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Volumetric efficiency of turbocharged engines

Thursday, March 1, 2018 - 08:00
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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?

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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.

Figure 1 - Scan data captured from a known good turbocharged vehicle
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