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Aviation History
1946
1946 - 1008.PDF
FLIGHT MAY 23RD, 194G WATER INJECTION FOR AIRCRAFT ENGINES constituent—methyl—has the advan-tage, requiring approximately 30 per cent less addition for the same freezingpoint. "Figs. 2, 3, and 4 illustrate theeffects of water, methyl, and ethyl alcohol mixtures on detonation-limitedi.m.e.p. These data were obtained using a single-cylinder superchargedCFR engine for the purpose of estab- lishing a working background forWright Cyclone engines. Fig. 2 indi- cates that the addition of water to fuelwill permit a variable increase in de- tonation-limited i.m.e.p. dependentupon the fuel-air ratio employed. It also shows that the percentage in-crease in detonation-limited i.m.e.p. is reduced with an increase in the rateof injection of water and fuel. Full- scale operation has shown that watermixtures in excess of 50 per cent by weight of fuel tend to drown out theengine combustion cycle when operat- ing at rich mixtures. This drowningtendency is best overcome by using water-alcohol mixtures instead of purewater, and by operating at best-power fuel-air ratios. "The advantage of water methylalcohol mixtures as compared with UJ.J / \ /1 / y y§110 &00 O75 O8OOB5 090-095 -tOO -1O5 MINIMUM OPERATING FUEL-AIR RATIO Fig. 4. Minimum fuel-air ratio re-quirements for increased power output ** using water-methanol fuel additions. pure water as a detonation inhibitor isshown in Fig. 3. Here, the addition of 50 per cent methanol by volume tothe water has resulted in a consider- able power increase. Although theethyl-water mixture appears to be superior to plain water at rich fuel-airratios, this conclusion is not borne out by full-scale testing. Full-scale engineinvestigations at the Wright Aero- nautical Corp. indicate that water-methanol mixtures are best suited as detonation inhibitors, with plain-water second, and water-ethanol mix- tures last." A conclusion which the authorswished to make at this point was that: '' pure water is approximately equalto fuel when used as an engine internal coolant at high power output. Thisfactor is important because it repre- sents a possible fuel cost saving ofapproximately 25 per cent with a small increase in fluid weight. Also, approxi- QX47 _J . fc~4S 2 43 —- 7O-3OMET •• WATER —1OO OC J > \ y WATER H _^ TANE, f / r '/ ^ / 1OO< I 80 4O Q 20 za: ZUJ O 1 -2 3 -4 SPECIFIC .FLUID, ADDITION LB./B.H.P/HR, Fig. 5. Effect of supplementary fluidaddition on cylinder head temperature and manifold pressure of a typicalCyclone engine. mately 1,000-2,oooft of engine criticalaltitude can be gained at take-off power by use of water cooling in placeof fuel cooling—a very important con- sideration for commercial aircraftwhich must be operated from airports of several different altitudes, and forwhich the possibilities of increased payload must be carefully con-sidered." Fuel Coolant The effects of water, water-methanol, and gasoline injection as an engine internal coolant are shown inFig. 5. The data were obtained at approximately 90 per cent of enginetake-off b.h.p. and a constant-datum fuel-air ratio of 0.080. The reductionin average cylinder-head temperature is approximately the same for fuel andwater additions. Water-methanol mixtures are less efficient from thecooling standpoint, i.e. approximately 80 per cent as efficient as water or fuelfor the 70-30 per cent water-methanol mixture used. A disadvantage of the use of fuel as >1OOO J 2 3 SPECIFIC FLUID ADDITION L&/6.H.P./HR. 7O-3O%'WATER METH 1OO OCTANE AVIATION. FUEU Fig. 6. Effect of supplementary fluid addition on the critical altitude of atypical Wright Cyclone engine. an internal coolant is shown by the5 to 10 per cent increase in engine manifold pressure requirements, repre-senting a loss in engine critical alti- tude. This loss, shown in Fig. 6,amounts to 1,000-2,oooft, depending upon the operating condition desired. Having established the effectivecooling possibilities of water injection, Fig. 7 has been prepared to illustratethe possible increase in a.m.p.g. ofe-* tained by reduction in cylinder-headtemperatures. These data are based on cruising conditions for a large com-mercial aircraft travelling at an indi- cated air speed of 225 m.p.h. The effect of cooling drag change atconstant thrust horse-power permits the calculation of decreased b.h.p. re-quirements for constant-speed flight, and therefore the decrease in gallons offuel per mile, or conversely, the in- crease in miles per gallon. The rela-tionship of cooling airflow, baffle pressure drop, and cylinder-head tem- 0- IO N \/ N /r y s\ oo; U o s S"-' - O ZO 4O 6O BO OO REDUCTION IN CYLINDER HEAD TEMP IN DEG F Fig. 7. The increase in a.m.p.g. madepossible by reduction of cylinder head temperatures. peratures permitted this data of Fig. 7 to be plotted. The most important results con-tained in the paper are summarized as follows: — 1. Water provides the greatest de-gree of cooling at high power out- put. 2. Water-methanol mixtures aresuperior to water-ethanol mixtures. 3. Water inethanol mixtures providegreatest increase in detonation- limited horsepower output.4. Water methanol mixtures provide the greatest saving in engine criti-cal altitude. 5. Pure alcohol additions should notbe used for aircraft engines—the' will cause a decrease in detonation-limited output when using 100 octane fuel.6. Water injection will permit the use of 91- and 87-octane fuels in placeof 100-octane for equivalent powe; output up to take-off horsepower.7. Water injection can be used most efficiently at lean fuel-air mixtures
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