I was reading the thread on using waste heat from vehicle engines to power small steam turbines and got to thinking about something that’s always puzzled me a bit.
Diesel-electric locomotion has been the preferred power source for US railroading for decades. My understanding is that it is much more efficient and reliable than all-mechanical drives and in particular, provides enormous amounts of torque for startup, useful for the sorts of heavy loads of a lengthy train.
Granted, highway tractor-trailer combos don’t haul anywhere near the tonnage of a railroad locomotive plus train, but it seems as though these efficiencies could transfer to an over-the-road application. Obviously, better minds than mine have concluded that this is not the case. My question, then is, why wouldn’t this work?
Note: here I’m talking about pure diesel-electric drives rather hybrids; diesel engine powers generator or alternator, supplying power to axle-mounted electric motors.
Well, I don’t know. There are diesel-electric hybrid buses in production now.
I’m wondering whether service costs might be a factor - all of these new designs incorporate batteries, and they will add to maintenance costs for trucks that log so many miles.
It’s just a WAG, but I’d guess it is due to the basic conservatism of the trucking industry, the cost of developing diesel-electric powertrains for OTR truck applications, and the additional maintenance expense and complexity of the system, i.e batteries, voltage regulators, electric motors, et cetera. Most independent truck drivers–which are admittedly a small part of the new purchasing market–work on their own vehicles. Large commercial operators typically have a series of small contractors who do maintenance and repair. Your basic modern Peterbuilt and Kenworth tractors are not significantly different in locomotive technology than they were thirty or forty years ago, save for using fuel injection instead of carburetors.
That being said, OTR Class7 and above vehicles are pretty much ideal for implementing a diesel-electric powertrain; the recovery from regenerative braking is significant, the torque losses from an electric powertrain will be much lower, and advances in battery and supercapacitor technology make the concept far more viable in a truck-sized vehicle than it was ten or twenty years ago. I’d be surprised if the major manufacturers like PACCAR aren’t looking at a diesel-electric powertrain at some level, but these companies are slow (and rightfully so) to adopt new technology and throw it out on the world. The expectation for lifetime on a Class 7 or Class 8 commercial long haul tractor is somewhere around half a million miles or more, and having to implement a recall or replacement campaign could bankrupt one of these companies; they’re not producing mass market autos for the consumer market with high volume, but a relatively modest volume of high markup tractors, and anything that cuts into their operating cost also cuts deeply into profit margins.
I think you’ve hinted at the answer here. Trucks don’t face anything like the starting problem of a large train, so have less need for high low-speed tractive effort.
They also operate on rubber tires, which means that the extra cost (in fuel) of the extra weight of a diesel-electric plant would be a serious liability. With steel wheel on a steel track, the corresponding percentage losses for a train would be much lower.
That said, a truck that does a lot of stopping and starting might benefit from some sort of regenerative braking system. (A long-haul truck would have little need for this.)
I believe the hybrid design that UPS is testing is a hydraulic hybrid, not an electric hybrid. In the hydraulic system, the vehicle stores energy by compressing a fluid, then using that fluid pressure during starts and acceleration to assist the internal combustion engine.
Interesting. In railroading, diesel-hydraulic locomotives were tried out on a few lines (most notably German-built Kraus-Maffei locomotives on the Southern Pacific) but my impression was they were mechanically troublesome and a bit too, well, different for the railroads to warm up to.
Meanwhile, I believe Xemamentioned extra weight for a diesel-electric vs. diesel-mechanical drive in a truck application. If we are talking about a straight diesel-electric rather than a hybrid (with heavy batteries), would this really be true?