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 test stand and coupled to a dynamometer, the job is a bit easier due to more 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 instrument power requirements, required installation space, and harsh underhood conditions.
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 gauge can be a gear gauge, a piston gauge, or a worm gear, to name a few. These meters are generally on the higher cost side 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 motor test houses, but not an ideal solution.
(2) Turbine flow meter, usually one of the most used flow meters to measure engine oil flow, mainly due to its low cost and small size. They can be purchased with relative ease of a large number of manufacturers and have a manageable pressure drop that is not too harmful to oil system performance. They are sensitive to fluid and oil flow profile effects, so it is extremely important to calibrate the flowmeter over the variable range of oil viscosity and with its upstream and downstream flow conditioning tubes installed. To ensure accurate results when used, it is essential to match the installed conditions as closely as possible. Very often specialized flow calibration laboratories will calibrate the flow meter with the same engine oil, at the same operating temperature and with the same inlet and outlet connections depending on the installed engine. Sometimes, this can be part of the motor assembly, such as an oil cooler or an oil filter set, usually the easiest part of the engine where the oil flow can be interrupted. Many flow meter manufacturers only provide a calibration certificate in water, which can lead to extremely large errors if used in motor oils.
(3) Coriolis meters. Very good high accuracy meters, which would only be suitable for fixed installations. These meters are still highly expensive and are large compared to turbine and positive displacement meters. These meters have progressed well during the last 10 years, but they can still be prone to poor installation, vibrations and zero drift measurement errors. Ideal for use on test benches where they are permanently installed, and with straight tube models now becoming the norm, they offer low pressure drop and excellent viscosity range.
(4) Characterization of pressure drop. An increasingly common method of obtaining accurate measurements without affecting oil system performance is due to the insertion of flow meters. The individual parts of the engine oil flow system are instrumented for pressure intake locations and each motor part has a flow characterized in a calibration bank for oil flow against the pressure drop at the required temperatures. Once each system has been characterized, the motor is reassembled and tested, and the recorded pressure measurements are used to derive the flow rate. 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 in the oil measurement system. Still very expensive to buy and currently not sufficiently proven in these applications to become an adequate competitor, although it progresses faster than most other techniques.
HAVING DECIDED ON YOUR FLOW MEASUREMENT METHOD and UNKNOWN the required level of flow meter calibration over the entire operating temperature range, you are close to taking a critical step forward in obtaining good flow measurement results. Engineers relatively new to engine lubrication studies should seek advice from the Flow Calibration Laboratory, which 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. Budget and inventory of flow meter hardware.
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 measurement process. Consider the influences of oil viscosity, oil temperature, the effect of meter installation on oil pump and flow meter performance, oil aeration, relief valve effects, and oil pressure. 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.