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Powertrain NVH

Engine noise (ICE) and transmission noise concerns the NVH experience of the vehicle propulsion system and the noise level and the sound quality are both very important. The main tests include sound pressure level and order content from the powertrain at different load conditions like drive away, steady state, acceleration part load and maximum load (WOT). The noise from all sources, engine, air induction, exhaust system, tail pipe noise, gear box, diesel knocking and accessories like cooling systems and fuel pump is transmitted both airborne and structure borne into the vehicle.

Typical challenges R&D deals with when working on Powertrain NVH
• Concept selection for engine installation and engine content, layout and isolation.
• Exhaust system installation and optimization.
• Transmission design.
• Auxiliary components concepts.
• Predictions of noise radiation.
• Compromise between NVH and other important attributes like performance, fuel consumption, emission.

Tomorrows challenges within Powertrain NVH
Accurate NVH modelling methods and early tests in engine test cells to predict vehicle noise and sound quality is extremely important. Some test data used in NVH driving simulators may also need to be binaural recordings.
Environmental friendly small engines with low noise levels for steady state driving but powerful temporary sound at acceleration and also new propulsion concepts belongs to the future.

Microphones used for Powertrain testing shall be robust and if they are used close to the engine also able to handle heat, dust and humidity. A small size is important for easy positioning in the engine compartment.

Free-field or random microphones are most often used and the choice depends on test procedures.

Engine noise

Engine NVH testing is a clearly defined process during the powertrain development phase. The powertrain as a system is first tested in NVH engine test cells for verification of system requirements and fine tuning of noise reducing measures. The vehicle integration is later verified with the complete car driven on a NVH Chassis dyno or on road.

Interior and exterior noise are both very important. The interior noise should be minimized during driving at constant speed but the powertrain shall produce a well-tuned sound during acceleration to provide useful and pleasing feedback to the driver. A brand identity is often desirable, for example a sporty sound.

The exterior noise is regulated by legislation to minimize noise disturbance in urban areas. Some of the most important test procedures and regulations i are ISO 362, Part 1: M and N categories, Part 2: L category and Part 3: Indoor testing M and N categories, SAE J1470, UNECE R51.03, regulation EU 54/2014.

These documents defines the needed facilities and test equipment as well as driving conditions and the corresponding maximum allowed noise emission.

A lot of measurements has to be taken to analyse sound pressure levels, engine order content and sound quality at different load conditions like drive away, steady state, acceleration at part load and wide open throttle etcetera. The balance between structure borne and airborne sound needs to be understood and measurements are of course also used for CAE correlations.

Challenges:

A lot of concept selections are critical for a successful engine sound; Engine installation, engine, mounts tuning, exhaust system optimization, isolation package tuning, and engine calibration. The final result shall be a perfect balance between NVH and other important attributes like performance, fuel consumption and emissions.

• Testing time has to be short since the access to prototypes is limited.
• Instrumentation must be fast and easy in a way to record repeatable results. 
• The microphones must be installed to minimize structure borne sound. 
• Microphone holders and cables shall not introduce any rattle noise. 
• The installation shall be safe for the test engineer during vehicle test.
• Calibration verification must be easy to perform.

The transducers are often moved to different positions so they must withstand both this and also occasionally to be dropped to the floor. Powertrain testing includes harsh environmental conditions as heat, dust and humidity and the microphones must also handle this.

Example:
A vehicle test on a NVH Chassis dyno includes typically 2-4 microphones inside the vehicle distributed between driver and passenger seats and if exhaust noise is also measured one or several microphones close to the tail pipe. The engine rpm is used to track the engine orders and can be registered with an optical or inductive sensor or from the CAN bus depending on the needed analysis resolution. The CAN bus also provides powertrain performance data to further increase the knowledge.

Transmission noise
Computer Model validation