What is the chemistry behind how combustion engines work?
In: Chemistry
Oxygen is compressed in a tight space then it combusts and pushes the piston of the engine back which spins the shaft and gets whatever you need to move.
Chemically speaking, you are combining a hydrocarbon (fuel) and oxygen, producing CO2 and water vapour as byproducts
e.g. methane is the simplest:
CH4 +2O2 = CO2 + 2 H2O
the other thing it produces is (a lot of) heat, which has the effect of increasing the pressure of the resultant gas mixture, which is what’s used to drive the pistons
You generally have 2 types of combustion engines: spark or pressure. (gas or diesel)
Both turn explosive energy into mechanical energy by using the micro-explosions in the cylinders to push on the pistons. The pistons then further transfer that energy, leading to the movement in the engine that makes it go.
So back to the chemistry part.
Spark engines are pretty easy. Take a volatile substane that ignites with flame, mix with enough air to get a nice reaction, then use the spark to create the explosion.
Compression engines, on the other hand, don’t use sparks at all. Instead they take a fuel that ignites under high pressure, and compress it until it explodes.
The ideal gas law tell us that 1 mole of a substance occupies 24 liters of volume as a gas at room temperature and pressure. At higher temperatures, like an engine, it wants to spread out and take up more space.
A mole is the atomic weight of a substance in grams.
A mole of petrol is about 114 g, but with a density of 0.7 cm/g this will occupy about 80 cubic centimetres, or 0.08 litres.
Turning liquid petrol into gas suddenly increases the volume it wants to occupy by at least 300 times.
A petrol engine takes in liquid petrol mixed with oxygen and uses a spark to explode it while the piston is in the top position. Making the cylinder as small as possible.
This exploding petrol suddenly wants to occupy hundreds of times more space, creating huge pressure, shoving the piston out of the way to create the space it wants.
The other end of the piston is attached to a crank shaft that ends up spinning as a result, giving you a power output drive shaft, that also returns the piston to the top position. Repeat a few thousand times a minute.
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