Twincharger
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 a mechanically driven supercharger, each mitigating the weaknesses of the other. A mechanically driven supercharger offers exceptional response and low-rpm performance as it does not rely on pressurization of the exhaust manifold (assuming that it is a positive-displacement supercharger such as a Roots type or twin-screw and not a Centrifugal compressor supercharger, which does not provide substantial boost in the lower RPM range). A turbocharger sized to move a large volume of air tends to respond slowly to throttle input while a smaller, faster-responding turbo may fail to deliver sufficient volume through an engine's upper RPM range. The unacceptable lag time endemic to a large turbocharger is effectively neutralized when combined with a supercharger which tends to generate substantial boost pressure much faster in response to throttle input. The end result being a zero-lag powerband with high torque at lower engine speeds and increased power at the upper 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 where two different kinds of compressors are used.
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.
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 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 compounding the pressurized air to the turbocharger inlet (yielding elevated intake pressures), or it can be bypassed and/or mechanically decoupled from the drivetrain via an electromagnetic clutch and bypass 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 turbocharger which produced 10 psi (0.7 bar) (pressure ratio = 1.7) alone blew into a supercharger which also produced 10 psi 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 and would be inefficient.
However, turbo and supercharger efficiencies do not multiply. For example, if a turbocharger with an efficiency of 70% blew on a Roots blower with an efficiency of 60%, the total compression efficiency would be somewhere in between. To calculate this efficiency, it is necessary to do the calculations of the 2 stages, first calculate the conditions of pressure and temperature at the exit of the first stage and starting from these to do the calculations for the second stage. Following the previous example, for a first stage of the turbocharger (efficiency of 70%, pressure ratio of 1.7) the temperature would reach the 88.5 °C (191.3 °F) after the first stage, to then enter the roots (efficiency of 60%) and leave at a temperature of 186.5 °C (367.7 °F). This is a total efficiency of 62%. A large turbocharger that produces 27 psi (1.9 bar) by itself, with an efficiency adiabatic of around 70%, would produce air only 166 °C (331 °F). In addition, the cost of energy to drive a supercharger is higher than that of a turbocharger; if it is ignored, the load of execution compression is eliminated, leaving only light parasitic losses of rotating the working parts of the supercharger. The supercharger can be disconnected even more electrically (using an electromagnetic clutch such as those used in VW 1.4TSI or Toyota 4A-GZE , although this is not because it is a double-load motor, but is intended to bypass the supercharger in low load conditions) which eliminates this small parasitic loss.
With series twincharging, the turbocharger can be of a less expensive and more durable journal bearing variety, and the sacrifice in boost response is more than made up for by the instant-on nature of displacement superchargers. While the weight and cost of the supercharger assembly are always a factor, the inefficiency and power consumption of the supercharger is almost totally eliminated as the turbocharger reaches operating rpm and the supercharger is effectively disconnected by the bypass valve.
Parallel
Parallel arrangements typically require the use of a bypass or diverted 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.
Efficiency
Efficiency plays a huge role in ruling whether a car is practical for everyday tasks or if it is more suitable for track days. To find an equilibrium between the two using a twincharged system a few upgrades will be needed. Upgrades usually include the installation of a larger Intercooler, a high volume water pump, new sensors to monitor air pressure and heat, and a programmable engine management system.
The most tedious in terms of physical labor is the installation of the intercooler. Intercoolers should be chosen by application and elevation. One example of an application for intercoolers is off-road vehicles. If a forced induction system is being used in an off-road vehicle, it is going to run hotter simply because of weight. The more weight being pulled along, the more work the engine must produce. If the intercooler is an air to water style system, it is imperative to have a clean source of air going into the core fins. Since this upgrade adds more coolant into the system it is important to be able to move the coolant throughout the entire system.
To obtain an optimal efficiency within the cooling system a high-volume water pump must be added. High performance water pumps have the ability to force larger quantities of water through the engine, superchargers, and turbochargers. The average stock water pumps usually work at a capacity of 35-50 gallons per minute depending on brand. If an aftermarket Pump is purchased and installed it can up that figure to 60-85 gallons per minute. This improves the cooling capabilities of the water pump for the forced induction system by 58%. To ensure that the cooling efficiency stays at its optimal level a series of checks and balances must be established.
Just like the human body an equilibrium must be maintained within an Internal combustion engine introduced to a forced induction system. Much like the bodies nervous system modern combustion engines have electrical sensors to keep all functions running within a certain tolerance zone. To keep track of all of the sensors in the system a programmable engine management system is implemented. This allows tolerances to be altered on the fly. This ECU is also the foundation for adaptive controls for the forced induction system. If one of the sensors reads that there is too much air in the system, the programmer can make it so that the system automatically compensates for the deficiency.
Disadvantages
The main disadvantage of adding any forced induction system 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.
Commercial availability
The concept of twincharging was first used by Lancia in 1985 on the Lancia Delta S4 Group B rally car and its street legal counterpart, the Delta S4 Stradale. 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, and Toyota MR2.
The Volkswagen 1.4 TSI is a 1400 cc engine – utilised by numerous automobiles of the VW Group – that sees use of both a turbocharger and a supercharger, and is available with eight power ratings:
| Power | Torque | Vehicles |
|---|---|---|
| 103 kW (140 PS; 138 bhp) at 5,600 rpm | 220 N⋅m (162 lbf⋅ft) at 1,500–4,000 rpm | VW Golf V, VW Jetta V, and VW Touran |
| 110 kW (150 PS; 148 bhp) at 5,800 rpm | 220 N⋅m (162 lbf⋅ft) at 1,250–4,500 rpm | SEAT Ibiza IV |
| 110 kW (150 PS; 148 bhp) at 5,800 rpm | 240 N⋅m (177 lbf⋅ft) at 1,500–4,000 rpm | (CNG version) VW Passat VI, VW Passat VII, VW Touran |
| 110 kW (150 PS; 148 bhp) at 5,800 rpm | 240 N⋅m (177 lbf⋅ft) at 1,750–4,000 rpm | VW Sharan II, VW Tiguan, SEAT Alhambra |
| 118 kW (160 PS; 158 bhp) at 5,800 rpm | 240 N⋅m (177 lbf⋅ft) at 1,500–4,500 rpm | VW Eos, VW Golf VI, VW Jetta VI, VW Scirocco III |
| 125 kW (170 PS; 168 bhp) at 6,000 rpm | 240 N⋅m (177 lbf⋅ft) at 1,500–4,500 rpm | VW Golf V, VW Jetta V, VW Touran |
| 132 kW (179 PS; 177 bhp) at 6,200 rpm | 250 N⋅m (184 lbf⋅ft) at 2,000–4,500 rpm | VW Polo V, SEAT Ibiza Cupra, Škoda Fabia II |
| 136 kW (185 PS; 182 bhp) at 6,200 rpm | 250 N⋅m (184 lbf⋅ft) at 2,000–4,500 rpm | Audi A1 |
Volvo produces a twincharged 1969 cc inline-four engine that is utilised in their T6, T8, and Polestar models. The T8 adds on the T6 with a rear electric motor.
| Power | Torque | Vehicles |
|---|---|---|
| 320 PS (235 kW; 316 bhp) at 5,700 rpm | 400 N⋅m (295 lbf⋅ft) at 2,200–5,400 rpm | T6 |
| 367 PS (270 kW; 362 bhp) at 6,000 rpm | 470 N⋅m (347 lbf⋅ft) at 3,100–5,100 rpm | Polestar |
| 408 PS (300 kW; 402 bhp) | 640 N⋅m (472 lbf⋅ft) | T8 (including rear electric motor) |
The Danish supercar Zenvo ST1 made use of both turbocharger and supercharger in its 6.8-litre V8 engine.
| Power | Torque | Vehicles |
|---|---|---|
| 1,104 hp (823 kW; 1,119 PS) at 6,900 rpm | 1,430 N⋅m (1,055 lbf⋅ft) at 4,500 rpm | ST1 |
Alternative systems
Anti-lag system
Twincharging's largest benefit over anti-lag systems in race cars 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.
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.
Twin-scroll turbocharger
A turbocharger with two sets of vanes for two operating pressures, can decrease lag by operating at different exhaust pressures.
Sequential twin turbochargers
Sequential turbocharger systems provide a way to decrease turbo lag without compromising ultimate boost output and engine power.
Nitrous oxide
Nitrous oxide (N2O) is mixed with incoming air, providing more oxidizer to burn more fuel for supplemental power when a turbocharger is not spinning quickly. This also produces more exhaust gases so that the turbocharger quickly spools up, 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.
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.
References
- grandJDM >> March Superturbo: Mighty Mite! (2007-12-09)