This website uses cookies to improve navigation. No personal data is held. Do you agree to the use of these cookies?    
Classic Engines, Modern Fuel

Comments on Topic: Cyclic variability spark plug gap

Share this topic item by email
Submitted by Rog


I have a few questions on cyclic variability:

What characteristic of modern fuel do you think increases the cyclic variability (reduced combustion eff.) at lower rpm, is it linked with the mixture (air fuel ratio), being less easy to control with the higher volatility at lower temps, with a carburetted engine?

Do you think the combustion chamber design (trying to target a combustable mixture at the spark plug)is more important, than intake manifold, for improving cyclic variability?

Also, several manufacturers have used a wider spark plug gap to address engine smoothness (cyclic variability), I guess this very slightly increases the ignition probabilty at the spark plug, where the mixture ratio is varying from cycle to cycle. Did you test that on the XPAG engine?



Share this topic item by email
Submitted by The Author


Your comment about spark plug gap is very sensible and one issue that would warrant further investigation. Unfortunately, we were only able to perform one set of tests on the engine. There was no opportunity to go back and investigate issues such as the one you raise in more detail.

I can only speculate on the reasons for modern petrol increasing the level of cyclic variability. I think you are correct in suggesting the higher low temperature volatility of modern petrol is the cause. The evidence from Manchester that supports this conclusion, is that fuels such as the Sunoco Optima 98, Avgas and Standard petrol with added kerosene burned better. All these had a lower percentage of front end components.

However, it is my view this is more to do with mixing and vaporisation in the cylinder rather than poor control of the inducted mixture. My reason is that the time for each cycle is so fast it is difficult to imagine how a mechanical device, such as a carburettor, could deliver a different mixture cycle on cycle. I would imagine variations in the mixture from the carburettor would have occurred over longer timescales. During the tests, other than vibration of the carburettor suction pistons, mentioned in the book, there were no noticeable variations in AFR or carburettor temperatures during a test run.

Turbulence and mixing takes place in both the inlet manifold and cylinder. While it was not possible for us to investigate the effect of combustion chamber design, the addition of the nebulizer (in the inlet manifold) certainly had a large effect. This demonstrates the importance of turbulence in the inlet manifold.

Speculating on the sparkplug gap. The tests with changing the sparkplug energy (Chapter 12) showed there was no effect. For this reason, I suspect that the spark plug gap will also have no effect. However, there are other factors which a bigger sparkplug gap could address. For example, modern fuel building carbon deposits on the plug that could “short it out”.

The second piece of evidence is from tests I performed. On the XPAG, the standard sparkplug sits in a “hole”. The electrodes do not penetrate the combustion chamber. I have tried longer nose spark plugs whose electrodes reached into the combustion chamber. While this “looks” a far better arrangement, it did not help improve the running. The longer nose plugs suffered from “wash down” where the inducted petrol was hitting the electrodes, possibly leaving them “wet” when they fired. In contrast the standard plugs would experience a greater level of turbulence as the mixture flowing over the “hole”, swirled and mixed around the sparkplug.

Hope this helps.


Please wait .....