How Air in Your Fuel Affects Performance

July 27th, 2011

By Matt Emery

Photography/Art: Courtesy of FASS

Correlation is a funny thing. Things sometimes go together in one way but are detrimental when joined in another. Take hydroelectric dams, for instance. Water force is used to spin turbines, which produces the electricity. But mixing water and that electricity is no good. The same goes for fuel and air. Everybody knows that it takes fuel, oxygen and spark to produce combustion, but having air in the fuel is detrimental to combustion. How can that be, you ask?

At its most basic explanation, a perfectly tuned combustion process needs precise amounts of all those aforementioned items, right? But they all have to be coming from their intended sources. Fuel from the line, air from the intake, that’s how it’s supposed to be. However, excess air in the fuel, which can be caused by many reasons, adversely disrupts the fuel’s flow rate, and therefore, its true pressure reading. The bottom line is that reduced or interrupted fuel pressure means less horsepower, plain and simple. And how does the air become trapped in the fuel, you might also ask?

Way back in 1965, Cummins released a service topic (File No. 5-135) that discussed the phenomenon of air/vapor in fuel. This article came about because of complaints the company had been receiving about fuel filters being only partially filled with fuel when removed. The service topic explained how air/vapor forms, which was the cause of the filter not being filled with fuel. The following is a quotation 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.

That’s a lot of words to say that the reason the filters weren’t full of fuel was that as the fuel enters the filter, the heavier fuel drops to the bottom of it, and any trapped air rises to the top. The rest of the stuff is interesting but really is the answer to a question that nobody asked. (Engineers—they’re all that way. You ask them what time the ferry gets in, and they tell you how to build a boat.)

Caterpillar also addressed the problem in 1990. Caterpillar released a special instruction (651-125, which stated,

Normally, No. 2 Diesel Fuel contains about 10% airs in solution, although the air is not visible.” It goes on to say, “When the amount of dissolved air exceeds 10%, fuel rate and power output are reduced.

That is, there’s always a certain amount of air trapped in the fuel when you pump it, so right out of the hole, you’ve got problems, because the trapped air is going to make your truck run less efficiently. We’ll leave the question of why we’re actually paying for air when we fill up for another day.

Fuel Air Separation Systems, or FASS, knows all this, and that’s why the company produces systems that will remove that air from the fuel stream. FASS explains that the problem does begin with the fuel but is really exacerbated right there, in the fuel tank. There are many reasons for the fuel tank being a problem area: hot fuel under pressure being dumped into the tank via the fuel injection return lines; fuel sloshing around in the tank while driving; less-efficient pumps (and by that we mean suction type, inline pumps— not in-tank pumps, which provide a more-positive flow pressure with less agitation); and the fact that most fuel tanks are mounted below the height level of the injectors. All of these things contribute to agitation, which then leads to additional unwanted air particles becoming trapped in the fuel flow. (Agitation may be great for washing your clothes, but it really messes up fuel-flow properties.)

To get the most horsepower out of an engine, it must have a continuous stream of correctly pressurized fuel. Some fuel pumps pull the fuel out of the tank via suction, which can cause slight cavitations. Nature abhors a vacuum, so when cavitations occur in the fuel flow, air fills that empty space. This causes air bubbles to form and/or causes the fuel to vaporize slightly. Both are bad for an engine that is looking to burn as much fuel as possible.

When vaporized fuel occupies the space in the tubing that should have pure fuel, the injection of fuel into the combustion chamber is delayed by a millisecond. That may not sound like a long time, but when an engine cylinder is starved of fuel, even for a millisecond, not as much enters the chamber—thereby retarding the timing and robbing the engine of its full power potential.

It’s also really bad for the injectors. Injectors depend on the fuel, not only for lubrication but also for help with how the injectors close.

What, you ask? Fuel flow affects how the injectors close and/or operate? It does, and here’s how: When an injector opens and closes so many times a minute, there is some wear and tear. It is the fuel, itself, that lubricates and cools the injector tip. However, when there is air in the mix (combined with the dropping overall quality of the fuel itself), it robs the injector of those essential lubrication and cooling functions. When pure fuel is present, the closing injector works more smoothly. When only fuel vapor is present, the injector closes harder or slams shut with much more force. All of this could cause potential damage to the injector tip.

Then, there is the chance that when the air/vapor mixture—rather than fuel—appears at the point of injection, it may actually combust within the injector tip. Fuel vapor is much less stable than pure fuel, and given that with fuel and oxygen present, essentially in a mist form, and with the heat involved inside an engine, all three components needed for combustion are present. Needless to say, this is not good and could cause outright damage to the injector tip—or at least cause the injector orifices to erode over time. Pre-combustion and increased impacting of the plunger can be the cause of blown injector tips, which could damage pistons and cylinder walls.

As we said, FASS, along with a few other companies, such as Air Dog, has designed systems that will remove that unwanted air from the fuel system. These systems accomplish this via a combination of filtration and compression.

Once the fuel enters the unit, it is first run through a fairly standard water separator filter. Past that though, the fuel pump pressurizes the fuel, which force-feeds it into a second filter housing. There, dirt particles are trapped in the filter and any air vapor rises to its top. This air/vapor mix is returned to the fuel tank. What’s left is pure fuel being forced from the bottom of the filter and fed eventually to the injectors. Pure fuel equals pure power.

And in this day and age of sky-high fuel prices, having pure fuel entering the engine improves mileage by 2 to 3 mph. When every bit of performance the fuel is capable of producing is being used to propel the truck, it’s going to run better. It just stands to reason that it would then take less fuel overall to go the same distance.

Now, we’ve gone over the hows, whys and benefits of an air/fuel separator; but is an air/fuel separator really necessary for everybody? When asked that question, even FASS’s Brad Ekstam (who’s naturally trying to sell you one) candidly replied, “I don’t know if you do. It depends on the truck and how many miles you drive per year.”

However, Brad does add that you do need one if you want to get the most power out of your truck, get the most mpg possible and also better protect those expensive new (and large) injectors you recently installed.

As we stated, there are a couple of companies that offer systems to remove air from fuel. The offerings of both are based on how much fuel flow is required.

FASS’s HD Series has a flow range of 90 to 260 gph and 8 to 70 psi—up to 160 psi when used with a FASS Fuel Pressure Regulator (FPR) with a boost compensation port. The HD has the largest filter capacity of the FASS line. The company’s Titanium Series has a smaller overall footprint than the HD Series. The flow is limited to 220 gph, but it will pump the same pressures as the HD Series. The Platinum Series features flow capabilities of 90 to 150 gph and pressures of up to 160 psi.

Air Dog offers its AirDog 150 and 100 models. As the designations imply, the 150 passes 150 gph while the 100 passes, you guessed it, 100 gph. Both have a preset pressure of 15 to 17psi.

This is the type of “performance” part that will work as a stand-alone part, as intended; it really comes into its own when it’s one piece of a high-performance engine. It sure couldn’t hurt to have one on a stock engine, because it should help both injectors last longer and improve mileage. An air/fuel separator may be just that edge you need when you get to the line and look over at that truck in the other lane. All things being equal, though, it stands to reason that the one who is pumping pure (or purer, anyway) diesel fuel into his engine will have an advantage over someone who isn’t.






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