Energy in Gasoline
Energy in Gasoline
The Department of Energy [DOE] web page says
that there are 124262 BTU or 36.4 kWh of energy
in a gallon of gasoline. They also say that 1 gallon
of diesel has 138691 BTU or 40.6 kWh of energy.
I met a petroleum engineer and asked him how
much energy was in a gallon of California gasoline
and he said about 107000 BTU depending on the
time of year. California gasoline is 10% ethanol
which has much less BTU/gallon then gasoline.
107000 BTU would be about 31.4 kWh.
A long time ago I was told that a gasoline engine
was about 20% efficient in city driving. I was told
that a diesel engine was about 30% efficient and
that a turbo-diesel could approach 40% efficiency.
A DC motor and controller in an electric car is
about 85% efficient. An AC motor and controller
can approach 95% efficiency. In addition, the
voltage needed to charge a batter is higher then
what the battery provides. The battery
charge/discharge cycle is about 80% efficient.
It takes almost 20 kW from the battery to keep a
small electric vehicle going at 60 mph on a level
road. That would be 20 kWh for 60 miles or 3
miles per kWh. That would be 25 kWh from the
wall to deliver 17 kWh to the wheels
To go 60 miles in an hour would require:
(17/31.4)/20% = 2.71 gal CA gasoline
(17/36.4)/20% = 2.34 gal AZ gasoline
(17/40.6)/30% = 1.40 gal Diesel
(17/40.6)/40% = 1.05 gal Diesel - Turbo
In other words:
60/20 = 3 miles / kWh EV
60/2.71 = 22.1 mpg CA gasoline
60/2.34 = 25.6 mpg AZ gasoline
60/1.40 = 42.8 mpg diesel
60/1.05 = 57.1 mpg turbo diesel
At a steady 60 mph on level road in a typical small
car that can hold 4 passengers and luggage.
Note that the calculations are a rough
“back of envelope” or “ball park” since the
power an actual vehicle requires depends
on weight, aerodynamics, tire rolling
resistance, driver ability, etc.
If the mpg of a vehicle is known, this
data can be used to estimate the kWh
needed in a gasoline to electric conversion.