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Classic Engines, Modern Fuel

Comments on Topic: Cyclic varaibility origin

Submitted by Anonymous
21-Aug-2020

Hi,

I understand that many problems we are facing come from the cyclic variability. Slow combustion, hotter combustion, making hot restart issue worst. But our engine were not subject to these problems in the 50's. So I'm wondering what is the physical property of the fluid that differs whith modern fuel ? Tuning the carburettor and the advance curve are only compensations but could we deal with the origin of the cyclic variability ? Atomizing and size of mixture droplets is a key as shown with the nebuliser. So I was thinking of a solution with a helix colar around the stem of the inlet valves or a rough surfaced jet or needle. 2 helix inserts in the inlet manifold? A special butterfly ? Googling those subjects leads to https://usalpc.com/swirling-intake-valves.asp. This guy also makes a intelligent plug that senses the denser mixture. I found also many topics about swirl porting head or dimple porting.

https://www.google.com/imgres?imgurl=https%3A%2F%2Fls1tech.com%2Fforums%2Fattachments%2Fgeneration-iii-internal-engine%2F477159d1416054723-thoughts-about-swirl-ramp-landspeed3i.jpg&imgrefurl=https%3A%2F%2Fls1tech.com%2Fforums%2Fgeneration-iii-internal-engine%2F1756233-thoughts-about-swirl-ramp.html&tbnid=HW-7Qd3Kg8_5KM&vet=12ahUKEwjw6c6Z_6zrAhUY-hoKHaH4CoAQMygPegUIARC2AQ..i&docid=be91iJ8JdlHwPM&w=720&h=540&q=porting%20swirl%20port%20heads&client=firefox-b&ved=2ahUKEwjw6c6Z_6zrAhUY-hoKHaH4CoAQMygPegUIARC2AQ.

Another two year project with Manchester university !

Laurent.

Submitted by The Author
22-Aug-2020

Laurent,

Thanks for these observations. These suggest other people are also aware of the issues caused by poor mixing. The irony of swirling valves is that, by increasing turbulence, they reduce the volume of air entering the cylinder. I.e. against the normal practice of race tuning an engine.

Older cars often had un-polished ports and mismatched inlet manifolds all of which increased turbulence. Like many others, I have smoothed my inlet and exhaust ports and ground the head & manifolds to remove any discontinuities. Worth it on the exhaust manifold but probably counterproductive for normal road use in the inlet manifold.

Your suggestions about rough surfaced jets or needles is something worth thinking about. These may improve atomisation and dispersion of the petrol without restricting the air flow into the engine.

One of my thoughts was to cut a spiral groove inside the jet, stopping just below the end. This would cause the fuel to swirl as it came up the jet. As long as the annulus at the top of the jet is not changed, this would not affect the mixture.

Possible something for somebody to try.

Paul

Submitted by Anonymous
22-Aug-2020

Thanks Paul for this answer. I can see that you already had ideas to improve atomizing.

Another solution could be a fuel additive. I know that fuel contains anti-foam. Could it be a reason ? Could we find something that would help the mixture ?

We need experts to solve our issue. Fluid mechanics, chemists. Unfortunately, I'm not.

Laurent.

Submitted by The Author
24-Aug-2020

Laurent,

Some time ago I read an article suggesting acetone reduced the surface tension of petrol. I tried to measure this effect using a capillary tube but I did not have an accurate travelling microscope so was unable to come to any real conclusions. The lower the surface tension of a fluid the more easily it will break up into droplets.

However, this paper shows that adding 3–10 vol. % of acetone to petrol reduced emissions. Although this paper focusses on the combustion mechanisms,the effect of acetone is very similar to, for example, the use of the nebulizer. This suggest it may reduce the surface tension, improve atomization and reduce cyclic variability.

Should you try adding acetone, I suggest you are very careful as it can be very damaging to paint!

Paul

Submitted by Anonymous
24-Aug-2020

Very interesting article. That is an idea. Many performance parameters increase but exhaust temperature also increases and that is far from what we need !

Nothing about the mixture control, injection, type of carburettor. Few details about the ingnition advance.

Also, surprisingly, the article does not mention the phenomenom of cyclic variability.

Paint can be protected or thoroughly avoided but acetone dissolves many plastic materials. What about the tank coating, the plastic filters, the hoses, the pump diaphragms...I won't try.

Laurent.

Submitted by The Author
25-Aug-2020

Laurent,

To answer a couple of your points.

  1. Exhaust temperature – the tests were run on full throttle (WOT). As adding acetone made the engine run more efficiently, it produced more power and hence a hotter exhaust. When you drive on the road, you use a throttle setting that produces the power needed to keep the car going at that speed. If the engine runs more efficiently, it produces more power for any throttle setting – hence you will need less throttle to maintain that speed. In turn this will produce a lower exhaust temperature. Improving the efficiency of the engine is beneficial.
  2. Cyclic variability is not a commonly known phenomena. It is very difficult to measure and I am not surprised this paper focussed on the chemical effects of the acetone rather than the physical effect of it reducing the degree of cyclic variability. I have contacted the author of the paper asking for his views on this.
  3. As you say acetone will dissolve most plastic materials. Even at 3% it could cause significant damage to fuel system components. However, this is something to try. All you need is a sealed container (preferably not plastic) a small quantity of petrol and some pieces of petrol hose, gaskets, etc. Leave them a few months and see how they turn out. It may be that at such low percentages acetone does not cause any damage.

Paul

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