Many aircraft engines use gear-driven generators. Why not combine the starter and generator for a car and have it gear driven? One thing to remember is that an alternator is constructed quite differently than a starter or generator.

Jim

 

A generator and an alternator are different? How? The basics are the same (or I've suffered from a huge amount of ignorance these past forty years or so). Actually, I know they are very similar to each other. Mom used to put alternators together and brought defective parts home to show us. I'd had generators apart since I was 13 or 14. The first thing someone would notice is the differences between the commutator (on the gen.) and the slip-rings or pick-up rings (on the alt.). What else is different?

Electrically, I'd give you that. Gotta have rectifiers to "filter" half of the sine wave...Alternators give us pulsating DC, not the same as what a generator delivers. Maybe it wouldn't work.

 

not trying to hijack but i think a bigger problem is why havnt they figured out a way to push start an automatic car????

but to the starter deal, the only way to start an engine is to turn it over to draw in and compress air for ignition. there are 2 ways to do this, well 3 if you count the hand crank. one use a starter to rotate the engine. two use an external power source to force air into the engine to start it rotating. but if you ask me id rather replace a 150 dollar starter then a 400+ dollar external power source.

in theory you could ignite fuel in a cylinder and get it to move the piston but the amount of fuel you would waste just getting it to wanna start would be to high i would think. and you couldnt use the cylinder that has compression because if the car sat over nite theres a good chance the pressure leakdown. unless the engine is in good running order, and we all know how many people acutually check things like this.

 

When I was 12, my cousin and I were the designated starters for my grandfather's tractor, we were cheap and available.

We each got to crank start it, until the designated starter #1 wore out and unit number 2(cuz) was put in place. So it was neither convenient nor easy.

Plus we quickly learned not to hang onto the handle when the engine kicked back. Those handles weren't called arm busters for nothin.

Thank god for electric start.


Jim Henderson

 

You guys have given me a great idea for a neat modification for the manual cranking system I came up with. Instead of turning the thing by hand (evidently a lot of trouble and dangerous), I propose hooking the metal rod up to a motor of some kind to turn it. It could even be turned by an electric motor powered by the battery that's already in the vehicle! What do you think?

 

Could always use compressed air to turn the vehicle over.

 

Just add an air compressor and a big tank, no real issue....

Back to the OP:

We still have electric starters because they are best, most practical, most reliable method currently available.

 

On my old lawn tractor with an old Kohler K341 engine, if you manually broke the breaker points when the engine was stopped and thepiston was close to TDC on the compression stroke, or just starting downwards, the engine would kick a bit, but the crank would only rotate maybe a quarter turn.

 

"Why not combine the starter and generator for a car and have it gear driven?"

Expensive gear train. Standard ring-gear reduction is dirt cheap, as are belt-driven alternators.

 

Mack used to have air starters available on some of their engines, I had a buddy who had one of them. Imagine the sound of a ~20 hp air wrench going off. When he started that thing brave men would dive for cover, mothers would pull their kids off the street, dogs could be heard howling in the distance...

 

I've got an air starter on one of my wheat trucks [Brigidier] It sounds like 200 people farting in the same key at the same time .........

I love to start it up on unsuspecting visitors!

 

Yeah, my buddy said he used to love starting it in highway rest stops and watching the people scatter....

 

This is how its done in Hybrid vehicles actually. The electric motor also starts the car and provides electrical power for the car when its not being used to assist the gasoline engine. I had a Civic hybrid, and the only belt driven accessory was the A/C compressor. IIRC, it had an electric water pump and electric power steering assist (not an electric power steering pump, it had an assist motor directly on the steering rack).

It was almost eerie starting the car, because there wasn't the characteristic starter noise. You'd turn the key and the motor would start running. Since it didn't require a large amount of current to start the car, it allowed the computer to turn the engine off when you came to a total stop, such as at a stop sign. When you lifted your foot off the brake, the car would start again automatically. Pretty cool really!

The fundamental difference between an alternator and generator is that the alternator uses a rotating field, thus only puts a relatively low amount of current through the brushes. On the other hand, the field on a DC motor is stationary, the stator, with its windings, rotates through the stationary field and the voltage is at the slip rings on the commutator. All of the load goes through the brushes, which means generator brushes are usually quite large and require more frequent replacement. Also, as the brushes and slip rings wear out, there can be some arcing, which causes a good deal of static through radios.

An automotive alternator does produce pure AC though, usually three phase. Some really, really old ones may be single phase, but they're not really suitable for modern electronics.

 

OK, so there really isn't much physical difference between the two, just the basic function of the parts. I thought someone was going to claim the two didn't share stationary and rotary windings (coils), brushes, etc.

You are right that an alternator produces alternating current, but from my very basic understanding of it, we do not use "half" of the current produced. That's where the rectifiers come in, I believe.

Right again about the brushes. I recall alternator brushes that were about 3/16th inch square, maybe less, and about 1 to 1-1/4" long compared to generator brushes of almost 1/4" by 7/8".

Along the lines of this topic, it would be interesting to know just how these things are handled in modern railroad locomotives.

Originally they coupled a huge generator to the prime mover. Later on these were replaced by similar sized alternators, but the drive motors (called traction motors in the industry) remained direct current. Recitifiers changed the AC produced to pulsating DC. These DC motors produce from 300 to 1,000 horsepower each (600 volts, up to 1,400 amps).

In addition to improved DC locomotives, there is a more improved AC type locomotive that has been around for a number of years. These are substantially more expensive compared to the DC locos due to computerized equipment used to control the AC power. I don't know details of these, I just run 'em.

I do know that for many years diesel-electric locomotives have had what is called dynamic braking whereby braking force is developed by electrically changing the traction motors into generators. The current produced is then routed to a grid which disipates the heat produced, much like a giant toaster. Huge blowers cool this grid.

Both AC and DC types have this feature and it is an invaluable operating tool when used in the right situations. In fact, dynamic braking has been so well developed and works so well that they have had to re-program it to keep from derailing trains because of too much braking force being developed too quickly. (If you try to stop a train, especially a very long or very heavy one all from the head end, and too quickly, the train will buckle in the middle somewhere and derail.)

On older locomotives there was a separate starter that engaged much like those on our cars and trucks. On the newer ones (GE's) the starting function is built into the alternators somehow. These are strange because once the start button is pushed or turned, it must be held on for up to 30 seconds, then you hear the main engine ever so slowly start to turn over, gaining speed as it nears a speed where the cylinders begin to fire. On another manufacturer's locomotive the placard says it has air-assisted starting, which, by the sound, it does. I assume the main starter is electric, but you can't hear it roll over.

So, maybe if we can discover now GE or EMD does it, all we have to do is downsize the design for our little engines.

 

All the current is used. The rectifiers are usually a bridge of some type utilizing both the positive and negative waves. jd