The truth about water injection in modern engines - Auto Expert John Cadogan

He is pretty good this guy - but he fucks up here and there...

For Instance:

"Question: On some jet engines there is an installation that is used for spraying water into the air intake for greater thrust during take-off, usually when the airplane is loaded rather heavily. What is the reason for it working? Is it the energy released from the water splitting and burning up alongside the jet fuel, or a greater expansion of water when turning from liquid to gas state than the fuel has? - Dennis S

Water injection does work for internal combustion engines - but comments like this make me wonder if they still teach science, frankly.

“Is the energy released from water splitting and burning up alongside jet fuel…” I despair for the future of humanity.

Water does not split up and burn. In an engine, or anywhere else. When your house is on fire and the fire brigade arrive, they spray water on the house specifically because water can’t burn any more. Water is already burnt. Water is not fuel. Water is exhaust.

Anything you can burn: gasoline, diesel, coal, wood, paper … one of the products is water.

And yet - water injection can yield a power gain in an internal combustion engine.
See, to an engineer or a scientist, water really is an amazing material. It really is the Swiss army knife of thermal shock absorption. It’s brilliant for removing heat and transporting it.

We sweat because evaporative cooling is an incredibly efficient mechanism for maintaining our core temperature. A small amount of water evaporating over our skin absorbs an incredible amount of heat…

...the dark side of which is: if you get wet in the bush and it’s cold, and there’s wind, and you can’t get dry, evaporative cooling is powerful enough to kill you.

Just to put some numbers on that, when liquid water absorbs heat, it pulls 4.2 kilojoules of heat out of the environment for every kilo that rises just on degree C. And when it changes phase, from liquid to vapour it absorbs 2.3 million joules for every kilo.

(A kilo of water is a litre - a bit over 1.5 Imperial pints - if you’re not from around here.)
If you can use air to supply the heat to warm up the water and evaporate it, the air loses the heat that the water gains. The air gets colder. It shrinks. So the same mass of air takes up less space.

And that’s good because there’s a finite amount of space inside your engine. Getting more air in there means you can burn more fuel. And that means you can make more power. And that means you can go faster than the other guy, in a competitive sense, which is always kinda nice ona racetrack.

Just putting some numbers on this. If you’ve got air compressed by a turbocharger, on the way into an engine, let’s say it’s at 60 degrees C - after the intercooler.

If you can do some latent heat of vaporization water evaporating applied science voodoo and suck another 20 degrees C out of the temperature…

... that’s … ahhh … ballpark, about 10 per cent temperature reduction (absolute temperature). So that should give you 10 per cent more air for any given volume, allowing you to burn 10 per cent more fuel, delivering 10 per cent more power.

And this is why engines have mass airflow sensors (MAF sensors). So they can tell the injectors how much fuel to deliver. Therefore, water injection is a hack that allows you to pump up the mass of air going into an engine.

So, how do you take advantage of this, in practice? You could inject water into the inlet airway. It would vaporise and increase the density of the air. But you’d need to arrange that so it vaporised ahead of the MAF sensor, obviously.

And you really would not want your water delivery system to malfunction in some unintended way, because if you tsunami the water in by mistake, you could catastrophically destroy your engine, because water is incompressible...

A safer alternative on a turbocharged engine is simply to spray water onto the exterior of the intercooler. Just increase the chill factor there. This is a dead easy mod - you’d just need a couple of garden irrigation sprayers, some matching tube, an old windscreen washer pump motor and tank from a wrecker, and a switch for the dash.

Obviously this works, because that’s basically what Subaru does, on the most powerful WRX STI engine ever - the S209. (That’s the one we don’t get here in Shitsville - it’s north American only.)
Subaru puts a 3.4-litre tank in the boot and a paddle for the driver behind the steering wheel, allowing the driver to activate the intercooler spray when a little extra urge is required. The effect is pretty immediate, and the engine adapts up in real time, because it just keeps listening to the MAF sensor."


You see when water is heated, it turns to steam - and steam has drops of water in it - it's called wet steam. So to stop the water droplets cutting through the valves, turbines and pipes, they heat it up to 400*C and it becomes dry steam....

But if you heat it past 450*C - the water starts to split up, creating oxygen and hydrogen - and that soaks an awful lot of heat to do this in a fire, but yes the oxygen and hydrogen do add fuel to the fire...

The main reason it's used in piston engines is that an engine will run happily at a certain air and fuel ratio - at a certain pressure - think turbo charging. BUT provided the engine is strong enough, you can FORCE way more air and fuel into the engine and make it produce enormous amounts of power - but this creates way more heat.

The injection of water and alcohol, was done in WW2 fighter aircraft for emergency boost - to run it way too hard for 3 minutes, to get the hell out of there etc., or have massive climb rates etc.

The alcohol (about 50%) was only added to the water to stop it from freezing at altitude. It's not an ideal coolant at most ambient temperatures...

IN jet engines on take off, some of the military ones, use water and alcohol injection to the flame holders, to cool the engine down, as on full power at ground level, there is an awful lot of fuel being burned and it's hot enough to melt the exhaust section of the turbine...

Jet engines usually run slightly lean and somewhat cooler... and they are speed controlled only by the amount of fuel being continuously burned.

So by turning the engines into way over powered blow torches, WITH water injection, they can produce enormous amounts of power and not melt.

Because jet engines run lean and cool and thus unburned oxygen is blowing out the exhaust - jet fighters frequently use the HOT and unburned oxygen in the exhaust gas, to inject fuel into - outside of the turbine, and this is called reheating boost, or an after burner....