helpful tips for your diesel fed engines:
ever think of mixing diesel with gasoline or kerosene.
wise old mechanic who has worked on Mercedes for years told me that
if I put one gallon regular gas to a diesel tankfull after about
every four tanks that it would perform essentially the same job
as a fuel injector cleaner at a fraction of the cost.
I would like to hear a technical opinion.
Bill Carson , e-mail
Bill, I’ll turn the answer to your request for a technical
opinion over to 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
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 USA.
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
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.
from TurboDieselRegister.com [http://www.turbodieselregister.com/mixing_gasoline_and_diesel.htm]