EPA Launches Heavy-Duty Hydraulic Series Hybrid Project for Yard Hostlers at Ports
05 September 2007
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| Chassis layout for the hydraulic series hybrid yard hostler. Click to enlarge. |
The US Environmental Protection Agency (EPA) has launched a project to develop hydraulic series hybrid systems for Class 6 port yard hostlers—the heavy-duty diesels that move goods and products from ships to trucks at ports.
EPA’s hydraulic series-hybrid drive technology is currently being tested in several UPS vehicles. (Earlier post.) The hydraulic hybrid offers an improvement in fuel economy of up to 74% compared to conventional diesel-powered versions of those trucks, depending on the operating mode .
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| The bent-axis pump/motor. Click to enlarge. |
The hybrid vehicles will use a diesel-hydraulic system that will combine the cleanest available diesel engine technology with components that use hydraulic fluid compression to store energy. Power to the drive axles is provided by a hydraulic pump/motor.
The goal is to develop a hybrid drive system that will include a diesel engine that meets the 2007 and 2010 on-road diesel standards. An engine meeting the 2010 standard will also achieve 93% reductions in NOx and 93% reductions in particulate matter compared to an ordinary diesel yard tractor. The hydraulic hybrid technology is expected to further reduce emissions by eliminating emissions from the internal combustion engine during idling.
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| Accumulators for energy storage. |
Hydraulic accumulators are used to store energy. As hydraulic fluid enters either accumulator, the nitrogen (N2) in that accumulator compresses and its pressure rises. Like other hybrid systems, energy saved when applying the brakes is reused to help accelerate the vehicle.
The system uses bent-axis pump/motors that can deliver 330 hp at 5,000 psi at 45 degrees and 510 hp at 7,000 psi at 45 deg. The current system is planned for maximum pressure of 5,000 psi. Future systems will utilize 7,000 psi.
The system adjusts to power demands using the variable position yoke assembly. At 0 degrees, no power is produced or absorbed; at 45 degrees, maximum power is produced or absorbed.
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| Hydraulic primary drive assembly integrated into rear differential. Click to enlarge. |
The hydraulic hybrid technology is expected to improve the fuel efficiency of the yard tractor by 50 to 60%, reduce or eliminate emissions during idling, and decrease brake wear.
Key partners in the project include the Port Authority of New York and New Jersey, A.P.M. Terminals, Kalmar Industries, Parker Hannifin Corporation and the Port of Rotterdam, with which EPA will share project information.
For the longer term, the EPA is interested in exploring a number of different high-efficiency engines for use in the series-hybrid architecture, including:
The EPA-International-Ford Clean Diesel Combustion Engine (earlier post);
Optimized E85 or M85 engines;
HCCI gasoline engines;
Free piston engines;
HyTEC—Hybrid Thermal Energy Converter that recovers energy from engine exhaust heat
Resources:
The hydraulic hybrid technology is expected to improve the fuel efficiency of the yard tractor by 50 to 60%, reduce or eliminate emissions during idling, and decrease brake wear.
Outstanding!
Posted by: yesplease | 05 September 2007 at 06:26 PM
Amazing improvement, although I wonder why it can't be pneumatic instead of hydraulic-pneumatic?
Posted by: Ben | 05 September 2007 at 08:16 PM
@ Ben -
this is a purely hydraulic hybrid. The accumulator is a pressure tank with an internal bladder. A fixed amount of gas mass (in this case, inert N2) is permanently trapped between them. As a volume of incompressible oil is pumped into the bladder, the bladder expands and compresses the surrounding gas - heating it in the process. By virtue of equilibrium, the interior of the bladder is at the same pressure as the gas. Note that even when the bladder is completely empty, the nitrogen can still be at high pressure, e.g. 200 bar. This sharply increases the load on the hydrostatic pump during recuperative braking and ensures it can always deliver useful amounts of power when operated in reverse as a motor. Single-stage pressure ratios of 200:1 are quite feasible, though internal leakage losses are substantial.
A purely pneumatic hybrid would also work in principle, but you would need to compress a much larger volume of gas and therefore suffer much higher heat losses. That means adding intercoolers. Also, in pneumatics, you cannot implement a 200:1 pressure ratio in a single stage.
Posted by: Rafael Seidl | 06 September 2007 at 10:16 AM
Ok I thought the N2 levels were variable. I was aware of the thermodynamics of air de/compression but since they do have a gas phase in there that is constantly changing pressure it will also absorb and release heat, unless of course the hydraulic fluid is acting as a temperature sink.
Posted by: | 06 September 2007 at 11:14 AM
It's possible that they allow the N2 mass in the accumulators to be varied, but that would be a system tuning parameter rather than something you'd do on each charge/discharge cycle.
The minimum pressure in the HP accumulator is given as 2000psi, rising to 5000psi when fully charged. If the gas temperature of the empty accumulator is 300K, adiabatic heating will raise that to 1080K. I'm not sure which bladder material would support such high temperatures. Plus, the hydraulic fluid would decompose if it ever got that hot. So I expect the flask wall is made of metal and double-walled, with coolant circulated between the walls.
A similar requirement for temperature management exists for the LP accumulator, but the much lower gas mass there means the amount of heat that needs to be wicked away is also much smaller. Forced air flow over the outside of a single-walled flask might be good enough.
Posted by: Rafael Seidl | 07 September 2007 at 08:18 AM
The minimum pressure in the HP accumulator is given as 2000psi, rising to 5000psi when fully charged. If the gas temperature of the empty accumulator is 300K, adiabatic heating will raise that to 1080K.
Really? The adiabatic exponent of N2 is around 1.4, right, so P * (V^1.4) = constant, and changing the pressure by a factor of c should change the temperature by a factor of c * (1 - 1/1.4). The final temperature would be a bit over 500 K.
Posted by: Paul Dietz | 07 September 2007 at 08:36 AM
also these processes are far from adiabatic in that heat will dispate out through the tank walls or the working hydraulic fluid. Now that I see how it works its definitly a brilliant power system, though hydraulic fluid will have more friction loss then pnumatic.
Posted by: Ben | 07 September 2007 at 09:45 AM
From the drawing, it appears that the high pressure N2 container is compressed 6 folds, to a maximum pressure of 5000 psi. 6^1.4 is 12.3 for an adiabatic process of N2 gas (diatomic molecule), so the uncompressed pressure of the N2 would be 5000/12.3=406 psi. If this 406 psi is at 300 K temp, then the final adiabatic temp at 5000 psi would be: ~600 K, or 327 C. Still pretty toasty for hydraulic fluids, but manageable due to the considerable thermal inertia of the hydraulic fluids if convective heat transfer within the fluid is fast enough.
Posted by: Roger Pham | 07 September 2007 at 03:56 PM