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> They said the Oxis battery would provide “in excess” of 500 Wh/kg, a number which appears to apply to the individual cells, rather than the battery pack, with

> They said the Oxis battery would provide “in excess” of 500 Wh/kg, a number which appears to apply to the individual cells, rather than the battery pack, with all its packaging, power electronics, and other paraphernalia. For comparison the energy density of gasoline is 12,200 Wh/kg https://xtronics.com/wiki/Energy_density.html https://xtronics.com/wiki/Energy_density.html

scotchmi_st 7y ago
Although it's more complicated than that, because IC engines are only have about 15% efficiency, whereas electric motors are up at about 80%.
- nradov 7y ago
  Real world efficiency for modern gasoline engines is closer to 20%. In hybrid applications they can get up to 35%.
  - djaychela 7y ago
    >In hybrid applications they can get up to 35%. But surely that is largely due to the electric element of the combined powertrain? The ICE part hasn't magically become more efficent?
    - tasty_freeze 7y ago
      The whole point of a hybrid is to run the ICE only in its most efficient region. At low loads the electric battery drives the car. If the battery gets too low, then the ICE runs, but rather than running at a low (and inefficient) load, it runs at its sweet spot and the excess power it generates charges the battery. When the demand is high, rather than running the ICE hard, it is kept at its sweet spot and the electric motor boosts power instead. So yes, the ICE part is more efficient in a hybrid, though it isn't magic.
    - aidenn0 7y ago
      The ICE is more efficient because the design constraints have changed. Here's a single example with the Prius: 1. The intake valve stays open for part of the compression stroke. 2. (1) means that the compression ratio for the compression stroke is lower than the compression ratio for the power stroke. 3. The efficiency of the engine is limited by the compression ratio of the power stroke 4. The compression ratio of the compression stroke is limited by engine knocking (if you compress a mixture of fuel and air too much, it will spontaneously combust) 5. So this engine can be made more efficient than an engine that closes the valve for the entire compression stroke 6. Some (non-hybrid) engines have variable valve timing and can do (1) some of the time, to a small extent. 7. Doing (1) to a larger extent makes the low-RPM torque very poor. 8. Electric motors have excellent low-RPM torque, so (7) is compensated for by having an electric motor run at low speeds.
    - rtkwe 7y ago
      Somewhat, by putting an intermediate step between the gas/diesel engine and the work the engine can be run more efficiently as well, instead of having to speed up and down at the whims of the driver it can run at a more steady and constant pace and the electric engine also allows regenerative brakes to be used which can recapture a lot of the energy lost in a normal diving situation during braking.
- userbinator 7y ago
  https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption#Examples_of_values_of_BSFC_for_shaft_engines https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... Over 30% efficiency for gasoline, and over 50% for diesels.
bretpiatt 7y ago
You cannot look at the energy density of the fuel by itself. Electric motors are lighter than ICE engines per HP and torque unit. Emrax 268 (electric engine designed for aircraft) - 6.12 HP/lb GE 90-115B (jet engine used in 777) - 6.10 HP/lb Junkers Jumo 205A (diesel engine used in Ju 86C-1) - 0.66 HP/lb Electric also scale down better so for light weight UAV a higher energy density battery will open up a bunch of use cases.
deleted 7y ago
[deleted]
korethr 7y ago
Absent notes to the contrary, I'm going to assume that's just the raw energy density. However, by my understanding the typical gasoline engine in a passenger car has an efficiency of something in the low-to-mid-30s percent. So, assuming 33%, that's 4026 Wh/kg of actually-useful energy density, the rest of the energy wasted as exhaust heat. Looking things up, I see different numbers for electric motor efficiency, ranging from the low 70s to the low 90s percent. As an overly simplified example, let's assume an electric motor in a car has an efficiency of 80% (in reality, they may be better). At that efficiency, a battery pack only has to get to 5032.5 Wh/kg to achieve the same practical energy density as gasoline, less than half the actual raw energy density. That is probably an easier number to reach than trying to achieve the same raw energy density of gasoline. Nobody has made this argument in this thread (yet, as I type this), but I've seen it made before and made it myself. Yes, at those energy densities, batteries are highly dangerous if something goes wrong (like a crash) and they release all their energy at once, but so is gasoline. At practical ranges, you're potentially sitting on a pile of high-explosives either way. But, I'd hope a 5032.5 Wh/kg battery is easier to make safer in a crash than 12,200 Wh/kg gasoline is.
- ProfessorLayton 7y ago
  It's worth noting that the latest generation of the Prius has reached 40% thermal efficiency [1] [1] https://newsroom.toyota.eu/new-2019-toyota-prius-with-intelligent-all-wheel-drive/ https://newsroom.toyota.eu/new-2019-toyota-prius-with-intell...
  - DesiLurker 7y ago
    I've also read good things about israeli aquaris engine. they've optimized it to brim for electricity generation for series EV applications & made it quite light/cheap. we'll see.
- the8472 7y ago
  On the other hand a plane loses weight as it burns fuel, this isn't true for batteries.
ogre_codes 7y ago
That's looking at the energy density of the fuel itself, not counting the weight of the engine, several gallons of oil, air filters, fuel pumps, etc etc. The weight of equipment required to extract that energy from gasoline is a lot higher than the weight of an electric motor.
Already__Taken 7y ago
90something% efficient electric motors vs 40-60% efficient gas engines however. If you can make up in other areas like drag in the design because of the different power plant it's not such a mental gap to close.

scotchmi_st 7y ago

Although it's more complicated than that, because IC engines are only have about 15% efficiency, whereas electric motors are up at about 80%.

nradov 7y ago
Real world efficiency for modern gasoline engines is closer to 20%. In hybrid applications they can get up to 35%.
- djaychela 7y ago
  >In hybrid applications they can get up to 35%. But surely that is largely due to the electric element of the combined powertrain? The ICE part hasn't magically become more efficent?
  - tasty_freeze 7y ago
    The whole point of a hybrid is to run the ICE only in its most efficient region. At low loads the electric battery drives the car. If the battery gets too low, then the ICE runs, but rather than running at a low (and inefficient) load, it runs at its sweet spot and the excess power it generates charges the battery. When the demand is high, rather than running the ICE hard, it is kept at its sweet spot and the electric motor boosts power instead. So yes, the ICE part is more efficient in a hybrid, though it isn't magic.
  - aidenn0 7y ago
    The ICE is more efficient because the design constraints have changed. Here's a single example with the Prius: 1. The intake valve stays open for part of the compression stroke. 2. (1) means that the compression ratio for the compression stroke is lower than the compression ratio for the power stroke. 3. The efficiency of the engine is limited by the compression ratio of the power stroke 4. The compression ratio of the compression stroke is limited by engine knocking (if you compress a mixture of fuel and air too much, it will spontaneously combust) 5. So this engine can be made more efficient than an engine that closes the valve for the entire compression stroke 6. Some (non-hybrid) engines have variable valve timing and can do (1) some of the time, to a small extent. 7. Doing (1) to a larger extent makes the low-RPM torque very poor. 8. Electric motors have excellent low-RPM torque, so (7) is compensated for by having an electric motor run at low speeds.
  - rtkwe 7y ago
    Somewhat, by putting an intermediate step between the gas/diesel engine and the work the engine can be run more efficiently as well, instead of having to speed up and down at the whims of the driver it can run at a more steady and constant pace and the electric engine also allows regenerative brakes to be used which can recapture a lot of the energy lost in a normal diving situation during braking.
userbinator 7y ago
https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption#Examples_of_values_of_BSFC_for_shaft_engines https://en.wikipedia.org/wiki/Brake-specific_fuel_consumptio... Over 30% efficiency for gasoline, and over 50% for diesels.

bretpiatt 7y ago

You cannot look at the energy density of the fuel by itself. Electric motors are lighter than ICE engines per HP and torque unit. Emrax 268 (electric engine designed for aircraft) - 6.12 HP/lb GE 90-115B (jet engine used in 777) - 6.10 HP/lb Junkers Jumo 205A (diesel engine used in Ju 86C-1) - 0.66 HP/lb Electric also scale down better so for light weight UAV a higher energy density battery will open up a bunch of use cases.

deleted 7y ago

[deleted]

korethr 7y ago

Absent notes to the contrary, I'm going to assume that's just the raw energy density. However, by my understanding the typical gasoline engine in a passenger car has an efficiency of something in the low-to-mid-30s percent. So, assuming 33%, that's 4026 Wh/kg of actually-useful energy density, the rest of the energy wasted as exhaust heat. Looking things up, I see different numbers for electric motor efficiency, ranging from the low 70s to the low 90s percent. As an overly simplified example, let's assume an electric motor in a car has an efficiency of 80% (in reality, they may be better). At that efficiency, a battery pack only has to get to 5032.5 Wh/kg to achieve the same practical energy density as gasoline, less than half the actual raw energy density. That is probably an easier number to reach than trying to achieve the same raw energy density of gasoline. Nobody has made this argument in this thread (yet, as I type this), but I've seen it made before and made it myself. Yes, at those energy densities, batteries are highly dangerous if something goes wrong (like a crash) and they release all their energy at once, but so is gasoline. At practical ranges, you're potentially sitting on a pile of high-explosives either way. But, I'd hope a 5032.5 Wh/kg battery is easier to make safer in a crash than 12,200 Wh/kg gasoline is.

ProfessorLayton 7y ago
It's worth noting that the latest generation of the Prius has reached 40% thermal efficiency [1] [1] https://newsroom.toyota.eu/new-2019-toyota-prius-with-intelligent-all-wheel-drive/ https://newsroom.toyota.eu/new-2019-toyota-prius-with-intell...
- DesiLurker 7y ago
  I've also read good things about israeli aquaris engine. they've optimized it to brim for electricity generation for series EV applications & made it quite light/cheap. we'll see.
the8472 7y ago
On the other hand a plane loses weight as it burns fuel, this isn't true for batteries.

ogre_codes 7y ago

That's looking at the energy density of the fuel itself, not counting the weight of the engine, several gallons of oil, air filters, fuel pumps, etc etc. The weight of equipment required to extract that energy from gasoline is a lot higher than the weight of an electric motor.

Already__Taken 7y ago

90something% efficient electric motors vs 40-60% efficient gas engines however. If you can make up in other areas like drag in the design because of the different power plant it's not such a mental gap to close.