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Peugeot Air Hybrid


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#1 blkirk

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Posted 23 January 2013 - 14:11

So, Peugeot claims to have invented a car that can be run on compressed air. They claim an 80% compressed air duty cycle in urban driving. The announcement is very short on technical details, but I find that claim highly suspect. I haven't whipped out my slide rule yet, but the energy balance for running on petrol 20% of the time and compressed air from regenerative braking 80% of them time just doesn't pass the smell test. It seems more plausible to run the engine at high load to generate compressed air instead of idling at stop lights to reach the 80/20 split, but we all know that's a great way to kill tank-to-wheel efficiency. I just don't see this working the way they say it does.

http://www.industryw...r-hybrid-engine

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#2 Joe Bosworth

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Posted 23 January 2013 - 17:41

Think in terms of gross efficiency of using HC fuel to either generate electicity which is then used to run an electic motor versus driving an air pump to compress air including adiabatic heat and then using the compressed air to run an air motor.

An air pump/motor system is not as efficient as a generator/motor combination ignoring adiabatic heat loss.

Regen energy can be designed into both systems and the same heat and o/a efficiency characteristics remain.

It doesn't take long for the laws of thermodynamics to show that the electric solution wins.

The only other factor to take into consideration is weight of gen/motor/battery versus pump/store/airmotor might be a little bit lighter and more space considerate but I haven't run a slide rule over this yet. There won't be much difference between the two systems in this regard.

Regards



#3 Greg Locock

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Posted 23 January 2013 - 22:43

Check out the long and horrible story of MDI's Air Car (also a French firm). The wiki talk pages include much amusement. The main problem if you don't use fuel is that the expansion system gets very cold, which need large heat exchangers to resolve, and the energy storage capacity is limited. If you do use fuel then efficiency bites you.

Against that, it is a nice low tech solution that should be robust if done properly. Nice to see Edwardian technology applied to cars in the 21st century. Again.



#4 gruntguru

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Posted 24 January 2013 - 06:06

The compressed air cycle can be used to convert waste heat to work at much higher efficiencies than is possible with a heat-engine cycle alone. Although not mentioned in the article perhaps Peugeot are utilising waste heat from the water jacket and exhaust to augment the air-motor work.

From Wikipedia "A 2009 University of Berkeley Research Letter found that "Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix." However, they also suggested, "a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles."

#5 Greg Locock

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Posted 24 January 2013 - 09:51

The compressed air cycle can be used to convert waste heat to work at much higher efficiencies than is possible with a heat-engine cycle alone. Although not mentioned in the article perhaps Peugeot are utilising waste heat from the water jacket and exhaust to augment the air-motor work.

From Wikipedia "A 2009 University of Berkeley Research Letter found that "Even under highly optimistic assumptions the compressed-air car is significantly less efficient than a battery electric vehicle and produces more greenhouse gas emissions than a conventional gas-powered car with a coal intensive power mix." However, they also suggested, "a pneumatic–combustion hybrid is technologically feasible, inexpensive and could eventually compete with hybrid electric vehicles."


Reject temp from an engine say 103 deg C. reject temp to atmosphere is 80 deg C, through a feasibly sized heat exchanger. Carnot will do the maths for you. Yes it is free energy, but as Hannibal Lecter says some mornings it isn't worth biting through the straps.

Edited by Greg Locock, 24 January 2013 - 09:53.


#6 gruntguru

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Posted 24 January 2013 - 22:39

Waste heat in the exhaust is a lot hotter than that say 600 C.

Air motors fed on compressed air at ambient temp have exhaust temps well below ambient. Heat can even be collected from the surroundings to re-heat between expansion stages and augment the efficiency. This is not "free" energy - it is eqivalent to heat that was lost to the atmosphere when the air was compressed.

Point being - the temperatures used for a Carnot calculation are not 103 and ambient.



#7 Greg Locock

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Posted 25 January 2013 - 00:41

Waste heat in the exhaust is a lot hotter than that say 600 C.

Air motors fed on compressed air at ambient temp have exhaust temps well below ambient. Heat can even be collected from the surroundings to re-heat between expansion stages and augment the efficiency. This is not "free" energy - it is eqivalent to heat that was lost to the atmosphere when the air was compressed.

Point being - the temperatures used for a Carnot calculation are not 103 and ambient.

Good points.

#8 kikiturbo2

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Posted 25 January 2013 - 22:46

here is the direct link...

http://www.psa-peuge...-system-article

with some more info.... It looks as a cheaper solution to the classic electric hybrid, and with the issue of battery recycling, PSA might even advertise it as a more eco friendly solution..

I think that it should be significantly lighter..

#9 gruntguru

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Posted 26 January 2013 - 02:04

Found this in a DOWNLOAD at the site linked by kiki. Staggering if accurate - looks like a 30 - 50% price increase over gasoline for a 50% fuel and CO2 saving.


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#10 gruntguru

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Posted 26 January 2013 - 03:15

No sign of wast heat recovery in any of the PSA material.

Wondering about the claimed fuel consumption advantage over electric-hybrid given the lower charge/recharge efficiency of the pneumatic cycle?

If the compressed air storage is short-term only, the cylinder might be storing hot air, improving efficiency somewhat?

Vehicle mass would be lower.

Regen braking could absorb energy at higher rates than a "battery-charging" regen system.

#11 kikiturbo2

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Posted 26 January 2013 - 17:16

Gruntguru.. Peugeot's own diesel hybrid is about 30% more expensive (real street price) than it's pure diesel equivalent (both at the same level of equippment, and diesel being an auto, hybrid being an robotized manual.)

however, hybrid will officially do 104 g/km CO2, automatic diesel 172, and manual diesel 145...

#12 manolis

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Posted 27 January 2013 - 06:18

No sign of wast heat recovery in any of the PSA material.
Wondering about the claimed fuel consumption advantage over electric-hybrid given the lower charge/recharge efficiency of the pneumatic cycle?
If the compressed air storage is short-term only, the cylinder might be storing hot air, improving efficiency somewhat?
Vehicle mass would be lower.
Regen braking could absorb energy at higher rates than a "battery-charging" regen system.


Hello.
Quote from Peugeot-Citroen official site:

Posted Image

the top figure is the most explanatory (the Low pressure storage is the blue tank, as the modified figure below shows):

Posted Image

Here is the hydraulic hybrid (or hydrid) of EPA-Chrysler (a few years ago):

Posted Image

They look quite similar.

If they are, then the air (or the Nitrogen) is used only as a "high capacity" spring.
The system is based on hydraulic oil and on hydraulic pumps / rotors.
The engine operates near the optimum load, with the surplus of energy consumed to pass oil from the Low pressure storage (Low Pressure Reservoir) to the Energy storage system (High pressure Accumulator).
With the oil that enters into the high pressure tank, the air (or Nitrogen) in the high pressure tank is further compressed storing energy for later use.

The pressure of the air (or nitrogen) in the high-pressure tank is necessarily high (some 300 bar) in order to store significant amounts of energy. This makes non-practical the direct compression of the air (or nitrogen) by a piston - rings - cylinder (think that during the combustion the pressure inside the cylinder is half). In comparison, the typical hydraulic pumps operate at, or above, 300 bars.
In the drawings of Peugeot there is no air-compressor.
The term "Hybrid-Air" of Peugeot is confusing.

Besides being cheaper and simpler and lighter and non-high-tech, the hydraulic hybrid (or hydrid) has, according EPA etc, another significant advantage over the electric hybrids: it can regenerate some 75% of the braking energy, while the electric hybrids can regenerate only 25%. If this is true, it justifies the high overall efficiency of the Peugeot-Hybrid-Air in the urban cycle.

By the way, the UPS made and tested - in real life - hydrid trucks; there are hydrid bicycles, too.

Thanks
Manolis Pattakos

#13 kikiturbo2

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Posted 27 January 2013 - 15:19

great explanation.. I wondered why peugeot mentioned hydraulic engines... thought it was some kind of translation mistake..

#14 bigleagueslider

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Posted 28 January 2013 - 03:15

The Peugeot hybrid system is not actually a "pneumatic" system. It is a hydraulic system that uses a compressed-gas accumulator.

#15 gruntguru

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Posted 28 January 2013 - 10:03

A quick calculation on the accumulator energy assuming 100 bar empty, 300 bar full and delta volume of 100 litres, gives energy storage of 2 MJ. Enough to supply 20 kW of power for 100 seconds.

#16 blkirk

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Posted 28 January 2013 - 17:45

That makes much more sense. So grunt, going with your numbers, if 20kW is all we need around town, then Peugeot are basically claiming that the car can run for about 2 minutes on the accumulated pressure. Then the IC engine will start up and recharge the system (while it drives the car) for 25 seconds. That might be a little annoying to live with, but people seem to be able to adapt to idle-stop engines.

Also, at least in electric hybrid cars, you can make the A/C electric. Does this thing come with electric A/C and a slightly larger battery to power it? Or has Peugeot come up with hydraulic powered A/C?

#17 manolis

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Posted 29 January 2013 - 05:23

That makes much more sense. So grunt, going with your numbers, if 20kW is all we need around town, then Peugeot are basically claiming that the car can run for about 2 minutes on the accumulated pressure. Then the IC engine will start up and recharge the system (while it drives the car) for 25 seconds. That might be a little annoying to live with, but people seem to be able to adapt to idle-stop engines.
Also, at least in electric hybrid cars, you can make the A/C electric. Does this thing come with electric A/C and a slightly larger battery to power it? Or has Peugeot come up with hydraulic powered A/C?


Blkirk, your 2 minutes estimation is pessimistic.
I think that 10 minutes, or 20 minutes, is closer to reality.

A modern medium size car needs some 18bhp (13.3 Kw) to keep a constant speed of 100 Km/h (flat road, no wind).
It is easy to calculate: with a fuel consumption, at 100 Km/h, of 4.5 lit/100 Km (3.3 Kg of gasoline) and a BSFC of 250 gr/KWh you consume 18bhp of power to cover the overall resistance / friction.

With 20Kw (27bhp) of power the car maintains a speed more than 120 Km/h on a flat open road. In this speed the overall resistance to the car motion is significantly increased (aerodynamic resistance). I.e. at 120 Km/h the energy consumed per Km covered is substantially increased.
With 120 Km/h in 100 seconds the car covers a distance of 3.3 Km.

The same car in the urban cycle has substantially less resistance (energy consumed per Km covered), but:

it has to accelerate and decelerate frequently,
the engine operates, most of the time, at light load (which means lower thermal efficiency),
during the frequent decelerations (braking from traffic light to traffic light), all the kinetic energy of the car is lost to warm the brakes and then the surrounding air.

Let’s suppose that the overall “resistance” (energy consumed per Km covered) in the town traffic (urban cycle) is as much as at 120 Km/h.
This means that, in the urban cycle, the car covers 3.3 Km with the 2 MJ of energy accumulated into the high-pressure tank.
With a mean speed of 20 Km/h the car operates without the engine for 10 minutes. Then the engine starts again to “recharge” the accumulator.

Now take into account the recovery of the braking energy. Suppose that from the energy provided to the car by the accumulator, the one third (33%) is consumed to cover the overall resistance (rolling resistance, aerodynamic resistance, transmission resistance) and the rest two thirds (66%) is consumed in the brakes.
With 75% of the braking energy recovered into the accumulator (as EPA etc claim), at the end of the 3.3 Km the car has consumed not the entire energy (2MJ) but only half (0.75*0.66=0.5, i.e. 50%).
This means that the car can make another 3.3 Km (i.e. it can move for another 10 minutes) before the accumulator is empty and the engine starts to “recharge it”.

If the 20 Km/h mean speed seems low, you can double it. Then the car operates with the accumulator for 10 minutes and covers the same 6.6 Km before “recharging” the accumulator.



A disadvantage of the "architecture" of the hydraulic hybrid of Peugeot (and of the similar hydrids) is the fact that they continue to use a conventional engine, a conventional gearbox and a conventional transmission (power shafts, differential etc).
Please take a look at the:

Posted Image

Posted Image

(the coontrollable color windows exe animation is at http://www.pattakon.com/pre/PRE19.exe )

The engine (a modified OPRE engine http://www.pattakon....attakonOPRE.htm ) integrates the hydraulic pump: the combustion pressure directly passes most of the energy to the oil (no need to transform the linear motion first to rotary motion). The engine operates - more or less - as a controlled free-piston engine. The crankshafts run - more or less - unloaded.

With a hydraulic motor/pump in the center of each driving wheel, the car needs not a transmission box, nor differential, nor power shaft etc.
The engine can be mounted anywhere in the car (even under the co-passenger seat).
The fact that the engine is disconnected from the driving wheels seems as a drawback (the energy passes necessarily through the accumulator). Considering the friction inside the conventional transmission, this is not a drawback any longer.



In places wherein cheap renewable energy is available, the "hydrids" can, optionally, operate without an IC engine (as the pure electric cars): The energy capacity of the accumulator increases. In the "gas station" (or in the garage of the owner) an electric motor drives the hydraulic pump of the "hydrid" and "recharges" - in a couple of minutes - the accumulator.



Gruntguru,
my thread about the Wankel is wrong.
Keeping the capacity unchanged, a decrease of the the breadth of the rotor by 2% causes a decrease of the surface to volume ratio by 1%.
I think you can explain it to the rest members.

Thanks
Manolis Pattakos

#18 ray b

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Posted 29 January 2013 - 06:10

why crank shafts at all
why not use hyd pressure to return the pistons
and do away with a lot of monkey motion

#19 ray b

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Posted 29 January 2013 - 06:18

my dad built a hyd drive car in the 40's
he worked on the B-29 powered nose wheel with bill stout [scarab and flying car guy]
so had access to surplus bits
he built the car himself then worked for ford to develop it a few years

anyway the car had wheel motors no brakes just regen back to a tank
air compressed in the oversize tank was the storage for the hyd fluid
to bad his pattens have long expired



Posted Image

Posted Image

Edited by ray b, 29 January 2013 - 06:45.


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#20 manolis

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Posted 30 January 2013 - 04:45

why crank shafts at all
and do away with a lot of monkey motion
my dad built a hyd drive car in the 40's


Because the crankshafts control the motion of the pistons in a simple and efficient way.


why not use hyd pressure to return the pistons
. . .
to bad his pattens have long expired


However, the hydraulic pressure is used to "return" the pistons.
Look at the animation: the low pressure (say at 75 bars) accumulator provides "automatically" oil - through the top one-way valves - to the external small-diameter hydraulic pistons; this way the two opposed pistons are pushed by the hydraulic oil and compress the "fresh" air inside the cylinder (compression stroke).
The crankshafts are, mostly, for the synchronization of the pistons (they handle only a part of the energy).

I would like to read the patents of your father. If you don't mind, please let me know either the patent numbers or his name.


PS. The crankshafts of the OPRE_Hydrid gif animation of my last post were "rotating" the wrong "direction". The gif animation is now corrected and runs as the original exe animation.

Thanks
Manolis Pattakos

#21 manolis

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Posted 31 January 2013 - 03:49

my dad built a hyd drive car in the 40's
he worked on the B-29 powered nose wheel with bill stout [scarab and flying car guy]
so had access to surplus bits
he built the car himself then worked for ford to develop it a few years

anyway the car had wheel motors no brakes just regen back to a tank
air compressed in the oversize tank was the storage for the hyd fluid
to bad his pattens have long expired


Ray b,

Your father's work / dreams / projects / achievements are amazing.
He was far ahead of his era.

You should gather every piece of information (and, it seems, there is a lot of it) in a web site dedicated in his memory.

Thanks
Manolis Pattakos

#22 bigleagueslider

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Posted 01 February 2013 - 04:06

The reason Peugeot chose this hydraulic hybrid system is because it can recover far more braking energy than any battery-electric system, it is cheaper, safer and more reliable than a comparable battery-electric system, and it does not require any scarce resources like rare-earths or lithium.

The horsepower of a hydraulic system is equal to efficiency x psi x gpm/1714.

#23 mariner

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Posted 02 February 2013 - 16:38

Its a very simple set up vs the Peugeot thing but way, way back GM lashed up an atr storage boost system for extra power.

It simply used a belt driven air pump and a storage tank to hold high pressure air.

At kickdown the air fom the tank was injected into a venturi near the carb and the venturi effect dragged in extra air for overtaking power.

Similar in its boost physics to the injectors used on steam egines to get fresh water into a boiler under pressure.


I don't think GM ever did much with it - the boring bar was cheap in those days

#24 J. Edlund

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Posted 02 February 2013 - 23:59

That makes much more sense. So grunt, going with your numbers, if 20kW is all we need around town, then Peugeot are basically claiming that the car can run for about 2 minutes on the accumulated pressure. Then the IC engine will start up and recharge the system (while it drives the car) for 25 seconds. That might be a little annoying to live with, but people seem to be able to adapt to idle-stop engines.

Also, at least in electric hybrid cars, you can make the A/C electric. Does this thing come with electric A/C and a slightly larger battery to power it? Or has Peugeot come up with hydraulic powered A/C?


No, the IC engine doesn't recharge the system, doing so would not be very efficient since the engine would have to produce more than 1 kWh to get 1 kWh back from the energy storage system. Instead, kinetic energy is used to charge up the system during braking just like any other hybrid. 2 MJ is by the way roughly the usable energy available in the Toyota Prius battery (which is about 5 MJ total, but cannot be fully used).

#25 manolis

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Posted 03 February 2013 - 07:07

No, the IC engine doesn't recharge the system, doing so would not be very efficient since the engine would have to produce more than 1 kWh to get 1 kWh back from the energy storage system. Instead, kinetic energy is used to charge up the system during braking just like any other hybrid. 2 MJ is by the way roughly the usable energy available in the Toyota Prius battery (which is about 5 MJ total, but cannot be fully used).


If the BSFC (brake specific fuel consumption: Kg of fuel consumed per KWh of mechanical energy provided) of the IC engine is constant along the usable revs (r.p.m.) and load range, then the hybrid / hydrid technology (excluding the regeneration of the braking energy) would be a waste of fuel and nothing more.

But the BSFC of the IC engines is highly variable along the usable rev and load range; it can easily fall bellow half of the peak when the engine operates outside a shot region of r.p.m. - loads.
The engine of the Prius has a peak BTE (the brake thermal efficiency is the part of the thermal energy of the fuel finally transformed into mechanical energy by the engine) of some 38%.
Outside a region of revs / loads the BTE of the Prius engine falls below 20%, even below 10%. It is because of the various losses (increased friction, increased pumping loss, increased thermal loss during combustion - expansion etc).
For instance, if you operate an engine at 20% load (almost closed throttle) and 3000 rpm, the BTE falls easily below 15%.

Instead of operating the engine at light load, the hybrid system of Prius keeps the engine at the optimum load (and so at high BTE) and stores the surplus energy into the batteries for later use.
The surplus mechanical energy is transformed initially into electric energy by an electric generator, then it is transformed into chemical energy into the batteries. Later the chemical energy is transfored back into electric energy and is provided to an electric motor that drives the car.
Every one of these energy transformations adds significant losses (especially the battery charge and recharge). But at the end, the Prius does achieve a significant reduction of the fuel consumption in the urban cycle as compared to a similar size conventinal car.

Even if only the 60% of the "surplus energy" is finally regenerated, the result is still good because the "surplus energy" was generated at high BTE.
For instance, if the 40% of the energy of the engine (operating at 38% BTE) is used directly to move the vehicle and the rest 60% is stored into the bateries, and if 40% of this 60% is lost in the various losses (electric motor, batteries, electric generator) with the rest 60% used later for the car motion, the "overall" BTE is (0.4+0.6*0.6)*38%=0.76*38%=28.8% which is still very good as compared to the "mean BTE" of the conventional cars.

Similarly, the hydrid (hydraulic hybrids) vehicles keep the engine running near the optimum BTE point and store the surplus energy into the high pressure accumulator for later use. And it seems the "charge" - "recharge" of a high pressure accumulator is more efficient than charging - recharging batteries.

The hybrid / hydrid technology is based on the weakness of the current engines to maintain a good BTE along a wide rev - load range.
With the Variable Valve Actuation systems, with the high compression ratio, with the Miller / overexpansion cycle etc things are substantially better than in the past, yet they are still not good enough to make the hybrid / hydrid tech useless.

Suppose an engine can keep its BTE from heavy to light loads and from medium to low revs. For a vehicle having such an engine the hybrid (or hydrid or ...) system adds nothing but losses (excluding the braking energy regeneration).


In order to keep high BTE at a wide rev - load range, an IC engine has to minimize the operating temperatures inside the combustion chamber (i.e. to minimize the themal loss), to minimize the pumping loss, to minimize the mechanical loss and to operate at high expansion ratios.

Take a look at the PatAir at http://www.pattakon....ttakonHydro.htm , an evolution of the MultiAir / TwinAir / UniAir of FIAT / Schaeffler-INA.

Posted Image

It does exactly the previous: substantially cooler cycle, no pumping loss, lower friction and high expansion ratio (and these under a true / easy / flexible digital control).

When combined with a simple / robust Variable Compression Ratio system like the Pat-Head VCR at http://www.pattakon....pattakonVCR.htm

Posted Image

besides the top (and "constant in a wide range") BTE, it can also offer top power desnity (heavily supercharged when necessary).

With the PatAir the hybrid technology is not necessary any longer (excluding the regeneration of the braking energy).
Is it?


Thanks
Manolis Pattakos

#26 Superbar

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Posted 03 February 2013 - 09:19

Would it make sense to add a system such as this to a BEV or a plug in hybrid (such as the Volt)?

#27 gruntguru

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Posted 04 February 2013 - 04:19

It wouid be marginal at best. Both systems already harvest a portion of braking energy and the only benefit of the air/hydraulic system would be to increase the % harvested. On the downside you would have a substantial weight and cost penalty.

#28 bigleagueslider

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Posted 06 February 2013 - 03:29

It wouid be marginal at best. Both systems already harvest a portion of braking energy and the only benefit of the air/hydraulic system would be to increase the % harvested. On the downside you would have a substantial weight and cost penalty.


gruntguru-

If I read your comment correctly, are you saying that a battery-electric system is lighter and lower cost than the hydraulic system PSA is proposing? In the article, PSA claims the opposite. PSA feels that their hydraulic hybrid gives a much better combination of cost, reliability, weight, and overall fuel efficiency.


#29 gruntguru

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Posted 06 February 2013 - 04:53

No.

Superbar was questioning the benefit of ADDING the pneumatic sytem to a Volt or BEV.

Edited by gruntguru, 06 February 2013 - 04:54.


#30 Superbar

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Posted 06 February 2013 - 10:39

No.

Superbar was questioning the benefit of ADDING the pneumatic sytem to a Volt or BEV.


I was thinking that maybe you could could find an economical benefit by reduced need for a big battery and thus bringing the price of the vehicle down. I am no engineer so I would not have any clue of the finer points. How compact you could make such a pneumatic system and how much adding it would let you downsize the drive systems in a BEV or plugin-hybrid. You are probably correct, I suspected as much myself. Thanks for the reply.

#31 Tony Matthews

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Posted 06 February 2013 - 11:25

No one seems concerned about the risk of the bends if a door is opened suddenly.

#32 gruntguru

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Posted 07 February 2013 - 02:02

I was thinking that maybe you could could find an economical benefit by reduced need for a big battery and thus bringing the price of the vehicle down. I am no engineer so I would not have any clue of the finer points. How compact you could make such a pneumatic system and how much adding it would let you downsize the drive systems in a BEV or plugin-hybrid. You are probably correct, I suspected as much myself. Thanks for the reply.

Good point. There have been numerous systems using two different storage media to combine the benefits of short-term-high-power storage/retrieval and long-term-low-power in the one vehicle. Way back when I was an undergrad, our Mech Eng dept built a hybrid electric car combining lead-acid battery and flywheel storage. Capacitors are used similarly for their high charge/discharge rates although the motor/generator size still dictates the upper limit. The pneumatic system offers fairly high rate and capacity for its mass and price but the totals for cost, mass and volume probably get out of hand once you combine pneumatic, electric and possibly ICE in the one vehicle.

#33 gruntguru

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Posted 07 February 2013 - 02:03

No one seems concerned about the risk of the bends if a door is opened suddenly.

:lol: :lol: :lol:
Only you Tony!

#34 J. Edlund

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Posted 07 February 2013 - 18:35

If the BSFC (brake specific fuel consumption: Kg of fuel consumed per KWh of mechanical energy provided) of the IC engine is constant along the usable revs (r.p.m.) and load range, then the hybrid / hydrid technology (excluding the regeneration of the braking energy) would be a waste of fuel and nothing more.

But the BSFC of the IC engines is highly variable along the usable rev and load range; it can easily fall bellow half of the peak when the engine operates outside a shot region of r.p.m. - loads.
The engine of the Prius has a peak BTE (the brake thermal efficiency is the part of the thermal energy of the fuel finally transformed into mechanical energy by the engine) of some 38%.
Outside a region of revs / loads the BTE of the Prius engine falls below 20%, even below 10%. It is because of the various losses (increased friction, increased pumping loss, increased thermal loss during combustion - expansion etc).
For instance, if you operate an engine at 20% load (almost closed throttle) and 3000 rpm, the BTE falls easily below 15%.

Instead of operating the engine at light load, the hybrid system of Prius keeps the engine at the optimum load (and so at high BTE) and stores the surplus energy into the batteries for later use.
The surplus mechanical energy is transformed initially into electric energy by an electric generator, then it is transformed into chemical energy into the batteries. Later the chemical energy is transfored back into electric energy and is provided to an electric motor that drives the car.
Every one of these energy transformations adds significant losses (especially the battery charge and recharge). But at the end, the Prius does achieve a significant reduction of the fuel consumption in the urban cycle as compared to a similar size conventinal car.

Even if only the 60% of the "surplus energy" is finally regenerated, the result is still good because the "surplus energy" was generated at high BTE.
For instance, if the 40% of the energy of the engine (operating at 38% BTE) is used directly to move the vehicle and the rest 60% is stored into the bateries, and if 40% of this 60% is lost in the various losses (electric motor, batteries, electric generator) with the rest 60% used later for the car motion, the "overall" BTE is (0.4+0.6*0.6)*38%=0.76*38%=28.8% which is still very good as compared to the "mean BTE" of the conventional cars.

Similarly, the hydrid (hydraulic hybrids) vehicles keep the engine running near the optimum BTE point and store the surplus energy into the high pressure accumulator for later use. And it seems the "charge" - "recharge" of a high pressure accumulator is more efficient than charging - recharging batteries.

The hybrid / hydrid technology is based on the weakness of the current engines to maintain a good BTE along a wide rev - load range.
With the Variable Valve Actuation systems, with the high compression ratio, with the Miller / overexpansion cycle etc things are substantially better than in the past, yet they are still not good enough to make the hybrid / hydrid tech useless.

Suppose an engine can keep its BTE from heavy to light loads and from medium to low revs. For a vehicle having such an engine the hybrid (or hydrid or ...) system adds nothing but losses (excluding the braking energy regeneration).


Thanks
Manolis Pattakos



Posted Image


No, hybrids such as Prius doesn't store excess energy from the engine in the battery in any significant amount as doing so would not be very efficient. The battery is also way too small for that, with a usable capacity of about 600 Wh. The battery is kept in a state of charge between 40 and 80%, with the control system trying to keep it at about 55% by controling engine and motor usage. At 40% the battery isn't used to drive the electric motor and at 80% it tries to discharge the battery by using the electric motor more.

I've posted the BSFC map from the Toyota Prius engine above, and the red line shows how the engine is used in the car.

If you look at the BSFC map you will notice a few things. For starters the engine is never used at a 20% load at 3000 rpm, if it were it would be about 21% efficient. Secondly, the impact of engine speed on engine efficiency is quite small within the rev range shown here. The impact of engine load on efficiency can be quite significant, but only at really low loads and these load points are generally avoided.

If we do a few quick calculations along the red "basic operating line" we will find that the engine produces about 2.5 kW at 20 Nm/1200 rpm with an efficiency of about 21% (this is just about 1.3 liters per hour). This increases to an efficiency of 28% at 5 kW and 34% at 9.5 kW, all at the same engine speed. Beyond 9.5 kW the engine speed will start to climb and engine efficiency will be in the 34-36% range. The maximum output on the red line is about 42.5 kW. So depending on how much power the car needs at any given time, the power will be adjusted following the "basic operating line". If the power need is lower the engine speed is reduced, it that is not enough the load is lowered and finally the engine can be shut down.

If the battery state of charge is low there will likely be some charging from the engine, but this is generally avoided.

#35 J. Edlund

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Posted 07 February 2013 - 18:56

gruntguru-

If I read your comment correctly, are you saying that a battery-electric system is lighter and lower cost than the hydraulic system PSA is proposing? In the article, PSA claims the opposite. PSA feels that their hydraulic hybrid gives a much better combination of cost, reliability, weight, and overall fuel efficiency.


Batteries in hybrids tend to be more limited by maximum charge/discharge (for acceleration/deceleration), while battery electric vehicles tend to be limited by battery capacity (to give enough range). With this hydraulic system in a hybrid there will be no need for a powerful motor, power electronics or a high power battery, but the battery electric vehicle will still require a motor/electronics and a high energy battery and 2 MJ more energy in such a battery would probably add no more than $300 to the production cost. I doubt you can produce this system for less than that.

#36 bigleagueslider

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Posted 08 February 2013 - 06:26

Batteries in hybrids tend to be more limited by maximum charge/discharge (for acceleration/deceleration), while battery electric vehicles tend to be limited by battery capacity (to give enough range). With this hydraulic system in a hybrid there will be no need for a powerful motor, power electronics or a high power battery, but the battery electric vehicle will still require a motor/electronics and a high energy battery and 2 MJ more energy in such a battery would probably add no more than $300 to the production cost. I doubt you can produce this system for less than that.


With production automobiles, cost is king. What PSA claimed in the article is that they can get more bang-for-the-buck in the combined driving cycle from the hydraulic system than from the battery-electric system. This would seem to be true since the hydraulic system can recover more braking energy, would be more durable, and would cost less than a battery-electric system. The hydraulic system would also present less of a control problem to integrate with the IC drivetrain. Also, battery-electric hybrids require separate electrically-driven, and more costly, accessories such as A/C or P/S. While a hydraulic system would not.


#37 Greg Locock

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Posted 08 February 2013 - 23:53

There's an interesting study which i need to find again that looked at the effect of DECREASING battery capacity (due to old age) on the fuel economy of the Prius. It turns out that the effect is quite small, because the biggest bang for your buck is to store the effect of one braking event, and to feed that energy into the next acceleration. So long as the battery is big enough tostore that one braking event, and be able to run the stop start strategy, that's all you really need.

So, given that Toyota aren't entirely potty, why did they fit a bigger battery than they really needed? I suspect paranoia about battery life in particular. The larger the battery, the more likely it will survive 5 years or 8 years or whatever their target is.

Therefore I think a relatively small energy capacity, combined with a high power handling ability, could offer a sensible solution.

Electrically the same is done using supercaps to buffer a smaller battery. We first used supercaps for this on the solar car in 1996. However we tore the system out for the race, a bit of modelling and a bit of engineering judgement showed that the benefits were marginal if you only do 3 stops per day!


#38 gruntguru

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Posted 09 February 2013 - 09:38

So, given that Toyota aren't entirely potty, why did they fit a bigger battery than they really needed? I suspect paranoia about battery life in particular. The larger the battery, the more likely it will survive 5 years or 8 years or whatever their target is.

They must have done their sums, but logic suggests they would have had a more marketable package if the battery was smaller:>
- lower weight (better performance, economy)
- cheaper car
- cheaper replacement battery (less anxiety over battery life, easier for Toyota to guarantee the battery)

#39 Greg Locock

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Posted 09 February 2013 - 23:25

I'm sure they did, but the longevity of the battery is not simply that one half the size will wear out twiice as quickly, I suspect it is a more aggressive function. As such a small reduction in battery size will cause a disproportionate increase in warranty problems.

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#40 gruntguru

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Posted 09 February 2013 - 23:39

I'm sure they did, but the longevity of the battery is not simply that one half the size will wear out twiice as quickly, I suspect it is a more aggressive function. As such a small reduction in battery size will cause a disproportionate increase in warranty problems.

So - half size battery plus some supercaps?

#41 Greg Locock

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Posted 09 February 2013 - 23:59

So - half size battery plus some supercaps?

Exactly. Some mobile phones do that for the same reason

#42 bigleagueslider

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Posted 10 February 2013 - 04:20

As I understand things, using capacitor storage also presents some unique issues. I believe that the voltage of a capacitor changes as it is discharged, which means that power electronics are needed to compensate for this effect. And these power electronics must have the capacity to handle the large transient amounts of power being transmitted.

#43 kikiturbo2

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Posted 10 February 2013 - 18:54

So, given that Toyota aren't entirely potty, why did they fit a bigger battery than they really needed? I suspect paranoia about battery life in particular. The larger the battery, the more likely it will survive 5 years or 8 years or whatever their target is.


do not underestimate the marketing dep't having a say.... a hybrid that does "5 miles on pure electricity" is better PR than "enough electric charge for one acceleration" :)
also I believe in the other answer which is that the battery life is much longer when they are working within a relatively small percentage of full charge.. i.e. not draining them fully on every acceleration..

#44 manolis

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Posted 11 February 2013 - 04:44

No, hybrids such as Prius doesn't store excess energy from the engine in the battery in any significant amount as doing so would not be very efficient. The battery is also way too small for that, with a usable capacity of about 600 Wh. The battery is kept in a state of charge between 40 and 80%, with the control system trying to keep it at about 55% by controling engine and motor usage. At 40% the battery isn't used to drive the electric motor and at 80% it tries to discharge the battery by using the electric motor more.


The 600wh battery capacity may seem "way too small", but is it? Think that 600wh usable capacity means that:

Consuming 36 KW of power on the electric motor, the battery is discharged in 1 minute.
Consuming 18 KW of power on the electric motor, the battery is discharged in 2 minutes.
Consuming 9 KW (i.e. 12 bhp) of power on the electric motor, the battery is discharged in 4 minutes (with 30 Km/h in four minutes they are covered 2 Km).

600wh means (600J/sec)*3600sec=2.16MJ (close to the hydraulic hybrid high pressure accumulator capacity as calculated by gruntguru).

Quote from the interesting PDF “Case study: Toyota Hybrid Synergy Drive” at http://www.ae.pwr.wr..._HEV_Toyota.pdf

(Page 11, for the PriusII):
The rated energy storage capacity is 6,4 MJ, the usable energy storage capacity is 2,56MJ.
This is enough energy to accelerate the car, driver and passenger up to 105 Km/h (without help from the ICE) four times.
Alternatively, it is enough to raise the car through200 vertical meters.
. . .
Discharge power capacity of the Prius II pack is around 20 kW at 50% SOC, with regenerative capability of 14,5 kW at 2C. The power capability increases with higher temperatures and decreases at lower temperatures.
. . .
The weight of the complete battery pack of the Prius II is 53.3 Kg.


Here is – slightly modified – the torque vs rpm plot of Prius:

Posted Image

They have been added the blue curves (constant- power curves).
The plot starts at 0 rpm.

According Toyota, the engine is allowed to operate only along the red curve.
How is this possible?
There is neither a CVT, nor a manual gearbox.

There is a planetary system (patent of the Bulgarian Antonov who, by the way, sued Toyota for unauthorized use of his intellectual property) by means of which the three parts (ICE, electric motor, electric generator) are connected to each other.

In order to have the ICE operating at the minimum BSFC (230 gr/KWh), the ICE provides 25 bhp at minimum.
The engine runs at 2000 rpm and 85% load.
How can all this power be consumed in the urban cycle?
Here is where the battery comes to absorb the surplus of energy, for as long as it is not charged. For instance, from the 25 bhp generated by the ICE, the 19 charge the battery, the rest 6 are used to move the car.
When the battery is charged, there is no other way than to stop the ICE and start moving the car by the battery. The other option is to throttle the engine in order to provide only 6 bhp, which means a substantially lower BSFC (like 400 gr/KWH at 2200 rpm).

Just follow the 6 bhp blue curve. This is what the conventional cars are doing in the slow urban traffic. Even with the best mechanical gearbox (CVT or conventional) the engine is operating away from its best BTE region. A good (efficient and quick) energy accumulator (like the NiMH battery of Prius or like the high pressure accumulator of the Peugeot-Air) makes the difference because it allows the engine to operate at maximum BTE (brake thermal efficiency) storing the surplus energy for later use.

When the battery is discharged, the ICE starts again.

That is, the battery is vital for the motion of the Prius hybrid.
With the planetary mechanism of Antonov, the Prius is like having an “ideal” CVT that can keep the engine operating along the red line.

Despite the inevitable loss of energy during the transformation of the mechanical energy into electrical, then into chemical, then back into electrical energy and finally into kinetic/mechanical energy, the fact that the engine is allowed to operate (when it operates) at the minimum BSFC covers the losses and lowers the overall fuel consumption.


The data-log graph below (quote from http://privatenrg.com/ ) shows that the battery of the Prius almost never rests:

Posted Image

Nearly half of the time the car goes with the ICE not operating: the average discharge current is 110A and the average power provided by the battery is 24 kW.
For the rest of the time, the battery is charged absorbing energy from the ICE (the average charge current is about 30A) or absorbing the braking energy (the average current is 90A, the average power is 21 kW).

So, the "small" capacity NiMH battery of the Prius is the "pivot" and works "overtime". It is repeatadly charged and discharged.
With the planetary system of Antonov the battery with the two electric motors / generators acts as an efficient electric CVT.
The engine can operate at, or near, its optimum BSFC only because the battery can absorb the surplus energy provided by the engine. Then the IC engine is stopped and the car moves by the battery for several minutes (urban cycle).


Suppose the battery of the Prius is removed. Can the Prius move?

Thanks
Manolis Pattakos


#45 gruntguru

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Posted 11 February 2013 - 23:42

Here is – slightly modified – the torque vs rpm plot of Prius:

Posted Image

They have been added the blue curves (constant- power curves).

Interesting that Toyota should publish this map on a Torque axis. Power vs speed is far more useful - particularly in this application where a power requirement would be specified and the corresponding speed and throttle would be selected. Also interesting, the operating line is almost constant throttle (apart from the vertical, constant-speed section).

Suppose the battery of the Prius is removed. Can the Prius move?

Only if an alternative electrical load is provided - a power resistor or perhaps even a short-circuit (since minimum electric motor RPM are required - dissipate the heat in the motor windings.)

Edited by gruntguru, 11 February 2013 - 23:43.


#46 Greg Locock

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Posted 12 February 2013 - 00:34

I am a bit surprised Antonov bothered suing, the prior art for the Prius architecture is well known. 1970 I think it was demonstrated, with lead acids. Presumably Antonov made lots of money and is now happily retired. Oh no, sounds like the court agreed with me.

If there is no battery then MG1 is free to rotate and will just be spun up by the engine. If you locked MG1 mechanically then the engine would stall on takeoff, and anyway could not be started.

THS is actually a good use of a differential as a mechanical mixer, and I'm puzzled why Manolis thinks it doesn't work as a CVT.