Commercial interest in hybrid vehicle technology has grown at a much more dramatic rate than was predicted a decade ago. Around that time, many industry observers were substantially more optimistic about a major leap from current petroleum based technology straight to hydrogen, fuel cells and bio fuel systems. However, it is now widely accepted that hybrid vehicles will have a significant role to play over the next couple of decades as these other technologies continue to be developed.
The development of power splitting transmissions (PST) has been a crucial feature in the technological success of hybrid driveline vehicles have played a key role in facilitating the management of the mechanical and electrical power flows, ensuring good drivability, providing improved economy and reducing emissions compared to conventional internal combustion engine vehicles.
The aim of HybriDrive project was to analyse the various PST systems, to compare the performance characteristics of competing designs and to make some observations about the future direction of the technology.
The first successful electromechanical PSD transmission is generally credited to TRW. The key feature is to use an epicyclic gear to combine the power from the engine and two MG units. Note that two MG units are required in order to exercise sufficient control of both speeds and torques in the system. The transmission effectively has two functions - it provides a continuously variable gear ratio over a certain range and it selects the balance of torques, and hence powers, applied to and from the drive wheels by the engine and MG units Fig 1). The Toyota Prius, introduced in Japan in 1997, used this transmission design, with an IC engine of 52 kW and MG units of 33 and 10 kW, and is now widely recognised as making it commercially successful.
Figure 1. Toyota Prius, THS arrangement
An alternative version of the single mode design by NexxtDrive for example, employs two epicyclic gears in conjunction with two MG units (Fig 2). This arrangement offers potential efficiency benefits over the single planetary gear systems because there is more flexibility in controlling the balance of power flows through the mechanical and electrical branches. It has an important benefit of providing two points at which the electrical path transmits zero power, and therefore no losses. If the system is designed so that these occur at common vehicle operating points, e.g. in the UK, 30 mile/h in urban driving and 70 mile/h motorway cruising, then considerable fuel efficiency improvements can be gained.
Figure 2. NexxtDrive twin epicyclic arrangement
A software tool, TSAT, was developed as part of the project specifically to analyse these types of system which comprise collections of epicyclic gears, brakes, clutches and motor generator units interconnected in a wide variety of possible ways.
Historically, the role of the power splitting transmissions in the overall development of hybrid vehicle technology has commonly been underestimated. But PSTs have actually played a crucial role in managing the electrical and mechanical power flows and ensuring good drivability and efficiency. One of the overall conclusions of the HybriDrive project is that whilst the first generation, single mode units have proved to be adequate in the small/mid size passenger car sector, dual mode systems are set to become dominant in the large car, SUV and commercial vehicle sectors.