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Comment by agumonkey | Hacker News Reader

I remember people saying that hybrid vehicle used ICE not to generate car scale torque, thus being build differently to improve the efficiency (mostly due to better control of thermal constraints ensuring better chemical reaction, higher conversion and less pollution). Do you know how true that is ?

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As of this year, and limited to three models exclusive to Japan: yes this happens.

The new 2023 model year Serena from Nissan uses a generator optimized engine in a series hybrid setup. The engine runs at a fixed rpm (except in extreme situations like accelerating a bunch on the highway going from 110 to 140) then it switches to a second higher rpm mode. I bought one, it's a lot of fun to manage the engines heat and battery room. My record is 38km per liter using local driving up and down our hilly community. Pretty incredible when you consider hills are the natural enemy to none hybrid millage.

The car accelerates like an EV. And with Japan's expensive electricity I'm paying only about double the millage cost of an EV. Despite the full size minivan costing us about 20k USD new.

Most hybrids, and all Toyota ones, are not using a special fixed rpm engines. They use a regular motor with different timings. The Nissan fixed rpm engine is actually less powerful than the electric motor powering the wheels. Thus at max acceleration you can deplete the battery faster than it is charged.

If we are just using it as a generator, a turbine engine makes more sense. Very efficient at fixed rpm. Many trains use that sort of hybrid setup. But if you put in enough battery to warm up a small proane fuel cell to warm up a larger propane fuel cell you could just fill up at the propane pump and still get battery efficiencies but with a lightening tank. And they think they have propane generation from environmental co2 over 90% efficient now too.

Turbine in a car is completely impractical. Some tanks, like M1 Abrams use a turbine and they are painfully loud and the exhaust is so hot it can literally set on fire anything too close to it. Imagine sitting in a traffic jam and lighting up a car behind you :)

oh right, I remember some guy mentioning fixed RPM helping a lot.

This is the “series hybrid” or “range extender” approach and the issue is that all of the engine’s power needs to be converted into electrical power and then back into mechanical power, even when running the traction motor directly off the generator output. This causes significant losses in efficiency which more or less cancel out the gains from being able to optimise the engine for a specific constant speed.

They might be referring to the Atkinson cycle engine, which is really just adjusting the valve timing. Lower power but higher efficiency.

https://www.autoweek.com/news/green-cars/a1832081/atkinson-c...

No, it means that you can leverage a more efficient part of an engine’s torque curve by supplementing with electric power at the low and high end (since motors have a flatter torque curve).

Not just that. It does require Atkinson cycle to reach the efficiencies they have. It just means that the compression stroke is shorter than the power stroke.

Basically with that cycle they can get massive compression ratios. As an example newer Toyotas are at 14:1 ratio with 41% thermal efficiency. That’s close to diesel territory on a gasoline car.

Naturally when you floor it it goes to Otto cycle and efficiency drops.

And, in the case of Toyotas eCVTs, keeping the ICE in the most-efficient-rpm sweet spot longer by allowing the ICE RPM to be pretty much independent of the wheel RPM. It's somewhat strange because RPM noise barely correlats with speed or acceleration anymore.

Also: start-stop-automatic for free ... I am still somewhat spooked when the motor just turns off going 70 km/h because the ECU decides that the ICE is not needed anymore.

Not true at all. Torque is basically just a number you can manipulate with gearing.

The Chevrolet Volt works like that.

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