Injection of steam to an internal combustion engine is the subject of a US patent; details are elaborated in the following extract from the patent description.

"[ 51 May 30,1972

United States Patent Boyer Shirrell....

mm. k w .mmm SH 44 35 99 NH 74 5 3 4 7 2 [73] Assignee: Boyer & Abbott Enterprises, Inc., Portland, Oreg. Primary Examiner-Al Lawrence Smith July 10 1970 Attorney-Lee R. Schennerhom [21] Appl.No.: 53,805

22 Filed:

Steam injection device for internal combustion engine

US 3665897 A

ABSTRACT

A steam generating tube is mounted in the exhaust manifold of the engine and supplied with water from a water tank. Wet steam generated in the tube passes through a steam separator and substantially dry steam is injected into the intake manifold of the engine to pass into the cylinders along with the fuel and air mixture from the carburetor. Water removed from the steam in the steam separator returns to the tank. The steam separator comprises a housing filled with stainless steel shavings, one end of said steam generating tube being connected with said housing. A tube to a steam control valve is connected to the top of said housing and a water return tube in the bottom of said housing is connected with said tank"

In the past, injection systems were limited just to inject water. The best known example is that of large aviation engines from the WWII era. Details are discussed in the entry: "Water injection (engine)" in the English edition of Wikipedia. Based on the information from historical records, we can conclude that the water injection systems were designed mainly to increase power and to prevent knocking in highly stressed, large displacement engines. The fuel efficiency was not considered as the main issue.

The recent implementations reported by BMW are also tuned to increase the fuel efficiency:

"In 2015 BMW has introduced a version of their high performance M4 coupe, the M4 GTS, that combines water injection with intercooling. The car was featured in the 2015 MotoGP season as the official safety car for the series and was released for the commercial market in 2016. Bosch, which co-developed the technology, offers a water injection system named WaterBoost for other manufacturers. The company claims up to 5% increase in engine performance, up to 4% decrease in CO2 emissions and up to 13% improvement in fuel economy.

(From the entry: "Water injection (engine)" in the English edition of Wikipedia)

I think I have seen BMW's system. Bosch, I believe, is the msnufacturer. In this system the water is sprayed together inside the cylinder with the air and burned fuel at the expulsion cycle. The idea is tompre cool the nect cycle and having a lower temperature one should be able to achieve higher compression ratio  and thus higher efficiency. This system does not mix water with the air- fuel mixture before ignition.

Yes, this is an old solution, reduce knocking, allow earlier ignition, and ultimately increase power. All technical details are discussed at

http://www.bosch-mobility-solutions.com/en/powertrain-electrified-mobility/water-injection/

It's odd. Because the humidity supress the temperature rise at compression stroke and increase the heat wasted with exhaust by increased heat capacity of air  fuel mixture. It leads inefficient combustion.

Small possibility it works is that the case the humidity can help supressing abnormal combustion, like knocking, through a certain chemical reaction control.

Many thanks to all of you for your comments. Please be aware that the steam injected is overheated to abt 300 C. This is the temperature that we have measured (infrared camera) on the exhaust that exchanges heat with the steam chamber. We are looking to another device called the HHO that injects hydrogen and oxigen into the air supply to the engine. Our study does not consider advancing or retarding ignition. This is controlled by the ignition circuitry / electronics of the gen set (500 KVA HFO turbocharged),