At the beginning of a new decade, the same priorities seem to be at the top of the agenda for vehicle manufacturers across the board: reduce emissions, improve fuel economy, and improve refinement. In the off-highway industry, great strides have been made in the past 10 years, but manufacturers are still being asked to go one stage further in all three areas and finding themselves facing trade-offs.
The upcoming U.S. EPA Tier 4 and EU stage IIIB emissions regulations are the most paramount hurdle to clear with test engineers and engine and vehicle designers looking at different solutions to achieve the upcoming, and also subsequent, standards. However, with changing emissions legislation, there are knock-on effects to current levels of NVH. Whether it be the noise generated by extra components on a selective catalytic reduction (SCR)-enabled powertrain or vibrations caused by higher cylinder pressures, a number of NVH challenges present themselves as a result of manufacturers being greener.
“The main aim for manufacturers is to reduce NOx as far as possible, and our job is to quantify the risks involved and make sure there are some developments to mitigate those risks,” said Gareth Strong, NVH Manager at Ricardo. “One of the impressions is that there doesn’t seem to be much of a push for weight reduction in off-highway vehicles and systems; more important are developments in SCR and EGR [exhaust gas recirculation].”
While there are noise problems relating to the extra components needed for the aftertreatment involved in SCR, things don’t get much easier when dealing with EGR.
“EGR is probably a bit easier to work with from an NVH point of view because there is less aftertreatment on the system, but if you are switching from a standard EGR to a high EGR to get the emissions down, you get differences between operating modes, which means different combustion noises,” said Strong. “If operators are using a piece of equipment that doesn’t sound how it normally does because of the different emissions regime, they may think there is something wrong with it.”
FEV is another testing operation whose NVH work is currently being influenced by emissions standards. “NVH and emissions requirements are often in conflict,” said Tom D’Anna, Supervisor, NVH and Driveline Systems. “Calibration changes required for emissions reduction often drive combustion noise levels higher.”
To combat this issue, FEV has created tools such as the in-house developed combustion sound level (CSL) to address the NVH/emissions balance. “CSL is a powerful tool that can be used to break down and address the combustion noise shares [mechanical, flow, and direct/indirect combustion],” said D’Anna. “It can be used in the test-cell environment to help refine the NVH characteristics of the engine before it is installed in the vehicle and also to tune the calibration to meet emissions requirements while understanding the NVH trade-offs.”
Another piece of legislation that FEV has to adhere to in its test and development work on NVH is related to pass-by and the 2000/14/EC directive from the European Parliament.
“This exterior noise procedure requires a large diameter hemispherical sound power array to be set up on an exterior noise pad that FEV has available,” explains D’Anna. “Once a baseline has been established on the pad, an array of nearfield microphones is used inside to more quickly assess various design iterations and changes without being constrained by the weather.”
Use of such an interior “sound pad” requires detailed frequency-domain-based correlation factors to be certain that improvements measured in the interior development pad can be realized during a certified test outside. Once correlation has been established, FEV uses its tools to address typical dominant sources, such as cooling fan and engine noise, to meet the required noise targets.