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Hydraulic Hybrid Vehicles | Hydraulic Engines | Hydraulic Hybrid Technology | Hydraulic Hybrid System For Four Wheeler | Series And Parallel Hydraulic Hybrid Power Train

Introduction To Hydraulic Hybrid Vehicles:

Hydraulic Hybrid vehicles use two sources of power to drive the wheels. In a hydraulic hybrid vehicle (HHV) a regular internal combustion engine and a hydraulic motor are used to power the wheels.

hydraulic hybrid system-Hydraulic hybrid vehicles-HHV-hydraulic motors to power wheels-accumulators to store the pressurized  fluid nitrogen gas

Hydraulic Hybrid system – Hydraulic Hybrid Vehicles – HHV – Hydraulic motors to power wheels – Accumulators to store the pressurized fluid nitrogen gas

Hydraulic Hybrid Vehicles system consist of two key components:

  • High pressure hydraulic fluid vessels called accumulators, and
  • Hydraulic drive pump/motors.

Working of Hydraulic Hybrid Vehicles System:

hydraulic-hybrid-retrofit-hydraulic hybrid system-HHS-regenerating braking energy

The accumulators are used to store pressurized fluid. Acting as a motor, the hydraulic drive uses the pressurized fluid (Above 3000 psi) to rotate the wheels. Acting as a pump, the hydraulic drive is used to re-pressurize hydraulic fluid by using the vehicle’s momentum, thereby converting kinetic energy into potential energy. This process of converting kinetic energy from momentum and storing it is called regenerative braking.

The hydraulic system offers great advantages for vehicles operating in stop and go conditions because the system can capture large amounts of energy when the brakes are applied.

The hydraulic components work in conjunction with the primary. Making up the main hydraulic components are two hydraulic accumulator vessels which store hydraulic fluid compressing inert nitrogen gas and one or more hydraulic pump/motor units.

How Hydraulic Hybrid Vehicles Work

Hydraulic hybrid systems can capture up to 70% of the kinetic energy that would otherwise be lost during braking. This energy drives a pump, which transfers hydraulic fluid from a low-pressure reservoir to a high-pressure accumulator. When the vehicle accelerates, fluid in the high-pressure accumulator moves to the lower-pressure reservoir, which drives a motor and provides extra torque. This process can improve the vehicle’s fuel economy by 25% to 30% compared to its non-hybrid equivalent.

The hydraulic hybrid system is made up of four components.

  • The working fluid
  • The reservoir
  • The pump or motor
  • The accumulator

The pump or motor installed in the system extracts kinetic energy during braking. This in turn pumps the working fluid from the reservoir to the accumulator, which eventually gets pressurized. The pressurized working fluid then provides energy to the pump or motor to power the vehicle when it accelerates. There are two types of hydraulic hybrid systems – the parallel hydraulic hybrid system and the series hydraulic hybrid system. In the parallel hydraulic hybrid, the pump is connected to the drive-shafts through a transmission box, while in series hydraulic hybrid, the pump is directly connected to the drive-shaft.

Types of HHV’s

There are two types of HHVs:

  • Parallel and
  • Series.

Parallel Hydraulic Hybrid Vehicles:

hydraulic hybrid cars-HLA system-pump mode to motor mode-parallel hydraulic hybrid vehicles-nitrogen accumulator pressure 5000 psi

Hydraulic hybrid cars – HLA system – Pump mode to motor mode – Parallel hydraulic hybrid vehicles – Nitrogen accumulator pressure 5000 psi

In parallel HHVs both the engine and the hydraulic drive system are mechanically coupled to the wheels. The hydraulic pump-motor is then integrated into the driveshaft or differential. In a regular vehicle, power is provided to the wheels by the engine through the transmission and drive shaft. In a parallel hydraulic hybrid there is a conventional engine and drive train system with hydraulic hybrid technology attached to the drive shaft.

Series Hydraulic Hybrid vehicles:

hydraulic hybrid vehicles-combines regular internal combustion engine- hydraulic motor as a accumulator-kinetic energy into potential energy to drive the vehicle

Series HHVs rely entirely on hydraulic pressure to drive the wheels, which means the engine does not directly provide mechanical power to the wheels. In a series HHV configuration, an engine is attached to a hydraulic engine pump to provide additional fluid pressure to the drive pump/motor when needed.

Hydraulic Hybrid Vehicles (HHV) Energy Management

A Hydraulic hybrid vehicles requires careful management of the kinetic and stored energy to minimize fuel consumption. The role of the energy storage device is to capture and release the kinetic energy of the vehicle, while the role of the engine is to provide the power to overcome the parasitic losses such as aerodynamic drag and rolling resistance. Furthering this concept reveals that the total energy of the vehicle, defined as the sum of the energy stored in the accumulator, the kinetic energy, and the gravitational potential energy, should remain approximately constant in an ideal hybrid operation.

Due to the energy storage and high power density of hydraulics, the engine operation is drastically different than a conventional drive train. The engine is not required to produce large amounts of power for rapid vehicle acceleration. Instead, the engine is sized to overcome the average parasitic losses and system inefficiencies at the maximum cruising velocity as well as the ability to climb a grade at a desired velocity. This means that the engine can be dramatically downsized from a conventional drive train without a negative impact on performance.

Advantages of Hydraulic Hybrid vehicles:

  • Higher fuel efficiency.  (25-45 percent improvement in fuel economy)
  • Lower CO2 (Greenhouse gas) emissions.  (20 to 30 percent)
  • Reduced operating costs.
  • Better acceleration performance.
  • Idle losses of the engine are avoided
  • Losses of the hydrodynamic torque converter are avoided
  • Throttle losses in the control of hydraulic cylinders are minimized
  • Auxiliaries like steering systems can cooling fans can be decoupled from the engine and optimized from an efficiency point of view

Why Hydraulic Hybrid Vehicles and not a Electric Vehicle?

There are currently many electric hybrid vehicles on the market. Toyota and Honda provide many compact vehicles that utilize the above technology using electricity instead of hydraulics. Even though the concept is very similar, the properties of hydraulics differ slightly. The main difference is that hydraulic systems are able to store more energy at a faster rate than an electrical system. This increase in energy storage is crucial to be able to effectively move and accelerate a heavy diesel engine.

There is no practical way to store the same amount of energy in an electrical system that would be stored in a hydraulic system. The cost would be far too great, and designing batteries and capacitors to make the system work would be very difficult and inefficient. However a hydraulic system is much larger than an electrical system which makes is much more reasonable for smaller compact cars which have less space to house a hybrid system, and which require far less energy to operate effectively.


What does HHV stand for?

HHV – Hydraulic Hybrid Vehicles

What is an HHV?

An HHV is a vehicle that uses hydraulic technology to power a vehicle

Why use an HHV?

HHV’s increase fuel economy up to 70 % with room for greater efficiency. They reduce CO2 emissions by 30 % and the owners save thousands of dollars over the vehicle’s lifetime.

How does HHV works?

When BRAKING the integrated engine and drive pump / motor slows the vehicle down by using the momentum from the vehicle to create pressure, storing it in the accumulator. This saved energy can accelerate the vehicle without the engine. Low pressure fluid is stored in the reservoir. The pump / motor uses high pressure fluid to power the wheels. When the engine is on its run only at its most efficient point.

2 thoughts on “Hydraulic Hybrid Vehicles | Hydraulic Engines | Hydraulic Hybrid Technology | Hydraulic Hybrid System For Four Wheeler | Series And Parallel Hydraulic Hybrid Power Train”

  1. good article. Could you shed some light on the exact working of the hydraulic accumulator as well as the limitations of this system

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