Manolis,
- in your view as to the dynamic/inertia torque implications of converting a Yamaha inline 4cyl 2-stroke ( TZ 500/750),
from the standard 180` crankshaft configuration ( where 2 cylinders must fire together),
to a 90` - 'crossplane' - crank, where each cylinder will discretely fire every 90` - instead.

Ta, in advance.

[manolis]

Hello J.A.W.

You write:
“Hi Manolis, as evidenced by both current F1 & recent undersquare B X S dimension/forced aspiration road cars,
- it has been shown that using extreme rpm as way of getting 4-strokes to do useful work is self-defeating.
The cost of the high quality components necessary for reasonable TBO, & poor low rpm torque mitigate against them.
For example, the high rpm 600/4 cylinder 4-stroke road bikes, ordinarily robust in everyday use, become very short-lived,
- when subjected to race-tune/high rpm stress, lasting only ~600 Km, so not even as long - as the oil in their crankcases.”


It is interesting that every time they see a threat coming (like the Bishop rotary valve in F1, for instance), they change the rules / laws of the “game”.


As explained in previous posts, the actuation of the poppet valves is the main obstacle / problem for making high revving engines having extreme bore to stroke ratios.

The fact that Ducati with their Desmodromic system pushed the “bore to stroke” ratio substantially higher (1.91:1 in the Panigale 1299) shows that this “method” works; however it requires a special valve train to actuate the poppet valves.

The most expensive motorcycle (US70,000$), the Panigale Suprleggera, uses titanium in the exhaust valves, too. Its rev limit is set at 12,500rpm (i.e. 25.3m/sec mean piston speed).

The high revving capacity does not mean that the driver has to keep the engine at extreme revs. Only when a lot of power is required, only then the engine runs at the extreme revs.

For instance, the modified to “pattakon VVA roller-version” Honda Civic 1,600cc (B16A2) runs nicely at low revs (in city traffic it is not required more than 2.0 - 2.5mm valve lift; the engine idles at 340rpm; the same engine turns to racing (12mm intake valve lift, lots of overlap, rev limiter set at 9,000rpm) any moment the driver presses deeply the gas pedal; Youtube video at https://www.youtube.com/watch?v=-zzW8YkReLU )
The torque curve is so flat that the driving at normal speeds soon gets boring as compared with the original Honda Civic 1,600cc wherein a deep hole at the transition point between the high-rpm mode and the low-rpm mode makes it operate as a 2-stroke.
More at http://www.pattakon.com/pattakonRoller.htm and http://www.pattakon.com/pattakonVtec.htm



I prefer it from a turbo making similar power.

Ducati also prefers their own way (Desmo) than using a turbo in their motorcycles.

On the other hand, do you see any reason for not combining a turbo-charger with a high revving engine?
For instance a Panigale Superleggera with a turbocharger?
Or a Bishop rotary valve F1 engine with a turbocharger?

There is solution to this “problem” of the conventional engines, too: a rule ordering that the racing engines are restricted to operate below, say, 15,000rpm!
With such stupid laws, eventually the racing F1 cars will be slower than sport cars made to go fast.


So the question turns to:
Why not to combine a PatRoVa cylinder head (which, among others, has not a mechanical rev limit) with extreme bore-to-stroke ratio (for reliable extreme revs) and a turbocharger on the same engine?



You also write:
“I would be interested, though Manolis,
- in your view as to the dynamic/inertia torque implications of converting a Yamaha inline 4cyl 2-stroke ( TZ 500/750),
from the standard 180` crankshaft configuration ( where 2 cylinders must fire together),
to a 90` - 'crossplane' - crank, where each cylinder will discretely fire every 90` - instead.”

The 4-cylinder in-line two-stroke turns to a dynamically “perfect” engine if the plane crankshaft is replaced by a cross-plane one (crankpins at 0, 90, 270 and 180 degrees around the crankshaft rotation axis) and a counter-rotating first order balance shaft is added:

The total unbalanced inertia force is eliminated.
The total unbalanced inertia torque is eliminated.
The total unbalanced inertia moment is eliminated.
The “gyroscopic rigidity” decreases due to the counter-rotating balance shaft.
And the engine becomes even-firing.

The smoothness of such an engines is similar to the smoothness of the best V-8 engines.

The only drawback I see is the added weight, cost and friction (counterbalance shaft and gearwheels for its driving)

Thanks
Manolis Pattakos

[J.A.W.]

Thanks Manolis.
As it happens, the Yamaha TZ inline 4 2T incorporates a 'jackshaft' between crank & gearbox,
- so this could be utilized for balance-shaft duties as well.

As for the complications & cost of turbo motorcycles, perhaps a 1/2 displacement Ducati could do, as was the case in the past,
with the Japanese makers, but there have been no new ones in the last 30 years ( 'cept the very expensive mech-supercharged Kawasaki).

Honda of course, with their corporate 4T fetish, wish they could make a turbo/dry sump/Ti component chain saw
to fulfill their line up of agri-business equipment, & replace 2T's - but cannot ( of course) make the business case - on cost grounds.

[Brian Coat]

For a given rated power output, the advantages of lower speed, more boosted vs higher speed, less boosted could include engine mechanical friction and noise?

[manolis]

Hello J.A.W.

The balance shaft has to rotate with the speed of the crankshaft at opposite direction.
Is the “crankshaft to jackshaft” transmission ratio 1:1 ?



Ducati can make the power of the Panigale with half capacity and a turbocharger. Also without Desmo.
However, what characterizes Ducati motorcycles today is their high revving big cylinders, which requires extreme bore-to-stroke ratios, which requires their Desmo system in the cylinder heads.

Ducati is challenging the rest engine makers saying: “Can you do the same?”

However, Ducati has reached their own limit.
1.91 bore-to-stroke ratio seems too much even for the Desmodromic valve train.
The “small” poppet valves of the 1199 Panigale were used in the “big” 1299 which has 4mm bigger bore.

The PatRoVa rotary valve design removes this limitation of the poppet valve engines allowing as high bore-to-stroke-ratios as desired.
The “no rev limit” characteristic of the PatRoVa rotary valve is challenging the engine makers (including Ducati) saying: “Can you do the same?”



As for the simplicity, low-cost and lightweight of the 2-strokes, think that the PatRoVa adds one only moving part (or none if you consider that a decent conventional 2-stroke needs a reed valve or a “rotary” valve).
With extreme bore-to-stroke ratio and way higher revs (the mean piston speed remains normal) the PatRoVa can make more specific power than the 2-strokes.

For instance, most racing 125cc 2-strokes use square design: 54mm bore, 54mm stroke.



Think of a racing 4-stroke PatRoVa 125cc having 76.5mm bore and only 27mm stroke, i.e. half piston stroke than the 2-stroke (which gives a bore-to-stroke ratio 2.8:1).

Thanks
Manolis Pattakos

[J.A.W.]

Actually Manolis..
..with the Panigale, Ducati is attempting to win another World Superbike ( production-based) title with a twin,
- which has been their tradition for a few decades now, but the other makers find the 1000cc 4 cylinder mills do the winning.

Here is a TZ 750 shown split, the transmission jackshaft is visible, it'll need some work to provide a counter-balance function.




Its B X S dimensions were 66 mm X 54 mm.


Interesting picture of the semi-mythical 'Ryger' there too, Manolis, no further data available from them yet, AFAYK?

Your claims to be able to beat the X 2 power stroke advantage of the 2T by doubling the rpm appears equally ambitious,
perhaps if you build a PatRova 4T chainsaw & ask for a Honda R & D technical test/review?
( & I still don't get why you would want to give that fundamental 2T advantage away, if the PatRova is capable of 2T function)

Here is a recent partial appraisal of current 2T - 'state of the art' - by technical doyen Kevin Cameron,
& he makes no mention of the 'Ryger' either..

http://www.cycleworld.com/another-path- ... ter#page-3

[J.A.W.]

For a given rated power output, the advantages of lower speed, more boosted vs higher speed, less boosted could include ( reduced) engine mechanical friction and noise?

Efficiency issues which are meaningful to current road car makers, to be sure - as well as for F1 , B-C,
( & even if the reduction of 'noise' levels - may be perceived as a -ve ambient factor in F1 - by some fans).

[Brian Coat]

Here is a question.

The pre-chamber ignition is a popular idea on the F1 engines and is all ready applied on some two strokes, so why is it not seen/proposed on high speed 2-strokes?

[Tommy Cookers]

.... converting a Yamaha inline 4cyl 2-stroke (TZ 500/750), from standard 180` crankshaft configuration (2 cylinders fire together),
to a 90` - 'crossplane' - crank, where each cylinder will discretely fire every 90` - instead.

there would probably be a difference in cylinder filling ie powerband (if induction tracts were to use a common airbox as normal today)

Gordon Blair wrote about this in the RE mag some years ago
showing big differences in induction pressure (and powerband) with various Ducati MotoGP V4 crankshaft configurations
and implying that this effect was the true cause of the 'big bang' myths

at these relatively low 2 stroke (TZ 500/750) rpm there is presumably relatively little vibration and 'gyroscopic rigidity'

imo the main point of a crossplane inline 4 is optimal time-spacing of the (reciprocation) inertial forces allowing a slimmer crankshaft
so less bearing friction
in a high rpm NA engine these forces will be far greater than the combustion forces

I wonder whether Kawasaki patented the Yamaha crossplane layout or the Fath/Kuhn URS layout that won WCs in the 60s