can someone explain exhaust staining?

[Killingholme]

Hi all,

As per the title, can someone explain the reason why exhaust staining on piston engined aircraft varies so much? Everything from carbon black, via rust, to a chalky white. In the dark recesses of my knowledge I suspect this is something to do with octane of the fuel and the mixture, but I don't really understand much more!!

I know this sounds like an elementary question, but I think understanding what causes all the different shades would help me decide how to weather my aircraft models- especially when I'm working from B&W photos

Cheers!

[bentwaters81tfw]

It largely depends on the type of aircraft and mission.

Black is the standard sooty exhaust, the brown tends to be heat burn on metal panels or where paint has been burned off the exhaust stack and it has rusted.

White staining is the tetraethyl lead which was added to the fuel.

If something like a Lancaster is on a high altitude mission over several hundred miles, the flight engineer will 'lean off' the mixture for best range and efficiency. A P-38 on long range escort will be the same.

Something like a fighter in a combat situation will have a darker stain due to engine settings, it's not really in a 'cruise' setting, so there will be no evidence of the lead in the exhaust stain.

Rather like cars before cats were fitted and lead was in the fuel. Short hops around town and you had a black exhaust, a long trip at a constant steady speed, and a correctly tuned engine would have a pale grey exhaust, or even nearly white if it had been thrashed over distance.

[John_W]

As I understand from other posts, Bombers and Transports have white/grey deposits, because the engines are run for range and efficiency, and the staining comes from the lead in the fuel.

Fighters are run at full revs for speed, so the fuel burns less efficiently and is therefore sooty and black/brown.

[brewerjerry]

Hi

There will most likely be a few aircraft with worn engines that burn/leak oil as well.

cheers

jerry

Edited January 4, 2016 by brewerjerry

[Graham Boak]

You'll find plenty of photos of bombers with dark stains, so it is never a simple matter. The RAAF Spitfires operating over New Guinea late in the war would "lean out" the engines, resulting in lighter stains than usual for the type. Always work from photos rather than simple assumptions.

[fingers]

Isn't it true to say that the Luftwaffe aircraft had dirty brown stains due to the quality, octane and base (coal as against oil) of the fuel they had available towards the end of the war?

[hacker]

exhaust stains are like brown marks on your underwear after the passing of a gaseous anomaly . Ok now l am leaving as l am in a mood to be silly

[Stonar]

The composition of the fuel, the engine and the way it was being managed would be the most relevant factors in the colour of the staining. The pattern also varied and was very type typical. It certainly did not always extend in a nice straight line behind the exhausts, nor was it always the same on both sides or from engines in different positions. I think that the advice already given above, to check references, is probably the most pertinent.

Cheers

Steve

Edited January 4, 2016 by Stonar

[Darby]

Isn't it true to say that the Luftwaffe aircraft had dirty brown stains due to the quality, octane and base (coal as against oil) of the fuel they had available towards the end of the war?

That was probably VW getting involved somewhere

Interesting thought though

[melvyn hiscock]

A friend of mine, who flew warbirds, told me that when you are taxiiing in a Corsair, you have to look down the side of the nose to see where you are going but also to watch the exhaust as you need to lean the engine on the ground. I would also do this in the Rearwin which would run rough on the ground as it sooted up unless you did this.

[bentwaters81tfw]

A friend of mine, who flew warbirds, told me that when you are taxiiing in a Corsair, you have to look down the side of the nose to see where you are going but also to watch the exhaust as you need to lean the engine on the ground. I would also do this in the Rearwin which would run rough on the ground as it sooted up unless you did this.

That's interesting, do you put the engine in 'full rich' for takeoff?

[fingers]

Isn't it true to say that the Luftwaffe aircraft had dirty brown stains due to the quality, octane and base (coal as against oil) of the fuel they had available towards the end of the war?

That was probably VW getting involved somewhere

Interesting thought though

I read it somewhere and if you look at the staining on some of there aircraft (one shot of a Ju 88 comes to mind) you can believe it.

[Killingholme]

Thanks guys, useful info to help me interpret reference photos- especially black and white ones!

Will

[Darwinism]

Some interesting posts here clarifying something that I'd clearly not thought enough about in the past. Thanks everyone - this info will help me with future builds.

[melvyn hiscock]

That's interesting, do you put the engine in 'full rich' for takeoff?

Yes, the problem is when it is idling. I had a long chat to Stephen Grey at La Ferte Alais this year which was enlightening too. On the Sea Fury you take off at full power because you need excess fuel to cool the engine as you have not got enough forward speed to cool the cylinders. This could well the same on the Corsair as that has a gert big radial too. Even little Lycomings are partly cooled by fuel, the mixture is usually too rich. I used to lean the Rearwin back a little in cruise keeping a good eye on the Ts and Ps.

[Wolfgang]

As others wrote, It depends on the kind of fuel, the engine settings and the surface and paint of the plane.

Especially the fuel and the engine settings can affect the staining considerable.

I am running an oldtimer and I can tell you from the exhaust staining If the engines runs in a good mixture. Black stains means more fuel and less oxygen, a rich mixture. The engine will be cooler but the fuel can wash out the oilfilm at the cylinder walls. Deer brown stains means the engine runs in a perfect mode with a well balanced mixture and grey or even whitish stains means the engine runs in a lean mixture. The engine consumes less fuel but more oxygen. the engine temperature rises especially at the heads and valves and can damage the cylinder head.

[3DStewart]

Isn't it true to say that the Luftwaffe aircraft had dirty brown stains due to the quality, octane and base (coal as against oil) of the fuel they had available towards the end of the war?

The staining under Luftwaffe aircraft, especially multi-engined aircraft could be astonishingly black and widespread, sometimes with an oily sheen to it. I understand this was because they used inverted V engines which leaked and burnt oil even when working correctly.

With regard RAF heavies you could get both white and black stains on the same aircraft:

[Black Knight]

Its not so much that the engines are running incorrectly when burning oil, at that time engines were designed to burn oil at a certain rate as upper cylinder lubricant, ala 2-stroke engines.

On my Austin 7s, the racing one burns 1pint per 100 miles, the tourer uses 1 pint per 500 miles, and the rally car uses about 1 pint per 200/250 miles.

In my motor club all the cars burn oil at various rates, even the RR cars.

[bentwaters81tfw]

Its not so much that the engines are running incorrectly when burning oil, at that time engines were designed to burn oil at a certain rate as upper cylinder lubricant, ala 2-stroke engines.

On my Austin 7s, the racing one burns 1pint per 100 miles, the tourer uses 1 pint per 500 miles, and the rally car uses about 1 pint per 200/250 miles.

In my motor club all the cars burn oil at various rates, even the RR cars.

Jaguar XK engines burned a pint per 500 miles from new.

[baldeagle]

Thanks for the Lancaster picture 3DStewart.Judging by the stain on the tailplane,it looks as if the starboard outer is due for replacement!

[Yankymodeler]

Do remember the color of the underlying surface can affect one's perception of the exhaust residue. Grey on an overall dark surface will appear to the eye as'darker' then the same grey on a light surface.

Eric aka The Yankymodeler

[303sqn]

Sooty deposits may originate from number of causes such as rich fuel/air mixtures but may also indicate that the fuel contains high amounts of aromatics, a characteristic of which is they burn with a smokey flame.

Aromatics, chiefly benzene and toluene, are a desirable component as they are octane good, about 105 RON. Before the introduction of tetra ethyl lead (TEL) from the USA, the method of increasing the octane of gasoline in the UK (and Europe) was to add Benzol. Benzol, produced by the eponymous British Benzol company, was a mixture of benzene and toluene extracted from coal tar. In the early days of motoring it was considered too strong for a normal engine and they started mixing it with petrol. In 1922 benzol fuel was replaced with a 50/50 mixture" of Benzol and petrol. Neat Benzol continued to be marketed as an effective anti-knocking additive. Benzol was also the basis of the so called “exotic fuel” used by the Supermarine entrants in the Schneider Cup races.

The white/grey deposits are lead bromide. Leaded petrol is produced by adding ethyl fluid. Ethyl fluid is a mixture of tetra ethyl lead (TEL) and the so called scavengers ethylene dibromide and ethylene dichloride. (Post war ethylene dichloride largely replaced ethylene bibromide for economic reasons. i.e., it was cheaper.) Lead has a very high boiling point, much higher than the combustion temperatures in the engine and the lead formed from the combustion of the fuel cannot be vaporised. The lead is deposited inside the engine and exhaust system causing problems such as fouling the spark plugs and burnt valves. A method of removing the lead had to found and the solution was to convert the lead into a lead halide (lead bromide or lead chloride), which has a lower boiling point. The lead halide vaporises at normal engine combustion temperatures and is expelled with the exhaust gases.

The quantity of scavenger added to TEL in ethyl fluid is calculated on the amount of lead present. The amount of scavenger that would theoretically convert all the lead present is called a 'theory'. Ethyl fluid was produced in a range of 1.0T to 1.5T but for aviation gasoline the limit is 1.0T. This is because the excess bromine forms hydobromic acid which reacts with the metal in the engine. Ethyl fluid was dyed so that lead petrol could be recognised. For aviation fuels the colour was blue.

[303sqn]

Isn't it true to say that the Luftwaffe aircraft had dirty brown stains due to the quality, octane and base (coal as against oil) of the fuel they had available towards the end of the war?

No. This is complete bunkum invented by idiots that know nothing about petro-chemistry and no idea what octane ratings are and has petrochemists rolling around the floor splitting their sides with mirth.

You cannot compare the octane ratings of automotive gasolines with aviation gasolines, octane ratings used in the UK, Europe and Australia and most other countries in the world with those used in the USA, Canada, Brazil, and you cannot compare the octane rating quoted for German wartime aviation fuels with those quoted by the Allies because they are all different, measured under different conditions.

The octane rating of petrol is measured in a test engine and is defined by comparison with a mixture of 2,2,4-trimethylpentane (iso-octane) and n-heptane that would have the same anti-knocking capacity as the fuel under test: the percentage, by volume, of 2,2,4-trimethylpentane in that mixture is the octane number of the fuel. For example, petrol with the same knocking characteristics as a mixture of 90% iso-octane and 10% heptane would have an octane rating of 90. A rating of 90 does not mean that the petrol contains just iso-octane and heptane in these proportions, but that it has the same detonation resistance properties. Because some fuels are more knock-resistant than iso-octane, the definition has been extended to allow for octane numbers greater than 100.

In 1926 Graham Edgar suggested using these two hydrocarbons because they could produced in sufficient purity and quantity and they both have similar boiling points, thus the varying ratios should not exhibit large differences in volatility that could affect the rating test. N-heptane was already obtainable in sufficient purity from the distillation of Jeffrey pine oil. 2,2,4-trimethyl pentane was synthesised and today is usually called iso-octane. Iso-octane is a highly branched alkane (paraffin) and branched alkanes burn better than chain alkanes. Iso-octane is given an octane rating of 100 and n-heptane zero. Automotive fuel octane ratings are determined in a special single-cylinder engine with a variable compression ratio ( CR 4:1 to 18:1 ) known as a Cooperative Fuels Research ( CFR ) engine. Only one company manufactures these engines, the Waukesha Engine Division of Dresser Industries, Waukesha. WI 53186.

Modern automotive fuels have two octane ratings, measured under different conditions. In this country, if you look at the octane ratings on the petrol pumps you will see “RON” next to them. RON is the Research Octane Number. The petrol also has another, very important, octane rating called “MON”. MON is the Motor Octane Number. During the late 1930s - mid 1960s, the RON became the important rating because it more closely represented the octane requirements of the motorist using the fuels/vehicles/roads then available. In the late 1960s German car manufacturers discovered their engines were destroying themselves on long Autobahn runs, even though the RON was within specification. They discovered that either the MON or the Sensitivity also had to be specified. The Sensitivity of a fuel is the difference between RON and MON. Because the two test methods use different test conditions, especially the intake mixture temperatures and engine speeds, then a fuel that is sensitive to changes in operating conditions will have a larger difference between the two rating methods. Modern fuels typically have sensitivities around 8 to 10. Today it is accepted that no one octane rating covers all use.

There is a third octane rating, called Observed Road Octane Number (RdON). It is derived from testing gasolines in real world multi-cylinder engines, normally at wide open throttle. It was developed in the 1920s and is still reliable today. The original testing was done in cars on the road but as technology developed the testing was moved to chassis dynamometers with environmental controls to improve consistency.

Visitors to the USA from Europe note that the octane ratings on petrol pumps are lower than those available in Europe. This is not an indication that they use lower octane fuels in the USA. In the USA you may see the symbol (R+M/2)on petrol pumps. (R+M/2) is the Anti Knock Index (AKI) and is the average of the RON + MON of the fuel. It is sometimes called the Pump Octane Number (PON).

AKI is used in the USA, Canada, Brazil and a few other countries and takes into account the fuel sensitivity. In 1994, there were increasing concerns in Europe about the high Sensitivity of some commercially available unleaded fuels. If the octane is distributed differently throughout the boiling range of a fuel, then engines can knock on one brand but not another brand. This "octane distribution" is especially important when sudden changes in load occur, such as high load, full throttle, acceleration. The fuel can segregate in the manifold, with the very volatile fraction reaching the combustion chamber first and, if that fraction is deficient in octane, then knock will occur until the less volatile, higher octane fractions arrive. With AKI high sensitivity will reduce the octane rating. The US 87 (RON+MON/2) unleaded gasoline is required to have a MON of 82+, thus preventing very high sensitivity fuels.

Aviation gasoline used in piston aircraft common in general aviation have different methods of measuring the octane of the fuel. The first number is the Aviation rating also called the Lean Mixture rating, and the second number is the Supercharge rating also called Rich Mixture rating.

The Aviation rating is determined using the automotive Motor Octane test procedure, and then corrected to an Aviation number using a table in the method - it's usually only 1 - 2 Octane units different to the Motor value up to 100, but varies significantly above that e.g. 110 MON = 128 AN.

The second Avgas number is the Rich Mixture method Performance Number (PN - they are not commonly called octane numbers when they are above 100 ), and is determined on a supercharged version of the CFR engine which has a fixed compression ratio. The method determines the dependence of the highest permissible power (in terms of indicated mean effective pressure) on mixture strength and boost for a specific light knocking setting. The Performance Number indicates the maximum knock free power obtainable from a fuel compared to iso-octane = 100. Thus, a PN = 150 indicates that an engine designed to utilise the fuel can obtain 150% of the knock limited power of iso-octane at the same mixture ratio. This is an arbitrary scale based on iso-octane + varying amounts of TEL, derived from a survey of engines performed decades ago. Aviation gasoline PNs are rated using variations of mixture strength to obtain the maximum knock limited power in a supercharged engine. This can be extended to provide mixture response curves which define the maximum boost (rich - about 11:1 stoichiometry) and minimum boost (weak about 16:1 stoichiometry) before knock.

The common piece of claptrap that the Germans were forced to use low-octane rated fuels is based on a very common misinterpretation about wartime fuel octane numbers. A common British aviation fuel of the later part of the war was 100/125. The misinterpretation that German fuels have a lower octane number (and thus a poorer quality) arises because the Germans quoted the lean mix octane number for their fuels while the Allies quoted the rich mix number for their fuels. Standard German high-grade aviation fuel used in the later part of the war (given the designation C3) had lean/rich octane numbers of 100/130. The Germans would list this as a 100 octane fuel while the Allies would list it as 130 octane.

After the war the US Navy sent a Technical Mission to Germany to interview German petrochemists and examine German fuel quality, their report entitled "Technical Report 145-45 Manufacture of Aviation Gasoline in Germany" chemically analysed the different fuels and concluded "Toward the end of the war the quality of fuel being used by the German fighter planes was quite similar to that being used by the Allies".

Between the wars Germany developed a number methods of producing fuels from coal. Simple distillation of lignite (brown coal) or bituminous coal provided poor quality products that could be used for diesel or lubricants.

Best known is Fischer–Tropsch developed by Franz Fischer and Hans Tropsch in1925. The process is a gas to liquid conversion of carbon monoxide and hydrogen that produces synthetic lubrication oils and synthetic fuel. Fischer–Tropsch plants using coal or related solid feedstocks have to first convert the solid fuel into gaseous reactants, carbon monoxide, hydrogen, and alkanes. The process is called gasification and the product is called synthesis gas (Syn gas). The under high pressure and temperatures the syn gas reacts forming mainly straight-chain alkanes of 10-20 carbon atoms suitable for diesel fuel. Small amounts of alkenes, alcohols and other oxygenated hydrocarbons are also produced. It is estimated that Fischer–Tropsch production accounted 9% of German war production of fuels and 25% of the automotive fuel.

The Bergious process was the most widely used method of liquid fuel synthesis

during the war. Patented by Friedrich Bergius in 1913, Theodor Goldschmidt invited him to built a plant at his factory in 1914. However, production did not start until 1919.

The process uses dried powder coal as the feedstock (lignite is preferred) which is reacted with hydrogen at temperatures of 400 to 500 C under high pressure (usually with a catalyst) to produce heavy oils, middle oils, gasoline, and gases. The heavy oils are recycled by mixing with the coal feedstock while the middle oils are hydrogenated to produce more gasoline. The reactor product is stabilized by passing it over a conventional hydrotreating catalyst to give a product stream high in naphthenes and aromatics, low in paraffins and very low in olefins. The different fractions can then undergo processing such as cracking and reforming to produce synthetic fuel of desirable quality. Platforming converts most of the naphthenes to aromatics and the recovered hydrogen recycled to the process. The final product contains over 75% aromatics with a octane rating of over 105 RON.

[Bear Paw]

There is some very good information here about how the exhaust staining looks on aircraft.

Thank you.

[bentwaters81tfw]

Very comprehensive articles, thank you. I'm no chemist, but I was aware of some of the above, particularly the knock effects on engines of the same nominal ratings from different producers. Your average Joe on the street would use fuels from different producers, then wonder why their engines behaved differently.