Turbo With an Alternator

[DragBean]

Thinking about putting a turbo in my car to overcome pumping losses and power an alternator so i can unbelt the regular alt.

The car has a carb. I'd maybe try to up to 6psi of boost on the engine for performance. I wonder if this could be done? Would the turbo spin at too high of rpms for the alternator?

[mazdamx640]

Given the design of a turbo this doesn't seen feasible. It might be possible to remove the intake side of the turbo and somehow rig the shaft up to a alternator but the drag created by the alternator would probably require the engine to be running at a high rpm in order to spool up the alternator there would be much more loss than gain. That doesn't mean that a turbo by its self is a bad idea for fuel economy if set up properly. I remember reading an article a couple years ago about a guy turboing his motorcycle for improved fuel economy I believe he called it improved volumetric efficiency

[oldtamiyaphile]

Turbos spin at 50,000 - 100,000 rpm. Alternators spin at 1600-12,000.

[niky]

Formula One cars use a device called the MGU-H (uh... Google says Motor Generator Unit - Heat) to harvest energy from the turbo and to enable quicker spool up at transitional rpm. So yes, the possibility is there.

But these units are pretty high-tech stuff... there's a big electric motor splitting the intake and exhaust sides of the turbo, spinning at 100,000 rpm.

I doubt you'll be able to make enough electricity off a regular turbo to replace your alternator... unless you're driving at full boost, all the time.

Turbo boost is "free" power only in the sense that the boost the turbo provides counteracts the extra backpressure created by the turbo. If you disconnect the intake side, you're just making the engine work harder to spin those exhaust turbines. Probably won't be as efficient as an actual alternator.

[RedDevil]

Wait a minute - you mean a turbo on the air intake, powering an alternator by the difference between atmospheric pressure before and the part vacuum behind the throttle valve, right?

That may work in theory, but you will have a hard time finding a fitting turbine.
Also it would only work on part load; if you idle not enough air is flowing through the turbine to keep it spinning despite the large difference in pressure, and at WOT there is no pressure difference to spool up a turbo.

I confess having thought along these lines myself.
At 2000 RPM at 50% load my 1339 cc engine pumps 22 liters of 50% vacuumized air per second, that dissipates about 1.1 kW of power over the throttle plate.
If you can get 25% of that in electricity, it may just be enough to sing it out for the day without an alternator, but you'd need to charge the battery at home. A bigger battery, preferably a lithium iron one, would be advisable in this setup.

Anyway, subscribed...!

[planejob]

Originally Posted by RedDevil

Wait a minute - you mean a turbo on the air intake, powering an alternator by the difference between atmospheric pressure before and the part vacuum behind the throttle valve, right?

That may work in theory,

I don't think that would work in theory either. You are restricting the air intake, making the air do "work" and "work" is never free. The engine would be doing the same or more work to make the same electrical power as a belted alternator.

Same theory as putting a windmill on top of your car. It's not free to push the blades through the air.

Putting half of a turbo on the exhaust side and running it through a planetary gear reduction seems pretty plausible.

Take a look at the centrifugal superchargers. They use a belt off of the accessory pulley to spool a compressor wheel... just use the same gearing and use an exhaust turbine to spool your alternator.

A broken centrifugal supercharger on eBay could yield some fantastic parts... I really like the idea.

[Crazyrabbit]

The turbo with an alternator is feasible and has been done. I don't know if they sell them now, but Garrett designed and built some turbos with a three phase motor/alternator between the turbine and compressor. Their objective was lag reduction, but with the appropriate controls, one of those would do what you want. That said, the design and development of such equipment is well beyond the capability of the vast majority of even professional engineers, it requires a team of engineering specialists. Like you might find at Garrett. As far as possible alternator speeds go, I have worked on the design, development, and test of alternators that operated WAY over 100,000 rpm. Windage, bearing, and magnetic hysteresis losses do add up, but I think you could go 1,000,000 rpm, if you could get the rest of the system to support that speed. That may have also have been done, on some space based cryo-coolers. There is a lot of stuff that never makes it to consumer media.

The comments about nothing being free are largely true, the only free energy I associate with a turbo, comes from the reduction of normal shock losses in the exhaust port due to the back pressure of the turbo. That shows up as about a 0.5% decrease in BSFC if you compare turbo, and non-turbo versions of the same engine from Yanmar, Kubota, etc.

[JRMichler]

Originally Posted by RedDevil

Wait a minute - you mean a turbo on the air intake, powering an alternator by the difference between atmospheric pressure before and the part vacuum behind the throttle valve, right?

That may work in theory, but you will have a hard time finding a fitting turbine.
Also it would only work on part load; if you idle not enough air is flowing through the turbine to keep it spinning despite the large difference in pressure, and at WOT there is no pressure difference to spool up a turbo.

I confess having thought along these lines myself.
At 2000 RPM at 50% load my 1339 cc engine pumps 22 liters of 50% vacuumized air per second, that dissipates about 1.1 kW of power over the throttle plate.

If you can get 25% of that in electricity, it may just be enough to sing it out for the day without an alternator, but you'd need to charge the battery at home. A bigger battery, preferably a lithium iron one, would be advisable in this setup.

Convert pumping losses into usable energy. If you could figure out a way to control such a power recovery gadget, it could replace the throttle plate. Maybe a supercharger connected to an alternator, with the alternator field controlled by the gas pedal?

[oil pan 4]

You still have pumping losses if its a gas engine.

Why not just get a turbodiesel (no isobaric losses), pull the belt off the alt and replace the alt output with solar panels and put the belt back on the alt for long night trips?

[niky]

Oh, intake?

To make 6 psi on an average-sized four-banger takes about 10-15 horsepower worth of power, going off of what Thomas Knight did with their electric superchargers back in the day.

Normal operating vacuum wouldn't even be able to turn such a thing, let alone produce meaningful power from it. If you've ever dismantled an air intake and put your hand over the intake hose or carb ("burping" the carb to clean it used to be such fun!), you'll know how little vacuum it produces at idle or low rpm... much less than a shop vacuum. Now those vacuum motors have the equivalent of about 2-4 horsepower... So say you have about 1 hp of suck available at cruise... then consider some of that suck has to go to intake air to power the engine... you're getting much less than that going to the intake alternator.

Consider it takes at least 2 hp to turn a regular alternator, and 10-20 hp at full electrical load, there isn't much of a comparison there.

More effective to run a generator off the exhaust side. Cap the turbine speed by braking it down with a generator when at full load... also eliminates the need for a wastegate, as you can control maximum boost pressure, as well.