The purpose of flow measurement is to measure the flow of fluids around the engine. The most common application is for measuring coolant andlor oil flows within the engine.
Coolant flow is a requirement if a thermal balance test series is to be undertaken.
Several methods and devices are used in the measurement of flow, such as the venturi gas meter (Figure 3.16) and the flow turbine (Figure 3.17). The venturi meter measures flow rate in terms of pressure drop across a venturi (or tapered throat) within a pipe. The turbine is fitted directly into a pipe to measure the flow rate of the fluid. The operating principle is that the flow impinges on the turbine blades, causing them to rotate. The rate of rotation is measured either mechanically or electrically. In the latter case, it works by giving off an electrical output directly related to the speed of the turbine, which, in turn, is directly related to the rate offlow. The electrical output then is measured using a frequency counter that can easily be converted into a flow figure (i.e., liters per minute).
The manufacturer performs the calibration.
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Instrumentation: Temperature, Pressure, Flow, and Calibration 5 3
Figure 3.1 6 Venturi gas metel: (Courtesy of Uni- versity of Sussex)
1,: .2'.
Figure 3.1 7 Turbine flow meter. (Courtesy of Uni- versity of Sussex)
Mass Airflow Sensors
This type of sensor (Figure 3.18) is used in the Bosch K type Jetronic fuel injection system and the RDA mass airflow system (CMC-Scotch Yoke three-cylinder applica- tion). It consists of an air funnel and a pivoting airflow sensor plate. A counterweight compensates for the weight of the sensor plate and the pivot assembly.
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54 An Introduction to Engine Testing and Development
Rotameters
A basic rotameter consists of a small float supported in a tapered glass tube by a flow of liquid or gas. In engine flow measurement, blow-by gases are directed through the calibrated tapered tube. Inside the tube, a calibrated pintle is held in suspension by the blow-by gases. The distance up the tube is proportional to the gas flow, thereby indicat- ing the rate of flow. Figure 3.19 shows an example of a rotameter.
Figure 3.19 Typical glass-walled rotamete,:
(Courtesy of University of Sussex)
Square Edged Orifice Plates
This is the preferred method of airflow measurement. The blow-by gases are directed into a parallel round tube. Positioned within the tube is a disc with a regular hole in it.
Pressure measurements are taken on either side of the orifice. By measuring the pres- sure difference across the orifice, it is possible to calculate the gas flow. (See British Standard 1024 and Figure 3.20.)
Figure 3.20 British Standard square edged
orijke plate. I
With reference to Figure 3.21, D (internal diameter of pipe) and 0.5D tappings measure the pressure difference between one pipe diameter upstream and a half pipe diameter downstream from the orifice plate. The presence of the orifice within a pipeline will cause a static pressure difference between the upstream side and the downstream side of the device.
The installation and use of orifice plates are documented in IS0 5 167- 1 : 199 1 (E) (also BS 1042: Section 1.1 : 1992). The information given here has been sourced from this document and refers only to D and 0.5D pressure tappings.
Instrumentation: Temperature, Pressure, Flow, and Calibration 5 5
D 0.5 D 5 D (Minimum distance for throttle)
4
X A B Y I
Boundary turbulence
Vena contracta NOTES: D
-
Diameter of flow stream (Point of minimum areaorlfice plate of flow stream) d
-
Diameter of square edged orificeV l
-
Velocity of flow before orifice V2-
Velocity of Row at Vena contracta P i-
Flow pressure before orifice P2-
Flow pressure at Vena contra- S-
Fluid densityQ
-
Fluld flow rateA B
-
Differential measurement (I) X Y-
Differential measurement (2)Figure 3.21 Standard airJlow installed or@e.
The mass flow rate through an orifice plate installation can be determined from the following equation:
where
C = coefficient of discharge
p
= diameter ratio = d/D d = diameter of orifice (d)D = upstream internal pipe diameter (d)
= expansibility factor Ap = differential pressure (Pa) p
,
= density of the fluid (kg/m3) qm = mass flow rate (kgls)The density can be evaluated from conditions at the upstream pressure tapping (pl= PI /RT). The temperature should be measured downstream of the orifice plate at a distance of between 5D and 15D. The temperature of fluid upstream and downstream of the orifice plate is assumed constant (Figure 3.22).
5 6 An Introduction to Enaine Testina and Develo~ment
Figure 3.22 Base airjow measurement system, 45-gallon fuel drum.
Figure 3.23 A Lucas- Dawe air massJlow meter:
British Standards orifice plate
'R
Airbox volume to be a minimum of 10 times the capacity of the engine in Air inlet order to damp cyclic pulsationsAtmospheric pressure P1
I
I engine
Lucas-Da we Air Mass Flow Meters
This flow meter originally was intended for use with engine management systems.
(These were superseded by the whetstone bridge hot wire systems.) However, this meter is better suited to laboratory use. The principle, as illustrated in Figure 3.23, shows the central electrode that is maintained at approximately 10 kV so that a corona discharge is formed. The exact voltage is varied so that the sum of the currents flowing to the two collector electrodes is constant. When air flows through the duct, the ion flow is deflected, thereby causing an imbalance in the current flowing to the two collector electrodes. The difference in current flow is proportional to the mass airflow rate.
Insulating supports
Collector electrodes