Defining a Measurement Circuit for Oil Pumps

"The automotive industry will see more change in the next 5-10 years than it has in the last 50," said GM CEO Mary Barra during her 2016 CES keynote. You're probably wondering, "That's a bold statement, but how is it relevant to me?" If you are involved in the design and testing of any powertrain component, it is especially relevant. The disruptive trends of electrification, car-sharing and autonomous vehicles collectively have a significant impact on the application, conditions, duty cycle and time-to-market for individual components. This is the first in a series of posts discussing the nuances of defining the right measurement circuit for testing current and future components, beginning with the oil pump.

The aforementioned trends have led to more sophisticated engine and transmission pumps. Mechanical oil pumps have widened operating envelopes with variable displacement configurations. Electric oil pumps are now being used in hydraulic actuation and cooling applications, beyond the typical lubrication purpose. The oil pump's wide range of use can make defining the ideal measurement circuit particularly difficult.

For starters, what pump type are you looking to test? Is it a Pump used in an engine or transmission?

This will largely influence your answers to the following questions.

What kind of test fluid are you planning on using?

There are a couple of reasons that this is important to know:

  1. You may want to be specific with the type of test fluid you use in order to accurately replicate the pump’s performance.

  2. The same grade of oil (i.e. 5W30) made by two different brands will have different types of additives, so they may behave differently depending on the operating conditions.

  3. Also, synthetic oil and conventional oil is an important question to answer. Although the two share most oil grades, they can have drastically different physical properties under extended operating conditions, and consequently act differently within an engine or transmission.

  4. What is the flashpoint? This will determine maximum test temperature, and the selection of heating hardware to avoid burning of the oil.

  5. What is the pour point? Determining this will indicate the minimum test temperature, and corresponding viscosity.

What is the temperature range that you are looking for?

This will get you the minimum and maximum density and viscosity. This range should match the temperature range that's obtained based on the oil grade.

What is the application?

  1. Performance

  2. Durability

  3. Production (i.e. end-of-line inspection)

This will contribute to determining the appropriate test profiles.

Is it a fixed displacement or variable displacement? And from there what test profiles are you looking for?

There is a long list of possible tests (ie. the SAE J2311 standard) to characterize or prove the durability of an oil pump, including:

  1. Waterfall test

  2. Sweep test

  3. High speed flow limit (High speed fill limit or HSFL Test)

  4. Durability test

  5. Pressure ripple test

  6. Cold start test

  7. Critical inlet test

  8. Auto prime test (Quick start test)

  9. Performance Test (Torque deterrent)

  10. Thermal shock test

  11. Aeration test

What is the rated pressure?

This determines what the measurement circuit needs to be rated for. This usually falls within one of the three following ranges:

  1. 34bar (maximum)

  2. 68bar (maximum)

  3. 103bar (maximum)

If your operating pressure requirement is less than the circuit’s written cap, then it is compatible.

For example, the 34bar measurement circuit would fit the requirements of most engine pumps because the operating pressure typically is approximately 10bar. 68bar would cover most of the conventional transmission pumps while 103bar is pushing the limit higher with newer hydraulic pump designs. 

What is the typical operating pressure?

Understanding pressure drop is vital to defining a circuit that will be suitable to test a particular pump. A pressure drop can be caused by a loss in total energy of a fluid in motion, usually the result of frictional losses in conduits, but often includes component losses (orifices, valves, etc.) and energy loss from work exerted on the system.

For the purposes of sizing your circuit, you’ll need to know these two operating points:

  1. Highest pressure drop at lowest flow rate.

  2. Lowest pressure drop at highest flow rate.

The maximum load and minimum load of the conditions above translate to the operating points of the hydraulic measurement system. 

A critical part of the hydraulic measurement system is the pressure control hardware, which simulates the varying resistances on the outlet of the pump that is being tested. Make sure that you select the right hardware for outlet pressure control. The different technologies will each have their own inherent stabilities in either open-loop mode (i.e. fixed position), or closed-loop mode (i.e. using feedback from a pressure sensor). The pressure control hardware must also match the rangeability of the system (ratio of maximum to minimum controllable conditions), as different technologies each have their own limitations.

When in doubt...

Defining the right test system for your pump can be confusing. The good news is that most oil pump perform within known testing envelopes. We've spent a lot of time identifying these envelopes and developing circuits for them. You may find a suitable circuit for your mechanical or electric oil pump application and save a lot of time.

I have worked together with our seasoned team here at ATA to produce this content and we feel that this should get you started on the right foot. Although, if clarification is required then please do feel free to contact us!