When is a hybrid a hybrid?

[IamIan]

Originally Posted by jamesqf

That would give it two distinct power paths, so it'd be a hybrid.

Even though all the energy still goes through the same electric drive motor in either mode?

Even if the source of that electrical energy is just another diesel generator?

Even if the source of that electrical energy is just another train braking?

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Originally Posted by dcb

No, because you can add a "granny gear" to a regular transmission, with a lot greater transmission efficiency than driving a generator driving a motor. The inefficiency of such a setup will marginalize/negate your ability to downsize the engine.

Something else occurred to me this morning ...

The GE Dash 9-44CW Diesel - Electric ( an older design ) produces Starting torque of up to 142,000 lbf from it's 4,400 HP electric drive motor.

That seems like a very tall order to me for a mechanically connected Diesel engine ... especially if we insist on not up-sizing the diesel engine any from its roll as a generator engine ... sense the Diesel literally has zero torque at zero RPMs... the initial breaking of rest mass inertia of the entire locomotive comes from a build of of rotating mass being connected to the stationary vehicle and all the mass of the load.

The clutch , flywheel , gear ratios , massive transmission components , enormous complexity of such a massive complex transmission ... that not only has to pull out from zero , but also shift gears smoothly and accelerate up to full speeds.

A mechanical transmission capable of performing this task ... I would anticipate not only being more expensive than the electric drive motor option ... but would also be much less reliable in the operating life of a locomotive.

Just like automotive engines that are mechanically connected the diesel engine such a design would no longer be operating the engine at it's peak efficiency , and the average operating efficiency of the diesel engine itself would drop considerably from the varying loads.

[dcb]

On the large end of things, it seems the norm is to use a diesel electric scheme for trains (or diesel hydraulic for mobile launchpads). Presumably for the reasons you cite, i.e. being able to make torque to get the thing moving and not break stuff (and still the electric/hydraulic motors need gearing). But they also need large engines to overcome the additional losses as well, or live with lower top speed, they don't "enable" a smaller engine per-se.

Moving something vs meeting equivalent acceleration and top speed parameters are not the same thing. If you have a 20hp motor and it can get your scooter up to 75mph with a gearbox, it will be lucky to do 60mph with a hydrostatic transmission, so you need to add a LARGER engine to get back to 75mph. An electric transmission (generator drives motor) like on a train would likely have more slippage and losses, and each half (generator,motor) has different peak efficiency parameters (load and rpm) most of the time.

The gearbox allows you to stay fairly close to peak engine bsfc without constant conversion losses, for vehicles suitably sized for a gear box. If you look at a somewhat typical bsfc map (i.e this saturn) you can see that you can stay within the 250g/kwh peak island from below 1500 rpm to above 3500 rpm. And the transmission does NOT drop to 1500 when you upshift from 3500.

As well with no batteries to charge, heating resistors just to run your diesel near peak bsfc is a losing proposition. I am having trouble thinking of a scenario where it would be better to increase the power and fuel demands of the engine to make waste heat and somehow expect that to use less fuel, given typical bsfc maps.

And charging/discharging batteries is not an efficiency free lunch either.

[jamesqf]

Originally Posted by IamIan

Even though all the energy still goes through the same electric drive motor in either mode?

Yes. Think about the way the power flows... And even in a Prius, the power from electric motor & IC engine join at the transmission, and is carried by the same driveshafts to the wheels.

Even if the source of that electrical energy is just another diesel generator?

Is your plug-in Prius still a hybrid if you live off-the-grid, and charge the batteries from a gasoline generator?

Even if the source of that electrical energy is just another train braking?

Whyever not? That's called efficiency - and of course it's fungible. The energy from one train braking goes into the grid, and gets split up among the myriad things the grid's running at any given instant...