Race Engine Technology issue 034 : NOVEMBER 2008
37
Porsche 911 road car turbo with variable geometry turbine (see also Figures 1 and 2)
INSIGHT : TURBOCHARGERS
Fred Tuerk: MAHLE’S STEEL BREAKTHROUGH THE COMMUNICATIONS HUB OF THE RACING POWERTRAIN WORLD TURBODIESEL
TECH SECRETS Inside the V12 sledgehammers BMW F1
POWER GAMES The road to victory
RACING
ON E85 AER’s V8 alternative
Jack Kane considers the state of the art in turbochargers, which are becoming ever more widely used as motorsport increasingly embraces energy efficiency
Improving the flow T
his article explores some of the current thinking in turbo- supercharger technology as applied to competition engines. For a review of turbocharger basics, see the accompanying sidebar. This article also refers to Tables Two and Three in
NOVEMBER 2008
the Focus: Advanced Metals article on page 26 of this issue (hereafter referred to as ‘Metals’), and to processes defined therein. There is a considerable amount of development work currently being done on turbochargers, which is being motivated primarily by these road-vehicle requirements: (1) the ability to operate reliably and continuously with higher exhaust gas temperatures (EGT), and (2) the ability to operate with higher compressor inlet temperatures and flowrates. The demand for higher EGT tolerance comes from increasing demand for better fuel economy in spark-ignition (SI) engines, which requires that the engines run much closer to stoichiometric mixtures rather than employ the richer mixtures used in the past to reduce EGTs. These days compression-ignition (CI) road car engines are invariably turbocharged and this still relatively fast developing technology is operating at ever- rising BMEPs, which means higher combustion temperatures, which means higher NOx emissions. The demand for higher inlet temperatures and flowrates comes from the high percentages (30-40%) of exhaust gas recirculation (EGR) required to control NOx emissions from CI engines. Although these motivators are coming from the production-vehicle
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TURBINES A turbocharger lives in a terribly hostile environment. The turbine is driven by exhaust gasses that can exceed 1025°C (1875°F) and which are very corrosive. Exhaust valves experience those same corrosive, high-temperature gasses, but exhaust valves do not approach the peak temperature of the exhaust gas. An exhaust valve in a competition engine spends at least half of the time on the valve seat (production engines more like two-thirds of the time). Valves continuously transfer heat through the stem to the guide, and when they are seated, they rapidly transfer heat into the cylinder head through the valve seats. Those cooling paths keep exhaust valve temperatures well below
Figure One: VGT at Low Flow (Photo: Porsche)
end of the spectrum, the resulting technology is or will soon be available for application into motorsports. An SI race engine won’t need to operate at stoichiometric mixtures, but the availability of turbines which can live with 1050°C (1925°F) EGTs will provide new opportunities for greater output. Currently, competition CI engines are not required to reduce NOx emissions (but that will surely develop as political correctness further invades motorsports), so the increased compressor efficiencies, flowrates and map widths can be used to provide greater intake density at the mandated manifold absolute pressure (MAP) limits. The increased compressor efficiencies, flowrates, and map widths developed for CI technology will certainly benefit competition SI engines in the same way.
EGTs. The turbine, however, lives in a continuous, high-velocity jet of those gasses. Although there is expansion across the turbine nozzle, therefore some cooling of the gasses, the temperature at the tips of the turbine rotor can approach exhaust gas temperatures. Further, the rotor system on many turbochargers operates well in excess of 100,000 rpm, and some approach 150,000 rpm. That imposes huge tensile loads from the centrifugal forces, as well as bending and vibratory loads. That environment requires the use of nickel-based superalloys for the turbine wheels. Those alloys can retain their high strength at these high temperatures. The turbines in most current production turbochargers are suitable for continuous operation at an exhaust gas inlet temperature of 950°C (1750°F). Production turbines are typically investment-cast from Inconel 713 C or 713 LC. The castings use HIP to improve their structure (see Table Two, Metals) and are heat-treated to the required strength level. Honeywell Turbo Technology (Garrett) supplies the turbochargers (TR30R) used on the stunning 5.5 litre CI Le Mans V12s of both Audi and Peugeot. They have fixed turbine nozzle geometry with wastegates. The turbine wheels in those turbos must operate continuously with EGTs up to 1050°C. Honeywell uses a superalloy material known as Mar-M-247 (developed by Martin-Marietta in the seventies for gas turbine engine blades, discs and burner cans). This material is a nickel-based alloy containing significant amounts of chrome, aluminium and moly (Table Two, Metals). In order to achieve the best properties in components cast from
Mar-M-247, NASA developed the Grainex process. This process uses traditional investment casting techniques, with the additional process of mold agitation during freezing to produce homogeneous grain inoculation, resulting in outstanding uniformity of grain structure and material properties. The part is HIP’d at 1185°C and 170 bar for four hours to minimize porosity, then solution treated for two hours at the same temperature, followed by 20 hours of aging at 870°C. That produces a room temperature UTS of 150 ksi, which increases up to 760°C. Variable geometry turbines (VGT) provide a substantial improvement in turbine efficiency and enable greater flexibility of operation. A large turbo with VGT can operate as if it were a smaller turbo at lower engine speeds. In many cases, the VGT can replace a wastegate. VGT turbochargers have been around for several years, but their applications have been somewhat limited by the EGTs they can survive. At present, VGT implementations are limited to continuous EGTs of 950°C (with an occasional spike to 980°C), as in the Porsche 911 twin-
Figure Two: VGT at High Flow (Photo: Porsche)
turbo system supplied by Borg-Warner. However, current development efforts are focused on producing VGT systems which will operate successfully at the 1050°C temperatures for which the newer turbines are designed.
VGT is implemented in different ways. One system uses a series of
movable vanes around the periphery of the turbine wheel (as shown at the beginning of the article). Each vane pivots on an axis parallel with the rotor axis. When the exhaust gas supply is low, the vanes pivot to a position which is a few degrees from perpendicular to the turbine wheel inducer vanes (Figure One). That gives the incoming gasses a strong tangential component to drive the turbine. The angle can be varied continuously, and at high exhaust flow they are nearly aligned radially with the outer contour of the turbine blades, giving the incoming gasses a strong radial component to drive the turbine, while offering a relatively large flow area to reduce backpressure (Figure Two). Although many such systems currently use non-cambered vanes (the chord line is straight), future developments will include cambered vanes to increase VGT efficiencies at the top and bottom ends of the operating range. These VGT systems can be electrically operated, providing even greater flexibility to an ECU-managed engine system. Another VGT system uses vanes which are attached to a ring surrounding the turbine wheel. These vanes have a fixed angular orientation. The ring and vanes move parallel to the rotor axis. The vanes orient the gas flow toward the turbine wheel blades, and the ring opens and closes the net nozzle area, dynamically altering the nozzle area, which changes the gas velocity therefore the turbine performance.
COMPRESSORS The compressor side of a turbocharger faces its own challenges. In most applications, the compressor is ingesting air at slightly more than ambient temperature, but the temperature rise across the compressor can be substantial (see the Basics sidebar). With ambient inlet air and a 4:1 pressure ratio at 80% adiabatic efficiency (AE), the compressor discharge temperature can exceed 400°F. However, the low temperature of the inlet air plus the fact that most of the temperature rise occurs in the diffuser, where velocity is exchanged for pressure, keeps the operating
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034 contents • INteRvIeW:
DR MaRIo tHeISSeN BMW’s Formula One Team Principal talks about powertrain development
• tuRBoDIeSeL tecH:
BoScH INJectIoN How Bosch is advancing the development of high pressure, common rail direct injection
• tuRBoDIeSeL tecH:
MaHLe pIStoNS We reveal in detail the development of steel pistons for the Le Mans racing turbodiesel V12s
• FocuS: aDvaNceD MetaLS In a world of constant materials development Jack Kane considers the best metal specifications for key race engine component applications
• FocuS: tuRBocHaRGeRS The turbo-supercharger is making a comeback in racing
• GReeN tecH: aeR v8 oN e85 This year AER has run its 4.0 litre V8 twin turbo on E85 in ALMS races; we investigate the issues that this causes a Prototype race engine
• DoSSIeR: eDL JuDD DB v8 We look in depth at this latest LM P2 engine from EDL
• INSIGHt:
poRScHe DIRect INJectIoN We investigate in unprecedented detail the story of how Porsche pioneered direct fuel injection for a five-figure speed V8
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ISSUE 034 race engine TECHNOLOGY NOVEMBER 2008
TURBODIESEL INJECTION AND STEEL PISTONS • PORSCHE DFI • JUDD V8 • MARIO THEISSEN • ADVANCED METALS • TRIUMPH 675 • TURBOS • AER V8 ON E85 • GT ENGINES
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