Electric cars / petrol comparison

trendie

Well-known member
#1
Trying to work out the efficiency comparisons of petrol v electric.

Found the article below:
https://cleantechnica.com/2018/03/10/electric-car-myth-buster-efficiency/

the salient paragraphs:
".... Conventional gasoline vehicles only convert about 17%–21% of the energy stored in gasoline to power at the wheels.”
...
An electric motor typically is between 85% and 90% efficient.
...
In a recent post for Quora, Brian Feldman, a robotics expert and entrepreneur, offered this explanation: “Consider the Tesla Model S, which has an available 85 kWh battery and a 265 mile range. Consider a similar gas-powered car, which gets 35 mpg. Gasoline contains about 33 kWh of energy per gallon. The Tesla uses 320 Wh/mile of energy (85 kWh/265 miles). The gas powered car uses 940 Wh/mile of energy (33 kWh/35 miles). Once the energy is on board (not counting the efficiency of the power generation, oil refining, or charging), the Tesla is using only about a third as much energy as the comparable gasoline-powered car.”

Heres my problem with the posted paragraphs.

it states an electric car can travel 265miles on an 85kWh charge. (320Wh/mile)
and petrol does 35miles per 33kWh. (940 Wh/mile)

But doesnt that imply 100% energy extraction?

I mean, if you have an 85kWh battery, and it is 90% efficient, you can only travel (265 * 0.9) miles = approx 230 miles.

If a petrol engine is only 20% (I know it says 21%!) efficient, then 33kWh gets you 20% of 35 miles, which is 7 miles.

Have I lost my ability to do arithmetic?
 

cbrads

Well-known member
Mar 11, 2014
1,161
252
93
NW England
#2
I'd question the figures, the energy density of petrol is much higher than the energy density of a Lithium Ion Battery - 45.7 MJ/Kg vs 0.3MJ/Kg - so there's over 100 times as much energy per unit mass in petrol.
 

tomorton

Well-known member
Feb 28, 2002
7,088
921
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Exeter
#3
This might be interesting as a theoretical exercise.

Surely a practical way of comparing electric v's internal combustion would be hours of travel on the road per hour off the road. A Tesla needs to spend more time re-charging than it does travelling.
 

trendie

Well-known member
#4
I was trying to work out how efficient the EVs are.

I was wondering if EVs just displace the pollution to the power station.
That is, instead of burning petrol in the engine, and the direct connection of the fumes to the car exhaust, EVs just burn the petrol in a distant power station, to produce the electricity, which gets channeled down a wire to a clean, shiny electric socket.
Thus allowing the user the false impression of being clean, whereas in fact the fumes / pollution are pushed "over the horizon", and disconnected from the end-user.

This now seems far more complicated than I first thought.