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Classic Engines, Modern Fuel

Comments on Topic: Cyclic varaibility origin

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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.

See: LS1Tech.com Thoughts about the swirl ramp

Another two year project with Manchester university !

Laurent.

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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

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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.

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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

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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.

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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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Submitted by awatkins
07-Oct-2023

Hi Paul --

I have the same question as one Paul asked: ". ...what is the physical property of the fluid that differs whith modern fuel ?"

Put another way, with all other conditions being equal, what is it about modern fuel that increases cyclic variability over that found with earlier fuel in the same engine?

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Submitted by The Author
07-Oct-2023

Hi,

Unfortunately, there is no one answer to your question. Fuel consists of a large number of different hydrocarbon molecules plus additives to prevent knock, etc. One factor is that modern fuel is more volatile at lower temperatures than earlier fuels. This will have two effects:

  1. A greater percentage of the petrol will evaporate in the carburettor and inlet manifold
  2. The evaporation profile of the liquid fuel in the cylinder will be different. Modern fuel will evaporate earlier in the induction / compression cycle than classic fuel. This will cool the gases reducing early stage thermal mixing.

These factors will affect how well and consistently the fuel evaporates and mixes with the air prior to the spark plug firing. A greater variation will increase cyclic variability.

The difference is not just between classic and modern fuel, it also appears to show itself with different brands or grades of modern fuel. I also own a Porsche 996 Turbo. This has a modern (2004) fuel injected engine. I can even notice differences in the way the engine runs between brands A and B of super grade fuel. I can also notice the same effect with my TC. Both cars run noticeably smoother (lower cyclic variability) on brand B. This suggests these brands use different additives that also impact the combustion profile.

Unfortunately, the petrol companies do not publish details of what goes into their fuel so it difficult to “point the finger” at any single component.

A simple answers to your question is: The differences in cyclic variability between classic and modern fuel is caused by both the difference in the volatility profile and the exact mixture of hydrocarbons and additives.

Paul

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