Aviation engineering advanced in stratospheric leaps and bounds during World War I. The Vickers F.B.5 biplane of 1914, for example, had a nine-cylinder 100hp rotary engine that allowed it to stagger to 70mph and a maximum altitude of 9000ft. It'd take some commitment to reach that altitude, mind, given that it took the fighter over 15 minutes to climb to 5000ft alone.
Just three years later, though, the company spun the prop of its advanced F.B.24C prototype for the first time. Suffice it to say that the difference was remarkable; for starters, it packed a 275hp V8 that aided Vickers in almost doubling the top speed of its aircraft to 130mph. This powerful fighter had a maximum ceiling of 23,000 ft, too - 14,000ft higher than the old F.B.5. Of considerable advantage was the fact that it would also take just 10 minutes to reach 10,000ft, allowing it to quickly intercept enemy aircraft.
Although the F.B.24C would never see service, other combat aircraft of the era were beginning to reach similar altitudes. Many, however, were encountering severe performance issues at these previously unreached heights - in particular, engine outputs were declining rapidly as the aircraft flew higher and higher. This was a result of the air density decreasing with altitude, meaning that the quantity of oxygen-laden air ingested by each of the engine's cylinder would drop as the aircraft climbed. Less oxygen present in the combustion chamber meant that less fuel could be burnt, resulting in less power being produced.
Fortunately, by this point, many specialists were working on turbocharging technology - and some, such as French engineer Auguste Rateau, were focused on applying it to aviation. Rateau, for example, patented a turbocharger set-up in 1917 that permitted 'means whereby the power of the motor may be maintained at any desired constancy while operating an aeroplane at varying elevations.'
American engineer Sanford Moss would soon further pave the way for turbochargers - then often referred to as 'turbo-superchargers' - by demonstrating a force-fed 27-litre Liberty L-12 in 1918. In naturally-aspirated form the engine would produce 230hp at 14,000ft but, with its turbocharger singing away, at the same altitude it would instead produce 380hp. The research carried out by Moss, who was working for American conglomerate General Electric, would underpin the company's further development and production of turbochargers - which culminated in it producing some 300,000 units during World War II alone.
There were limitations to what could be achieved with a single turbocharger using the technology of the era, though - and it was consequently difficult to get a turbocharger to perform well in a range of conditions. A large and heavy turbocharger might produce excellent boost at maximum engine speed, when the exhaust gas flow was high, making it ideal for combat applications. Elsewhere in the engine's operating range, a large turbocharger's low output and sluggish response could otherwise make accurate control of the aircraft tricky.
Alternatively, a smaller turbocharger with a lighter rotating assembly might respond well at low engine speeds but quickly become inefficient - or even overspeed and fail - when the throttle was firewalled. Consequently, as the demand for improved performance over a wider range of altitudes increased during World War II, General Electric began to employ both turbochargers and superchargers in compound configurations; the supercharger would provide prompt response at lower altitudes, while the turbocharger would deliver more boost at higher speeds and heights.
Aircraft that featured compound-charged engines with GE turbochargers included the P-47, P-38, B-17, B-24, B-29 and B-32 - but they could be difficult to manage and, in some cases, notoriously unreliable. The Lockheed P-38, in particular, became renowned for its finickity force-fed Allison V-1710 engines - which caused pilots and ground crew so much grief that Lockheed engineers were keen to replace the Allisons with Merlins just two years after the P-38 went into service.
The pilots were crying out for an automatic engine control system, too, such as the innovative 'Kommandogerät' engine management system offered for the BMW 801. In any case, though, the use of these forced induction set-ups allowed for increased power - particularly at altitude - despite the frequently poor intake and cylinder head designs, and inaccurate fuel and ignition systems of the era.
In an effort to curtail some of the problems, though, General Electric set about coming up with ways to optimise the output and response of turbocharging and supercharging systems. In particular, GE engineer Chester Smith - who had been working on compressor technology for several years - had been looking at ways to deliver better performance across a wider range of altitudes.
Smith had conceived a new configuration that made use of a standalone turbocharger and a power recovery turbine - which would use energy from the exhaust gas to physically drive the engine via its crankshaft, increasing its output. This approach, later dubbed 'turbo compounding', helped convert more waste energy into useful output.
He quickly realised that, in applications featuring an exhaust-driven compressor, it would be beneficial to be able to alter the percentage of exhaust gas flowing to each of the turbines. 'Since the speed and required power of the compressor increase as the altitude increases,' wrote Smith in 1939, 'a greater fraction of the gas will usually be diverted to the supercharging turbine as the altitude increases - and a correspondingly smaller fraction will be available for the compounding turbine. Unlike the supercharging turbine, however, the speed of which increases with altitude, the compounding turbine - being geared to the engine shaft - operates at substantially the same speed at all altitudes; at least, the variation in its speed is much less.'
Smith subsequently proposed an adjustable valve that would allow for exhaust gas to be diverted in the desired amount to the respective turbines; as a result, the output of the turbocharger could be altered depending on the required power - while the remaining portion of exhaust gas would flow through the power recovery turbine instead of simply being dumped out of a wastegate.
Moss, who himself was continuing to develop advanced turbocharging technology, had latterly been experimenting with pairs of turbochargers that were connected to the same intake and exhaust systems. Flat out, this configuration would often work - but, in certain conditions, one turbocharger could stall and would promptly be driven backwards by the pressurised intake air from the other turbocharger that was at full chat.
The solution was to extend the concept mooted by Smith to a twin-turbocharged configuration, fitting control valves to both the inlet and outlet of one of the turbochargers in the system. When the load on the turbochargers decreased, and before one stalled, both valves would close simultaneously - so that the remaining turbocharger would continue operating at peak efficiency without discharging boost through the compressor of the other turbocharger.
This set-up, patented by Moss in 1942, describes what would be recognised today as a 'parallel-sequential' system; so configured, both turbochargers operate when the engine load is high but only one would be used at lower outputs. Later, 'series-sequential' systems would follow - in which the first turbocharger would be shut down once enough exhaust gas was being produced to drive the second turbocharger at full power.
Moss even sought to combat the problem of sluggish response when the other turbocharger was called upon. 'Preferably, the arrangement is such that the valve, when moved to closed position, does not completely close the conduit but permits sufficient gas to flow past it to operate the supercharger at idling speed,' he said. 'The uncovering of the aperture permits sufficient air through compressor II to prevent its overheating; preferably, the idling speed at which the turbocharger is operated is sufficiently high to enable it to again take its part of the load promptly when occasion demands.'
Alas, the advent of the jet age curtailed much further research into large, complex aero engines. Not that the technology was dead in the water; the Nordberg Manufacturing Company - which specialised in diesel engines, among other heavy equipment - would later also submit a patent for a dedicated sequential turbocharging system which drew on GE's earlier innovations. This patent, filed in March 1952, described a set-up in which 'for starting and low load, one or the other of the block valves will be closed so that the entire exhaust will pass through one of the turbochargers only'.
In these parallel-sequential systems, the turbochargers would be the same size. In a series-sequential system, though, the first turbocharger would typically be a smaller unit. This more compact turbocharger would have a low boost threshold, spooling easily at low engine speeds to deliver quicker response and increased engine output. Its smaller, lighter components would also have little inertia - helping minimise lag when the turbocharger was spooled and more power commanded.
In any case, there were numerous benefits to sequential turbocharging - including improved response and air flow. Certain engines also lent themselves, in terms of packaging, to suiting power-dense twin-turbocharged configurations particularly well - instead of a single large turbocharger.
Turbocharging eventually made the leap in earnest, to the automotive world, with the launch of the Chevrolet Corvair Monza Spyder and Oldsmobile Jetfire in 1962 - at which point all the issues experienced during the turbocharged WWII era began being repeated. Compression ratios that were too high, turbochargers that were improperly sized, wastegate control problems, anti-detonant injection requirements, fuelling issues and more all rose their ugly head again; before long, much aviation expertise was being drawn upon.
As the turbocharged era of the 1970s ably proved, throttle response - or the lack of it - could be a real issue. Porsche, which had been experimenting with turbocharging for some time, unveiled its first turbocharged production car - the 911 Turbo - in late 1974. To the uninitiated, a conventional rear-engined 911 could prove somewhat of a handful. Ramp up the power, add a hefty dose of lag and you had yourself the fine makings of a prompt exit through the nearest hedge.
Porsche was all too aware of this, leading it to patent a sequential system of its own in the late 1970s. Ultimately, as the company strived to make the power delivery of its turbocharged flat-six engines more progressive, its development and advancing technology culminated in the first production sequentially turbocharged car - the Porsche 959 of 1986.
Its 2.85-litre engine featured a pair of KKK K26 turbochargers, of which only one would be used initially. As the engine speed increased, and more exhaust gas was produced, the second turbocharger would be brought online - helping the Porsche punch out 444hp at 6500rpm and 369lb ft at 5000rpm, with far better response and behaviour through the engine's rev range.
Like others that would later adopt the technology, Porsche's patents would reference Nordberg's sequential system - and, in turn, GE's. The concept was even making its way into diesel trucks by this point, with Volvo patenting a sequential system in 1985 and previewing a sequentially turbocharged diesel in 1987 - although, even then, its engineers had reservations about the complexity and bulk of such a set-up.
Regardless, other manufacturers were soon to follow in Porsche's footsteps as they battled to improve the performance of their turbocharged cars. Mazda introduced sequential turbocharging in the twin- and triple-rotor Eunos Cosmo in 1990 and would also use sequential technology in the twin-rotor RX-7 that followed. Toyota, not one to be left behind by its rivals, also adopted sequential turbocharging for use in its now-legendary Mk4 Supra of 1993 - followed by Subaru, in its Legacy, in 1994.
While they delivered impressive performance, ever-advancing and more efficient single- and twin-turbocharger systems made sequential technology increasingly redundant. The complexity and cost of such systems further contributed to their downfall; any RX-7 owner, for example, will be able to regale you with nightmarish tales about maintaining the 'rat's nest' of vacuum lines required for the sequential turbocharging system to function properly.
That said, many engines - particularly industrial diesels - still make use of sequential set-ups. Despite the potential drawbacks, many modern car manufacturers still also employ sequential turbocharging from time to time. Companies including Mazda and GM have recently patented new systems using the concept, too, so there may well be further life in the decades-old system yet.