Ignition Dynamics III - CAFE Foundation

More documents

Recommendations

Info

165016001550150014501400135013001250Figure 13C.B 8.0 gphAt points A, B, and C, turned offthe 25° BTDC Bendix 1200magneto to run on the 40° BTDCEIS-1 electronic ignition only.ceptible rise in EGT when turning offthe magneto, but still no significantchange in TAS.At point “C”, at 7.0 gph (~150° Flean of peak EGT), there is a morenoticeable rise in EGT and fall inTAS when the magneto is turned off .This again indicates that, in the SEconfiguration with 40° electronictiming, the magneto only contributessignificant ignition at the slowestburning,leanest mixtures.See Figure 1 3 C, and point “E” inFigure 1 3 A. Calculation shows that,at 2550 RPM, the advertised sparkduration of 1.8 milliseconds for theEIS-1 using 0.030" spark plug gapwould provide a continuous sparkfrom 41° BTDC to 13.5° BTDC.With the SE system, this would overlapthe 25° BTDC spark delivered bythe magneto. Especially with richmixtures, the flame front that originatesat 40° BTDC from the SEbottom spark plug might be expectedto partially engulf the magneto's topspark plug electrodes before they firetheir spark. Such circumstances mayburden the magneto coil with havingto fire its spark into a somewhat increasedpressure environment,increasing the possibility of stray arcingin its distributor or coil. Althoughwe saw no evidence of arcing andknow of no reports of it thusfar inusers of the SE system, it is suggestedthat the magneto-fired sparkplug gaps be kept properly smallwhen using the SE system.Figure 1 3 D depicts the effects ofturning off the manifold pressuremodulatedtiming advance of theEIS-1. This changes the timing of thebottom spark plugs to 25° BTDC fromtheir advanced setting of 40° BTDCand retains dual ignition operation.The results show a consistent but mildrise in EGT at every fuel flow settingexamined, indicating the delay in theburning of the fuel. This is accompaniedby the familiar drop in TAS at allexcept the 8.0 gph (stoichiometric)setting (point “C”, Figure 13D), wherethe airspeed actually rose by almost1.0 mph. This rise of TAS demonstratesbetter power with dual ignitionat 25° BTDC at these particular settingsthan when the bottom plugs werefiring at 40° BTDC, i.e., it is closer tothe MBT timing. As was shown inFigure 1 3 B, point “B”, the increasedpower from an optimized timing setting(closer to MBT) more than compensatesfor the wasted energy ofhigher EGTs that accompany such retardedtiming.The ‘burn speed’of the 8.0 gph stoichiometricmixture is slower than the9.5 gph best power mixture. The lossof TAS at 9.5 gph (Figure 1 3 D, point“B”), differs from the r i s e in TA Sfound at 9.5 gph in Figure 1 3 B, point“B”, where only the top spark plugswere firing. To explain why this situationreverses at 9.5 gph, recognize thatflame spread is faster when originatedat two spark plugs than when originatedat only one. Thus, with b o t hspark plugs firing, 25° BTDC advanceis slightly too early for MBT at 9.5gph (Figure 1 3 d, point “B”) whereaswith only the top spark plugs firing,the slower flame spread places 25°timing close to MBT. The MBT s e t-ting at 9.5 gph with all spark plugsTime, secondsTAS, 11.0 gphTAS, 8.0 gphTAS, 7.0 gphC 7.0 gph= EGT #4 (leanest)= EGT #2 (richest)= EGT#4 (leanest)= EGT#2 (richest)A 11.0 gph12001550 10 20 30 40 50 60 70 80 90 100Change in EGT and TAS during operation on EIS-1 only, 3 different fuel mixtures: MooneyN6057Q, 12/15/01. 12,500' density, w.o.t., 2550 RPM. Bottom spark plugs on ElectroairEIS-1 at 40° BTDC. Top spark plugs on Bendix 1200 mag @ 25° BTDC timing.185175165firing thus would likely be about 20°BTDC. This is true even though theEGT is shown to rise as the 40° BTDCtiming is cut to 25°.Simply modulating ignition timingaccording to manifold pressure andRPM ignores the direct and profounddependency of MBT timing on thefuel/air mixture. Figure 1 3 c l e a r l ydepicts this dependency and suggeststhe need for a sensor such as an exhaustoxygen sensor to modulateignition timing with mixture. A l-though such sensors are purported tobe problematic in engines that useleaded fuels, hopefully this technicalproblem will soon be solved for aircraftengines.Though firing at the same 41° timingat 15,000' as the DE system, theSE system probably operates as if itsignition were timed later due to theslower burning that results when early
firing is initiated only at the SE bottomspark plug. Thus, a slightincrease in timing advance relative tothe DE system might be expected tohelp extend the lean misfire limit ofthe SE system and improve its poweroutput. Figure 1 9 appears to confirmthis idea by showing SE data for both36° and 40° BTDC timing at 12,500’altitude. Note here that the airspeeddeveloped with mixtures near thelean limit is slightly better at 40° timingthan at 36° timing and that theconverse is true at rich mixtures.Also note the expected increase inC H T and decrease in EGT with the40° timing.In Figure 1 9, the 36° timing wastoo rough running at 6.0 gph to obtaina stable TAS. We estimate thatthis would have yielded about 15mph slower than the TAS of the 40°timing at 6.0 gph. The use of a0.060” electrode gap on the bottomspark plug of cylinder #4 (the leanestcylinder) caused it to prematurelymisfire relative to the lean limit foundusing the standard 0.031” gap. Presu m a b l y, the diminished sparkduration of the larger gap did not lastlong enough to reliably ignite thevery scarce fuel molecules at theleanest mixtures.Textron Lycoming's Flyer newsletterstates that they allow "leaning topeak EGT at 75% power and belowon our direct drive normally aspi -rated engines." 4 They also state that"leaning past the peak (EGT) is notrecommended".Wide experience has shown that,when a reasonably even mixture distributionexists, conventional aircraftengines can safely operate continuouslylean of peak EGT with thefollowing important stipulations:1) It must be at less than 70%p o w e r. For normally aspirated engines,it is generally at altitudesabove 10,000 feet where w.o.t. settings,which are best able to giveeven mixture distribution, deliverno more than 70% power.2) It is verified by EGT m e a s u rementsthat all cylinders are at least25° lean of peak EGT. Ideally, them u l t i - c y l i n d e r E G T / C H T g a u g eshould have an alarm capabilitysuch as those built by JPI and KSAvionics.16501600155015001450140013501300B 9.5 gph1250= EGT #4 (leanest) = EGT#4 (leanest)= EGT #2 (richest) = EGT#2 (richest)AD 7.0 gph11.0 gph12001550 10 20 30 40 50 60 70 80 90 1003) All cylinder’s CHTs ’ must bewithin acceptable limits for c o n t i n u-ous operation (generally below 420°F) when measured by an accuratemulti-cylinder CHTgauge.4) T h e re must be no evidence ofnoticeable roughness.O t h e r e l e c t ronic ignition systemsmay perform diff e rently than theEIS-1 system tested here and maybe the subject of future study by theCAFE Foundation.BIBLIOGRAPHYFigure 13D.3 Klaus Savier, personal communication.11/08/01.4 Lycoming Flyer--Key Reprints,page 41. 1996. Te x t ron Lycoming, 652C 8.0 gphAt points A, B, C, and D, turned off the M.P.advance so that 38-40° BTDC EIS-1electronic ignition went to 24.5° BTDC. The25° BTDC Bendix 1200 magneto remained on.Time, secondsTAS, 11.0 gphTAS, 9.5 gphTAS, 8.0 gphTAS, 7.0 gphChange in EGT and TAS when ignition timing reduced from ~40° to 24.5° BTDC, 4 differentfuel mixtures: Mooney N6057Q, 12/15/01. 12,500' density, w.o.t., 2550 RPM. Bottomspark plugs on Electroair EIS-1. Top spark plugs on Bendix 1200 mag @ 25° BTDC timing.185175165O l i v e r S t reet, Wi l l i a m s p o rt, PA. 17701.Page 41.The CAFE Foundation wishes tothank Jeff Rose at Electro a i r ® f o r p rovidingthe electronic ignition system forthis study. His experience, advice andguidance were indispensable in performingthis work. The CAFEFoundation has no financial interest inE l e c t ro a i r, LightSpeed Engineering,Unison, A e rosance, Te l e d y n e Te c h n o l o-gies, Lycoming, Continental, GeneralMotors orGAMI jectors.Internet links:cafefoundation.orgelectroair@juno.com (Electroair ® )LightSpeedEngineering.com
Page 1 and 2: AIRCRAFT PERFORMANCE REPORTSponsore
Page 3: loss of power due to the wasteful l

Ignition Dynamics III - CAFE Foundation

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?