jetlude

iTrader: (9)

Posted this in the single turbo section already and am posting here as well to get some more input.
Can someone explain (in detail) why is it so detrimental to a rotary engine for the trail plug to fire before the leading while under boost. This could be unintentionally caused by a number of reasons like failing lead ignition or fouling/failing lead plugs.
Is not the trail ignition usually already firing usually in a retarded (safe) position relative to TDC? Is it because of where the flame front will be initiated relative to the rotor surface? I read something about the squish area and knock.

Please explain.

BLUE TII

iTrader: (9)

Can someone explain (in detail) why is it so detrimental to a rotary engine for the trail plug to fire before the leading while under boost.

This question does not specify when the spark plugs are firing, just their firing order.
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The Leading plugs may (and do) fire Before Top Dead Center (BTDC) as the combustion is on the leading tip of the rotor pushing it in the direction of rotor travel.

The Trailing plugs must fire After Top Dead Center (ATDC) or the combustion will be on the trailing tip of the rotor pushing it opposite to the direction of travel.
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So, if your Trailing spark sparked at its normal (ATDC) time and then your Leading sparked after (some 20 or more degrees retarded) it wouldn't hurt your motor except for complete lack of power and high EGTs from poor combustion.
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However, if your Trailing plugs sparked some 20 degrees to early (before the Leading plugs sparked) the combustion would be pushing the trailing rotor edge backwards against its current rotation. The rotor has enough inertia to continue its correct rotation and so the trailing combustion chamber is decreasing in size (this is still on the compression stroke since it is BTDC) and the combustion expansion is trying to expand, so you have extreme pressure which leads to detonation (very fast combustion/explosion instead of regular slower flame front travel).
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So, how does negative split work (where Trailing fires fires before Leading at low load)?

At low load the size of the combustion volume (amount of fuel & air) is low enough that the expanding combustion charge has time to shoot through the slot in the rotor from the Trailing side of the rotor housing minor axis to the Leading side of the rotor housing minor axis as the Trailing rotor edge continues its normal rotation.

At high load the rotor slot is too much of a restriction to combustion flow to allow the higher combustion volume to shoot through the gap in time.

jetlude

iTrader: (9)

Thank you for the reply. Is clearer to me now that you mention where the combustion is occurring relative to the trailing edge of the rotor and its rotation.