Ignition Timing

[Founder]

If the ignition timing is advanced, the spark will occur:
A - Nearer to TDC
B - Furthur away from BDC
C - After the inlet valve opens
D - Furthur away from TDC.

Correct me if I'm wrong but doesn't the spark ignite the mixture just after
TDC during the compression stroke. If the timing is advanced, it's moved
forward in time that is... the spark would occur nearer to TDC...

Correct answer according to test is D...

[moogleman]

The spark ignites the mixture just as the piston is completing the compression stroke, so just before TDC.
By advancing the timing you are providing the spark earlier in the compression stroke, at a point where the piston is further away from TDC.

I think thats right, although its been a while since I studied it in depth so someone else will probably be in a better position to answer than me!

MM

[Dick Whittingham]

Absolutely right MM. The spark occurs before TDC. Under power it can be as much as 30deg BTDC. At idle it should be less, perhaps 10deg BTDC. Reasom: It takes a finite time for the flame front to cross the combustion chamber and burn all the fuel, so at high RPM you have to start the fire earlier.

Dick W

[howflytrg]

Moogle is correct. The spark occurs before TDC as there is a lag between the spark and the combustion of the air/fuel mixture. If the spark occured at or after TDC there would be a loss of the power that could be exerted on the piston head during power stroke.

[Founder]

Thanx a lot for clearing up the matter =)

Many thanx
/Tim

[EchoMike]

The answers given are correct, the problem is the poor phraseology of the question.

Advancing the timing means moving the spark event further in advance of other "events" in the engine, this "event" specifically being TDC on the compression stroke.

Except if we waited until AFTER TDC on the compression stroke to fire the plug, advancing the timing would then place the ignition event closer to TDC. The engine would run with the spark after TDC, but not well and would produce little power.


I even wonder why they ask this question of a PPL candidate - how many modern training airplanes (i.e built in the last 50 years) have their spark advance adjustable from the cockpit? (Other than by indirect connection to the throttle, which controls engine speed, and that is what determines spark advance - spring loaded weights in the magnetos.)


One wonders if the writer of the question has ever actually SEEN an internal combustion engine . . . :ugh:

Best Regards,

EchoMike (who has seen more than a few of them, even including one or two on airplanes)