Car engine efficiency?

Question

On a answer it was stated that a electric car would need aprox 40HP to achieve 75MPH, where as normaly 80hp min in a petrol engine. Is this because the petrol engine is so inefficient?

Answer 1

an electric drive train is configured differently then a normal internal combustion motor. there is also the difference in gearing, weight, type of terrain that the car needs to cover. the 80 HP motor will not slow on hills as quickly as a 40 HP.  

Answer 2

Gas cars break down like this:

1/3 of the energy goes out the radiator as waste heat

1/3 of the energy goes out the exhaust pipe as waste heat

1/3 of the energy actually turns the crank shaft

Now subtract from that final third the power to turn the water pump, oil pump, alternator, power steering pump and air conditioning.  Then loose some more energy through friction losses in the drive train.  If the car has an Automatic Transmission (A/T) add a little bit more loss to turn the fluid pump and the energy loss in the torque converter while the car is stopped.  That constant push you feel while the A/T is in gear is lost energy exiting in the form of heat in the transmission fluid.  That's why A/T have oil coolers.

So only 20% to 25% of the total energy in that gallon of gas actually goes to pushing the car forward.  I've seen it as low as 15%.  See the waste?

While in an electric car you have very little waste heat to get rid of.  There is only 1 moving part on the motor.  Also electric motors are much more efficient at converting input energy to torque.  And torque is what pushes you down the road, not RPM or horsepower.