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|Diesel Fuel Discussions|
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|Diesel Fuel Terms||Cold Weather Q's||Using Fuel Additives||Using Other Fuels|
|Longer Discussions from the mail list archives:|
|On-Road vs Off- Road fuel||Alternative Fuels||What is a Cetane Rating?|
For everything you ever wanted to know about diesel fuel,
see the Cheveron Diesel Fuels Technical Review
|Sulfur content||Affects wear, deposits, and particulate emissions. Diesel fuels contain varying amounts of various sulfur compounds which increase oil acidity. Legislation has reduced the sulfur content of highway fuel to 0.05% by weight. Off road fuel has an average of 0.29% sulfur by weight.|
|Cetane Number||A measure of the starting and warm-up characteristics of a fuel. In cold weather or in service with prolonged low loads, a higher cetane number is desirable. Legislation dictates the Cetane index should be 40 or above.|
|Aromatic Content||By definition, aromatic content is characterized by the presence of the benzene family in hydrocarbon compounds that occur naturally in the refining of diesel fuel. In the chemical make up of fuel, the heavier aromatic compounds of toluene, xylene, and naphthalene are also present. Limiting these aromatic compounds has the effect of reducing burning temperature and thus NOx formation.|
|Cloud & Pour Point||Affect low-temperature operation. The cloud point of the fuel is the temperature at which crystals of paraffin wax first appear. Crystals can be detected by a cloudiness of the fuel. These crystals cause filters to plug.|
|API Gravity||Related to heat content, affecting power and economy. Gravity is an indication of the energy content of the fuel. A fuel with a high density (low API gravity) contains more BTU's per gallon than a fuel with a low density (higher API gravity).|
|Ash||Measures inorganic residues - The small amount of non-combustable metallic material found in almost all petroleum products is commonly called ash. Ash content should not exceed 0.02 mass percent.|
|Water & Sediment||Affect the life of fuel filters and injectors. The amount of water and solid debris in the fuel is generally classified as water and sediment. It is good practice to filter fuel while it is being put into the fuel tank. More water vapor condenses in partially filled tanks due to tank breathing caused by temperature changes. Filter elements, fuel screens in the fill pump, and fuel inlet connections on injectors must be cleaned or replaced when they become dirty. These screens and filters, in performing intended function, will become clogged when using a poor or dirty fuel and will need to be changed more often. Water and sediments should not exceed 0.1 volume percent.|
|Viscosity||Affects injector lubrication and atomization. The injector system works most effectively when the fuel has the proper "body" or viscosity. Fuels that meet the requirements of 1-D or 2-D diesel fuels are satisfactory with Cummins fuel systems.|
|Carbon Residue||Measures residue in fuel - can influence combustion. The tendency of a diesel fuel to form carbon deposits in an engine can be estimated by various tests to determine the carbon residue after 90% of the fuel has been evaporated .|
Q: How much does a gallon of diesel fuel weigh?
Q: What is premium diesel fuel?
Using Fuel Additives:
Cummins says that fuel additives are not needed in their engines. Almost all diesel technicians (including many Cummins service technicians) agree that while they are not needed, additives help reduce problems caused by fuel contamination, cold weather, and lack of lubricity in modern lo-sulfur fuels. I received this from Jerry, a service manager at a John Deere dealership, and the proud owner of a Dodge Cummins diesel pickup. Here is what Jerry had to say:
"As you know sulfer is no longer available in diesel fuel. This was the main lubricating property of the fuel. Since then the injection pump shops have been flooded with work because of the lack of lubrication in diesel fuel. John Deere recomends using Stantadyne year round fuel conditioner with every fill up. The injection service that I use strongly recomends this treatment, or something with LUBRICATION ADDITIVE in all diesel engines. They have "fixed" many fuel systems by dosing them with this additive. I myself add it to every tank. This conditioner costs around $0.08 per gallon to use however, there is a noticable increase in horsepower and fuel economy. Also, since sulfur was removed the pour point of diesel fuel was raised about! 20 degrees F. This means that the pur point of #2 fuel 3 years ago was 20 degrees F. Today it is now 40 degrees F. This fuel conditioner lowers the pour point 40 degrees F., that goes for both #2 and #1 fuels. I am not trying to sell you guys on this specific fuel conditioner, but I feel that it is something that diesel owners should be aware of to try to lower the risk of premature injection pump and injector failure. I do not however know Cummins standpoint on this issue. But as I said the injection service that I use recommends additive for ALL injection systems."
For a discussion of additives and their effects, see this
Standyne's Supplements Information
Cold Weather Diesel Fuel Questions
Q: Should an additive be used in the fuel to keep it from clogging the fuel system?
Q: Is cold weather fuel "different" from summer fuel?
One TDR member works for a fuel pipeline company that blends fuels for northern
climates.He posted this information to the TDR forum:
Most fuels are blended by the refinery. Our #2 has a flashpoint of 138°F to 148°F in the winter months and 150°F to 170°F in the summer. We add 1.25 gallons of Paraflow 527 per 1000bbls of each reciept. Our straight #2 diesel has a cloudpoint of -10° to-15°.Our premium diesel has a extra additive package and is good to -20° to -25°. Some customers from colder climates in our area will custom blend their fuel to their climate. This is done by the carrier(truck) at the rack and is usually 70/30 or 75/25. To make it short.if you live in a colder climate you are probably getting blended fuel. I use a regular diesel with 50ml of Amzoil Cetane booster in every other tank. I have not had any problems down to -30°.
Our #1 diesel runs at 110°F to 130°'F flash and 41.0 to 44.0 gravity at 60° - all depending on the season. #1 is a hotter fuel and lighter than #2.More Cold weather hints:
> do you find that the Ram keeps everyone's toes warm
> when running down the highway in sub-zero stuff? Or does anyone put
> any type of cover over the radiator to up the temp a bit?
Always use a winter front when traveling in the cold... Gas, diesel or whatever... It doesn't take long for fuel lines to gel... I'm not sure what its like in the US, but in Canada, the gas stations will switch to a winter blend of Diesel to prevent gelling... I believe that the switch occurs as soon as temps start dropping to < 32F... Carry a bottle of diesel anti-gel with you (but check to see what Cummins recommends).. You may want to make sure and run down your tank of diesel that you've bought in warmer climates (once again this comes from sheer ignorance of how things work in your home area), and fill up once you hit the snow belt...
Using Other Fuels in a Ram Diesel
|For the "inline" P7100 pump ('94 to 98.5 12 Valve engines). P7100 pumps are internally lubricated by engine oil.|
These fuels are "OK": #1 & #2 Diesel, 1K & 2K Kerosene, Jet-A, Jet A-1, JP-5, and JP-8.
|NOT OK under any circumstances: Jet-B, JP-4, and Cite|
|The "rotary" VE pump (pre '94) and electronic VP-44 (98.5+ 24 valve) VE and VP-44 pumps are fuel lubricated.|
#1-D Diesel or #2 Fuel Oil (diesel) only
|Alternate fuels listed for the P7100 pump may be used in the VE pump if if 5% lube oil is added.|
The P7100 inline pump is internally lubricated by engine oil, while the VE and VP-44 pumps are fuel lubricated. The "lighter" fuels, such as Kerosene, Jet-A, and JP-5/8, don't lubricate enough.
Note: there have been some reports of sticking plungers in some early P7100 pumps. There are many commercial fuel additives which contain a pump lube - to avoid plunger trouble, consider using an additive when the engine is running on an alternate fuel.
Fuel Tidbits from the Bradley Bits - a news bulletin for Bradley Fighting Vehicles
JET A-1 fuel is essentially identical to JP-8 except
it does not have three additives required in JP-8: a fuel system icing inhibitor,
a corrosion inhibitor, and a static dissipater additive. Hot JP-8 fuel reduces
the life of fuel pumps on some diesel engines, so avoid using JP-8 or Jet A-1
extensively in hot heather, and keep the fuel tank as full as possible to reduce
Diesel fuel #2 and water separate completely; JP-8 fuel and water do not. JP-8 becomes cloudy when contaminated with water; and the cloudier the fuel, the more severe the contamination. The Ram fuel/water separator will not be able to remove suspended water from JP-8 or Jet A-1.
Some Questions answered by Josh Berman of Cummins:
> Dodge says to run #2 diesel under most conditions. In "arctic"
conditions (<10 °F), others
> recommend 50% #2, and 50% K1 OR #1 diesel. Is the 1st statement true? I know the second
> is because its in the owners & service manuals.
Blending is better than running on fully on #1 or K1, because it will keep your power output closer to that of running on #2 Diesel.
> What if any problems will be caused by extended operation on
other than #2
> diesel? I'm more concerned about short duration needs... ie out in a rural
> area and unable to find diesel, but can find a K1 pump.
If you have a '94 to '98.5 12 Valve truck, then you should have no problems running a tank of K1 (2K Kerosene, Jet-A, Jet A-1, JP-5, or JP-8) through your truck. You might find that it performs differently (ie: #1 Diesel gives lower power than #2 Diesel), but it should not cause any damage to your engine. (Note: Jet-B, JP-4, and Gasoline are NOT to be used in any models. Dave)
However, if you have a 1989 to 1993 Ram or a 1998 and up 24 valve
engine, then we DO NOT recommend using any fuel other than #1 or #2 Diesel.
In an emergency, you can use 1K or 2K Kerosene, Jet-A, Jet A-1, JP-5, or
JP-8 ONLY IF YOU BLEND 5% NEW LUBE OIL with the fuels (ie: 20 gallon fill-up,
add 1 gallon new lube oil). Adding the lube oil is VERY IMPORTANT, as running
on the lighter fuels without additional lubrication will cause excessive wear
in your fuel pump, which is not a warrantable repair.
CAUTION: Running fuel other than #1 or #2 diesel w/5% lube oil should only be done if there is ABSOLUTELY NO OTHER CHOICE. Fuel pump damage due to inadequate fuel lubrication IS NOT COVERED BY WARRANTY, we don't recommend "lighter fuels + 5% lube oil" unless you're in a really desperate situation.
Q: WHAT'S THE DIFFERENCE BETWEEN HIGHWAY AND OFF-ROAD FUEL?
On September 9, 1999 I called the local fuel oil supplier to discuss fuels, DOT inspectors, and the perils of having dyed fuel in a tank. Here are the high points:
Off road Diesel #2 and fuel oil # 2 differ only in the tax applied at the time of sale. Both are dyed red.
Kerosene #1 and #2 are lighter than #1 and #2 diesel fuel. Most kerosene is dyed red.
The old method of winter treatment using 1 gal of kerosene to 10 gal of diesel can get a driver into trouble unless it is dispensed as clear kerosene from a pump which charges road tax.
Any red dye in a tank of fuel is detectable by the sampler the DOT uses, even when diluted by a large quantity of undyed fuel. As little as 1/2 qt of ATF in a tank of fuel will be detected as untaxed fuel and can cause a major headache for the driver.
In VA, fines for using dyed fuel (untaxed) begin at $1000and go up rapidly from there. Road checks for untaxed fuel began in northern VA, and have now spread throughout the state.
Subject: Re: [RAM] Highway diesel vs.
Date: Mon, 12 Jan 1998 09:27:06 -0500
From: KI4CY <my email address>
> A farming friend of mine w/o internet access who also owns a
> is running off road diesel in his truck.
> My question is this: is there a difference between off road diesel vs.
> road diesel besides the dye and the road taxes. I've heard that the off
> road stuff will clog catalytic converters and warned him about this.
On road fuel has a low sulfur content to prevent catalytic converter damage. The sulfur will deactivate the catalyst and lead to soot clogging of the converter core.
The process of producing the lower sulfur content raises the Cetane rating of the fuel, but lowers the fuel lubricity. The seal shrink&swell characteristic of low sulfur fuel is different than the high sulfur stuff. Some engines need the higher Cetane of low sulfur fuel. The off road high sulfur fuel I buy for my tractors has a Cetane rating of 40, while the low sulfur highway fuel has a rating of 45.
Some older injector pumps (not the '94+ Ram) need the lubrication and swell characteristics of the high sulfur fuel or they fail. Many of the older automotive injector pumps (Ford, Dodge, Mercedes) required a rebuild when the fuel was changed. I am trying to gradually wean my tractors to low sulfur fuel so that they won't need a rebuild when the high sulfur stuff disappears suddenly the way off road leaded gasoline did.
When the law was written that changed the fuel sulfur levels, the USDOT had no authority to test for dyed fuel. The EPA had sole responsibility for testing and enforcement. Since then, many states have begun random testing at weigh stations to ensure their cut of the road fuel tax. Testing is still spotty and infrequent, so the chances of an individual pickup truck or car being caught are very low. The state safety inspection stations have never checked the diesel fuel in my car or truck. Only a few highly publicized cases have have made the news in an effort to scare people into compliance.
The bottom line: Off road fuel has no sulfur restriction, and extended use of this fuel can lead to catalytic converter failure. If you have no catalytic converter and the Cetane rating of the fuel is good enough, the fuel will not harm the engine.
Subject: Re: [RAM] Highway diesel
vs. offroad diesel?
Date: Tue, 13 Jan 1998 09:28:19 -0500
From: KI4CY <my email address>
> I use on road fuel in my truck and your response about cetane
> and lubricity generated several additional questions.
> 1. If 40 cetane has more lubricity than 45 cetane, should I stick with 40?
Here are the answers as I understand all of this. If I goof, Joe D. can straighten us out.
The lubricity differences between high sulfur and low sulfur fuels is related to the lubricating value of sulfur (BTW sulfur compounds are what make 90W gear oil stink). Most highway fuel has another lubricant added to keep the injector pump plungers from sticking. The P7100 injector pump used on 94+ Rams uses internal lubrication from the engine oil pump and additional lubrication is not needed.
The Ram/Cummins specifies a minimum Cetane rating of 42. It will run on lower Cetane fuel, but may produce some smoke and have lower fuel mileage. Cetane is (sort of) an indication of the delay between fuel injection and fuel ignition, and the higher cetane of low sulfur fuel is a byproduct of the refining process to remove sulfur. The lower the cetane rating, the longer the ignition delay. For fuel economy and emissions, longer ignition delays produce effects similar to late pump timing. The higher cetane requirement allows manufacturers to use later and more precise injection timing to meet emissions standards.
> 2. The station I usually visit advertises their diesel as "premium".
> Does this equate into a higher cetane rating?
Usually, but "premium" is not very clearly defined.
> 3. I don't recall seeing a cetane rating on the pump at the
> have never looked for it either. Are cetane ratings posted?
Most fuel stations post the cetane rating and a "low sulfur highway fuel" certification.
> 4. Related to question 3, I do recall seeing something like
> Diesel #2" posted on the pumps at the station. What exactly does this
> mean with respect to sulfur and cetane rating?
It means that the sulfur level is approved for highway use and catalytic converters. Diesel #2 is heavier than #1. It also (usually) has lower Cetane and more heat energy per gallon than #1. #2 will tend to form wax crystals and gel at temperatures below 10 degrees F. Most winter fuels are a blend of #1 and #2, but when it gets really cold I add fuel conditioner to avoid clogging the fuel filter with wax crystals.
> 5.<SNIP> Here's the question. Does the "around
town" driving, where the
> motor just barely gets up to normal temp, tend to clog the converter?
> Any preventative measures?
If the converter runs below operating temperature for extended periods, it can clog. My opinion is that in your case the engine should be run at operating temp for at least 1/2 hour at least every other week. I try to take my truck for a one hour drive every couple of weeks to keep the batteries charged and the oil dry. I know it's a drag to force ourselves to drive these trucks, but it's a sacrifice we all should make. ;-)
Here is a posting I found on a web forum and saved long ago (probably from ag-online)
Subject:RE:low sulfur diesel
Date:Sat Oct 19 08:24:20 CST 1996
posted by:firstname.lastname@example.org (Garret)
I believe the concerns over low sulfur diesel are unwarranted. The sulfur-containing species constitute only a minor fraction of the overall molecular composition. The lubricity of the fuel is most influenced by the paraffin content, which is related to the cetane number. High cetane fuels have a lower aromatic content which can adversely affect lubricity. In other words, the quality and cleanliness are much more likely to affect pump performance than the minor sulfur constituents. I am a fuels chemist and diesel truck/tractor owner. I would not hesitate to use the reformulated fuels
For a discussion of low sulfur fuel lubricity, see this discussion on the TDR forum.
|From TDR #4: Fuel differences due to the sulfur reduction in diesel
Pour Point - The pour point of low sulfur fuel is nearly the same as high sulfur fuel.
Cloud Pount - The cloud point of low sulfur fuel is slightly higher, but not enough to cause a filter plugging concern
API Gravity - The hydrogeneration process used to remove sulfur from fuel does reduce the density of the fuel but, it increases the energy content. The net result per unit volume volume density loss versus the energy content gain, is a less than 1% decrease in energy.
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)
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.
Date: Tue, 13 Jan 1998 22:33:21 EST
From: Drdonnelly <Drdonnelly@aol.com>
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".
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: B.Hamilton@irl.cri.nz (Bruce Hamilton)
> What is the Cetane rating in Diesel fuel and what are the consequences
> 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 )
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@example.com>
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.
Subject: Re: octane and cetane ratings
Date: Fri, 17 Jan 1997 12:33:21 -0800 (PST)
From: Blaine Hufnagle <firstname.lastname@example.org>
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
>> 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
>> 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"
> 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.
> 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.
> 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
> 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.
I think I covered all the bases there... :-)
Last update October 2, 2001