Starter generators

[snj5]

Many small engines, such as tractors, use a single starter/generator for both functions, also sometimes called motor generators. Very large starter generators are used on turbine engines. Is there any experience or advice on adapting a starter generator to a low compression aircraft engine? The wiring seems pretty straight forward.

Thanks!
rt

[rwanttaja]

Many small engines, such as tractors, use a single starter/generator for both functions, also sometimes called motor generators. Very large starter generators are used on turbine engines. Is there any experience or advice on adapting a starter generator to a low compression aircraft engine? The wiring seems pretty straight forward.
rt

I believe Starter/Generators are limited to a generator, instead of an alternator, so the amount of power produced is less (especially at low RPM). It would eliminate the Bendix gear for a direct-drive gear, which might have some reliability issues. Could belt-drive the thing, but I worry whether the belt would slip on a cold-weather start.

Modern starters and generators are fairly light, so the advantages probably aren't there. Turboprop engines are so small that it probably makes sense from a real-estate aspect to use starter-generators.

But I'm just guessing, mostly....

Ron Wanttaja

[WLIU]

As Ron noted, you only can have an electric device that is wired as a starter, also be a relatively inefficient generator, not an alternator. And as Ron noted, generators require RPM to do their job. A turbine has LOTS of RPM and horsepower available so the generator gets the job done well enough. In that application the combined function saves weight over two units.

Using one on a recip you get all of the disadvantages of having a generator, and you have an inefficient generator. A recip has less horsepower to give away. Having operated with a generator, I really like my alternator. And there are some pretty light weight ones out there.

The other factor that counts is that our recip engine designs predate turbines. So if you look at how the starter and generator or alternator connect to the rest of the engine, you see that the generator/alternator drive train is not strong enough, and geared correctly, to allow the starter action to be applied there. And you see the manner in which the start connects to the drive train has gearing that would require a generator or alternator to operate at MUCH higher RPM's than are practical today. So unless someone does a clean sheet recip engine design, the engines that we live with today can not be adapted to combine the starter and generator.

Hope this explanation helps,

Wes
N78PS

[Darryl MacDonald]

Many small engines, such as tractors, use a single starter/generator for both functions, also sometimes called motor generators. Very large starter generators are used on turbine engines. Is there any experience or advice on adapting a starter generator to a low compression aircraft engine? The wiring seems pretty straight forward.

Thanks!
rt

The older Cub Cadet riding mowers used a starter generator on them. They had a pretty neat little mechanism on the camshaft that would open the exhaust a bit at cranking speed for a compression release. When the RPM got over about 50, centrifical force would let the mechanism retract and the exhaust valve would operate normally. The output of the generator wasn't much over about 25-30 amps at full throttle and you had to have a voltage regulator with it. It would keep the small mower battery charged quite well. Not sure if the starter mode would have enough oomph to push even a low compression A/C motor past the compression stroke or not. If it would turn it over, you'd have a pretty good set up if you aren't running a full IFR radio stack with strobes, rotating beacon, etc.

[1600vw]

Gas powered Golf Carts use this type of system. Some even run backwards. This is how they would go in reverse. No transmission, the engine ran in reverse.

Its been years since I have worked on any of this, but if my memory serves me right that Generator was then hook to a rectifier, regulator. This Starter-generator was belt driven.

[Matt Gonitzke]

The older Cub Cadet riding mowers used a starter generator on them. They had a pretty neat little mechanism on the camshaft that would open the exhaust a bit at cranking speed for a compression release. When the RPM got over about 50, centrifical force would let the mechanism retract and the exhaust valve would operate normally. The output of the generator wasn't much over about 25-30 amps at full throttle and you had to have a voltage regulator with it. It would keep the small mower battery charged quite well. Not sure if the starter mode would have enough oomph to push even a low compression A/C motor past the compression stroke or not. If it would turn it over, you'd have a pretty good set up if you aren't running a full IFR radio stack with strobes, rotating beacon, etc.

The output of those is no more than 15A. I doubt they'd turn over an aircraft engine. They are also extremely heavy.

[Jeremy Leasor]

A number of cars were fitted with Dynastarts and other variations of starter generators in the 1920s. We have one on a 1920 40HP Renault and but my experience of this, confirmed by what I have read from when they were new, is that they are very heavy and not particularly good at either task.