Hybrid hydraulic vehicles use stored hydraulic energy to reduce fuel consumption. It works similar to regenerative braking used in hybrid electric vehicles (HEV). The difference is that, instead of charging a battery during braking, a hydraulic fluid is pressurized. It is this pressurization process that slows the vehicle down. This pressurized fluid then releases its energy when the vehicle accelerates, allowing most of the braking energy (which would otherwise be lost) to be recouped. This reduces fuel consumption. The figure below shows a schematic of a hybrid hydraulic system.
In the high-pressure accumulator and low-pressure reservoir, the blue represents Nitrogen gas, and the red represents hydraulic fluid. Usually a bladder of some sort is used to separate the hydraulic fluid from the gas. The bladder contains the Nitrogen. It contracts and expands as hydraulic fluid enters and exits the accumulator, respectively.
Hydraulic fluid is much easier to pump than a gas would be, but it cannot be compressed. However, a gas can be compressed and is much better at storing mechanical energy than a fluid. Therefore, a gas – fluid combination is ideal. The Nitrogen gas acts as a gas “spring” which stores and releases energy as the hydraulic fluid shuttles back and forth, in and out of the high-pressure accumulator. Nitrogen gas is used because it is inert and non-explosive at high pressures.
To deliver the necessary power, the pressure inside the high-pressure accumulator must be very high, as much as 5000-7000 psi. The pressure inside the low-pressure reservoir is much lower, 100-200 psi, and serves to provide the necessary pressure differential as the hydraulic fluid is pumped into and out of the high-pressure accumulator.
The accumulator and reservoir are typically constructed out of carbon fiber material which is high-strength and much lighter than steel. Air Cars, which run on compressed air, also use carbon fiber tanks.
When the brakes are applied, the driveshaft drives the actuator in pump mode and the hydraulic fluid is pumped from the low-pressure reservoir into the high pressure accumulator. When the vehicle accelerates, the motor mode is activated and the hydraulic fluid flows out of the high-pressure accumulator, and into the low-pressure reservoir. This powers the driveshaft and transfers torque back to the wheels. The actuator is a pump/motor whose function is determined by the direction of flow of the hydraulic fluid.
As mentioned, the Nitrogen gas acts as an effective energy storage and release medium, similar to a spring. As the high-pressure accumulator is filled with hydraulic fluid during the pumping (braking) stage the Nitrogen compresses and increases in pressure, which increases the pressure of the hydraulic fluid by the same amount. Once the braking action stops a valve shuts off the flow so that the hydraulic fluid remains in the accumulator tank, under pressure until it’s needed. When this valve is released the high pressure fluid flows back through the actuator, enabling it to function as a motor, and the vehicle accelerates.
The hybrid hydraulic system is very effective, with efficiency around 70%. This is comparable to the efficiency of regenerative braking in HEVs, assuming that all the braking energy goes into the hydraulic system. In reality, there are instances where standard friction brakes must be used in addition, because the regenerative system cannot satisfy the braking requirements by itself. For performance, and even safety reasons, a vehicles brakes cannot rely solely on a regenerative system.
Nevertheless, a substantial reduction in fuel consumption can be realized with a regenerative braking system, either electrical or hydraulic.
The use of hybrid hydraulic technology is best suited to larger and heavier vehicles because it can handle the high braking power and acceleration requirements. Hybrid hydraulic systems have a much higher power density than their electric counterparts, meaning they can handle a higher rate of energy input and output delivered during braking and acceleration. However, the energy density of hybrid hydraulic systems is low compared to a regenerative HEV system. This means that for high rates of energy exchange a hydraulic system works best. But for maintaining energy output for longer periods of time, an electric system is best. Batteries used in HEVs work best for sustained charge and discharge at rates that are not very high.
Given the low energy density of a hybrid hydraulic system, it may not be feasible to downsize the vehicles engine with the intention of using stored hydraulic energy to give it a “boost” when required, such as when climbing a hill. For this reason the engine used must match the full power requirement of the vehicle. However, with a HEV system the engine can be downsized since the high energy density of the batteries can make up the power difference when required. And by design, this can allow the engine to operate within its most efficient power bands, further increasing fuel efficiency.
In smaller passenger vehicles, hybrid hydraulic is incompatible with the tighter space restrictions – in particular due to the size of the accumulator tanks. In passenger vehicles space is a premium. On the other hand, electric regenerative braking is used for passenger HEVs because it is compact in size and the lower power density requirement is well suited to battery capacity. In addition, hybrid hydraulic systems tend to be noisy, which interferes with the design criteria of passenger vehicles.
Optimizing Performance
The efficiency of a hybrid hydraulic system is lower at smaller (braking and acceleration) loads than at higher loads. The typical delivery truck benefits most from a hybrid hydraulic drive, operating at moderate speeds in urban environments, and making frequent stops.
In delivery trucks, the fuel economy can increase between 28 – 48% (ref: http://www-personal.umich.edu/~hpeng/VSD_from_AVEC_HHV.pdf)
Comparison With Regenerative Braking In Hybrid Electric Vehicles
Looking at http://www.fueleconomy.gov, the fuel economy of a 2008 Toyota Camry hybrid is 33 mpg for city driving, and 34 mpg for highway driving. For a regular 2008 Toyota Camry, the fuel economy is 21 mpg for city driving and 31 mpg for highway driving. Both are equivalent mid-size cars, front-wheel drive, with 4 cylinder, 2.4 L engines, and with automatic transmissions. The electric regenerative braking represents a 57% increase in fuel economy for city driving and a 10% increase for highway driving. This compares well with the fuel economy increase in hybrid hydraulic vehicles.
As mentioned before, the problem with using HEV technology in heavier vehicles, such as trucks, is that batteries are not well suited to very high charging and discharging rates. Ultra-capacitors, however, can possibly eliminate this problem in the future, allowing electric regenerative braking to be used for heavier vehicles, as they can rapidly store a high charge and rapidly discharge it. This would better match the high power requirements of braking and acceleration cycles, in heavier vehicles.
Good links for more information:
http://auto.howstuffworks.com/hydraulic-hybrid1.htm
http://www.epa.gov/otaq/technology/420f06043.htm
http://files.harc.edu/Projects/Transportation/HydraulicHybridsGray.pdf