Ignition Dynamics III - CAFE Foundation

1700165016001550150014501400135013001250190185180175170165160155Figure 19. SE ignition: EGT, CHT and TAS, timing and plug gap effects.ÖÖÖÖ15005.5 6.5 7.5 8.5 9.5 10.5 11.5Fuel flow, gallons per hour (gph)ÖTAS 36° adv.TAS 40° adv.ÖTAS 36°/0.060"EGT2 36°ÖÖÖÖÖCHT4 36°CHT4 40°CHT4 36° @0.060" gapEGT2 40°EGT4 36°EGT4 40°EGT4 36°/0.060"420400380360340320300280260240220200Mooney N6057Q: IO-360A1B6X engine. 12,500' density altitude. ElectroairEIS-1 firing bottom plugs using 0.031" gap, Bendix 1200 magneto @ 25° BTDCfiring top plugs using 0.018" gap. Wide open throttle, 2550 RPM, All CHT's corr.to 100° F day. Cyl # 4 (leanest) bottom plug gap = 0.060" on 1/10/02 flight only.Aircraft weights: 2472 lb. @ 36°, 2537 lb. @ 40°, 2490 lb. @ 0.060" gap.Figure 19 Explanation:The 40° timing produces a hotterCHT than 36° timing. This is a finding consistentwith most of the data from other flights inwhich advancing the timing increases CHT.Likewise, the 40° timed EGTs are generally coolerthan when at 36° timing. More of the heat ofcombustion goes into the cylinder head and lessremains in the exhaust gas when using 40° timingthan with 36° timing.The TAS with 40° timing is fasterthan with 36° timing at their lean misfire limit of6.5 gph. The TAS with 40° spark advance wasslower than with 36° advance when using ROPmixtures. This is presumably because rich mixturesburn faster, calling for less timing advanceto optimize power.When the #4 bottom spark plug gapwas set to 0.060”, the engine reached its leanmisfire limit sooner, with perceptible roughnessbeginning at 7.0 gph rather than the 6.5 gph limitfound on the other flights that used the standardgap of 0.031”. The large gap misfired badly at6.5 gph with a large loss of TAS and majorengine roughness.The 40° timing allowed the engine tocontinue running without severe roughness atjust 6.0 gph. However, at 5.8 gph, the roughnessbecame prominent and uncomfortable as EGT #440° plummeted due to frequent cycles of severemisfire.