GASOLINE AND DIESEL
(Issue 26, pages 14 – 15)
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.
would like to hear a technical opinion.
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:
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.
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.
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
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.
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
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.
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.
do not attract in this case. Use any of the diesel fuel additives available to
clean out carbon deposits, not gasoline or alcohols.
we’re on the subject of fuels, let’s discuss another common question. What
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.
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.
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.
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.
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.
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.
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.
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.
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.
Click on any link below for more TDReprints
GASOLINE AND DIESEL
(Issue 26, pages 14 – 15)
AND THE MARKETING OF AMERICA
(Issue 31, pages 126 – 129)
(Issue 28, pages 110 – 112)