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 power train is supplemented by the addition of the hybrid hydraulic system. In a series hybrid hydraulic system, the conventional vehicle drive line is replaced by the hybrid power system. The energy is transferred from the engine to the drive wheels through fluid power. These systems have one thing in common: the recovery and storage of energy normally wasted as heat from friction pads during braking. The objective of a hybrid transmission is to store and re-use as much as possible of this wasted energy.
Hydraulic hybrids
A pump/motor unit is the primary working mechanism in a hydraulic hybrid system. Depending on the vehicle’s instantaneous needs, it can pump hydraulic fluid to high pressures for storage in an accumulator, or it can operate as a motor powered by the accumulated high-pressure fluid. While in pumping mode (during slow down), it provides a decelerating force that absorbs energy from the drive line 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. In a parallel hybrid hydraulic system, the conventional vehicle power train is supplemented by the addition of the hybrid hydraulic system.
This system comprises two parts: regeneration and launch assist. 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 pressurises the system. The regenerative braking captures about 70% of the kinetic energy produced during braking. In launch assist, during acceleration, fluid in the high-pressure accumulator is metered out to drive the pump/motor as a motor. The system propels the vehicle by transmitting torque to the drive shaft.
In a series hybrid hydraulic system, the conventional vehicle drive line is replaced by the hybrid power system and energy is transferred from the engine to the drive wheels through fluid power. In driving mode, a series hydraulic hybrid power system combines a combustion engine and a hydraulic propulsion system to replace the conventional drive train and transmission. The vehicle uses hydraulic pump/motors and hydraulic storage tanks to recover and store energy. The engine operates at its ‘sweet spot’ of fuel consumption, facilitated by the continuously variable transmission functionality of the series hybrid hydraulic system and by regenerative braking. Fuel economy improvement is significant. In regeneration mode, the vehicle recovers and stores energy, as is the case with parallel hydraulic hybrid vehicles.
Electric hybrids
An electric motor/generator recovers energy normally lost during braking and stores the energy in batteries. The energy is used to power the vehicle at given speeds, or to operate for periods with electric power only. The system also provides energy for engine-off operations, further reducing fuel consumption and emissions caused by idling. Hybrid electric systems have much higher energy storage capacity, but generally have low to moderate power capabilities.
The parallel electric hybrid retains a conventional drive train while adding the ability to augment engine torque with electrical torque. It incorporates an electric motor/generator between the output of an automated clutch and the input to the transmission system. In a series hybrid electric system, the conventional vehicle drive line is replaced by the hybrid power system and energy is transferred from the engine to the drive wheels through electrical power. Hybrid electric vehicles require an unprecedented level of integration.
Choosing between these hybrid systems depends on application. The hydraulic system is more suitable for vehicles needing high power levels for short durations, like refuse trucks, whereas electric systems are better for vehicles that require low power levels for sustained periods of time, like utility boom trucks and urban delivery vehicles. Parallel hydraulic hybrid power makes a lot of sense for a heavy vehicle like a refuse truck that has an intermittent duty cycle. The system can capture the braking energy and recycle it over and over to improve operating efficiency. In addition, the parallel hydraulic hybrid system can offer the flexibility of two operating modes: ‘economy’ and ‘performance’.
When in ‘economy’ mode, the energy stored in the accumulator during braking alone is used to get the vehicle moving. When the accumulator is exhausted, the engine takes over to continue the acceleration, a process that results in increased fuel economy.
In ‘performance’ mode, acceleration is provided by the energy stored in the accumulator and the engine. Again, when the accumulator is exhausted, the engine is solely responsible for acceleration. While fuel economy improvements are also seen in performance mode, the greatest benefit is increased productivity. Trials with Eaton’s Hydraulic Launch System, HLA have demonstrated improvements in fuel economy, productivity, brake service and reduced noise.
Series hydraulic hybrid systems have a broader range of applications than parallel hydraulic hybrids, though - as with all hybrids - benefits will be highest in vehicles that operate in ‘stop-and-go’ duty cycles. The series hydraulic hybrid is an ideal ‘upgrade’ for material handling equipment already equipped with a hydrostatic transmission. With this system, the vehicle uses hydraulic pump/motors and hydraulic storage tanks to recover and store energy (motors and batteries in an equivalent electric hybrid vehicle). Some series hydraulic hybrids provide engine management, which targets operation at the most economic engine condition for maximum fuel economy and reduced emissions. They can provide up to 40-50% fuel savings.
Considerable emphasis is being placed upon parallel hybrid electric systems because they provide the best balance between costs and benefits. Eaton’s version of this type couples a vehicle's diesel engine with an electric motor/generator and batteries, driving via the company’s Fuller UltraShift automated manual transmission and clutch automation system. A permanent magnet traction motor-generator and inverter are integrated with high-power lithium-ion batteries to deliver a cost-effective and reliable hybrid system for commercial vehicles. Like series hydraulic hybrids, series electric hybrid systems provide engine management, which targets operation at the most economic engine condition for maximum fuel economy and reduced emissions.
Electric or hydraulic
Both electric and hydraulic regenerative systems have a future but 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. Therefore the specific vocation or driving cycle of the vehicle determines whether a hybrid hydraulic or hybrid electric system is best suited for the application.
Juergen Jasche and Robert Golin are with Eaton Hydraulics