Perfecting the tricky in-and-out balance of valvetrain performance
Ask any automotive engineer which sub-system in the combustion engine makes the most difference today concerning performance and they most likely will answer: the valvetrain. Valvetrains are the gatekeepers regulating the flow of the fuel and air to the engine. Since they control the ingredients, so to speak, valvetrains are one of the key areas where engineers can tweak the design to shave off those critical emission and fuel efficiency points to meet tougher global regulations.
At the Renault Technocentre in Lardy, a little outside of Paris, testing engineers take valvetrain performance seriously. Very seriously. Which they should since the valvetrain, a complex sub-system of camshafts, cams, and valves, controls the critical air-fuel mix in the cylinder. The amount of air that passes through the valves and how far the valves can open determine vehicle performance. Too much air can cause problems in an engine – everything from unwanted noise to potential damage – and not enough will decrease engine power and ultimately the performance. Valve timing is extremely critical: the faster the valve opens and closes, the better the engine performs. This affects fuel efficiency and emissions as well – both hot topics in the automotive world at the moment. Especially with all the current focus on downsized engines, it is logical that the valvetrain along with the other subsystems are optimally designed to make a difference.
In the valvetrain development process, a variety of tests are required to design, develop and validate new concepts or modifications to existing valvetrain combinations. These tests can include fatigue tests, functional tests or misfire tests. One of the more important tests is the dynamic performance test, where valvetrain kinematics and dynamics are tested at operational speeds to determine, amongst other things, the fatigue life of the valvetrain.
So how exactly does one test the longevity of a valvetrain? A rather time-consuming and complicated testing process, the valvetrain is isolated from the engine and run on an electrical motor to test its performance and see how long it can run. During this test, huge amounts of data are acquired and processed during the various runs.
Managing this immense quantity of data is a challenge in itself. To do this, Renault uses LMS Test.Lab and the LMS SCADAS Mobile hardware acquisition system. During the tests, the LMS system can even check the operational behavior of the running valvetrain on the test bench to make sure that it continues to operate ideally without deformation or breakage.
Ready-to-go test campaigns
Simplicity of the testing routines and ease-of-use of LMS SCADAS acquisition systems means that most valvetrain measurement campaigns can be executed by an operator. For more complex testing, the engineering team at Renault can easily modify parameters, creating new routines in LMS Test.Lab based on Visual Basic scripts. Even if these new routines use non-standard functions, they can still be easily integrated in the Renault test process. And since LMS Test.Lab runs on Windows, there is additional simplicity in terms of reporting. The team at Renault saves additional time creating reports and exchanging data in a straightforward Microsoft Excel file.
Another reason LMS Test.Lab is so attractive for valvetrain testing is that it offers a high level of measurement capabilities, such as high sample rates. It is no problem to take a large number of measurements on the running valvetrain, like valve speed and applied forces on the various components like the rocker arm or the valve spring. The influence of various parameters, like oil pressure, can be experimentally verified as well. Multiple metrics are calculated based on measured valve motion. At the Renault testing center, engineers even measure the exact valve displacement as well as its velocity with a laser-mirror system rather than classic sensors to get the highly precise results they need.
It’s all in the timing
The ultimate goal in valvetrain testing is that the valves open and close perfectly. And the task of tracking down those tricky anomalies and physical phenomena – those little unexpected malfunctions – fall on the test engineers. This is why measuring is so critical. Measurements are the only way to analyze each and every event in the valve motion cycle, from the valve opening duration and valve-lift and valve bounce to the valve opening and closing angle, pre-lift loss, valve seating velocity and contact forces.
One rather too familiar example of a tricky anomaly would be valve bounce: a phenomenon where the valve does not remain properly closed due to inertia and resonance effects from the metallic valve springs. To understand the impact of this phenomenon, engineers look for this event in the valve cycle. The metrics are calculated using angle domain data to separate the different cycles, independent of operational speed. A high pulse/rev encoder is mounted on the camshaft, allowing accurate time-to-angle conversion. These metrics are evaluated over the complete operating range, from low to high rpm. At higher rpm/s, the valve will bounce after closing as valve seating velocity will increase, also resulting in increased valve impact noise. Accurate measurements help analyze and reduce the potential valve bounce.
Full measurement flexibility
Besides specific angle domain analysis, engineers at Renault still perform numerous traditional NVH analyses on the valvetrain. Signature testing or modal analysis is part of the routine valvetrain development process. Engineers use different systems for data acquisition and analysis: one to analyze specific phenomenon in angle domain, such as valve bounce and one to analyze the frequency domain for signature and modal testing.
“What is so good about our new LMS Test.Lab system is that with only one measurement, we can analyze the data in the angle, frequency and time domain. Previously we could not do this. This saves us a lot of time because is it is no longer necessary to perform the measurement more than once. Once the test is done and the data is available, we run the different analyses. We have all the information that we need to limit valve bounce,” commented Mr. Stéphane Lusson, test manager for valvetrain control at the Renault Technocentre.
Using the same data for further analysis does not only represent huge time savings, it is also guarantees that the exact same data, measured in the exact same condition, is used for different analysis. Variations due to differences in the data acquisition process belong to the past.
“Another important benefit is that the testing resolution significantly increased. This is extremely detailed test work and we need to know what happens to the valve right before it closes. This wasn’t a problem for LMS Test.Lab, which can handle higher sample rates. We have been able to achieve a much higher level of precision, which gave us useful data to determine the ideal valve displacement profile and prevent troublesome valve bounce,” concluded Mr. Lusson.