Always wondered why it doesn’t make sense to use diesel/electric in large trucks. Assumed it’s because cost/complexity doesn’t out weight benefits, but not sure why. Thanks.
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Heat engines suck at producing torque. Really, the engine in your car or truck doesn’t have the balls to get your car to even move from a stop. It has to go through a transmission, RE: a torque multiplier, typically near 4:1 for first gear, AND THEN AGAIN with the differential or transaxle usually near 2:1.
AND EVEN THEN, the damn thing has a minimum speed, or it stalls. This means you need the ability to slip. This is done with either a friction plate like a clutch, or a torque converter – as in an automatic transmission, which is a hydraulic pump.
And then you’ve got to consider that engines produce optimum torque at a specific RPM. So how do you balance engine speed and torque with slip? The clutch is out, because you’ll burn it up long before you get a 2 mile long train moving. That means the train would need an automatic transmission.
And how many gears would the damn thing need? Remember the power band on that engine is very narrow. And how big would the transmission have to be, physically? I’ve heard that the US Navy has gearing between the engines and the props on their ships, and they’re so big, and so expensive, the Navy doesn’t actually own that part of their ship – they have a lease agreement for the gears with the ship builder. Insane! You’re talking about adding a gear box that’s going to be the size of an SUV to a freight train.
Compare that to an electric motor. You can run the diesel generator at optimum speed to produce electricity, and it can’t stall pulling the train. Electric motors produce 100% of their torque at 0 RPM. That’s damn useful for pulling a heavy train. You can omit the torque multiplier because you can more efficiently scale the electric motor itself.
There are efficiency concerns; You’re converting rotational energy to electrical energy back to rotational energy, whereas the diesel drives the wheels with rotational energy, end to end, you only suffer parasitic losses. But it’s safe to conclude that engineers smarter than us have done the numbers, and the electric strategy won out. If I recall my tidbit of train history I do have, I do believe diesel direct drives were tried in the early days, and they ran into all sorts of mechanical and engineering problems.
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