Tuesday, December 11, 2007

Future of Engine Thermodynamics: The Twincharger

Inside VW’s New “Twincharger” TSI Engine

The TSI Twincharging systems

VW’s goal for its new dual-charged (“Twincharger” in VW marketing-parlance) engine (earlier post) was to combine the low-end power boost provided by a mechanically-driven compressor (supercharging) with the higher-end increase provided by an exhaust turbocharger (turbocharging) to enable the downsizing of the engine for a given application while maintaining the driving experience for consumers.

Put another way, downsizing delivers comparable (or better) performance with lowered fuel consumption and emissions.

The first instance of this new Twincharged TSI engine family is a 90-kW (121-hp) 1.4-liter model that delivers a torque corresponding to a 2.3-liter engine, but with 20% less fuel consumption. Compared to the 2.0-liter FSI engine in the Golf, the power and torque gains are clear, although the decrease in fuel consumption is more modest. (See chart below.)


Golf GT 2.0 FSIGolf GT 1.4 TSITSI %
Displacement 1,984 cc 1,390 cc -30%
Cylinders 4 4
Compression 11.5:1 10:1
Boost Pressure 2.5 bar
Power 110 kW (148 hp) 125 kW (168 hp) +14%
Torque 200 Nm 240 Nm +20%
0–100 km/h 8.8 s 7.9 s -10%
Maximum speed 209 km/h (130 mph) 220 km/h (136 mph) +5%
Fuel consumption 7.6 l/100km 7.2 l/100km -5%
Fuel economy 31 mpg US 32.7 mpg US +5%
CO2 182 g/km 173 g/km -5%

Super-and turbo-charging systems are designed to force more air into the cylinder, thereby enabling more combustion and delivering more power—but also consuming more fuel than a comparable naturally-aspirated engine. However, the increase in fuel consumption of a charged engine is more than offset by the overall decrease in fuel consumption resulting from using a smaller engine.

For example, the 1.4-liter TSI is 39% smaller than the 2.3-liter FSI, but consumes 20% less fuel. As long as a downsized TSI is used to replace a larger FSI, there is a net gain in efficiency.

As a starting point for developing the Twincharged family, VW selected the direct-injection FSI from its EA 111 engine series as used in the Golf.

The basic FSI 1.4-liter engine (1,390 cc) is a 66-kW (88-hp), four-valve, four-cylinder engine. Note that the Twincharger 1.4-liter TSI offers 36% more power than its FSI cousin of the same displacement: 90 kW vs 66 kW.

To support the twincharging concept, VW engineers had to deliver a new, highly-resilient gray cast-iron cylinder crankcase to withstand the higher pressures, a coolant pump with an integrated magnetic clutch and supercharging technology.

VW also modified the injection system, introducing its first multiple-hole, high-pressure injection valve with six fuel outlet elements.

The injector, like that in the naturally-aspirated (non-charged) FSI engines, is arranged on the intake side between the intake port and cylinder head seal level.

To support the wider variability in the quantity of fuel needed across the range of operation (from idling speed to the 90-kW peak power output) to optimize the twincharging, VW increased the maximum injection pressure to 150 bar.

For the compressor, Volkswagen engineers chose a Roots-type supercharger (also known as a “blower”). Unlike some other types of supercharger, a Roots supercharger doesn’t actually compress air within the device. With two counter-rotating lobes, it moves a fixed volume of air per rotation (“fixed displacement”). Compression occurs in the intake manifold.

Roots superchargers can deliver a large amount of boost even at low engine speed. The main disadvantage is that they create a lot of heat.

Air flow through the VW Twincharged TSI. Click to enlarge.

The compressor and the turbocharger are connected in series. A control valve ensures that the fresh air required for a given operating state can get through either to the exhaust turbocharger or the compressor.

The control valve is open when the exhaust turbocharger is operating alone. In this case, the air follows the normal path as in conventional turbo engines, via the front charge-air cooler and the throttle valve into the induction manifold.

The compressor is operated by a magnetic clutch integrated in a module inside the water pump. Under turbocharging conditions, the clutch disengages the compressor.

The maximum boost pressure of the Twincharger is approximately 2.5 bar at 1,500 rpm, with the exhaust turbocharger and the mechanical supercharger being operated with about the same pressure ratio (approx. 1.53). The compressor alone delivers a boost pressure of 1.8 bar even just above idling speed.

A conventional exhaust turbocharged engine without compressor assistance would achieve only a pressure ratio of about 1.3 bar.

The more rapid response of the turbocharger enables the compressor to be depressurized earlier by continuous opening of the bypass valve. Compressor operation is restricted to a narrow engine map area with predominantly low pressure ratios and, therefore, low power consumption.

In practice, this means the compressor is only required for generating the required boost pressure in the engine speed range up to 2,400 rpm. The exhaust turbocharger is designed for optimum efficiency in the upper power range and provides adequate boost pressure even in the medium speed range.

For acceleration, an automatic boost pressure control decides if the compressor needs to be switched on to deliver the tractive power required, or if the turbocharger alone can handle the situation.

The compressor is switched on again if the speed drops to the lower range and then power is demanded again. The turbocharger alone delivers adequate boost pressure above 3,500 rpm.


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