I have to say at this point that water injection is by no means a new technology. The first time it was used in gas turbines, and then in spark ignition aircraft engines to increase power. Here's an interesting link:
Frank Walker - "What can I do about this problem?"
As of late, water injection has been commercially used for the past 4-5 years in marine diesel engines.
Saad
Not too sure about the CaCO3 residue. I've thought about it at the start. But then that it might not deposit on anything. As water evaporates in a gaseous medium (in the burning gases) any residue that forms will be dust like, and will hopefully get expelled via the exhaust.
That said, I will open up the head of the test engine at the end of my research studies and see if there has been any degradation or deposit. Degradation of the cylinder liners mostly since I've been troubled by water in the oil. As for now, the engine is running quite happily. It still produces the peak torque as given by the manufacturer even after 3 hard years of life as a research engine.
Buksh
Good of you to pass by. Drawbacks? Here are a few I can think of:
Water consumption rate. Depends on the water fuel ratio, but it can easily go upto 200%. 160 litre water tank for 80 litres of fuel.
Aformentioned water / oil mixing. Although I think it can be solved with further development
Control system. Again, can be developed if automotive manufacturers are interested.
Requirement of distilled water? Maybe, maybe not, endurance tests will have to be done to see if the presence of CaCO3 actually poses a threat to the engine.
Other than that, I can't think of any more. One thing is for sure, I'd rather use water injection than fuel enrichement to counter knock in a turbo charged petrol engine. Rally cars have been using it for quite a long time and its only recently that the WRC banned it.
As for diesel engines, almost all automotive manufacturers are interested in water injection because it is a potentially relatively inexpensive system to reduce emissions.