Pinger wrote: ↑Fri Sep 07, 2018 11:01 am
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)