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Model Forum / Radio Controlled / Air Models / June 2004electronic ignition
Though not necessarily specific to an aircraft engine, I've run across some information concerning an after-market ignition system for Zenoah engines that has me a bit baffled. It may very well just be that my comprehension of the subject is more limited than I already thought. Anyway, the ignition is from MSD and specifically designed for the small Zenoah 2-strokes found on go-ped brand 2 wheel scooters. In stock form, the engine has a fixed magneto ignition. The MSD is a CDI type ignition with adjustable retard from .5 to 1 degrees per every 1000 rpm. The installation manual for the MSD ignition says to set the static timing at about 27 degrees BTDC. Then adjust the retard rate for best performance. This means the ignition will retard the timing as engine rpm increases. I was thought it was desirable to advance the engine timing for the upper rpm for best performance and have less timing for starting and low rpm operations? The reason I am inquiring concerning this subject is because I am interested in adapting the small MSD ignition for installation on another brand of small 2-stroke and not a Zenoah as it is designed. The engine is not for an aircraft. Here is a link to the manual for the MSD ignition. http://www.factorydirectperformance.com/frm20347.pdf There is a chart indicating the timing retard curve on the seventh page of the manual. I don't understand why it would be desirable to retard the timing at higher rpm. This is contrary to what I though was appropriate. Thanks for any input. John John <nope@INVALIDCHAR.com> scribed in <10df59vn124sic6@news.supernews.com>: > I don't understand why it would be desirable to retard the > timing at higher rpm. This is contrary to what I though was > appropriate. after ignition the fuel takes time to burn at higher RPM, the piston is rising faster. thus you need an earlier spark so that the charge still has time to burn to produce max power on the falling piston before the exhaust port opens. now is that called advance or retard? I get them confused.... swarf, steam and wind -- David -:- the email address is real but not forever /"\ http://terrapin.ru.ac.za/~iwdf/welcome.html \ / ASCII Ribbon campaign against HTML E-Mail > - - - - - - -> X If you receive email saying "Send this to everyone you know," / \ PLEASE pretend you don't know me. > John <nope@INVALIDCHAR.com> scribed in > <10df59vn124sic6@news.supernews.com>: [quoted text clipped - 10 lines] > > now is that called advance or retard? I get them confused.... "Advance" is to move the ignition point earlier in the cycle, which is what you're describing. "Retard" is to move the ignition point later in the cycle. The confusion often arises from how it's indicated. Which is more "advanced": Firing at 5degrees BTDC or 10degrees BTDC?  SignatureThe Raven http://www.80scartoons.co.uk/batfinkquote.mp3 ** President of the ozemail.* and uunet.* NG's ** since August 15th 2000. > ... Which is more >"advanced": Firing at 5 degrees BTDC or 10 degrees BTDC? I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is 5 degrees later in the cycle, therefore more "retarded." Marty | I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is | 5 degrees later in the cycle, therefore more "retarded." It's BelowTDC, not BeforeTDC. That would be true if the piston were supposed to be rising during the spark. But you can see how an explosion before TDC would be counter-productive, even to the point of being damaging. Thus, the more advanced cycle would be the one that fired at 5° BTDC, and 10° would be a more retarded cycle. I've always thought a better way to describe it would be ATDC (AFTER TDC). Below TDC could be interpreted either way, before or after, if you didn't realize that the ignition is supposed to take place as the piston falls. Kev "Kevin M" <kevmit@swball.com> posted message ID<Tl2Cc.2416$297.2259@newssvr23.news.prodigy.com>on Tue, 22 Jun 2004 22:25:55 GMT >| I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is >| 5 degrees later in the cycle, therefore more "retarded." [quoted text clipped - 11 lines] > >Kev I'm gonna have to call BS on this. While I'm no expert on model engines, I am an expert on automotive gasoline engines, all of which are currently 4-stroke. On an auto, BTDC does, in fact, stand for Before Top Dead Center. The spark fires while the piston is still rising, and for the reasons stated elsewhere in this thread. If you have references indicating otherwise for model 4-stroke engines, I'd sure like to see it. You win the cookie. BTDC=Before Top Dead Center If you look at the timing wheel on a 4 stroke auto engine, 0 is top dead center. Advanced timing (10 degrees BTDC, for example)will be at the pointer before 0 will, if you turn the engine the way it is supposed to turn. Don't know who taught the Kevin his engines, but I don't want him/her working on MY cars.... Roger > "Kevin M" <kevmit@swball.com> posted message > ID<Tl2Cc.2416$297.2259@newssvr23.news.prodigy.com>on Tue, 22 Jun 2004 [quoted text clipped - 24 lines] > If you have references indicating otherwise for model 4-stroke > engines, I'd sure like to see it. >Below TDC could be interpreted either way, before or after, if you didn't >realize that the ignition is supposed to take place as the piston falls. I've always heard/read before TDC...I have also heard/read that ignition starts before TDC and contunues until after TDC where the exhaust port opens. I could point you to several articles on the subject, but if you do a google search on it, you can find them on your own. Good luck! FredD > | I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is > | 5 degrees later in the cycle, therefore more "retarded." [quoted text clipped - 11 lines] > > Kev BelowTDC? Are you serious? I'm pretty sure that is not correct. John >| I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is >| 5 degrees later in the cycle, therefore more "retarded." >It's BelowTDC, not BeforeTDC. As Todd and Red have already said in reply to you, I respectfully disagree. Here's a professional site discussing how NASCAR Cup engines use VERY advanced timing (29 degrees or so before top dead center): <http://www.circleracingonline.com/technical/msd_1.html> >That would be true if the piston were supposed to be rising during the >spark. But you can see how an explosion before TDC would be >counter-productive, even to the point of being damaging. Fuel doesn't "explode." It burns. As someone pointed out earlier in this thread, it takes time for the flame front to spread across the combustion chamber. Start the burn too soon and you get "knocking" (very unhealthy). Start it too late and you waste fuel and power. >... I've always thought a better way to describe it would be ATDC (AFTER TDC). People do use ATDC--and BBDC and ABDC, too: <http://www.rbracing-rsr.com/camshaft.html> <http://autorepair.about.com/library/glossary/bldef-783.htm> When starting an engine, it's very, very helpful to retard the spark (have it come later rather than earlier). That helps prevent backfiring. Marty > | I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is > | 5 degrees later in the cycle, therefore more "retarded." > > It's BelowTDC, not BeforeTDC. Not in any book I have ever read on the subject > That would be true if the piston were supposed to be rising during the > spark. But you can see how an explosion before TDC would be [quoted text clipped - 7 lines] > > Kev Complete bollocks. IC engines fire before the piston completes the rise, in order that the delay in flame propagation through the mixture gives peak pressures at or around TDC. To early, and the detonation retards the compression stroke, and you get knocking, too late and the mixture is not up to pressure before the power stroke has already nearly completed, and you end up with burning gases rushing through the exhaust ports, which are then burnt. Timing needs to be more advanced the faster the RPM, as the flame propagation delay is a constant time interval, not a constant angular issue :-) > | I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is > | 5 degrees later in the cycle, therefore more "retarded." > > It's BelowTDC, not BeforeTDC. Actually, it's BEFORE. > That would be true if the piston were supposed to be rising during the > spark. Which is exactly what's done. > But you can see how an explosion before TDC would be > counter-productive, even to the point of being damaging. True, but the flame front takes time to travel and ignite all the mix SO at higher RPMs you advance the ignition point so as to get a chance to light the mix before the piston is back on it's way down..... > Thus, the more > advanced cycle would be the one that fired at 5° BTDC, and 10° would be a > more retarded cycle. Sorry, that's not right. > I've always thought a better way to describe it would be ATDC (AFTER TDC). They do use the term ATDC but there are very few instances where you'd want to light the mix after the piston has reached max compression and is now decreasing. > Below TDC could be interpreted either way, before or after, if you didn't > realize that the ignition is supposed to take place as the piston falls. Ignition on 99% of all four strokes starts before the piston reaches TDC. SignatureThe Raven http://www.80scartoons.co.uk/batfinkquote.mp3 ** President of the ozemail.* and uunet.* NG's ** since August 15th 2000. > > ... Which is more > >"advanced": Firing at 5 degrees BTDC or 10 degrees BTDC? > > I guess 10 degrees Before Top Dead Center. 5 degrees BTDC is > 5 degrees later in the cycle, therefore more "retarded." Yup, that's basically it. SignatureThe Raven http://www.80scartoons.co.uk/batfinkquote.mp3 ** President of the ozemail.* and uunet.* NG's ** since August 15th 2000. > Though not necessarily specific to an aircraft engine, I've run across > some information concerning an after-market ignition system for Zenoah [quoted text clipped - 27 lines] > > John From the manual for the MSD ignition... "The MSD Kicker Ignition allows you to adjust the rate of retard from .5° per thousand rpm to 1° per thousand rpm. This can be done with just a simple turn of a screw." "MSD recommends that you start out at 27° BTDC. This will be approximately 7° advanced over stock." "For best performance set the static timing as high as possible without having detonation or signs of over heating. This should be in the 25 to 29 degree range." "The Kicker Ignition has a built in timing adjustment. This adjustment provides a rate of retard as rpm increases. With the adjusting screw In the full counterclockwise position the Kicker Ignition will retard the ignition timing at a rate of ½°per 1000 rpm. With the timing adjustment screw in the full clockwise position the MSD Kicker Ignition will retard the timing at a rate of 1°per 1000 rpm. Figure 8 shows the timing curve of the Kicker Ignition." >> Here is a link to the manual for the MSD ignition. >> http://www.factorydirectperformance.com/frm20347.pdf There is a chart >> indicating the timing retard curve on the seventh page of the manual. AH! That explains everything! The MSD ignition is a problem-solver for folks who have hopped up their engine in some way. Here is the first two sentences of the manual, which you left out of your quotation: "The MSD Kicker Ignition System is a high output multiple spark capacitive discharge ignition system (MSD) with the ability to adjust the timing curve for different engine combinations. If you increase the compression, run high octane fuel, advance the initial timing, or have ported your engine, then you will need to adjust your timing curve accordingly." >> I don't understand why it would be desirable to retard the timing at >> higher rpm. This is contrary to what I though was appropriate. The MSD folks are comparing their stuff to a standard timing cure; after you've made one of the tweaks that affects timing, you use the MSD stuff to shift the curve to fit the hopped-up engine. Same thing happens with Harleys: if you put two plugs in the heads, you then have to retard the ignition from the standard curve because now you've got two flame fronts spreading the fire instead of one. In the initial setup of the MSD system, you start with static timing that is advanced 7 degrees over the standard system: "MSD recommends that you start out at 27° BTDC. This will be approximately 7° advanced over stock." Then you go for a ride. If you get detonation (ignition too advanced), then you start tweaking for retardation at higher RPM to solve the problem of detonation. Marty |
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