Mixing Gasoline with Diesel Fuel
My 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
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
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
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
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 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.