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Old 01-26-2011, 03:18 PM   #1
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Adding gasoline to diesel fuel... Not good

Brian Kmetz. As a mechanical engineer, Brian's daily task at
work is to extract BTUs through oxidation from mass quantities of
methane and fuel oils. Needless to say, he knows how the fuel "stuff"
works. Brian writes:

We hear this one all the time. Another version is to add one gallon
of gasoline to 20 gallons of diesel fuel as a cheap easy anti-gel for
winter fuel. I'll include alcohols in this discussion because a lot
of guys add it instead of gasoline. Both fuels have the same
detrimental effect on diesel fuel and are very close in weight and
BTU content.

The mechanic meant well and probably never saw a fuel pump or
injector failure due to improper blending of fuels. But that doesn't
mean one is not risking damage, even in small dosages.

Gasoline and alcohols hit diesel fuel right where it hurts the most.
Those light thin fuels will lower the cetane number and lubricity. To
explain how octane and cetane DO NOT work together, I'll have to
review more crude oil and fuel fundamentals.

The light distillates that gasolines are made from have a natural
high-octane index. The middle distillates that diesel fuels come from
have a high cetane index. The octane and cetane indexes are INVERSE
scales. A fuel that has a high octane number has a low cetane number,
and a high cetane fuel has a low octane number. Anything with a high
octane rating will retard diesel fuel's ability to ignite. That's why
each fuel has developed along with different types of engine designs
and fuel delivery systems. Gasoline mixed in diesel fuel will inhibit
combustion in a diesel engine and diesel fuel mixed in gasoline will
ignite too soon in a gasoline engine.

A lot of old-time mechanics added some gasoline to diesel to
supposedly clean the carbon deposits out of the cylinders. I have
never read anything that said it worked. Gasoline will make the fuel
burn hotter, and hotter burning fuels burn cleaner. That's probably
where the theory got started. In the older diesel engines that
belched lots of black smoke even when properly tuned, the result of
adding gasoline was probably more white smoke instead of black. This
might lead one to believe the engine was running cleaner. Maybe so,
probably not. Here's what happens.

Gasoline will raise the combustion temperature. This might or might
not reduce carbon deposits in the cylinder. This also might or might
not overheat the injector nozzle enough to cause coking on the
nozzle. That's a clogged injector tip in layman's terms. The fuel
being injected is the only thing that cools the nozzle. Diesel fuel
has a lower combustion temperature than gasoline. The fuel injectors
depend on the fuel burning at the correct rate and temperature for a
long life. If the combustion temperature is raised long enough, the
gums and varnishes in gasoline will start to cook right in the fuel
injector and turn into carbon. These microscopic carbon particles
will abrade the nozzle. High combustion temperatures alone will
shorten fuel injector life, gasoline makes the problem worse.

Gasoline and alcohols do have an anti-gel effect on diesel fuel, but
these fuels are too thin and will hurt the lubricity. Alcohols work
as a water dispersant in small amounts, but also attract water in
large amounts. Diesel fuel is already hydrophilic (attracts water) so
why add to the problem. The old timers got away with this because
high sulfur diesel fuel had enough lubricity to take some thinning.
Today's low sulfur diesel fuels have adequate lubricity, but I
wouldn't put anything in the tank that would thin out the fuel,
reduce lubricity, or attract water.

Opposites do not attract in this case. Use any of the diesel fuel
additives available to clean out carbon deposits, not gasoline or

While we're on the subject of fuels, let's discuss another common
question. What is cetane?

Cetane is to diesel fuel what octane is to gasoline. It is a measure
of the fuel's ignition quality and performance. Cetane is actually a
hydrocarbon chain, its real name is 1-hexadecane. It is written as
C16H34, or a chain of 16 carbon atoms with 34 hydrogen atoms
attached. All HC chains are also referred to as paraffins. Cetane is
a hydrocarbon molecule that ignites very easily under compression, so
it was assigned a rating of 100. All the hydrocarbons in diesel fuel
are indexed to cetane as to how well they ignite under compression.
There is very little actual cetane in diesel fuel.

All the hydrocarbons in diesel fuel have similar ignition
characteristics as cetane. Cetane is abbreviated as CN. A very loose
way to think about cetane is if the fuel has a CN of 45, then the
fuel will ignite 45% as well as 100% cetane. Diesel engines run just
fine with a CN between 45 to 50. There is no performance or emission
advantage to keep raising the CN past 50. After that 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. The minimum CN at the pump is supposed
to be 45. The legal minimum cetane rating for #1 and #2 diesel is 40.
Most diesel fuel leaves the refinery with a CN of around 42. The CN
rating depends on the crude oil the fuel was refined from. It varies
so much from tanker to tanker that a consistent CN rating is almost
impossible. Distilling diesel is a crude process compared with making
gasoline. Gasoline is more of a manufactured product with tighter
standards so the octane rating is very consistent. But, the CN rating
at the diesel pump can be anywhere from 42-46. That's why there is
almost never a sticker on a diesel fuel pump for CN.

Premium diesel has additives to improve CN and lubricity, detergents
to clean the fuel injectors and minimize carbon deposits, water
dispersant, and other additives depending on geographical and
seasonal needs. More biocides added in the south in summer, more ant-
gel added in the north in winter. Most retailers who sell premium
diesel will have little brochures called POPs (Point of Purchase) at
the counter explaining what's in their fuel. Please don't ask the
poor clerk behind the counter any technical questions after reading
this discussion. All they need to know how to do is sell you beer,
milk, cigarettes, lottery tickets, and take your money.

Texaco and Amoco are two big names who sell premium diesel in limited
markets. Amoco primarily sells its Premier to specialized industrial
and agricultural markets. I cannot get either in my area. Most fuel
retailers buy additives or buy treated fuel. In the Northern plains
states, Koch is a well-known marketer of premium diesel. I buy it
when I travel into Northern Wisconsin.

Because there are no legal standards for premium diesel yet, it is
very hard to know if you are buying the good stuff. I have good news.
An ASTM task force has drafted standards for premium diesel. When the
new specifications are accepted, information will have to be posted
on the fuel pump. Retailers will no longer be allowed to label cheap
blended diesel as `premium.' They will have separate pumps with clear
labels on both informing the customer what is being sold. The
marketing and labeling will be the same as with regular and premium
gasoline. Retailers selling the real thing use this system now.
Enforcement of all fuel standards is done at the state level in the

Diesel fuel is an international commodity for industry. Therefore,
you should be picky about where you fill up. Shop for price from a
large volume retailer so you have the freshest fuel. That's about the
best advice I can give.

The 1994 legislation and reformulation of diesel fuel in North
America is due to an international effort for lower emissions.
Cleaner diesel emission laws are on the way. Diesel fuel is going to
be reformulated into a cleaner fuel in general. Without getting too
technical (this is over-simplified and very generalized), diesel fuel
for the most part is made up of two different hydrocarbon families:
paraffins and aromatics. The paraffins have a naturally high cetane
index, burn clean, but cause the annoying gel problem in winter. The
aromatics have a naturally high lubricity, low cetane index, and
cause a lot of diesel emissions and soot. Reformulated diesel will
have a higher paraffin content, higher cetane number, and a much
lower aromatic and sulfur content. It will also be more prone to
jelling and have a lower lubricity. Big oil is working on improved
additives as I type this.

The reason nothing has happened yet is because of infighting in the
EPA on its new Tier II Emissions standards for gasoline and diesel.
Ultra-clean technology for gasoline and diesel engines is almost
ready to go, but the refiners have to lower the sulfur level
drastically in both fuels. The EPA should formally set something by
year 2000.

Brian Kmetz
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Old 01-26-2011, 08:58 PM   #2
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Re: Adding gasoline to diesel fuel... Not good

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Old 01-27-2011, 09:37 AM   #3
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Re: Adding gasoline to diesel fuel... Not good

Thanks for that information Frank! Do you know anything about using ether to start a diesel on cold days? My bus is equiped with a built-in ether spray bottle, but I've heard that using ether on a diesel engine is not a good idea.
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Old 01-27-2011, 11:54 AM   #4
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Starting aids

Most old truck drivers will advise, just enough, for starting a cold engine. Starting aids are a great tool, but excessive use does bad things. There are basic rules of adding start fluid. Never add start fluid to a stopped engine. Add short spurts of fluid to a turning engine. Introduce fluid into air inlet for engine. Try to start engine for about 1 minute, rest 1 minute, try to start again. If no start rest at least 3 minutes. Without adding any start fluid, try the start for a minute and then rest a minute, then the engine may start with no aid. Attempting to start a diesel engine with a low battery is not a good choice. After owning a diesel for some time, a good start method becomes known. My Ford 7.3 requires a quick squirt every morning and starts great all day with no more start fluid. That is dependent on the temperature. Currently at 33 degrees, fluid is required. Starting fluid is a starting aid, not a fuel. Use carefully and sparingly. Less is more..... Frank
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Old 01-27-2011, 08:13 PM   #5
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Re: Adding gasoline to diesel fuel... Not good

Liquid Wrench works pretty well and is easier on the rings than ether.

But a 7.3 should start EASILY at 32 degrees without ether! Sounds like you have some problems...
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Old 01-29-2011, 01:56 PM   #6
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Re: Adding gasoline to diesel fuel... Not good

Old MBZ owner's manuals said that gas could be added to diesel to winterize the fuel. But that was with the older higher sulfur diesel.
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Old 02-01-2011, 08:38 PM   #7
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Re: Adding gasoline to diesel fuel... Not good

I had a 6.2 chevy's fuel gel once, not good, a trucker said to mix kerocene with it, but i don't know if it was recommended. I had a clutch put in a GMC 4x4 W/ 6.2, the genius left the ground wire off the glow plug controller so he apparently used ether (even though a sticker on the air cleaner warned against it "immediate engine damage may result") The warm engine (when I picked it up) started right up, a few days later (in winter) no start. When I tried to remove the just replaced glow plugs the would not come out of the holes in the head. Further research revealed that the hollow tube glow plugs (sort of like a water heater element) which protruded into the "pre-combustion chamber" were mushroomed and would require both heads removed to fix. The "mechanic" denied using ether (of course) and accepted no blame.
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Old 02-02-2011, 06:48 AM   #8
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Re: Adding gasoline to diesel fuel... Not good

Winter-blend diesel is frequently nothing more than #2 fuel mixed with 10-20% K-1 works fine.

For that matter, a diesel will burn straight kerosene (or even jet fuel).
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