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  • Sparking plug washers.

    I read recently that for engines with a cht probe beneath the plug, one ought to remove the washer. It makes sense to me. I could get better starting but I don't want to burn a piston. Any opinions?

  • #2
    Sparking plug washers.

    Is the washer not there to ensure an airtight seal and ensure it stays in place to potentially prevent it loosening, can't comment on probe etc, but the plug would probably sit slightly lower in its seat so don't know if that would be better or not, I suppose if the probe did the job of the washer etc it would be right to remove to keep the plug in same position as it was supposed to be in, no doubt some one will have the proper tech answer,

    Regards

    Chris

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    • #3
      Sparking plug washers.

      I would have thought so too, but a couple of sources suggest differently. I suppose the aluminium fitting doubles as a sealing washer.

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      • #4
        Sparking plug washers.

        I had also heard of removing the crush washer on the plug(s) being used for CHT sensors so as I had a few minutes, I looked it up.

        The Installation Manual for the more popular Rotax 2-strokes available here: http://docusearch.flyrotax.com/files/pdf/d00287.pdf does not seem to indicate that is the case on pages 50 and 59 of the 68 page document. There's no mention either, in my Simonini Victor 2 engine handbook.

        Another old wife's tale exposed!

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        • #5
          Sparking plug washers.

          It makes no mention of it, certainly, but I'm not entirely convinced.

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          • #6
            Sparking plug washers.

            If you want my humble opinion its to leave the washer on the plug.

            The plug washer is designed to seal and the CHT probe is designed to get hot.

            It will make no difference to the compression or the plug.

            If you leave the washer out and the plug leaks then the plug will run hot and loosen off until its loose enough to blow out when you should concentrate on the big field below ;-)
            ( after all its a two stroke )
            Mick Broom
            Member 909
            Shadow G-MWTN

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            • #7
              Sparking plug washers.

              Of course I want your opinion. Compression is not the issue. Position of the spark in the combustion chamber is as if it can be optimised, the engine runs better and lasts longer. A millimetre is significant in this regard so I am interested to know for sure.

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              • #8
                Sparking plug washers.

                Hi Tom,

                If you are concerned about the position of the spark as its a Rotax I would look at the different track records of the twin spark to the single spark engines as the easiest way of seeing a physical difference.

                If you run a twin spark with one plug not firing then you get a power drop so proving your point that it matters for performance. Less performance means less heat and less load on the components so from a life point of view it should last longer everything else being equal.
                If you look at the torque curve for a twin spark to a single spark head if there is a difference its shown in favour of the single spark. Theory would suggest that this is what to expect.
                All these observations are on a lot bigger changes in position than 1mm.

                Then there is the mixture strength giving different burn rates which if the power output is to go by would lead to greater changes inside the engine than 1mm on the plug position and so have a greater effect on the life and performance of the engine.

                My Rotax 503 is a twin spark version because I wanted the oil injection but run with a single spark head just on one set of coils. It saves about 1.5Kg in weight and gives the best power and because its lost its points has better reliability. Its been running forever and best fuel can be down to 3.5Ltrs /H in comps.
                These changes are possible because its SSDR

                Its running platinum plugs which just means they last longer with a small increase in clean running. I have in the past played with different types of plug and different positions and the differences are very small together with the variations on the spark timing well in excess of 1mm.

                Hope this helps.

                Added

                This is still only my humble opinion and you are correct to pursue advice until you are happy and confident as its you as pilot who is responsible for the condition of your plane and its suitable for flight.

                Safe flying
                Mick Broom
                Member 909
                Shadow G-MWTN

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                • #9
                  Sparking plug washers.

                  "Less performance means less heat and less load on the components so from a life point of view it should last longer everything else being equal."

                  It does not follow that reduced performance means less heat or load, nor longer life. When we know that the fueling is the same, we can reasonably conclude that the fuel is not being burnt at the correct time or rate to produce power efficiently. With a single spark, it is likely that combustion is occurring over a longer period, continuing to heat the piston face and the edge of the exhaust port.

                  "If you look at the torque curve for a twin spark to a single spark head if there is a difference its shown in favour of the single spark."

                  What is it that is considered 'favourable' about the torque curve for the single plug engine?
                  Pete T.

                  "A closed mouth gathers no feet".

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                  • #10
                    Sparking plug washers.

                    I would agree with Peter. Igniting the mixture in two places causes the burn to be faster but the torque produced greater because the energy is pushing the crown of the piston down with greater pressure, I would have thought. As he says, optimising the burn is better because it reduces bore wash,cold seizures and other nasties. If two sparks improve the combustion then you are getting more power per cc of fuel. Placing the spark in the right place optimises the flame front and, most importantly to me the starting which, despite a new ignition block remains either brilliant or a five minute ordeal, not helped by only having a short distance available for string pulling.

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                    • #11
                      Sparking plug washers.

                      Hi Peter,

                      I did say everything else being equal meaning the only thing that has changed is less power. I consider if its producing less power it may be expected that its producing less heat and the components are under less load which in my opinion should result in longer life.

                      The torque curves I was referring to is the ones issued by Rotax for both the single and twin spark engine of 503. To be honest I view both just a guide at best.

                      I was making the point that I thought moving the spark position down by 1mm would make no material difference to the engine , what is your opinion on this?

                      Sorry if I was not clear in my statements.
                      Mick Broom
                      Member 909
                      Shadow G-MWTN

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                      • #12
                        Sparking plug washers.

                        Mick,

                        When you said you've tried different plugs, did you include iridium in that? I've put iridiums in my 462's and they definitely do improve starting. Because the 462 is a points engine I had to gap them down from the .040 they came with to around .020 or less, but with the fatter spark iridiums give the engines now start easier than they did with the normal NGK's I used to use. As for the electronic ignition 447 in my minimax, I haven't decided yet whether to bother with them or whether to leave well alone.

                        Best regards,
                        XL's forever! Well, one of them anyway. It's all I can afford, not to mention the Raven and the Mini-Max. Oh, and I almost forgot the Spectrum as well :-)

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                        • #13
                          Sparking plug washers.

                          Hi Bob,

                          Iridium plugs are good and if the mixture is good will last some time.

                          I think you are getting better starting because of the closer gap reducing the requirement to get a spark, this in theory will have a small knock on effect at speed but a spark is a spark and petrol/air being what it is you will end up with a bang.

                          If its working why change ;-)
                          Mick Broom
                          Member 909
                          Shadow G-MWTN

                          Comment


                          • #14
                            Sparking plug washers.

                            Hi all,

                            As someone who's spent a lifetime tinkering with 2 stroke motorcycles and having read several manuals on tuning them, I can vouch for the fact that a single spark plug is more efficient in terms of retaining combustion chamber shape and the correct swirl characteristics. Having two spark plug heads and their electrodes poking into the combustion chamber doesn't do the flame propagation any favours, but of course from the point of view of reliability (notably in aviation engines), having two plugs firing from independent ignition circuits is invaluable and to be honest we're looking for reliability and not the last ounce of BHP. ALL aviation engines should be under-stressed for obvious reasons.

                            I swear by Iridium plugs. They provide a really fat spark and to be honest I don't think they have as long a life as the standard type, but the increase in performance and ease of starting is well worth the extra sponduliks to buy them and change them when the maintenance schedule demands. The difference in my Yamaha RD 350 LC in particular was remarkable after fitting them.....much harder acceleration and the exhaust really crackled. I'm not sure if the economy improved - maybe it did but I didn't care! On reflection, if the engine is working more efficiently, less throttle would be required to produce the same power so I suppose economy should be improved along with performance.

                            As for the washer question - even if combining the washer with the CHT ring DOES withdraw the plug by 1mm the performance loss would be absolutely minimal - the compression ratio being very low anyway. Fit Iridium plugs and in my humble opinion it'll make up the difference tenfold!

                            Happy landings,

                            Phil.

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                            • #15
                              Sparking plug washers.

                              The voltage required to jump a gap is the same for the same gap when considering a spherical spark gap. The discharge is affected by the more pointed iridium electrode, causing it to fire at a lower voltage and therefore advancing the spark a very (probably immeasurably)small amount. That ought to improve starting.

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