Train Air Compressors

[broncoman]

Does anyone know why the air compressors on trains are electric and not directly engine driven?

Dave

[ericsp]

Why make things unnecessarily complicated?

[mackb4]

      Some of your older units are shaft driven directly from the rear of the "prime mover" or motor.The SD-40's and the GP-38's I know the NS have are.And why newer engines use electric comp. rarther than the shaft driven more than likely has something to do with the efficiency of the motor.Since the engine is already producing electricity,it would make scence to use it,and not drag down the engine and make it use more fuel.It is big bussiness in the railroad locomotive war to get more hours and miles out of a gallon of fuel.On the shaft driven comp's. the more throttle you give them,the quicker it charges the train line pressure.On the engines with electric comp's. max pumping pressure is acheived in the #1 throttle position.

[mvlandsw]

Most compressors were driven directly by the diesel engine. Electric motor drive is used now so the compressor does not have to turn when air is not needed. This saves fuel and wear on the compressor.  Mark

[Modelcar]

Whether the air compressor is drawing it's power mechanically or from it's electrical source produced by the prime mover it will still cost a bit of horsepower...It's not free to pump air pressure and will use a bit of fuel.

[Hugh Jampton]

Whether the air compressor is drawing it's power mechanically or from it's electrical source produced by the prime mover it will still cost a bit of horsepower...It's not free to pump air pressure and will use a bit of fuel.

Yes, but, if the comp is driven from the prime mover either it will run all the time (and that needs power which is fuel and increases wear) or some kind of clutch will be required (an added expense and maintenance issue). Plus there's only so many places in the engineroom where it can be situated.

Electric drivem comps drive teh compressor directly and only run when needed and the comp can be placed anywhere.

[Modelcar]

....Oh yes, I agree....I'm not saying one way is better than another...Another poster had mentioned one might as well use the electric  version in place of the mechanical connection and save on fuel since the electricity was available, etc....

[jchnhtfd]

It's a variety of the various factors cited... first, if you have a mechanical drive, you have maintenance associated with it -- either the compressor runs all the time, in which case you  have added maintenance on it plus unloaders and other oddments or, if it doesn't, you have a clutch and clutches are a nuisance, to put it mildly.  If it's electric, a pressure switch trips, the motor starts, and the pressure builds.  Simple.  There's one in every gas station in the country!

Second, if it is electric, it can be built to do a really good job of stuffing air -- all the time.  The mechanical ones, as has been noted, if the engine is running at 1 can take quite a while to charge a train line, as they are a compromise between cycling too fast at say 6, 7, or 8 and not getting the job done at all.  The electrics can be 'just the right size'.

[broncoman]

Thanks for the info guys.  I hear the compressors kick on on the GE units all the time and I wasn't sure if the EMD units were electric or mechanical it seems to make sense since you are going to pay for it one way or another.
Are the compressors pretty much like shop air compressors, radial or screw type, or are they an inline cylinder arrangement like the big bendix equipment compressors or are they a multi-stage affair.

Dave

[Modelcar]

.....And both methods requiring horsepower to produce compressed air for the train line....That's all I'm referring to.

[Randy Stahl]

Thanks for the info guys.  I hear the compressors kick on on the GE units all the time and I wasn't sure if the EMD units were electric or mechanical it seems to make sense since you are going to pay for it one way or another.
Are the compressors pretty much like shop air compressors, radial or screw type, or are they an inline cylinder arrangement like the big bendix equipment compressors or are they a multi-stage affair.

Dave

I shudder everytime I hear that sound , starting the air compressor takes alot of current , each compressor has it's own cycle skipper to control the speed , alot of eletronics to do a simple job . Alot of wires to get blown off .

[oltmannd]

An unloaded shaft driven air compressor at 900 RPM consumes about 15 HP. 

That's a lot of wasted energy!

Add to that the extra fuel it costs to run the engine in N4 no load to pump air vs. idle.   An those elec motor driven air compressors are worth the added complexity!

[ericsp]

An unloaded shaft driven air compressor at 900 RPM consumes about 15 HP. 

That's a lot of wasted energy!

Add to that the extra fuel it costs to run the engine in N4 no load to pump air vs. idle.   An those elec motor driven air compressors are worth the added complexity!

I would think that the air compressor driven directly off of the motor would be more complex.

[broncoman]

I can't speak to the locomotive ones but on trucks and equipment its a very simple thing.  They bolt up the engine and when they are not used the valves are open so there is no resistance for them to pump against.  The only parasitic loss is the draging of the pistons up and down.  In trucks they can last for up to 300,00 mi if the engine is taken care of.

Dave

[oltmannd]

An unloaded shaft driven air compressor at 900 RPM consumes about 15 HP. 

That's a lot of wasted energy!

Add to that the extra fuel it costs to run the engine in N4 no load to pump air vs. idle.   An those elec motor driven air compressors are worth the added complexity!

I would think that the air compressor driven directly off of the motor would be more complex.

Driving the air compressor off the front end of the engine is really simple.  A shaft goes between a coupling on the engine and a coupling on the air compressor.  Once you get the shaft aligned, you're set to go for years!

A motor driven compressor requires a motor and motor controls to operated the motor at 1x engine speed or 2x engine speed.  More parts, more cost, more complex.

[Modelcar]

....In modern engine building times how does GE and EMD choose to do it....?  What is the norm....?

[JonathanS]

Going way back to the Westinghouse and Ingersoll Rand era of diesels you will find that some actually used two of the cylinders on the engine block for the air compressor.  While others had air compressors that were driven by small gasoline engines.  And some, like New York Central's 1920's diesels, had both.