Monday, August 29, 2005

Twin-Turbocharged FSI Engines

Text from Volkswagen AG.
“Twincharger” offers maximum power with minimum consumption

Compact high-performance power pack with 7.2 l/100 km (39.2 mpg) consumption

Very high power output per litre (90 kW / 121 PS) from a series production four-cylinder engine


WOLFSBURG, Germany - At the International Motor Show (Cars) in Frankfurt, Volkswagen is presenting to the general public a ground-breaking innovation in the drive sector: The world’s first twin-turbocharged FSI engine – the “Twincharger”.

The compact 1.4 litre direct-injection engine develops up to 125 kW / 170 PS and has a maximum torque of 240 newton metres in the range from 1750 to 4500 rpm thanks to the combination of an exhaust turbocharger with a mechanically driven compressor.

The 1.4 litre engine delivers a power output of 90 kW / 121 PS per litre, representing a peak value for a series production four-cylinder engine. Furthermore, the “Twincharger” delivers a torque corresponding to a naturally aspirated engine with a swept volume of approx. 2.3 litres. And its fuel consumption is around 20 percent lower.

Another performance variant of this innovative TSI engine with 103 kW / 140 PS (maximum torque 220 newton metres) will be available from early 2006, initially in the Touran compact MPV, and after that the Golf will also be available with this engine.

THE TASK
Reducing consumption values –
Increasing driving performance


It is the declared objective of European car makers to reduce CO2 emissions. This will be done in various steps, to a value of 140 grams per kilometre. Reduction in CO2 emissions goes hand-in-hand with a reduction in fuel consumption. Achieving this ambitious target will require a combination of the latest engine technology with driveline optimisation.

However, this is not enough. As well as the consumption reduction, it was specified that there had to be a full torque characteristic combined with a high standard of quality and a long service life. In addition, the engine had to be compact to allow it to be integrated into many different vehicle concepts. And, it would have to be designed to enable straightforward production in high quantities. Another target was concerned with resolving numerous conflicting objectives in an innovative way. To cut a long story short – we succeeded.

THE CONCEPTUAL IDEA
A compact FSI engine with twin,
different supercharging as an approach to the solution



The most effective way to reduce consumption is referred to as downsizing. A reduction in cubic capacity and therefore lower friction losses result in a low specific consumption, which equates to better efficiency. However, an engine with a low cubic capacity only meets the current requirements for active road safety and pleasurable driving to a very limited extent. As a result, the objective can only be achieved by supercharging. Classic turbo engines with a small cubic capacity supercharged using exhaust turbochargers have only been used to a very limited extend in the past since they have low moving-off power and are therefore less acceptable. This problem can be solved by a mechanically driven supercharger that supplies additional fresh air to the engine even at low speeds. The challenge was to combine these two systems in a rational way.

The only candidate for injection technology was the FSI technology that is now used by Volkswagen in numerous model ranges. Experience gathered during the last few years by engine developers at Volkswagen in this injection technology had revealed that FSI could be ideally complemented by the two different supercharging techniques, the result being a previously unheard of increase in efficiency.

This gave rise to the world’s first direct-injection SI engine with twin supercharging for use in high-volume series production – the “Twincharger”.

THE IMPLEMENTATION
Compressor for power at low speeds,
turbocharger for power at high speeds


The choice for the basic power unit was the FSI from the EA 111 engine series as used in the Golf in power levels of 66 kW / 90 PS (1.4-litre) or 85 kW / 115 PS (1.6-litre). The 1.4-litre engine is a four-valve four-cylinder engine with a swept volume of 1390 c.c., a cylinder gap of 82 millimetres and a bore/stroke ratio of 76.5 to 75.6 millimetres. The focus in developing the “Twincharger” engine was placed on designing a new, highly resilient grey cast iron cylinder crankcase in order to withstand the high pressure of up to 21.7 bar over long periods, a water pump with integrated magnetic clutch and supercharging technology.

However, the injection technology was also modified. A multiple-hole high-pressure injection valve with six fuel outlet elements is used for the first time in the 1.4 l TSI engine. The injector, like that in the naturally aspirated FSI engines, is arranged on the intake side between the intake port and cylinder head seal level. The quantity of fuel to be injected between idling speed and the 90 kW/litre output power requires a wide variability in the fuel flow through the injectors – given a sufficient mixture preparation time after completion of injection under full-load conditions on the one hand and idling speed with reproducibly low injection volumes on the other hand. The maximum injection pressure was increased to 150 bar in order to achieve this wide range of throughflow. Furthermore, only FSI technology made it possible to achieve a compression ratio of 10:1 which is high for supercharged engines.

The Volkswagen engine developers selected a compressor with a mechanical belt drive in order to increase the torque at low engine speeds. This is a supercharger unit based on the Roots principle. One special feature of the compressor used is its internal step-down ratio on the input end of the synchronisation gear pair.

The exhaust turbocharger also kicks in at higher engine speeds (with wastegate control). The compressor and exhaust turbocharger are connected in series in this case. The compressor is operated by a magnetic clutch integrated in a module inside the water pump. A control flap ensures that the fresh air required for the operating point can get through to the exhaust turbocharger or the compressor. The control flap 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.

One of the major challenges facing the development was to achieve the best possible interplay between the two superchargers arranged in series. Only when both units – the compressor and the exhaust turbocharger – complement one another optimally can the small power unit achieve its required, level torque characteristic over a broad engine speed range in conjunction with a previously unheard of increase in efficiency.


THE RESULT
A twin-turbocharged FSI
with two power levels


The ambitious objective of squeezing an output per litre in excess of 90 kW per litre swept volume out of a 1400 c.c. engine could not be achieved with single-stage supercharging alone. However, an upstream compressor enables the boost pressure buildup of the exhaust turbocharger to be significantly increased.

The maximum boost pressure of the “Twincharger” is approx. 2.5 bar at 1500 rpm, with the exhaust turbocharger and the mechanical supercharger being operated with about the same pressure ratio (approx. 1.53). A straight exhaust turbocharged engine without compressor assistance would only achieve a pressure ratio of about 1.3 bar here. The more rapid response of the exhaust turbocharger enables the compressor to be depressurised earlier by continuous opening of the bypass valve. This means compressor operation is restricted to a narrow map area with predominantly low pressure ratios and, therefore, low power consumption. Consequently, the disadvantage of the mechanical supercharger system in terms of consumption can be limited.

In practice, this means the compressor is only required for generating the required boost pressure in the engine speed range up to 2400 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. In dynamic driving, this is inadequate for the specified in-gear acceleration values in the low engine speed range. In these driving situations, the compressor is engaged to permit a spontaneous boost pressure buildup. The way in which these two systems complement each other means there is absolutely no turbo lag. The compressor is no longer needed above an engine speed of 3500 rpm at most, as the exhaust turbocharger can definitely provide the necessary boost pressure even dynamically during the transition from coasting to full-load operation.

THE DRIVING EXPERIENCE
High performance and torque
produce low consumption and enormous driving pleasure


The compressor, with its high ratio of 1:5 in relation to the crankshaft, delivers a boost pressure of 1.8 bar even just above idling speed. This provides the power needed when moving off. An electromagnetic clutch integrated in the module of the coolant pump is responsible for switching the compressor on and off. It is driven by an additional belt. A torque of 200 newton metres is available at a speed of only 1250 rpm – and all the way through to 6000 rpm. In dynamic compressor mode, the automatic boost pressure control decides whether the compressor will be switched on in accordance with the tractive power required, or if the turbocharger alone can generate the necessary boost pressure. 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 3500 rpm.

In practice, the “1400 Twincharger” drives like a big naturally aspirated engine with 2.3-litre cubic capacity. This is because the maximum torque of 240 newton meters is available from 1750 rpm to 4500 rpm. The boost pressure gauge installed as standard in the cockpit of the Golf GT 1.4 TSI is the only signal of the furious activity being undertaken by the superchargers and the complex procedure of harmonizing both systems taking place under the engine hood. The driver likes it, because when the needle is fully deflected then the acceleration really presses the occupants back into their sports seats (fitted as standard).

Power/torque characteristic of 1.4 TSI 125 kW

The smooth torque characteristic allows the driver to refrain from gear changes whilst still driving briskly. It goes without saying that the “Twincharger” is much more free revving than a diesel engine. Indeed, the 1.4 TSI has a maximum speed of 7000 rpm. Thanks to this outstanding engine performance, overtaking manoeuvres on country roads are particularly enjoyable and much more rapid than is the case with a naturally aspirated engine. The value for in-gear acceleration from 80 to 120 km/h (50 to 74.5 mph) in fifth gear in 8,0 seconds can only serve as a reference here. Active safety has seldom been improved in this way without having an effect on consumption.

This is because very low consumption values are possible due to the generous torque and the high level of power that allow a correspondingly relaxed driving style. In the Golf GT, the 1.4 TSI gets along with only 7.2 l/100 km (39.2 mpg) of Super Plus petrol. This is about 20 percent less than in a naturally aspirated engine with comparable torque and power and a cubic capacity of approx. 2.3 litres. In interurban transport, indeed, the “Twincharger” veritably sips only 5.9 l/100 km (47.9 mpg).

In combination with the direct shift gearbox available for the “Twincharger” from early 2006 onwards, the power developed by the 1.4 TSI will be appreciated even more due to the gearshifts without any interruption in traction. And what is more, the advantage in terms of consumption, far from being reduced by this innovative automatic, is in fact increased.

It is possible to activate the winter programme using a switch in front of the selector lever in the centre console of the Golf GT to prevent too much torque being sent to the front wheels on a snowy or icy road. This reduces the moving-off torque and therefore prevents the drive wheels from spinning.

The second power variant of the TSI reveals that this innovative engine technology is not only intended for a sporty model variant but will also be used across the board. With 103 kW / 140 PS and a maximum torque of 220 newton metres, this engine variant will also appeal with its smooth and masterly engine performance. This variant of the TSI will be used first in early 2006 in the Touran.

QUALITY AND PRODUCTION
High-quality materials and assembly
assure a long service life


The selection of materials that are resistant to high-temperatures does more than make it possible to keep consumption down to the best possible level at high speed. In spite of the high output per litre, the high pressure level in the engine and possible engine speeds of up to 7000 rpm, the “Twincharger” is designed for a long service life – with the same criteria that apply to all power units from Volkswagen. More than 250 prototype and pilot series engines have been put through their paces in all necessary test cycles. Every single component of this new power plant has been designed for the engine service life and has come through its baptism of fire. Endurance runs corresponding to a mileage of 300,000 km (186,420 miles) have been successfully completed. The cylinder crankcase is made from grey cast iron and guarantees complete operating reliability even at the high peak pressures of up to 120 bar. The highly qualified personnel at the Chemnitz Engine Works use optimised production processes and the latest measuring technology to ensure that these high-tech power plants are assembled without defects.

Comparison of performance and consumption values


Golf GT 1.4 Twincharger

Golf 2.0 FSI

Swept volume

1390 cc

1984 cc

Bore / stroke

76.5/75.6 mm

82.5/92.8 mm

Valves per cylinder

4

4

Compression

10.0 : 1

11.5 : 1

Supercharging

Compressor
Turbocharger

-

Boost pressure

2.5 bar

-

Power

125 kW / 170 PS at 6000 rpm

110 kW / 150 PS at 6000 rpm

Torque

240 Nm at 1750 – 4500 rpm

200 Nm at 3500 rpm

0 – 100 km/h
(0 – 62 mph)

7.9 s

8.8 s

In-gear 80 – 120 km/h (50 to 74.5 mph) 5th gear

8,0 s

9.0 s

Maximum speed

220 km/h
136 mph

209 km/h
(130 mph)

Consumption

Urban

9.6 l
29.4 mpg

10.6 l
26.6 mpg

Interurban

5.9 l
47.9 mpg

5.9 l
47.9 mpg

Overall (l/100 km / mpg)

7.2 l / 39.2 mpg
(98 ROZ)

7.6 l / 37.2 mpg
(98 ROZ)

CO2 emissions combined

173 g/km

182 g/km

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