Hydraulic hybrid vehicles (HHVs) use a pressurized fluid power source, along with a conventionalinternal combustion engine (ICE), to achieve betterfuel economy and reductions inharmful emissions. They capture and reuse 70–80% of the vehicle's kinetic braking/decelerating energy and potential descending energy[1] compared to 55% for electric hybrids.[2] For trucks and buses, this can also be less expensive than electric systems, due to the price ofbatteries 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 heavyvehicle classes.
Hydraulic hybrid vehicle systems consists of four main components: the working fluid,reservoir, pump/motor (in parallel hybrid system) or in-wheel motors and pumps (in series hybrid system), andaccumulator. In some systems, a hydraulic transformer is also installed for converting output flow at any pressure with a very low power loss.[3] In anelectric hybrid system, energy is stored in thebattery and is delivered to theelectric motor to power the vehicle. Duringbraking thekinetic energy of the vehicle is used to charge thebattery through theregenerative braking. In hydraulic hybrid system, the pump/motor extracts thekinetic energy duringbraking to pump the working fluid from thereservoir to theaccumulator. Working fluid is thus pressurized, which leads toenergy storage. When the vehicle accelerates, this pressurized working fluid provides energy to the pump/motor to power the vehicle.[4] For a parallel hybrid system, fuel efficiency gains and emissions reductions result from reducedmechanical load on the internal combustion engine due to thetorque provided by the hybrid system.
TheUS 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.[1] The CO2 emissions of the same demonstration delivery truck were claimed to be over 40% lower, and thehydrocarbon andparticulate matter production were also much lower (50% and 60% respectively).[1]
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.[1]
Like theelectric 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 differentialtransmission 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,[5] 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.[3] 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.
In some cases hydraulic hybrid systems may be more cost-effective thanelectrical hybrid systems because no complicated or expensive materials (such as those required forbatteries) are used. However, in most designs the pressure tanks of accumulators are made ofcarbon fiber that make the pressure tanks somewhat expensive, but the price of carbon fiber has been forecasted to drop as economies of scale and manufacturingenergy efficiency is reduced by 60% according toOak Ridge National Laboratory can lower the cost of manufacturing the tanks.[6]
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.[7][2][8]
Reduced cost, complexity, and weight for additionalpower take-off 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.