Engine oil flow measurement: which flow meter?
One of the most difficult flow measurements for the automotive engineer is measuring engine oil flow under operating conditions. With the motor mounted on a dyno and coupled to a dynamometer, the job is somewhat easier due to the increased space available, but careful selection and installation of the flow measurement system is nonetheless required. Flow measurements required in a vehicle’s engine compartment are further restricted due to the power requirements of the instrument, the required installation space, and the harsh conditions under the hood.
What are the flowmeter options? Well, industry standards seem to exist to some extent, but this does not mean that they are correct. Let’s look at some options.
(1) The positive displacement meter, can be a gear meter, piston meter, or worm screw, to name a few. These meters are generally higher in cost and can cause problems when installed due to their relatively high pressure drop. Although they are quite good at operating at wide viscosities, they do suffer from meter slippage at low viscosities. Meters vary in size, but can often be installed without the need for flow conditioning. Used by some engine test houses, but not an ideal solution.
(2) Turbine flow meter, generally one of the most widely used flow meters for measuring engine oil flow, mainly due to its low cost and small size. They can be purchased relatively easily from a large number of manufacturers and have a manageable pressure drop that isn’t too detrimental to oil system performance. They are sensitive to the effects of oil and fluid flow profile, so it is extremely important to calibrate the flow meter in the variable oil viscosity range and with its upstream and downstream flow conditioning tubes installed. To ensure accurate results when in use, it is essential to match the installation conditions as closely as possible. Very often specialized flow calibration laboratories will calibrate the flow meter with the same motor oil, at the same operating temperature and with the same inlet and outlet connections as the installed motor. Sometimes this can be part of the engine assembly, such as an oil cooler or oil filter assembly, usually the easiest part of the engine where oil flow can be interrupted. Many flow meter manufacturers only provide a certificate of calibration in water, which can lead to extremely large errors if used on motor oils.
(3) Coriolis meters. Very good high precision meters, which would only be suitable for fixed installations. These meters are still very expensive and large, compared to turbine and positive displacement meters. These gauges have progressed well over the past 10 years, but can still be prone to poor installation, vibration, and zero drift measurement errors. Ideal for test bench use where permanently installed, and with straight tube models now becoming the norm, they offer low pressure drop and excellent viscosity range.
(4) Characterization of the pressure drop. An increasingly common method of obtaining accurate measurements without affecting oil system performance due to the insertion of flow meters. Individual parts of the engine oil flow system are instrumented for pressure tap locations and each engine part has flow characterized on a calibration bench for oil flow against pressure drop at required temperatures. Once each system has been characterized, the engine is reassembled and tested, and the recorded pressure measurements are used to derive flow. This method is typically used in critical engine lubrication studies, where the insertion of a flow meter would have a detrimental effect on the oil flow system.
(5) Ultrasonic clamp on meter. Although it is a non-intrusive device, due to its straight pipe installation requirements, it means that the system becomes intrusive to the oil metering system. Still very expensive to buy and currently not sufficiently proven in these applications to become a proper competitor, although it is progressing faster than most other techniques.
Having decided on your method of flow measurement and carried out the required level of flowmeter calibration across the full range of operating temperatures, you are close to taking a critical step towards obtaining good flow measurement results. Engineers relatively new to engine lubrication studies should seek advice from the Flow Calibration Laboratory, who should have adequate experience in this type of flow measurement application and be in a good position to find the best solution for their application. flow meter hardware budget and inventory.
Other issues to consider. The complex nature of engine oil flow measurement means that it is very difficult to obtain accurate oil flow readings from a running engine, but the informed engineer can make sound engineering judgments about his or her measurement process. Consider the influences of oil viscosity, oil temperature, the effect of meter installation on the performance of the flow meter and oil pump, oil aeration, relief valve effects, and the hysteresis, the effects of fuel dilution, engine oil levels, etc. The next series of discussions will delve into the implications of these parameters in more detail.