Mixing Gas with Diesel - on Purpose?
February 01, 2023
At the sister company, Geno’s Garage, the phones are answered by a staff that’s heard it all. The frequency of phone calls asking what to do about “gasoline added to my diesel fuel tank” is always a stressful situation for the truck owner.
In the next issue of the TDR, we take a look back 10 years ago and note the topic-of-the-day. Wouldn’t you know it, “gasoline mixed with diesel.”
So, is there an easy answer to the follow-up question; “How much is too much?” Nope. Each situation is as unique as the personality of the caller, the tolerance for risk, the age of the truck/engine, the travel situation, etc. What the caller is really looking for is the shared experience of others.
To provide that data, let’s take a look at the text from the forthcoming TDR magazine:
Gasoline Mixed with Diesel
The “10 Back” review of the year 2003 found an article about mixing diesel fuel and gasoline. It was good information, and I repeated the 2003 article in its entirety. Now, 20 year later, fuel systems have evolved, and the guidance given in Issue 39 (and repeated in Issue 79) does not hold true with today’s electronic fuel injection engines. Follow along with me as we go back 20 years for the TDR member question and the technical answer from TDR member and petroleum engineer, Brian Kmetz. Following is an abbreviated Q&A.
My wise old mechanic who has worked on Mercedes for years told me that if I put one gallon of regular gas to a tank full of diesel 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 BTU content.
The mechanic meant well and probably never saw a fuel pumps 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 alcohols.
Editor’s Note: Inquiring minds want to know, should the gas-in-diesel mistake occur and the mistake be caught early in the filling process, is there a point of too much danger? Because of the liability I could not get an answer “for the record.” However, the consensus among many in the diesel business was that a five-percent or less mixture is acceptable. This goes hand-in-hand with the story that Brian related to you in his second paragraph, “The mechanic meant well . . . but that doesn’t mean one is not risking damage, even in small dosages.” Amounts greater than five-percent lower the flash point of the gasoline/diesel brew. The resulting brew makes the fuel hazardous to handle as well as making the brew below diesel specifications, thereby degrading combustion and increasing wear.
And, the 20-year later update: Without a detailed chemistry lesson, Brian Kmetz gave you the technical overview. But, as you read, old-school habits, stories and folklore still exist. It is obvious to me that starting with the 1998.5 24-valve engine and its electronic VP-44 injection pump the “gasoline brew for injector cleaning” is an absurd practice. Old habits die hard?
I’m hopeful you found the review of Brian Kmetz about old school mechanics’ tales and gasoline mixed with diesel to be a helpful reminder.
Turbo Diesel Register Staff
This article was published in the Turbo Diesel Register Issue #119/ February 2023
In the next issue of the TDR, we take a look back 10 years ago and note the topic-of-the-day. Wouldn’t you know it, “gasoline mixed with diesel.”
So, is there an easy answer to the follow-up question; “How much is too much?” Nope. Each situation is as unique as the personality of the caller, the tolerance for risk, the age of the truck/engine, the travel situation, etc. What the caller is really looking for is the shared experience of others.
To provide that data, let’s take a look at the text from the forthcoming TDR magazine:
Gasoline Mixed with Diesel
The “10 Back” review of the year 2003 found an article about mixing diesel fuel and gasoline. It was good information, and I repeated the 2003 article in its entirety. Now, 20 year later, fuel systems have evolved, and the guidance given in Issue 39 (and repeated in Issue 79) does not hold true with today’s electronic fuel injection engines. Follow along with me as we go back 20 years for the TDR member question and the technical answer from TDR member and petroleum engineer, Brian Kmetz. Following is an abbreviated Q&A.
My wise old mechanic who has worked on Mercedes for years told me that if I put one gallon of regular gas to a tank full of diesel 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 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 pumps 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 alcohols.
Editor’s Note: Inquiring minds want to know, should the gas-in-diesel mistake occur and the mistake be caught early in the filling process, is there a point of too much danger? Because of the liability I could not get an answer “for the record.” However, the consensus among many in the diesel business was that a five-percent or less mixture is acceptable. This goes hand-in-hand with the story that Brian related to you in his second paragraph, “The mechanic meant well . . . but that doesn’t mean one is not risking damage, even in small dosages.” Amounts greater than five-percent lower the flash point of the gasoline/diesel brew. The resulting brew makes the fuel hazardous to handle as well as making the brew below diesel specifications, thereby degrading combustion and increasing wear.
And, the 20-year later update: Without a detailed chemistry lesson, Brian Kmetz gave you the technical overview. But, as you read, old-school habits, stories and folklore still exist. It is obvious to me that starting with the 1998.5 24-valve engine and its electronic VP-44 injection pump the “gasoline brew for injector cleaning” is an absurd practice. Old habits die hard?
I’m hopeful you found the review of Brian Kmetz about old school mechanics’ tales and gasoline mixed with diesel to be a helpful reminder.
Turbo Diesel Register Staff
This article was published in the Turbo Diesel Register Issue #119/ February 2023
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