Parallel-Hybrid Transmission
On any given day a passenger vehicle can be found stuck in traffic, navigating surface streets or traveling the freeways. Conventional vehicles meet these varying demands in a manner somewhere between two extremes: the "Corvette" approach, and the "Geo Metro" approach. The "Corvette" approach, although most fun to drive, is no where near efficient as its "Geo Metro" counterpart. The "Geo Metro", although more efficient, ... well you know the rest. What if you could have both great performance and efficiency in a vehicle? In order for hybrid vehicles to seriously compete with the conventional types, they must be equally capable of meeting operator demands. With a few caveats specific to transmission design, the parallel-hybrid is capable of both high performance and high efficiency. By letting the electric motor dominate during the transients of urban driving and high-speed passing, a relatively small diesel engine may be used for steady-state demands.
The purpose of the transmission is to adapt the multiple power inputs of the hybrid vehicle to the demands made upon the drive wheels by the operator. Each component adds several parameters, and the sum of these results in the overall design.
The choice of a large capacity AC induction motor reduces the need for the multiple gear ratios common to conventional transmissions, even when compared to the DC motor most often found in an electric vehicle. That this motor has an efficiency peak centered at 8000 rpm, however, means that the overall reduction from the motor to the drive wheels must be higher than that of a conventional transmission.
The TDI engine to be used is also well suited to a minimum of multiple ratios, since the torque peak is separated from the power peak by half the available rev range. Consequently, the hybrid transmission is currently designed as a mere 2-speed.
Operator demand is the broadest parameter set influencing transmission design. It is quantified as a driving cycle; plotted as vehicle velocity over time. The various driving cycles may be broadly categorized into either urban or highway types. For this reason as well, the hybrid transmission is currently designed as a 2-speed.
There is yet another reason for a 2-speed, if the hybrid is to perform a variable speed/ torque splitting function. The need for this function would occur when vehicle velocity is too low and/or too variable to maintain the state of charge of the batteries, e.g., a traffic jam. From the standpoint of transmission design, the simplest solution would be to pull off the road, disengage the drive-wheels, and let the engine restore the state of charge at optimum revs for both engine and motor/generator. However, the necessity of doing this removes the parallel hybrid from the realm of functional equality with a conventional vehicle.
The accompanying drawing shows the current state of transmission development. The central shaft connects the axle shaft to the engine shaft and the motor shaft. The motor has a primary reduction of 3:1 with respect to the engine shaft. The engine and/or motor torque may be connected to the axle shaft at either 1:1 or 2:1. Disengaging the small central shaft gear from the central shaft converts it to an idler, which allows engine starting and motor/generator charging when the vehicle is at rest. The final reduction is as a conventional transverse FWD application.
Applying a concentric hub to the engine shaft with a speed dependent clutch, permits variable drive wheel speeds via the 2:1 reduction, as well as accommodating generator speeds via the 1:1 reduction. It is important to note that the velocity in the 2:1 gear set will vary from a zero to 4500 rpm as restricted by the engine, while the motor/generator speed will never fall below 6000 rpm. This makes the restoring state of charge possible in stop-and-go driving situations. While actuated, the engine will seem as if at high idle: with additional engine speed causing the vehicle to move from rest. This "traffic-jam" mode will provide a maximum vehicle velocity of 50 mph and have a range limited only by the fuel available.
The transmission housing is the most recent addition to the project. FEA was used to optimize wall thickness and bearing mounts, which makes the stiffness/weight ratio as high as possible, given the material limits of aluminum alloy.
HEV Team
Department of Mechanical Engineering
San Diego State University
5500 Campanile Dr.
San Diego, CA 92182-1323
Fax: (619) 594-3599
E-mail: hev@kahuna.sdsu.edu