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Dual ignition system

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MissSoph created the topic: Dual ignition system

Doing some practice questions and had this one come up....
A secondary advantage in fitting the dual ignition system on an aircraft engine is to provide for...
1. More uniform cylinder head temp
2.reduce oil consumption
3.improve engine performance through improved combustion efficiency
4. Reduce chance of spark plug failure.
So I had 4.... and it was 3....
Could I be right in thinking that the ignition system includes the spark plugs and magnetos... that’s why 3 is the answer
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  • John.Heddles
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  • ATPL/consulting aero engineer
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John.Heddles replied the topic: Dual ignition system

First off, what is the ignition all about ?

You guessed it - providing a spark at the right time to get the combustible mixture in the chamber burning away merrily. This it does by starting a fire at the spark plug. Result is that a fairly rapid flamefront moves across the mixture volume.

Now, we can improve the burning process a little if we start more than one fire in the chamber volume. How to do this ? Easiest way is to have two spark plugs so that we can start two fires. Result is that the mixture ends up burning better and we get a little bit more energy released usefully from the burning mixture and a bit more power out of the cylinder's efforts.

A side benefit is that having two separate spark systems gives us some redundancy - ie if one plays up we have the other one there to make sure the fire still gets lit.

Looking at the alternatives, we should follow the standard approach to multichoice questions -

(a) read the question stem and figure out what the examiner might be looking for, while paying particular attention to key words and phrases.

(b) read EACH alternative answer and ask yourself "Could this answer be correct ?". If so, why ? Then ask whether it could be incorrect and, again, why.

(c) repeat this exercise for EACH and EVERY alternative answer.

(d) if you are lucky you end up being able to discard all but one answer as being incorrect with the remaining answer having been assessed as being correct. Easy situation in this case.

(e) if you end up not being able to get rid of all the distractors and, say, you end up having to decide between two .. it is easier to make a call between two than it is for four. Remember you don't get penalised for a wrong answer but you do get NO marks if you don't answer the question. Ergo, if you end up with, say, two answers and you can't figure out which is the one with which to run, you make a guess. Again a guess between two has much better odds that a guess between four.

So let's look at the answers on offer -

1. More uniform cylinder head temp Maybe ? But, then again, CHT is a pretty broadbrush measurement and we understand that the basic carby engine doesn't have a really good control over cylinder mixtures so, really, this answer doesn't offer any real advantage either to the designer or the operator. I'd be pretty comfortable binning this alternative.

2.reduce oil consumption Now, just how is this going to work ? It doesn't make any sense at all. Bin it.

3.improve engine performance through improved combustion efficiency This is sounding a bit more promising. Using two spark plugs will give a better burning performance and will provide a bit more power (as shown by the RPM drop checks). This option sounds very interesting.

4. Reduce chance of spark plug failure. How might this work ? Sounds a bit like black magic to me. The second spark plug knows nothing about the other one. Bin it.

So, at the end of the review, we can get rid of three alternatives as being not much use to the question. That leaves us with only one, and that one we were quite happy with as being a contender for the correct answer. It now becomes a programmed decision process and the answer selected is 3.


Could I be right in thinking that the ignition system includes the spark plugs and magnetos

Ah, if we take out the spark plugs and magnetos ... there's not a lot left in the ignition system for us to think about ...

Engineering specialist in aircraft performance and weight control.
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