Author Topic: Diesel-electric semi-trailer trucks  (Read 1016 times)
dor123
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Diesel-electric semi-trailer trucks « on: July 16, 2022, 10:44:18 AM » Author: dor123
I'm wondering why semi-trailer trucks aren't driven like diesel locomotives, i.e. by diesel-electric transmission? These trucks are very heavy, almost as locomotives and with a direct mechanical diesel transmission, they needs a gear with more than ten speeds and even . Diesel-electric semi-trailers would not only be easy to drive, since there are no gear box, but it would have better acceleration and more fuel economy as an electric motor have more torque to drive the truck when it isn't moving than a diesel engine, hence diesel locomotive have diesel-electric transmission.
Your opinions?
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Re: Diesel-electric semi-trailer trucks « Reply #1 on: July 16, 2022, 10:57:10 AM » Author: rjluna2
We do have diesel-electric vehicles available, dor123.

https://en.wikipedia.org/wiki/Diesel%E2%80%93electric_transmission#Road_and_other_land_vehicles
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Re: Diesel-electric semi-trailer trucks « Reply #2 on: July 16, 2022, 11:51:24 AM » Author: dor123
With buses, they would be more likely series hybrid, as buses aren't as heavy as semi-trailers and dump trucks, and city buses can drive with an electric motor alone fed from the batteries. With intercity buses and coaches, the problem operating them electrically from batteries is the distance they are driving, so hybrid is more suitable here.
Since heavy vehicles like semi-trailer trucks and mechanical engineering vehicles like bulldosers and military vehicle, have very slow acceleration as a result, diesel-electric transmission is very suitable for them, rather than direct diesel transmission, like most uses actually. In the article you linked, no semi-trailers appears.
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Re: Diesel-electric semi-trailer trucks « Reply #3 on: July 17, 2022, 04:20:03 PM » Author: Medved
Maybe except the really heavy mine trucks or similar, the mechanical transmission is still the lightest way to get the power to the wheels. And with normal road trucks, the machine weight is what you can not load as cargo, so not getting paid for. Becsuse practically all around the world the total mass of the tractor+trailer combination is restricted by legislation (exact number differs among countries, but practically everywhere is a limit), so the lighter the vehicle alone, more cargo it can carry, so more money it generates. So the heavier electrical power transfer is used only in offroad vehicles, like big mining or construction dump trucks.

Yes, it means something between 12 to 20 gearing ratios, but the trick is, these are not "1 from 20" gears (like small cars use to have 1 from 5 or so), but rather build as two transmissions in a cascade: One "5-speed" (plus R) and the second 4-speed (hi/low range, 2 halves; or there are actually two stages for these, one for hi/lo, second for "halves"). That way with rather few shiftable gears (10 in total for the 20-speed one) you may attain 20 ratios forward plus in theory 4 gearing ratios in reverse.
Historically in US trucks with manual transmissions the 5+R uses to be controlled by the typical "H" pattern, the auxiliary (hi/low), European use to employ "preselect" type of control (you first selec the gear you want to shift to by sequential buttons/lever, then depressing the clutch actually actuates the shifting itself), or the whole system (clutch plus the 2 or 3 shifting stages) are controlled as an automatic.
Practically the same approach is used with many epicyclic gearboxes (mechanical part of common torque converter based automatics, when they have more than 6 gearing ratios).

The locomotives tend to use the electric transmission, because first the weight is not an issue there (the locomotives need to use additional ballast anyway to get the pulling force from wheels to the rail) and second because there you need to distribute the power in a controllable manner (to adopt for different pressure between front vs back axle) over 4 or 6 axles in 2 bogies, what becomes quite simpler with electrical.
But many rail vehicles (mainly motorized coaches, even some german locomotives) use just pure (hydro-) mechanical transfer, like an ordinary car automatic transmissions with a torque converter (that is the "hydro-" part in the classification). This is for vehicles, where the mass is not needed (not pulling much else, or when the high power is needed for high speed and not that much static pulling force).

And by the way some hybrid cars (especially Toyota) use transmission systems, where (even when the battery is not handling any power) part of the power goes from engine to the wheels pure via mechanical gears, partly via generator/motor pair. That way it is able to limit the power through the electrical part (so not gettoo heavy), yet attain practically continuously variable ratio, all with just fixed ratio mechanical gearing (although differential).
This setup is still heavier than pure mevhanical transmission, but when the battery power gets involved (so the electric motor/generators need to be there anyway), it simplifies greatly the mechanical part.
Some makers use for their 4x4 hybrides mechanical power transfer to one axle (usually front) and electric for the second one. That way they are able to get rid of at least the drive shaft tothe back and go around the Toyota patents. But again makes sense only when the electric part is involved anyway because of the battery traction...
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Re: Diesel-electric semi-trailer trucks « Reply #4 on: July 17, 2022, 10:42:57 PM » Author: funkybulb
Trucks in the US can only leagally carry 80,000 pounds  otherwise
 U have get special permission to go over.    What have change in trucking in the US is having smaller auxillery power unit
 To provide Air conditioning and 120 and 12/24 volt power to
Things while main engine off.   Trucker used to leave trucks
 Main engine running 24/7 for long distant driving.   So it not uncommon for semi trucks to carry 3 diesel engine two on the
 Truck and 1 on semi trailer to keep food cold.    US right now
 Is expermenting with battery operated electric trucks.  I think
 It will be good for local deliveries that about it .
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Re: Diesel-electric semi-trailer trucks « Reply #5 on: July 18, 2022, 11:25:01 AM » Author: Medved
Well the APU has many other advantages over just the main engine than just the fuel savings. It is easier to start even when batteries get weak due to e.g. low temperatures (even the small remaining power is sufficient to stsrt the smsll APU engine) and then preheat everything, allowing to "wake up" the truck even at colder weather, or just not that big deal to keep the APU running and so not allowing everything to freeze up in the first place. Consequently it allows the use of smaller starting batteries (do not need that much low temperature margin for cold start, plus the APU generated energy help with the starting directly), which more than offsets the weight of the APU itself.
And for the driver it means greater comfort, as it is way quieter than the main engine. And it could be left running even when the keys are removed from the "ignition", so driver may hide them, so better security against truck hijacking/theft when the driver is sleeping or out of the vehicle,...
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