Author David Cave's 1910 4hp Waterloo boy. Poor running got David looking closely at ignition dwell.
Some time back, I acquired a 1910 4hp battery and coil powered Waterloo Boy engine with an interesting problem. Today I say it had an interesting problem, at the time I used less complimentary terms.
It’s a big engine with good compression, difficult for a 70-plus-year-old man to start. I’d pull it up on compression slowly, then give it all I had to pull it over before it kicked back. It seemed to have no retard mechanism. Most often it would get off a weak hit and then come around and get off a good hit. The engine speed would then be in the neighborhood of 200rpm-300rpm. At that point the engine would quit firing, even though the igniter was tripping, and the RPMs would rapidly start dropping towards zero. At about one revolution per second, just as I thought it was dead, it would have another good hit. With that hit the RPMs went back up again — and we’d start the process all over again: a hit to gain RPM, then die till the RPMs were low. This would go on about as long as I’d let it.
Looking for a Solution
I spent the better part of a day playing with the mixer, choke, cam timing, valve timing, igniter timing and about anything else I could think off. Finally, in desperation, I got my ohmmeter out and checked the dwell angle. The igniter points closed at 35 degrees BTDC and the igniter tripped at 20 degrees BTDC. I had only 15 degrees of dwell.
In the early 1900s, battery- and coil-powered engines kept igniter points open as long as possible in order to conserve battery power. With my points being closed only 15 degrees of crank shaft rotation, the actual clock time the points were closed was very short. At 300rpm the igniter points were closed only 8ms (0.008 second) before the igniter trips, but at the much lower speed of say 60rpm, the points were closed for 42ms (0.042 second) before the igniter trips.
A screenshot from David's Oscilloscope showing the rise of coil current across time in the coil used on the Waterloo Boy engine.
Why is dwell, or the amount of time the points are closed before the igniter trips, important? When the igniter points close, 6 volts appear across the coil and the coil current begins to ramp up. The current ramping up after voltage is applied to a coil is a rather long, slow process; it is not instantaneous. The graph shown is the actual current in the coil I was using versus time. Each horizontal box is 10ms (0.010 second) of time while each vertical box is 200ma (0.2 amps) of coil current. When I was trying to start the engine, the RPMs were near zero so the points were closed a long time and the coil ramped up to 1.2a when the igniter tripped (six vertical boxes on the graph). If that first one or two low rpm hits abruptly got the engine up to 300rpm (for example), the points were closed for only 8ms and the current only gets to about 350ma (0.35a), not enough for a good hot spark. With not enough coil current for a good hot spark the engine tries but won’t fire, the RPMs fall, the time that the points are closed before the igniter trips increases with the falling RPMs, which results in the coil current increasing with falling RPMs. Finally, the RPMs would get low enough and the coil current high enough for a hot spark. Clearly the dwell had been incorrectly set in the past.
The solution then became straightforward; close the igniter points earlier and give the coil more time to ramp its current up. After adjustment the points now close at 75 degrees BTDC with the igniter trip still at 20 degrees BTDC, giving me 55 degrees of dwell. At 300rpm the points are now closed for 31ms before the igniter trips and the coil current ramps to 800ma. The engine now starts reliably and runs beautifully.
The original engine tag on David's 1910 4hp Waterloo Boy. running poorly when acquired, it now spins over happily at any speed thanks to proper dwell setting.
You might ask why the spark is so dependent on coil current. As the current ramps up in a coil it is storing energy in its magnetic field. This stored energy becomes the totality of your spark energy and is I2L (coil current squared times the coil inductance). At 300rpm and 15 degrees of dwell the current was 350ma at igniter trip. Now at 55 degrees dwell the current is 800ma. Since the coil energy goes as current squared my spark is now five times hotter (0.8/0.35)2.
Engine enthusiast Dave Cave is a retired electrical engineer living in Arizona. Contact him at JDengines@cox.net