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Author Topic: What are the pros and cons of hydraulic hybrid vehicles?  (Read 15250 times)

Offline peppercorn

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There seem to be other good arguments for hydraulic hybrids:

  • Li-ion battery has an energy density of 0.72MJ/Kg.
    Air compressed at 30MPa has an energy density of 4 MJ/Kg.
     
  • Carbon composite air tanks are light, yet strong enough to survive major impacts.
    Plus certainly no more expensive than batteries if mass produced.
     
  • Hydraulic motors & pumps are approaching efficiencies given by electric motors.
     
  • Engine heat can increase work by pre-heating compressed air.
    Cold air can cool engine intake air for bigger bangs.
    Cabin A/C can be cooled by expanded air.
     
  • A lot less nasty chemicals and longer 'battery' lifetime.
     


Disadvantages compared with electric hybrids are:

  • Electrical systems will quickly sap power from normal sized car battery if vehicle is running in engine-off mode.
     
  • Compressed air cylinder is still capable of explosive failure in extreme circumstances.
     
  • Regenerative braking cannot be just the motor running in reverse as with electric.
     
« Last Edit: 02/10/2009 08:18:45 by chris »


 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #1 on: 18/08/2009 19:08:36 »
4 MJ/kg may not be quite right

I can only get 458 kJ of work out of 1 kg of air at 30 MPa, and that's assuming it ends up at 1.1 atmospheres, which is a bit optimistic. The other problem is that to get the 458 kJ, you have to supply 500 kJ of heat. That could be problematic at low ambient temperatures. The French air car guys have something they call a "rejuvenator". It burns petrol to heat the compressed while it's expanding when the vehicle is on the open road.

I ran the numbers on a thermodynamic calculator. If you want to try it, there is a free version available at http://www.thermofluids.net/
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #2 on: 18/08/2009 20:41:49 »
D'ya know what, I  thought that was optimistic as well!
Serves me right for trusting an unsubstantiated web source!

I suppose I wanted it to be the case.

Although, surely you only have to supply all that heat if the compressed air has cooled back to ambient between compression and re-expansion.

And the other advantages still stand...
« Last Edit: 18/08/2009 20:48:42 by peppercorn »
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #3 on: 18/08/2009 21:14:41 »
The 4 MJ/kg might be the total thermal energy relative to absolute zero?

Thermodynamics is a b**ch. If you don't let the air cool down (assuming you can prevent it from cooling down) you'll need a much bigger tank, unless you want to store the air at a much higher pressure. (Temperature affects pressure.)

I think these could be some of the reasons we don't see too many (or any?) air cars on the roads, despite the fact that several companies have been working on them for years. Compressed air is convenient for a lot of applications, but it does not seem to make a very effective "battery".

An interesting comment from the lead engineer on the GM Volt - He pointed out that the gigantic battery assembly, that takes up a substantial chunk of the volume and weight of the vehicle, is equivalent to one (US) gallon of gasoline.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #4 on: 19/08/2009 09:42:34 »
If you don't let the air cool down you'll need a much bigger tank, unless you want to store the air at a much higher pressure.
Damn! I kind of know this stuff, but I get selective memory when I want an idea to work! I couldn't find the section for calculating this energy on the thermo page link you sent, but am still surprised at how much heat is required. Luckily heat engines do have a lot of spare heat hanging around!


...the gigantic battery assembly, that takes up a substantial chunk of the volume and weight of the vehicle, is equivalent to one (US) gallon of gasoline.
True, but the 'motor' section is much lighter and the 'gallon' will be used much more efficiently.
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #5 on: 19/08/2009 18:44:15 »
I think he already factored in the efficiency. Could be wrong, but I think what he meant was the battery would take you as far as a gallon of gasoline - about forty miles with a hybrid.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #6 on: 20/08/2009 17:56:41 »
I'm right in thinking there will be more than enough heat from a IC engine to reheat the expanding air during normal operation, am I not?  I don't have the figures, but it would stand to reason, no?

It was mentioned that some French guys [& they are?] are using petrol to heat the air - assuming this is for a straight air engine & they are paying duty there must be cheaper fuels - like natural gas.


Regarding overcoming the problem of electrical drain (lights, radio, etc) in a hybrid mode where IC engine is switched off, I was reminded of the turbocharger-generator idea - this uses the IC engine exhaust gas to spin a high-speed generator.
Example here.

This effectively replaces a standard belt-driven alternator for a normal engine.
I envisage for this hybrid design the turbo-gen comes fitted with a disengageable compressor stage (which the pure electric generation unit doesn't require). When the IC engine is off [whilst stationary or running on air only] the turbo-gen can run as a small gas-turbine (bypassing the ICE) with an independent combustor.  This allows continuous electrical generation, plus heat for the 'rejuvenator'.
« Last Edit: 20/08/2009 17:58:39 by peppercorn »
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #7 on: 20/08/2009 20:29:56 »
I think there would be plenty of heat from the IC engine (I forgot you were talking about a hybrid.)

French company link http://www.mdi.lu/english/  Also TATA in India are doing similar stuff.
Good point about natural gas. Don't know why not.

Not sure I'd worry about the battery drain. A battery will keep lights etc running for a long time. Or were you thinking of dumping the battery altogether?

 

lyner

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #8 on: 21/08/2009 09:46:52 »
You'd need to define what you mean by 'plenty of heat'. How many kW of motive power would it represent?
Also, there's no chance (and no point!) of getting rid of a battery. Just as they're getting smaller and higher capacity. You can always expect to want to run things with the 'engine' off.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #9 on: 24/08/2009 13:12:48 »
Good point about natural gas. Don't know why not.
...A battery will keep lights etc running for a long time...
Natural gas is not available on most forecourts - I'm guessing that's their thinking. Still!
Admittedly, batteries will keep things going for quite a while, but I've read that this is one major argument that car companies have claimed makes petrol-hydraulics hybrids unworkable.


You'd need to define what you mean by 'plenty of heat'. How many kW of motive power would it represent?

Yes, figures on this heating power (kW, BTU's, etc) available at the exhaust manifold would be useful. Clearly, taking too much heat from here would be counter-productive - stalling the exhaust gas flow.


You can always expect to want to run things with the 'engine' off.

That's what I was saying. I definitely wouldn't suggest getting rid of the battery (except in a fuel cell car).
I was suggesting another way of charging it (& running electrical systems) when the petrol engine is 'off' (specifically for a non-electrical hybrid).
I envisage a gas turbine-generator configuration (as explained in previous post)  with 'free' heat for getting extra work from the compressed air supply.
 

lyner

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #10 on: 24/08/2009 13:46:37 »
Your argument might be more convincing if you could produce an approximate Energy Budget, including parts costing. The numbers are particularly important in a thread which deals with efficiency.
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #11 on: 25/08/2009 03:04:15 »
Your argument might be more convincing if you could produce an approximate Energy Budget, including parts costing. The numbers are particularly important in a thread which deals with efficiency.

Peppercorn's call of course, but if he intends to develop commercial applications of his technology, he should keep trade secrets of that nature to himself, or share them only under a strict confidentiality agreement.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #12 on: 25/08/2009 12:57:11 »
Your argument might be more convincing if you could produce an approximate Energy Budget, including parts costing. The numbers are particularly important in a thread which deals with efficiency.

Peppercorn's call of course, but if he intends to develop commercial applications of his technology, he should keep trade secrets of that nature to himself, or share them only under a strict confidentiality agreement.

Aw well, I appreciate the protectionist attitude, but I'm sure any section of my overall design will infringe tons of current patents anyway!

I have hunted around a bit for numbers on efficiency, but find the concept of calculating such an energy budget (even with all data to hand) a bit overwhelming.

I need info on:
Free-piston Diesel engines - specifically driving a high pressure pneumatic pump.
Estimates on kW of waste heat - mainly exhaust, but cooling system also.
Energy available from regen braking plus weight/cost of suitable air pump.
Details on how driving 'cycle' & style effects each component.
Air reservoir cylinder cost, size & weight.
Air reheating requirements for required displacement in hub motor.

I have found on the Artemis website details for their hub motors

Artemis Digital Displacement motor effic plot
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #13 on: 25/08/2009 17:31:48 »
Don't be so sure you can't get a patent! You don't have to prove anything about the efficiency to get one. Mind you, filing patents gets "spendy" really fast.

The thermofluids.net calculator should help you find a lot of the answers. At least it will help to make sure you are not violating the laws of thermodynamics. The calculator thingy takes a bit of time to learn, but once you get the hang of it, it's really easy to examine a proposed system. I liked it so much I splashed out on the "PRO" version.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #14 on: 26/08/2009 11:01:40 »
The thermofluids.net calculator should help you find a lot of the answers.

I have tried using their site. I went to the 'gas dynamics' tab, but found it confusing. I entered temp & pressure and some numbers came up.

Can I use the table calc changes in energy density?
Is this the table you used to get your earlier figure of 458kJ/Kg - if so how?

h1 shows Air @ 20MPa & 300°K has 1.86kJ/Kg?

I'd appreciate some pointers.
Thanks in advance.
« Last Edit: 26/08/2009 11:16:02 by peppercorn »
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #15 on: 26/08/2009 18:31:53 »
I used one of the daemons. I'll try to reconstruct the data and send it to you so you can plug it in and see what happens. Stay tuned.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #16 on: 04/09/2009 12:41:28 »
Here is an article explaining the efficiency limits of air-powered cars:

Thermodynamic Analysis of Compressed Air Vehicle Propulsion

The figures are most relevant to simple air-only designs (non-hybrid) with ambient reheating.

An IC engine hybrid design should be able substantially boost the power (~12MJ from a 300 litre tank) by employing waste exhaust heat to increase work available to much nearer the theoretical maximum (51MJ).

There is still also the question of how to utilise heat produced from the regenerative braking system.
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #17 on: 10/09/2009 01:06:23 »
Nice paper Peppercorn. Are there any more like it?

I'll try to run the numbers through the tool and see if they agree.
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #18 on: 10/09/2009 02:22:16 »
Peppercorn - The numbers in the paper seem to be accurate.

One thing I don't think it mentions is that 300 liters of air at 300 bar weighs 107 kg.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #19 on: 10/09/2009 12:16:45 »
Peppercorn - The numbers in the paper seem to be accurate.

One thing I don't think it mentions is that 300 liters of air at 300 bar weighs 107 kg.


A fair point!
Although how much does the equivalent Desiel engine plus a full tank of fuel weigh?
 

Offline syhprum

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #20 on: 10/09/2009 13:10:13 »
At one time compressed engines were used by navies to power torpedoes but they have now been superceded by electric motors or Hydrogen peroxide engines.
The military use highly compressed helium as a energy storage medium but it is a very expensive way to do it
 

Offline Geezer

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #21 on: 10/09/2009 16:30:40 »
Peppercorn - The numbers in the paper seem to be accurate.

One thing I don't think it mentions is that 300 liters of air at 300 bar weighs 107 kg.


A fair point!
Although how much does the equivalent Desiel engine plus a full tank of fuel weigh?

Well, the diesel fuel and tank will weigh an awful lot less. That amount of air only produces about 15 kW.H. Without doing the math, that's probably equivalent to about 10 L of diesel.

An equivalent diesel engine would not really be all that heavy if it only produces (say) 25 kW. It likely will only need three cylinders. Also, the expansion engine(s) that produces the power might be lighter than the diesel engine, but it's still going to weigh quite a bit. Then you have to allow for heat exchangers etc. And, the weight of the air tank is going to be considerable if it's going to contain that volume of air at 4300 PSI.

I'm not saying it's impossible to make it work, it's just that there are a lot of things to factor into the equations that tend make it less attractive the more you look at it.
 

Offline peppercorn

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #22 on: 22/09/2009 23:11:53 »
Found this on green-autoblog comments page for now Saab 9-x air concept car. Have just scanned through it, but seems to make some good points:

"Accumulators are used differently in a car than batteries. They are significantly larger in size and weight, but can be charged and discharged much faster - like (electric) Ultra-capacitors. The fast charging means, the entire braking energy can be absorbed (pumped back) into the accumulator when braking from e.g. 60mph to a stop. For the purpose of reaching highest efficiency, the hydraulic motor(s) should have high peak power (e.g. 800 hp for 3 seconds) to absorb all the braking energy. This concept also simplifies the drive train since no friction brakes are required.

When driving at 35 mph (constant speed) a medium size car needs only about 6-8 hp (1% of the installed power). The mileage of a hydraulic hybrid will drop under these conditions because of the decreasing efficiency of the hydraulic motors. The variable displacement of the motors - one of its strength - allows to bridge most of the span, but 1% of maximum power is still low for good efficiency.

The efficiency of a good insulated accumulator is 95% to 97%, about the same as a batterie (without the loss of the power electronics).

In a hydraulic hybrid, the combustion engine and car can operate independent from each other. The engine needs only to pump the average consumed energy into the accumulator and can be relatively small (30-40 hp). When charging, the engine can run at its best point of efficiency - high power and low speed (e.g. 85% hp, 2000 rpm). It would be designed for these conditions only and becomes simple and efficient. The fuel consumption can be half of that when compared with a conventional engine of a car operating under the same conditions. (EPA Cycle) The engine is turned off when the accumulator is charged.

The energy stored in the accumulaltor is practically twice as much as absorbed during braking. This is sufficient to drive about two to three miles, but is only a fraction of a slow charging battery needed in an electrically driven car (40-100 miles). Therefore, size and weight of the accumulator would be about the same or less. The intensity of energy supply from the accumulator and the high power of the hydraulic motors result automatically in excellent driving dynamics.

Hydraulic drives have typically gears to reduce the rpm. Generally, the gears improve the efficiency, weight and cost, and are not shiftable.

Hydraulic hybrids can adopt to all driving conditions very well, using only a small, but very fast energy storage device. A large, but slow reacting device (battery) would not be able to provide comparable benefits. All other hydraulic drive train components are noticeably smaller and lighter. The combustion engine would weigh about half and the hydraulic motors less than 1/10 of electric motors.

The overall savings in weight and size can be significant, so are the savings through the recuperation of the entire braking energy - about 1/3 in city driving and nearly 20% overall. The reduction in fuel consumption of the engine are state of the art, but can't be utilized because of the limitation of current drivetrains.

Together, the improvements result in a significant increase in mileage and reduction in emissions. 100 mpg for a conventional car with a hydraulic system are realistic."
 

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Re: What are the pros and cons of hydraulic hybrid vehicles?
« Reply #22 on: 22/09/2009 23:11:53 »

 

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