Suppose I wish to backyard-engineer my own electric turbocompounder:

I take a turbo and attach a reduction gear to the turboshaft, and an alternator to the reduction gear. The electricity this setub generates goes directly to an electric motor geared directly to the crankshaft.

According to good ol' Nick Tesla, alternating current is more efficient than direct current, so I disable the alternators rectifier and run the AC to the electric motor.

Problem is...since the turbo is spooling up and down, the frequency of the alternation goes up and down. If I am using an electric motor "used to" the American AC frequency (60 hertz, IIRC), will the homebrew set up still work well when the AC is putting out above and below 60 hertz?

The efficiency of AC power over DC power comes from the ease it can be stepped up to high voltages - reducing current and transmission losses.

For automotive purposes there's almost no difference between AC and DC efficiencies - assuming you'r putting power directly from the "generator" to the motor.

For your idea, you'd probably be better off with a DC system. As long as there's enough voltage there to keep the motor spinning faster than the engine, it will contribute to moving the car.

The diodes in the alternator would keep the motor from trying to back drive tha alternator if there wasn't enough voltage. Thus, you'd have a fairly simple system.

Trouble is, by the time you gear reduce a turbine, and feed it into a generator, there isn't going to be much power left. Gearboxes that get the 100,000 ish RPM of a turbocharger down to a rational speed for an alternator are notoriously inefficent - and pretty delicate.

Check into how jet aircraft generate electricity. It seems like they bleed off some hot gasses from the jet and spin a turbine. Might be some surplus parts out there.

Back to the AC motor question...The power frequency is directly related to the motor RPM. For most US motors, 60 hertz corresponds to 1800 or 3600 RPM, with some oddballs at 1200 RPM. The actual rated speed is a little less than that - that's called slip, and it's how the rotating field produces torque - the armature is always trying to keep up with the spinning field.

It's somewhat tuned, too - efficiency starts to drop somewhat once you're off frequency. "inverter duty" motors have some design features to lessen the effects of that. Many industrial speed controls adjust motor speed by varying the power frequency.

For the amount of power you'd scavenge, you might consider putting it into teh electrical system, rather than a separate motor. Electricity used in the car and for battery charging comes from the alternator - which gets the power from the crankshaft. Energy that teh alternator doesn't have to generate is power that didn't get sucked off the crankshaft - which means it's either available to move the car, or reflected in less throttle - and fuel - to the engine.

Stuff like alternaotrs and water pumps are called parasitic losses - for the obvious comparison. These accesories leach power off the engine.

Eric K

quote:

The diodes in the alternator would keep the motor from trying to back drive tha alternator if there wasn't enough voltage. Thus, you'd have a fairly simple system.
Eric K

HEY! if the diodes were disabled...allowing the motor to drive the alternator and thus the turboshaft...then the turbo would have eliminated lag; instant useable boost at all times!

There's some people working on electric boosters for turbos to eliminate boost.

It seems like Delphi licensed it from a turbocharger maker.

'course, under these circumstances, you're sucking crankshaft horsepower to spool the turbo - which eliminates the efficiency gains that turbos are useful for in the first place.

Eric K