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Heavy-Duty Air-Hybrid Engine Prototype Shows Up to 18% Improvement in Fuel Economy, Depending On Drive Cycle

Initial simulation studies of a prototype heavy-duty air-hybrid engine being developed by engineers at Volvo Powertrain, UCLA and Sturman Industries show a 4-18% improvement in fuel economy over a wide range of driving cycles. The amount of improvement depends on the cycle.

The addition of a low-temperature EGR circuit could boost that improvement by another three to four percentage points, according to the research team. The goal of the project, co-funded by the DOE, is to develop and demonstrate a heavy duty air-power-assist (APA) engine for a refuse vehicle with a 15% improvement in fuel efficiency and with emissions that meet the 2010 regulations. (Earlier post.)

Hyungsuk Kang from Volvo Powertrain presented the current results of this APA project at the DEER (Diesel Engine-Efficiency and Emissions Research) conference in Detroit.

The APA engine uses braking energy to work as a compressor, pumping compressed air into an on-board air tank (Air Compressor mode, or AC mode). During acceleration, the stored compressed air powers the engine—with or without burning diesel fuel—to get up to speed, or until the compressed air is depleted (Air Motor mode, or AM mode). Once the vehicle is moving along the engine converts back to a conventional diesel engine.

During engine acceleration, the high boost pressure also helps reduce the emissions of particulate matter. In addition, smaller quantities of fuel burned in each cylinder lead to lower peak temperatures, resulting in lower NOx formation. The APA technology also facilitates greater use of the engine brake, which in turn helps save fuel and reduces the frequency of brake service with its associated costs.

The APA engine is based on an 11-liter in-line six-cylinder diesel engine modified with a hydraulic valve actuator system from Sturman, an Amplifier Piston Common Rail System (APCRS) from Bosch, and a control system developed by UCLA. Compression ratio for the engine is 16:1. (Tsu-Chin Tsao, professor of mechanical and aerospace engineering at the UCLA Henry Samueli School of Engineering and Applied Science and his students are also working with Ford on a prototype light-duty air-hybrid system based on a 2.5-liter V-6.)

The variable valve actuator and the mode-switching controls are key enablers for the air-hybrid—in AC mode, the engine switches to a two-stroke mode, according to Kang. Other critical elements are the engine valve stem seals and external switching valves.

The air tank is designed to operate at 20 bar maximum pressure. Currently, in the experimental work, the system works at just 13 bar. The air tank also has a relief valve that can trigger at 15 bar.

Comments

HealthyBreeze

This has some things in common with the hydristor hybrid that's been in development for awhile. I wonder how that development is going?

BlackSun

The beauty is in the simplicity. The downside is the efficiency improvement is incremental, not dramatic.

Rafael Seidl

The idea of pneumatic recuperation has been discussed on this site before, albeit in the context of hybrid F1 race cars.

Pneumatics are simpler and a lot cheaper than electric recuperation of brake energy, but you need a fairly large well-insulated accumulator if you want to store more than a miniscule amount of energy. Intercooling would increase tank capacity. As with any thermal and/or mechanical energy storage system, the main hurdle is crash safety (cp. superflywheels, hydraulic hybrids).

HDVs already feature pneumatic systems for their brakes, fed by a small compressor. They also feature hydraulic retarders for controlled hill descents. In theory, a pneumatic recuperation system could replace both, with compressed air released into the atmosphere once the accumulator is full. Noise levels would have to be managed, though.

HDVs also feature turbodiesel engines. Rather than waste the pressurized air on an air motor mode, it could be used to mask turbo lag. This would allow heavily laden commercial vehicles to accelerate from low speeds with reduced emissions of NOx and particulates.

Engineer-Poet

Who says the air would be wasted in air-motor mode?  It goes out the exhaust, which helps drive the turbo.  If the transition to fuel-burning operation isn't an all-or-nothing event, the air-expansion cylinders would increase the turbocharger throughput while taking no air from the intake manifold.  This would increase boost (and efficiency on those cylinders) while also delivering some power with no fuel cost.

Rafael Seidl

@ Engineer-Poet -

perhaps "wasted" was too strong a word. It's just that even in the best of scenarios, pneumatic recuperation of brake energy will support air motor operation for just a very small number of seconds. Unless very high pressures are achieved, the rated power in that mode would be quite small. Also, the exhaust from an air motor is cold and near atmospheric pressure - not what you want for spooling up a turbo.

Engineer-Poet
  • pneumatic recuperation of brake energy will support air motor operation for just a very small number of seconds.

    How many seconds of operation do you need to get a garbage truck from one house to the next?


  • Unless very high pressures are achieved, the rated power in that mode would be quite small.

    Not necessarily. That depends on BMEP, which is determined by the expansion ratio. The motor can always admit air for up to the full expansion stroke.


  • Also, the exhaust from an air motor is cold and near atmospheric pressure
    1. Cold, but it will be mixing with exhaust from the fuel-driven cylinders and the turbocharger doesn't care (pressure times volume is all it cares about).
    2. Only near atmospheric pressure if the expander is operated that way. If air is admitted for the entire expansion stroke, the exhaust will be close to the supply pressure.  This means that useful energy can be recovered down to very low supply pressures.
Being able to use a slightly modified diesel engine as a regenerative brake would be a big advantage for some users, and the possibilities for idle-off operation and short air-driven movements would also be es in some (most?) situations.

Engineer-Poet

It also occurs to me that there are additional possibilities here.

  • The compressed air could be cooled by engine coolant going into storage, but heated by heat from the engine exhaust on the way back to the engine.  This could give it a regeneration ratio greater than one!
  • Applications would include city and school buses as well as delivery and garbage trucks.
  • Cranking and performing initial acceleration on air would eliminate combustion noise.
  • Starting the engine while already at operating speed would guarantee good compression and low smoke emissions.
The scheme doesn't have as much promise for cutting fuel consumption as PHEV, but it would be a lot cheaper and in some ways better than standard hybrid (exhaust-heat recovery).

Paul Dietz

Another possible advantage of compressed air hybrids would be the speed with which the tank could be charged in a 'plug-in' variety of the system. This would require sufficient capacity (and hence energy density) to make it worthwhile, though.

Jens Stubbe

We had busses using similar technology for a while here in Copenhagen. I think they were Volvo's but the noise was deafening. Which is why I think the idea was abandoned.

wxfman

Compressed Air energy source has more applications than electrical, and without the need for a heavy expensive battery bank, motor, mech energy conversion to electical and back penalty, and all that complicated control stuff. The current Air-hybrid is only first generation, E-hybrid on level 4 or 5 now. Besides the obvious regeneration of lost energy, some of the untouched applications for on-board or off-board compressed air are:

1) Unlimited turbo booster power (lots of O2 in CA)
2) Bumper or chassis stiffeners (also air storage)
3) Tire air maintenance and blowout compensation
4) seats, air bags, air shocks
5) electric motor of all size replacements with light pneumatic motors
6) Mobile high pressure compressed air source
7) compression starter
8) road whisker air brush (for pushing slick water gravel debris out of tire way on the fly)
9) impulse air engine using air-hammer
10) windshield wipers
11) iterative compression of compressed air to boost storage capacity, increase supply pressure
12) fuel jet thrusters for emergency maneuvering
13) etc .. better stop now

The compressed air tank is not an efficiently managed resource currently, and that opens up another whole area, similar to battery bank management. Smaller tanks can be used, and with fast actuating valves, air hammer principles can be used to multiply the compressed air to targets from much lower compressed air sources. Much like waves passing over each other, with correct timing, digital pneumatics provides another universe of products, much more efficient use of compressed air, see above air impulse mode. Current analog use of compressed air just first generation. Lots of potential still untouched. Air hybrid is great technology, you are just seeing the rough first draft. Like most technologies, takes vision and patience. Dream a little.


wxfman


Compressed Air energy source has more applications than electrical, and without the need for a heavy expensive battery bank, motor, mech energy conversion to electical and back penalty, and all that complicated control stuff. The current Air-hybrid is only first generation, E-hybrid on level 4 or 5 now. Besides the obvious regeneration of lost energy, some of the untouched applications for on-board or off-board compressed air are:

1) Unlimited turbo booster power (lots of O2 in CA)
2) Bumper or chassis stiffeners (also air storage)
3) Tire air maintenance and blowout compensation
4) seats, air bags, air shocks
5) electric motor of all size replacements with light pneumatic motors
6) Mobile high pressure compressed air source
7) compression starter
8) road whisker air brush (for pushing slick water gravel debris out of tire way on the fly)
9) impulse air engine using air-hammer
10) windshield wipers
11) iterative compression of compressed air to boost storage capacity, increase supply pressure
12) fuel jet thrusters for emergency maneuvering
13) etc .. better stop now

The compressed air tank is not an efficiently managed resource currently, and that opens up another whole area, similar to battery bank management. Smaller tanks can be used, and with fast actuating valves, air hammer principles can be used to multiply the compressed air to targets from much lower compressed air sources. Much like waves passing over each other, with correct timing, digital pneumatics provides another universe of products, much more efficient use of compressed air, see above air impulse mode. Current analog use of compressed air just first generation. Lots of potential still untouched. Air hybrid is great technology, you are just seeing the rough first draft. Like most technologies, takes vision and patience. Dream a little.


Roger Pham

wxfman,

Let me quote for you again Jen Stubbe's previous comment:
"We had busses using similar technology for a while here in Copenhagen. I think they were Volvo's but the noise was deafening. Which is why I think the idea was abandoned."

I've been to the tire shop and can recall how loud these airmotors used to power the wrench can get. Certainly, airhammer motor won't get anymore quiet.

"Fuel-jet thruster"? I don't think that the Batmobile is a street-legal vehicle!
"Road whisker airbrush": Great for throwing gravels into the windshield of the vehicle behind you. Hey, a surefire way to get rid of tailgaters.

You sure have great imagination. :-)

bahahahaha:P

Dream a little i like it:)
<3

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