2 stroke thread (with occasional F1 relevance!)

[J.A.W.]

T-C, are you suggesting that a 2T inline 4 with a 90` 'crossplane' crank configuration ought to be able to utilize an
interlinked ( rather than a discrete carb/intake per cylinder) induction system, allowed by the sequential timing ?

My thoughts were directed to the smoother power impulses ( instead of running as paired 180`twins) being beneficial
in terms of inertia torque/power impulses at the driven tyre footprint, & throughout such transmission, both for adhesion,
& durability of components.

[J.A.W.]

This oversquare B X S V4 design features 90` firing :

http://www.aaenperformance.com/V4_racing_engine.asp

[Tommy Cookers]

T-C, are you suggesting that a 2T inline 4 with a 90` 'crossplane' crank configuration ought to be able to utilize an
interlinked ( rather than a discrete carb/intake per cylinder) induction system, allowed by the sequential timing ?

No
iirc airboxes work best with slugs of air taken by the engine in a sequence progressing from one end to the other, not hopping to and fro
though iirc MotoGP Ducati with what might be called 'big bang' cranks seemed to work well at low rpm and badly at high rpm
maybe irrelevant here

the TZ 750 etc seem to have the crankshaft in reverse rotation - this somewhat related to the inlet porting ? (as in late race twins also)
this should help mid-corner but probably hinder corner entry - there's no free lunches
the jackshaft is surely not 1:1 from the crankshaft as a 'tooth hunting' ratio is conventional (actually it runs faster than the crankshaft ???)
non-1:1 may still be beneficial to vibration (if weighted to oppose the net vibrational moments of a crossplaned crankshaft of course)
the result could reasonably be regarded as increased vibrating moment but at a much lowered frequency
rather like a sub-harmonic
so the vibrations felt at engine frequency would probably be reduced by this

of course the original machine has no vibrations at engine frequency, vibration will be at twice engine frequency (the first harmonic)
crossplaning would eliminate this (but unless a counterbalance shaft effect is added would have vibration at engine frequency)
I suspect that at 10500 rpm on a TZ750 a bit of vibration is not the most important aspect of your situation

btw
Yamaha started out using 56x50.6 and 61x59.6 for the 250 and 350 twins
later they standardised the stroke at 54 and used 54, 64, and 66.4 bore for the different capacities

[J.A.W.]

T-C, the TZ inline 4 does also utilize the jackshaft for driving the ignition, so it must 'mark time' - as such.

Crankshaft rotation opposite to direction of travel was supposed to reduce the propensity to wheel stand on acceleration
- which could be problematic - with those short wheelbase/hard torque hit/pre-electronics suite machines,
( & the 90` L-twin Ducati crank went 'backwards' too, but due to right side final drive, rather than by racing design intent ).

[Tommy Cookers]

T-C, the TZ inline 4 does also utilize the jackshaft for driving the ignition, so it must 'mark time' - as such.

this was what I assumed at first, but then found catalogues referring to ignition equipment fitting crankshaft tapers
no matter, a 1:1 jackshaft could be made as a counterbalance shaft for a crossplane crank engine - this would need 4 expansion chambers of course

but a N-N-S-S crank could also be treated this way
this would need only 2 expansion chambers and demonstrate worthwhile space saving eg for aero purposes as for a notional F1 twostroke
as rectangular (rather than round) cross section chambers should work quite well with the 2 exhaust streams
a twostroke should be viable with such a crank, due to the moderate rpm and more frequent and smaller power impulses

regarding expansion chambers, even NA, the mean exhaust pressure is above atmospheric ? - useful for an exhaust turbine
but they should also work in a turbocompound application as F1 ? - usefully, these would need less space

btw - a crossplane 4 stroke 4 .....
is surely less vibratory (without a counterbalance shaft) than the conventional 4 (without its usual 2 counterbalance shafts @ 2x rpm
and more powerful than the conventional when that has those usual cb shafts in place

[J.A.W.]

As it happens T-C, the early TZ fours used a space saving group of flattened ( rectangular cross-section) expansion
chambers,(all underneath the chassis) but it was found that the exhaust pulse-waves caused the fattened sections to flex & break up.

Noted tuner Kevin Cameron produced an alternative pair of 2-into-1 pipes ( as Konig had used also on their flat 4) for the TZ,
which were effective, but these never quite as sharp as a discrete set of individual chambers, once room was found for them.

[manolis]

Hello Tommy Cookers.

You write:
“the TZ 750 etc seem to have the crankshaft in reverse rotation - this
somewhat related to the inlet porting ? (as in late race twins also)
this should help mid-corner but probably hinder corner entry - there's
no free lunches
the jackshaft is surely not 1:1 from the crankshaft as a 'tooth
hunting' ratio is conventional (actually it runs faster than the
crankshaft ???)
non-1:1 may still be beneficial to vibration (if weighted to oppose
the net vibrational moments of a crossplaned crankshaft of course)
the result could reasonably be regarded as increased vibrating moment
but at a much lowered frequency
rather like a sub-harmonic
so the vibrations felt at engine frequency would probably be reduced by this”

No.

Any ratio different than exactly 1:1 is not working.

Suppose you have a 1% difference than 1:1 transmission ratio between
the jackshaft and the crankshaft.

This means that after 100 rotations of the crankshaft, the jackshaft
is at the same “phase” relative to the crankshaft.

This also means that after 50 crankshaft rotations the jackshaft is
180 degrees out of phase from the crankshaft.

Think about it.
The engine runs at, say, 3,000rpm, i.e. with 50 rotations per second.
Every even second the engine runs perfectly balanced.
Every odd second the engine has a burst of unbalanced inertia moment.
And the frequency of the variation is 0.5 per second.

I couldn’t think a worst “balancing”.

So the transmission ratio needs to be exactly 1:1.
If it is not, the three gearwheels (two on the crankshaft, one on the
jackshaft) need to be replaced.
If required, the final transmission has to be aligned, too, to give
the same overall transmission ratios.

From the photos in the internet the ratio seems as 1:1.

Thanks
Manolis Pattakos

[manolis]

Hello J.A.W.

The optimum bore to stroke ratio in the 2-strokes is the 1:1.

In the past they tried oversquare and subsquare designs, however for
racing applications where the peak power is what counts above all, the
bore and the stroke are about equal in the 2-strokes.

This is not the case in the 4-strokes wherein higher bore to stroke
ratios give more specific power.
The obvious limitation for not going beyond 2:1 bore to stroke ratios
in the 4-strokes (note: the famous Panigale 1,299 already uses 1.91:1
bore to stroke ratio) is the valve train.
The more you increase the bore, the bigger the valves you need.
The bigger the valves, the bigger the valve lift (otherwise the bigger
valves are meaningless).
The bigger bore comes with shorter stroke (same cylinder capacity),
which means ability for higher revs keeping the mean piston speed into
the reliability region.
However, the bigger (i.e. heavier) poppet valves with the higher valve
lift cannot follow the increase of the revs.

This problem / limitation of the 4-strokes is solved by valve trains
rid of reciprocating parts.

And with 2.8:1 bore to stroke ratio, the 4-stroke makes more power
than a 2-stroke (same mean piston speed).

I hope it is more clear now.

Thanks
Manolis Pattakos

[J.A.W.]

Hi Manolis,

Thanks for adding the jackshaft dual-duty timing analysis.. however..
..your theory about an extremely oversquare B X S ratio allowing the super-high rpm naturally aspirated 4T..
.. to thus be able to best the racing 2T for specific power output..
..is easy to state - but has never been achieved in reality, on any scale, from tiny model IC engines right up to marine mills.

& the cost of the super high quality/ short TBO recip' components necessary for such high rpm ( needed due to 1/2 time torque) is cruelling,
a point realized by the recent trend in F1 rules from high rpm limitation > forced induction/hybrid power.

[gruntguru]

Compared to 1.0 B/S ratio, at 2.8 the rpm is doubled for the same piston speed so TBO is similar. Of course a rotary valve is needed for valve train durability and freedom of combustion chamber design.

The four stroke also wins on trapped mass and EGR but loses on supercharge available to two strokes via expansion chamber exhaust.

[J.A.W.]

Compared to 1.0 B/S ratio, at 2.8 the rpm is doubled for the same piston speed so TBO is similar. Of course a rotary valve is needed for valve train durability and freedom of combustion chamber design.

The four stroke also wins on trapped mass and EGR but loses on supercharge available to two strokes via expansion chamber exhaust.

Are you sure gg, on the TBO factor regarding the forces spinning the crankshaft, ultra short stroke item notwithstanding,
at those super high rpm rates, & esp' if they are transferring gas-flow/torque/BMEP at such a high potential too?

[Tommy Cookers]

going to a b:s ratio of 2.8 (from 1) allows 'only' 67% more rpm for the same acceleration of the reciprocating parts ie piston and rods
the TBO will be driven by such accelerations and related stresses, not by piston speed

piston speed issues are not a hard limit on rpm, and thereby on power
piston etc accelerations are a hard limit on rpm (and TBO), and thereby on power
(unless we have the typical 2 stroke, whose port sizes are limiting the useful rpm to a level that's quite easy on the rods etc)

imo it is laughable to imply that reciprocation stresses as drivers of TBO are not usually dominant as limiters of power
'peak' NA F1 had a much higher piston acceleration (and much lower TBO) than Nascar, but the piston speeds were the similar
anyway, high piston speed increases frictional losses
http://www.epi-eng.com/piston_engine_te ... _to_f1.htm

Honda, having used 2 rods per piston in the 1970s NR500 in reaching a then-unprecedented b:s ratio of 1.8 (safe to 23000 rpm)
in 1983 etc apparently used a further-unprecedented b:s ratio of 2.3 in the F1 V6 turbo (90x39.2) with conventional pistons and rods

the conventional 2 stroke gains more than the 4 stroke from reducing cylinder size and increasing cylinder number
because the increase in port area relative to displacement is greater
(than a similarly-treated 4 stroke's increase in valve area relative to displacement)
such an engine can have 2 or more simultaneous-firing cylinders for each of a number of common crankcases and expansion chambers

yes, this below tells me the TZ750 jackshaft is 1:1 - it's interesting in other ways, too
http://www.cycleworld.com/2016/01/18/ya ... n-insights

btw
(re assuming that the rotary valve is beneficial to all engines)
Ducati wins in Moto GP without (according to Blair) needing to use pneumatic valve actuation
NA F1 reached a VE of 130% without even using variable-length induction systems

there was a production RV-engined car in the 1920s (and a GP one ???)
and quite recently eg from Goodwood we saw 1920s Voisin double sleevevalve road and GP cars
(also Fiat and another 2 stroke GP car existed then)

[gruntguru]

going to a b:s ratio of 2.8 (from 1) allows 'only' 67% more rpm for the same acceleration of the reciprocating parts ie piston and rods
the TBO will be driven by such accelerations and related stresses, not by piston speed

I knew someone would pull me up on piston acceleration.

(re assuming that the rotary valve is beneficial to all engines)
Ducati wins in Moto GP without (according to Blair) needing to use pneumatic valve actuation

. . . . and without needing to use desmodromic valve actuation?

NA F1 reached a VE of 130% without even using variable-length induction systems

130% is impressive. Do you know how much of that flow was trapped and how much was scavenged?
btw. Don't forget that variable-length induction doesn't improve peak VE, it just extends optimum wave tuning over a wider rev range.

(Variable induction might offer slight improvements in peak VE if the fixed induction engine design was compromised elsewhere (valve timing, port size, exhaust etc) to broaden the power band.)

[Brian Coat]

Re Vol Eff

This paper has some interesting real data.

http://www.f1-forecast.com/pdf/F1-Files ... P2_10e.pdf

Of course the in runner pressures, relative to valve events, are what drive N/A vol eff. in the >> 100% region.

[J.A.W.]

This article discusses some of the technical issues with extreme bore-to-stroke dimensions & piston acceleration.
( Also notes Ducati desmo-tech ).
http://www.grandprixengines.co.uk/note13.pdf

& from Eastern Europe, this interesting paper includes some recent Russian research into stratified charge 2T port injection.

http://trans-motauto.com/sbornik/1-2015.pdf