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Discussion Starter #1 (Edited)
In sea going vessels it has for while been several ships that uses diesel electric propulsion. In short, the diesel engines are used solely to crank alternators that charges their large battery packs.

The propeller is turned by one or more electric motors.

Picture this kind of setup for tractor truck, max gross weight 80 000 lbs, but realistic combined weight more like 40-45 000 lbs. This tractor truck has special hauling 99% of the time, so we don't need to build for 80 000 lbs. The truck seldom goes full highway speed for more then a 50-100 miles and the rest of the time in slower speed, rugged terrain etc.

It seems Tesla 3 gets 4.1 miles per kwh. Lets assume the tractor truck is at least ten times less efficient. The diesel engine is v8 550 hp. Around 1350-1500 RPM engine runs the most efficient, uses less diesel and makes less noise. Of course for limited time engine RPM can increase upwards to 2500 RPM for shorter periods of time.

This project got three goals. Short, mid and long term goals.

  • Short term: Get things working for shorter trips for test purpose to learn more.
  • Mid term: Using the tractor truck for 2-300 miles trips at combines weight of 40-45 000 lbs - where the rear drive shaft is removed and rear wheels are all electric and front wheels can be driven by the diesel engine. Turning it into 6x6 in rough conditions. 6x6 usage will be short bursts only, when conditions require 6x6 to go anywhere.
  • Long term: A 100% green alternative without emission, purpose build to fit the users need, a 100% electric conversion with ability for 6x6, and a range of maximum 200 miles per charge
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I imagine the two rear axles will have between 800-1000 hp combined electric propulsion. No drive shaft. And no gearbox or clutch. The alternators should have enough charging power to constantly top up the battery pack when vehicle is used. Further in there will be a period when the diesel engine will make the truck FWD at will. That will be for testing purpose, with the torque from the electric powered two rear axles 6x6 might not be required but we will not know before we test it out. Chassis is fairly easy to change. We can add length if need be to accommodate battery packs, rather then stacking those up high behind the sleeper. Some room will be gained from ditching the drive shaft, clutch and gearbox.

  1. What are the limiting factor for charging the battery pack on the go from the diesel engine, the chosen alternators or the power of the diesel engine?
  2. What kind of alternators would one have to use to get a useful vehicle?
  3. What capacity battery pack should I use for the tractor truck? Remember, the battery pack will be the reserve the electrical motors tap into, the battery pack will be charged when stationary and charged all the time when vehicle is moving, by the alternators connected to the diesel engine.
 

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In sea going vessels it has for while been several ships that uses diesel electric propulsion. In short, the diesel engines are used solely to crank alternators that charges their large battery packs.

The propeller is turned by one or more electric motors.
Ships rarely have battery capacity, although some do now. Without a battery, they work just like rail locomotives and heavy mining trucks with similar diesel-electric drive systems.

With a battery, this is a hybrid powertrain of the series type. That's commercially available, although typically only used in buses.

A series hybrid is usually too expensive and not efficient enough to make sense, especially in a highway vehicle.

The diesel engine is v8 550 hp. Around 1350-1500 RPM engine runs the most efficient, uses less diesel and makes less noise. Of course for limited time engine RPM can increase upwards to 2500 RPM for shorter periods of time.
A substantial advantage of a series hybrid is that the engine speed can be whatever is ideal for the engine. It only makes sense to run the engine at the speed which provides best efficiency for the current power output required.

Just because I'm curious... what engine is being used? Large truck diesels are usually inline-six configuration, not V8.

  • Mid term: Using the tractor truck for 2-300 miles trips at combines weight of 40-45 000 lbs - where the rear drive shaft is removed and rear wheels are all electric and front wheels can be driven by the diesel engine. Turning it into 6x6 in rough conditions. 6x6 usage will be short bursts only, when conditions require 6x6 to go anywhere.
Why would you attempt to drive one axle mechanically? That would require adding an entire transmission. Why not use electric drive for all axles?

I imagine the two rear axles will have between 800-1000 hp combined electric propulsion. No drive shaft. And no gearbox or clutch.
Why so much power? Tesla announced a lot of power for their "Semi", but that's just to get attention from people who might buy Tesla stock but know nothing about commercial trucking.

And you do need a gearbox per motor to reduce the electric motor speed to the axle speed. It doesn't need to have multiple ratios.

The alternators should have enough charging power to constantly top up the battery pack when vehicle is used.
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  1. What are the limiting factor for charging the battery pack on the go from the diesel engine, the chosen alternators or the power of the diesel engine?
  2. What kind of alternators would one have to use to get a useful vehicle?
Since the alternator (and you should probably call it a generator, because alternating current output is not desired) exists to convert engine power to electrical power, sizing the generator is entirely about engine output power and speed. This assumes that you get rid of the idea of a mechanical drive to one axle, which is just an unnecessary complication. Obviously both the engine power and the generator capacity are limits - whichever one is lower is the limitation.

What do you mean by "kind" of alternator (generator)? Almost any current design would use a 3-phase permanent magnet synchronous machine, but I don't know if you're asking about type of machine, sizing, configuration (radial or axial flux), specific brand...

Is there a reason to consider multiple generators? Just that you can't get one big enough? Getting components in a small quantity for a project like this will be a challenge.
 

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I've been thinking more about this idea of driving just the front axle mechanically. Arranging a transmission and driveline for just occasional and perhaps temporary use doesn't seem worthwhile, and driving the front axle is particularly awkward. So I have a suggestion to consider:
Drive the leading axle of the rear set (also known as the intermediate axle) with a conventional transmission and shaft, and drive the other axles electrically as a parallel hybrid.

The steps could be
- conventional operation as a 6X2 until the electric drive works (or as a backup of it fails)
- add electric drive to the rearmost axle, perhaps with the generator on a PTO port, as a parallel hybrid with the mechanical drive as the primary for efficiency; the electric drive would assist traction (as a 6X4), enable regenerative braking, and allow power level smoothing.
- extend the electric system to the front axle to make it a 6X6 for traction as required, and improve regenerative braking
- possibly replace the engine, transmission, and mechanically driven axle with more battery capacity and an electrically driven axle

I don't see much sense in taking the final step to a fully battery-electric vehicle, whatever steps are ahead of it. The design would be compromised by the need to accommodate the engine (and transmission, fuel, exhaust, etc) in earlier stages, and all of the effort of making a complex hybrid system work would be wasted. It would make more sense to me to just start fresh with another truck for the electric version... or just buy one, since they're already available.
 

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Fundementally, series hybrids are more expensive and less reliable (due to the complexity and the fact we're not professional auto engineers) than the base ICE vehicle.

Also, taking a system with two energy conversions (chemical to heat, heat to mechanical) and adding another two (mechanical to electrical, electrical to mechanical) plus potentially two more for the battery storage (electrical to chemical and chemical to electrical)... every conversion is less than 100% efficient and it's unlikely you'll be able to re-tune the ICE to gain enough efficiency in the original two conversions to compensate for the losses in the 2-4 new conversions.

Since your vehicle is a truck, I suspect you may do enough milage for a slight improvement to repay within the life of the vehicle, but most haulage operators wouldn't risk the downtime, since time is money.
 

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Discussion Starter #5
Great responses.

Ok, so what you say is to drop the battery pack and just have the diesel engine turning the generator?

The idea for having the front axle driven by the diesel engine was I guess an attempt to keep cost down. As that would save on motor and controller, and possible also on the generator(s). After reading your comments I see that is not the way to go, and will keep all axles turned by electric motors.

The amount of power was loosely based on a kit that is available from a stranded project. Because that project never got anywhere we can get motors and controllers for cheap.

The gear ratio will be locked. Single gear reduction from the motor to the axle. Motor RPM will take care of the rest.

Regarding the "what kind of generator", it was meant as where should I look for generators, and what type of generators will work for this project.

@brian very nice. For sure that is a more useful approach, keeping the rear drive until the diesel electric is working 100%. Also makes sense what you say about the final step, 100% electric. There are already trucks out there, and more will come over the next few years. So probably smartest to keep truck diesel electric.

Thanks for the replies, valuable all of them.
 

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Ok, so what you say is to drop the battery pack and just have the diesel engine turning the generator?
No, sorry, my comment apparently wasn't clear. I was just saying that typical diesel-electric ships (and locomotives) don't have batteries.

A system like this with no battery is just an inefficient continuously variable transmission, and in a road-going truck there is no point in that. To be a useful hybrid it needs energy storage.
 
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