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Detonation/pinging - internal pressure before flash

Discussion in 'Technical and Troubleshooting Torque' started by Ljiljan, Sep 19, 2011.

  1. Does anyone happen to know what pressure on the fuel mix in the piston will cause pinging? I'm assuming it greatly depends on the ratio of the fuel/air mix (I'm guessing a rich mix will be more susceptible) but I guess to make the answer a bit simpler a normal mix is assumed.

    Not all that important, just completely out of interest.
    Thanks all.

  2. Sounds lean got a leak?
  3. Dont you mean temperature causing pinging?

    Rich mixtures should be less prone to pinging, as is the thought in stock tuned turbo vehicles.

    Stoichiometric mixtures actually run hot, even though they are technically the most efficient.
  4. Advanced timing can also affect it
  5. \/ \/ \/
    Besides that, I'm not sure. In my experience it's easiest to induce pinging when having a very open throttle at low engine speeds. This lead me to think that it might be more applicable to rich mixtures - although quite possibly just overcompression due to having the valves open for longer.
    Possibly, diesels detonate because the fuel doesn't vaporise and so the temperature has to get hot enough to light the liquid, yes?

    Ultimately it amounts to the same thing, particularly in a diesel

    My question comes because I've been given a design task and in the outline it specifies a normal tune combustion pressure of 1.1 MPa (wishful thinking, maybe before combustion) but has a poor tune combustion pressure of 2 MPa. Initially it seemed odd to me that the normal tune would have a lower pressure than a bad tune so I decided to put the question out there.
  6. I don't think there's any hard and fast rule. As well as mixture strength, combustion chamber shape has a huge influence. For example, it's entirely possible to lower the compression ratio on a two-stroke by putting in a thicker head gasket but make it more prone to pinking due to losing the squish band.

    As another example, the ancient Morris Oxford upon which I cut my driving teeth had a CR of about 2.5:1 once you took into account all the leakage and wear but would still pink like buggery at big throttle openings 'cos the combustion chamber design of the old BMC B-Series lump was rubbish. It had a nice little nib of cast iron between the valves that almost seemed designed to cause detonation by glowing incandescent if the engine was worked hard. Which brings up the other critical point of heat management, which a lot of older engines, car and bike, don't do very well.

    Conversely, modern engines can get away with 10:1 or more thanks to superb combustion chamber design and effective cooling systems, aided by knock sensors in the engine management systems which retard the ignition timing if they detect the hammerblows of detonation.

    If you really want to get a good handle on this stuff, I strongly recommend reading pretty much anything by Harry Ricardo, the man who, quite literally, wrote the book on how the petrol engine actually works, in the early 20th Century. I have to admit that I've only read his autobiography, but even that provides a fascinating insight into a time when research on the internal combustion engine was only just moving from trial and error to the truly scientific.
  7. See: Adiabatic Compression.
  8. I do understand the concept quite well.

    My question was that assuming modern engine technology and quality, what is the point where the pressure and hence temp gets high enough to cause the detonation.
  9. It would depend on the octane rating of the fuel. IMO.

    Also, At wide open throttle but low engine speed, youre trying to suddenly dump a huge amount of air into the cylinder. This leans out the mixture. Fuel flow into a cylinder in a carburetted engine is dependant on airflow(venturi effect) through the carb, too much air too quickly and the petrol cant keep up with the amount of air your letting in.
  10. I would think the main/base factors purely in that respect would be the nominal flashpoint of the fuel in question, along with the ratio and volume of the mixture at a particular throttle opening (and rpm), and the hottest spot inside the combustion chamber, which pretty much makes up the basic Fire Triangle that makes it all happen, then pulls it's corners this way and that with any changing variables. I imagine the air/fuel mix variation could alter the flashpoint of a given fuel. Possibly also the speed of detonation might contribute to the severity.

    Obviously for a given compression ratio, how much it gets squeezed depends on how much is let into the cylinder on that particular cycle. As well as revs playing a part in how quickly that happens (along with how even the mix is after getting swirled about), piston speed at a given rpm of the engine design in question might also add another variable to that aspect.

    Ambient air temperature should factor too, and in theory, any particles in the chamber (carbon build-up or what might get past the air filter) could be another potential ignition source. O2 systems are similarly at risk from particles under rapid compression causing detonation if a valve is opened quickly.

    I suppose pre-ignition in the common low rpm scenario is also somewhat relative, since "pre" is also relative to how much advance the spark has, and I think the recent sytems more actively alter that than just by outright rpm, both to keep optimal running and prevent these problems. The EFI on the recent BMW bike systems seems to manage very good fuel economy, which I think is made possible by both minimising the passive contributors as well as actively monitoring and countering the variable ones, so it gets away with leaning off quite a bit.

    Maybe if the speed of detonation that's normally taken into account when calculating the optimal ignition advance can be altered by any of a number of factors (like a change of fuel), then maybe a more trigger-happy detonation even after the spark could be less than ideal, although that shouldn't technically count as Pinging.
    • Like Like x 1
  11. Thanks Wayned.
    Let me just make sure I understood you correctly. At full noise a WOT (carb'ed), while dumping a greater amount of fuel into the chamber actually increases the air ratio of the mixture? Hmm, ok. Surely this would have negative effects at high engine speeds?
  12. On your original question, the RON would dictate the point at which it combusts, but there are plenty of factors that would play into it, at best I reckon you could say something like "Under ideal conditions, XXRON fuel will combust under XX pressure".

    It won't happen at high RPM, it's only when you got from closed or partly open throttle to WOT.
  13. My understanding of the reply was that at low speed when you open the throttle all the way his bike backfired/detonation, his assumption was that the WOT caused too much fuel and a rich mixture.
    In this instance, Sudden WOT at low speed, will cause too much air and a lean mixture until the airflow increases enough to draw more fuel through. The higher you are in the rev range, the less effect sudden WOT will have because youre not going from near closed to full open. your going from needle jet + main jet to full main, not from Pilot or very early needle jet to full main jet. CV carbs operate a little differently, the CV design slows everything down a bit.

    A very crude analogy is to picture a bath tub half full of water with a ring of sand a couple of inches from the plug, the Plug is the butterfly controlling the airflow, water is the air and sand is the petrol. When you pull the plug, (WOT) the water near the hole goes down first, (air into intake) as its flowing the water starts to pick up the sand (fuel) and drags it down the plug hole as well, That first bit of water (air) hasnt been able to pick up any sand, (petrol) and you get a lean mixture until everything reaches a balance.