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.