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A supercharger system which boosts the engine via a direct drive from the engine crankshaft has been identified as a possible solution to improve low-end torque and transient response for a conventional turbocharged SI engine. However, the engine equipped with a fixed-ratio supercharger is not as fuel-efficient especially at high load and low speed due to the fact that a large portion of the intake mass air flow has to recirculate through a bypass valve causing inevitable mechanical and flow losses. In addition, the fixed drive ratio of the supercharger which is mainly determined by the full-load requirements might not be able to provide sufficient over-boost during a transient. The fact that a clutch may be necessary for high engine speed operation on the fixed-ratio supercharger system is another issue from the perspective of cost and NVH performance.

Hybrid drivetrain systems are becoming increasingly prevalent in Automotive and Commercial Vehicle applications and have also been introduced for the 2009 Formula1 motorsport season. The F1 development has the clear intent of directing technical development in motorsport to impact the key issue of fuel efficiency in mainstream vehicles. In order to promote all technical developments, the type of system (electrical, mechanical, hydraulic, etc) for the F1 application has not been specified. A significant outcome of this action is renewed interest and development of mechanical hybrid systems comprising a high speed composite flywheel and a full-toroidal traction drive Continuously Variable Transmission (CVT). A flywheel based mechanical hybrid has few system components, low system costs, low weight and dispenses with the energy state changes of electrical systems producing a highly efficient and power dense hybrid system.

Application of high torque capacity Infinitely Variable Transmissions (IVT's) utilizing Torotrak variable ratio traction technology have delivered 20% fuel economy improvement in vehicles ranging from V8 SUV's to diesel city buses. The fuel economy improvement results from a combination of optimised engine operation and significant reduction of launch parasitic losses by the replacement of the torque convertor with a “Geared Neutral” capability inherently provided by the IVT architecture. The conventional Torotrak Variator comprises two toroidal cavities with three rollers per cavity. Each roller is attached to a reaction piston via a carriage assembly, resulting in the active division of the transmission load across the six available power paths. Hydraulic pressure is applied to each roller piston assembly, defining the torque reacted within the Variator.

With the increasing focus on vehicle fuel economy and powertrain efficiency, pressure charging of engines is becoming increasingly prevalent. Building on the successful application of the full-toroidal traction drive CVT in both mechanical hybrid systems for Formula 1 Motorsport and mainstream road cars and as a transmission system for low power and torque systems in the Outdoor Power Equipment market sector, the Torotrak variable drive technology is now applied as a variable drive for supercharger and turbo compounding systems providing enhanced operation and maximising system efficiency. This paper considers the options for providing a variable drive for pressure charging systems to further optimise powertrain efficiencies.

Replacing a conventional automatic transmission with a high torque capacity, full-toroidal traction drive Infinitely Variable Transmission (IVT), delivers fuel economy benefits approaching those achieved by hybrid drivelines when a suitable balance between battery life and fuel economy is implemented, but with minimal additional on-cost, weight and complexity. These benefits are achieved by optimizing the engine operating conditions, eliminating the torque converter and removing torque interruptions during the gearshifts. Recent application of a non-optimal IVT to an Optare Solo bus (an 11,300kg European Midi-Bus) produced fuel savings of 19% over the standard Allison 5-speed automatic transmission. These measurements were independently performed by the internationally recognized Millbrook Proving Ground, UK.

The Torotrak full toroidal traction drive technology has been proven in a range of applications, the most familiar being a V8 SUV where double figure fuel economy improvements were delivered in a durable, smooth and refined package. Novel transmission architectures, new approaches to roller control and improvements in hydraulic design and operation have delivered significant parts count and cost reductions together with system efficiency, performance and package improvements. The result is a family of new transmission design concepts in both ‘clutch start’ Continuously Variable Transmission and ‘geared neutral’ Infinitely Variable Transmission format. Automotive applications are from high torque RWD / SUV vehicles to low torque FWD vehicles. This paper describes the new designs, transmission layouts and roller control mechanisms, with particular focus on FWD / transverse applications in the A, B and C sector vehicles.

The IVT control system can be viewed as having two distinct roles, namely that of steady state and transient torque management. Steady state management functions consist of setting engine power and transmission reaction torque to achieve optimal fuel economy, emissions and driver demanded wheel torque. The transient torque management function defines additional engine and transmission reaction torques, based on known inertia and plant responses, to manage the transition between these steady state operating points according to the driver's wishes, subject to plant constraints. This paper gives a basic overview of the IVT steady state control functions, leading onto a detailed description of the transient torque management function. The software functions are illustrated in block diagram format, using simulation verification data plots and vehicle data logs for validation purposes.

This paper reports the results arising from Torotrak's efficiency optimization program based upon the current Torotrak Series III Infinitely Variable Transmission (IVT) prototype (Fig.1), which was designed for the North American SUV (Sport Utility Vehicles) and luxury car market segment. System optimization is achieved in a number of ways including a variable hydraulic endload system, optimized variator design (in order to minimize variator power loss), minimized referred (or reflected) inertia and reduced power recirculation. The results are presented for the following attributes: Contact/Variator Module/Transmission efficiency. Powertrain specific fuel consumption (SFC). Fuel economy. U.S. City, Highway and Metro Highway drive cycle improvements compared to the standard Ford Expedition 4AT. Vehicle performance. Launch and acceleration performance compared to the standard Ford Expedition 4AT.