What is the function of a piston?

Answer 1

Original Answer:

A piston compresses gas and air causing tiny little explosions, which turn the turbines that run the wheels.

My Answer:

Um, no. There's no turbine in an internal combustion engine. Except in a turbocharger, and that's external anyway. And the explosion (one per cylinder per revolution) isn't really tiny.

The piston does compress the air and fuel. In a diesel, this compression causes enough heat to ignite the fuel. (See PV=NrT) In a gasoline engine, a spark plug is required to ignite the fuel/air mixture. This ignition causes a fairly large explosion which releases lots of energy and pushes the piston in the opposite direction, thus turning the crankshaft and flywheel and clutch/torque converter and etc. all the way to the tires on the road. The piston facilitates combustion and harnesses the energy from said combustion. (Unless you drive rotary! LOL) The piston also expels waste fumes from the cylinder. I'll simplify. In a four cycle engine, each piston does four jobs:

1. As the piston is drawn down (away from the head), it creates vacuum. During this stage, the intake valves remain open and fuel/air is drawn in from the intake manifold.

2. All valves close and the piston is driven up (toward the head) to compress the mixture. The spark plug ignites the mixture right at the beginning of stage...

3. The mixture explodes and the piston is forced down (away from the head) by the expansion of gas and heat. The energy from this expansion is translated into torque by the momentum of the moving piston.

4. The piston is pushed upward (toward the head) to evacuate the exhaust through the open exhaust valves and into the exhaust manifold or header pipes.

Bear in mind that each piston is doing only one of these jobs at a time. Imagine a four cylinder motor... At any given moment each cylinder is doing one of these jobs. More accurately, there are four jobs and four pistons, and none of the jobs are done more than once at a given instant. Your firing order will directly relate to which piston fires off its fuel at which particular instant. At any given, frozen instant in time, one piston is being pushed, one is drawing in, one is compressing, and one is exhausting. (in a 4cyl engine).

So keep in mind, each piston goes through the four cycles, in order, for each complete combustion process. Bear in mind, also, that one complete combustion process of each piston requires two complete rotations of the crankshaft. A piston, along with a connecting rod, will convert the force of an expanding gas into a rotational energy.