Accurate Measurement and Analysis of Pulsating Flow in Diesel Engines
Overview: The normal operation of a diesel engine results in pulsating flow conditions which can impair the accuracy and repeatability of the flow metering system (venturi meter and DP sensor). A number of steps can be taken to lower the pulsation effect and/or eliminate them from the processing of the signal from the venturi meter. The following information is provided based on actual testing the PFS has done as well as a review of pertinent industry papers on the subject.
Pulsation Science: The term “pulsation” is generally used to indicate the presence of sound waves traveling through a gas. These waves are generally referred to as acoustic waves although they are at a pitch too low for the human ear to detect.
Acoustic waves consist of waves with alternating high and low pressure regions. When present, the acoustic wave is superimposed over the static pressure in the system. The pressure difference as measured from the highest pressure in the wave to the lowest is defined as the peak to peak pulsation amplitude.
Acoustic waves move through the gas at a speed which is determined by the compressibility factor of the gas which in turn is affected by, among other things, the temperature, pressure and specific gas composition.
It should be noted that the acoustic waves do not transport gas from one location to another – the gas vibrates back and forth for a short distance as each wave moves past.
Because the actual or true gas flow moves at a much slower speed, the gas has little effect on the waves.
Diesel Engine Pulsating Flow
Engine developed pulsations are generally the result of two conditions:
- Cylinder movement/compression and decompression.
- Flow restrictions or obstructions that may cause the flow pattern on the upstream side of the venturi meter to become unstable which then results in pressure fluctuations which result in acoustic wave formation.
When pulsations or pressure waves encounter a change in pipe cross section, such as is the case with a venturi meter or orifice meter, the wave may be partially or totally reflected. This can result in additional or secondary wave formation and is often called “noise”. Pulsation noise can be particularly high when the frequency is at an acoustic resonant frequency that is the same as the piping and/or the pressure transducer – the result is an increased pressure pulsation amplitude.
Pulsating Flow Error Sources
There are at least four basic error sources to consider:
- Errors developed at the point of pressure sensation within the meter caused by acoustic waves.
- Errors developed in the pressure tubing and/or sensor caused by vibration or the result of acoustic resonant frequency with other parts of the system.
- Instability of the jet or flow pattern effect on the venturi meter signal as the distorted flow profile moves through the meter.
- Square root averaging errors as follows:
- Generally, flow through a venturi meter is considered as an “average flow rate or Q” where Q x Time – equals – Total Flow.
- The instantaneous flow rate is proportional to the square root of the pressure differential.
- Therefore, the average flow rate is proportional to the average of the square roots of the differential pressure and not to the square root of the average of the differences.
While the effect of the above on steady state flow conditions is small, they can result in considerable errors on pulsating flow conditions..
Generally speaking, under pulsating conditions, the differential reading from the venturi meter is in error to the plus side. Also, it is generally agreed that SRE (square root error) decreases with higher differential because the SRE is proportional to the square of the ratio of the peak to peak differential divided by the static, non-pulsating differential.
Gauge line errors (impulse lines) can develop as a result of acoustic effects in the sensing lines. If the acoustic resonant frequency of the gauge line matches a pulsation frequency in the main line, the amplitude can be further amplified. Some success in lowering the effect is to make the impulse lines as short as possible and increase their interior diameter. As well, pulsating conditions in the main line can induce acoustic waves in the gauge lines which raise or lower the observed static pressure at the transducer. In some cases, this can, in turn, create higher or lower differentials at the transducer location than exist at the pressure taps on the venturi meter.
- The impact on the system accuracy must be evaluated to see if the pulsating conditions result in unacceptable measurement errors.
- The DP sensor should be calibrated for the application differentials if at all possible.
- The natural frequency of all components should be determined and evaluated.
- Sensing line size, length and overall geometry should be evaluated.
- Upstream flow pattern effects should be evaluated.
- Dampening of the DP sensor should be considered.
- The engine mass gas flow should be checked against an “in-system” “off-engine” measurement standard which allows for correction and/or compensation by the on-board computer.