Yr Right, Progressive....Thanks, Gents, but had the cranking-speed inhibited the Impulse-coupling, I'd have said so
There seems tobe some ignorance on that score.....So.....
The timing on an Aero-engine is, in technical terms, a compromise.
Retarding, via Impulse is , really a Kludge.....because of low cranking speed, the fixed-timing flame front would develop and cylinder -pressure would rise unacceptably, whilst the piston was still coming up to TDC...the delay introduced by the impulse,starts the burn later in the cycle, thus allowing the main "push " of the combustion to push the piston down the correct side of TDC. A secondary effect is the rapid "flick" to the Mag, giving a much stronger spark.
This particular correspondent had difficulty starting and due to the effect we've all seen....prop momentarily stopping as starter overcomes too-early ignition....(Don't believe me? crank with mags off...youll get it slowing, but no "kickback") So he changed the starter.
AFTER changing -out the faulty plugs, the engine now starts promptly.
The impulse coupling is a red-herring, you only need it because the cranking-speed is so slow that A- the mag doesn't generate a spark, due to the flux break being too slow.
B-cylinder combustion pressure rises too quickly, whilst piston is still BTDC.
To explain my remarks re-timing......
Fixed timing is a compromise, Fortunately, the average piston aero-engine spends most of it's duty-cycle at constant speed....therefore, ignition timing can be optimised for that speed.
Back in the day, cars had a manual advance-retard and a manual mixture control...some also had a hand-throttle. Agood driver used these controls to get the best out of the crude engines of the pre-war cars.
Anyone who has ridden a motorbike with Mag ignition and manual A-R ,knows how this affects Kick-starting, their shins and their ankles
The Panther 600 "sloper" single was capable of launching you over the handlebars, if you forgot to retard!
Plugs...I grew up believing the maker's tosh.....massive electrodes were needed 'cos they'd otherwise glow red-hot and cause "pinking" AKA pre-ignition or running-on The design of the ceramic insulator, was to ensure the plug got hot enough to burn off oil and lead deposits, whilst remaining cool enough to not pre-ignite.
Fine-wire plugs defy that logic and give a service-life far in excess of the usual massive-electrode alternatives.
Yes, I'm in agreement with LOP as well. Scientifically, it makes sense....ROP doesn't....It's supporters fail to explain what happens to the "extra" fuel....(hint...It carries on burning after the exhaust valve opens and the piston boots it out of the combustion chamber)
OTOH, the LOP charge has used most of it's energy, pushing the piston down
Gents, thanks for a very interesting and reasonably civilised debate.