How Chrysler's Hydraulic Hybrid Works: Energy From Pressure

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Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

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Chrysler has come out of hibernation with a bang, as far as green and more fuel-efficient vehicles are concerned.

The company announced Wednesday that it would partner with the U.S. Environmental Protection Agency to build and test prototypes of a different kind of hybrid vehicle, one that accumulates energy not in a battery pack but by compressing a gas hydraulically.

CEO Sergio Marchionne joined U.S. EPA Administrator Lisa Jackson for the announcement, made at EPA laboratories in Ann Arbor, Michigan. They spoke in front of a silver 2011 Chrysler Town & Country minivan, representing one vehicle that would be tested with the new technology.

Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

Enlarge Photo

Surfeit of hybrids

During the last 18 months, while the company restructured after its bankruptcy, integrated its product development with that of its new partner Fiat, and embarked on a crash program of refreshing its existing models, only a single "green" vehicle was known to be on the drawing board.

That's the low-volume electric conversion of the Fiat 500 minicar, which will be engineered in the U.S. for buyers here.

But in the space of only a week, Marchionne has said the company will launch a Chrysler 300 Hybrid by the end of next year and a hybrid minivan as well. The EPA announcement makes it a triple.

2011 Chrysler Town & Country

2011 Chrysler Town & Country

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Engine, but no battery

The system in question, originally developed at the EPA labs, uses engine overrun torque to capture otherwise wasted energy, as do conventional hybrid-electric vehicles. The engine is Chrysler's standard 2.4-liter four-cylinder, the base engine in its minivan line.

But rather than turning a generator, that torque powers a pump that uses hydraulic fluid to increase the pressure inside a 14.4-gallon tank of nitrogen gas, known as a high-pressure accumulator.

That compressed gas, stored at pressure as high as 5,000 pounds per square inch, represents energy waiting to be released. When the tank is sufficiently charged, that pressure is released to power a hydraulic axle motor that turns the wheels. The engine remains off as long as there's sufficient power to operate the motor.

Determining the possibility

Despite the hoopla around the announcement, though, any hydraulic hybrid minivan is still far from production. The partnership will simply "determine the possibility of adapting a hydraulic hybrid system" to certain vehicles.

Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

Chrysler's Sergio Marchionne & EPA Administrator Lisa Jackson announce hydraulic-hybrid program

Enlarge Photo

That translates to basic conceptual, design, laboratory, and prototyping work to reduce the system's size and complexity. Chrysler's projections say the system could raise fuel efficiency as much as 30 to 35 percent overall, and potentially more than double the city gas mileage. It would also be cheaper than a hybrid-electric system for the same vehicle.

The tests will be completed by July 2012, at which point Chrysler will decide whether the system is viable for production. If so, its next-generation minivans will be designed to accommodate the necessary tanks and pumps.

Garbage trucks, delivery vans

Presently, the few hydraulic hybrids on the road are found in large, heavy commercial vehicles where the cost of a suitable battery pack would be too steep. Garbage trucks, delivery vans, potentially even transit buses offer the best fit for the technology. The EPA is working with Eaton on such commercial-vehicle tests.

Hydraulic hybrids are not well suited to smaller vehicles--subcompacts to midsize--because the hydraulic equipment and storage tanks occupy too much space. For those vehicles, other technologies, from start-stop and smaller and more efficient engines to full electrification, make more sense to increase energy efficiency.

Chrysler's system, says the company, is destined for "large passenger cars and light-duty vehicles," meaning minivans and perhaps future delivery vans.

[EPA, Chrysler, Detroit News, Automotive News (subscription required)]

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Comments (27)
  1. I had no idea that hydraulic hybrids had progress this far. Eaton apparently has shipped 4500 vehicles and covered 100 million miles.

  2. Sounds like an effort to reinvent the wheel which is a bit ironic for a car company. How is this better than Toyota's hugely successful hybrid system that works in small cars too and is available now and not maybe somewhere beyond 2012 at a price that may or may not be lower than conventional hybrids will be at that point. Is Chrysler really thinking some cumbersome mechanical device is going to impress buyers at a time when more and more hybrids will feature plugs? I think this decision was more driven by pressure than Chrysler's cars will ever be.

  3. If they used a spring type accumulator, then there would be no losses of energy due to heat dissipation. It's not a problem for stop and go vehicles in city driving, etc., but some stored energy would be lost if the vehicle sets overnight.
    Such an accumulator would allow pre-compression by plugging it in, becoming a plug-in hybrid without batteries.

  4. Chris O: "How is this better"
    Answer: It uses a tank of nitrogen to store the energy, instead of batteries made with expensive rare earth metals. As stated, it is for larger vehicles where a very large and expensive battery would be required. A nitrogen tank is clearly less costly on a vehicle like a garbage truck that already has extensive hydraulic power systems.
    Seriously why not RTFM before making such comments. It's all in there.

  5. This is fully functional deal, Compressed air can provide a lot of stored energy and a low failure rate compared to items like batteries.

  6. what FT said (Why is there only a 'flag' button?)

  7. This seems to be very similar:

  8. @FT:" batteries made with expensive rare earth metals" really? I'll go ahead and assume that the rest of your argument has the same merit...

  9. I did some analysis for a renewable energy project using compressed air. I was surprised by two things. First the storage of energy was as expensive as Li-Ion batteries. This is due to the fact that high pressure tanks (5000-10000 psi) which are rated for transportation use are extremely expensive carbon-fiber type. This might come down significantly as high volume use comes along. The second thing that surprised me was how inefficient it was due to the energy lost in the air compression (as mentioned by another commenter).
    On the other hand, if Eaton is having success with it, then perhaps I am missing something (of course I know that Eaton is using hydraulic, not pneumatic. But in their system all the energy is stored pneumatically. You cannot store energy hydraulically (incompressible don't cha know))
    John C. Briggs

  10. @John Briggs
    the case being made is that Li-Ion batteries for a garbage truck are astronomically expensive compared to a regular car sized pack. Did your research only involve economics of a smaller vehicle? The article states that a 14gal tank at 5000psi is cheaper than a battery plant for a hundred ton vehicle like a garbage truck or city bus. Also it seems logical to me that using hydraulics to compress the gas instead of pneumatics would be more efficient, as you point out, gases are compressible, so the mechanism doing the compressing is itself subject to being compressed. this would imply that not all the force is making it to the storage tank. Pascal's law: A force applied to the surface of an incompressible fluid at rest will be transmitted equally, and undiminished, throughout the fluid and to the walls of its container.

  11. John Briggs, if a pneumatic system quickly releases its energy, as it does in rapid stop-and-go operation of garbage trucks, then heat dissipation losses are minimal. I am suggesting using a spring type accumulator, in which the hydraulic piston pushes against a spring instead of compressing air. In that way, you could pre-charge your system using an electric compressor without lossing any of the charge by setting out overnight.

  12. Hydraulic accumulators have better power density, electric batteries have better energy density. In other words, an accumulator can absorb and deliver energy faster than a battery, but a battery can hold more total energy. So the decision to go electric or hydraulic depends less on the mass of the vehicle and more the overall hybrid strategy. Do you want to charge your vehicle overnight and slowly use that charge during the day, then electrics are your answer. However, if your hybrid strategy is kinetic energy recovery, power compensation during shifts, or a fully decoupled drivetrain with CVT (aka. series), then hydraulics may be a better solution.

  13. First, isn't it kind of worrisome to pressurize a tank you're carrying with you to 5000psi? What if you get rear ended? Turbo boost?
    As to compressing a spring vs air, wouldn't you need one heck of a spring? It would need a long stroke and some think coil; that sounds very very heavy and with a spring like that you'd have to go to great lengths to keep it tamed during an accident, no?

  14. PSST... Chris O... here you go buddy...
    (And don't forget, when you get angry for no valid reason, just calm down, count to 10, and Google)

  15. Yes a 5,000 psi storage tank would be dangerous in a collision, but an accident sufficient to damage the tank would also blow up a gasoline tank, which would contain MUCH more energy AND be flammable.

  16. Can anyone explain how the power brakes and steering are supposed to keep working while the engine is off and the "axle motor" is moving the car?
    Electric hybrids know how to do this.
    In the absence of a solution, this thing is useless.

  17. @David Gorgen: I'd presume they work just as they do in a hybrid vehicle: The power steering is operated by an electric motor rather than an engine-driven hydraulic pump, and the brakes are regenerative unless full friction braking is required, when an electric booster (I believe) kicks in.

  18. @Michael --- 5000 psi is pretty significant. It is not as dangerous as it sounds, though. Carbon fiber tanks don't explode when ruptured. They split apart. Also, Nitrogen is not an explosive gas. Sure, you wouldn't want to be right in front of a carbon fiber tank releasing 5 kpsi of compressed Nitrogen, but if you're in the car, the tank is under the car, and it's hit hard enough to rupture, you have far more serious problems to worry about.

  19. I favor hydraulic in larger trucks, but not cars. It sounds like they may have found a way to make this more cost effective, so more solutions that work help.

  20. @John V.: OK, I guess that's plasible. I was assuming that a standard 12v battery (the only source of electric power when the engine's off) wasn't enough margin to do this safely...certainly it isn't much, compared to what a traction battery can supply. However I guess they could specify an enhanced battery, and/or a system to restart the engine (getting alternator power back) if the battery is flagging.

  21. There are some attempts at a "smart alternator" sort of BAS light that will start/stop and still have power to do so with a larger, say...36-48 volt battery. With that they could run power steering, AC and keep the automatic transmission hydraulics flowing at stop.

  22. @David: You already have a powerful battery -- it's called the accumulator. Brakes and power steering can be directly supplied by the accumulator. AC can be driven by a hydraulic motor. And the main pump attached to the engine can double as a hydraulic motor for starting. In this configuration, the engine's job is to ensure a minimum operating pressure to keep all the systems on-line. Unifying the vehicle's hydraulics may present some reliability challenges, but it also eliminates a handful of auxiliary hydraulic pumps and electric motors from the parts list.

  23. if this technology is succesful in the current vehicles they're testing, chrysler will soon mold the tank into the frame of the smaller vehicles or some other way but they will use it .

  24. Toyota's Hybrid battery has a very limited lifespan and is way expensive so when the battery is dead your car is worth $500 :)
    Also, I think Hydraulic will offer much more raw torque should you need it.
    Seems like a great option for full size pickup... but would be funny to gun-it and hear nothing but birds chripping in the background.

  25. @CyberPine: Actually, I wouldn't be so sure about that "limited lifespan" for the Toyota battery:
    And also:

  26. I feel you are definitely headed in the right direction. Full-size cars and minivans even full-size vans should be considered for hybrid power. I would like to see more plug-in hybrids. The vehicle should run on electric the batteries can then be recharged on the go using any number of fuels. I have several of my own designs that I would very much like to discuss with Chrysler's engineers. I have always and I hope To Always drive Chrysler automobiles. Having read this article I see you are on the right track. It excites me to see such innovative thinking.using energy from hydraulic pressure is brilliant. If you can generate that pressure without too much energy cost you definitely have a winner. I have a design that uses hydraulic pressure

  27. US gvmt funded research by Ingocar Valetin is to create a new design car, maximizing the packaging of gas-hydraulic hybrid components. Bulky components integrated into the chassis. 130mpg in tests by using a large hydraulic accumulator which is the structural chassis of the car. Small hydraulic driving motors are within the wheel hubs reversing to claw-back kinetic braking energy. The aim is 170mpg in average driving conditions. Energy created by shock absorbers and kinetic braking assists in charging the accumulator and is sized to run for 15 mins when fully charged. Charged accumulator has energy storage of 670HP, and produce 0-60mph in 5 secs using 4x4 drive.

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