Twincharging represents a sophisticated method of forced induction in internal combustion engines, distinguishing itself from more common single-compressor setups. This system ingeniously combines two different types of compressors: an exhaust-driven turbocharger and a mechanically driven supercharger. The core principle behind twincharging is to leverage the strengths of each compressor while simultaneously mitigating their individual weaknesses,
thereby offering a more comprehensive solution for engine performance and efficiency. Unlike twin-turbo arrangements, which use multiple turbochargers, twincharging specifically refers to the integration of these two distinct compressor technologies.
Understanding the Series Twincharger Configuration
The most prevalent form of twincharging is the series arrangement, where the output of one compressor feeds directly into the inlet of another. In this setup, a supercharger is typically connected to a medium- to large-sized turbocharger. The supercharger plays a crucial role in providing nearly instantaneous manifold pressure. This immediate boost effectively eliminates what is known as turbo lag, a common issue where a turbocharger takes time to reach its optimal operating speed and deliver full power. By providing initial boost, the supercharger ensures a responsive engine feel right from low revolutions per minute (rpm).
Once the turbocharger has accelerated to its operating speed, the system offers flexibility. The supercharger can either continue to compound the pressurized air to the turbocharger inlet, leading to elevated intake pressures, or it can be bypassed. Bypassing the supercharger, often achieved through an electromagnetic clutch and a bypass valve, increases induction efficiency. This mechanism allows the engine to transition from supercharger-dominant boost at low speeds to turbocharger-dominant boost at higher speeds, optimizing performance across the engine's operational range.
Continuous Compounding and Boost Pressure
Beyond the bypassable series configuration, other series arrangements exist where both compressors remain in continuous operation without a bypass system. In these setups, compounded boost is consistently produced because the pressure ratios of the two compressors are multiplied, rather than merely added. To illustrate, if a turbocharger independently generates 10 psi (0.7 bar) and feeds into a supercharger that also independently produces 10 psi, the resulting manifold pressure would be approximately 27 psi (1.9 bar), significantly higher than the 20 psi (1.4 bar) that would result from simple addition. This continuous compounding allows for the generation of boost pressures that might otherwise be inefficient or unattainable with other compressor configurations.
However, it's important to note that while pressure ratios multiply, compressor efficiencies do not. For instance, if a turbocharger operates at 70% efficiency and feeds into a Roots supercharger with 60% efficiency, the overall compression efficiency would fall somewhere between these figures. Calculating this total efficiency requires determining the pressure and temperature conditions at the exit of the first stage and then using these as the starting point for the second stage. In the example given, a 70% efficient turbocharger would raise air temperature to 88.5 °C (191.3 °F) after the first stage. This air then enters the 60% efficient supercharger, exiting at 186.5 °C (367.7 °F), resulting in a total efficiency of 62%. For comparison, a large turbocharger producing 27 psi (1.9 bar) on its own, with about 70% thermal efficiency, would only heat the air to around 166 °C (331 °F).
Efficiency and Cost Considerations
Another critical aspect of twincharging, particularly with superchargers, is the energy cost. The energy required to compress air with a supercharger is generally higher than with a turbocharger. This is because superchargers are mechanically driven by the engine, creating a parasitic load. When the supercharger is not actively compressing air, there is still a small parasitic loss from rotating its working parts. This residual loss can be further minimized or eliminated by completely disconnecting the supercharger using an electromagnetic clutch, a feature seen in engines like the VW 1.4TSI or Toyota 4A-GZE, which bypass the supercharger under low-load conditions.
From a cost and durability perspective, series twincharging allows for the use of a less expensive and more robust journal bearing turbocharger. Any sacrifice in boost response that might typically be associated with such a turbocharger is more than compensated for by the instant-on nature of positive-displacement superchargers. While the added weight and cost of the supercharger assembly are always factors, the inefficiency inherent to the supercharger is significantly reduced once the turbocharger reaches its operating speed and the supercharger is effectively disengaged by the bypass valve. This strategic combination ensures optimal performance and efficiency across a wide range of engine demands.
