Hacker News

points by mahboi 1 day ago | 0 comments

This would only make sense if the EV efficiency gain were due to lightness of the vehicle or better aerodynamics. It's definitely not the first, and don't imagine the second is a big factor but maybe I'm wrong. I'd even expect EV to help here because a heavy trailer takes more energy to get moving that can be partially recouped when regen-braking.

Is it just that these EV trucks don't have multiple gears to tune themselves for towing? Like how many EV cars are geared for city speeds but suffer on the highway. I know dual-motor ones have different ratios, but that's still 2 speeds vs 6-10. Edit: Could also be about engine tuning.

[–] porphyra 1 day ago

The second is in fact a big factor. The vast majority of a car's drag is aerodynamic drag since it grows quadratically with speed; rolling resistance and stuff are much smaller at highway speeds. In order to compensate for the tiny capacity of batteries, EVs try really hard to squeeze out better efficiency in aerodynamics and other sources. That's why EVs have flush door handles and stuff.

[–] mahboi 1 day ago

That would explain it if it's large. But if this matters so much, why don't the diesel trucks make all the same resistance/aerodynamics improvements? The only EV-specific advantage there is not needing air intake... again I have to say, intake drag doesn't sound huge but maybe I'm wrong.

[–] WALoeIII 1 day ago

The large fleet operators do care about this and do go out of their way to improve aero. Look at a UPS 18-wheeler and you'll see trailer skirts, no bug deflectors, window buffeters, etc.

Ironically, the private owner/operators are most often the ones installing accessories to make their trucks look cool and sacrificing immensely for it.

[–] pge 1 day ago

Vehicles have two primary components of resistance, air drag and rolling resistance. Rolling resistance is a function of weight, and air drag is a function of the frontal surface area (and to a lesser extent the total body surface area). For a small passenger car, rolling resistance is much lower than for a large truck, so they get much greater efficiency gains from reducing air drag. For a large truck, the rolling resistance is so much greater that the gains from reducing air drag are not large. That's why big tractor trailers don't have the same optimized aerodynamics that a tesla or a prius has.

[–] Mawr 1 day ago

1. No real market pressure — if you cared about fuel burn rate you wouldn't be looking into buying a truck in the first place. EVs on the other hand live or die by their battery range.

2. When optimizing, you want to start with the biggest problem and work your way down. ICEs are so laughably inefficient that improving aerodynamics just won't have that much of an impact.

[–] nicoburns 1 day ago

A large amount of the energy of an ICE is wasted as heat and noise. Some of that is a constant factor that isn't sensitive to load. So when increase load, it doesn't make as much of a difference to energy used.

By contract in EVs, much more of the energy is going towards moving the vehicle and therefore both weight and air resistance greatly affect the amount of energy use.

[–] mahboi 1 day ago

The heat and noise do scale with how much gasoline you burn. That's part of the whole ICE being less efficient than battery in terms of input chemical energy vs output mechanical energy. It cancels out when you're calculating regular vs towing range. The engine is the same efficiency either way.

[–] mahboi 1 day ago

Oh, it's not the same efficiency either way though, got some good explanations elsewhere

[–] thecompilr 1 day ago

You can’t recoup the braking energy when the trailers have their own brakes.

[–] mahboi 1 day ago

True that