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National Advisory Committee for Aeronautics, Technical Notes - Valve Timing of Engines Having Intake Pressures Higher than Exhaust

With the wide acceptance :of the- gear—driven super—
charger as s.tandard equipment for aircraft engines, it is
so easy to_ obtain almost any- desired increase in power by
increasing the supercharger gear ratio, that the problem
of increasing power has been merely one of making an en-
gin e that will stand up under the conditions of operation.
Unfortunately, the pr oblem of obtaining reasona'ole fuel
consumption Iwith such "boosted" engines is not so simple.

Hany engineers hav e been interested in the possibil—
ity of obtaining increased power from supercharged en—
gines by filling the clearance volume with combustible
mixture by means of the available difference_in intake and
exhaust pressures. To accomplish this, it _is necessary
merely to allow the exhaust valve to remain open _later,
and to .open the intake valve earlier in order that there
shall be a considerable period at the end of the exhaust
stroke when both valves are .dpen to permit the charge to
flow through.

The purpose of this investigation is to determine
with a.fain degree of approximation the possible improve—
ment in performance by using a large amount of valve over-
lap on a supercharged engine. If it were possible to
scavenge the clearance space completely, we .would expect
an increase in the indicated horsepower equal to 1/63.

As in the two— —cycb.e engine, complete scavenging is not
possible since there will always be mixing of the fresh
charge with the exhaustpgas. Conseguently, this figure
should be taken as'the maximum increase obtainable in the
ideal case. If the fuel is mixed with the air'outside the
cylinder and if any considerable degree.of scavenging is
obtained, a considerable quantity of fuel is almost cer—
tain to-escape through the exhaust valves However, with
fuel injected directly into the cylinder,-it is possible
to.delay the injection'ofl fuel until after.the-exhaust
valve has closed and therefore a reasonable fuel consump—
tion can be maintained with a high degree of scavenging.

A series of tests was made-on the H.A.C.A. universal
single—cylinder engine of 5— inch bore and 7— inch stroke
with varying amounts of overIlap fIrom Q to lQOo at five
different intake pressures. Air was supplied under pr-es—
sure to the engine from a sep_arateIly driven Roots type
supercharger. A 55—gallon drum Was in-terposed between the
supercharger and the engine to proinde a receiver from
which the engine could draw ':its Charge.