1. Field of the Invention
This application is directed to hydraulic hybrid vehicle technology, and in particular to a drive system thereof.
2. Description of the Related Art
Significant interest has been generated, in recent years, in hybrid vehicle technology as a way to improve fuel economy and reduce the environmental impact of the large number of vehicles in operation. The term hybrid is used in reference to vehicles employing two or more power sources to provide motive energy to the vehicle. For example, hybrid electric vehicles are currently available that employ an internal combustion engine to provide power to a generator, which then generates electricity to be stored in a battery or storage cells. This stored power is then used, as necessary, to drive an electric motor coupled to the drive train of the vehicle.
There is also interest in the development of hybrid hydraulic vehicles, due to the potential for greater fuel economy, and a lower environmental impact than hybrid electric vehicles. Inasmuch as the present invention is directed to innovations and improvements in hybrid hydraulic technology, where reference is made to hybrid vehicles, or hybrid technology, it may be assumed that the reference is directed to hydraulic hybrids in particular, unless otherwise noted.
Hybrid vehicles may be grouped into two general classes, namely, parallel hybrid and series hybrid vehicles. Parallel hybrid vehicles are vehicles employing a more or less typical engine, transmission, and drive train, with hydraulic components operating alongside. For example, FIG. 1 shows what is commonly referred to as a launch assisted vehicle 100. The vehicle 100, shown in wire-frame to illustrate selected components, includes an internal combustion engine 102, a transmission 104, a drive shaft 106, differential 108, drive axle 110, and drive wheels 112, as may be found in conventional vehicles. However, the vehicle 100 also includes a hydraulic pump/motor 114, in this case coupled to the drive shaft 106, and high and low pressure hydraulic accumulators 116, 118.
A hydraulic pump/motor is a device that functions as a motor when fluid from a high-pressure fluid source is used to impart rotational force to an output shaft. On the other hand, if rotational force is applied from an external source to rotate the shaft, the device may be used as a pump, to pump fluid from a low pressure fluid source to high pressure.
During normal operation, the vehicle 100 operates in a manner similar to conventional vehicles. However, when the vehicle operator applies the brake, the pump/motor 114 is coupled to the drive shaft 106 such that rotation of the drive shaft 106 provides energy to draw fluid from the low pressure accumulator 118 and pump the fluid at high pressure to the high pressure accumulator 116. Engagement of the pump/motor 114 in this manner creates drag on the drive shaft, which is transferred to the drive wheels 112, slowing the vehicle 100. In this way, a portion of kinetic energy of the moving vehicle is recovered and stored as hydraulic fluid under pressure. When the vehicle 100 is pulling away from a stop, or accelerating, the pump/motor 114 is again coupled to the drive shaft 106, while the pump/motor 114 is switched to motor mode, in which pressurized fluid drives the pump/motor 114, which in turn adds rotational energy, or torque, to the drive shaft 106. In this way, the pump/motor is utilized in these two modes such that energy that would otherwise be lost to friction in the brakes of the vehicle is stored, to be released later to assist the vehicle 100 in accelerating.
According to another parallel hybrid scheme, the engine of a vehicle is used to drive a pump to pump fluid at high pressure into an accumulator. This is done during periods when the vehicle is cruising at a steady speed, or otherwise demanding less than the engine is capable of providing when operating at its most efficient load.
It is known that internal combustion engines used in motor vehicles are required to have a maximum output capacity that far exceeds the average requirements of the vehicle, inasmuch as such vehicles occasionally require power output levels far exceeding the average power output. For example, during acceleration from a stop, or for passing, etc., much more power is required than during periods when the vehicle is cruising at a steady speed.
By using excess capacity of the engine to drive the fluid pump, the load on the engine can be increased to a point where the engine operates at a high level of fuel efficiency, while the excess energy is stored as pressurized fluid. Again, the energy stored as pressurized fluid may then be used to supplement the engine during periods when power requirements of the vehicle exceed the engine's maximum efficient output. This scheme may be implemented using a configuration similar to that shown in FIG. 1, in which the single pump/motor 114 is used to provide all the pumping and motoring function, or a second hydraulic pump may be provided, which is configured solely to be coupled to the engine 102 for the purpose of pumping fluid to the high pressure accumulator 116.
Other parallel hybrid configurations are also known in the art, and will not be discussed in detail here.
Series hybrid vehicles have no direct mechanical link between the engine and the drive wheels of the vehicle. They do not employ a transmission or drive shaft as described with reference to parallel hybrid vehicles. In a series hybrid vehicle, a hydraulic pump is coupled directly to the crankshaft of the engine of the vehicle. All of the energy output of the engine is used to pump fluid from a low pressure accumulator to a high pressure accumulator. A second pump/motor is coupled to the drive wheels of the vehicle, and is driven by pressurized fluid from the high pressure accumulator. In such a vehicle, the engine may be operated with a load, and at a speed selected to provide maximum efficiency and fuel economy, without regard to the constantly varying speed of the vehicle itself.
The configuration and operation of parallel and series hybrid vehicles are described in detail in the following references: U.S. Pat. No. 5,887,674, U.S. patent application Ser. No. 09/479,844, and U.S. patent application Ser. No. 10/386,029, all of which are incorporated herein by reference, in their entirety.
Although hydraulic drive equipment has been used on commercial and off-road equipment and mobile devices for many years, hydraulic drive equipment has not yet found successful commercial application for on-road, private and multi-passenger vehicles as part of a “hybrid” powertrain. Such lack of implementation of hydraulic drive equipment in passenger vehicles has thus far prevailed in the prior art despite the tremendous fuel economy benefits that could be obtained for such vehicles through use of a hydraulic hybrid powertrain. As is known in the art, a principal obstacle to implementation of hydraulic drive equipment in passenger vehicles as a hybrid powertrain is the challenge of packaging the added hydraulic equipment (e.g., pump(s), motor(s), accumulators, hoses) in addition to conventional drivetrain components (e.g., engine, transmission, differential, etc.) into the very limited space generally available to such components in conventional passenger vehicle frames and styles. Furthermore, the increase in cost and weight created by the addition of hydraulic equipment to the conventional drivetrain components in such vehicles somewhat offsets the benefits of a hydraulic hybrid drivetrain, by reducing the fuel economy benefits of the technology (due to increased vehicle weight) while simultaneously increasing vehicle cost.