Where does the energy in a gallon of gasoline go when I drive my car?

Question

Energy only changes forms, so if I put in about 100,000,000 joules (about the energy in a gallon of gasoline) to driving my car for 20 minutes - where is that energy now? Some has been converted to heat, noise, and I guess air currents - but that doesn't seem to be enough to account for 100,000,000 joules - what am I missing? I haven't necessarily driven the car up a hill to create gravitational potential energy so where is the energy in the system after I have driven on a gallon of gas?

Answer 1

Three words: Internal. Combustion. Engine. Look it up.

Answer 2

You are right to wonder...

The internal combustion engine is a great waste and very inefficient.

It was wonderful when it replaced the horse and the supply of fuel was unlimited (relative to the demand).

Now it is like the old saying, "too much of a good thing..."!

Almost all the energy of the internal combustion engine goes to heat. After that mechanical energy to drive the moving parts and the car itself. Typically, about 20% of the energy of combustion drives the car.

See the link for a brief summary of this (under, "Ratings of Efficiency")

Answer 3

All of the energy eventually becomes heat.  That's what entropy is all about.  The heat goes out the exhaust pipe as hot gases, off of the radiator into the air, into the air from hot brake pads or shoes, heat in the tires from friction, even direct heat from air molecules striking the car as it moves.  Even though some of the energy causes the car to move, the car eventually stops because all that energy is lost as heat.

Answer 4

Good question.

It pretty much all goes to heat in the end.

About 70% of it goes directly to heat after burning.  This heat is mainly sent out the exhaust pipe, but some of it is pumped out of the radiator and some of it disperses to ambient air around the engine and exhaust system.

The remaining 30% goes into work (but that isn't the end of it).  The expanding gas of the burning gasoline drives the wheels.  This driving is only necessary to accelerate the car.  If there was no hill, and you were already at speed, and there was no air resistance, and no rolling friction, and no friction in the wheel bearings, then you would go forever at that speed and never need any more gasoline.

But there are all those friction.  At highway speeds air resistance is the biggest decelerator.  At lower speeds elevation change and accelerating back to speed after braking are big decelerators.

The energy not lost in the efficiency of the engine and the drive train goes into accelerating the car to get it moving or to keep it moving.  In the case of the elevation change, the kinetic energy is converted into potential energy.  In all the frictional losses, including air resistance, the energy is dissipated as a heat form.

Technically I suppose there are also some entropy term losses but I don't think they are significant for what you are asking about.

Mostly, its just friction of all kinds.

Answer 5

a joule isnt that big of an energy figure. as you said, more than half of it is wasted as heat, noise, and friction (air resistance, mechanical etc....), and the rest is used towards functional mechanical work. getting you from point a to point b.