Designing quiet off-highway equipment

  • 30-Jan-2012 02:30 EST
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The “art” created from this noise runup plot taken at SwRI is actually a plot of noise as an engine starts at low speed and full load, accelerates to maximum speed, engine load drops to zero, and ramps back down to low speed (x-axis is time). Log frequency can be seen on the y-axis, straight lines indicate resonant responses of engine components, and those that shift with time indicate engine-speed-dependent noise, such as engine firing order at low frequency, gear meshing frequencies, or turbocharger blade-pass whistle.

Off-highway vehicles can generate a lot of noise. Many improvements are being made to engines, structures, and systems related to NVH.

Whether in smaller vehicles working in urban areas and/or enclosed spaces or in larger vehicles, quieter systems will eliminate the need for hearing protection and cut down on fatigue caused by long exposure to high noise or vibration levels.

While palliative treatments have been used for some time and are very familiar, many companies address noise problems during the design stage. Thomas Reinhart, an Institute Engineer in the Engine Design & Development Department at Southwest Research Institute (SwRI), discussed with SOHE noise improvement projects done as part of contract R&D work for customers trying to make engines quieter for trucks and construction equipment applications.

In the past several years, engines in off-highway vehicles have evolved to be fully electronically controlled. Some electronic fuel systems such as high-pressure common rail greatly reduce noise levels and change the sound quality compared to engines with mechanical controls in addition to significantly impacting engine performance and emissions.

One example is a feature called pilot injection that injects a small amount of fuel early before the rest of the fuel is sprayed into the already-burning mixture. Reinhart explains that it produces “a much quieter combustion than if you put all the fuel in at once.”

But there are trade-offs.

“Here is a feature that allows you to reduce noise, but you can’t use it 100% of the time. It must be switched off under some operating conditions for emissions reasons,” said Reinhart. “Developing a careful schedule of when and how to turn on and off pilot injection is important.

“To minimize the step change in noise when pilot is turned on or off, the injection timing and injection pressure parameters must be carefully selected. Fuel system control schedules are developed over the entire speed and load range. Changes made to improve engine noise must be carefully balanced with performance and emissions requirements.”

Passenger-car diesel engines not only have to meet high customer expectations of good NVH characteristics but also very stringent emissions standards, so many engines use more than one combustion mode. One emissions control system using a lean NOx trap occasionally runs rich for a few seconds to regenerate the trap. Rich operation is very different from the traditional lean operation of a diesel engine, resulting in a substantial change in engine noise.

Reinhart explained that “if drivers don’t do anything to change the operation of the engine, but the noise suddenly changes, they will assume that something is broken.” As a result, development engineers need to make combustion mode changes, such as a rich pulse event or turning pilot on or off, as transparent to the operator as possible.

In some cases, it is not possible to make the louder operating mode as quiet as the quieter operating mode. In these situations, the solution is to make the quieter mode louder.

“The passenger-car diesel folks have dealt with this challenge for a few years now, but it is coming to the off-highway world in the next few years. Emissions controls and regulations will force you to have more than one combustion mode,” he said.

Some NVH changes are adopted from the small vehicle (automotive) market. But Reinhart explains that how, when, and whether OEMs incorporate NVH improvements into off-highway vehicles depends on many factors.

Where the vehicles will be marketed makes a difference. If an OEM intends to sell a machine in geographical regions with different noise requirements, it must weigh cost effectiveness and other considerations and decide whether to produce one or two versions, for the locations with stricter requirements.

There are two main cost factors in NVH: the cost of implementing the NVH solutions in production and the cost of engineering time to develop the solutions. The size of an OEM, experience level in analyzing and solving NVH problems, and volume of machines expected to be produced help a company determine its engineering budget. If a company builds five machines a year, it can’t spend a lot on engineering capability. Most smaller OEMs take the attitude that “if a problem arises, we will worry about it then.”

“Large companies tend to base designs on past experience and can determine a list of design criteria for developing a quiet [product],” said Reinhart. “Large companies that have engineering capabilities with NVH tend to design-in low-noise features from the beginning, and smaller companies who may not have as much engineering capability tend to fix problems as they come up, which can be expensive. But so is engineering experience.”

Not all companies have the resources to determine the return on investment for noise-reduction designs. But for those that do, the new materials, composite structures, electronics, and design modeling are making advances in NVH noise reduction better every year.

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