Twincharger
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Twincharger refers to a compound forced induction system used on some piston-type internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other. A belt-driven supercharger offers exceptional response and low-RPM performance as it has no lag time between the application of throttle and pressurization of the manifold. Combined with a large turbo which would offer unacceptable lag and poor response in the low-RPM range, the proper combination of the two can offer a zero-lag powerband with high torque at lower engine speeds and increased power at the higher end. Twincharging is therefore desirable for small-displacement motors (such as VW's 1.4TSI), especially those with a large operating RPM, since they can take advantage of an artificially broad torque band over a large speed range.
Twincharging does not refer to a twin-turbo arrangement, but rather when two different kinds of compressors are used.
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[edit] Technical description
A twincharging system combines a supercharger and turbocharger in a complementary arrangement, with the intent of one component's advantage compensating for the other component's disadvantage. There are two common types of twincharger systems: series and parallel.
[edit] Series
The series arrangement, the more common arrangement of twinchargers, is set up such that one compressor's (turbo or supercharger) output feeds the inlet of another. A sequentially-organized Roots type supercharger is connected to a medium- to large-sized turbocharger. The supercharger provides near-instant manifold pressure (eliminating turbo lag, which would otherwise result when the turbocharger is not up to its operating speed). Once the turbocharger has reached operating speed, the supercharger can either continue contributing pressurized air to the turbocharger inlet (yielding elevated intake pressures), or it can be bypassed and mechanically decoupled from the drivetrain via an electromagnetic clutch and bypass valve or one-way valve (increasing efficiency of the induction system).
Other series configurations exist where no bypass system is employed and both compressors are in continuous duty. As a result, compounded boost is always produced as the pressure ratios of the two compressors are multiplied, not added. In other words, if a supercharger which produced 10 psi (0.7 bar) (pressure ratio = 1.7) alone blew into a turbocharger which also produced 10psi alone, the resultant manifold pressure would be 27 psi (1.9 bar) (PR=2.8) rather than 20 psi (1.4 bar) (PR=2.3) This form of series twincharging allows for the production of boost pressures that would otherwise be unachievable with other compressor arrangements.
However, the efficiencies of the turbo and supercharger are also multiplied, and since the efficiency of the supercharger is often much lower than that of large turbochargers, this can lead to extremely high manifold temperatures unless very powerful charge cooling is employed. For example, if a Roots blower with an efficiency of 60% blew into a turbocharger with an efficiency of 70%, the overall compression efficiency would be only 42% -- at 2.8 pressure ratio as shown above and 20 °C (68 °F) ambient temperature, this would mean air exiting the turbocharger would be 263 °C (505 °F), which is enough to melt most rubber couplers and nearly enough to melt expensive silicone couplers. A large turbocharger producing 27 psi (1.9 bar) by itself, with an adiabatic efficiency around 70%, would only produce 166 °C (331 °F). Additionally, the energy cost to drive a supercharger is usually several horsepower, thus if it can either be disconnected electrically (using an electromagnetic clutch such as those used on the VW 1.4TSI or Toyota's 4A-GZE) or allowed to freewheel and vent to the atmosphere, several horsepower can be gained independent of the efficiency gain by switching to one compressor.
Thus, switching the supercharger off at a certain boost or RPM threshold is most desirable, since a large, inexpensive journal bearing turbocharger can be used which will provide more than enough pressure and flow at any RPM for most twincharged motors. However, a smooth switchover can be very difficult to accomplish for non-OEM twincharging applications.
[edit] Parallel
Parallel arrangements typically always require the use of a bypass or diverter valve to allow one or both compressors to feed the engine. If no valve were employed and both compressors were merely routed directly to the intake manifold, the supercharger would blow backwards through the turbocharger compressor rather than pressurize the intake manifold, as that would be the path of least resistance. Thus a diverter valve must be employed to vent turbocharger air until it has reached the pressure in the intake manifold. Complex or expensive electronic controls are usually necessary to ensure smooth power delivery.
[edit] Disadvantages
The main disadvantage of twincharging is the complexity and expense of components. Usually to provide acceptable response, smoothness of power delivery, and adequate power gain over a single-compressor system, expensive electronic and/or mechanical controls must be used. In a spark-ignition engine, a low compression ratio must also be used if the supercharger produces high boost levels, negating some of the efficiency benefit of low displacement.
[edit] Commercial availability
The concept of twincharging was successfully used by Lancia in the 1980s on the Lancia Delta S4 rally car. The idea was also successfully adapted to production road cars by Nissan, in their March Super Turbo [1]. Additionally, multiple companies have produced aftermarket twincharger kits for cars like the Subaru Impreza WRX, Mini Cooper S, Ford Mustang, Nissan Skyline GT-R, Toyota MR2, as well as the GM 3800 Engine, as in the Pontiac Bonneville SSEI, Pontiac Grand Prix GTP, and the Chevrolet Cobalt SS among others.
The Volkswagen 1.4TSI is a 1400 cc engine that utilizes both turbocharger and supercharger. It produces 170 bhp (127 kW; 172 PS) at 6000 rpm and 240 N·m (177 lb·ft) from 1500 to 4750 rpm.
[edit] Alternative systems
[edit] Anti-lag system
Twincharging's biggest benefit over anti-lag systems now in race car applications is its reliability. Anti-lag systems work in one of two ways; by running very rich AFR and pumping air into the exhaust to ignite the extra fuel in the exhaust manifold; or by severely retarding ignition timing to cause the combustion event to continue well after the exhaust valve has opened. Both methods involve combustion in the exhaust manifold to keep the turbine spinning, and the heat from this will shorten the life of the turbine greatly.
[edit] Variable geometry turbocharger
A variable-geometry turbocharger provides an improved response at widely-varied engine speeds. With variable-incidence under electronic control, it is possible to have the turbine reach a good operating speed quickly or at lower engine speed without severely diminishing its utility at higher engine speed.
[edit] Nitrous oxide
Nitrous oxide (N2O) is mixed with incoming air, providing more oxygen to burn more fuel for high power when a turbocharger is not spinning quickly. This also causes the turbocharger to quickly accelerate, providing more oxygen for combustion, and the N2O flow is reduced accordingly. The expense of both the system itself and the consumable N2O can be significant.
[edit] Water injection
For more engine power, and to augment the benefits of forced induction (by means of turbocharging or supercharging), an aftermarket water injection system can be added to the induction system of both gasoline and diesel internal combustion engines.
[edit] References
- ^ grandJDM >> March Superturbo: Mighty Mite! (2007-12-09)
