The Ford Barn

rotorwrench · 01-15-2020, 06:18 PM

The impulse coupling is there to delay the spark to a more retarded position so the starter won't get busted when it tries to kick back at full advance.

Mags are great for aircraft since they only run at a certain rpm most of the time. When there are large variations in rpm such as driving on the street, a mag is not as friendly to the overall function. Street cars need some spark control to prevent detonation and to increase fuel mileage. The aircraft engines can be leaned for altitude so having two magnetos is no big deal with them. They are set for full advance. Impulse couplings are not used much anymore due to their tendency to fail and cause problems. Most aircraft have one or two electronic starting aids that run through a set of retard points built into the magneto just for the start phase. The starting aids only work when the starter is actuated.

This thread is a few years old & brought back up by a newby that doesn't want to post a new thread.

johnneilson · 01-15-2020, 06:46 PM

What is the purpose of the impulse coupling on a magneto?

A mechanical device, known as the impulse coupling or starter, is installed between the engine drive and the magneto proper on a majority of rotary magneto installations. Its primary function is to intensify the ignition spark at low rotative speeds in order to facilitate engine starting.

Learned this from an old aircraft mechanic, as he said, hand propping ain't easy.

J

rotorwrench · 01-16-2020, 09:54 AM

It just delays it. It doesn't really make the spark any hotter than it normally is. The aircraft type mags that I'm more familiar with, do have a spring effect that flips the mag through E-gap to reverse the polarity quicker which give a tad bit more effective spark than just slowly turning it through e-gap would. Once the engine starts, the couplings flyweights deactivate the retard sequence and return the spark to full advance.

01-15-2020, 06:18 PM	#21
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	Re: wico jem 1590 magneto by ford The impulse coupling is there to delay the spark to a more retarded position so the starter won't get busted when it tries to kick back at full advance. Mags are great for aircraft since they only run at a certain rpm most of the time. When there are large variations in rpm such as driving on the street, a mag is not as friendly to the overall function. Street cars need some spark control to prevent detonation and to increase fuel mileage. The aircraft engines can be leaned for altitude so having two magnetos is no big deal with them. They are set for full advance. Impulse couplings are not used much anymore due to their tendency to fail and cause problems. Most aircraft have one or two electronic starting aids that run through a set of retard points built into the magneto just for the start phase. The starting aids only work when the starter is actuated. This thread is a few years old & brought back up by a newby that doesn't want to post a new thread.

01-15-2020, 06:46 PM	#22
johnneilson Senior Member Join Date: Jun 2010 Location: 34.22 N 118.36 W Posts: 1,056	Re: wico jem 1590 magneto by ford What is the purpose of the impulse coupling on a magneto? A mechanical device, known as the impulse coupling or starter, is installed between the engine drive and the magneto proper on a majority of rotary magneto installations. Its primary function is to intensify the ignition spark at low rotative speeds in order to facilitate engine starting. Learned this from an old aircraft mechanic, as he said, hand propping ain't easy. J __________________ As Carroll Smith wrote; All Failures are Human in Origin.

01-16-2020, 09:54 AM	#23
rotorwrench Senior Member Join Date: Jun 2010 Location: San Antonio, Texas Posts: 16,436	Re: wico jem 1590 magneto by ford It just delays it. It doesn't really make the spark any hotter than it normally is. The aircraft type mags that I'm more familiar with, do have a spring effect that flips the mag through E-gap to reverse the polarity quicker which give a tad bit more effective spark than just slowly turning it through e-gap would. Once the engine starts, the couplings flyweights deactivate the retard sequence and return the spark to full advance.