Miguel928

I found out that on a airplane jet engine the electric starter is also the electric generator for the plane. The same equipment can normally work both ways like any electric engine. So the question is why this is not used for internal combustion engines?

I think that the answer lies on the different torques and rpm on each cases. For the engine it is requested a hi torque at low speed to start. And then a transient regime over a wide speed range. On the jet the rotation speed is stable and there is no need for hi torque on start.

Any way can some one with a electric background explain this better.

Thank you all.

Imo000

Here’s a simple explanation. If it was possible car manufacturers would have done it. If you try to keep the starter on all the time to generate electricity, it will over heat and burn up in a matter of minutes.

Alan

What you said is correct - its the relative speeds & gearing that are wrong but also the nature of the power generation. A motor used in reverse is a DC dynamo type generator - its output voltage & power capability is dependant on its rotational speed. In a car the desired voltage is relatively fixed (nominally ~12v) and the current loading is mostly not related to speed and yet is widely variable depending on equipment usage: lighting & headlights, windshield heaters, wipers, blowers, fans etc.

Car manufacturers developed the alternator (a multi-phase AC generator) which can be equipped with voltage regulation (via electro-magnetically controlled field windings & rectifier diodes) to:

1) Maintain its voltage generation in the range of ~ 12.5v - 14V DC over a wide range of engine rotational speeds from idle to 6(+)K rpm (often the generator itself rotates @ 2x engine speed)

2) Adapt its total power generation by varying its loading on the engine per rotation to maintain the regulated voltage over a wide range of supply currents, (via control of the field coil excitation).

Alan

dr bob

Many of the hybrids combine the starter and alternator functions, and use the same systems to help move the car along.

The 'starter' on your stationarly gas turbine engine may or may not be an actual auxiliary electric motor. Depends on the size of the turbine generally. FWIW, if a turbine is small enough to spin with a separate motor, that seems to be the preferred method. Getting the variable-frequency drive for starting coordinated with the excitation and switching used for generating is no small task, so the KISS principle applies whenever possible.

The larger frame machines take megawatts of starting energy. IN an area where the grid is soft, starts are carefully coordinated to prevent serious dips in voltage. It's fun stuff!

michael j wright

Also if you have ever seen a starter on the inside after it has hung up you will find that the high speed will normally expand the windings and basically looks like spagetti ! Now on the other hand I have read in Tec pub's that they are working on just that idea, and may be just around the corner. You thougt alt's and starter's have been pricey before, just wait it's going to get a lot worse!

ZEUS+

An automotive generator and starter(the old days) are similiar in construction. You can apply power to the generator and it spins. As Alan pointed out, the increased electrical demands require a more powerful constant. An alternator turns unrectified ac to rectified dc. Modern alternators are high tech and electrically quiet. The size of a dual purpose unit would be one drawback.