Reference Air Mass Flow Meter for engine test benches

Overview of the reference air mass flow meter

A precise reference air mass flow meter is the most important basis to optimize modern engines on the test bed. At the same time it is unfortunately one of the most challenging sensors to select and apply. Several special requirements need to be taken in account:

• High dynamics
• Big measurement spans
• Massive pressure- and flow pulsations
• Small pressure losses
• High precision

New deltaflowB solves these requirements perfectly well! Apart from the air flow measurement, deltaflowB can even be used for exhaust or blow-by measurements.

deltaflowB is working on the dp principle which offers a number of advantages.

• Insensitivity against fouling and dust
• Huge pressure and temperature range
• High precision

deltaflowB optimizes the dp-technology in many points to fit the special needs of engine test beds.

Primary element

For deltaflowB, we do not use a classical primary element (orifice, venturi or nozzle), but a newly developed combination of a low-pressure-loss-venturi and a wing sensor. This combination reduces the remaining pressure loss to less than 10% of the measured differential pressure. Therefore at the highest flow rate, the pressure drop is still below 20mbar.

Sensors

Commercially available pressure-, differential pressure- and temperature-sensors are by far too slow for the dynamics of engine test beds. In addition to the slow transmission of measured values to the test bench, the main problem is that motor pulsations can’t be detected by standard sensors. The result is errors depending on the pulsation strength and frequency (square root error).

The CAN-based sensors from systec Controls are measuring up to 2000 kb/s and are using highly developed filter strategies to separate pressure and flow pulsations and to calculate precise air mass flows.

Redundand, digital flow calculation

The deltaflowB is equipped with a triple sensor mount. This redundancy has several advantages for the user:

• Plausability check of the 3 x 3 measured values (p, dp, T)
• Automatic maintenance and recalibration suggestions
• High accuracy by averaging

Since the triple measurement takes place at different locations in the venturi, the user is also informed about the flow conditions in the measuring device. Highly disturbed inlet conditions will lead to a deviation between the three sensors.

Flow computing

The sensors are connected via a CAN interface on a master PC, which is doing the signal analysis. The user-friendly software shows the operator both - the individual measured values of the three sensor locations as well as the filtered average values. The flow computing PC and the control terminal are designed as a 19" rack version. The connection to the test stand DCS is usually carried out via the second CAN interface of the PC. Other outputs are optionally available via additional I/O cards.

Advantages od deltaflowB compared to thermal mass flow meters

Thermal mass flow meters have a number of disadvantages compared to the deltaflowB. First of all, as a result of the single-point measurement, the thermal sensors require ideal flow conditions. In order to produce these, long inflow distances and flow straighteners are necessary, which negate the advantage of the low pressure losses of the thermal air mass sensors. The deltaflowB has low requirements on the inlet conditions due to the venturi structure. The deltaflowB is supplied with a straight inflow section of around 10D. This is good enough for all common upstream piping.

Since pulsations in thermal flow meters lead to severe measurement errors, pulsation dampers (volumnia) must be inserted between the motor and the thermal sensor. This reduces their dynamics. With the deltaflowB,  precise measurements are possible even with pulsations. And the measurement is highly dynamic and does not need any
dampers.

Last but not least, thermal flow meters must be recalibrated up to four times a year, depending on the accuracy requirements. This is not the case with a deltaflowB. The intelligent signal processing of the deltaflowB, compares the redundant sensors and automatically recommends the recalibration, typically every 2 years.