Delivery Powertrain Using Series Hydraulic Hybrid Technology

Published Date: 02 Nov 2017

With the increase of gasoline cost, due to decrease of natural resources, and the necessity of environmental friendly solutions we have assisted in the last decades, to the raise of new technologies that use another kind of source to make cars move. In this way, it is natural that investigators development investigations for the alternative systems and also companies, with focus in automotive industry, put in the market revolutionary solutions for vehicles that help saving money with the fuel and are less aggressive for the environment.

This Paper describes one of the hybrid option Series Hydraulic Hybrid Vehicle, it describes its state of art of this technology, the benefits, the cost, and gives the possible design

1. Introduction

Fossil fuel reserves, gasoline in special, are becoming increasingly scarce around the world with rapid economical development of poor countries, which greatly influences gasoline prices in areas all over the world. Not only that, but burning fossil fuel increases the pollution of the planet, thus accelerating the climate change and the global warming effect that are now on the verge of taking over the earth. There are a lot of alternatives to internal combustion engines, like fuel cell vehicles, electric vehicles and some of the most popular hybrids today combine traditional I.C.Engines with alternative power plants such as electric motors which could decrease the gas prices and reduce the pollution. Paper describes one of the hybrid option Series Hydraulic Hybrid System. which uses the same principle as electric hybrids, but instead of using batteries, they use a Braking energy regeneration using a hydraulic system, this system is compose of lightweight components and clean fluid to power the vehicle while it’s at slow speed. Paper describes their benefits and demerits when compared with the other kind of vehicles that are environment friends.

2. Background Research

A hybrid vehicle is defined as one that has more than one source of power. In a hydraulic hybrid system the engine is coupled with a hydraulic pump/motor assembly and hydraulic accumulator. whereas in electric hybrid systems the engine is coupled with an electric generator/motor and batteries or other electrical storage devices. Both technologies are available in either series or parallel configurations. In a parallel hybrid hydraulic system, the conventional vehicle powertrain is supplemented by the addition of the hybrid hydraulic system. In a series hybrid hydraulic system, the conventional vehicle driveline is replaced by the hybrid power system. The energy is transferred from the engine to the drive wheels through fluid power. Commonality of these systems is recovering and storing energy normally wasted in heat and friction pads during braking. The objective of a Hybrid Transmission is to store and re-use as much as possible of this wasted energy.

2.1 Hydraulic Hybrids

A pump/motor unit is the primary working mechanism in a hydraulic hybrid system. Based on vehicle needs, it can pump hydraulic fluid up to high pressures for storage in an accumulator. Conversely, it can operate as a motor powered by the accumulated high-pressure fluid. While in the pumping mode (during slow down), it provides a decelerating force that absorbs energy from the driveline and consequently aids in braking. In the motoring mode it returns energy to the vehicle in the form of accelerating torque and thus reduces the load on the engine.

3. Design of Series Hydraulic Hybrid Powertrain

In a series hybrid hydraulic system, the conventional vehicle driveline is replace by the hybrid power system and energy is transferred from the engine to the drive wheels through fluid power

Fig 3.1 Series Hydraulic Hybrid

Driving:

A series hydraulic hybrid power system combines a combustion engine and a hydraulic propulsion system to replace the conventional drivetrain and transmission. The vehicle uses hydraulic pump/motors and hydraulic storage tanks to recover and store energy. It makes the engine operate at its "sweet spot" of fuel consumption facilitated by the continuously variable transmission (CVT) functionality of the series hybrid hydraulic system and by regenerative braking. In a series hybrid hydraulic system, demonstrated fuel economy improvement is significant.

Regeneration:

During braking, the vehicle’s kinetic energy drives the pump/motor as a pump, transferring hydraulic fluid from the low-pressure reservoir to a high-pressure accumulator. The fluid compresses nitrogen gas in the accumulator and pressurizes the system. The regenerative braking captures about 70% of the kinetic energy produced during braking.

Fig.3.2 Typical arrangement of the component

4. Comparisons with other Hybrid Technology

In this session, it will brief us about few illustration/comparisons are made with the some other hybrid technology systems.

4.1 Energy Storage Device Comparison – Power vs. Energy

4.2 The RDS (REGENERATIVE DRIVE TECHNOLOGY)

During the braking the vehicle kinetic energy is converted into mechanical energy which drives the pump/motor, transferring hydraulic fluid from low-pressure reservoir to high pressure accumulator. The fluid compressors the nitrogen gas in the accumulator and pressurizes the system. About 70 percent of kinetic energy is captured during braking with this RDS system. During the acceleration, the high pressure in accumulator is used to drive the motor. Following picture shows the schematic diagram of RDS.

Fig 4.2 Schematic of RDS

4.3 Superior Power to Weight Ratio

Compare to the hybrid electric systems, the RDS systems has an advantage of superior power to weight ratio. Meaning – heavier the vehicle the greater the advantage it becomes. The hybrid systems require high power, controllers and storage systems to drive these heavy vehicles. This requirement is easily achieved by the hydraulic hybrid compare to the hybrid electric systems.

The advantage of hydraulic accumulators is that they can cope with energy flow rates well in excess of braking requirements. In contrast, batteries in hybrid electric systems must be large and heavy to absorb energy at the rate demanded by braking. Accordingly, these systems require a battery that will only be partially charged by braking rendering up to 85% of the battery's capacity redundant, and the spare storage capacity cannot be efficiently utilised. On the other hand, accumulator storage can be matched exactly to any application requirement without redundancy.

4.4 Weight Advantage

Hydraulic hybrids are considerably lighter than electric hybrids for urban commercial vehicles. This weight advantage maximizes payload, improves fuel economy, providing a considerable market advantage. Electric hybrids use either batteries or super capacitors as their storage whereas the RDS uses accumulators. Typical types of batteries used for vehicles are nickel metal hydride (NiMH) and sodium-nickel-chloride (NaNiCl) which have been used in the graphs below. For the same type of vehicle an electric hybrid using NaNiCl batteries would weigh double that of a hydraulic hybrid RDS system.  A NiMH system would be around 95% heavier than the RDS and a super capacitor would be more than 40% heavier.

Even when comparing series systems shows that hydraulic hybrids are significantly lighter. If the RDS was presented on the graph below it would be represented at the 100% mark.

4.5 Storage Efficiency

Storage efficiency refers to energy efficiency as it travels from the engine to storage out, or from driveshaft to driveshaft. It is described in the illustration below for hydraulic hybrids (RDS) and electric hybrids.

4.5.1 Hydraulic Hybrid (RDS) 4.5.2 Electric Hybrid

The graph below shows that RDS is much more efficient than other types of electric hybrids.

For the same power requirement, a typical hydraulic installation:

is considerably lighter (up to 900kg)

has better energy storage efficiencies

does not introduce 650 volt components and operating systems

has a fraction of the heat loss and dissipation into the vehicle maintenance regime

integrates with existing vehicle hydraulic accessories (e.g. cranes)

4.6 Cost Advantage and Availability

Hybrid hydraulic systems are cheaper to buy and own than other hybrid systems. They are also reliable. Normal vehicles require only minor adoptions. In comparison, while hybrid electric systems consist of components with a long track record of performance, they also have a limited battery life, and require significant development to improve efficiency, power to weight ratio, volume production and cost.

4.7 Safety Advantage

The RDS technology is very safe. The pressure in hydraulic accumulators is easily and safely discharged for storage and maintenance. In addition, the RDS technology is easily programmed to automatically discharge when the vehicle is switched off. Thus there is no risk for self-ignition, fire or shock. By contrast, hybrid electric systems use high voltage, high current circuits which pose several significant safety hazards.

4.8 Environmental Impact

The RDS being a pure and clean technology, the components contains long-term, readily reprocessed materials and fluids. While the batteries used in the electric hybrids contain toxic materials, ranging from rigorous acid to heavy materials, which are ruinous to the environment when prepared and predisposed of. Thus lifespan environmental footprint of HH components is significantly less when compared to the hybrid electric components.

4.9 Environmental Operating Conditions

The tolerance of the components of hydraulic hybrids operating in extreme conditions like heat, cold and moisture is better when compared with the hybrid electric systems.

4.10 Maintenance

The hydraulic components of the RDS technology will fit in flawlessly into the existing maintenance practices of fleet owners. The wide spread use of hydraulic hybrids in automotive industry, so no new skills or tools are needed to service the vehicles. If there is a problem, the system can turn itself off (or be turned off by the operator) until repairs are carried out. This allows the vehicle to continue working productively and with all the original vehicle capabilities.

4.11 Versatility

Cranes and other lifting devices, which are integrated with hydraulic ancillaries, are easy to assimilate with these hydraulic hybrids components.

4.12 After-Market and Retrofit

The RDS technology is promoted in kit form to make after-market and retrofit fittings very user-friendly. The minimal system changes required during the installations, which is a significant advantage over other systems available in the market.

5. Summery

In conclusion, the hydraulic hybrid powertrain uses regenerative system has a future, for heavy vehicles with significant start/stop operation, hydraulics are better at capturing lost energy than electric systems. The real advantage of hydraulics is in its power density. A hydraulics system is capable of transferring energy very quickly. Also, the rate of energy transfer in a hybrid electric system of a similar size is much lower. Series hydraulic hybrid systems have a broader range of applications, though as with all hybrids benefits will be highest in vehicles that operate in stop-and-go duty cycles. They are very good fit for parcel delivery vehicles.