Volumetric efficiency of turbocharged engines

March 1, 2018
Performing a VE calculation on a boosted engine is not much different that doing so on other engines. A known good 2012 Mini Cooper S with a 1.6 liter turbocharged engine will be used to illustrate the technique.

Volumetric efficiency, or “VE,” is a measurement of how well a pump can move a liquid or gas compared to its physical limitations. If one were to perform an internet search, the results would include things like oil drilling platforms, hydraulic rams and more. Automotive technicians have also been using this measurement for many years to diagnose engine breathing problems. After all, isn’t an engine just an air pump?

Two years ago, I penned an article “Asthmatic Engines,” (March 2016) on this very topic. However, its subject matter pertained to naturally aspirated engines. The technique lends itself well to these engines and assists in quick and efficient diagnosis of engine performance issues and some MIL (Malfunction Indicator Lamp) illumination complaints. With the increasing popularity of turbocharged applications in the last decade or so, there needs to be some adaption of this technique in order to use it correctly on a forced induction engine. Let’s do a quick review of volumetric efficiency as it pertains to naturally aspirated vehicles.

VE review

VE is calculated using three basic inputs: engine displacement, engine RPM and measured airflow. With these three pieces of information, we can calculate how much air the engine should pump at the given RPM and compare that number to the amount of air that was actually measured entering the engine. The VE calculator is recommended and outputs the value in a percentage. The higher the percentage the better the flow and vice versa. If we want to be a little more precise, some calculators allow us to enter information that effects air density, such as barometric pressure, air temperature and even humidity. A general rule of thumb is a VE number of 75 percent or higher is acceptable, but this number will vary with different engine applications. An OHV V6 engine may be only 75 percent to 80 percent when it is functioning to the best of its ability while a DOHC in-line 4-cylinder engine (with better airflow) may be more like 80 percent to 95 percent.

Once the VE number is obtained, and a few extra data PID’s (Parameter Identifiers) are observed, a diagnostic direction can be quickly chosen. For example, if a vehicle has low VE while the throttle is wide open and the oxygen sensor reads very lean the fault is more than likely an air metering issue. On the flip side, if a vehicle has a good VE number and the oxygen sensor still reads very lean during wide open throttle, then the fault is most likely a fuel delivery issue. For more detail on this technique as it pertains to naturally aspired engines, refer back to the March 2016 issue of Motor Age.

Introducing turbochargers

With the addition of turbochargers, we now have to account for air being forced into the engine above and beyond what the engine alone would normally flow. The results are VE numbers in excess of 100 percent. One of the problems that these high numbers present is “how high is known good?” Without knowing what good is, we cannot make a good diagnostic decision using the VE value. Some applications may yield a VE number somewhere around 125 percent while other applications may push 300 percent. With this wide of a range we have to make some adjustments to our calculations to level the playing field. Once we do this we can use the values to make some diagnostic decisions and move our way down a logical diagnostic path.

The additional data PID that is required to accomplish this on a boosted engine is boost pressure or intake manifold pressure. Air temperature is also a desired addition to the equation but is not necessary. The only tool, besides a scan tool, that is required to calculate a VE number on a forced induction engine is a VE calculator that allows a boost pressure or barometric pressure input. Most calculators that I have seen do not provide this option so they cannot be used for this task regardless of how well they may work on a naturally aspirated application.

If it can be obtained, desired boost or a published maximum boost specification would be beneficial to the process. If this information is available, it can be compared to the maximum boost achieved during a test drive and the result can be an additional piece of information used during diagnosis

Calculating VE

In a way, calculating VE this way is kind of like tricking the calculator a bit. What I mean is, during wide open throttle on a naturally aspirated engine we have barometric pressure in the intake manifold. A general barometric pressure is either built into the calculator behind the scenes or input by the technician and is required for the calculation whether it can be seen or not. On a turbocharged engine, the vehicle may be operating at the same barometric pressure as its non-turbo counterpart, but is there still barometric pressure in the intake manifold when it is floored? Not if the turbo is boosting it. So let’s lie to the calculator and tell it what the boost pressure is instead of the barometric pressure. After all, boosted or not, the intake manifold pressure is what the calculation actually requires so it does not matter which label the calculator assigns to it, boost or BARO. We will need to perform this calculation twice. First, the calculation will be performed using the barometric pressure and we will label this value “VE.” Second, the calculation is repeated with the actual boost pressure instead of the barometric pressure. The second value will be labeled “Adjusted VE.”

A note on conversion and baseline pressure

Some vehicles, or scan tools, may display information in different units of measure. These situations may require some conversion. For example, one vehicle may display approximately 99 kPa (kiloPascals) for barometric pressure and 34 kPa while idling. Another vehicle may display 29.4 inHg (inches/mercury) of barometric pressure and 10.1 inHg while the engine is idling. The first vehicle may be easier to grasp because 100 kPa is one atmosphere and the engine creates vacuum, hence the lower kPa number, while idling. The second example could appear as a good barometric pressure but have poor engine vacuum at idle or 10.1 inHg while a standard specification of 18 – 21 inHg is expected. In all actuality, if we were to connect a vacuum gauge to this engine we would read 19.3 inHg, well within the acceptable range. In this case, 10.1 inHg is 19.3 inHg less than our BARO of 29.4 inHg. These readings are a result of the scan tool displaying absolute pressure instead of gauge pressure/vacuum. How the vehicle and scan tool display this pressure information, as well as the units of measure supported by the VE calculator being used, need to be understood and converted correctly for the VE number to mean anything at all.

Known good calculation

Performing a VE calculation on a boosted engine is not much different that doing so on other engines. A known good 2012 Mini Cooper S with a 1.6 liter turbocharged engine will be used to illustrate the technique. In addition, the procedure will be done using an aftermarket scan tool using only global OBDII data. A recording of a wide-open throttle test drive has been made and data PID’s are observed (Figure 1) at the engine’s highest boost. There are two data PID’s not included in the image: a BARO (Barometric Pressure) of 98 kPa and an IAT (Intake Air Temperature) of 19 degrees Celsius.

First, a non-adjusted calculation (top of Figure 2) is made by inputting engine displacement, engine RPM, MAF reading, IAT and BARO. Boost numbers will be entered later. In this case the actual VE of the engine is 179 percent. For the second, or adjusted, calculation the actual boost pressure will be substituted for the barometric pressure (bottom of Figure 2). The new adjusted VE calculation is 103 percent. This adjusted number is a much better way to determine if the engine is breathing properly. Since different boosted applications are all over the board when it comes to the raw numbers, the adjusted number actually mean something to the technician regardless of application since it can be compared to a general good specification of 90 percent to 100 percent. However, care must be used in a low-boost situation since this number is a good indication of VE under current boost, not desired boost.

You may have noticed that the calculated load and the absolute load PID’s were also included in the screen capture. On this vehicle, the absolute load PID was very close to our raw VE number and the calculated load PID is very close to our adjusted VE calculation. I would like to say these PID’s can be substituted for our calculations, but when failures occur these numbers can stray from the actual calculations. In a nutshell, these PID’s should be used with a little caution.

While diagnosing breathing problems, another data PID that we need to observe is fuel trim. It is important to observe these values during closed loop operation which will most likely not be at peak boost, rather a bit before the peak boost in the recording. For the known good vehicle, the fuel trim numbers were normal.

The next step is to determine how this information is useful for diagnosing a failure. To accomplish this, a few broken cars will be used for comparison and to observe how different faults effect data and VE.

Leaking boost

When a vehicle has a boost leak, or a leak between the outlet of the compressor and the intake valves, the scan data will change. All of the air entering the turbocharger passes through the MAF sensor and is reported to the PCM. However, some of the air is forced out of the boost leak while the remainder is forced into the engine. This will cause the PCM to over fuel the engine and the VE number to be falsely elevated. Basically, too much air is measured, too much fuel is injected and too much airflow gets entered into our calculation.

The result of a boost leak (Figure 3) can now be spotted with VE and a scan tool due to high adjusted VE and the resulting negative fuel trim numbers. For the sake of full disclosure, the scan tool capture in this case is actually the same vehicle used earlier in the article for our known good before a boost leak repair had been performed. It should also be noted that the load PID’s did not change very much and is a perfect example of what was mentioned before — use the load PID’s with caution.

Air metering

A vehicle with an air metering issue, such as a faulty MAF sensor or broken turbocharger inlet tube, will exhibit different behavior. In the case of false, or pirate air as it is sometimes called, the MAF sensor does not measure all of the air entering the turbocharger. Extra unmetered air is drawn in through the leak and the MAF sensor reports an incorrect value to the PCM. In the case of a faulty MAF sensor the results will be the same as long as the MAF sensor is under measuring the air. These metering issues will affect the VE and the adjusted VE measurements since the MAF signal is an input for the calculation as well. Both values will be lower than normal. In addition, much like their naturally aspirated counterparts, fuel trim numbers will be positive to compensate for the unmeasured air that is entering the engine.

No boost

The next example (Figure 4) is taken from a 2011 Mini Cooper Convertible with the same 1.6 liter turbocharged engine. The test drive is performed and it is confirmed that the vehicle has very low power. It should be obvious that the engine definitely has lower than normal load values. However, our VE calculation yields a value of 95 percent and the adjusted VE is calculated to be 90 percent.

VE and adjusted VE numbers that are very close together, and in the good range of 90 percent to 100 percent, are a good indicator that the engine can breathe to its full capacity but it just is not being boosted. Remember, if a VE and adjusted VE calculation both have BARO/Boost pressure inputs that are nearly the same both calculations will be as well. Additionally, even though the air flowing through the engine is not the desired amount, it is still being metered correctly so the resulting fuel trim numbers will be normal.

Restricted exhaust

Exhaust restrictions can have unusual effects on turbocharged engines. An exhaust restriction effects the engines ability to exhale as well as slowing the turbocharger’s turbine due to the reduced exhaust flow. Basically, an exhaust restriction will cause both the raw VE number and adjusted VE number to be low due to the fact that the engine cannot breathe correctly. However, since the reduced airflow is still being measured correctly by the MAF sensor, the engine will still be fueled correctly and the fuel trim numbers will remain near normal.

Diagnostic application

All of the information that we have covered is useless unless we can apply it to our diagnostics. To view what has been covered so far, let us look at all of the data in chart form. The chart (Figure 5) allows us to visualize the relationships, and more importantly the differences, between the four faults that have been discussed.

Using only three pieces of data, all of the faults exhibit different results. For example, a wide-open throttle test drive that yields a low VE value, a low adjusted VE value and positive fuel trim numbers would indicate high potential for an air metering issue. These results do not match any of the other faults on the chart. The next logical diagnostic step would be to inspect the air ducting between the MAF sensor and the turbocharger air inlet as well as testing the MAF sensor itself. If the same test drive were to yield different results, the chart would be consulted and the appropriate next step would be performed.

Parting thoughts

Only four faults were covered here and we technicians know there are more possibilities: restricted intakes, PCV issues and more. We also know that there are always vehicles that are the exception to the rule. If the chart works the majority of the time then it has served its purpose. If it does not find the fault with a vehicle quickly, then the worst thing we have to do is perform more testing to isolate the issue.

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