Re: Regenerative Braking
Posted by:
Peter Brow (IP Logged)
Date: February 12, 2003 10:01AM
<HTML>Hi David,
I haven't seen the articles in question, but somebody on this or the SACA forum said that a locomotive equipped with a diesel exhaust recompressor did show lower fuel consumption than the plain steam locomotive. However, as I recall, it was stated that the cost and maintenance of the equipment didn't justify the expense.
Energy is indeed lost in the compression process, so even if all the exhaust steam is recompressed, the diesel/steam hybrid would eat more fuel than a plain diesel operating at full load. Compared to a diesel engine operating at very low load, though, there might be an improvement, if the steam could be accumulated and the compressor cycled on/off with diesel engine operating only at full load. An exhaust steam accumulator would take up a lot of space though. Maybe if the exhaust steam were condensed by absorbing its heat into a storage medium, and then the heat turned back into low-pressure steam to recompress? This has more heat losses, though, and the whole procedure gets pretty Rube Goldberg, with lots of stages, loses, and costly extra equipment.
Regenerative braking is a bit different. Even if it is relatively inefficient (whatever system is used), it wastes less energy than a total-loss friction braking system. Making it both continuously variable and economically feasible is the hard part, and though interested in the various possibilities, I have serious doubts.
Something like the proposed Ford hydraulic regenerative braking system (which I mentioned recently at the SACA forum) seems to me the most efficient/promising of the various options, or maybe a "big clock spring" like Dick suggests, which may not be as impractical as it sounds. The energy storage capacity needed for an automobile "regenobrake" system isn't too big -- enough to decelerate/accelerate the car one time from/to 65 mph, maybe a few hundred feet of travel.
Perhaps designing the steam engine to compress air properly to (help?) brake the car, and storing the air in a tank? Then the compressed air would run the engine briefly on acceleration. Storage tank might not have to be very big. Keeping air out of the steam/water system would be a must. Air from tank could run thru a tube coil preheated (to 300-500°F) by boiler or engine exhaust, to make up for some of the compression/expansion losses.
Haven't done the math, but depending on the air pressure and exhaust temp, that might "recoup" more energy than was put into compressing the air -- ie, brake for 200 feet, then run for 300 feet before switching back to steam. Or, decelerate from 60 mph, then accelerate to 70 mph on air, by recovering some extra waste heat in the process? Doubtful, though -- air compression temp would have to be lower than exhaust temp, and there are exhaust pressure losses.
For now, I'd bet on the hydraulic regenobrake system a la Ford. Still many problems, eg, you're only regen-braking with two wheels, how do you transition smoothly from 2-wheel regen braking to 4-wheel friction braking when the accumulator fills up, etc.. And will the equipment cost be less than that of the fuel saved? A worthwhile system seems way off in the future, if ever.
Intriguing possibilities, though. Ford claims their hydraulic system could increase city fuel mileage by 20-25%.
Peter</HTML>