Boeing 787 Dreamliner, Fuel Costs, Electric Systems, Auxiliary Power Unit and Lithium-Ion Batteries

Posted by PITHOCRATES - January 23rd, 2013

Technology 101

Auxiliary Devices reduce the Power Output of the Engine available to Drive a Car Forward

If you’re middle age (or old) you may remember looking under the hood of a car.  When you could see things.  In the days of rear-wheel drive cars and big engines.  The front of the engine had a power takeoff pulley attached to the crank shaft.  The thing the pistons spun when it converted reciprocal motion into rotational motion.  Wrapped around that pulley were a lot of belts.  Sometimes three or more.  They transferred the rotational motion of the crankshaft to auxiliary devices.

These devices included the water pump that pumped engine coolant to remove the heat of combustion.  An alternator to generate electric power.  A power steering pump to make steering easier.  An air pump to inject air into the exhaust system to help complete the combustion process to reduce emissions.  (An electronic air pump has since replaced this belt-driven device.)  And an air conditioner compressor.  All of these devices reduce the power output of the engine available to drive the car forward.  Requiring more fuel.

Today’s cars have a lot more stuff under the hood.  Engines are often mounted transversely.  And the multiple belts have been replaced with one serpentine belt that winds around all of these auxiliary devices.  And engines are smaller.  With on board computers that maximize the power output of smaller engines.  That drive lighter cars.  But one thing hasn’t changed.  When you turn on the air conditioning you can still hear the engine labor under the additional load.  While burning more fuel.

The Boeing 787 Dreamliner can do what other Planes can do while Burning less Fuel

In the airline industry the greatest cost is fuel.  So anything that allows airlines to burn less fuel greatly interests the airlines.  And it’s why pilots do careful calculations to determine how much fuel to carry.  That is, to determine the absolute minimum amount of fuel to carry.  If it were up to pilots they’d top off the fuel tanks.  But if they did that the airlines could not operate profitably.  Because you have to burn fuel to carry fuel.  And the more fuel you carry the more you have to burn.  Increasing your fuel costs to the point an airline loses money.  Especially if you’re landing with a lot of fuel in your tanks.  So pilots load less fuel than they would want.  Because to get a paycheck their company has to operate at a profit.  But it doesn’t stop there.  Not for aircraft designers.

Designers have been using more plastic in airplanes.  Because plastic is lighter than metal.  So engines can burn less fuel.  These composite materials are also stronger than metal.  So less of them can replace equivalent metal components.  So engines can burn less fuel.  Airlines have also been charging more for carry-on luggage.  In part to help offset their rising fuel costs.  And in part to encourage people to carry less onto the airplane.  So engines can burn less fuel.  Then Boeing raised the bar on burning less fuel.

The Boeing 787 Dreamliner is a remarkable design.  Remarkable because it delivers what airlines want most.  An airplane that can do what other planes can do.  But does it while burning less fuel.  Boeing has used more composite material than any other manufacturer.  Making the 787 the lightest in its class.  And lighter things allow engines to burn less fuel.  But Boeing did more than just make the airplane lighter.  They used electric systems to replace hydraulic and pneumatic systems normally found on an airplane.

The 787 Dreamliner uses Lithium-Ion Batteries to start their Auxiliary Power Unit

Hydraulic and pneumatic systems bleed power from the aircraft engines.  As the engines drive pumps and compressors for these systems.  By converting these to electric systems more of the power of the engines goes to producing thrust.  Which means they burn less fuel to fly to their destination.  They even replaced the pneumatic starters (that spin the engines during starting) with a combination electric starter/generator.  Saving weight.  And reducing the complexity.  By replacing two parts (pneumatic starter and electric generator) with one (combination starter/generator).

To start the aircraft engines they first start the auxiliary power unit (APU).  The APU is typically mounted near the tail of the aircraft.  The APU provides power, lights, heating, air conditioning, etc., when the main engines aren’t running.  Some provide back up power (electric and pneumatic) should the main engines fail in flight.  The APU also drives an air compressor to provide the pneumatic power to spin the main engines for starting.  Going to all electric systems (except for the engine anti-ice system) removes the air compressor from the APU.  Reducing the weight.  And they further reduced the weight by making another change.  To the battery that starts the APU.

The 787 uses lithium-ion batteries.  Which can provide the same power larger batteries of different technologies can provide.  As lithium-ion batteries has a very high energy density.  But with great energy density comes great heat.  Some of these batteries have actually caught fire.  In electric cars.  Laptop computers.  Cell phones.  Even in Boeing 787 Dreamliners.  They’re not sure why.  And they’ve grounded the fleet until they figure out why.  It may be because they are overcharging.  Or that there are internal shorts causing a thermal runaway (releasing all the stored energy at one time).  Or the caustic electrolyte is leaking and causing a fire.  Until they determine what the problem is the 787 will remain grounded.  Making it very difficult to enjoy the cost savings of that remarkable design.

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