2 stroke thread (with occasional F1 relevance!)

[J.A.W.]

Check this high-efficiency turbo-compound 2T CI design.. killed by French disinterest, nearly 70 years ago..

http://www.flightglobal.com/pdfarchive/ ... 00265.html

[Pinger]

Have all turbo compound engines been forced induction?
All the 2T ones I've ever seen are, and looking at the Curtiss Wright 4T unit it appears to have been developed from a turbocharged engine with a supercharger added when the turbochargers were replaced with the pressure recovery turbines. If so, possibly the SC was there for high altitude performance rather than necessity. Would the engine (at sea level) worked without a SC? Is forced induction an absolute necessity for a 2T turbo compound and an option for a 4T turbo compound? Any ideas anyone?

[Tommy Cookers]

Have all turbo compound engines been forced induction?
All the 2T ones I've ever seen are, and looking at the Curtiss Wright 4T unit it appears to have been developed from a turbocharged engine with a supercharger added when the turbochargers were replaced with the pressure recovery turbines. If so, possibly the SC was there for high altitude performance rather than necessity. Would the engine (at sea level) worked without a SC? Is forced induction an absolute necessity for a 2T turbo compound and an option for a 4T turbo compound? Any ideas anyone?

the only CW turbocompound I know of was the 15000 TurboCompound versions of the Wright 3350 cu in Duplex Cyclone
all WW2 era US 'turbocharged' engines had a standard mechanical first stage of supercharging and a turbo second stage
the turbo was a design add-on to the standard engine - at low altitude the turbo did nothing
though usually the US used 1 or 2 stage 2 speed mechanical supercharging when it wanted good altitude performance
the Wright TC had 1 stage 2 speed and was originally for ocean patrol ie endurance slow cruise at sea level
giving there its famous 0.38 lb/hp-hr at about 1.15 bar absolute manifold pressure
very lean and 'overgeared' aerodynamically so low rpm and little or no throttling (as all proper aircraft engines)
(btw JAW's French wonderengine imagined bsfc is little better in TE terms as the Wright and diesel fuels differ in specific HV)

however Wright had rather huge takeoff power (run very rich at about 2.2 bar manifold absolute and high rpm)
inconveniently for followers of engine design aircraft engines were designed primarily for high t/o power via high manifold
the 2.2 bar dictated a CR of 6.7 and the PRTs recovered a lot of power otherwise wasted by the low CR's recovery in-cylinder
remember at speed and altitude 'waste' exhaust power is equally well recovered by 'jet' action alone - without PRTs
but Wright were designing for power recovery at low speed and low altitude
if designing for sea level endurance only Wright might have gone NA and used 11:1 CR
when the PRTs would have been much less useful

the later airline Wright TCs had 7.2 CR but no headline bsfc exists
maybe 30000 non TC Duplex Cyclones were made - the TC was an option fit to this standard engine
these mechanical engines often used ADI ('WI') + high manifold for high t/o etc power (efficiency then being unimportant)

[Pinger]

Thanks TC.
Kind of confirms what I'd been thinking - the added back pressure of PRTs require forced induction just so the engine can breathe.
Looking at it from a 2T perspective ie a 'simple' crankcase scavenged unit, exhaust back pressure higher than crankcase compression would kill it stone dead. So much energy going to waste - but unavoidable.
The only road going turbo compound engine I'm aware of is a Scania truck engine, but again, forced induction, the turbocharger getting first dibs on the exhaust stream as it exits the cylinders.

[Tommy Cookers]

fwiw ..... regarding back pressure
Wright explained and showed that the PRTs worked on blowdown ie mean exhaust pressure was not more than ambient
(even though for practicality only 12 cylinder's exhaust blowdown pulses were optimally treated by the exhaust system)

iirc wuzak recently posted in the Ferrari PU thread a link showing the quite convincing NACA TN paper
from which one can infer the PRT recovery potential at zero BP
an earlier NACA TN on NA PRT claims a 9% power boost - it's less convincing eg if accidental pulse tuning effects occurred

in working PRT weakens? pulses that otherwise give 'jet' thrust at high speed or 'tuned exhaust' crankshaft power boost
so an NA 2 stroke might be designable for PRT use but this might not be competitive with conventional design types
isn't it just false economy to have PPT but no supercharging ?
raised exhaust pressure/density improves recovery thermodynamics as blowdown to ambient is aerodynamically choked
especially in a 2 stroke ?

[J.A.W.]

@ Pinger, a 'boosted' 2T - 'senses' any added pressure/flow as essentially 'ambient' - relatively speaking.

The Napier Nomad 2T CI - ran a fairly low static comp-ratio & had an auxillary S.I. starting system to run..
- 'til the turbo boost built-up - as rpm climbed.

(@ T-C, the Wright blow-down turbo-compound was only efficent over a narrow operating range,
compared to the 2T CI turbo-compound units).

See here: http://www.flightglobal.com/pdfarchive/ ... 02503.html

[Tommy Cookers]

the Wright efficiency increases with altitude and speed (for any power)
the setrighted-in-concrete 0.38 lb sfc is the 1951 headline value for minimum power cruise at sea level - for patrol
most of the customers (military transport then airlines) would have always flown at higher powers and altitudes
and whatever sfc values stated were for crankshaft power - they simply didn't and don't include exhaust thrust
most of the airline customers had the later (1956 ?) 7.2:1 CR and improved DI

your 1954 Napier Nm 6 seems to be the Nomad II not the 1951-announced Nomad (now called Nomad I)
the Nomad II had multi-stage compressor and multi-stage turbine
its sfc is lb/hr-eshp - it's based on crankshaft power + exhaust thrust power at some speed (cruise, static or whatever)
but remember any engine gives free exhaust thrust (eg your RR 30% at speed in the Hornet) but this wasn't included as hp
btw presumably Napier used kerosene (Avtur) as any Nomad aviation customers surely would have

broadly, the Nomad's CI made it a 2000 rpm engine but the Wright's SI made it a 3000 rpm engine
aircraft engine design is biased towards high takeoff power - hence the vague Nomad offer of compounded afterburn
some sources have a late Wright version cleared to 4250 t/o hp on WI (Gunston gives 3700 for similar established versions)

[J.A.W.]

Well T-C, do feel free to link your sources for the Wright's merits..

However, if you care to read this 'Flight' article:

http://www.flightglobal.com/pdfarchive/ ... 01215.html

You'll duly find your Nomad misapprehensions - duly corrected..

[Tommy Cookers]

the Nomad II has maxima of 3050 shp and 320 lb (static) exhaust thrust = 3135 equivalent shp
eshp is what Napier uses to get specific fuel consumption
this says that at c.120 mph the Nomad II will propel a plane exactly as would a 3135 shp engine with no exhaust thrust

Nomad II has 3 stage PRT, a higher CR and higher AFR (than Wright) so the Wright will have relatively more exhaust thrust
at takeoff power the Wright PRTs over 500 hp, at slow lean cruise about 100 hp
early WW2 exhaust thrust was equivalent to 15% of shp at c.375 mph and late WW2 to 30% of shp at c.500 mph
suggesting a 'static' thrust value equivalent to of 5-7% of shp at c.120 mph

so Wright's headline 0.38 lb/hr/shp (based on 1600 shp and wrongly assuming no exhaust thrust hp) should be ......
replaced by c. 0.36 lb/hr/eshp (1600 shp plus actual at 120 mph exhaust thrust equivalent hp) for comparison with Nomad

eshp was introduced for turboprops to represent better their practical performance (they normalise for 150 mph)
their v. high AFRs give high mass low speed exhaust ie disproportionate thrust power at low speed
(it's said a 1000 shp piston engine exhaust thrust at speed will make it go faster than a 1000 eshp turboprop)

note to self - diesel, Avgas and Avtur are so close in mass-specific HV that an engine's sfc is a tolerable indicator of its bte
115/145 Avgas might be a bit lower in HV (though there was a minimum HV specified)
Avgas has a lower stoichiometric mass ie the AFRs may be a bit higher than is usual for gasoline

note to JAW - there's about 20 NACA TNs and Reports (on exhaust recovery/propulsion) in NASA and Cranfield archives

[J.A.W.]

This page from 'Flight' : http://www.flightglobal.com/pdfarchive/ ... 01222.html
discusses the comparative features & provides figures showing the 2T CI advantages over the Wright PRT.

This page: http://www.flightglobal.com/pdfarchive/ ... 01223.html
gives the fuel use costing comparison.

[gruntguru]

This seems to suggest BSFC (using shp - left side diagrams) below 0.35 for cruise at all altitudes.

[Pinger]

in working PRT weakens? pulses that otherwise give 'jet' thrust at high speed or 'tuned exhaust' crankshaft power boost
so an NA 2 stroke might be designable for PRT use but this might not be competitive with conventional design types
isn't it just false economy to have PPT but no supercharging ?

Not a false economy if the aim is a turbocompound 'light' at sea level, no exhaust thrust required.
Merely a turbine in the exhaust stream extracting energy that can be coupled to the crankshaft and not the second in series with an upstream turbocharger or incurring the losses of driving a mechanical supercharger.
Were the pulses being harnessed without incurring high backpressure (which would require higher scavenge pressure) maybe viable but for the probably slight power gain, the additional mechanical complexity of the PRT is likely more ironmongery than the gain justifies.

[Pinger]

Finally read through the '1000hp 2T diesel helicopter turbocompound' paper and in this paragraph:

'' The back pressure
imposed by the turbine on the engine cylinders establishes the
lower pressure level of the operating cycle in the cylinders.
This also determines the degree of supercharge necessary to pass
the required quantity of air through the engine.''

which refers to Napier's Nomad, I have an answer to my question in the post above. Any backpressure incurred by a PRT would require a corresponding increase in scavenging pressure - a near impossibility with the crankcase as transfer pump. Equally hard to imagine a PRT could be contrived to harness the pulse energy without creating backpressure.

Interesting paper though. Surprised by how little attention is given to the dynamics of valve operation at 6000rpm. Talk of the use of sleeve valves if necessary but even there, the emphasis is on thermal not mechanical limits.

[roon]

My understanding of sleeve valves is they do not move greatly (compared to piston travel, for example). 6k RPM still slow in this context, would be my guess.

[J.A.W.]

See here: https://www.youtube.com/watch?v=YEaE6BHyjzg

2-piece piston designed to reduce mass & improve dimensional-control/sealing - via even thermal expansion.