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Most diesel-electric trains are not hybrids, they have no batteries and therefore always get power from the single source, the diesel engine. There have been at
by SigmundA 6y ago
Most diesel-electric trains are not hybrids, they have no batteries and therefore always get power from the single source, the diesel engine. There have been attempts to make them hybrid by putting a car in that contains a lot of batteries for say storing power down a mountain to use going back up the next mountain rather than converting the power to heat in the dynamic braking grid. My understanding is so far it is not worth the cost or complexity for the minimal savings.
Diesel-electric is actually less efficient than a mechanical coupling at steady speed due to double conversion losses which is about 80% efficient vs high 90's for gears through standard mechanical transmission. Trains use the an electric drivetrain because they need precise traction control with huge torque to get the train moving, a mechanical transmission with torque converters, clutches and gears would be difficult to route power and wear out too easily due to the low speed lugging. Once up to cruising speed however it would be more efficient.
This is why there are no hybrid semis or even diesel electric semi's, standard mechanical transmission are more efficient and hybrids only have significant gains in stop and go traffic where as for mostly highway travel the hybrid would normally disengage and simply be dead weight in a parallel hybrid or simply a efficiency drain in a series hybrid. This is also why most hybrid cars are not series, they mechanically couple the engine to wheel for efficiency once up to speed.
Planes again would have little gains from being a hybrid as they would spend most of their time at cruise running the combustion engine and a straight mechanical connection from engine to prop is much more efficient and lighter than any electric double conversion scheme.
Electric planes might make sense because there is no double conversion or engine in the plane itself, but you still have the issue of energy density with batteries.
- deeg 6y agoExcellent and well-written answer. I hadn't considered the dead weight of the generator. It's more proof that questions with seemingly simple answers rarely exist in the real world. As a friend of mine (a data analyst) likes to say: "the real world is messy".
- SigmundA 6y agoYes most people don't seem to realize the hybrid system tends to just shut down at highway speeds. The reason a hybrid car has good highway mileage is because it has skinny tries and good aerodynamics, if you ripped out the hybrid gear its would get even better highway mileage due to less weight. There is another aspect specifically of gas hybrids they typically use Atkinson cycle engines to increase efficiency but have reduced torque. This makes it ideal for a hybrid setup as the electric motor can make up for the lack of torque off the line. Notice there are basically no diesel hybrids, this is because a diesel engine is more efficient than a Atkinson cycle gas and has even better torque than a Otto gas but they weigh more and cost more, so it makes little sense to add more weight and cost to a diesel drivetrain that is already very efficient and has plenty of torque. City buses are the exception due to the extreme stop and go low speed duty cycle that can really take advantage of a hybrid drivetrain.
- hinkley 6y agoYou can have Atkinson cycle with diesel, so 1) do they, and 2) how much does that offset the transmission losses?
- SigmundA 6y agoI have never heard of a Diesel-Atkinson and technically Diesel is a cycle, so if its a Atkinson burning diesel fuel its no longer a Diesel engine although the terms get fuzzy between the fuel and cycle.
- Reason077 6y ago> "they typically use Atkinson cycle engines" Only the early-model Toyota Prius used Atkinson cycle engines, up until 2003 or so. You'd be hard pressed to find a modern hybrid using Atkinson cycle. Any modern Prius uses conventional engines with variable valve timing.
- SigmundA 6y agoWell they use the variable valve timing to operate in a Atkinson cycle but can change valve timing back towards Otto. Most still seem to refer to these engines as Atkinson cycle even Toyota themselves: "The Prius retains its 1.8-liter VVT-i equipped Atkinson cycle petrol engine (2ZR-FXE)" https://global.toyota/en/detail/9827044 https://global.toyota/en/detail/9827044 https://en.wikipedia.org/wiki/Toyota_ZR_engine#2ZR-FXE https://en.wikipedia.org/wiki/Toyota_ZR_engine#2ZR-FXE
- Reason077 6y agoBattery-electric trains (not hybrids) are certainly a promising emerging technology, however. Countries like the UK and Germany are looking to replace diesel trains with battery-electric on non-electrified branch lines: https://www.railwaygazette.com/battery-traction-agreement-signed-as-hitachi-sees-market-for-more-than-400-trains-in-the-uk/56885.article https://www.railwaygazette.com/battery-traction-agreement-si... Battery-electric trams are also a thing: https://www.railtech.com/infrastructure/2019/12/12/first-uk-wire-free-tram-line-opens-in-birmingham/ https://www.railtech.com/infrastructure/2019/12/12/first-uk-...
- SigmundA 6y agoYes similar to planes its an energy density and cost issue for the batteries although weight is less of a concern. I would imagine you could do some interesting things too like have the train run through an electrified section to charge itself and then run through a non-electrified section on batteries.
- MLR 6y agoThat's basically the plan for the UK network going forward, electrification has been ludicrously expensive so most of our rail network (in miles) is serviced by diesel trains, even some of the major lines have had full electrification indefinitely delayed. It also allows un-electrified or inactive railway lines to be incorporated into urban metros much more easily and we're doing that as well.