Twincharger

A twincharger is the denomination by the Volkswagen Group of a combination of a turbocharger and a supercharger in an engine. The 1.4 L TSI engine used in some current automobiles like the Volkswagen Golf, Touran employs both methods of forced induction, with the supercharger boosting low-end power while the turbocharger takes over at the high end. VW family of TSI engines also employs Fuel Stratified Injection.

The concept of a twincharger was pioneered by Lancia in the 1980s on the Delta S4 rally car and picked up by Nissan in the March compact car. In addition multiple aftermarket companies have made twincharger kits for cars like the Subaru Impreza WRX, Mini Cooper Type S, Skyline GT-R, Toyota MR-2, etc.

The most common type of twincharging system is a Roots type supercharger paired up with a medium-large sized turbocharger. The supercharger will give better power at low RPM while the turbocharger will give better power at medium-high RPMs. The main drawback of this configuration is that at high speeds, the supercharger (as it is driven by belts) will increase drag upon the engine, limiting the top end power. The Volkswagen Twincharging System overcomes this by locating the supercharger's pulley on a clutch system, which is automatically engaged and disengaged by the computer. This allows the car to keep its top-end performance and automatically switch to supercharging at low RPM.


Drawbacks

There are a few drawbacks to this setup, such as the supercharger still creates drag in the airflow system, even when the clutch is off and the turbocharger is on. It can also cause excessive pulley/belt wear as the clutch engages/disengages at usually very high engine speeds. The transition from the supercharger engaging/disengaging can also be quite rough as well. Lastly, as turbos continue to flow more air as engine speed rises, the turbocharger will eventually outflow more air than it is receiving from the supercharger unless complex piping is used to bypass which can be quite costly and inefficient in a small engine bay.


Solution

The solution to these problems is quite simple, switch the places of both force induction units. By having a turbocharger feed air into a supercharger you gain several benefits as well as minimize any drawbacks from a conventional twincharger setup. The benefits are quite obvious, the supercharger will generate a set amount of boost (determined by pulley size) almost instantly, thus causing the engine to create more exhaust flow and spin the larger turbo quicker. As engine speeds start reaching past the superchargers efficiency under normal circumstances, the turbocharger is providing increasing amounts of air to the supercharger where it would normally run out. All pressure the supercharger receives from the turbocharger is further compressed by whatever the supercharger pulley is rated for. This helps reduce drag on the engine and top speed restrictions isn't as limited.

Simply put, the twincharger concept is a cheaper alternative to the high cost of Nitrous used in many race cars as it provides the instant power without excessive re-filling cost. The twincharger concept itself has not been used on many production cars simply due to the complexity and cost of implementing this system on a vehicle. Another reason simply put is the concept itself is really only viable on sports cars with relatively small displacement engines with high revving capabilities. The main purpose of twincharging is to provide a broad power band, in which the vehicle would perform in just about any RPM speed. This concept is relatively pointless on V8 engines or any vehicles with high torque in stock form as they already possess broad power bands.


Twincharger Vs. Anti-Lag System

Twincharging's biggest benefit over Anti-Lag Systems now being employed in quite a few race cars is the amount of power and reliability it provides. Anti-Lag Systems depend on excessive engine misfiring to keep the turbochargers spooled and thus to maintain the vehicles optimal powerband. Excessive misfiring will cause excessive heat and damage to the turbo manifolds, turbochargers as well as damaging the engine. Anti-Lag Systems make passing Emissions testing next to impossible due to the excessive heat damaging the catalytic converters as well, the Twincharger system does not rely on misfiring at all thus increasing longevity. There are several things such as fuel economy, etc. that are severely compromised.


Other solution

The simplified blow-through system as defined above where the turbocharger feeds the supercharger is inherently flawed as of course the overall airflow is still defined by the rated flow of the supercharger itself, negating any benefits of using the supercharger as the means of purely boosting low engine speed air flow, as it restricts the top end air flow to the same magnitude. A purely supercharged engine would be more efficient. There will be some increases in the SC flow as the air fed to it by the TC becomes more pressurised, but the returns are diminishing and efficiency is reduced.

All systems utilising the twincharging method have a bypass valve that is electronically controlled. At a certain boost and engine speed, the supercharger is bypassed allowing all air to flow past it rather than through it, dedicating the high-flow, high-boost top end power duties to airflow through the turbocharger. The twincharge (or compound charge) systems of today rely heavily on an empirical method where load sites, speeds and airflow characteristics are mapped onto the ecu, which then controls a) the SC bypass valve, and b) the SC clutch. This method can be understood to be not unlike normal engine mapping where fuel and ignition timing are mapped to similar parameters, stored in lookup tables on the ecu, then read and outputted according to what the engine sensors are reading.

As a purely mechanical system, both blow through and suck through systems have their drawbacks when designing a suitable method of transition between the two modes. Historically, compound charged systems relied on the SC blowing into the TC, then when the TC was sucking on the SC hard enough to create a vacuum before the TC intake, a spring loaded valve placed before the TC would open allowing the extra air flow that the turbocharger reqested to be drawn around the SC instead of through it, whilst still using up the boost that the SC was providing. Similarly, the suck through method has a spring loaded valve placed after the turbo charger and before the supercharger, which would open and allow the TC to blow any extra pressurised air around the SC when the SC presented a restriction. These methods of system design are mechanicaly simple, but do not lend themselves to stop start driving where a no-boost moment can be experienced at partial throttle, when the useful boost in the intake escapes back past the partially open valve. It is however suited to cars that spend more time fully accelerating, as during a full bore gearchange the boost would be allowed to vent anyway.