2 stroke thread (with occasional F1 relevance!)

[J.A.W.]

T-C, what you wrote then was largely incorrect, & still is, bump or no..

N/A 2-strokes with their X2 BMEP advantage inherently don't need to rev hard like 4-strokes,
& so can avoid that distorting/expensive feature of radically short B X S ratios & still keep
sensible piston speeds..

It was the rapid advent of new-fangled turbines that killed large piston aero-engines..

The Napier Sabre design was subject to a "Chinese copy" effort by R-R - in response to their
failure with the Vulture & failure to get their conventional V12 Griffon to match the Sabre's power..

R-R made a bit of a hash of their H-24 Eagle, however having snatched Whittle's turbine,
it didn't matter, & they hadn't really been too keen on the developing the Crecy, either.

[wuzak]

@wuzak
surely the Crecy was not actually rated at the levels you suggest ?
ie your quoted values are for predicted performance far greater than was ever demonstrable in sustained running ?
yes, I have the book

I doubt that they did the actual tests required for an official rating.

So I'm not sure if they are predicted or real measured values. I would have to read the book again, rather than just picking out data.

Crecy main engine No 10 recorded 1798bhp in 1944. So the ratings for the normal engine would seem to be correct.

The turbo-compound engine ratings would seem to be estimated from the bhp plus the compressor power.

[Tommy Cookers]

@J.A.W
the main point intended was that the port area relative to demand will tend to become .....
favourable in a poppet valved engine if the b:s ratio is unusually large, and
unfavourable in a cylinder wall-ported engine if the b:s ratio is unusually large

at low b:s ratios the conventional 2 stroke will have enough port area and will beat the 4 stroke for specific power
at very high b:s ratios it won't
forget the bmep and rpm, the 2 stroke needs to flow the same amount of air (or more) as the 4 stroke
yes, the current F1 mandated b:s ratio is much lower than the extremely high ratio mandated before 2014
my post was suggesting that the 2 stroke dominance would not apply at that ratio
but the current F1 ratio is a lot higher than usual for 2 strokes (except where minimal bulk is the dominant factor)

clearly the open sleeve valve (as in the Crecy) offers improved port area in a wall-ported engine
as do poppet valves combined with the usual piston-controlled wall porting

the compounded engine with a large proportion of turbine power may be the most efficient (SI) engine
not a surprise, if it has 2 matched compression stages and 2 matched expansion stages
but F1 rules are written to prevent things with these proportions

[J.A.W.]

T-C, 4T poppet valve N/A race engines were forced by the demands of high rpm breathing to
run "unusually large" bore-to-stroke ratios, yet still could not compete with 2Ts on specific output.
No N/A F1 mill achieved the reliable/rideable ~440hp/litre that 2T GP bikes made.

Poppet valves also have a number of fundamental issues - some of which may be partly
addressed by materiel development in mass/temperature resistance/operating shock loadings..
.. but that valve head obtunding the port has always been /& is still.. a problem..

[wuzak]

T-C, 4T poppet valve N/A race engines were forced by the demands of high rpm breathing to
run "unusually large" bore-to-stroke ratios, yet still could not compete with 2Ts on specific output.
No N/A F1 mill achieved the reliable/rideable ~440hp/litre that 2T GP bikes made.

It would be interesting to see the comparison of BSFC between the 2 stroke GP bike and the 4 stroke GP bikes (though they rarely were tuned for max hp) or the F1 engines of the era (850-900hp V-10s at the end of the 2 stroke Moto GP era).

[J.A.W.]

W, AFAIR, the 2T GP bikes were not run to a fuel flow limitation formula, but were octane limited/sans TEL,
& also had to race at a mandated minimum weight somewhat higher (~15%) than technically needed..

A 125cc 2T GP race cylinder was gas flowing ~ as much as the classically developed 500cc 4T GP Manx Norton,
& 'bout what a current ( rules limited) 250cc 4T GP race cylinder can do..

Development of 2T harmonic pulse flow not only improved max output ( 125cc single bettering earlier 125cc 4 cyl)
but the wider powerband allowed tractable riding - even when limited to 6 gear ratios,
with accurate fuel ratio metering - of course - being part 'n' parcel of this process.

Earlier, thirstier, 2Ts often relied on excess fuel as a form of internal coolant, but the more highly
developed units were effectively coolant temperature regulated, running with closer tolerances,
& systems to prevent closed throttle/over-run/over-rev seizures - which were problematic in earlier decades..


I have a set of engine performance curves from Kawasaki for their 3 cyl 750cc 2T road-bike of ~40 years ago,
it was of simple piston-port design, softly tuned to a fairly low specific output ( ~100hp/litre), & the graph
shows a best SFC of ~300 grams per hp/hr in the rpm zone (@ ~6,500 rpm) between max torque & max hp..

Do you have figures for the Crecy, W? AFAIR, the 4T R-R Merlin has a cruise SFC of ~245 grams per hp/hr,
& the sleeve valve Bristols & the Sabre both did rather better than the Merlin SFC-wise..

[wuzak]

The Crecy had specific fuel consumption of ~0.76pt/hp/hr, which is ~0.684lb/hp/hr at crusing rpms and WOT. Or ~310g/hp/hr. Which is considerably worse than a Merlin.

Minimum BSFC was at approximately 80% power.

The Merlin wasn't spectacularly great either.

[J.A.W.]

Thanks W,

& hence the descriptor of "Sprint engine" for the Crecy, with that comment about part throttle flow issues..
(Got any WOT/max boost SFC figures for the Merlin?)

That's why tuned port/duct & asymmetric inlet/trapping port exhaust valves were developed for 2Ts..

I have owned/operated numerous Yamaha 350 2T twins.. .. & over a 15 year period (`70 -`86) even the road-bike
sports-standard production versions increased hp by about double ( to ~60hp), & improved overall fuel economy
( esp' part-throttle), by a like amount.. ..by incorporating reed inlet & drum rotary exhaust valves..
..a pretty good effort..
..& its really too bloody bad that..
.. a current emissions compliant 'clean-green & mean' version with DFI - aint in the showrooms today..
..I'd buy it..

[wuzak]

From the RRHT:

Merlin XX
(Moderate gear):
Max t/o (SL) - 1280bhp, 3000rpm, 12lb boost, 0.64-0.71 pt/bhp/hr (0.576-0.639lb/hp/hr, 261-290g/hp/hr)
Max climb - 1220bhp, 2850rpm, 9lb boost, 0.57-0.63 pt/bhp/hr (0.513-0.567lb/hp/hr, 233-257g/hp/hr)
Max cruise - 1080bhp, 2650rpm, 7lb boost, 0.54-0.59 pt/bhp/hr (0.486-0.531lb/hp/hr, 220-241g/hp/hr)
(High gear):
Max cl - 1130bhp, 2850rpm, 9lb boost, 0.63-0.70 pt/bhp/hr (0.567-0.630lb/hp/hr, 257-286g/hp/hr)
Max cr - 1015bhp, 2650rpm, 7lb boost, 0.57-0.64 pt/bhp/hr (0.513-0.576lb/hp/hr, 233-261g/hp/hr)

[J.A.W.]

Thanks W, those are interesting figures for an early 20 series Merlin,
& a max of 290g/hp/hr is getting close to ~300/310 level noted above for the basic 2Ts..

Have you got any SFC consumption numbers for a later Merlin..
.. running at closer to 100hp/litre.. ..at or above + 25lbs boost?
..did they stabilise SFC-wise with the after/intercoolers, or get thirstier when generating more power?
( Diminishing returns via Increased internal friction/pumping losses?)

I note that the SFC for the sleeve valve Sabre - improved with higher boost levels, even at cruise ratings..

[tok-tokkie]

J.A.W. Your comment about no DI two strokes currently available. Are the EU regulations and the California regulations such that two strokes are outlawed explicitly or is it just the emission regulations that prevent them?
Do the EU regulations outlaw carburetors now? (Here in ZA the EU regulations have been adopted & we no longer have any carb'd bikes available, classics like the Kawa KLR are gone now).
Lastly, why are engine types abbreviated to 2T & 4T instead of 2S & 4S?

[J.A.W.]

No T-T, there are indeed - current production emissions compliant DFI 2Ts available,
- but as largish/powerful units - only water/snow craft..

There are 2T motorcycles too, even in Europe (Aprilia, Jawa, Ossa, KTM, Peugeot,Yamaha) but mainly dirt bikes..
..or wee mopeds.. running Orbital-type DFI..

BMW, via Husqvarna - had a multi-purpose ( road/trail/enduro) emissions compliant DFI bike ready to
go a year or so back, but then sold Husky to KTM, who have since sat on it - "for future use"..

KTM could present a clean-green-mean 900cc 180hp DFI triple 2T based on their proven 300cc 2T single cylinder..
.. in a ~140kg chassis.. that'd be a bit of fun..
..or BRP Can-Am could adapt the existing Rotax DFI 600/800 twin used in the Ski-Doo snow-craft..

Cycle-World magazine in the USA has even suggested that Polaris ( Victory/Indian motorcycles) ought to
use their wealth of 2T production knowhow to sell a new 2T road bike..

AFAIK, there is no technological obstacle to it happening, its mainly a marketing/liability/profit issue..

2T/2S abbreviations are both used, take your pick..(& why not 2C - for Two Cycle?)

[wuzak]

Thanks W, those are interesting figures for an early 20 series Merlin,
& a max of 290g/hp/hr is getting close to ~300/310 level noted above for the basic 2Ts..

Have you got any SFC consumption numbers for a later Merlin..
.. running at closer to 100hp/litre.. ..at or above + 25lbs boost?
..did they stabilise SFC-wise with the after/intercoolers, or get thirstier when generating more power?
( Diminishing returns via Increased internal friction/pumping losses?)

I note that the SFC for the sleeve valve Sabre - improved with higher boost levels, even at cruise ratings..

No, I do not have any SFC figures for 2 stage Merlins.

I think, in general, they will be worse at low altitudes than teh single stage engines and better at high altitudes.

[Blanchimont]

Lastly, why are engine types abbreviated to 2T & 4T instead of 2S & 4S?

Well, it's probably a German thing where T means Takt, which is the same as stroke.

Regarding the bsfc of the two and four stroke motorcycle engines, this document(page 16) from http://www.grandprixengines.co.uk states that in 2002, both 500cc 2S and 990cc 4S engines were allowed and that the 4S type was more efficient. The limits on fuel consumption(24l for 4S and 32l for 2S for the race) seem to confirm that.

[wuzak]

Lastly, why are engine types abbreviated to 2T & 4T instead of 2S & 4S?

Well, it's probably a German thing where T means Takt, which is the same as stroke.

Regarding the bsfc of the two and four stroke motorcycle engines, this document(page 16) from http://www.grandprixengines.co.uk states that in 2002, both 500cc 2S and 990cc 4S engines were allowed and that the 4S type was more efficient. The limits on fuel consumption(24l for 4S and 32l for 2S for the race) seem to confirm that.

And the 4 strokes had 40-50hp more, not even achieving (or trying to) the specific power of F1 engines at the time.