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How Chrysler's Battery-Less Hybrid Minivan Works
thecarchik writes "Chrysler announced Wednesday that it would partner with the US 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. 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."
The amount of energy you can store in a 14 gallon hydraulic accumulator is pretty small. Even if they're cranking the pressure up to 6-7,000 psi the energy density is around 50kw-sec/gallon or somewhere around the equivalent of a car battery.
Perhaps not pointless. In the city, it's the start-stop aspect which is the mileage killer. Regenerative systems capture some of the energy used to decelerate, and use it to re-accelerate later. This is responsible for a large part of the efficiency of electric hybrids in city usage. I'm not sure if the hydraulic system described in TFA is linked to braking, or would by nature of its design capture energy during deceleration, but if so it would definitely help in city use. In fact, that may be the only place in which it shows gains, but let's not underestimate that. Most minivan use IS city use.
There is also the advantage that it's not based upon rare earths or lithium, which have their own political "sourcing" issues and their own limitations on how much is available. In short- to medium-term timeframes, that could be more important than ultimate efficiency comparisons with electric hybrids.
The safety concern is a serious one. Unlike present applications mentioned in TFA (garbage trucks, busses), there is much less structure in a minivan-sized platform to protect the pressure vessel. Anyone remember the Pinto problem? This is solvable, though it will require more structure (meaning more weight) to protect it. Overall, the hydraulic subsystem + the weight of the protective structure are probably less than the weight of the electric subsystem including its batteries, so that may be a net gain over electric hybrids, but we won't know til we see specs.
Everybody gets what the majority deserves.
If you have ABS, you already have something like this in your car. It's a little (1qt) metal sphere with a rubber diaphragm in it. It holds about 3,000PSI of Nitrogen in order to cycle the ABS when it activates.
As for the safety...well... how safe is it to carry around 20 gallons of highly flammable gasoline?
FTFA:
That compressed gas, stored at pressure as high as 5,000 pounds per square inch, represents energy waiting to be released.
Not sure I'd want to be an a 1.0 version consumer vehicle with that much pressure without some serious discussion about the safety precautions to prevent or mitigate "unexpected pressure drops". Can someone who's got more experience with the fluid mechanics add to this?
Scuba divers drive around with aluminum cylinders containing air at 3,000 PSI. Safety "burst" discs are built into the regulator of the cylinders so that if over pressurization occurs they rupture. The results are frightening and embarrassing but its only air and not shrapnel since the cylinder remains intact. I expect there are similar technologies in the pressure vessels in these cars.
In an accident, it will remain intact. If not, then the car won't pass standard safety tests, and the manufacturer knows it won't sell. In the event that some freak crushing blow strikes the tank (like, for example, getting caught between a freight train and a reinforced bunker, or perhaps dropped from an airplane) It'll most likely burst open at the one spot that the engineers intentionally design to be slightly weaker than the rest of the case, which conveniently releases the contained gas in a harmless direction.
5000 PSI is like having an average American car, including all passengers, with all its weight sitting on a single square inch. That's the maximum operating pressure, implying that the tank itself will actually hold significantly more pressure before having any problems. I feel pretty confident that the engineers involved know what they're doing, and can prevent catastrophic failure during a collision.
You do not have a moral or legal right to do absolutely anything you want.
I routinely work with compressed gases (~2500psi, medical oxygen on an ambulance). The tanks are tremendously well-built, and if you drop one you're worried about the valve because it protrudes - not the tank itself. And by my envelope calculations, there's something like 603k pounds trying to turn my tanks inside out.
Yes, I'd want to be damn sure I knew what that tank was doing, and how well it was built - but we're pretty good at making pressure vessels that won't rupture on their own, and equally good at making ones that are solid enough to withstand impacts.
Frankly, 15 gallons of gasoline worries me more. The kind of impact that would rupture a tank would aerosolize the gas, and I'd rather be in an explosion than an explosion with fire.
I have developed a truly marvelous proof of this comment, which this signature is too narrow to contain.
http://en.wikipedia.org/wiki/Compressed_air_car
The compressed air car has been under development for a long time. It shows great promise but nobody yet has been able to make a practical vehicle.
The advantage of a hybrid vehicle is that it doesn't have to store enough energy for a complete trip. In particular, it stores energy (thereby heating the engine) and releases energy (thereby cooling the engine) over a short period of time. The pure compressed air vehicle has the problem that the engine is permanently in cooling mode. If the engine is hot, because it has just been compressing gas, it is far more efficient. The longer it operates as an engine, the less efficient it becomes.
The advantage of compressed gas for short time energy storage is that the storage is simple and does not take much sophisticated material as compared with batteries.
People raise the problem of a tank of gas stored at very high pressure. The hybrid vehicle doesn't need as big a tank. Also, they've been working on this for a long time. The problem is basically solved. It isn't nearly as much a problem as a tank of gasoline.
The problem can be with the T. The hot compressed gas cools to ambient over time, dissipating energy (seen as a loss of pressure). I suppose, though, the energy is used before much heat has a chance to leak away. Barring that the limit on efficiency is the mechanical losses in the motor you drive with the gas.
You don't need particularly high pressures to make it theoretically efficient. You may be thinking of heat engines based on Otto (piston) or Brayton (turbine) cycles where efficiency is related to the pressure and temperatures at combustion, the higher the better.
Equine Mammals Are Considerably Smaller
What people sometimes forget about is that such a cycle can be theoretically 100% efficient: it's called the reversible adiabatic process -- completely reversible! As long as your gas storage system is well insulated and has low thermal masses, that is. You simply compress and heat up the gas and store it. Later on, you decompress and cool down.
Think of a gas sealed in a well-insulated, low thermal mass cylinder. You do some work to move the piston in, the gas heats up and compresses. You release the piston, the gas does the same work going out as it expands and cools down. If the system is perfectly isolated and there is no friction, you get exactly the work you put in.
This has the theoretical potential of being a rather nifty thing, but I don't know how the practical (engineering) side of things works out. It may be impractical, or may be not. Time will tell.
A successful API design takes a mixture of software design and pedagogy.
For those who are not into car repair et al, Audi used hydraulic pressure accumulators for power brake assist. It's a great system, particularly for turbocharged cars, which spend a considerable amount of time in normal driving with low or no manifold vacuum (which is created by the pistons trying to draw air past a restriction, aka, the throttle vane. That big round thing your brake master cylinder comes out of? That's the vacuum servo. It uses surface area to multiply force from the vacuum.) Citroen used the same idea to power the extensive hydraulics used in their famous suspension systems. Mercedes did as well for their cars which had hydraulic power windows (!!), door-closers, and suspensions. Nowadays, the idea of hydraulic assist has largely gone by the wayside, with auxiliary electric vacuum pumps used where necessary. It's a shame, because the hydraulic system had a HUGE amount of reserve; you could pump the pedal hard almost thirty times.
The reservoirs are lovingly nicknamed "the bomb" by enthusiasts and owners of mid-80s-to-early-90's Audis, strictly on appearance; they look sort of like a large-ish cartoon bomb. I have NEVER heard of one exploding or failing (in terms of the pressure vessel, say, by cracking) in any way, and they've been in use for almost thirty years.
The way they DO fail, very predictably, is via the internal bladder that separates the nitrogen charge from the hydraulic fluid. Eventually the bladder fails, or the nitrogen simply diffuses through the bladder. Also, hydraulic systems are pretty horribly unreliable; with age, everything rubber fails eventually. Citroen did a pretty good job of proving that too, but on Audis, pretty much all the hydraulic hoses eventually fail. The hazard, in this case, is that when this system fails, it'll dump gallons of very slippery hydraulic fluid all over the road. If you're lucky, it won't also spray it all over, say, your hot exhaust. Atomized oil is pretty damn flammable.
Another danger: with the Audi system, all you had to do was pump the brake pedal until it was hard, and the system was safe to work on. This system would involve higher pressures and larger quantities of fluid...and it would become a real danger for anyone working on the car to do so with the system charged, as fluid over a certain pressure will either break skin or worse. I imagine they'll develop an easy way to discharge it, but people are still idiots.
The thing is also going to be a total bitch in a fire; I'm sure they'll put a pressure relief on the nitrogen side, but even then, you've got 10-15 gallons of flammable oil to deal with.
I really don't see Chrysler having any incentive to make the thing more durable than Audi/VW/Citroen did. It'll be made so it lasts about 60-70K, and then you'll be looking at replacing a huge, high-pressure tank. Expect the hilarity 3-4 years from whenever they go on sale, probably sooner.
Please help metamoderate.
CNN describes the tank as a "bladder".
Damn! Now that's two bladders I'll be emptying when an accident occurs.
Have gnu, will travel.
Adiabatic heating on compression would be pretty serious. A diesel engine only has 15:1 to 20:1 compression ratio, and develops enough heat thereby to ignite diesel fuel. In this system we are looking at upwards of 300:1. The temperature would be absolutely fierce.
If on the other hand you design the system to dissipate the adiabatic heat, you are rejecting a good proportion of the compression energy, which then you will not get back on expansion. So either you must withstand incredible heat in the system, or you sacrifice efficiency.
The mirror image is adiabatic cooling on expansion. If you do reject the adiabatic compression heat, then on expansion you will have problems with supercooling and moisture freezing.