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Help Understanding Ignition Timing

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Help Understanding Ignition Timing

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Old 03-13-18, 06:11 AM
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Help Understanding Ignition Timing

Hi Guys,

I'm a little confused as to how the timing works on the rotary. I have done a fair bit of researching but have yet to come across a basic explanation on TDC, BTDC, ATDC and how the degrees of ignition work.

In my point of view, logic tells me any spark before TDC will cause knock. So why is it that advanced timing is used? Is it the shape of the rotor face the directs the combustion towards TDC?

As RPM goes up, Timing goes up, is that to compensate for the amount of time the ignition system has to work?

On the Power FC, the more positive timing you have the more the rotor is BTDC therefore any negative timing would be making spark once the rotor has passed TDC?

Lastly I've got a picture of half a motor that I have drawn rough angles on according to TDC being 0, Is my understanding correct? BTDC degrees being positive on the PFC and calling anything BTDC "advancing the timing"?

I have so many questions but let's leave it at that for now..

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Old 03-17-18, 11:25 AM
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the part you are missing is that it takes time for the fuel and air mixture to burn/combust.

you are correct that we do not want combustion to occur before TDC, but since the engine is moving, we need to start combustion before TDC. the faster the engine goes, the earlier we need to fire the spark plug/initiate combustion.

the speed of combustion varies with mixture, very lean and very rich mixtures burn a little more slowly. also chamber pressure has an impact on combustion speed, more pressure = faster.

there is a rotor position to the housing where there is the most leverage available, and we are trying to get peak combustion pressure to happen at that point, through all the RPM ranges and throttle openings.

for further reading http://foxed.ca/rx7manual/manuals/RE...amoto-1981.pdf
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Old 03-19-18, 09:01 PM
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Also, you marked housing is wrong. The eccentric shaft is turning three times faster than the rotor and timing is in eccentric shaft rotation, not rotor rotation. So, 21 degrees advance eccentric shaft would be 7 degrees at the rotor.

Also, the rotary engine has two top dead centers and two bottom dead centers. When comparing to a piston engine, TDC for power stroke is piston at the top with the compressed fuel/air mixture (right side of your picture). TDC is also the end of the exhaust stroke, beginning of intake (left side of your pic). BTC is the end of the power stroke (bottom of you pic) and also the max volume on the intake stroke (top of your pic). This all coincides with the individual faces but, I think you'll get it.

Last edited by TonyD89; 03-19-18 at 09:11 PM. Reason: second paragraph
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