High Octane Fuel and Cooling

[wiking85]

I'm aware that higher octane fuels produce higher horse power via increased compression ratios, which allow for more efficient piston cycles. This AFAIK means more revolutions for less fuel, but does that increased compression and higher potential RPM create a cooling issue? Could and existing engine designed around lower octane fuel be tuned to handle higher compression ratios and absorb the resulting RPMs, or would it require strengthening of parts and a better cooling system? Basically does there need to be a specially designed mark of an existing engine to handle that higher octane fuel for higher horsepower or can the lower octane designed 'out of the box' engine handle the extra RPMs and compression?

As an example would the DB601E engine with 1350 hp based on 87 octane fuel and an improved cooling system over the higher octane designed DB601N have been able to take the C3 94(?) octane fuel (or even the RAF 100+ octane) with just re-tuning and develop 1500hp, or would it have to 'slow down' its rpm to stay at the 1350hp mark with same RPM, but just be more efficient in producing that horse power rating?
Thanks for any help understanding this issue.

 

[mikewint]

Don't pretend to be an engine expert but as you compress air the temperature increases. The higher the compresion the higher the heat and fuels begin to ignite BEFORE they are supposed too, i.e. the application of a spark. This pre-ignition (dieseling/engine knock) occurs as the piston is rising causing violent vibrations within the cylinder that can shatter an engine. Low octane fuel easily ignites whereas high octane fuels are more difficult to ignite therefore they resist dieseling and require an actual spark to ignite. With computerized engine controls a wider octane range can be utilized as the computer adjusts the engine controls (such as timing) for the lower or higher octane fuel.
My car will utilize either low or high octane fuel as the onboard computer senses the octane level and adjusts engine parameters accordingly BUT low octane fuels will still give decreased performance as compared to a higher octane

 

[GregP]

Higher octane doesn't allow for more revolution. It allows for a higher cylinder pressure before detonation sets in. That is, the ropm would stay the same, but you could run higher manifold pressure, which makes more power.

So, you could run the propeller at slightly coarser pitch, theoretically going a bit faster at the same rpm.

RPM alone won't cause detonation, it is directly related to the pressure in the cylinder.

Once you get to an octane rating of 100, anything higher is not an octane rating, it is a performance rating. The highest actual octane rating is 100. A 130 / 150 grade fuel has a performance rating of 130 when run lean and 150 when run rich.

 

[wiking85]

What is the difference of a 'lean' or 'rich' mixture? More fuel relative to air? What does performance rating mean in this context?
Edit:
https://en.wikipedia.org/wiki/Compression_ratio

A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. This occurs because internal combustion engines are heat engines, and higher efficiency is created because higher compression ratios permit the same combustion temperature to be reached with less fuel, while giving a longer expansion cycle, creating more mechanical power output and lowering the exhaust temperature. It may be more helpful to think of it as an "expansion ratio", since more expansion reduces the temperature of the exhaust gases, and therefore the energy wasted to the atmosphere.

To me this reads as the higher compression resulting from better fuels means actually less heat due to less fuel being used to create the detonation and overall greater efficiency. So it would seem that higher performance fuel then means that the same engine properly tuned for greater compression would result in greater power without the need for the engine to be altered in its basic construction, i.e. the DB601E using C3 fuel would achieve 1500 hp (roughly) if properly tuned and compressed without any cooling system changes.

Am I correct in my understanding?

 

[mikewint]

Octane ratings are measured 3 ways:
RON - Research Octane Number: A test engine with the ability to vary compression is run at 600rpm no load conditions with the fuel to be rated. The compression at which the engine begins to knock is determined and compared to mixtures of ISO-OCTANE and HEPTANE knock tendancies. If the test fuel produces knock at the same point as a 90% Iso-Octane/10% Heptane mix the test fuel is rated at 90 Octane. The actual content of the tested fuel does not matter. It's resistance to pre-ignition is the same as 90-10 Iso-Octane/Heptane mix.
MON - Motor Octane Number: Similiar to the RON test except the test engine can also vary ignition timing. Plus it is run at 900rpm under load conditions. The point at which the test fuel produces knock is determined and compared to mixtures of Iso-Octane/Heptane. Since the test conditions are much more severe MON numbers are 8 - 10 LOWER than RON numbers.
PON - Pump Octane Numbers: In the US you will find the formula written on the pump, i.e. (R + M) / 2. Or simply the average of the fuels RON and MON ratings
Avgas - AViation Gas: Exists in many types with dyes added to tell one from another. The major difference is that Avgas still contains TEL (TetraEthylLead).
100/130 (green) Avgas: The 100 is the octane rating of a LEAN mixture and 130 the octane rating of a RICH mixture. The RICH simulates a supercharged condition with a rich mixture, elevated temperatures, and a high manifold pressure. In other words the engine is producing Maximum power, e.g. taking off or climbing. For simple crusing, less power is required and the engine runs a LEAN mixture.
RICH vs LEAN - Chemically Iso-Octane combines with Oxygen to produce Carbon Dioxide and Water. This complete combustion requires a 14.7 : 1 mass ratio of oxygen to fuel. RICH mixtures are any ratio with a higher fuel mass and LEAN would be the reverse. RICH mixtures produce INCOMPLETE combustion and Carbon Monoxide and or Pure Carbon are produced (the BLACK exhaust smoke produced at a hard acceleration).
Most aircraft now use 100LL (One Hundred Low Lead) with reduced amounts of TEL
As with Mogas the trend is towards Lead-free Avgas such as 91/96 UL (colorless) produced in Sweden

 

[mikewint]

Wiking, just read your post. OCTANE number has nothing whatsoever to do with energy content of the fuel. Ethanol added to gasoline increases the Octane rating but reduces the energy content of the fuel. Octane number just measures resistance to pre-ignition

 

[Shortround6]

You have a couple of things going on. Greg is quite right when he says that higher octane or performance number fuel allows for higher pressure in the cylinder with-out detonation or pre-ignition (knocking) setting in.
In an unsupercharged engine this allows more compression to be used and better fuel economy (more power for the same amount of fuel). 80 octane and 100 octane (or higher PN) having the same BTUs per gallon. Since you are burning the same quantity of fuel the heat load is the same and you are getting more power at the crankshaft (prop) for the same amount of heat, No increase in cooling needed.
In a supercharged engine it allows more boost to be used and this changes things a lot. An engine running 12lb of boost vs 6lb of boost is pushing almost 30% more fuel and air through the engine at the same rpm and so the heat load goes up 30% which means a change in cooling is definitely needed.

The next thing going on is that raising the the pressure in the cylinder before the spark plugs fire also raises the peak pressure in the cylinder even if it doesn't change peak temperature much. It doesn't matter if you raise the compression or raise the boost at the same compression, you are going to have more pounds per sq in acting on the cylinder head, walls and most importantly, the piston top. This gives you the extra power but it also increases the load on the piston top, the piston rings (blow by) the wrist pins, connecting rods, crankpin and bearings, crankshaft, crankshaft bearings and even the crankcase. If everything can handle the increased load all well and good. If just one link fails at the higher pressure/load the engine comes apart.

At the end of the war there was a push to move from 100/150 fuel to 115/145 fuel for several reasons, not the least of which was that the 115/145 fuel allowed either higher compression or higher boost pressure at lean settings for better cruise.

You are correct in the lean and rich settings. There is only a certain range of mixture at which gasoline will burn. Too much fuel results in no burn and too little will result in no burn. When running lean you have unused air but the best economy. Running rich uses ALL the air but has unburned fuel in the exhaust. The unburned fuel can carry heat away and act as a coolant.

Not all fuel acts differently in lean and rich mixtures. You can get the cooling benefit running rich from just about any fuel but the amount of compression allowed (or boost) changes very little from lean to rich. Certain compounds in fuel will allow the fuel to behave differently under high pressure when running lean or rich and these compounds allow such things as 100/130 and 115/145 fuel.

I hope this helps.
The chances of re-tuning an engine in the field to use a different grade of gas are about zero however. Most aircraft engines in WW II used fixed ignition timing unlike cars that use variable ignition timing. Trying to fool around with carb jetting or the fuel injectors is also not a real good idea. Aircraft engines often were NOT running at the most optimum settings but since the engine had to run at not only different rpms and throttle settings but with intake air varying in by well over 100 degrees F and air pressure varying widely (back before engine computers cars used in the mountains were often re-tuned for the lower air pressure) things could get a little chancy letting squadron mechanics fool around with the ignition timing and mixture settings on their own.
That said some American engines were given two specs or operating instructions. One for stateside use using 91 octane fuel for training and moving from US base to US base and another set for overseas/combat use.

edit, I agree with what Mike is saying too. But in some cases aircraft engines were run so rich that raw, unburned fuel was coming out the exhaust pipes in addition to the soot and carbon monoxide.

 

[Bretoal2]

In my opinion the answer to the original question is very simple.

About cooling: a gasoline engine has an efficiency of about 25%. This means that 75% of the fuel energy is transformed into heat.

Increasing the octane number of a fuel does not increase its energy content.

Increasing the compression ratio of an engine and its manifold pressure increases efficiency just a bit.

So in the end, the answer is mandatory : more power = more calories to evacuate.

And about load ... it is obvious that more power = more mechanical stress !

Regards

Alain

 

[rinkol]

The case of the DB 601E is interesting since there was apparently a high compression version, the DB 601R (not the pre-war racing engine). This was apparently used in some HS 130 protoypes, but was said to be unreliable, though little seems to be known with any certainty.

 

[Balljoint]

In a normally aspirated engine, i.e. nonsuperharged, higher compression is fairly simple. A given fuel charge will be confined to a smaller volume when combusted. The smaller volume develops a higher pressure with the same heat and combustion products. The greater pressure acting on the piston develops more force, or torque at the crankshaft, resulting in more power. Somewhat counterintuitively, since the higher compression develops more work from the fuel charge, the exhaust gases are cooler as is the engine heat load in general, though there may be localized higher temps at the head for example during combustion.

A boosted engine usually has a lower mechanical compression ratio but a larger fuel mixture charge can be pumped in by the supercharger. This generates more heat and is less efficient in that a smaller percent of the greater amount of heat energy is utilized as the piston moves through the power stroke.
Higher octane rating is a measure of the fuel's ability to tolerate higher pressure and temperature from higher compression rating and/or greater intake fuel charge.

It's really not all that complicated –I just make it seem that way it seems.

 

[parsifal]

I always believed that higher octane rated fuel, with lead added to the fuel, actually slowed the burn rate of the fuel. Fuel without lead, overall delivers less power because the ignition is not sustained and much energy is lost in the more rapid reaction of the fuel to ignition. . what us british types refer to as "standard" fuel actually ignites more or less instantaneously, whereas a slower burning fuel, with an ignition inhibitor added burns longer, allowing the full effects of the long stroke of the pistion to be utiilised.

i dont know about heating effects, but i do know if you run standard fuel in an engine tuned for super grade fuel, you will get a ping in the engine, and likley valve or pistion damage. Invariably running the wrong fuel will raise the operating temperature of the engine because it is impossible to get the ignition timing optimised

 

[Balljoint]

I always believed that higher octane rated fuel, with lead added to the fuel, actually slowed the burn rate of the fuel. Fuel without lead, overall delivers less power because the ignition is not sustained and much energy is lost in the more rapid reaction of the fuel to ignition. . what us british types refer to as "standard" fuel actually ignites more or less instantaneously, whereas a slower burning fuel, with an ignition inhibitor added burns longer, allowing the full effects of the long stroke of the pistion to be utiilised.

i dont know about heating effects, but i do know if you run standard fuel in an engine tuned for super grade fuel, you will get a ping in the engine, and likley valve or pistion damage. Invariably running the wrong fuel will raise the operating temperature of the engine because it is impossible to get the ignition timing optimised

Higher octane can result from slower burning fuel, usually cyclic or more branched hydrocarbons. However, TEL is different. Fuel charge detonation occurs when, under high temp and/or pressure, the oxygen become hyper reactive, i.e. free radicals. The lead in TEL sops up free radicals before detonation occurs. However, if the fuel mixture pressure and/or pressure aren't great enough to produce detonation for a given octane rating, higher octane is immaterial.
The burn rate of the fuel is independent of TEL provided octane, but often the more complex high octane hydrocarbons are somewhat slower burning. AVGAS has large molecules for a low vapor pressure at altitude and is notoriously slow burning. Not a good choice for a race car even if it's high octane.

 

[Shortround6]

There may be some confusion as far as slow burning and auto-ignition temperature. A higher auto-ignition temperature does not necessarily mean that fuel is slower burning once it does ignite.

You have a lot of things going on at the same time and sometimes in direct conflict.


Most large aircraft engines had a bit of a problem with fuel burn due the large size of their cylinders. You want the fuel burn to be pretty much finished with the Piston about 20 degrees past top dead center. The gas is still expanding and pushing on the piston but burning large of amounts of fuel once the piston is rapidly accelerating down the bore doesn't get you much except a very impressive exhaust flame. Most aircraft engines used dual ignition not only for reliability but for a wider flame front which means less time for the flame front to cross the cylinder. It is this speed of flame front travel that put a limit on the combination of cylinder bore size and rpm. Triple ignition could raise the limit/s but the complication of triple ignition was not thought to be worth it. Perhaps if the high PN fuels had not become available?

High PN fuel does two things, it doesn't start burning before the spark plugs fire (and in some engines they don't fire at the exact same time, depends on location and desired flame front shape and travel) and as the flame front moves across the cylinder it further compresses the remaining fuel/air mixture and the high PN fuel resists igniting ahead of the flame front. Obviously a cylinder full of mixture auto-igniting before or just as the piston hits TDC is a lot more harmful than the last 5-10% of the cylinder diameter auto-igniting as the piston is already moving downward although this can lead to local hotspots in the cylinder or piston leading to failure or even a source of ignition independent of the spark plugs if it goes on long enough.

 

[GregP]

Hi Mike,

14.7 : 1 by mass is about ideal, depending on air pressure. But you have rich and lean backwards.

Rich means too much fuel, so there is unburned fuel in the exhaust. Lean means too much air. That burns all the fuel and creates VERY high exhaust temperatures ... and can melt things. Most mixture controls adjust the fuel added to the intake flow.

The richest it has to be is rich at sea level or slightly below. It will be run leaner everywhere else. The leanest it has to be is the leanest setting the manufacturer recommends for EGT at the anticipated service ceiling.

Mixture controls can be pushed in or pulled out directly and also have a fine screw thread that can be used to turn it in or out a small bit at a time. That way, you can easily find the highest EGT (Exhaust Gas Temperature) reading and then go rich or lean according to manufacturer's recommendations.

If I recall correctly, I used to run the C-172's about 50 - 75° rich of peak. Many engines today are run lean of peak EGT for economy ... but it will cost you in heat stress on the engine. Running rich of peak will usually mean longer TBO than running lean of peak. The manufacturers say differently, but they sell engines ... and have no access to your bank account unless you are buying an engine or are buying parts for your engine.

Make your own choice there ... I run rich of peak and pay a bit extra for fuel. Never had an issue with it in 30+ years of flying, and never found metal in the oil filter yet. Also haven't yet burned through an exhaust pipe.

Hope to keep that record going but you can't ever tell, can you?

The future looks like diesels running on Jet A, and they probably have a whole new set of things to learn. I hope it isn't too expensive to learn it.

 

[Trilisser]

Greg, flying lean of peak is senseless as the optimum (minimum) specific fuel consumption is obtained at peak EGT. By operating lean of peak you simply get less power and higher sfc.

As for the overall topic, by coincidence I just recently obtained volume 1938 of Shell Aviation News from a library. One issue has an excellent article by Sir H. R. Ricardo on aiorcraft engines. He emphasizes that raising the compression ratio is a very poor way of increasing power. It raises BMEP a little but the peak pressure goes through the roof while the oppiste happens if the CR is kept low and the MAP is increased.

By the way, anyone willing to see what PN is a fuel of less than 100 octane number, there is a formula: 2800/(128-octane number). This formula was in Maxwell Smith's "Aviation Fuels".

 

[FLYBOYJ]

A great article on the pros (and cons) of operating lean of peak.

Dogfight: Running lean of peak - AOPA

Personally I fly slightly rich of peak, watch my EGT gauge (which has a pre-set mark for lean of peak set during ground runs at my local airport) and lean as required. I operate at 5600' AGL and this setting will change if I fly into lower terrain. The important thing here is not to operate so lean or rich that you are damaging your engine. I do notice that people who continually fly at sea level are less likely to monitor their EGT and lean for altitude.

 

[Trilisser]

Well, that article does not disprove my claim: The best fuel economy is obtained at peak EGT. And the problems listed speak volumes of the crappity of Lycocraps and Contishits pretending to be aero engines while being fishnet weights.

 

[FLYBOYJ]

Well, that article does not disprove my claim: The best fuel economy is obtained at peak EGT. And the problems listed speak volumes of the crappity of Lycocraps and Contishits pretending to be aero engines while being fishnet weights.

That's your opinion. Can better engines be designed and build for the GA market? Yes. Is it worth the certification process and headaches for those investing in new piston engine designs? Probably not for an engine operating on 100 LL. Rotax has cut into the market but even then they are limited to a small portion. In the mean time Continental and Lycoming has the corner on the GA engine market until some other company steps to the plate. With 100 LL going away, now is the time to do this.

As I told you previously, I manage 7 tow planes, all operate 180 Lycomings, all are used to tow gliders and all have consistently made TBO (we even take them to 2300 hours per a Lycoming SB). In 10 years I've had only 2 cylinder heads go bad before TBO, so much for crappy unreliable Lycomings.

 

[Trilisser]

There were numerous better GA engines in existence, like Argus, Hirth, Kinner, Jacobs, DH Gipsy. The latter was cleared to 1500 hrs TBO in 1944 in wartime conditions with wartime oils and mostly used fuels well below 100 octane with competitive sfc. My favourite would be a 5- to 7-cylinder radial with turbocharging, liquid-cooling, direct injection and sleeve valves.

 

[FLYBOYJ]

There were numerous better GA engines in existence, like Argus, Hirth, Kinner, Jacobs, DH Gipsy. The latter was cleared to 1500 hrs TBO in 1944 in wartime conditions with wartime oils and mostly used fuels well below 100 octane with competitive sfc. My favourite would be a 5- to 7-cylinder radial with turbocharging, liquid-cooling, direct injection and sleeve valves.

For one reason or another they did not go over well in the civilian market after WW2 (at least in the US market). All if not most of those engines were certified by the CAA (forerunner of the FAA) so I think it was a matter GA manufacturers choosing to go with opposed engines (Franklin was another opposed engine manufacturer that is no longer with us) with Lycoming and Continental getting a corner on that market. Additionally one has to consider manufacturability, maintainability and product support once the engine is in the field. I know a few guys who have taken the Hirth's out of Bü 131s and replaced them with Lycomings because of issues with replacement parts.