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Comment by bee_rider | Hacker News Reader

On a completely tangential note, has anyone ever tried a hybrid that was like a diesel-electric train? The engine just charges the battery. IIRC it can be much simpler, no need for gearing at all, the engine just runs at a constant speed and the battery handles delivering variable amounts of power.

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Called a series hybrid, and it has been done. However gears are more efficient than a generator -> motor, and charging a battery adds even more losses. Thus if the engine is running anyway you are better off just mechanically connecting it to the wheels.

Trains don't do the above in large parts because the gears needed either wouldn't fit in the allowed space. (we may not be able to make them large enough either - that problem is solvable but may not be worth it)

I wonder if fuel efficiency just doesn't matter enough for trains, since they're already so efficient at turning motive energy into motion. Other costs may dominate.

Cost matter, but the dominate costs are important. The steam engine was more efficient than diesel when they scraped all the steam engines for diesel - steam engines need a lot more labor and so were more expensive despite using less fuel.

At that scale I think ICE engines preferably run at the same RPM continuously, and a gearbox and a clutch would be massive and require a lot of maintenance. The electric motor gives you the huge amount of torque you need to get a train going from stand-still. Not sure what actually tips the scale though.

This is correct or close too. The small steel on steel contact surface has so much less friction, that some conversion losses are fine in exchange for massive torque, and less moving parts (The diesel electrics replaced steam locomotives over maintenance cost/operation not pulling power).

The main thing for them is torque. Need a whole buttload of that to get it going. The efficiency of the train then comes from steel wheels on steel rails really. Electric motors is for the torque.

I have understood that idea as with some ships using similar systems is that this allows generators to run at optimal speed for efficiency which is rather tight band when you are looking for few of the last percentage.

With cars the speed range is much larger.

Series hybrids, where only electric motors drive the wheels, are becoming more common: Nissan now sells many models under their e-POWER brand.

Efficiency seems to match or exceed conventional hybrids in city driving, and only slightly less efficient for highway driving. And people like the instant torque and the smooth “EV like” driving feel.

I've got a Nissan Epower Kicks. I bought it exactly because the technology was different than standard Hybrid cars.

Here in Mexico there's no infra for fully electric cars, and they are still way too expensive.

The Nissan setup is pretty cool in that the generator is quite small, and the car doesn't need all the mechanical parts of an ICE car. It also gives us the range of a standard ICE car. So far it has been pretty good.

Series hybrids do have efficiency advantages in some situations -- for example, they can run in EV mode at any speed, and they can also have more freedom to run the engine at speeds independent of the vehicle speed, which can enable it to be programmed to prefer running it at speeds dictated by efficiency rather than by the drive wheel speed.

The place where they fall behind is at steady state on the highway -- but all of the series hybrid systems on the road have a solution for this problem too! They typically have clutch that engages a one-speed direct drive from the engine to the wheels. This skips the double-conversion losses at highway cruising. Then if you give it some gas to accelerate, the clutch disengages and you go back to full double-conversion again.

This was addressed explicitly in the video. It's far less efficient end-to-end, even though the gearing is theoretically simpler. Trains do it because diesel engines just can't produce the torque you need to move a train (at least, in the form factor of a locomotive), so they need to use electric motors.

He makes that argument but IMO it's not particularly well founded. He talks about his old Chevy Volt and guesses about the new Nissan series hybrids while ignoring Honda's current lineup of series hybrids. The Civic hybrid* meets or beats the EPA ratings on both the Corolla and Camry hybrids when on the same size wheels (18").

I agree in principal that there's efficiency to be gained by minimizing conversion losses, but Honda may be clawing that back with larger and more efficient motor-generators that only package well because no planetary gear set is required to connect everything.

* Honda hybrids do have either one or two clutches to mechanically connect the engine to the wheels at fixed ratios for highway cruising, but their city EPA numbers are still very competitive.

Rumor mill, but the Mazda RX9 may have their rotary engine powering the battery, which then delivers energy to the drivetrain.

"In that concept, the rotary engine functions as a generator to produce electricity for the battery and electric motor, rather than mechanically driving the wheels."

Excited to see if this becomes a reality

more: https://uscarcover.com/blog/2026-mazda-rx9-review-rotary-is-...

one of the primary arguments of the video is that ecvt is a strictly superior solution. (and that diesel electric exists for massive torque conversion in trains, but loses efficiency)

The Chevrolet Volt and Honda’s recent hybrids work this way. They are mechanically even more simple than Toyota’s drivetrain.

The engine doesn’t run at a constant speed though, it responds to the amount of electrical power needed.

And the BMW i3. It had a 2-stroke engine that allowed you to charge the battery.

However, in the USA, in order to get EV status, it was nerfed where it was only allowed to use its engine to charge the battery once you went below a 30% state of charge, and next to that, the fuel capacity was electronically limited.

A neighbor of mine had one, and the engine couldn't keep up with charging the battery to move the car on the freeway + running the AC, because it wasn't powerful enough.

In the EU version, this wasn't a problem, because you could set up the motor to run to maintain a charge (instead of only allowing it to run below 30%).

Edit: 30% state of charge, not 10%.

RAM and Ford are about to launch range extended EV pickups with 609 miles of Range and 14,000lbs of towing

https://expeditionportal.com/the-extended-range-ev-influx-is...

These are range extenders, and were used with the Fisker Karma, BMW i3 (as an option) and, most recently, the Mazda CX-30 with a Wankel engine.

The latter was sold in the UK/EU and was on the market for two years only.

Related: Honda's 2 Motor Hybrid System Explained. https://www.youtube.com/watch?v=-P_VChtMGK8

That is a series hybrid.

The Chevy Volt was one and the current Honda CRV is another. Both of them work mainly by the gas engine driving one of the electric motors as a generator while the other motor drives the vehicle. They have a simple eCVT transmission. However, both vehicles have a mode where they directly engage the engine to the transmission at highway speed cruising because that is more efficient.

Nissan has a series hybrid system that they have used in the Note that is only the series hybrid without the direct connect mode. That saves some money.

That's called a series hybrid: https://en.wikipedia.org/wiki/Hybrid_vehicle_drivetrain#Seri...

Ford is replacing their Lightning EV with an “EREV” gas generator.

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