Figure 4 - Gas Bearing Piston with Scotch Yoke Mechanism (Courtesy FICHT Gmbh)

achieved by narrower passages to increase the exhaust gas velocity and use of a smaller turbine area. In order to prevent turbocharger overspeed, a waste-gate is used to vent excess pressure at the high power end. If the turbocharger match 1s for a coast-to-coast truck which operates a high percentage of time at full power and constant speed, then the turbocharger would be selected which had a high efficiency for the mass flow and pressure ratio at those conditions. In this case the gas passage would be larger and the turbocharger designed to operate without the necessity of a waste gate. This turbocharger would tend to be sluggish at low power levels.

There are several methods or systems which improve the turbocharger matching to the diesel over a wide range of operating conditions. These include variable geometry, sequential turbochargers, multi-stage turbine, the Hyperbar system and the "3 Wheel" turbocharger concept. There are several techniques used to vary the geometry of the flow path to the turbine. One technique is to install turbine inlet guide vanes, which are also called stator nozzles, that are pivotable about a midspan axis. These vanes are connected through linkages to a rotatable ring which positions the inlet guide vanes appropriately for the mass flow rate. This is one system for achieving variable geometry turbocharging. There are several other schemes of achieving variable geometry for the turbocharger. One is a mating plate which has non-pivoting stator nozzles fixed to a sliding wall plate. This scheme is designed such that the fixed noz.zle plate is mated with a similar parallel plate that has recessed mating slots for the nozz.le blades. Relative movement between the plates varies the stator width and consequently the flow area. Other schemes involving controlled blockage of part of the turbine inlet flow area have been advanced.

The more conventional method of turbocharging is the constant pressure system whereby each cylinder exhausts Into a common header. This system results in mixing losses in the manifold. A pulse turbocharging scheme is more efficient but requires separate piping from each exhaust port to the turbine. Another approach, a compromise between constant pressure and pulse turbocharging, is the pulse converter which includes a junction of at least 2 pipes, each from 1 to 3 exhaust ports, and is connected to the turbocharger. This junction could convert some of the exhaust pulse pressure into kinetic energy. By converting the high momentum of the gas flow through the noz.zle, the effect of a pressure pulse entering the junction on the mixing losses of the other manifold pipes is reduced. This pulse converter would enter the turbine as a single entry. Volvo is developing the pulse converter concept for heavy duty truck diesel engines. The pulse converter concept is compatible with sequential turbocharging.

Major improvements in diesel engine fuel efficiency can be obtained by applying the latest design techniques to improve the turbine and compressor aerodynamic efficiencies. The 3-D finite element codes used in the gas turbine compressor development field have demonstrated very impressive aerodynamic efficiency gains. The overall efficiency of a turbocharger is the product of the turbine adiabatic efficiency, the turbocharger mechanical efficiency and the compressor adiabatic efficiency. Current overall turbocharger efficiency is in the 60 percent range. Advanced aerodynamic efficiency designs should be able to increase the overall turbocharger efficiency to near 75 percent, or greater.

Improvements in efficiency can be obtained by reducing turbine and compressor clearances and making the flow path much more aerodynamically smooth. Cast in-place ceramics, such as aluminum titanate (A12 TiOc) can be part of the exhaust path such tnat heat loss is reduced and aerodynamically efficient surfaces are improved.

One of the more interesting advances in turbochargers has been the emergence of ceramic turbochargers. Sintered silicon nitride rotors are 1/3 lighter than metal rotors. This reduced inertia is a bigger plus for the automobile as it should improve acceleration response. The heavy-duty truck engine turbocharger would be more demanding than the auto application. However, there are several other advanced material candidates for turbocharger rotating components. Squeeze castings recently introduced in piston development could provide advantages in turbocharger applications. Fiber or whisker reinforced ceramics or metals extend design capability and reliability. Nickel-aluminum alloys provide advantages. Rapid solidification technology (RST) could provide aluminum alloys with silicon content up to 40 percent with an accompanying increase in high temperature strength. RST technology allows manufacture of alloys unobtainable by conventional powder technology. For example the conventional powder manufacturing methods are limited to 16 weight percent of silicon in aluminum. Fabrication of rotors from powders is a major challenge.

Turbocharger bearings should be reviewed. Current oil lubricated sleeve bearings allow oil leakage into the intake air which contributes to particulate emissions. Foil bearings, gas bearings and roller bearings (steel or ceramic) are some of the candidates.

Sequential turbocharging involves cutting in 2 or more turbochargers connected in parallel to a common , exhaust manifold. All the turbochargers are in operation at the engine rated speed. The number of turbochargers cut in is determined by the engine power level. Thus the individual turbochargers can be operated closer to their higher efficiency conditions. This can be shown in figure 5 which shows an MTU sequential turbocharging system.

The hysteresis cut-in, cut-off approach prevents a hunting effect at a particular power setting. MTU data show that sequential turbocharging not only provides a significant engine efficiency improvement but also increases the turbocharger output particularly at the lower power level compared with an equivalent single turbocharger. Sequential turbocharging thus permits use of turbochargers at their most efficient operating range with reduced thermal load thereby extending service life. Sequential turbocharging provides excellent low speed performance and response and can provide advantages in emission control. Control of the m • 0

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