how does a gas engine convert heat to energy?

I have a class in school where we have to describe how a car converts heat into energy. i already have the firing sequence right, but i need to know how that makes energy.

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Nothing really makes energy. Rather there are processes, both natural and artificial, which convert energy, from one form to another.

The mechanism used in an internal combustion engine is exploding fuel, inside a cylinder, with a piston on top of it.

When the fuel burns, the chemical energy in the fuel is converted to heat energy.

Next the heat energy in the hot gases (i.e. burned fuel) causes these gasses to expand. The only way the gasses can expand is by pushing on the piston, and moving it. By this action, some of the heat energy in the hot gasses is converted to useful mechanical energy in the moving piston, and the temperature of the gases actually decrease somewhat as the gases do work on(i.e. give mechanical energy to) the piston.

Actually the heat energy in the hot gasses, can be thought of as kind of "disorganized" mechanical energy. Microscopically the gas molecules are moving, but moving in all directions, and constantly colliding with the walls of the cylinder. The sum of all these tiny microscopic collisions give pressure.


The gasses push against the piston, and the walls of the cylinder too, although the piston is the only part that's free to move.

That's the critical part: Only one wall of the box, the bottom of the piston, is allowed to move, so that's the part that does move.

I mean aside from the valves and stuff. Technically those are kind of "movable walls" too. But the valves stay shut during the power stroke, when the explosion happens. The power stroke is where the magic happens, the magic of turning heat energy into mechanical energy.

The same mechanism is used to propel bullets out of guns, and artillery shells out of, um, larger guns. Same thing. Chemical energy turns to heat. Then hot gasses do work on a bullet, pushing it down a cylinder.

There was line in that novel, "Fight Club",


something about a gun being a tool for focusing the energy of an explosion in one direction. This observation was profound.


Here are the basics. All internal combustion engines have this in common: Suck, squeeze, bang, blow. Nice and memorable. Suck: Intake of air fuel mixture. Squeeze: Compression of air fuel mixture. Bang: Ignition of air fuel mixture. Blow: The now burned gases need to be exhausted. Rinse and repeat.

Heat energy is created and converted in the Bang stage. The air fuel mixture burns, creating heat. Heat expands gases, forcing the piston down because it has nowhere else to go. The piston then pushes the connecting rod, which pushes the crank shaft. The crank shaft converts the linear motion of the piston and connecting rod into rotary motion.

seandogue2 years ago

It uses thermal gas expansion to drive a cam shaft via the piston. In other words, thermal gas expansion drives a piston piston which turns a shaft which drives a set of gears which then drive the wheels

BTW, heat is energy. You're simply converting one form of energy (thermal energy) into another (mechanical energy)

Vyger2 years ago

The text book for the class should have this spelled out in it. Read the book.

galvatron522 (author)  Vyger2 years ago

there is no text book, it's like shop.


Wired_Mist2 years ago

Heat is the Bi-Product

The combustion of the fuel causes the pressure in the cylinder to Rise. That in turn forces the Piston down and turns the crank shaft.

A steam engine is just about the only engine that is designed to convert Heat to Mechanical motion.

A controlled and harnessed explosion. I remember a show talking about different fuels that can be used. There was an engine developed that worked on coal dust. Another on grain dust, the same stuff that sometimes blows up grain elevators. They also used gun powder. The problem with those fuels is not that they don't work because they do, the problem to overcome is fuel delivery. Powders are slower to get into the cylinder and so slow down the top speeds of the engine. Modern high end engines use superchargers that push the fuel in under pressure because that is the performance limitation.

Heat IS energy ! Internal combustion engines, like steam engines though are LIMITED to the maximum amount of USABLE energy they can provide by something called the Carnot efficiency. The engine burns fuel (source of chemical energy), creates thermal energy and turns that into mechanical energy. The obvious point at which it happens in a four stroke engine is ?????