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Classic Engines, Modern Fuel

Comments on Topic: Isn't this just "induction", "compression", "ignition" and "exhaust"?

Submitted by The Author

No. The way an engine runs is far more complex. This chapter descrides these processes in very simple terms that will allow you to understand why modern petrol causes the problems it does and help aviod the damage it can cause.

Submitted by The Author

What is Cyclic Variability and why is it bad for an engine.

Submitted by Guy

Hi Paul, many thanks for the book, a really great insight. Just interested in the difference between Pre Ignition (ignition caused by glowing spark plug or glowing carbon etc) and detonation of pockets of end gas through heat/pressure after spark plug ignition. Can you explain the differences between pre ignition and detonation a little more, please?

Submitted by The Author

Thanks for the positive comment about the book.

Pre-ignition caused by a glowing spark plug or glowing carbon etc. is basically the same as pre-ignition caused by the detonation of pockets of mixture. The symptoms are the same as is the potential damage it can cause to the engine. What is different is the ignition source.

In both cases the mixture starts to burn from either a single or multiple points before the spark plug fires. This can lead to an uncontrolled burn and a rapid increase in cylinder pressure before the piston has reached TDC.

In the case of glowing carbon deposits on the piston crown, valves or cylinder head. If these are sufficiently hot to exceed the auto-ignition temperature of the petrol / air mixture, they will initiate the burn. The analogy is the glow plug used in some model petrol engines.

During the compression stroke, things are happening very quickly. The pressure in the cylinder does not rise equally at all points. A compression wave is formed as the piston rises which can cause pockets of mixture to become overly hot. If the compressive heating raises the temperature of these pockets above the auto-ignition temperature, they will begin to burn.

The only real difference is in the way this pre-ignition can be addressed. Hot carbon deposits may be the result of a heavily coked up engine or Cyclic Variability producing a large number of late combusting cycles. Detonation of pockets of the mixture may be caused by a low octane rating for the petrol.

Submitted by Guy

Thank you Paul, for your quick response. Very interesting, as I hadn't seen it from that way before. We build race engines for racing in the VSCC type events (cannot wait to be able to race again)! We supercharge engines, play with rolling roads and dynos, and the fun thing is seeing how increasing compression ratios, playing with cam timing, playing with inlet and exhaust systems translate into more power/torque. We have supercharged an Xpag engine recently in an MGTC, which performs faultlessly in the South of France, even in difficult traffic. You book is excellent as it consolidates our feeling about fuel in a scientific way. We have been altering fuel systems to create circulation to tank, and even giving people lessons on leaving bonnets open, and using plant sprayers with water to cool things down before a restart! We have played with octane booster and retarding ignitions a little when detonation happens, and have a detonation sensor we can bolt to the engines, with headphones that enable us to hear it in real time whilst driving. NGK created a mesmerising video about pre ignition, and knocking pre and post the spark plug starting the flame front, I thought you might like to see it. Hope you are keeping well, and entertained. Thanks again for the book, it will become a must read for all owners and fettlers!

the link to video below, very slick from NGK.

Submitted by The Author

Another interesting YouTube shows inside a running engine .You can see many of the features described in "Suck, Squeeze, Bang and Blow". Watch for:

  • the mist of petrol entering the cylinder.
  • If you watch carfully you can also see a droplet of fuel bouncing off the inlet valve.
  • See how the mist clears as the petrol vapourises.
  • See the small blue inition point when the spark plug fires and relatively how slowly it grows.
  • Finally see the yellow flames from the rich mixture during the power stroke.
Please wait .....