Defining hydraulic hybrid designs — the differences between parallel and series systems

July 13, 2009
Hydraulic hybrid vehicles that combine hydraulic, electrical, and mechanical components to improve vehicle efficiency and performance generally are either series or parallel hydraulic ...

Hydraulic hybrid vehicles that combine hydraulic, electrical, and mechanical components to improve vehicle efficiency and performance generally are either series or parallel hydraulic systems. The more common system of the two is a series system, which is analogous to a diesel/electric locomotive. In brief, this system uses a conventional internal combustion engine to power a hydrostatic pump of variable displacement which supplies hydraulic pressure to one or more hydraulic motors that propel the vehicle.

The parallel hydraulic hybrid system is similar to many of the electric hybrid vehicles on the market today wherein a conventional internal combustion engine can drive the vehicle by itself or be assisted by another power system that can be mechanically coupled or decoupled from the main conventional driveline. In the parallel hydraulic system a hydraulic motor/pump acts in place of the electric motor.

The details of how the parallel hydraulic hybrid system operates and ways in which it can be implemented are the topic of this paper.

Parallel hydraulic hybrid background

A parallel hydraulic system consists of the following components:
Conventional Automobile Drivetrain — The automobile drivetrain comes in three main configurations today, all of which can be fitted with a parallel hydraulic hybrid system.
A front-wheel drive will typically have a transversely mounted engine, transmission, transaxle, and output shafts;
A rear-wheel drive vehicle will have a longitudinal mounted engine, a transmission, a differential, and output shafts;
A four-wheel drive system will typically have a longitudinally mounted engine, a transmission, a transfer case, a front differential, a rear differential, and front and rear output shafts.

Hydraulic Pump/Motor — The main hydraulic component of a parallel hydraulic hybrid vehicle is a hydraulic pump/motor. The pump/motor is a variable-displacement, axial-piston type pump. This type of closed-circuit pump has been used reliably in various hydraulic applications for many years. Depending on which port is pressurized, the pump/motor can operate in both counterclockwise and clockwise directions and as a pump or motor with the same efficiency of between 85% and 90% depending on speed. Because of this characteristic, the pump/motor can either add power to the system or be used to pump hydraulic fluid to a high-pressure accumulator and store that energy for use in its opposite mode of operation.

Hydraulic Accumulator — The hydraulic accumulator stores energy for use by the pump/motor to drive the system without the aid of an internal combustion engine.

Example — A rear-wheel drive vehicle fitted with a parallel hydraulic hybrid system. In a very simple and effective system, the pump can be coupled to the drive shaft after the transmission and before the differential or final drive, via a belt and clutch. In this arrangement if the pump/motor clutch is disengaged the vehicle can operate exactly as it was originally engineered to operate. Negligible bearing losses that support the clutch and pump/motor are the only efficiency losses to consider. These losses are considered negligible because modern day ball bearings or tapered roller bearings are usually better than 99% efficient. The pulley ratio at this junction is configured so that the pump/motor can most efficiently add power over its speed range. Because the full torque of the hydraulic pump/motor is available at zero rpm, one gear ratio can be used to accelerate the vehicle over a wide range of speeds and be disengaged at its maximum speed. The engine then takes over. With one reduction roughly equivalent to a standard transmission’s second gear ratio, a hydraulic pump/motor with the same power output as an internal combustion engine can accelerate a vehicle much more quickly than an internal combustion engine using two or three gears because of this relatively constant torque output.

Real-world illustration

To illustrate how the system works, let's assume that the driver starts the vehicle in exactly the same way that he or she typically would without a parallel hydraulic hybrid system. The vehicle is simply started and put in drive. As the driver starts down the road and the brakes are applied approaching the first stop sign, the internal combustion engine turns off and the conventional transmission shifts to neutral. The pump/motor — which is directly coupled to the wheels through the differential — uses the braking energy to pump fluid in proportion to how quickly the driver wants to stop. This fluid is pumped to a high-pressure accumulator, and energy is stored. If the accumulator reaches maximum capacity before the vehicle has stopped, conventional brakes are used to slow the vehicle to a stop.

When the driver accelerates from the stop sign, rather than accelerating the internal combustion engine, high-pressure fluid from the accumulator is routed to the pump/motor to run the unit as a motor. The pump/motor propels the vehicle from stop to at least 40 mph before the accumulator is fully discharged, and before the engine starts and begins to propel the vehicle. At this point the pump clutch is disengaged and the hydraulic system is entirely decoupled from the system. The internal combustion engine is able to run in a narrow range of speeds that are close to its most efficient operating speed.

Fuel is saved by allowing the engine to turn off when it isn’t in use, and allowing it to run in a narrow speed range. In this way the parallel and series systems are very similar. No extra load is added to the conventional drive train by the parallel system.

Different mechanical configurations allow for improved drivability and further efficiency gains. In a rear drive system, a pump can be added to the front wheels via a belt, clutch, and differential, to recover more braking energy and also to apply speed matched power to the front wheels. Because of the added traction of the front wheels, more power can be absorbed in a stop and more power can be added in an acceleration cycle.

An in-line configuration where the pump is added after the engine and torque converter but before the transmission would allow the pump to operate through a greater range of speeds before requiring assistance from the engine. This configuration would also allow the engine to drive the pump and charge the accumulator at very low flow rates. The amount of power that the pump requires from the engine comes from the equation

hp = psi × gpm / 1714,

where hp is the horsepower required from the engine,
psi is the system pressure in lb/in.2,
gpm is the pump flow rate in gal/min, and
1714 is a constant.

As mentioned above the pump is a variable-displacement type that allows the pump to pump any amount of fluid between zero gallons per minute and its maximum value, which is dependent on the physical size of the pump. While the main pump can be made to pump no fluid at all, there is an integrated gear pump within the unit that allows the pump to operate, and keeps it lubricated. This pump has a constant flow rate of 0.53 gal/min at a pressure-relieved pressure of 300 psi. This pump also powers the hydraulic logic controls. If the pump is not pumping any fluid at all, the pump losses can be calculated according to the equation:

300 psi × 0.53 gpm / 1714 = .092 hp

or roughly one-tenth of a horsepower. This is a very small amount of power, comparable to a standard alternator or air-conditioning unit used on a vehicle. The advantage of this configuration is that the pump can be made to pump fluid at the optimum rate for the engine’s fuel use maps with the transmission in neutral and the vehicle at a stop. That energy is stored in the accumulator and allows the vehicle to make a full acceleration cycle using the pump only. The accumulator can be charged in the same way while the vehicle is moving, and the system is now behaving very similarly to a series system.

For increased performance, the pump and internal combustion engine can be used simultaneously to power the vehicle. In many cases, this essentially doubles the power of the vehicle (depending on the pump size). This is not possible in a series system. Because of the additive nature of the parallel system the main conventional power plant can be undersized. With a well-designed system, the conventional drivetrain only needs to drive the vehicle at cruising speeds where maximum power is not needed. Also, the parallel system has inherent efficiency gains over a series system. In the series system power has to be routed through a pump to a hydraulic motor. Both of these units have approximately an 85 to 90% efficiency, so their combined efficiency becomes on average 85%. This is the best efficiency that a series system can realize. The parallel system on average can be 92.5% efficient, or when not in use no less than 99% efficient depending on the size of the internal combustion engine.


In conclusion, the parallel hydraulic hybrid drivetrain is a highly flexible and efficient system. Because it works in parallel with a conventional drivetrain, there are numerous mechanical configurations that allow the system to greatly improve the efficiency of the standard drivetrain, without ever loading the system more than it otherwise would be. Depending on how the electrical controls are arranged, a parallel system can serve to lessen the load on a conventional internal combustion engine at all times.

This flexibility allows tuning to increase performance and driveability of the vehicle and eliminate many failure modes that would render the vehicle unable to drive. The purpose of the system is improve existing vehicle design in aspects of efficiency (fuel efficiency reduced at least 50%), emissions (reduced), performance, and driving experience.

For more information, contact Dan Johnson, CEO of Lightning Hybrids at (800) 223-0740 or [email protected] or Bonnie Trowbridge, VP Business Development, at (303) 519-4144 or [email protected] or visit

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