[tzs]

> Afterall, if you can make an ICE match an electrical power plant's carbon emissions, electric cars make very little sense in the short to mid term (until the marginal power is guaranteed to be sustainable)

Doesn't the need for a car engine to be light enough and small enough to work in the car mean that power plants will almost always be able to be cleaner?

[shagie]

You can make the car engine more efficient by making it be a small generator that is working at optimal RPMs rather than the variable RPMs driving the car itself.

Consider some of the various hybrid approaches.

Honda has the IMA. The insight is an ICE car with an "underpowered" gas motor that has an electric motor to assist it. If it runs out of gas, it's out of gas and doesn't move.

Toyota's Prius is an electric car with a gas motor that switches on when optimal. If you run out of gas in the Prius, the car will go for some further distance until the battery goes dead.

The Chevy volt is an electric car with a gas generator (and a trick that can shunt some power from the gas generator to driving the car). https://www.cnet.com/roadshow/news/chevy-volts-engine-more-t...

> When the battery is depleted, the range extender engine kicks in to generate electricity for the motor, as GM noted in its press materials. But when the battery is depleted and the car is running at 70 mph or above, the planetary gearset transmits additional motive force directly from the engine to the wheels.

... however, this also should take into account the efficiency of the power grid too.

https://www.epa.gov/egrid/power-profiler#/

A hybrid car in part of the grid that is heavily coal can be efficient in terms of CO2 than an electric car because it is burning gas more cleanly than the grid is burning coal.

[sbaiddn]

This is an excellent question but a massive one to answer. Probably the length of a scientific paper. It'd involve thermo, engineering economics, etc.

To keep it short, yes there is a return to scale, but it's a diminishing one. Gas turbines run at about 55% thermal efficiency. Large of small it matters little, that limit is set by the blade materials' melting temperature (which sets longevity).

Your car typically runs at about 30%, its efficiency partially offset by heat losses to the cylinder walls (ie the larger the better), but is mostly set by engineering decisions other than fuel efficiency (one big cylinder has less surface heat loss than four little guys but would be unbalanced)

But there are so many legacy design decisions in an ICE that no longer apply if we have hybrid drivetrains and ammonia/urea injection (to mitigate NOx from high compression).

The Prius challenged a lot (but not all) of these decisions and remains, in my opinion, the most revolutionary car of the past 50 years.

I really believe that an fossil fuel car engine can get an efficiency within the transmission losses of the best gas power plant. But even if ICE development were frozen, hybrids still make more sense, from a CO2 POV, than EVs