Hybrid Technologies for Commercial Vehicles
In spite of the hype about hybrid cars by manufacturers, they have been slow in integrating hybrid technologies into commercial vehicles such as trucks or buses. This is because manufacturers are divided over their opinions regarding beating the fuel efficiency of diesel engines with current hybrid technologies. There are also manufacturers who are just waiting for advancements in fuel/cell technologies which can be incorporated with current commercial drive-trains.
The fact is, incorporating hybrid technologies in commercial vehicles is not as simple as implementing hybrid technology into consumer cars. One cannot just scale down the internal combustion engine and then add an electric motor to the bus or truck. Heavy workhorse vehicles like buses, trucks or vans require a certain amount of power for them to climb a hill. Having a smaller engine will diminish the power reserve that these heavy vehicles require. Furthermore, they will also need a large electric motor and sizable battery packs to accommodate the already heavy vehicle hence placing more stress on the power plants and reducing the cargo capacities of the vehicle.
Cost is another major determinant which impeded the adoption of hybrids into commercial vehicles. Hybrid buses and trucks all feature expensive technology like energy storage system and power system which require additional capital outlay. Companies wishing to adopt hybrids into their traditional diesel fleets will have to consider the return on investment (ROI) for the hybrids. However with factors like fuel price volatility, lower maintenance costs and government incentives, more and more companies are studying the possibility of adopting hybrid technology.
In order to realize the benefits of hybrid technology in commercial vehicles, it is essential that the technology match the particular duty cycle required by the vehicles. For low speed, stop start drive cycles, hybrid buses perform better than traditional diesel buses, especially when coupled with technology like regenerative braking which recoup back some of the energy lost during the braking cycle.
Key Systems and Components
In commercial vehicles with electric hybrid systems, the energy storage unit is the most expensive part of the system, it can account for one- third of the total hybrid system cost. Nevertheless, as more and more vehicles are incorporating hybrid technology, the cost of battery packs will reduce.
Ultra-capacitors is an alternative to using battery packs. They store energy physically unlike batteries which store energy chemically. This shortens the charging and discharging cycle time. However the disadvantage of ultra-capacitors is the fact that they store a lesser amount of energy making them more suited for vehicles that require power rather than endurance.
With hydraulic hybrids, the core components are the hydraulic pump cum motor, low pressure reservoir and the high pressure accumulator. This technology has been adopted in construction vehicles for quite some time already. However, the integration of hydraulic technology into a hybrid drivetrain is still in the early stages of development.
Nevertheless, the low cost and robustness of the hydraulic system makes it an excellent candidate for commercial vehicle hybrid technology development. According to the US Environmental Protection Agency (EPA), the payback period for electric commercial hybrids can range from 5 to 10 years while for commercial hydraulic hybrids and with economy of scale, the system will just take around three years to achieve payback.
Software and Control systems
At the core of the hybrid drive technology, for both electric and hydraulic hybrids, is the software and control system. It is the software and control system which allows the vehicle’s energy to be utilized to the full. The control system provides the connectivity between engine, transmission energy storage unit and electric motor or pump motor in the case of hydraulic hybrids. The software helps to configure the functioning of the various components efficiently.
Parallel System, Series System, Dual Mode Systems
Electric hybrid systems for commercial vehicles can be categorized as Parallel systems, Series Systems or Dual Mode systems.
Parallel Hybrid System
Parallel hybrid systems function with the combustion engine and electric motor connected in parallel. They can either be used together or separately. As such, this kind of system can be equipped with just a single electric motor, saving on cost. In addition, the motor can be integrated into the transmission housing which allows existing transmission designs to be used with current technology on driving dynamics.
“GM Allison Hybrid Ep40”
This is a Dual Mode parallel hybrid system that is incorporated in more than 2500 buses in cities like London, Istanbul, Chicago, Washington DC, Philadelphia, Houston and Seattle. The system is a variable speed hybrid system that automatically selects to run in either parallel or series mode to reduce emissions, fuel consumption and noise level. The GM Allison system comprises of the vehicle drivetrain – the GM EV DriveTM module, a DPIM (Dual Power Inverter Module) and energy storage unit which consist of nickel metal hydride batteries.
Serial Hybrid Systems
The serial hybrid system is normally the system that is incorporated into buses. It doesn’t have a mechanical link from the combustion engine to the wheels. The combustion engine is only for the generation of power. The propulsion is driven by the electric motor connected to the wheels directly or through a driveshaft with power generated by the combustion engine. The advantage of this is that the combustion engine will always operate at the optimum level. There is no stress loads imposed on the combustion engine as a result of heavy loads or high speeds run.
“BAE Systems HybriDrive”
BAE Systems HybriDrive system have been used for quite some time in Northern American cities and Canada like Houston, New York, San Francisco, Ottawa and Toronto. The system is a series hybrid system that is incorporated into the Orion-VII NG hybrid electric bus. The system consists of a diesel combustion engine, electric motor and energy storage unit which are all managed by a computerized control system.
The diesel combustion engine, smaller than those found in a typical bus, powers the generator. Propulsive power is provided by the electric motor. As the diesel engine is not connected directly to the electric motor or wheels, it has less stress load and requires less maintenance. There is also no transmission in the Orion-VII NG as the electric motor is connected directly to the wheels.
Dual Mode Hybrid Systems
With a Dual Mode Hybrid System, part of the combustion engine power is transferred to the electric motors and then transmitted to the output shaft. Another portion of the power is transfer mechanically. In essence, the distribution of the propulsive power is done via two mode, one electrically and another mechanically. With this system, the transmission plays an important role in the relocation of power between the mechanical and electrical drivetrain.
“ArvinMeritor Dual Mode System”
This system had been developed to be used in the U.S. Class 8 line hauling operations. The system is also presently being tested by the giant retail chain WalMart. The diesel cum electric parallel hybrid system is now used in a Class 8 tractor powered by a Cummins diesel engine. The tandem axle and regenerative braking system is provided by ArvinMeritor. The hybrid drivetrain utilize the electric motor chiefly during periods of heavy power usages.
The mechanical propulsion system will provide propulsive power once the vehicle starts moving and has reached optimum highway speed. During hill climbing, additional power is provided by the electric motor. In addition to providing propulsive power, the diesel also provides generative power to charge the energy storage unit when the vehicle is travelling at highway speed. The regenerative braking system also captures some of the energy released during braking and is stored in the energy storage unit.
Currently, the battery that is the most popular for hybrid vehicles is the lithium-ion battery. With regards to current battery technology for hybrid systems, the only areas that require addressing for the short term are:
- Anode Selection
- Cathode Selection and
- Cell Packaging
However, as hybrid technology advances more, battery manufacturers are also under intense produce to produce better and more cost effective batteries. This had brought about a demise of smaller manufacturers and an upturn for larger manufacturers due to economy of scale. This includes companies like:
- Exide Technologies
- GM Yuasa
- Johnson Controls-Saft
Depending on the technology and volume, battery prices which represent a major cost component in hybrid systems for commercial vehicles is predicted to go down.
Read more about battery technology here.
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4th International Congress Automotive Battery Technology
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