Supercharger designed to avoid turbo lag

Typically, a three-cylinder 1-litre engine will now replace a four cylinder 1.6-litre model, with a pressure charging device added to emulate the performance of the larger engine, while retaining superior fuel economy. However, mass uptake of this technology is hampered by the issue of 'turbo lag' that conventional boosting devices produce. The driver experiences this as poor responsiveness and driving feel at low speed and so finding a low cost solution to this problem is an industry priority. As a leading developer of gearless traction drive technologies, Lancashire-based Torotrak has sought to address this problem with its V-Charge variable drive supercharger for gasoline and diesel engines. V-Charge allows downsized engines to maintain their superior emissions levels, whilst retaining the driving feel, urgency and enjoyment of a larger engine. Compact and easy to install, V-Charge offers a substantial cost advantage over more complex devices. By accurately and rapidly varying boost pressure independently of engine speed, V-Charge ensures that the engine responds instantly to any driver demand. The optimum mass of air can always be delivered to the cylinders, at any engine speed, without any of the lag often associated with turbochargers. This remains true even at very low engine speeds where conventional boosting systems struggle. The second generation of the V-Charge (called the V-Charge V2) was presented at the Dresden Supercharger Conference recently. The new unit is 2kg lighter and 50mm shorter than the earlier, proof-of-concept design and is production-feasible, with the potential to use die castings, pressings and powder metal components in its construction. V2 also has improved efficiency, lower power consumption and a greater drive ratio spread than its predecessor. "Since successfully demonstrating the proof-of-concept design in a 1.1 litre car that drove like a naturally aspirated 1.6 litre, we have put our emphasis on optimising the design for production," explained Torotrak product director Andrew de Freitas. "By integrating the design more fully and using purpose-designed components to replace proprietary parts, we have reduced the parts count, size and weight of the unit, making it lighter and easier to package." V2 is 50mm shorter, 25mm narrower and depth is reduced by 20mm compared to its predecessor, making it easier to install on the front end accessory drive (FEAD) of an engine and simplifying the engine bay integration task. Its weight will ultimately reduce from 7kg to 5kg. V2 also outperforms the earlier units. By using an electric oil pump instead of a mechanical type, the power consumed in lubrication drops from 1kW to just 30W. Ratios are now changed by electro-mechanical control with a 10W actuator instead of hydraulics, and no power is required to hold the unit at a given ratio. Hydraulic requirements, and power consumption, have been further reduced by introducing a ball-and-ramp end load system to the variator assembly, instead of applying hydraulic end load. The ratio spread has been increased from 6.25 to 8, allowing greater boost at low engine speeds without an efficiency penalty at higher speeds. The traction epicyclic assembly, which provides a fixed step-up ratio to the compressor impeller, has been reduced in complexity. "Our push for improved overall efficiency reflects the importance of minimising parasitic losses, not just when the unit is boosting, but when off-boost as well," said de Freitas. "We also have the potential to disconnect the drive at small throttle openings. In this respect we have a big advantage because superchargers normally generate a huge inertial shock on the FEAD when re-engaging, but our variable drive can reduce the ratio and the referred inertia from the supercharger at the moment of re-clutching." The variable drive in the V-Charge system allows the supercharger to increase engine torque from 0-95% in less than 400ms, cutting the time-to-torque by up to 70% compared with the latest state-of-the-art single turbocharger technologies. Torotrak believes that V-Charge is the most cost-effective way to produce the level of performance enhancement required on the latest generation of downsized engines.