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air compressor on new amtrak locomotives

Air pressure above a certain point releases the brakes.

The engineer pumps up the train brake line to release the brakes and fill the small reservoirs on each car. Then when he "sets some air" by blowing down the pressure on the brake line, the three-way valve on the car channels air pressure from the reservoir to the wheel brake cylinders. The more he sets in one application, the higher the pressure applied to the wheel brakes.

Breaking the airline or putting the brake system into "emergency" applies the full reservoir pressure to the wheel brakes.

Now a clever listener might ask "Then how can the engineer start a train that's stopped on a grade?"

Reply to
Jim Stewart
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That's more-or-less what I said, but it's more complicated than you imply. Essentially the air both APPLIES and RELEASES the brakes.

The locally STORED air in the main reservoir is what provides the force needed to APPLY the brakes. A DROP in pressure in the train-line is what triggers the brake application (via the triple-valve as previously described). Once the pressure in the train-line is restored to normal, the pressure in the brake cylinders is vented (again via the triple-valve), and the brakes are released.

What's important to understand is the pressure in the train-line is used to CONTROL the brakes, NOT to provide the force to apply them.

The brakes are NOT "spring loaded", and held "off" by the air in the train line. It's only PREVIOUSLY stored air in the reservoir that can a provide the force to apply the brakes (I suppose that could be thought of as an air-spring). However, once that reservor air is used up, you're in BIG trouble.

Once the main reservoir is depleted, as from too many brake applactions in a short period of time, the brakes become inoperative regardless of train-line pressure. Then you have a run-away train. NOT good!

It's the engineer's job to manage his available air supply to assure adequate braking at all times.

Dan Mitchell ============

Reply to
danmitch

All makes good sense except the "vampire" comment. Whatever the power it takes to run the air compressor, it HAS to be taken from the Diesel engine, by whatever means ... there's nowhere ELSE to get it from. That's the same issue as electric applicances on automobiles.

The next question is the relative efficiency of the power transfer ... whether electrical or mechnical. I suspect that's not a whole lot different.

That leaves reliability and maintence as the big issues, and, as you point out, electric compressors likely win out in those areas. since they're the common type on newer locomotives, that pretty much answers THAT question.

Dan Mitchell ============

Reply to
danmitch

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