Re: [Small4-strokeEngines] Serving Two Masters
- Maybe use a rheostat or something like it and determine at what voltage ignition failure begins under full load (this might vary with temperature?) and what safety margin you want. And also determine at what voltage starting becomes difficult if that is an issue. Then use the same device to discharge the battery at 400 ma (or whatever) and determine how long you have. You can probably get a voltage/time discharge curve from the battery maker, but better to determine it experimentally for the individual battery.... (Of course, a simple resistor will not give a constant discharge current as the voltage drops, but with a few manual adjustments you could likely get close enough.)
At 06:06 PM 3/2/2013 +0000, you wrote:
No man can serve two masters, or so the saying goes. I am torn between building a 1/2 VW engine and an industrial V-Twin engine. Actually I am semi-retired so I have the time to work both projects. I hope to avoid the two masters syndrome. They are all engine conversions for airboat, ultralight, hovercraft, etc. My focus is on a legal (254 lb) ultralight engine. The 37hp 1/2VW at 80-85 lbs turning a direct drive prop is one example. I am working with a Kohler V-Twin I would like to see be competitive with the 1/2 VW for a little less money and weight.
The Kohler is on the test stand running in the light direct drive hand prop configuration. I really like the way it sounds with the straight pipe exhaust. And the carb with accelerator pump really throttles up nicely. I also like my two DC CDI ignition system with a li-ion battery pack which does a nice automatic spark advance. The ignition running 400 milliamp and the battery at 6800 milliamp is over 10 hours using my grade school math? I would like to hear from an expert how many hours might be expected from the battery before my ignition quits in the air.
Norm Heistand my .02
Regarding your battery life question, It sure seems like your math is correct. There are a couple of things to be aware of before you do that long cross country over the swamps...
First, the CDI Ignitions don't have constant current draw. The current draw has a small constant drain even if the engine isn't running, and another "induced drag" current draw caused by firing the plugs. The "induced load" will vary in proportion to the engine's RPM. With the engine cruising at 3500 RPM, expect 5X the "Induced load" compared to idling at 700 RPM.
Another consideration is that using an ammeter (even digital) to measure the current might give a reading lower than the actual value. The electronic ignition probably won't pull a smooth continuous supply current. Rather, the input current will probably have "spikes" right after each plug fires and the ignition is re-charging it's capacitor. These spikes might be too fast to be fully recorded by the current meter. The result is that more current is actually drawn than the meter indicates.
How to get around this? Some of the more expensive meters will record these spikes, and the manufacturer of the ignition should be able to supply you with real life current draw / RPM curves.
You could also smooth out the CDI input current spikes by temporally connecting one of those huge capacitors the car stereo guys put on their amplifiers. To protect your current meter from damage, use this process: Connect the capacitor to the CDI's input using short & heavy wires. Then using standard wire sizes, connect one side of the capacitor to the battery's minus terminal, and the other side of the capacitor through a light bulb (rated for the battery voltage) and then to the battery's positive terminal. Expect the bulb to flash as the capacitor charges. Then connect the current meter in parallel with the light bulb, and remove the bulb leaving only the current meter to complete the circuit to the capacitor. Now when you run then engine, the current meter should provide reliable numbers.
Two more point to consider: First, The CDI will charge it's capacitor to a constant energy level. If the Input voltage to the CDI drops because the battery is running down, the CDI will compensate by drawing more current. This will cause the battery to discharge even faster. This means one can't easily use a voltmeter to gauge the remaining run time. Second, Remember that batteries loose a little of their capacity each time you recharge them. A great battery this year might not be up to the task a year later.
Having dual ignitions, you could possibly gain some margin of safety by having two completely separate ignition & battery systems.
Run them both for takeoff & landing, but run just one when cruising. If the battery runs down, the engine will probably run rough before it quits. If that happens, switch to the other system and land as soon as practical. Alternatively, keep a spare battery as a backup, and use a cross-over switch to swap it with the primary battery. However, if air restarts are not practical, you need to think about how to do the switchover well before there is a possibility of an engine out.
My recommendation is to always do an extended test run to validate whatever you conclude.