could cars run on water

[BrookeMarie]

No, cars cannot run on water alone. The idea is a persistent myth, often based on a misunderstanding of hydrogen technology. While water contains hydrogen, which is a potential fuel, a standard internal combustion engine cannot use H₂O directly. The concept typically involves electrolysis, a process that uses electrical energy to split water (H₂O) into hydrogen and oxygen gases. The hydrogen can then be used in a hydrogen fuel cell to generate electricity to power an electric motor. However, this process requires a significant external energy input to work, making it an energy storage method, not a source.

The fundamental issue is the First Law of Thermodynamics: energy cannot be created or destroyed, only converted. Splitting water consumes more energy than you can get back by burning the hydrogen. Using a car's battery to perform electrolysis while driving is inherently inefficient, like using a bucket with a hole in it. True hydrogen cars, like the Toyota Mirai, store pre-produced hydrogen in high-pressure tanks, avoiding the inefficiency of on-board electrolysis.

Historically, scams claiming "water-powered cars" have surfaced, often using slight-of-hand tricks like adding a metal fuel (e.g., aluminum) that reacts with water to produce hydrogen, but the metal is the actual fuel. For a consumer, converting a car to run on a water-electrolysis system is not feasible, reliable, or safe due to the risk of hydrogen explosions.

Technology	How It Relates to Water	Feasibility for Powering a Car	Key Challenge
Internal Combustion Engine	Cannot combust water.	Impossible.	Water in the fuel system causes severe engine damage.
On-Board Electrolysis	Splits water into H₂ and O₂.	Highly impractical and inefficient.	The process consumes more energy from the battery than the hydrogen produces.
Hydrogen Fuel Cell Vehicle	Uses hydrogen (not water) to create electricity; water is the only emission.	Commercially available but limited.	Requires a massive infrastructure for hydrogen production and refueling.
Adding Water-Methanol Injection	A small water mix is injected into the engine to cool combustion.	Possible as a performance enhancer, not a fuel.	Does not replace gasoline; only improves efficiency and power in high-performance engines.

The realistic path forward for water's role in transportation is not as a fuel, but as the harmless exhaust from hydrogen fuel cell vehicles, which represent a promising zero-emission technology.

[MacDaniel]

Nope, it's a sci-fi dream. My uncle was convinced he could retrofit his old truck with a "water kit" he found online. He spent a bunch of money and time, and it just never worked. The science doesn't add up. You can't get more energy out of water than you put into breaking it apart. It’s like expecting a battery to charge itself. Real hydrogen cars are cool, but they’re filling up with pure hydrogen at special stations, not just tapping into a garden hose.

[LeJohn]

As a concept, it's fascinating but physically implausible. Water molecules are very stable; breaking the bonds between hydrogen and oxygen demands a substantial energy input. Any system claiming to run a car on water alone is essentially proposing a perpetual motion machine, which violates core laws of physics. The energy required for electrolysis would always exceed the energy output from the hydrogen, resulting in a net energy loss. The term "water-powered car" is a misnomer; the energy comes from whatever source powered the electrolysis, like the electrical grid.

[AveryRose]

Think of it this way: water is ash, and gasoline is the wood. You can't re-burn ash to get another fire. Water is the "ash" leftover after hydrogen has burned. So trying to use it as a fuel source is backwards. The realistic technology is the hydrogen fuel cell, which does the opposite: it combines stored hydrogen with oxygen from the air to produce electricity, and the only thing coming out of the tailpipe is clean water vapor. That's the only legitimate way water is involved in powering a car—as the exhaust.

[SawyerMarie]

From a chemistry standpoint, water is an oxidized fuel. The hydrogen in it has already reacted with oxygen, releasing its energy. For it to be a fuel again, you must reduce it, which requires an energy source more potent than what you hope to get back. This makes it an energy carrier, not a source. While electrolysis powered by solar or wind energy could produce "green hydrogen" as a sustainable fuel, the process is done at an industrial scale, not inside your car's engine bay. The inefficiencies and safety concerns of on-board hydrogen generation make it impractical for personal vehicles.