Usual disclaimer regarding the energy density of gasoline: current gasoline engines have an efficiency of 10-30% while EVs are around 90%.
So to reach similar kWh/g we're looking at ~3k Wh/kg
So to reach similar kWh/g we're looking at ~3k Wh/kg
Hybrids running on battery are about as efficient as an EV. When you enter the highway, they turn into ordinary ICE engines. If you use your prius exclusively for traffic in your neighborhood, you only use petrol to charge the battery, which is efficient and about as good as it gets with a hybrid. Unless you can plug it in of course. It won't use any petrol at all in that case.
I believe this is because the electric motor can handle the torque variation so the engine runs in a more consistent optimized torque band
Some of the Chinese plugin hybrids are starting to do this. Those basically are EVs with a generator bolted on to extend the range to something crazy like well over 1000 miles. These vehicles are far simpler to build and mechanically a lot simpler. And the generator is of course an optional extra if your battery is big enough. More like a safety blanket for the range anxious.
Edison trucks (a Canadian startup) is doing putting generators on their trucks. They've built and designed their own electrical logging truck designed for the extremes of that business. Pretty awesome vehicle. Effortless pulls huge loads over unpaved roads. Basically, it has no gears so it is super simple to operate and it drives like a car. A pretty sporty one even. The generator is just there for when they go out into the middle of nowhere where they can't charge the truck. It's purely a range extender.
If you think about it, you could just buy an off the shelf generator, shove it on a trailer and hook it up to your EV. If you have a EV pickup, you can just sacrifice some space in the back. This isn't that hard. You only need a few kw of output from the generator. A simple, small, and cheap one would be good enough.
The reason companies like Toyota are trying to sell you a much harder to maintain and much more complicated solution instead is not because they haven't thought about it but because they have a large sunken investment in ICE engine manufacturing that they want to milk for a bit longer.
The Mazda's rotary gets ~70-75HP if the internet is to be believed. But oh, unlike many PHEVs that have 30-ish miles of range it has 50, so it's got a decently larger battery and starts to get into the 'Where GP is right about plug-in hybrids being marketing fluff'.
My general understanding is that they are a very unpleasant experience in such a state despite whatever range advantages you are given. Because of the relative weight of the batteries to give even 100 miles of range, the small engine isn't that useful on the highway and on a steep hill even less so.
Can a typical EV battery charge while driving or would they need to be redesigned for this purpose?
I think this strategy works super well for electric semis.
I think it is being proposed for electric container shops as well.
Finally, I had hopes those ultra compact rotaries would get commercialized as rechargers for phevs and EV range extenders (I think there was a darpa project mentioned two years ago for a novel inside out rotary).
Honestly the emissions for a recharger that gets used maybe 5-15% of the miles driven by a phev aren't that big of a deal.
If it speeds hybrid and full electrification, and the engine runs in a pretty ideal rpm that is lower emission anyway, then civilization is coming out ahead.
Nissan sells this as their E-Power system Hybrid. The Chevy Volt plug-in hybrids were primarily series hybrids when in hybrid mode.
Running the gas engine to charge the battery and drive the wheels is not as efficient as just using a large EV battery but the trade offs in cost, weight, and range may make it worthwhile for now.
Not necessarily. A pure Series hybrid, at least so far, haven't been able to match up to the series-parallel setup Toyota and Ford is using. And part of the reason for that, is they typically have a 'direct drive' mode, such that the engine is directly connected to the wheels; at that point you can have far less energy lossage via the mechanical drivetrain compared to the loss of mechanical->electrical, even if that electrical was getting dumped straight into the electrical motor.
> If you think about it, you could just buy an off the shelf generator, shove it on a trailer and hook it up to your EV. If you have a EV pickup, you can just sacrifice some space in the back. This isn't that hard. You only need a few kw of output from the generator. A simple, small, and cheap one would be good enough.
The relative efficiency of your typical generator is probably not on the order of most car engines, I'd wager. Especially, again, going back to Toyota/Ford hybrid designs, the engine is can do an atkinson cycle which is more efficient (at the expense of larger engine size to power ratio.) Willing to bet most generators don't have nearly as much/good 'scrubbing' equipment for the exhaust compared to a modern car either.
> The reason companies like Toyota are trying to sell you a much harder to maintain and much more complicated solution instead is not because they haven't thought about it but because they have a large sunken investment in ICE engine manufacturing that they want to milk for a bit longer.
I do still agree with most of this, but TBH a series-hybrid setup (at least for the single speed Aisin units and Ford's HF transmissions to date) can be in many ways wayyy simpler than most modern cars. Lots of em don't even have direct injection, let alone turbochargers, and the gearbox itself is simpler than any modern automatic.
That being said, both the figures mentioned are to me a little bit optimistic.
I don't know about Teslas, but my fuel economy presents itself like this(figures are in litres per 100km):
-City driving: ~5 + ~100ml to bring the engine to working temperature. Checks out to 7 on a 7km drive and falling with distance.
-Highway, so maintaining real 120-140km/h (speed limit around here), 6.3-6.5. Absolute worst was 7.8 during a snowless -20°C night.
-Backroads doing 70-90km/h, average trip speed 50km/h, and here is where I think hybrids shine - 4.0-4.2.
-Hypermiling record: 3.7 as I was steadily rolling at 20-30km/h to a highway onramp a few kilometres away.
Overall fuel economy is nice, but what I like about this car the most is the ease of manoeuvring on the parking lot and very little vibration when the engine is running.
On the other hand with electrical cars that I rented and from what I heard from friends the real range was shorter by 20% or more.
[0] This uses the 3kWh/kg that was provided above and a density of gasoline of .75g/mL
units
You have: 0.7429 g/mL * 3 kWh/kg
You want: kWh/L
* 2.2287Modern cars with good high-compression engines have efficiencies over 40%.
A fuel cell with hydrocarbons could reach efficiencies of 60% or more.
So no lithium battery can reach volumic energies or specific energies comparable to what can be achieved with hydrocarbons.
The reason to use lithium rechargeable batteries is to obtain a better total efficiency of using energy, not the hope that it is possible to match the densities achievable with energy stored in hydrocarbons.
Among lithium rechargeable batteries, the lithium-air batteries should achieve the best energy per mass, perhaps also per volume.
Usually the weak point of metal-air batteries is the power per mass or the power per volume, because the reaction with air is slow, therefore the electrical current density in the electrodes is low, so to obtain a given amount of power requires great areas for the electrodes.
A fuel cell with hydrocarbons would have a slightly better efficiency than the best mobile thermal engines, e.g. of 60%, while the ideal energy per mass ratio is more than double for hydrocarbons in comparison with lithium-air batteries, so even with a better efficiency lithium can never match hydrocarbons in usable energy per mass, not even in lithium-air batteries.
The claim from the parent article is wrong and it is based on an incorrect method for computing the ideal energy per mass ratio for lithium-air batteries.
Can you elaborate for laypersons such as myself?
To do that, you need to expend roughly the same amount of energy that is needed to first liquify and then solidify the oxygen.
In fancy chemistry-speak it's called "entropic loss". You do gain some of that energy back when the battery is charged, as oxygen goes from a well-ordered solid state into the gaseous state. But it's not 100%.
That claim is based on dividing the stored energy by the mass of lithium, which is incorrect.
The product of the reaction, i.e. lithium oxide, is stored in the battery, so a lithium-air battery can never be lighter than the lithium oxide.
Because the mass of lithium oxide is what counts, the energy per mass of pure lithium, which is indeed not much less than for gasoline, must be divided by a factor that varies between 2.14 and 5.57, depending on the construction of the lithium-air battery.
The best value of 2.14 is when the discharged battery contains only Li2O. The worst value of 5.57 is when the discharged battery contains only lithium superoxide, LiO2.
In the parent article, they claim that their discharged battery contains mostly Li2O, with only small quantities of peroxide Li2O2 and superoxide LiO2, but the exact amounts of peroxide and superoxide have not been measured.
So when computing correctly the energy per mass ratio, for lithium-air batteries it is limited to a value less than half of that for hydrocarbons. In practice batteries need a lot of materials besides the active reactants, so the achievable energy per mass ratio will be several times lower.
The advantage of hydrocarbons, regardless whether they are used in living cells, thermal engines or fuel cells, is that their reaction products are eliminated into the atmosphere, so their mass does not matter. The energy per mass for carbon atoms in hydrocarbons and for lithium atoms in lithium metal is approximately the same, but with lithium it is impossible to neglect the mass of the oxidant, like with carbon, because the reaction products cannot be dumped outside.
So for any battery except for fuel cells, what counts is the sum of the masses of the reactants, e.g. lithium + oxygen in the best case, or e.g. zinc + manganese in the cheap non-rechargeable batteries. It is wrong to compute the minimum mass of a battery by using only the mass of one of the reactants, like in the parent article, instead of both masses.
This paper directly contradicts this claim with actual measurements of efficiency.
> The energy efficiency of the first cycle was 92.7%, and it gradually dropped to 87.7% after 1000 cycles.
Which is centered just above the 90% mark the person you are replying to gave.
For any kind of battery, there will be a power threshold over which a fuel tank + a turbo-generator will be smaller and lighter.
So a useful comparison would determine those power thresholds.