Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com - View Single Post - Test: Alternator vs. no alternator = 10% MPG gain @ 70 km/h

TimG · 03-04-2010, 02:19 PM

There was some interest back in the mid-seventies about putting the parasitic losses of
alternator, water pump, power steering pump, and A/C on "bottom cycle" power. They
experimented with a Rankine cycle using freon as the working fluid, kind of running an A/C compressor as a motor:

ht tp://tinyurl.com/yzomksc

I think it went away with the memory of waiting in lines to buy gas. The peltier, being
solid-state, would be a cleaner, more reliable solution. Making them even larger than the
University experiment would allow adding DC motors to run the other parasitics besides just replacing the alternator.

One thing that hasn't been addressed (in this thread anyway) is replacing the alternator with a more efficient unit. Stock ones are only about 50%, emphasis is on reliability and
inexpensive manufacturing instead. There is a design called an axial flux alternator that has been developed primarily for windpower generation but would be easily adapted to automotive (actually has been for the military). It uses Permanent rare earth Magnets to provide the field, arranged on the inside of two steel discs. The stator is merely copper wire coils (aircore) potted in epoxy between the (field) rotor discs. Peak efficiencies as high as 95% are obtained with careful design and construction by home builders. You can't control output by switching the field, but you can turn phases (coils) on and off. With RPM varying approx 1:8 between idle and top speed but electrical demand varying by much less, something like a unit with six coil phases would match capacity to demand very closely. Using Pulse Width Modulation on one coil while switching the others would be even finer control.

Another loss is the rectification method. Diode banks in standard alternators are set in big heat sinks for a reason- they have about a 2V drop across them, so about 1/8 of the power you generate is lost to heat right there. FET transistors are now cheap so that a switching circuit to convert the AC to DC would be fairly simple and inexpensive to make. Cut that loss down to a fraction of a volt.

03-04-2010, 02:19 PM	#65 (permalink)
TimG EcoModding Lurker Join Date: Mar 2010 Location: Beaverton, Oregon Posts: 15 Thanks: 0 Thanked 4 Times in 3 Posts	There was some interest back in the mid-seventies about putting the parasitic losses of alternator, water pump, power steering pump, and A/C on "bottom cycle" power. They experimented with a Rankine cycle using freon as the working fluid, kind of running an A/C compressor as a motor: ht tp://tinyurl.com/yzomksc I think it went away with the memory of waiting in lines to buy gas. The peltier, being solid-state, would be a cleaner, more reliable solution. Making them even larger than the University experiment would allow adding DC motors to run the other parasitics besides just replacing the alternator. One thing that hasn't been addressed (in this thread anyway) is replacing the alternator with a more efficient unit. Stock ones are only about 50%, emphasis is on reliability and inexpensive manufacturing instead. There is a design called an axial flux alternator that has been developed primarily for windpower generation but would be easily adapted to automotive (actually has been for the military). It uses Permanent rare earth Magnets to provide the field, arranged on the inside of two steel discs. The stator is merely copper wire coils (aircore) potted in epoxy between the (field) rotor discs. Peak efficiencies as high as 95% are obtained with careful design and construction by home builders. You can't control output by switching the field, but you can turn phases (coils) on and off. With RPM varying approx 1:8 between idle and top speed but electrical demand varying by much less, something like a unit with six coil phases would match capacity to demand very closely. Using Pulse Width Modulation on one coil while switching the others would be even finer control. Another loss is the rectification method. Diode banks in standard alternators are set in big heat sinks for a reason- they have about a 2V drop across them, so about 1/8 of the power you generate is lost to heat right there. FET transistors are now cheap so that a switching circuit to convert the AC to DC would be fairly simple and inexpensive to make. Cut that loss down to a fraction of a volt.