Eli5: Why do automobiles operate with diesel or petrol but not with water?
In: Physics
Water is not a fuel.
In the context of fuel, you can think of water as being the unusable ash produced by burning hydrogen.
You can use electrolysis to break water apart into hydrogen and oxygen, but it takes more energy to do that then you will get back from burning them again. This is why we can’t just invent a car that uses a tank of water to make its own fuel.
You need something that is flammable. In a car engine, fuel vapor is injected into pistons. The vapor is exploded which forces the piston up and the piston on the opposite end down. These explosions happen rapidly pushing the pistons up and down. These pistons are connected to a shaft which spins and is connected to the axeles which spin the wheels.
Water can’t create these high pressured explosions.
There have been cars run on water in the past. The very first cars in fact were steam powered and there have been experimental cars which use a process called electrolysis to extract the hydrogen from water to use in a combustion engine, but these cars aren’t as practical and efficient as standard fossil fueled combustion cars. If you want to use a combustion engine it’s best to use something that’s highly flammable from the start.
Explaining like you’re five:
Water is stubbornly lazy and doesnt want to change using it’s own energy.
It only changes into other things when something else helps. Such as electricity helping the bonds between hydrogen and oxygen break. Doing all the work.
Other fuels are full of energy and only either needs a little nudge to work (for petrol), or works well under pressure (with diesel). They then release all that stored energy (by burning), change into a different form with a lower energy state.
Once upon a time they did… kinda. Steam powered engines use expanding water vapor to drive pistons, but the heat to turn the water to steam was normally a coal or wood fire in the boiler.
In the future, they may be… kinda. Hydrogen fuel cells have been developed that burn hydrogen and discharge water instead of noxious fumes, but the race is on between pure electric and hydrogen cells, and right now it’s looking like electric will win simply bc the infrastructure is already there.
It could be argued they are right now… kinda. The electricity for an electric car comes from somewhere, and if your electrical grid is powered by a dam… well then your car is powered by the stored potential energy of a massive amount of water.
Water cannot “burn” like traditional (fossil) fuels, but any hope of extracting energy from it at all, in some other way, can only be crushed by chemistry. A water molecule contains three atoms: an oxygen atom and two hydrogen atoms, which bond together like magnets. Breaking those bonds will always take more energy than you get back. A water molecule is very stable. The energy needed to separate the atoms is greater than what you get back. Plus there’s a more volatile problem: hydrogen is dangerously flammable. Without the right safety measures, things can go kaboom very fats.
Both diesel and petrol are combustible, which means that as a liquid they burn and as a gas they explode.
Car engines are called internal combustion engines (think explosion inside engine) because they take a fuel, like petrol, and make it to explode inside a small space.
When the fuel explodes it releases energy that moves parts of the engine and makes the wheels of the car turn.
Water isn’t combustible. It doesn’t burn or explode when it touches fire, so it won’t release energy, and won’t move the parts of the engine that turns the wheels.
-Water molecules are very very stable
-hydrogen is extremely dangerous
– even if you derived a system that splits up the oxygen and hydrogen and does end up producing energy? The entire energy required for the process would be far greater than the energy you produce for the water engine
To be a usefull fuel stuff has to be able to burn.
There is no chemical reaction that would release energy from water, as water is already a very stable molecule.
When you burn diesel or petrol one of the results is actually water. So running a car on water is like trying to run a campfire on ashes.
Vehicles with traditional engines use a chemical process called combustion to generate power.
They take a fuel molecule and react it with oxygen to release heat and generate pressure.
Now consider water – H2O. It already contains oxygen – it’s actually mostly oxygen by weight. It’s not combustible, it’s hydrogen that has already been combusted.
You can’t burn it again, so no more energy can be realistically extracted from a water molecule unless you have something much more dangerous than oxygen around to react it with.
Internal combustion engines use fuels that explode so that small explosion drives the pistons in the engine.
Water doesn’t readily expel its energy like that. The energy required to break the bonds of the water molecule are much higher than the energy it would release.
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