Manifold pressure, fuel mixture, and water injection

[Harmon Sommer]

Older military aircraft engines were rated for power output using avgas
which was classed as having performance  numbers of, for example '100-130'
or '115-145'. I think this meant that using a baseline fuel, mixture,
and manifold pressure (100 octane-stoichiometric-atmospheric?) it would
develop baseline horsepower. Then the test fuel would be run with and
power output would be measured at various levels of boost and various
fuel/air mixtures to determine the power the engine could develop with lean and rich mixtures.

In his book 'Turbochargers', Hugh MacInnes quotes from Sir Harry Ricardo's
book 'The High Speed Internal Combustion Engine' of some work Ricardo did
with water injection. Following is an excerpt:

"In this case, running throughout at a speed of 2,500 RPM and with a
compression ration of 7:1, the engine was run on an economical mixture,i.e.
about 10% week, and supercharge applied to the first instance of
detonation, which occured when the BMEP had reached 168 pounds per square
inch. The mixture strength was then increased, step by step, and more
supercharge applied until the same density of detonation was recorded; this
process was continued until was reached at which no further enrichment was
effective. In fact, after 60% excess fuel, not only did further enrichment
have no effect but there was even some indication that it increased the
tendency to detonate.

A finely pulverized water spray was then delivered into the induction pipe
which served to suppress detonation, in part by the intercooling it
provided, and in part by the influence of steam as an anti-detonant, and so
allow further supercharging. This was continued progressively, admitting
just sufficient water at each stage to ward off detonation until a BMEP of
290 pounds per square inch was reached, which was found to be the limit of
the dynamometer. At the same time, it was noted that, with the addition of
water, the influence of steam as an anti-knock allowed the fuel/air ratio
being much reduced.

>From this curve .... it will be seen that under these operating conditions
a limiting BMEP that could be reached with 87 octane petrol alone at an
economical mixture strength was 168 psi. By enriching the mixture to the
limit of usefulness, the BMEP could be stepped up to 237 psi. By the
introduction of water, it could be further stepped up 290, and probably
more; at the same time teh fuel/air ration could be reduced once again; in
fact with water injection, no appreciable advantage was found from the use
of an overrich fuel/air mixture. It will be noted that the total specific
consumption of liquid, i.e. fuel plus water, is not so very much greater
than when running on a very rich mixture of fuel alone."

Has anyone done any research/published on this topic recently?

Thx, Harmon