About the Air in Fuel

The Damaging Effects of Air in Fuel

As we've researched the problems of air/vapor on engines, we have found many books and troubleshooting manuals concerned with the same problem. We've also discovered that some of the world's most well known leading manufacturers have service topics concerned with air/vapor and fuel delivery. You'll see that fuel supply woes aeffects of airnd air/vapor are discussed heavily in relation to many problems. We've discovered the following companies diagnosing many problems and relating air/vapor to be a major culprit:

  • Bosch
  • Caterpillar
  • Cummins
  • Detroit Diesel
  • ADS, Association of Diesel Specialist
  • Racor/Parker
  • Milwaukee School of Engineering
  • More to come with research

Huge corporations and schools from around the world recognize air/vapor in fuel as a problem. We've listed only some of the symptoms air/vapor has on engines to help you better understand how serious the affects can be. You will be able to see these topics discussed throughout our documentation. Other factors can create these same symptoms however; we do know that air/vapor will cause these symptoms:

  • Low Horsepower
  • Low Fuel Mileage/Poor Fuel Mileage
  • Excessive Smoke
  • Injector Failure
  • Hard Start or Will Not Start
  • Engine Surges at Low Idle
  • Warm Engine Runs Rough or Misfires in Operating Range
  • Rough Idle
  • Engine Will Not Reach Rated Speed When Loaded

In 1990 Caterpillar released a Special Instruction, 651-1250, stating: "Normally No. 2 Diesel Fuel contains about 10% airs in solution, although the air is not visible." Caterpillar also states: "When the amount of dissolved air exceeds 10%, fuel rate and power output are reduced." In slight contradiction to that statement, through our studies and feed back from customers, we've come to one of two conclusions; first, less than 10% dissolved air in fuel still affects the engines performance adversely and/or two, there is always at least 10% dissolved air in fuel. It may be a little of both.

January 2006, Caterpillar released another publication, SENR9620-02, discussing the damage caused by air/vapor in fuel to the EUI injectors. Note: These types of injectors are also found in the Ford Powerstroke Engines. The following are quotes from this publication: "If fuel supply pressure is too low, or if fuel flow is restricted due to plugged fuel filters, a vacuum bubble implodes and causes internal damage to the injector. The implosion actually blasts small amounts of material away over time and results in CAVITATION EROTION. This erosion can damage high-pressure sealing surfaces, causing excessive high-pressure fuel leakage during injection and significant loss of performance. The injector must then be replaced to restore performance." They go on to say, "Air bubbles in the supply fuel can contribute to injector tip failures. Tip failures may result in major mechanical damage to the piston and liner, turbocharger and cylinder head." "An air bubble in the tip provides no fluid damping allowing the check to impact the tip with up to 50% greater force." Air/vapor in fuel is very inconsistent. There are many variables to take into consideration, "fuel temperature, pressure on the fuel, specific gravity and the amount of aeration to which the fuel has been subjected," according to the Cummins service topic. While the vehicle is in motion we believe it would be easy to estimate there is more that 10% air/vapor in fuel.

The number 1 schools of hydraulics in the country, the Fluid Power Institute at MSOE (Milwaukee School of Engineering) estimates "approximately 75% of hydraulic system failures are a direct result of air".

In 1965 Cummins released a Service Topic, File No. 5-135, and discussed air/vapor in fuel in this document. This article came about to address complaints they were receiving about the fuel filters being only partially filled with fuel when removed. They explain how air/vapor form, which was the cause of the filter not being filled with fuel. Note: if the fuel supply is above the filter (the fuel line runs from above into the filter) this phenomenon will not be apparent, however the problem still exists. The following is a quote from the Cummins Service Topic 5-135: "The source of the vapor is the fuel itself. Like water, fuel contains a certain amount of dissolved air depending upon the fuel temperature, pressure on the fuel, specific gravity and the amount of aeration to which the fuel has been subjected. Reducing the pressure on the fuel or increasing the temperature of the fuel releases the air. The amount released depends upon the degree of fuel saturation with air and the magnitude of pressure reduction or temperature increase."

Keep in mind that there are many changes that one can do to enhance performance; pistons, cam, number of valves, position of injector, timing, etc. Of all of these changes there are 3 constant variables that directly affect the outcome; exhaust restriction, air supply/ condition, fuel supply/ condition. Cummins stated 4 variables related to the amount of air/vapor in fuel, we'll cover 3 of them:

  • Pressure on the Fuel
  • Fuel Temperature
  • Aeration to which the fuel has been subjected

First, we'll discuss "pressure on the fuel". Considering that fuel is a liquid the same principals apply; place a liquid under a vacuum, the boiling point will be lowered, vapor will develop. Place the liquid under pressure; the boiling point will be raised. As a vehicle travels into higher elevation atmospheric pressure is reduced. Atmospheric pressure has a direct relationship to the vacuum necessary to draw the fuel to the supply pump or injection pump. The more atmospheric pressure applied to the fuel level, less vacuum is needed creating fewer vapors. Reducing the atmospheric pressure to the fuel level requires an increase in vacuum, producing more vapors. This is part of the reason why engine performance is lost in higher altitudes.

Second, we need to factor in the" fuel temperature". Cooler (thicker) fuel produces fewer vapors while warmer (thinner) fuel will produce more vapors. The cooler the fuel is the more viscous it will be, the warmer the fuel the thinner it will be. Remember as the sulfur is removed it has a thinning affect to the fuel, thus producing more vapor. To summarize this;

Cool Fuel

  • Thicker
  • Less Vapor
  • More Entrained Air

Warmer Fuel

  • Thinner
  • Less Entrained Air
  • More Vapor

Recognizing the conditions of the test cell vs. real world application introduces a whole gamete of variables affecting the fuel supply/delivery to the engine.

In a manufactures test cell, if you ever have the chance to go on a tour, you will recognize optimal engine conditions. Optimal engine conditions equal optimal engine performance. Optimal engine performance consists of optimal fuel supply. Manufactures' tests vary slightly from one manufacture to the other but are configured with these basics:

  • Fuel tank is usually at least 10 feet above the engine
  • Fuel tank is stationary
  • Return fuel is returned to a separate tank

With the optimal fuel supply achieved, the fuel injection system can perform the job for which it was designed, keywords, optimal performance. That is to deliver virtually pure fuel to the injectors at the proper pressure; the injectors can then deliver the proper amount of fuel at the proper time into the cylinder for a more complete combustion. Understand fuel injection systems were designed to inject a predetermined amount of fuel at a predetermined time.

Give thought to the real world applications. As you might have noticed, we are unable to raise our fuel tanks higher than they are.

  • Fuel tanks are usually below the engine and separated from the engine by 5 – 15 feet
  • Vehicles are in motion
  • Hot return fuel is routed into the fuel supply tank