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DeTRacted

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  1. Don't forget the elephants though. (unseen university emoji)
  2. I can't help thinking this would be better discussed in the common-room but: I understand your point John and am familiar with resonance and harmonics and with pulses in cables which is a good analogue as the physics is similar. The reflection of fast pulses is much used in time-domain reflectometry and that goes for optics too because the same thing holds for fibres and light pulses. Similarly, standing waves are set up in cables under the right conditions but these must be steady-state. I can’t help feeling we are describing the same thing from different directions as it were. Standing waves are an effect, not a cause and do not exist in isolation. They are the interference pattern from the interaction of incident and reflected travelling waves (continuous sinewaves, not pulses) . It follows that If there is no wave travelling downwards and no reflected wave travelling upwards, there cannot be a standing wave. Considering each header pipe, I believe in practice any standing wave would be severely disrupted - often - by the short pressure pulses of gas, with a repetition rate and duration which will vary with engine speed - so the conditions are far from steady-state. There will be more pressure pulses occurring at different times and feeding from the other end of the pipe as the other cylinders exhaust so the result is likely to be very complex. If it was one pipe per cylinder all the way down things would be simpler to comprehend.
  3. Worbarrow bay is a special place. Lots of fossils. There is a small headland that juts out on the east side of the bay and on the east side of that there is -or used to be- a large wave-worn ammonite in the rock, about two feet across. It must be 40 years since I was last there so maybe it has gone now.....
  4. I’m not sure about that John. The diagrams you show are not of a travelling wave but a static or ‘standing wave’. You will only get a standing wave if the excitation is continuous at exactly the right rate - like blowing a flute. I think an exhaust is much more like a tubular bell where the exhaust pulses are the stick. There may be a resulting resonance in the tube but it would be more of a decaying sinusoid following each pulse. As with a bell, the pulse repetition rate which does the excitation does not change the frequency of the resonance. The exhaust pulse must travel - after all the gas does have to get out. Another analogue is of sending an electrical pulse down a cable. Provided the pulse rises quickly and is shorter than the transit time in the cable it will travel along largely unchanged until it meets a change in impedance (e.g. pipe diameter or joint in an exhaust). Then some of the energy continues but some reflects depending on the degree of impedance mis-match. You can also see this with a skipping-rope held at both ends, where if you tweak one end sharply you can watch the wave travel down the rope and then come back with reduced amplitude. The faster the pulse rise-time and the shorter its duration the more pronounced any reflection will be so for an exhaust it should really only be a high-revs thing. The reflected energy will interact with following pulses in a complex manner depending on when they meet. Surely tuning is the trick of getting the pulse to arrive back at the valve with the right polarity and spacing to give some rarefaction. ( I don’t think you can get a flute to suck at the player’s lips can you ?)
  5. An ordinary full-size momentary centre-off toggle switch but I did extend the toggle.
  6. I wanted a two-way centre-biased momentary toggle switch for my J-type box, flick down for OD on , flick up for OD off, plus the logic from the interlocks. That way I don't need a warning lamp as operation is obvious, so this is what I did:
  7. Yep. 12 dots - four on each of the top, middle and bottom horizontal lines.
  8. They aren't actually moving around, it is just that you can't distinguish them unless you are looking directly at them. Something to do with less detail being available in peripheral vision?
  9. ...and your concern was correct as witness the Iranian centrifuge plant full of Seimens PLCs, sabotaged by the Stuxnet virus. The problem isn't necessarily the system itself either but the daft b*****s who insist on it being connected it to the internet, though even an air-gap doesn't help there is an unprotected USB socket.
  10. The problem with the latching relay is you need a pulse to re-set it when the interlock opens, for the 'logic' function - which is what the transistors do. I don't think there is any other way to do that. You also need to find a way to automatically ensure it always starts in the 'off' position, because as-is it stays where it was last set. That circuit is for a TR 'box with the off-in-neutral function. In your sketch, if the latching relay is ON and the interlock kicks RL1 off because you change gear, the next time you operate the switch it turns the latching relay off again so you don't get OD. You would have to push the button twice to get overdrive on again. I think you might soon find that irritating - sometimes one push needed, sometimes two.
  11. Here is a way of having sequential on/off switching with a single momentary switch (in this case on the gear-stick but it could be anywhere). It uses a bi-stable relay and was designed for a J type but is easy to adapt for an A or D type. The two transistors shown could be replaced by a single TIP120 :
  12. Are you over-thinking this? I can't see what you are trying to achieve with the third relay. If you have the interlock opening between 3rd/4th the circuit automatically drops out as you go through neutral without the need for another relay, since power for RL1 is removed when the interlock opens. What you don't have without the clutch switch or an equivalent on the dashboard, is any means of turning off the overdrive yourself except by changing gear. (By the way, the TR/saloon box already has interlock opening in neutral between 3/4 AFAIK - at least, the ex-2000 box in my '3A does)
  13. ... and dare I say it , rather OTT in my opinion (but each to his own). Perhaps it's because my electronics training came long before PICs were even dreamed of but I was always taught to keep things as simple as possible, commensurate with the function required. Two relays is all you really need for the 'logic' as per Andy's circuit above. Even if you want to get complicated and have only one momentary switch to operate it on and off, the most you need is a couple of transistors in addition. Since this is a fixed-function device there is absolutely no need for programmability, and voltage glitches don't affect relays.
  14. Ah you can't do the both-through-the-interlock suggestion then. The solenoid takes too much current.
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