In today’s cars gasoline and diesel engines, what are the percent efficiencies when burning fuel?

Mr. USA questioned:

For model, a gallon of gasoline possesses 124,884 BTU of energy, what percentage is lost due to heat, friction and incomplete combustion? How about for diesel engines? Typical numbers delight. Be grateful you.

hows a car engine work in regard to fuel efficiency?

Rich questioned:

i watched a TV show days gone by where a driving lecturer was instruction how to get the best fuel efficiency out of a car. he confirmed that if your foot isn’t on the gas, then the injectors will be switched off and no fuel will be used? but if a car is idling it is subdue burning fuel right? so does the engine know the difference between idling when the car isn’t tender and when the car is in gear but tender below momentum and not life obsessed by its engine?