SUPERIOR DOWNSIZING
3.2 Historical background: turbocharging for high specific output
The amount of power an sI engine can produce is directly proportional to the amount of charge it can consume. Hence an engine with a larger swept volume, or one operating at higher rotational speed, will generate more power than a smaller or slower one. similarly, pressurizing the intake charge (‘pressure charging’) in order to increase its density was recognized as a means of increasing the specific power output from the very first days of the internal combustion engine. Gottlieb Daimler was granted a German patent in 1885 for a supercharger applied to an internal combustion engine [2] and Louis renault patented a supercharger based on a centrifugal air pump feeding the mouth of a carburettor in 1902 [3]. In 1907 Lee Chadwick developed a supercharged engine receiving compressed air from a single-stage centrifugal compressor driven via a belt from the engine flywheel and by 1908 had progressed to using a three-stage supercharger in a racing car in which he won a major hill-climb event [3]. In the background, a swiss engineer, alfred Buchi, realized that, rather than using the brake power of the engine to drive the compressor, it could be coupled via a shaft to a turbine powered by the exhaust gas, utilizing exhaust gas enthalpy that would ordinarily be lost. Buchi obtained a patent for the concept in 1905 [4] but the technology was not immediately adopted by his employer, sulzer Brothers.
The increase in engine specific power output obtained from pressure-charging was attractive in early aerospace applications as a means of increasing wing loading and aircraft performance, shortening take-off distance and increasing operational altitude ceiling prior to the advent of the gas turbine [5]. The
royal aircraft Factory (later renamed the royal aircraft establishment, or r.a.e.) began development of aero-engine supercharging in 1915 and, during 1918–25, began experiments with turbocharging (or turbo-supercharging1) [6]. although the r.a.e. terminated the development of turbocharged aero engines to concentrate on supercharging after this period, mainly due to bearing failures and the lack of affordable high-temperature alloys for the turbine wheels, in 1923 the first application of turbocharging in a car occurred when the aero engine designer and racing car driver Major Frank Halford equipped a 1.5 litre in-line 6-cylinder engine of his own design with a purpose-built turbocharger [7].
Halford’s engine, as shown in Fig. 3.1, being intended for racing, was specifically designed for high performance, incorporating double overhead camshafts, inclined valves and twin-spark ignition, adopted from his experience of aircraft engines. The engine also benefited from Halford’s knowledge of the development of high-octane fuel. The subject of synergies between engine architecture and turbocharging will be returned to later in this chapter, but it
1 originally the term ‘turbo-supercharging’ was used to denote a particular embodiment of supercharging in which the compressor power is obtained from an exhaust-driven turbine. The term has become shortened to ‘turbocharging’, whilst ‘supercharging’ is generally understood to imply that the compressor is mechanically driven from the engine crankshaft.
3.1 Turbocharged racing car engine developed by Major Frank Halford in 1923. Note the adoption of double overhead camshafts and twin spark plugs and the separation of the manifolds for each set of three cylinders. The use of an underslung intercooler can also be discerned. Source: Taylor [7]; (courtesy Douglas R. Taylor and The Rolls-Royce Heritage Trust).
is remarkable that many of the features we now associate with turbocharged engines and their fuels were adopted in the so-called ‘Halford special’ engine over 85 years ago.
The Halford special engine, understandably, initially suffered from under-development of the turbocharger which was eventually replaced in this application by a roots-type supercharger.2 although both Bristol and rolls-royce worked on developmental early turbocharged engines around the 1920s, their production aircarft engines were supercharged. Mercedes built the first supercharged production vehicles during this decade. Production turbocharger applications were first seen in diesel engines powering ships in 1925 [8] and in the 1930s turbochargers with axial turbines were used in marine, rail and large stationary applications and the first production aero engines appeared in a version of the allison V-1710 in 1938.
although trucks with turbocharged diesel engines started to appear in the 1950s, and Fred agabashian took pole position at the Indianapolis 500 race in a car fitted with a 6.6 litre turbocharged Cummins diesel engine in 1952 [3], it was not until 1962 that the production car with a turbocharged engine appeared in the form of the Oldsmobile Cutlass Jetfire, closely followed by the Chevrolet Corvair Monza Spyder. These vehicles were fitted with turbocharged 3.5 litre V8 and 2.7 litre flat-6 gasoline engines respectively but production of the engines was abandoned after only a few years. offenhauser turbocharged engines appeared at Indianapolis 500 in 1966 and won the race in 1968 [3]. Following this, BMW and Porsche were inspired to start developing their own turbocharged racing engines, culminating in the 1100 bhp Porsche 917s of 1973.
Following this racing background the resurgence in turbocharged automotive engines was led by BMW in 1973 with the 2002 Turbo (see Fig. 3.2), followed by Porsche with the 911 Turbo in 1974. some of these early engines exhibited significant levels of ‘turbo lag’ (i.e. a significant delay in the demand for air at the throttle pedal and it being delivered to the engine by the charging system). This phenomenon was largely due to the fact that many of these early engines employed a ‘floating’ turbocharger.
In such a turbocharger the turbine is sized to provide sufficient power to drive the compressor at maximum air flow conditions only, and there was no way of providing excess power to increase the boost pressure above the floating level at lower mass flows. This issue was, in large part, addressed by the invention by saab of the ‘wastegate’, which is a controlled bypass around the turbine to allow some exhaust gas enthalpy to be intentionally
‘wasted’, so permitting the aerodynamic contraction of the turbine to enable
2 The roots ‘blower’ or supercharger was originally patented in 1860 by Philander and Frances roots as a machine to help ventilate mine shafts.
it to be made to provide a higher expansion ratio at lower mass flow rates.
A significant reduction in the inertia of the turbine, which contributes the most significant portion of the assembly inertia, is also possible and the combined effects allow the engine to be brought on boost with much less delay [9]. The disadvantage of adopting this technology is that the turbine stage then provides a higher back-pressure on the cylinders at high engine power, which inhibits scavenging and increases pumping losses (both to the detriment of fuel consumption). However, the driveability improvements were considered worthwhile, and the wastegated turbocharger has become nearly universal. This subject will be returned to later. With the provision of adequate driveability, the application of turbocharging became more widespread with manufacturers such as alfa romeo, audi, Buick, Ford, Lotus, Maserati (with the first ‘bi-turbo’), Mazda, Mitsubishi, Nissan, Pontiac, renault, Toyota and Volvo becoming associated with it in the late 1970s and early 1980s [10]. The first automotive turbodiesel was introduced in 1978 with the Mercedes-Benz 300sD and today virtually all automotive diesel engines are turbocharged.
This plethora of automotive applications was in part catalysed by the beginning of the ‘turbo era’ in Formula 1, which was led by renault introducing their 1.5 litre turbocharged V6 to compete with 3.0 litre naturally
3.2 BMW 2002 tii turbo engine with floating (non-wastegated) turbocharger (courtesy BMW AG).
aspirated engines [11]. an application of the renault Formula 1 turbocharged engine, fitted to a Lotus 97T, is shown in Fig. 3.3. The inherent advantage of turbocharging in low-exhaust back-pressure applications such as racing cars and aircraft – that the absence of a full exhaust system means that the pre-turbine pressure is low – could then be fully exploited and very rapidly nearly all of the Formula 1 grid turned to turbocharging. Driveability again became an issue because of the sheer levels of boost pressure being applied:
in excess of 4 bar boost pressure above ambient was not uncommon [11].
This in turn led to the research and development of mitigating technologies including compound charging [12], which has only just begun to be applied to road vehicles [13]. Turbocharged rally cars followed from the introduction of the technology in the wider automotive field and Audi, with the Quattro, were the first to enter a turbocharged car in the FIA world championship, They were soon followed by Peugeot, Lancia and others.
With the banning of turbocharging in Formula 1 for the 1989 season there was a general retreat from the technology for automotive applications, largely because it was associated with driveability and fuel consumption issues (arising from the need to use an exhaust system with proper silencing and latterly exhaust gas catalysis). While modern ‘torque-based’ engine management systems and supporting technologies such as camshaft phasing devices have permitted the successful development of downsized port fuel injected (PFI) engines with excellent driveability, it is with the advent of
3.3 Renault turbocharged V6 Formula 1 engine fitted to a Lotus 97T racing car (courtesy William Taylor/Coterie Press Ltd).
direct injection that turbocharging has truly become a fuel consumption improvement technology.