In Front of the Flywheel - October/November - 2020

Forced Induction Volumetric Efficiency

My Apologies

All of us are human. Being able to admit when you made a mistake is an admirable trait. This month I am going to address turbocharged vehicles. But before I do, I want to correct an issue that I may not have covered completely in last month’s article.

In the article “MAF Sensor Diagnosis” I had a section titled “Double the Displacement.” It referred to a 2.0 Liter Ford and stated that 4 grams per second of air flow at idle was the approximate expected value. This information was completely correct. After the article was published, I had a conversation with a friend, who also happens to be a highly respected industry professional, and he pointed out a missing piece from my statements.

The original writing addressed MAF readings at idle based on the displacement of the engine. I basically mentioned two things: 1: The expected airflow measurement from the MAF sensor at idle should be twice the displacement of the engine. 2: This is a mediocre test for diagnosing a MAF sensor issue.

From those previous points, number 2 is correct. However, point 1 needs a bit of clarification. “Double the Displacement” works very well on smaller displacement engines. The example I used was a small engine and the data worked well. But when we have larger engines the “Double” rule does not apply. When an engine gets into the 3.0 Liter range, or higher, the airflow measurement starts to shift. It becomes more like 1 gram per second per liter instead of double.

To Illustrate my point, the aforementioned 2.0 liter Ford (used in the last article) should flow about 4 grams per second at idle. But, if we were to idle a 6.0 liter engine, we are now expecting just over 6 grams per second. 12 grams per second, 6 x 2, would be excessive.

There are a few important things that we can take away from this. First, we do not normally use airflow measurements at idle to diagnose MAF sensor issues. Second, what I wrote last month about the “Double the Displacement” technique was correct for smaller displacement engines but is not accurate for engines once they get in the 3.0 liter range or higher. Third, and most importantly, networking with other industry professionals, being able to admit when you are wrong and learning from your mistakes can get you a long way. I apologize for being incomplete when I wrote about GPS at idle. Time to move forward.

Volumetric Efficiency on a Forced Induction Engines

In last August GEARS article I suggested that a volumetric efficiency (VE) measurement could be a valuable diagnostic tool for testing MAF sensors as well as other engine breathing issues. This month, and most likely the next month, we will play with forced induction vehicles equipped with MAF sensors.

The Physics

If an engine is not boosting then it behaves the same as a naturally aspirated engine. This statement pertains to a VE measurement as well as fuel trim behavior. Once boost occurs operating conditions change. So, everything we have covered in the past for a naturally aspirated engine still applies with some new additional behaviors during boosted conditions on a forced induction engine.

Any manufacturer can boost an engine to improve VE. Let us break this down. Our atmosphere is somewhere in the neighborhood of 14.5 pounds per square inch (psi,) depending on your altitude, and this is the pressure that is pushing down on our bodies at any given time. This is also the amount of pressure that is being applied to every naturally aspirated engine’s intake manifold. Forced induction changes the game.

Specifications/Scan Data

Specifications are quite often key in our diagnosis of many vehicle systems. Data PID’s on a scan tool have gotten better over the years and prove to be a source of valuable diagnostic information. I personally love when a data PID says target/command and another data PID says actual. These data PIDs make our life easier on some manufacturers. When it comes to forced induction, understanding of pressures, airflow and the PCM’s response are required to make all of this data useful. Not all manufacturers provide us with the data we would like to see. In addition, I have yet to see a manufacturer that publishes VE specifications.

Back to Physics

If our barometric pressure is 14.5 pounds per square inch, and our vehicle is capable of producing 14 pounds per square inch of boost, would it be safe to say that two atmospheres of pressure are entering our engine under a full boost situation? Twice the air means twice the fuel and we generate power. In this situation, if everything is working correctly, we would expect a 200% VE reading when we plug the numbers into a calculator.

I have seen MAF equipped diesel engines boost 30 psi. If we follow the previous statements, that equates to three atmospheres of pressure, one atmosphere of natural pressure and two atmospheres on top of that, what do you think? It does not take a rocket scientist to figure that three times the pressure equates to 300% expected VE. This is just one reason why we have forced induction.

Measuring Volumetric Efficiency with Forced Induction

Measuring VE on a forced induction is easy but it needs to be done twice. The math is actually a bit complex, but thanks to computers we have apps that can perform these calculations for us. It is important to know that whichever calculator you decide to use there needs to be a barometric pressure (BARO) input. Not all calculators offer this option.

Record a wide-open throttle test drive, as you would with a naturally aspirated vehicle, and plug the numbers into a VE calculator as usual. This will yield a “raw” VE number. When the data is entered into a VE calculator it should be greater than 100% depending on the amount of boost. At this point we have no idea what a known good number is for the particular application.

In order to illustrate my point, we are using a Chevy Cruze equipped with a 1.4 liter turbocharged engine and in Figure 1 you can see that the raw VE is 155%. (Figure 1).

Second, plug the same numbers into the VE calculator but enter the boost pressure into the barometric pressure field (Figure 2.) This will compensate for the new, or actual, pressure in the intake manifold and will yield an “adjusted” VE value that should be near 100%. The “raw” and “adjusted” VE numbers, coupled with fuel trim numbers, will be valuable during diagnosis. In this case, the raw VE number is above 100% and the adjusted VE number is 94%. (Figure 2).

On a naturally aspirated engine, it is pretty easy to predict what a VE number should be. On a forced induction engine, the waters get a bit muddy. Known good results for the “Raw” calculation should be above 100%. How far above 100% depends on the amount of boost. The “Adjusted” calculation should be near 100% regardless of how much boost is happening. This calculation helps level the playing field when specifications are absent. These two calculations, along with fuel trim behavior, can be used together to point us in a diagnostic direction. For example, a raw VE calculation of near 100% and an adjusted VE calculation of approximately the same would indicate that no boost is occurring.

Predicting VE During Boost

Predicting boosted VE can also be valuable. Even before plugging numbers into a calculator, the same scan data recording can be used to predict what the VE should be. The next example is a 2012 Mini Cooper with a 1.6 liter turbocharged GDI engine. A test drive was performed and the data shown in figure 3 was gathered. Not shown in the data is a BARO reading of 98 kPa. (Figure 3).

The procedure to predict VE is easy, simply divide boost by BARO (boost/ BARO). In this case, 171 divided by 98 equals 1.75. Move the decimal place two spaces to the right and our prediction is that this engine should have 175% VE under these current operating conditions. Figure 4 shows the raw VE calculation using the same numbers.

The raw VE calculation for this Mini was 179% and our predicted VE was 175%. I’d say that is pretty darn close. If the numbers stray too far from what we predicted this indicates the presence of an engine breathing or air metering issue.

In the next issue we will expand on how these two calculations, coupled with fuel trim numbers, can point us in a logical diagnostic direction.