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WHAT ARE HYDRAULIC HYBRID VEHICLE SYSTEMS?
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Hydraulic Hybrid systems offer a robust, cost-effective means to achieve significant fuel economy and exhaust emissions reductions in both the near-, mid- and long-term. The first Hydraulic Hybrid systems to be commercialized will likely be hydraulic regenerative braking systems. During braking, the vehicle’s kinetic energy is captured as the pump/motor pumps hydraulic fluid from the low-pressure reservoir to the high-pressure accumulator. The fluid compresses nitrogen gas in the high-pressure accumulator and pressurizes the system. This recovered energy is used to supplement or substitute the engine’s power during acceleration.
During acceleration, fluid in the high-pressure accumulator drives the pump/motor as a motor, thus providing torque to the driveshaft. These systems leverage proven technologies and can be commercially available before the end of the decade. One key advantage of the Hydraulic Hybrid regenerative braking system is that it can be installed in the existing powertrain without substantial modifications to the existing vehicle architecture.
This provides OEMs with a near-term lower risk and lower cost hybridization option on a wide range of vehicles ranging from Class 2B through Class 8. This technology is particularly beneficial on vehicles with a significant stop/start duty cycle. Advantages to the user are reduced fuel use and reductions in maintenance, particularly brakes.
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How Does a Parallel Hydraulic Hybrid System Work?
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How Does a Series Hydraulic Hybrid System Work?
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Advanced Hydraulic Hybrid control systems derive their fuel economy improvement from the use of four design and control strategies: 1) recovery and reuse of over 70% of braking energy (known as regenerative braking), 2) optimization of engine operation at the “sweet” spot, 3) reduction of engine operation (e.g. engine is shut-off when the vehicle isn’t moving, so there is never any engine idling), and finally, 4) improve engine efficiency from enabling new innovative efficient engine concepts.
The Parallel Hydraulic Hybrid System utilizes design strategy 1). The Series Hydraulic Hybrid System takes advantage of all four strategies.
There are a number of possible mechanical arrangements, but the simplest implementation is where the conventional vehicle transmission and driveline components are replaced by a system much like a hydrostatic transmission. With this arrangement the engine drives a hydraulic pump, and hydraulic motors drive the road wheels. Energy from braking can be stored in a hydraulic accumulator for re-use in the same way as in a parallel hydraulic hybrid system.
For road vehicles to be successful with this arrangement both mechanical and hydraulic efficiencies of the pump and motor units must be considerably higher than the current state of the art hydraulic pumps and motors used in industrial, off-highway and aerospace operations. With such a system, the cost and weight of the hydraulic system elements is offset by the elimination of the transmission and driveline components, and very significant fuel savings can be achieved with a wide range of vehicle types and operational cycles.
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