What is the purpose of dynamic braking? - Trains Magazine

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Posted by mvlandsw on Monday, November 29, 2004 12:21 AM

QUOTE: Originally posted by dwil89 Originally posted by Overmod . It should be remembered that the effectiveness of conventional DB tapers off with speed, and often can't be used below a minimum speed. For example, it's possible in theory to fiddle with the motor fields to allow dynamic to work down to relatively low speeds, but this adds complexity and possible points of failure to the design. quote] In older diesels, the effectiveness ( or lack thereof, in certain situations) of dynamics below a certain speed would be more pronounced, especially in mountainous areas.... Conrail's SD40-2 were fitted with what was called 'extended range dynamic braking' which allowed the dynamic braking to be effective at lower speeds than standard dynamics....Norfolk Southern still uses ex-Conrail SD40-2's in helper service over Horseshoe Curve, and the helpers are also used on the downgrades for dynamic braking....The extended range comes in handy here, especially when trains start descending the east slope out of Gallitzin down track one, otherwise known as 'the slide' which has a 10 or 12 mph maximum downgrade speed limit....I don't know how modern dynamics on SD70's and GE Dash Nines equate to the older dynamics, are they equal to older extended range dynamics, or are the older extended range more effective, maybe someone could answer that part of it....Dave Williams The newer engines have extended range dynamic brakes that are much more effective than those of the SD40-2's. The dynamics on the AC units are effective almost to a stop. Member since September 2003 21,048 posts Posted by Overmod on Sunday, November 28, 2004 6:36 PM The Jacobs Compression Brake (which I believe was actually 'invented' by Clessie Cummins) is specifically designed for direct-drive diesel engines -- it uses the considerable compression of the engine to resist rotation of the driveline, and anything that allows 'slip' in that driveline... for example, a non-locked torque converter... will degrade the available brake performance. My understanding of the brake was that the hydraulics didn't hold the exhaust valve closed, but rather opened it late in the compression stroke. At the same time, the fuel injection to the cylinder concerned was effectively cut off. Note that the compression heat keeps the cylinder in question 'hot' so the engine will promptly fire when the brake is disengaged and fuel injection is resumed. There would be little point in putting a compression brake on a railroad locomotive equipped with a Voith transmission, when the transmission itself is tremendously more effective -- and much quieter, too -- as a method of braking. The comparatively slow engine maximum speeds permitted on locomotive diesels might restrict the effective braking effort too, I'd think. Theoretically, it would be possible to brake a K-M diesel-hydraulic by engaging the reverse, then throttling up the engine and engaging (by filling, IIRC) one of the converters. The drawback here is the speed (slow) with which the mechanical reverse engagement on each truck was accomplished, and the need to get everything done and locked before applying power (this is also the reason why switching with a K-M was not a particularly desirable task -- and you thought slow-loading diesel electrics were a problem!) -- not to mention that the reverse engagement might have to be done with the unit stationary... no synchromesh! The DH643s also had Voith transmissions, and would have been poor contenders for compression brakes. Perhaps someone has distinctive knowledge of 'dynamic brake' operation on these locomotives. Member since January 2004 From: NY 913 posts Posted by dwil89 on Sunday, November 28, 2004 6:19 PM Originally posted by Overmod . It should be remembered that the effectiveness of conventional DB tapers off with speed, and often can't be used below a minimum speed. For example, it's possible in theory to fiddle with the motor fields to allow dynamic to work down to relatively low speeds, but this adds complexity and possible points of failure to the design. quote] In older diesels, the effectiveness ( or lack thereof, in certain situations) of dynamics below a certain speed would be more pronounced, especially in mountainous areas.... Conrail's SD40-2 were fitted with what was called 'extended range dynamic braking' which allowed the dynamic braking to be effective at lower speeds than standard dynamics....Norfolk Southern still uses ex-Conrail SD40-2's in helper service over Horseshoe Curve, and the helpers are also used on the downgrades for dynamic braking....The extended range comes in handy here, especially when trains start descending the east slope out of Gallitzin down track one, otherwise known as 'the slide' which has a 10 or 12 mph maximum downgrade speed limit....I don't know how modern dynamics on SD70's and GE Dash Nines equate to the older dynamics, are they equal to older extended range dynamics, or are the older extended range more effective, maybe someone could answer that part of it....Dave Williams David J. Williams Member since November 2003 From: West Coast 4,122 posts Posted by espeefoamer on Sunday, November 28, 2004 5:57 PM Did the KM and ALCo diesel hydraulics used by the SP use a Jake Brake? If so they must have sounded awesome coming down off Donner Pass or the Front Range of the D&RGW[8D]! Ride Amtrak. Cats Rule, Dogs Drool. More great sites from Kalmbach Media