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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."
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?
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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.
If it's holding 5000 PSI it will be pretty difficult to crush.
Generators cause drag so you loose some energy but this type of system would add friction into the mix which would waste more energy. Seems more like an energy shell game with looses from friction along the way.
Why is it that this system would necessarily waste more energy than a electrical system? You say that this system would add friction, which is just another word for the "drag" that the generator adds in an electrical system. Why is this more of an energy shell game than an electric hybrid? It's just replacing the generator/battery combo with a compressor/accumulator combo.
Assuming that it's mostly a short-term compress/decompress cycles, as long as the accumulator is well insulated to prevent heat loss, it should be fairly efficient. Perhaps more efficient than a battery.
This article suggests that a hydraulic/compressed gas system can have 75% energy recovery for start/stop conditions as compared with 15 - 20% for a gasoline-electric hybrid:
http://www.scientificamerican.com/article.cfm?id=hydraulic-hybrid-vehicle
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.
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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.
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A two-ton SUV moving at 80 miles an hour agrees with you, but doesn't give a fuck
SCBA tanks are required to be tested every 5 years at 5/3 of their rated pressure. I wonder if the Chrysler tanks will be due similar scrutiny...
In terms of "bladder," it's probably not a misnomer: Similar to an expansion tank on a hot water system, or a pressure tank on a well system, the factory-installed nitrogen will be separated inside the tank from the newly-introduced compressed gas by rubber.
FWIW.
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Now there's an idea! If crushed in a wreck, it would be holding more energy. Storing the energy of wrecks could become the new eco-friendly feature in cars.
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.
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Generators cause drag so you loose some energy but this type of system would add friction into the mix which would waste more energy
A bigger engine with 2 extra cylinders (to match the performance of this 4-cylinder hybrid) also adds friction, and it does so all the time the engine is running.
I would assume that this gas compressor can be disengaged with a clutch when not needed, so the friction losses could actually be less overall for the same max power output.
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.
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Wait a minute... isn't that why people didn't want hydrogen cars in the first place?
No, it is because pure hydrogen has a lower energy density the hydrocarbons and it's highly difficult to store hydrogen (the tiny bastard uses the pores of the steel container to escape). See hydrogen storage.
What about supercapacitors?
Expensive like hell.
Those would be much safer than high-pressed nitrogen.
Would it, now? Just what you think happens when the hundreds of ampere*hours discharges through you body in the shortest time possible? Ah, you say: why through my body and not through the car's body? I ask you in return: why the nitrogen tank should explode instead of releasing all the gas through a "sacrificial valve"?
Just because it constitutes 78.08% by volume of Earth's atmosphere doesn't mean we should trap it inside high-pressure cylinders. What will PETG say?
Who? The polyethylene terephtalate glycol?
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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.
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I hope there's a turbo button that vents the nitrogen to a rocket nozzle for when you want to pass someone.
Only if your van uses a 486 CPU. After that, it's just nothing or adjusting your BIOS setting (which requires a cold-reboot most of the time).
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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.
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So if I can't start my engine and my pressure tank is empty, how do I jump start it? Connect a high-pressure line from another vehicle with the same kind of accumulator? Or do I have to tow-start it?
Would help if you'd have had baked beans for the dinner a night before. Though... mileage may vary.
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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.
I'm actually wondering about the opposite of most of the energy being released as heat. When you depressurize a small can of compressed air, it will freeze the can, as well as the object you are pointing it at. How much heat could a 14 gallon tank suck up if were to depressurize rapidly?
As a scientist who lives off the grid on solar PV (for decades now), I've pretty much investigated every way there is to store energy, and it's not so simple a problem. Vanadium redox batteries (utterly impractical for autos and that membrane ain't cheap) look about the best so far in terms of simple and good while being efficient. Most things that do heat storage are only efficient if huge enough that the surface area to volume ratio can be really small.
The above approach might work out fine for small amounts of energy and for short times, however, and having some is better than nothing -- it probably is pretty reliable unlike most batteries which tend to have much shorter cycle life than is claimed. I think we're going to see a big backlash against battery cars at some point because of that one.
Why guess when you can know? Measure!
UPS and the EPA announced something very similar two years ago.
http://articles.cnn.com/2008-10-28/tech/ups.hybrid.trucks_1_hybrid-trucks-hydraulic-hybrid-hydraulic-fluid?_s=PM:TECH
But they're not *that* bad, just lousy. More like you lose about 40% round trip, not 75% as you say, and that 40% is at end of life, when you get disgusted and buy new ones. Now that's at lower cyclic rates, eg capacity in 10 or more hours. The Li Ion ones are better for fast things, but still not great. But neither do you lose half each way....until they're about at end of life. The real sad story is that they're not going to have the claimed cycle life and a lot of buyers of cars where most of the cost was that battery are going to be real unhappy when they find out it doesn't live all that long and costs nearly the full car price to replace...even though like any car, the value of the basic car goes down quick the instant it comes out of the showroom.
The issue with a bunch of the other storage mechanisms is explosion risks. Gasoline burns, but only as fast as it can get air. Pressurized things (look at the safety history of early steam) let all the energy go in a fraction of a second....so, maybe OK in a system buried in a pit or something, but not so great in a crash. In a normal crash, you only release at most about 10 seconds worth of full engine power in the crash alone -- then maybe the fuel burns slowly. (reality isn't televised) Now consider what would happen if it was an hour's worth, all the car's potential energy released at once like a broken flywheel would do....not a pretty thought.
Which is another reason why things like liquid fuel will be around awhile, even though there are plenty of reasons to object to them. You don't have to carry the oxidizer, which with gasoline works out to about a 15::1 weight advantage...You burn about 16 times the weight of air (or at least that weight, which includes the nitrogen, goes through the system) for every weight of gasoline, which is what makes IC engines practical at all. With a battery, you have to carry both, in effect. With mechanical storage, you always have full total energy ready to go "bang" in an instant, not so safe.
We have a long way to go to get out of the woods on this problem, I've been studying it for a lifetime, and there's nothing new on the periodic table that's going to magically solve this anytime soon. LiIon already has nearly the energy density of high explosives....that's about the limit of chemistry, real or imaginable.
Why guess when you can know? Measure!
This sounds reminiscent of the starting system used in the good old Yakovlev Yak-52 aircraft. They first started flying them in 1976 according to Wikipedia .... ah well, what is old is new again.
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. 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.
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Mythbusters successfully demonstrated that a SCUBA tank, which have pressures up to 30 MPa (4400 psi) and internal volumes up to 18 liters, will turn itself into a missile if its regulator catastrophically fails. The tank proposed in TFA would have a pressure of 34 MPa and a volume of 54 liters, meaning that it will store even more energy.
An over-pressurized liquid nitrogen tank caused major damage to a Texas A&M building when it failed (read: exploded). According to the engineer's report (pdf):
Pictures of the devastation are included in the report. This tank, like all compressed gas cylinders, had both a safety relief valve and an emergency blowout disc. The explosion only occurred when both of these safety features were compromised due to improper maintenance.
While the hybrid's gaseous nitrogen tank is substantially different than the liquid nitrogen tank described above, it is safe to say that compressed gas cylinders are dangerous beasts. Unless you're planning on participating in the Hybrid Space Program, I would suggest steering away from vehicles like this.
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It's a good system that was implemented years ago by these guys:
http://www.permo-drive.com/tech/index.htm
From my quick perusal the systems look the same.
They even sold it to the US military for use in their FMTVs.
I wonder if the Chrysler tanks will be due similar scrutiny...
doubt it. scba tanks are strapped to our backs and providing us life-air while deep under water. a tank failure would probably kill a diver so this extreme cautionary preventative maintenance is warranted.
bite my glorious golden ass.
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This engine is a split-cycle four stroke air hybrid that fires After Top Dead Center (ATDC) effeciently. The engine already holds far more pressure than standard combustion engines and reduces NOx by up to 80% and CO2 emissions by ~30% over similar hybrids and standard combustion engines - without the need of an ancillary system for power management (an electrical system for example). The engine presses out nearly 100% of the gas from the exhaust piston which leads to far much better optimization.
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.
On the other hand, any thermal leak is a very big energy loss. You're running a heater and throwing away heat on "charging", a cooler and then absorbing heat into the cooled gas on "discharging".
This is why compressed air is a rotten energy storage and transport medium. (In factories, however, it IS used as an energy transport medium because the inefficiency is offset by various design advantages in the devices it powers - typically linear actuators, large clutches on stamping presses, compact refrigeration and air cooling (using vortex/swirl tube refrigerators and vortex entrainment air pumps), and light but powerful hand tools.)
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Source please? Last I've heard, Nickel-based chemistries (early hybrids such as the Prius use(d?) Ni-MH) achieve 90% charging efficiency if fast-charged (that is, the battery stores 90% of the energy provided to it). And Li-ion's charge efficiency reaches an impressive 99.9%.
While compressed air may have many advantages over modern batteries, charging/discharging efficiency is unlikely to be one of them.
It's a good concept as an alternative to using generators to store the energy as electricity in batteries. The safety issue that people keep bringing up about the tanks exploding is pretty far fetched. When I was a mechanic Nitrogen pressure cylinders were used extensively back in the 70s on diesel engine equipped vehicles to create "hydro-boost", a device that did not require vacuum like a conventional power brake system. I never heard of any mechanics being hurt with these nor did I hear of one exploding. As a technology high pressure cylinders have been mastered. For example, I have used Oxygen tanks for oxyacetylene torch, brazing etc that were manufactured by the Third Reich in Germany. These tanks are still used and are recognized by the swastica near the top of the tank. The real hazard with tanks of this type is -- as someone else mentioned -- that the valve might be broken off, creating a rocket projectile of enormous power. No such hazard exists with the Chrysler project. Also the proposed compressed Nitrogen method of regenerative power saving is more reliable and does not require expensive replacement of all those batteries every 5 years, which is the real killer of all electric cars and is why they will never live up to expectation. Chrysler is on a good track with this and I commend them for their engineering and research and development.
TFA refers to using the "engine overrun torque". Presumably they're referring to the situation where you take your foot off the gas pedal and engage a lower gear, thus causing the momentum of the vehicle to drive the engine - colloquially referred to as "slowing down using the gears".
I see three problems with this. One, most US cars are automatics. Two, your typical minivan driver thinks one or the other pedal has to be pressed flat to the floor at all times. Three, your typical minivan driver doesn't understand what gears are. And four (I'll come in again) even if she did understand what gears are, she'd have to put her cellphone/pizza/lipstick down in order to move the lever (point two notwithstanding).
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It's the size that gets me. A quart-sized accumulator is no big deal. They're used in construction equipment for ride-smoothing or for emergency operation in any number of hydraulic circuits (just remember to isolate the accumulator before doing any maintenance, as any number of amputees will tell you).
But 14.4 GALLONS at 5000 psi? Surface area squares as you double any dimension so that 5000psi is going to be acting on a LOT of surface area. The energy being stored is huge, which I guess is the idea.
As a scientist that has been studying how to store energy for decades you should know the difference between a method of storing energy that is theoretically 100% efficient, and one that it isn't. Most batteries are not theoretically 100% efficient. Adiabatic compression of an ideal gas is. How close you can get to 100% in reality is an engineering problem. With batteries there is a theoretical limitation that won't allow 100% efficiency.
i'm pretty sure I build these accumlator tanks at my job, they are wreck proof, bulletproof, and fireproof they are tested well above their rated pressures, we even blow one up out of every batch just to see what it would take (i'm guessing these are 15,000 psi + to explode, but it could only be 10-12 I'm not sure what configuration we used to build these) and there is a bladder installed after it leaves the factory
Whereas this tank, being much larger tank, holding much higher pressures, inside a metal cage holding maybe 4 or 5 people, charging down a busy carriageway at 70MPH, won't at all have the capacity to maim or injure significantly many more people if it fails than one SCBA tank? Pull the other one.
This will be tested up the wazoo, and then some.
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I do agree that I was ahead of myself. Now for nitpicks:
Aerogel isn't a daydream. You can buy it. Costs reasonable amounts, even. If you wanted to insulate particularly hot CPU/GPU heatsinks in a laptop from the bottom of the case, a few mm of aerogel would be my choice, at a cost of maybe $10 or so. Maybe not in a $200 netbook, but Apple sure could pull that off if they needed to. Hot heatsinks are much easier to cool.
Superconductors are problematic due to rather theoretical reasons, too: good luck when you lose cooling (see what happened to LHC). Supercapacitors and low ESR don't mix. You get one or the other, and I think the reasons are to do with fundamental properties of the class of materials used in their design.
I don't really think that electric hybrids are very good at what they claim to do. Practical -- sure, but not very good at all. Unfortunately, neither are hydraulic hybrids, even 100% efficient ones, or really any other kind of hybrid, and I think that's where the buck stops.
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Not quite. Overdrive is a larger ratio gear. Back in the olden days, top gear ( in the car, not the TV series ) was a 1:1 ratio between the input and output of the transmission. Overdrive is a bigger ratio than that. It, and the 1:1 that "D" gives you can have a "freewheel" to them, which is more like what you describe ( allows the engine to go slower without that causing compression deceleration ). So, if you commonly run in "D" rather than overdrive, you might be wasting a bit of fuel on a lower gear ratio than is most efficient.
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