How Chrysler's Battery-Less Hybrid Minivan Works 347
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."
They've solved some serious problems (Score:5, Informative)
http://en.wikipedia.org/wiki/Compressed_air_car [wikipedia.org]
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.
Re:It's worse then that. (Score:5, Informative)
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.
Re:Sounds inefficent (Score:5, Informative)
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.
Re:Compressed gases aren't *too* bad (Score:5, Informative)
Finally someone who has something intelligent, constructive and relevant to say rather than the myriad of knee-jerk, living in mom's basement, I watch Discovery Channel experts.
Are you on the right site?
Perfectly safe, not reliable. (Score:5, Informative)
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.
Re:Chemical battery efficiency is quite poor (Score:4, Informative)
The typical chemical battery used in hybrids have very poor efficiency. It stores only 50% of the energy given and releases only half of the stored energy
Nope. Lithium-ion is around 90+ percent efficient round trip. See the note in the wiki article [wikipedia.org]. Lead acid is around 70% round trip. Molten sodium is a very old technology that is actually quite safe, but has durability and power density problems.
Flywheels are great, but they're really scary. Flywheel hybrid research was mostly stopped when a wheel blew up and killed a technician at Chrysler. The problem with compressed air is that their is heating of the air during compression and cooling during expansion. If that heat does not stay in the air, there is efficiency loss.
What we need is really electricity priced the way cell phone minutes are sold. Peak hour, off peak and night rates. Then there will be an incentive for people to buy these things to store cheap electricity at night and use it in the day and reduce the grid load on hot summer days.
There already is for large industrial customers. The smart grid would bring that to homes. One of the consequences is negative electricity prices due to excess wind power. Even so, I did some bath that showed you could expect to make around 0.1 and 0.2 dollars per kWh of capacity per day. That's around 30-70 dollars a year per kWh. The cheapest batteries I know of are around 50-60 dollars per kWh and will be toast before they pay for themselves.