Engine Oil Flow Measurement – Which Flowmeter?

One of the most difficult flow measurements for the automotive engineer is to measure the engine oil flow rate under operating conditions. With the engine mounted on a test bench and coupled to a dynamometer, the job is slightly easier due to the increased space availability, but none the less, careful selection and installation of the flow measurement system is required. Flow measurements required in the engine bay of a vehicle become further restricted due to instrument power requirement, installation space required and the harsh under bonnet conditions.

What are the flowmeter options? Well, industry standards seem to exist to some extent, but this does not mean they are correct. Let us look at some options.

(1) The positive displacement meter, this could be in the form of a gear meter, piston meter or helical screw to name a few. These meters are generally on the higher cost side and can cause issues when installed due to their relatively high pressure drop. Although quite good at operating across wide viscosities they do suffer from meter slippage at low viscosities. Meters vary in size, but can often be installed without the need for any flow conditioning. Used by some engine test houses but not an ideal solution.

(2) Turbine flow meter, generally one of the most widely used flowmeters for engine oil flow measurement, mainly due to its low costs and small size. They can be purchased relatively easily from a host of manufacturers and have a manageable pressure drop which is not too detrimental to oil system performance. They are sensitive to oil and fluid flow profile effects, so it is extremely important to calibrate the flow meter across the varying oil viscosity range and with its installed upstream and downstream flow conditioning tubes. To ensure accurate results when in use, it is essential to match the installed conditions as close as possible. Quite often specialist flow calibration laboratories will calibrate the flow meter on the same engine oil, at the same operating temperature and with the same inlet and outlet connections as per the installed engine. Sometimes this may be part of the engine assembly, such as an oil cooler or oil filter assembly, generally the easiest part of the engine where the oil flow can be interrupted. Many flow meter manufacturers only provide a calibration certificate on water, which can lead to extremely large errors if used on engine oils.

(3) Coriolis meters. Very good high accuracy meters, that would only be suited to fixed installations. These meters remain highly expensive and are large in size, compared to the turbine and positive displacement meters. These meters have progressed well over the last 10 years, but can still be prone to poor installation, vibration and zero drift measurement errors. Ideal for test bench use where they are permanently installed, and with straight tube models now becoming a norm, they offer low pressure drop and excellent viscosity rangeability.

(4) Pressure Drop Characterization. An increasingly common method for obtaining accurate measurements without affecting the oil system performance due to insertion of flowmeters. Individual parts of the engine oil flow system are instrumented for pressure tapping locations and each engine part is flow characterized on a calibration bench for oil flow against pressure drop at the required temperatures. Once each system has been characterized the engine is reassembled and tested, and the recorded pressure measurements are then used to derive the flow rate. This method is generally used on critical engine lubrication surveys, where the insertion of a flow meter would have a detrimental effect on the oil flow system.

(5) Ultrasonic Clamp on meter. Although a non intrusive device, due to its straight pipe installation requirements, it does mean the system becomes intrusive into the oil measurement system. Still highly expensive to purchase, and presently not sufficiently tested in these applications to become a suitable contender, although progressing faster than most other techniques.

Having decided on your method of flow measurement, and undertook the required level of flow meter calibration across the range of operating temperatures, you are close to taking a fundamental step forward in acquiring good flow measurement results. Engineers relatively new to engine lubrication studies should seek advice from the Flow Calibration Laboratory, who should be suitably experienced in this type of flow measurement application and be in a good position to find the best solution for your budget and flowmeter hardware inventory.

Other Issues to bear in mind. The complex nature of engine oil flow measurement means it is very difficult to get accurate oil flow readings from a running engine, but the informed engineer can make sound engineering judgements on their measurement process. Keep in mind the influences of oil viscosity, oil temperature, meter installation effect on flowmeter and oil pump performance, oil aeration, relief valve effects and hysteresis, fuel dilution effects, engine oil levels, etc. The next series of discussions will delve deeper into the implications of these parameters in more detail.