- Jun 15, 2006
Capturing the power of hydraulics
UPS, one company that could clearly benefit from reducing fuel consumption is investigating a number of approaches, with this article calling the brown company pretty green. One of their approaches, hybridization of the powertrain, an approach successfully demonstrated in a number of passenger cars, is one of the few viable options to improve heavy vehicle fuel economy. Hybridization enables the regeneration of large amounts of braking energy, due to the large mass of these vehicles. This is clearly an opportunity for fuel economy improvement, but it is also a challenge. Due to the large amount of energy generated in regenerative braking, the power flowing through the hybrid system can be very high. For just this reason, hydraulic systems make an attractive choice when considering heavy vehicles, and UPS is partnering with the Environmental Protection Agency to investigate these systems. Aside from being technologically attractive, hydraulic hybrids are also cheaper than electric hybrid systems.
Hydraulic components are characterized by high power density, and hydraulic accumulators can accept high frequencies and high rates of charging and discharging. While batteries in combination with electric motors are common in passenger car hybrids, they do not allow for high rates of charging and discharging, and they have lower power density in comparison. The main drawback of the hydraulic systems is their low energy density. While the system has significant power, the total energy stored in a hydraulic hybrid system is relatively low, requiring careful design of the control strategy. The low energy density also makes the hydraulics more suitable for large vehicles. The accumulators, which are often significantly larger than a battery pack, can be packaged more readily on a large vehicle. The control strategy for the diesel engine and hydraulic components needs to be carefully optimized to allow for the highest possible fuel economy, with the smallest possible accumulator size. While there are currently no commercially available hydraulic hybrids, attempts at creating such a vehicle date back as early as 1979.
Before we talk more about some vehicles currently being developed, we need to look at the architecture of hybrid vehicles. When considering hybrid vehicles, two approaches are generally taken. The most common architecture is the parallel hybrid. In a parallel hybrid, like the Toyota Prius, the vehicle can be powered directly by the internal combustion engine, the electric motor, or both at the same time. In parallel hybrids, there is a direct mechanical connection between both power sources and the driven wheels. A second architecture is called the series hybrid. In a series hybrid the vehicle can only be powered by the alternate power source. In case of a series electric hybrid, the vehicle can only be powered by the electric motors. In case of a series hydraulic hybrid, the vehicle would be powered by the hydraulic motors. In a series hybrid, there is no longer a direct mechanical connection between the internal combustion engine and the driven wheels. So why are these series hybrid systems considered? The main reason can be attributed to increased engine control. When looking at the fuel economy map of any engine, both diesel and gasoline, the consumption will vary widely depending on rpm and power. In a series hybrid vehicle, the control algorithm can maintain the internal combustion engine at or close to its most efficient operation, regardless of the demand from the driver. While the series hybrids can probably offer greater fuel economy benefits, they are more complex.
So why aren't any of these vehicles available yet to you and me? The vehicles they are being considered for aren't generally cars you and me would buy. Ford showed off their Mighty Tonka 350 concept truck with a parallel hydraulic launch assist system. The Environmental Protection Agency has demonstrated the technology in a Ford Expedition, which would get 32 mpg in city driving and 22 mpg on the highway, compared to 13 mpg and 20 mpg for the conventional gasoline-only version. Eaton, an EPA partner, is fielding 20 hydraulic hybrid garbage trucks in the second half of 2006, and UPS expects to start using a hydraulic hybrid truck this yeas. Lets hope these make it to the market sooner rather than later.