50DN Operating Speed

[Kenny]

Maybe someone could help me to understand how a 50DN alternator direct mounted to the rear of 6V92 can put out 270 amps. From what I can see the alternator is direct coupled to the camshaft which on the 2 stroke engine rotates at the same speed as the crankshaft. Being that the normal operating range going down the road is 1500-1800 rpms would mean the alternator is spinning at the same speed. Performance curves I've seen for the 50DN indicate 1500-1800 rpms would only produce approx 150-200 amps. The 50DN needs approx 5000 rpms to produce the 270 amps. What is it I don't understand? Or will I never get the 270 amps?

Kenny 

[luvrbus]

On the end of the camshaft is a round gear and the alternator has a small gear that runs inside the big gear it is called a planetary gear system



good luck

[bevans6]

I have it in my head that the alternator is running at 2.5 times crank speed, but I don't know if that is perfectly accurate.  Clifford is right as usual, there is a step up gear arrangement for the alternator, details are in the Detroit engine manual.  FWIW the air compressor is cam driven at cam speed, and the power steering pump is driven off the cam gear by a step up gear at higher than cam speed.  On the belt drive alternator like I have the drive is 1:1 and the speed increase is done using unequal size pulleys.

Brian

[Kenny]

I thought that may be the case (alternator driven off a planetary gear). It's not very clear in the MCI manual I have. Thanks for the education. Now I'm more dangerous.
Kenny   

[artvonne]

  Camshafts run at 1/2 crankshaft speed. If you see 1800 rpm on your tach, the cam is turning 1/2 that speed, or 900 rpm. Im guessing then, if peak output is 5000 rpm, the gear ratio to the alternator is stepped up something like 5:1.

[wildbob24]

Art,

That's true on 4 stroke engines, but on the DD 2 strokes,  crank speed and camshaft speed are the same.

Bob

[Boomer]

Many MC-8's & 9's also used an adaptor between the cam gear and the alternator.  This is an item that can cause you trouble.  If there is a bearing or gear failure in the adaptor the debris goes right into the engine gear train and CAN cause instant engine failure and ruination.  Because of that, we had a company policy of overhauling both alternator and adaptor at 100,000 miles.  If you are beyond that, or don't know how many miles are on your alt, it's a lot cheaper to rebuild those two items than the engine.

[artvonne]

Art,

That's true on 4 stroke engines, but on the DD 2 strokes,  crank speed and camshaft speed are the same.

Bob

  Hmmmmm. I am thinking maybe I should do more studying and less thinking/commenting, lol. But I dont recall reading that tidbit anywhere in the manual, so I was probably bound to insert my foot in my mouth sooner or later.

[wildbob24]

Art,

No harm done. It's all part of the learning process.

Bob

[Geoff]

I used to have a 24v gear-driven 50DN on my RTS, and when I would come to a stop and let it idle the "generator stop" light would come on indicating the the solenoid relay would disengage the alternator and all the 24v accessories (mainly the air condition fans and compressor).  From what I have gathered the 50DN only produces 270 amps at high idle or full running speed, not idle.

[norules]

50DN spec's