AIRCRAFT RESEARCH REPORT TRIAVIATHON Ignition Dynamics I TROPHY CAFE FOUNDATION BY BRIEN SEELEY AND THE CAFE BOARD PRESIDENT Brien Seeley VICE PRESIDENT Larry Ford TREASURER Scott Nevin SECRETARY Cris Hawkins CHIEF TEST PILOT C.J. Stephens TEST PILOT Otis Holt DIRECTORS Otis Holt Jack Norris Bendix 1200 magneto, Stephen BRIEN SEELEY Williams Ed Vetter This is the first of three would improve the perfor- shelf, bolt-on products. Al- Scott Nevin CAFE Foundation research mance of light aircraft while though their popularity is Jo Dempsey reports on aircraft ignition reducing their fuel consump- increasing, wider use of elec- Bill Bourns systems. Part I discusses ba- tion. One such innovation tronic ignition systems in Darrel sic operational and design was the use of electronically aircraft has been limited by Harris considerations while Part II advanced ignition timing concerns regarding battery presents comparative flight when operating with reduced dependency, reliability and test results. Part III will pre- manifold pressure at very lean the safety of using ignition sent the results of ignition mixture settings. Some of the timing advanced beyond that system bench testing. A more aircraft that won the CAFE specified on certificated en- comprehensive version of 400 races employed this tech- gines. Claims of up to 20% each report will be available nique to significantly improve greater fuel economy and up at www.cafefoundation.org their MPG score. to 7% increase in horsepower In the last 10 years, high have been made for these sys- BACKGROUND energy electronic ignition tems.2,3 The CAFE systems for reciprocating air- Foundation performed exten- The CAFE 400 flight effi- craft engines, particularly in sive flight performance ciency races of the 1980's experimental aircraft, have testing of electronic ignition encouraged innovations that been developed into off-the systems during 2001. To properly evaluate the test results, a re- trodes to prevent stray arcing. Its coil COMPARATIVE AIRCRAFT view of basic spark ignition engine is capable of generating as much as FLIGHT EFFICIENCY, INC. operation is helpful. 30,000 volts.5 The CAFE Foundation: In the quest for better fuel econ- A Non Profit, All Volunteer, Tax-exempt MAGNETOS omy, automotive engineers have Educational Foundation developed distributorless ignition sys- 4370 Raymonde Way In conventional aircraft magnetos, tems that allow the use of even higher Santa Rosa, CA. 95404. a high voltage arc crosses the spark energy sparks without unwanted arc- FAX 707.544.2734 plug electrode gap at the desired igni- ing. High energy sparks are better Aircraft Test Facility, Santa Rosa Airport tion timing point, typically at 25° able to ignite very lean mixtures. 707.545.CAFE (hangar, message) before the piston reaches top dead America Online: [email protected] center (25° BTDC). For the sake of COMBUSTION DYNAMICS simplicity and to avoid detonation, the 25° BTDC timing is held constant at Ideally, the rise and fall of cylinder IMPORTANT NOTICE all power settings and RPM's. pressure is timed by the ignition event Any reproduction, sale, republication, The peak voltage demanded from a so that as much of the pressure as pos- or other use of the whole or any part magneto by a properly gapped aircraft sible is used for the mechanical work of this report without the consent of spark plug seldom exceeds about of pushing the piston. The ignition the Experimental Aircraft Association 12,000 volts. The spark voltage and timing that accomplishes this is called duration depend upon several factors "maximum brake torque" (MBT) tim- and the CAFE Foundation is strictly as shown in the table below.4 ing. MBT timing must also account prohibited. Reprints of this report for the piston’s ever-changing me- may be obtained by writing to: Sport Makes Makes Aviation, EAA Aviation Center, 3000 An increase in: chanical leverage on the crankshaft voltage duration during the expansion stroke. For a Poberezny Road, Oshkosh, WI. Plug electrode gap increase decrease given engine at a given power setting, 54903-3086. Altitude, non-turbo decrease increase MBT timing causes the peak cylinder Mixture richness increase decrease pressure to occur slightly after top ACKNOWLEDGEMENTS dead center, often in the range of 11°- Water vapor increase decrease 16° ATDC. Igniting the mixture too Compression ratio increase decrease This study was supported by grant early or too late causes a reduction in funding from the Experimental Air- Air temperature decrease increase power. Likewise, if the mixture burns craft Association and by FAA Gap airflow increase decrease too slowly, burns incompletely or ex- Research Grant Number 95-G-037. tinguishes prematurely, power loss The CAFE Foundation gratefully ac- Because the spark plug electrode will occur. Deviations from MBT knowledges the assistance of Klaus gap is a main determinant of the volt- timing of +/- 3° BTDC produce only Savier, Ben Ellison, Lyle Powell Jr., age required to ‘fire’ the spark plug, 1-2% reduction in torque, so that cycle magnetos typically use smaller gaps to cycle timing accuracy of 1° should Paul Loewen, Tim Davis, Steve Pen- (~0.018") than high energy ignition generally be adequate to sustain near ning, Wyman Shell, Wayne Reece, systems (~0.030"-0.040"). If the spark optimum torque.6 See Figure 1. Walt McPeck, Bob Pixton and Anne plug gap is set too wide, the higher The details of the propagation of Seeley, EAA Chapter 124, and the voltage discharge that this demands the flame front created at the spark Sonoma County Airport FAA Control from the magneto's coil may ulti- plug exert an important influence on Tower Staff. mately cause the coil to short fuel economy and power. The speed internally, overheat and fail. Gaps that and direction of the flame front's dis- SPONSORS are too small, although they increase persion are mainly influenced by gas Experimental Aircraft Association spark duration, are subject to fouling flow (swirl and turbulence), fuel/air Federal Aviation Administration and consequent misfiring. It is there- mass ratio, and the homogeneity, at- fore very important to set the proper omization and exhaust gas dilution of Engineered Software “PowerCadd” and WildTools electrode gap when servicing aircraft the mixture. Ideally, the fuel mixture Bourns & Son Signs spark plugs. burns smoothly and completely at the AeroLogic's Personal Simulation Works Use of larger magneto coils with proper rate and does not explode or DreeseCode Software-www.dreesecode.com higher voltage capability is limited by detonate. the tendency for stray arcing to occur within the magneto’s distributor, espe- FUEL MIXTURE engine power all vary with changes in cially at high altitude where it takes this ratio. When flying at altitudes less voltage to initiate such a stray arc. Most reciprocating aircraft engines above about 5000 feet, appropriately The Bendix 1200 magneto, shown in are equipped with a mixture control to leaning the mixture improves power, the first photo, was introduced circa allow adjustment of the leanness or economy and engine vibration. 1965 as a modern “high altitude” mag- richness of the inducted fuel to air Gasoline powered engines typically neto. It uses an extra large high mass ratio (or weight ratio). The ex- operate with fuel/air mass ratios that voltage coil and has exceptionally haust gas temperature, speed of vary from a lean value of about 0.056 wide separation of its distributor elec- combustion, ideal ignition timing and to a rich value of about 0.083. For Figure 1. Maximum Brake Torque (MBT) Timing Effects Actually, the best economy mixture is that which delivers the lowest value in Torque pounds of fuel per horsepower per 100 3 hour for the engine as a whole, a term 50° defined as the engine's brake specific adv. fuel consumption, or b.s.f.c. Piston 30° 2.5 98 adv. engine b.s.f.c. values can range from 10° about 0.38 to 0.60 lb./hp/hr. and vary adv. 2 with mixture, manifold pressure, RPM 96 and ignition timing. The b.s.f.c. MBT serves as a yardstick for the fuel effi- timing 1.5 ciency of different engines and power 94 settings and is an important determi- linder pressure, MPa nant of range, payload and cost of 1 y C operation. 92 x An engine’s best (i.e., lowest) 0.5 b.s.f.c. occurs at a particular MBT ig- Relative torque, percent of maximum x nition timing that will vary depending 90 x TDC upon the engine’s power setting. The 0 0 -60 -50 -40 -30 -20 -10 0 10 20 30 40 50 60 70 80 more the mixture is leaned from stoi- Crankshaft position, degrees chiometric, the more advanced will be its MBT ignition timing, because the Spark advance effects on cylinder pressure and torque. "X" signifies ignition point. leaner the mixture, the more slowly it takeoff at full power near sea level, As the mixture is leaned from this burns. fuel/air ratios are often enriched to as "best power mixture" point, power and Figure 2 indicates that, for the high as 0.11 so that excess fuel can airspeed steadily diminish, while CHT CAFE testbed aircraft, best MPG and maximum relative CAFE score occur serve as a coolant and help prevent rises slightly to peak at around 50° detonation. at an EGT that is over 100° LOP, ROP. The flight data in Figure 2 illus- where the peak CHT has fallen by Depending on the fuel's chemical trate these phenomena. composition, a fuel/air ratio of about about 50° F. This unusually lean mix- A fuel/air ratio of about 0.059 gives 0.065 to 0.067, produces a "stoichio- ture operation is made possible by metric mixture", meaning that it has an EGT about 50° "lean of peak" some modifications made to the test the chemically correct proportions of (LOP) and this has popularly been engine’s induction and exhaust sys- fuel and oxygen.