Hydraulic hybrid vehicles (HHVs) use a pressurized fluid power source, along with a conventional

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

(ICE), to achieve better fuel economy and reductions in harmful emissions. They capture and reuse 70–80% of the vehicle's kinetic braking/decelerating energy and potential descending energyHydraulic Hybrid Vehicles
US EPA compared to 55% for electric hybrids. For trucks and buses, this can also be less expensive than electric systems, due to the price of batteries required for the latter. Hydraulic hybrid vehicle systems can also weigh less than electric systems, due to the high weight of the batteries. This can lead to a lower impact on payload capacity, especially for heavy vehicle classes.

Principle of operation

Hydraulic hybrid vehicle systems consists of four main components: the working fluid,

reservoir A reservoir (; ) is an enlarged lake behind a dam, usually built to water storage, store fresh water, often doubling for hydroelectric power generation. Reservoirs are created by controlling a watercourse that drains an existing body of wa ...

, pump/motor (in parallel hybrid system) or in-wheel motors and pumps (in series hybrid system), and accumulator. In some systems, a hydraulic transformer is also installed for converting output flow at any pressure with a very low power loss.HyDrid brochure
, Innas BV In an electric hybrid system, energy is stored in the battery and is delivered to the

electric motor An electric motor is a machine that converts electrical energy into mechanical energy. Most electric motors operate through the interaction between the motor's magnetic field and electric current in a electromagnetic coil, wire winding to gene ...

to power the vehicle. During braking the

kinetic energy In physics, the kinetic energy of an object is the form of energy that it possesses due to its motion. In classical mechanics, the kinetic energy of a non-rotating object of mass ''m'' traveling at a speed ''v'' is \fracmv^2.Resnick, Rober ...

of the vehicle is used to charge the battery through the

regenerative braking Regenerative braking is an energy recovery mechanism that slows down a moving vehicle or object by converting its kinetic energy or potential energy into a form that can be either used immediately or stored until needed. Typically, regenerativ ...

. In hydraulic hybrid system, the pump/motor extracts the

during braking to pump the working fluid from the

to the accumulator. Working fluid is thus pressurized, which leads to

energy storage Energy storage is the capture of energy produced at one time for use at a later time to reduce imbalances between energy demand and energy production. A device that stores energy is generally called an Accumulator (energy), accumulator or Batte ...

. When the vehicle accelerates, this pressurized working fluid provides energy to the pump/motor to power the vehicle. For a parallel hybrid system, fuel efficiency gains and emissions reductions result from reduced mechanical load on the internal combustion engine due to the

torque In physics and mechanics, torque is the rotational analogue of linear force. It is also referred to as the moment of force (also abbreviated to moment). The symbol for torque is typically \boldsymbol\tau, the lowercase Greek letter ''tau''. Wh ...

provided by the hybrid system.

Efficiency gains

The US EPA claims that in laboratory tests, the city fuel economy of an urban delivery truck was 60–70% increased miles per gallon versus a similar, conventionally powered internal combustion truck. The CO₂ emissions of the same demonstration delivery truck were claimed to be over 40% lower, and the

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

and

particulate matter Particulate matter (PM) or particulates are microscopic particles of solid or liquid matter suspended in the air. An ''aerosol'' is a mixture of particulates and air, as opposed to the particulate matter alone, though it is sometimes defin ...

production were also much lower (50% and 60% respectively). The EPA calculated for this test vehicle, the hybrid technology added a cost of about US$7,000 over a comparable conventional truck, while the lifetime fuel savings over 20 years were estimated above $50,000.

Types of hydraulic hybrid vehicles

Like the electric hybrid system, there are several possible drivetrain architectures. In a parallel hydraulic hybrid vehicles, the pump/motor is typically installed between the engine and gearbox, or between the gearbox and differential transmission box. The role of pump/motor is to provide assistance to the engine during acceleration and recapture energy under braking that would otherwise be lost as heat in the conventional brakes. As with electric hybrids, the pump/motor may or may not be able to drive the vehicle alone with the engine off. In a series hydraulic hybrid vehicle, the pump/motor directly connects to the driveshaft, or the in-wheel motors provide driving torque directly to the wheel. The internal combustion engine is only connected to a pump, and is set to operate in its most efficient power range to maintain the optimal hydraulic pressure in the accumulator. The traction motor must supply all the torque required to propel the vehicle, meaning maximum acceleration performance is available with the engine running or stopped. Its main disadvantage is in steady-state cruising, where the double conversion of energy introduces additional losses.

Advantages and disadvantages

In some cases hydraulic hybrid systems may be more cost-effective than electrical hybrid systems because no complicated or expensive materials (such as those required for batteries) are used. However, in most designs the pressure tanks of accumulators are made of

carbon fiber Carbon fiber-reinforced polymers (American English), carbon-fibre-reinforced polymers ( Commonwealth English), carbon-fiber-reinforced plastics, carbon-fiber reinforced-thermoplastic (CFRP, CRP, CFRTP), also known as carbon fiber, carbon comp ...

that make the pressure tanks somewhat expensive, but the price of carbon fiber has been forecasted to drop as economies of scale and manufacturing energy efficiency is reduced by 60% according to

Oak Ridge National Laboratory Oak Ridge National Laboratory (ORNL) is a federally funded research and development centers, federally funded research and development center in Oak Ridge, Tennessee, United States. Founded in 1943, the laboratory is sponsored by the United Sta ...

can lower the cost of manufacturing the tanks. Hydraulic hybrids recover, or harvest, the vehicle's kinetic energy during braking and decelerating significantly more efficiently than electric systems; hydraulic hybrids can recover up to 70–80% of the vehicle's kinetic energy compared to 55% for electric hybrids. Reduced cost, complexity, and weight for additional

power take-off A power take-off or power takeoff (PTO) is one of several methods for taking power from a power source, such as a running engine, and power transmission#Mechanical power, transmitting it to an application such as an attached implement or separate ...

devices such as water pumps, hydraulic lifts, and winches. Technical challenges with hydraulic hybrid vehicles include noise, size, and complexity. Technical advances, such as very Large Diameter, Flat Format (LDFF) hydraulic motors which produce very high torque in limited drive line space, enable heavy vehicles like refuse trucks and city buses to be fitted with hydraulic hybrid systems. Sophisticated control software results in hydraulic hybrid vehicles which are safe, driveable, reliable and efficient.

References

External links

Fleet Test and Evaluation Project
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National Renewable Energy Laboratory

United States Environmental Protection Agency Hydraulic Hybrid Research Page
Hydraulic Hybrid Technology - A Proven Approach
United States Environmental Protection Agency, March 2004
The ERS (Energy Recovery System)
, Lightning Hybrids
What are mechanical batteries?
Lightning Hybrids
Emissions Testing Results: Hydraulic Hybrid Delivers Major Emissions Reductions
Lightning Hybrids

Eaton Corporation (Requires QuickTime plug-in to view) * https://www.innas.com/hydrid.html, Innas BV
EPA Innovates Hydraulic Hybrid System
autoMedia.com
Hydraulic hybrid retrofits
KersTech Vehicle Systems *
IFPE High Efficiency Hydraulic Hybrid Drive System
NRG Dynamix (Formerly Hybra-Drive Systems LLC), December 12, 2007 * {{DEFAULTSORT:Hydraulic Hybrid Automotive technologies Hybrid vehicles Hybrid powertrain