gkamysz

My Feedback: (19)

Auto ignition temperature of castor is 450 degC. I don't think that castor is having any effect. We know that the castor isn't burning in our engines. It can't vaporize in our engines. It may work in an injected diesel where vaporization is mainly done by the injector. Cetane rating describes the ignition property of fuel. If it's not buring what difference does it make?

The castor will probably settle out at lower temperatures in a straight kerosene mix. I had this problem when i was running my diesel this spring with lower ether content mixes. With enough ether the castor would stay in solution.

Did I read the your Youtube right in that with 20% amsoil mix the engine would not throttle at all?

Greg

AndyW

My Feedback: (1)

So a Cetane Index is a mathematical determination whereas a Cetane Number is determined experimentally. And note the last sentence. Cetane boosters do a magic that is not refelcted in the Cetane Index but do affect the Cetane Number.

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cetane number
A rating on a scale used to indicate how quickly a fuel will ignite in a diesel engine, a figure of merit for diesel fuel as the octane number is a figure of merit for gasoline burned in spark ignition engines. A diesel engine run on a fuel with a lower cetane number than it was designed for will be harder to start, noisier, operate roughly and have higher emissions.

In a diesel engine, the fuel is ignited by hot air; the air is heated by compression. The fuel is injected into this hot air just before the piston reaches top center. (“Top center� is the moment when the piston has traveled into the cylinder as far as it can go, and compression is at a maximum.) Ideally, ignition should begin just as the piston reaches top center. If it does not, the entire charge of fuel may have time to become thoroughly mixed with air, and when it does ignite, the pressure rise will be much steeper than it would have been had ignition occurred earlier. So a desirable property of a diesel fuel is that it ignite quickly.

In the 1930's the Cooperative Fuel Research committee sought a way of expressing the tendency of a diesel fuel to ignite quickly, and their work was taken up by the ASTM. A substance that ignited very quickly, cetane (n-hexadecane) was arbitrarily given a rating of 100, and a substance that was slow to ignite, alpha-methylnapthalene (later called 1-methylnapthalene), was assigned a rating of zero. Values on this scale were named “cetane numbers.�

A specially-designed engine with adjustable compression is used to determine a fuel’s cetane number. The fuel being tested is injected at 13° before top center. (This describes a particular moment in the engine’s cycle. “13°� refers to the rotation of the crankshaft.) The engine’s compression ratio is then adjusted until the fuel ignites at top center. Retaining this compression ratio, the engine is then run on various blends of cetane with 1-methylnapthalene, until a blend is found for which ignition occurs at top center. The cetane number is the percentage by volume of cetane in the mixture that has the same performance as the fuel being tested.

In 1962, difficulties in handling alpha-methylnapthalene and its expense led the ASTM to replace it with a secondary reference fuel: 2,2,4,4,6,8,8-heptamethylnonane (also called isocetane). Heptamethylnonane was assigned a cetane rating of 15, based on engine testing. Since 1962 engine tests of diesel fuel have ordinarily been conducted with blends of cetane and heptamethylnonane. In calculating the cetane number allowance is made for the 15-cetane rating of the heptamethylnonane, to keep the scale as it was.

Cetane Index
Often the cetane number is not determined experimentally by an engine test. Instead an estimate is made from the fuel’s specific gravity and the temperature at which half of a sample will boil away.1 Such estimates are called cetane indexes, not cetane numbers. An improved method relies on the temperatures at which 10%, 50% and 90% of the sample boils away.2 Incidentally, the various additives used to increase a fuel’s cetane number have no effect on its cetane index.

AndyW

My Feedback: (1)

And these guys talk about adding kerosene to diesel fuel to boost cetane ratings??

http://www.thedieselstop.com/archive...o=&fpart=1.htm

That implies that diesel fuel is inferior to kerosene for our purposes. BUT, this is all heresay. got to try it to believe it.

AndyW

My Feedback: (1)

Hi Greg,

Thanks for the info. As we mull and speculate, we have fun.

About settling. We are having temps in the 80s and 90s now so that may certainly be a factor. And settling even happens to glow fuel. I had some 60% nitro fuel with 25% castor last year that I ran on my TD .010s and .020s during a "warm spell" in the winter. Temps were just above freezing and yes, the castor was settling out. At anything significantly above freezing, no problem. Never saw this problem with any other fuel at low temps. Going to pop that fuel jug into the freezer to test. Will report.

Actually, the Amsoil percentage was 10%. If I misstated, I'll fix it. But yes, throttling disappeared with the high Amsoil mix. The question is, did that happen due to chemistry or was it that the main needle setting took the idle setting out of range. I didn't have a chance to play with the airbleed setting on this run. Rain and other obligations, just then, forced a shutdown of activity.

The high Amsoil issue is going to be more fully explored with engines that were actually MEANT to be throttled, like the Norvels and others. The PAW is FORCED to be throttled against its wishes, it seems. The point here is that engine design, as much as anything determines how well it can be throttled and the PAW is a poor frame of reference, in my opinion.

More testing fun and reports will follow.

Addendum:

Reduced compression, as in this case, will necessitate the needle to be leaned out by a good measure. This would take the idle mixture lean and the engine quit in just that manner, abruptly and without warning. So, I'm speculating that the idle went overlean and killed the engine.

AMB

Andy Having paw s from 03 thru 40, I think you may be on the right track one the carb issue it is quite dated and a little crude but it does work and has for 50 years, howevery I mounted a MVVS carb on my paw 40 and of course ran tank pressure, a whole new ballgame throttle well no loadups great idle. As pointed out wide open in c/l use the orginal PAW carb is fine RC use is a different
animal martin

gkamysz

My Feedback: (19)

Are saying that the castor is being burned in your engines? I'm confused at this point.

That thread is interesting but it's quite old. Now we have ultra low sulphur diesel which is supposed to have a higher CN. They never given the actual rating because law only requires a minimum. Many references said premium diesel would have 5 point high CN. There are so many references to No.1 diesel being the same a kerosene that there might not be much of a difference.

One thing that stands out in my mind is synthetic diesel fuel. I read a couple reports that said it has a CN of 70+. I have no idea where to get the stuff though.

Greg

[email protected]

Greg, I purchased some SyDiesel fuel from Hiper Fuels back in 2003. Cetane 63 and 20500 btu/pound. Sulfer free and will not oxidize. The winter of 2003-2004 I mixed up some fuel with kerosene and a batch with SyDiesel. Same percentages of ether, castor and ignition improver in both mixes. I used an reproduction Oliver Jaguar 2.5 cc late 1940's long stroke/side port design with many runs as a test engine. I used an APC 12/7 prop and a 1/2 ounce tank. I ran a couple of tanks of Olde English mix thru the engine just prior to test runs with the two fuels I mixed. I ran three runs with each fuel, alternating fuels each run. The SynDiesel required a bit more compression but I did not change the needle valve. Averages for the kerosene based mix were 4,900 rpm and 7 minutes and 51.66 seconds on the half ounce tank. With the SynDiesel I got a 5,100 rpm and 6 minutes and 9 seconds. Since at the time I was testing fuels for A Texaco I stuck with kerosene. Recently I tried a couple of different engines with the SynDiesel mix. One turned up a bit better with the SynDiesel and the other about the same on both fuels. I think a Drone was one of the two I tried, but do not remeber the other engine. I just checked Hyper Fuel's web site and the price of their SynDiesel has gone thru the roof. www.hiperfuels.com I have purchased methanol from them in the past to mix FAI glow fuel. Jack

AndyW

My Feedback: (1)

Martin, Greg,

See my addendum in the last post. Throttling is my favourite subject and this has given me some ideas to try. And YES, modern throttles can do wonders,,, you just HAVE to have a way to adjust the idle mixture. Can't understand why the guys that design and make engines can't be convinced of that.

No, I don't think that the castor is being burned but someone mentioned that it has a cetane rating similar to diesel/kero. What does that mean? How does that work? Does that work in our engines? Does it help as it seems to do here? My search for that answer didn't get much except what I found and posted.

Kelly mentions 70 weight, non detergent oil from the FBO for aviation use. This was a tip given to me many years ago that I had forgotten. Good to know and to try but my idea was to find a solution to diesel fuel from local, generally available sources. Hardware, automotive and the like. So far, the oil is the issue as well as the cetane booster. I haven't given up on the MEKP as it was mentioned that some brands are quite diluted so you don't know where you're going. To try will be 10% MEKP on my benchmark .15

And here's where we get insight into the cost of R & D. Imagine the sheer number of permutations of,,, % of the various ingredients, brands, TYPES of oil, types/brands of kero/diesel, types/brands of cetane boosters, types/brands/sizes of engines. The combinations could take us into the thousands. So we try to narrow the experiment down to what we know. Trouble arises there too though, as in, if we KNOW that castor won't blend with kero, we'll never try it and will be held back.

All we can do is poke and prod at the problem with a stick relentlessly until the beast wakes up and talks to us.

And synthetic diesel fuel at cetane 70? Oh boy, THAT I have GOT to try. [X(]

AndyW

My Feedback: (1)

Jack,

Great to hear from you.

From their website,

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Out of Stock - This item is backordered and we do not have any timing on availability. Syndiesel (R) Synthetic Diesel Fuel/Additive. 63 Cetane Number, 20,300 Btu/pound, for European (MB & TDi) and other High Performance Diesel Engines and Standby Diesel Generators. Specific Sins: Eliminates SMOKE, quiets engine, sulfur free, reduces exhaust temperatures, stops detonation, quick power response - NO hesitation, pleasant 'candy' smell, no more gums, no sludge. No Oxidation - It will last for ten years in storage. Fuel stays clear and bright. Best of all the GEL POINT is -67F - perfect for standby or emergency generator fuel supply. Perfect for cold weather applications or winterizing a diesel engine powerplant. Flammable Liquid. $20 UPS/FedEx HazMat Fee is included in price above. Shipped in a UN/DOT APPROVED CONTAINER. Made in the USA. This product can be shipped Next Day, Second Day and 3 Day Select ($10 per container extra charge) as well as UPS Ground. Shipping is additional for all products. No shipment outside of USA. Taxable for sales in Texas. 5 Gallon

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What would we DO without the stink??? But, I like candy.

But again, this is something you have to order. If you can run diesel on local ingredients, maybe more will join the fold.

colingw

Hi Andy,

Thanks for the additional info. I'm curious about this talk about Castor not mixing without the ether, as I thought you had tried that and it worked?

PAW's recommended prop sizes are here:
http://www.eifflaender.com/techdets.htm

I suspect the upper end of prop sizes is open ended. Please bear in mind that the smaller size PAWs are commonly used in events like combat and team race, where high revving engines are the norm.

I would not have any issues with putting an 8x4 on my 80DS, or a lower pitch 11" on my 19 if it were appropriate for the flight characteristics of the model. I still think a 9x6 or 9.5x6 master airscrew was the sweet spot for my 19DS - but when I get it in a plane will try a 10x4 to see how I go. For C/L stunt, a large diameter relatively low pitch prop at around the 9000rpm mark seems ideal.

Those other Redline products you point out look interesting. The Redline stuff is locally available - they list a couple of stockists which are only about 5km from where I live (and on the way to my daughter's school).

cheers,
Colin
Re-learning C/L in Brisbane, Australia

AndyW

My Feedback: (1)

Colin,

I put my 20/80 mix in the freezer four hours ago and this is what I found. Hard to see in the pics but we definitely have separation. The curious thing is that it looks more like a 40 oil by 60 kero mix at this point. This bottle was mixed last year and only tried this last little while. It never got below 50F in the basement and I had no separation. Well below freezing, however and we have a problem,,, or so it seems. Will be taking this frozen mix to the field right now and try to run it before it warms up too much. Yes, I'll give the jug a good shake.

colingw

Ok, so I'm thinking if the fuel is at relatively warm, and you shake well before fueling, there's probably not going to be a problem?

AndyW

My Feedback: (1)

I went flying just after posting the pictures of the frozen fuel. By the time I got to the field, (10 minutes) the fuel was still very cool. I gave it a shake, looked good so I fuelled up and flew as normal. This fuel was as cold as you might get with winter flying. The oil went back into solution at a fairly low temp. The ambient would have to get REALLY cold for any separation to take place. Much like my experience with the 60% nitro mix.

The engine was my old .074 that still gives good bang for the buck. Compression had to be set a full half turn in from a running setting. High ether prime was used and it took three prime bursts before she lit off on the fourth. Not too bad,, considering the cold fuel.

Also, once the cylinder was hot, the engine would light off with just the prime provided by blocking the muffler while spinning the engine with the starter. A real, no muss start, once the engine was up to temperature. Kelly's preheat ideas have merit. A hot spot contra-piston might work or even an unenergized glow plug installed in a disc. Yes, this was tried last year but NOT with a turbo plug.

So it ain't over yet. But I'm real happy that I can run without ether. I just have to fly all this stuff for a season to work out any unforeseen bugs. So far, only the .074 has flown with this unusual mix. Next step is to turn down the fins and see what that might do.

colingw

Andy,

Thanks for all the effort you're putting into trying this, and documenting.

When I get time, I'm going to try my PAW 19 on some Castor + Kero + one of the Redline boosters. I've just bought a tachometer so I can actually see what the engine's doing.

FWIW, with my PAWs I NEVER had to resort to cylinder priming. The .19 in particular was always happy to light off from cold with just a couple of chokes, even when inverted. A very easy to use engine. It got so I could leave it with the compression & needle valve set right for my flying and rarely need to even touch it.

I notice you're still using ether in the prime. Do you think we can get to the point where we simply don't need ether even for starting?

AndyW

My Feedback: (1)

Colin,

Yes, to the "NO ether, anywhere in sight".

The last run on the PAW was with the 10% Amsoil mix. This was ALSO used for the prime. Notice the deep red colour of the mix in the prime bottle at the very beginning. THAT, is with 20 oil, 10 Amsoil and 70 kero. No ether to dilute the colour.

Note the ice cube on top of the cylinder. The first try was a no go but a bit more compression and she lit right off, with no problems. Note that I got on the compression screw right away. Trial runs, before the video was taken, showed that a start could be had with a full half turn increased compression but that nearly immediately, you had to back off and carefully note the tone and adjust. But, it doesn't take long for all to stabilize to get your final setting.

I'm glad to hear that the old caveat is dying. No doubt there are very good reasons for it, the engine guys do want your engine to run and last a long time for you. But once you get past the learning curve, much convenience and joy can be had with a starter. And it doesn't have to be electric, a spring will do.

colingw

Great,

So that just leaves me needing an equivalent of the Amsoil Cetane Booster that is available here in Oz land. Hopefully one of those Red-Line products will do the trick.

--Col

AndyW

My Feedback: (1)

OK, Colin, let us know how you made out. Lots of stinky power guys in Oz, I understand.

colingw

Thanks Andy. Probably be a couple of weeks before I find time to do anything.

I just read the MSDS and technical details for those Red Line boosters. They seem to give a similar order boost in Cetane numbers to Amsoil. (3-9 for the 85 Plus). Have also emailed the Oz distributor of Amsoil to see if I can get Amsoil Cetane Boost given that its known to work.

The redline products all contain Octyl Nitrate as well.

AndyW

My Feedback: (1)

From Wikipedia:

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Cetane number or CN is to diesel fuel what octane rating is to gasoline. It is a measure of the fuel's combustion quality.

Cetane is an alkane molecule that ignites very easily under compression, so it was assigned a cetane number of 100. All other hydrocarbons in diesel fuel are indexed to cetane as to how well they ignite under compression. The cetane number therefore measures how quickly the fuel starts to burn (auto-ignites) under diesel engine conditions. Since there are hundreds of components in diesel fuel, with each having a different cetane quality, the overall cetane number of the diesel is the average cetane quality of all the components.

A fuel with a high cetane number starts to burn shortly after it is injected into the cylinder; it has a short ignition delay period. Conversely, a fuel with a low cetane number resists auto-ignition and has a longer ignition delay period.

To measure cetane number properly is rather difficult, as it requires burning the fuel in a special, hard-to-find, diesel engine called a Cooperative Fuel Research (CFR) engine, under standard test conditions. For most practical purposes, fuel-users control quality using the Cetane Index, which is a calculated number based on the density and distillation range of the oil. There are various versions of this, depending on whether you use Metric or Imperial units, and how many distillation points are used. These days most oil companies use the '4-point method'.

There is very little actual cetane in diesel fuel. Diesel engines run just fine with a CN between 45 to 50. There is no performance or emission advantage when the CN is raised past 50; after this point, the fuel's performance hits a plateau.

Diesel at the pump can be found in two CN ranges: 40-46 for regular diesel, and 45-50 for premium. Premium diesel has additives to improve CN and lubricity, detergents to clean the fuel injectors and minimize carbon deposits, water dispersants, and other additives depending on geographical and seasonal needs.

Some fuel additives used to raise the cetane number are eg. alkyl nitrates and di-tert-butyl peroxide.

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So, Amsoil cetane booster is an octyl NITRATE and MEKP is methyl-ethyl-ketone-PEROXIDE. And of course, there's the traditional amyl NITRATE.

But note that there's no performance advantage when the CN number is raised past 50.

As we all know, with ether, we get an improved cetane rating. As we add 2% amyl nitrate to that, we boost the CN a bit more. AND, as we know, over 2% can cause problems. Is the issue too high a cetane number? Or is the issue the MANNER in which too much AN raises the cetane rating. I wish I had some to test 10% in a no ether fuel. When I added way more MEKP in the one test, it didn't help but it didn't hurt.

It would be interesting to test ALL the ignition improvers in various concentrations with various ether and no ether formulas. These combinations could reach into the hundreds though.

But what if you could get this stuff called cetane and play with that? Say, add it to methanol and create a new fuel base. Gasoline? Acetone? With a CN number of 100, it might not take much to raise the cetane rating of these "octane" fuels to 50 for use in diesel engines.

AndyW

My Feedback: (1)

From Wikipedia:

First to distill fuel from crude oil, it seems,

http://en.wikipedia.org/wiki/Ignacy_%C5%81ukasiewicz

Found while browsing for factoids on kerosene.

AndyW

My Feedback: (1)

This implies that a standard glow plug with its platinum element CAN have a catalytic action with diesel. A cold start glow plug, installed into a contra-piston may make an all kero fuel that much more user friendly with a lower compression ratio and with no need to alter the compression to start.

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Diesel engines
Glow plugs are used to heat the combustion chambers of some diesel engines in cold conditions to help ignition at coldstart. In the tip of the glow plug is a coil of a resistive wire or a filament which heats up when electricity is connected.

Glow plugs are required because diesel engines produce the heat needed to ignite their fuel by the compression of air in the cylinder and combustion chamber. Petrol engines use an electric spark plug. In cold weather, and when the engine block, engine oil and cooling water are cold, the heat generated during the first revolutions of the engine is conducted away by the cold surroundings, preventing ignition. The glow plugs are switched on prior to turning over the engine to provide heat to the combustion chamber, and remain on as the engine is turned over to ignite the first charges of fuel. Once the engine is running, the glow plugs are no longer needed, although some engines run the glow plugs for between 5 and 10 seconds after starting to ensure smooth and efficient running and sometimes to keep the engine within emissions regulations (combustion efficiency is greatly reduced when the engine is very cold). During this period, the power fed to the glow plugs is greatly reduced to prevent them burning out by overheating.

Indirect-injection diesel engines are less thermally efficient due to the greater surface area of their combustion chambers and so suffer more from cold-start problems. They require longer pre-heating times than direct-injection engines, which often do not need glow plugs at all in temperate or hot climates even for a cold start.

In a typical diesel engine, the glow plugs are switched on for between 10 and 20 seconds prior to starting. Older, less efficient or worn engines may need as much as a minute (60 seconds) of pre-heating.

Large diesel engines as used in heavy construction equipment, ships and locomotives do not need glow plugs. Their cylinders are large enough so that the air in the middle of the cylinder is not in contact with the cold walls of the cylinder, and retains enough heat to allow ignition.

Modern automotive diesel engines with electronic injection systems use various methods of altering the timing and style of the injection process to ensure reliable cold-starting. Glow plugs are fitted, but are rarely used for more than a few seconds.

Glow plug filaments must be made of materials such as platinum and iridium that are resistant both to heat and to oxidation and reduction by the burning mixture. These particular materials also have the advantage of catalytic activity, due to the relative ease with which molecules adsorbed on their surfaces can react with each other. This aids or even replaces electrical heating.
[edit]
Model engines
In model aircraft, and similar applications , glow plugs are used for starting as well as continuing the power cycle. The glow plug consists of a durable, mostly platinum, helically wound wire filament, within a cylindrical pocket in the plug body, exposed to the combustion chamber. A small direct current voltage (around 1.5 volts) is applied to the glow plug, the engine is then started, and the voltage is removed. The burning of the fuel/air mixture in a glow-plug model engine, which requires methanol for the glow plug to work in the first place, and sometimes with the use of nitromethane for greater power output, occurs due to the catalytic reaction of the methanol vapor to the presence of the platinum in the filament, thus causing the ignition. This keeps the plug's filament glowing hot, and allows it to ignite the next charge. Since the ignition timing is not controlled electrically, as in a spark ignition engine or by fuel injection, as in an ordinary diesel, it must be adjusted by the richness of the mixture, the ratio of nitromethane to methanol, the compression ratio, the cooling of the cylinder head, the type of glow plug, etc. A richer mixture will tend to cool the filiment and so retard ignition, slowing the engine, and a rich mixture also eases starting. After starting the engine can easily be leaned (by adjusting a needle valve in the spraybar) to obtain maximum power.

Retrieved from "http://en.wikipedia.org/wiki/Glow_plug"

gkamysz

My Feedback: (19)

Amsoil Cetane Boost is basically pure octyl nitrate. I found other diesel fuel additives but none were pure. The 16 oz Amsoil will treat up to 200 gallons so use that as a reference. Most other additves are only good for 20-30 gallons for a bottle. So we could assume that a whole bottle of typical additves raises CN the same amount as 10-15 percent volume of pure octyl nitrate. This is in the range I've seen in some MSDS that actually specified now much octyl nitrate was in the bottle.

Our engines are a little different than injected diesels It's hard to say weather higher CN is good or not any better. Too much cetane improver is bad because it costs more, even if it doesn't change how the engine runs. Again, if synthetic diesel with CN of 70 is superior but costs 15USD/gallon and you have to order, there is little point. If kerosene or diesel and cetane booster works, then I don't think I want gasoline or acetone and something which is hard to get. I think I read the same article you did about gasoline and diesel fuel mixing. It's interesting because I read something that said if you put gasoline in an injected diesel it ignites too quickly after injection and will cause engine damage. I did find something that said cetane index of gasoline is 26-28. Gasoline burns at higher temperatures. and cooling will become a problem as I mentioned earlier. Gasoline does not have the lubricity of kerosene or diesel so we would need more oil. And quite honestly I would rather have spark igntion if I was running gasoline to allow proper timing maps and simplified starting.

Greg

gkamysz

My Feedback: (19)

The glow plug idea for starting is fine with me. Fuel economy is better than glow and fuel is much cheaper. The glow plug is easy to do in a two stroke. Providing for a glowplug and compression adjustment in a four stroke will be a challange.

Greg

grant-RCU

My Feedback: (10)

I wonder if a bigger 2 stroke glow or gas engine would work even better with straight diesel or kerosene fuel since its about 5 times the volume of air vs a .15 sized engine.
I am thinking like a 120 glow or a 18 to 25cc gas string trimmer engine with diesel heads. This would be real great for the gas engines because they would be real light with out the mag and no electronic ignition. plus the low oil content of the fuel.

AndyW

My Feedback: (1)

That's a good thought. To test and compare, I have the Norvel .06 and the Norvel .40 a difference of over 6 to 1.

But more food for thought on cetane, octane, kerosene, diesel, gasoline.

++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++++++++++++++++++++++++++++++++++++++ ++++++++++++++

Q: What is the difference between #1 Diesel and K1 Kerosene?

This response from From Marketing Fuel Tech Service may help explain some of the differences:

K1 kerosene is a low-sulfur kerosene that is made for use in space heaters, lamps, etc. - and not for use in vehicles or generators. It is also not taxed so would be illegal to use in "on-road" vehicles.

Lower lubricity is likely as the viscosity decreases. While this may not cause catastrophic instant damage, it could cause long-term wear of pumps, etc. Four semi-annual surveys for years 1990-1992 showed national averages as such for viscosity (represented in milliPascal-seconds (mPa . s)(=centipoise) cSt)

Diesel # 1 1.33
Diesel # 2 3.20
Kerosene 1.63
Both Kerosene and Diesel # 1 are less dense than Diesel # 2 and will thus have a slight reduction (~3%) in BTU per gallon. This would likely be reflected in lower fuel economy.


--------------------------------------------------------------------------------

Cetane number is a measure of ignition quality.
RTML Off road Fuel Thread evolves into a technical Cetane discussion

Subject: Re: [RAM] Highway diesel vs. offroad diesel?
Date: Tue, 13 Jan 1998 22:33:21 EST
From: Drdonnelly <[email protected]>
To: RTML

I have just a few comments to add for clarification:

cetane rating improves with more unbranched, saturated hydrocarbons (more or less, waxes). this is detrimental to the cloud and pour point requirements for winter fuel. Winter fuel has more aromatics and branched molecules. Hence, the cetane rating of #1 usually is lower than #2. Winter fuel feed stock usually has a wide boiling range, so the wax crystals tend to be smaller and more easily handled by the cloud and pour point depressants that are added for winter use. As Dave stated, the cetane rating may improve with the process (hydrogenation) often used to help remove sulfur. This is because unsaturated hydrocarbons in the fuel stock are also hydrogenated, making more of the straight chain saturated molecules that raise the overall cetane rating. The severe hydrogenation conditions also hydrogenate much of the aromatics that would have improved lubricity.

Alkyl nitrates such as 2-ethylhexylnitrate improve cetane ratings, and are generally the way "premium" diesel fuel cetane ratings are increased over that of the "regular" fuel feedstock used to make the premium fuel. Premium diesel fuels may also contain such additives as antioxidants, antirust agents, corrosion inhibitors, and de-emulsifiers. As Dave noted, there is no clear-cut definition and standard for calling diesel fuel "premium".

Joe
---------------------------------------------------------------
From A sci.energy Newsgroup Posting: (This info is similar to Joe D's article in TDR issue 18)

Subject: Re: Cetane rating in Diesel Fuel
Date: Wed, 29 Jan 1997 16:10:27 GMT
From: [email protected] (Bruce Hamilton)
Followup-To: sci.energy

> What is the Cetane rating in Diesel fuel and what are the consequences of
> raising or lowering this rating? What does adding kerosene do to this figure
> and what is the rating of heating oil in this application?

The cetane number measures the ignition quality of a diesel fuel.

It is the % volume of cetane ( n-hexadecane, Cetane Number = 100 ) in alpha methyl naphthalene ( Cetane Number = 0 ), that provides the specified standard of 13 degrees ( crankshaft angle ) ignition delay at the identical compression ratio to that of the fuel sample. These days, heptamethyl nonane - with a Cetane Number of 15 - is used in place of alpha methyl naphthalene because it is a more stable reference compound.

It is measured in special ASTM variable compression ratio test engine that is closely controlled with regard to temperatures ( coolant 100C, intake air 65.6C ), injection pressure ( 1500psi ), injection timing 13 degrees BTDC, and speed (900rpm ). The compression ratio is adjusted until combustion occurs at TDC ( the ignition delay is 13 degrees ). The test is then repeated with reference fuels with five cetane numbers difference, until two of them have compression ratios that bracket the sample. The cetane number is then determined by interpolation.

Now, if the fuel is pure hydrocarbons ( does not contain cetane number improving agents like alkyl or amyl nitrates ) then the Cetane number can be predicted fairly well using some physical properties, such as boiling point and aniline point.

It's obvious from the above that the higher the cetane number ( 100 = normal alkane, 15 = iso-alkane ), then the lower the octane number ( 100 = iso-alkane, 0 = normal alkane ). This is because the desirable property of gasoline to prevent knock is the ability to resist autoignition, whereas for diesel, the desirable property is to autoignite. The octane number of normal alkanes decreases as carbon chain length increases, whereas the cetane number increases as the carbon chain length increases. Many other factors also affect the cetane number, and around 0.5 volume % of cetane number improvers will increase the cetane number by 10 units. Cetane number improvers can be alkyl nitrates, primary amyl nitrates, nitrites, or peroxides.

In general, aromatics and alcohols have low cetane numbers ( that's why people using methanol in diesels convert it to dimethyl ether ).

Typically engines are designed to use fuels with Cetane Numbers of 40-55, because below 38 a more rapid increase in ignition delay. The significance of the cetane number increases with the speed of the engine, and large, low speed diesel engines often only specify viscosity, combustion and contaminant levels, as Cetane Number requirement of the engine is met by most distillate and residual fuels that have the appropriate properties. High speed diesel engines ( as in cars and trucks ) virtually all are designed to accept fuels around 50 Cetane Numbers, with higher numbers being a waste.

However, Cetane Number is only one important property of diesel fuels, with three of the others being also very important. Firstly, the viscosity is important because many injection systems rely on the lubricity of the fuel for lubrication. Secondly, the cold weather properties are important, remember that normal alkanes are desirable, but the desirable diesel fraction alkanes have melting points above 0C temperature, so special flow-enhancing additives and changes to the hydrocarbon profiles occur seasonally. That's why it's never a good idea to store diesel from summer for winter use. Thirdly, diesel in many countries has a legal minimum flash point ( the minimum temperature it must attain to produce sufficient vapours to ignite when a flame is applied. In all cases it's usually well above ambient ( 60C+, kerosene is 37C+, whereas gasoline is typically below -30C ), and anybody mixing a lower flash point fraction with diesel will usually void all insurance and warranties on the vehicle. The recent increase in blending fuels has resulted in significantly more frequent analyses of fuel tank contents from diesel vehicle fires.

From all of the above, you can see some common factors emerging, larger normal alkanes are desirable, and they also burn with a less smoky flame and have higher flash points than gasoline and kerosene, making them also desirable for home heating fuels, however the relatively expensive Cetane Index improvers have no value in heating fuels.

So heating oils are often a slightly different fraction, and may have differing additives ( for cleaner combustion ) to fuels used for high speed diesel engines. For low speed ( large, stationary and marine engines ), they often use the cheapest residual fuel oil available, as do the larger heating boilers - so there is commonality of fuel as size increases.

Details of the important, specified properties of various grades ( 1D, 2D, 4D ) of diesel fuel oils can be found in the Annual Book of ASTM Standards. ASTM D975-93 " Standard Specification for Diesel Fuel Oils", as can the fuel oil specification for grades 1, 2, 4, 5, and 6 in " Standard Specification for Fuel Oils ASTM D 396-92. Note that ASTN D975-93 actually defines the low temperature requirements by dividing the USA into regions. It is possible for a fuel to meet both specifications, but the diesel specification may have additional requirements such as Cetane Number and Cloud Point ( temperature at which the fuel goes cloudy ), whereas the Fuel Oil may have additional limits on the distillation properties, and viscosity at 100C. A fuel has to be tested for all the criteria in each specification grade before it can be said to comply with the relevant grade in each specification.

The interchanging and dilution of fuels is performed by suppliers, taking into account the effect on all of the above, but especially flash point, as that is closely regulated in many countries. Adding kerosene and gasoline to diesel can have dramatic, adverse effects on the flash point, with minimal gains in the flow properties if the fuel already contains flow-improving additives. Regardless of what other people may advise, check your insurance policies before embarking on experimentation. These days, assessors for both vehicle and insurance companies these days are far more aware of the signs of the dilution of expensive diesel fuel by cheaper lower flash fuels. Some countries, like NZ, avoid this by having diesel cheaper than gasoline at the service station, and imposing taxes based on distance traveled ( as measured by hubometers on vehicle wheels), number and location of axles, axle loads, and gross vehicle weight, as they more accurately indicate road damage potential.

For people that are interested in diesel fuel properties and the effects on engine performance, the following are good sources.

Internal Combustion Engine Fundamentals
John B. Heywood
McGraw Hill ISBN 0-07-100499-8 (1988 )

Automotive Fuels Reference Book
Keith Owen, Trevor Coley
SAE ISBN 1-56091-589-7 (1995)

Modern Petroleum Technology
edited by G.D.Hobson
John Wiley & Sons ISBN 0-471-262498 (1984 )

Bruce Hamilton



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Joe Donnelly's Cetane Discussion which appeared in TDR issue 18

(you do subscribe to the Turbo Diesel Register, don't you??) ( TDR homepage)

Subject: octane and cetane ratings
Date: Fri, 17 Jan 1997 08:15:58 -0800
From: "William H. Cole" <[email protected]>
CC: [email protected]
To: cummins

There seems to be some interest in octane and cetane ratings, and some mystery about them too, so I thought I would throw in my 2 cents worth on the subject.

Gasoline is made up of the petroleum fraction that boils below 200 degrees centigrade (390 F). Aviation gas has a smaller boiling range (38-170 C, 100-340 F), leaving out the lowest boiling components that are in auto gas, largely because of extreme volatilities they would have at the altitudes involved in flying. The two tests used to determine "research" and "motor" octane differ in the load on the test engine (more load for the motor test). Both octane and cetane tests are described by, and conducted according to specifications of, the ASTM (American Society for Testing Materials). The standard test compound is "iso-octane" as oil men call it. Chemically it is not iso-octane which would be 2-methylheptane, but rather 2,2,4-trimethylpentane, a highly branched eight-carbon hydrocarbon. Gas engines knock less on branched hydrocarbons, although the straight distillate of raw petroleum tends to contain mostly straight-chain hydrocarbons in this low-molecular-weight range. Cracking and catalytic reforming processes are used to increase the percentage of branched hydrocarbons to improve octane ratings. Av-gas usually has no olefins (alkenes) because they tend to form gums and have poor antiknock characteristics. Aromatics, such as benzene and toluene, have good octane ratings under load (rich conditions) but act more like olefins under lean cruising. Toluene has research/motor octane ratings of 120.1/103.5; benzene has 114.8 motor octane, compared to "isooctane" which is set arbitrarily at 100 on both scales. In 1922, tetraethyl lead was found to improve anti-knock characteristics of gas. This became more important in the 1930s because the increased demand for gas led to use of cracking processes that produced more gasoline from crude oil, but of lower octane ratings. Standards for octane ratings over 100 are made from "isooctane" with tetraethyl lead added (1% = 108.6; 2% = 112.8; 3% = 115.5, etc.).

Crude oil has more of the branched, cyclic, and aromatic hydrocarbons in the higher molecular weight range where Diesel fuels are obtained. Diesel fuel, and fuel oil, have a boiling range of about 175-345 C (350-650 F) The standard for Diesel fuel ratings is "cetane" or n-hexadecane. This is a straight-chain, 16 carbon hydrocarbon with a short-delay period during ignition, and its rating is set at 100. Heptamethylnonane is a highly branched 16 carbon hydrocarbon with a long-delay ignition, and cetane rating set at 15. Diesel fuels largely contain molecules having 10-20 carbons, whereas gasoline components have mostly 12 or fewer carbons. Diesel fuel power in terms of heat content is increased by saturated hydrocarbons, but these are prone to form waxes at low temperatures. Ignition performance is improved by straight-chain hydrocarbons, such as cetane.

As mentioned above, crude oil is just the "opposite" of what we want--it has a lot of straight chain small molecules where we want branching, and it has a lot of branched, cyclic, and aromatic (highly unsaturated) heavy molecules, where we would prefer straight-chain saturated molecules. One "legitimate" reason for Diesel fuel price increases is the cost of removing sulfur to meet EPA requirements.

The "bottom line" is that the best Diesel fuel would have a lot of "waxes" or saturated, straight-chain molecules, up to the limit of the cloud point and pour point allowed by ambient conditions. The other "stuff" helps with viscosity, pouring, lubricity, etc. but is largely there because that is what is available. It should be apparent that a poor Diesel fuel would be made up of small molecules with a lot of branching and unsaturation--that is, a pretty good gasoline!

To use our "normal" frame of reference, we know that gasoline ignites very easily, and is very volatile. Diesel fuel is much less volatile--it stays on you when you spill it during fueling the truck, even after you try to wipe it off. Diesel fuel also ignites much less easily. So, if you put some of the above "pretty good gasoline" in your Diesel, it would ignite so explosively that the heads would pop off the engine, etc. That would be "powerful" but in the explosive sense. It would have less heat content (the useful kind of power), because of the smaller, unsaturated (aromatic, etc.) molecules, so it would decrease fuel mileage, if the engine could stay together. Now you see why #1 Diesel and winterized Diesel fuel decrease your fuel mileage. To improve the cloud and pour points, lower the viscosity, and increase volatility to compensate for low ambient temperatures, smaller molecules, and ones that tend to stay liquid at lower temperatures (branching and aromaticity help here) are used in the fuel. They do the needed job, but have less heat content (often expressed in BTUs or British Thermal Units). Basically, the characteristics that make more power are more carbons and more hydrogens per gallon, and saturated molecules have more hydrogens.

Joe Donnelly


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Another Cetane Discussion from the Cummins Mail list
Subject: Re: octane and cetane ratings
Date: Fri, 17 Jan 1997 12:33:21 -0800 (PST)
From: Blaine Hufnagle <[email protected]>
To: cummins

Now I've got to throw in my buck-fifty... :-)

At 11:45 1/17/97 -0800, Walt wrote:

>> recall that 'octane-rating' is a measure of the fuel's resistance to
>> auto-ignition, aka detonation and/or pre-ignition)...

Need to Clarify here, a bit Walt.

Auto-ignition is desired combustion by self-ignition of fuel.

Detonation is destructive combustion by simultaneous ignition of multiple flame kernels within the "wedge" at the top of the compression stroke. Usually caused by carbon deposits, or having high enough compression that your temperature is above the ignition point of the fuel at some point before the REAL ignition signal.

Pre-ignition is combustion by the REAL ignition signal that is simply too early in the combustion stroke.

Now, to the human ear (a problematic sensory device at best) detonation and pre-ignition sound the same, and in many respects, they are; they're both mightily damaging to engines. BUT, if you look at each "knock" on an oscilloscope, they are VERY different (but unfortunately I can't remember exactly HOW).

(It's sort of looking at a very fast, but smooth sinusoidal pressure increase versus a sharp "impulse" spike.)

Both result in tremendous pressure spikes (instead of smooth, but fast rises) that can seriously damage an engine, up to and including breaking wrist pins.

>> us to this statement -- increased cetane rating is analogous to
>> decreased octane-rating, since cetane-rating is a measure of
>> auto-ignition capability and octane-rating is a measure of resistance to
>> the same.

When looking at only autoignition capability, this is correct.

>> In light of this, I question whether gasoline would even
>> ignite in a Diesel cylinder (where's Mike Smith? he's undoubtedly done
>> it ).

Oh, yes. Joe pointed out that diesel's vapor point is about twice that of gasoline.

Remember, now, that liquid fuel will NOT burn in a combustion chamber environment (maybe even at all). It must be forced into a phase change into a vapor, which will readily burn. Now, in a gasoline engine, pressures and temperatures are such that the remaining liquid fuel is vaporized well and "predictably" during the compression stroke, thus being rendered "burnable" at the correct moment during the cycle: having maximum BMEP at approximately 76 degrees ATDC. Putting your timing signal too early (pre-ignition) moves this pressure point up, so that you're pushing down on what amounts to a column instead of a crank arm.

Detonation results (usually) from having combustion chamber deposits that serve as secondary ignition sources. Multiple flame kernels result, and the pressure waves generated during combustion interfere with each other, thus producing the "knocking" sound, and raising the BMEP up a couple of orders of magnitude higher than it need to be, or what the engine is designed for, and usually WAAAY too early.

In a diesel engine, only the air is compressed during the compression stroke. The higher compression ratio means that the temperature increase gets it well above diesel fuel's vapor point. The fuel is injected AS A LIQUID and thus must be vaporized almost instantaneously at the top. The fuel begins to burn as soon as it vaporizes, but the phase change must be occurring at the same time, thus resulting in a combustion dynamic that produces a much longer power stroke duration: A controlled "long-duration" burn versus a controlled "explosion;" both terms are mis-nomers to an extent, since the time interval we're talking about is on the order of about 30 milliseconds.

(also why a diesel motor produces soot: the soot is the un-burned, carbonized liquid fuel.)

Now, the Cetane rating will determine the ability of the diesel fuel to undergo that vaporization in a smooth manner. A higher cetane rating will vaporize LESS readily, but has more BTU's per gallon than a lower cetane rating. Thus it's ability to give you more bang for the buck, pun intended. BUT, high-cetane fuel's low vaporization qualities make it a terrible fuel, combustion wise, for cold temperatures, especially starting environments. During starting, a diesel engine can't generate enough compression heat to heat the air AND the cylinder walls, AND the piston crown, AND the head assembly. The cranking rate is too slow to keep the process in the "adiabatic" range; that is, the heat also heats the environment, not just the system. Thus some of your heat of vaporization is sucked out of the air by the motor itself. The fuel doesn't vaporize, and the liquid fuel doesn't burn. Note that during a normal running stroke, the process happens so fast that there's no time for the heat to transfer to the walls; the process remains in the adiabatic range, and will easily vaporize the fuel, even when running cold.

Now, Joe wrote:

> Walt, there are several things going on here. At "reasonable" ambient
> temperatures, gasoline components are well over their flashpoints, so it
> is very easy to ignite them.

True. TOO easy in many cases over about 80 degrees ambient.

> Compression ignition and spark ignition are two different animals. The former
> depends a lot on ignition temperature of the fuel, that is, the temperature at which
> there is spontaneous combustion.

I *think* you may have these backwards. If that were the case, then the diesel motor would require the spark ignition and the gasoline motor would be auto-igniting.

Remember that compression ignition has no ignition source, and (of course) must auto-ignite. You've got to be well above the ignition temperature to do that, not just above the flashpoint.

> I can assure you that gasoline will ignite in a Diesel--all too well at
> common ambient temperatures. You can sweep up the pieces later.

Here's why: Gasoline's autoignition temperature, when looked at in a combustion cycle, occurs WELL before the piston comes even CLOSE to TDC. So far ahead, in fact, that it can make the motor run BACKWARDS if it were possible. So, assuming you've got enough inertia (or other power strokes) to FORCE that piston UP, and you've got that sudden BMEP increase from the gasoline *exploding* (NOT burning, since it's VERY uncontrolled) you've got to have that pressure go *somewhere.* Thus you blow the head gaskets, and in worst-case scenarios, actually dropping heads (and hoods, and whatever else happens to be in the way) on the other side of the garage.

> The octane rating won't help you worth a darn, because you are so far over
> ignition temperature in the Diesel combustion chamber.

Bingo!

> Octane rating refers more to rate of burning, or smoothness, under spark ignition,
> at the lower combustion chamber temperatures associated with gas engines.
> And, as you noted, some resistance to pre-ignition.

Whoops... Remember, Detonation, in this case. Perhaps a philosophical difference, but a very IMPORTANT philosophical difference.

> pre-ignition is generally associated with a "hot-spot" such as carbon
> deposits that are "always" present in gasoline combustion chambers, and
> can serve as weak ignition initiators.

Or not so weak initiators, depending upon your conditions.

Remember that high temperatures actually HELP diesel combustion, while high temperatures badly hurt gasoline combustion; thus a car's tendency to "knock" more when it's hot outside.

*whew!*

I think I covered all the bases there... :-)
-blaine http://acs.tamu.edu/~bnh5940/home.htm