1. VWPT EA211 MOBILE ENGINE NVH TEST REPORT
Chengdu, June 2013

2. Rotas Analysis Methods (Overview)
The Rotas noise analysis performs a rotationally synchronous re-sampling of the noise signals. This allows calculation of exact order spectra and derived values.
Time Signal (rotational synchronous)
Order Spectra
Traditional
Spectral analysis
→ RMS, Peak, Crest, Kurtosis
→ spectral values
New: Short time spectral analysis
Recording over speed
Cycle Spectrograms
Order Tracks, Band Sum Tracks
Ramp Spectrograms
→ “speed bands”

3. Cycle Spectrogram
Short-time spectra are computed over the revolution of the engine. Several spectra are averaged and indicate spectral abnormalities at certain angles.
Spectral and timing information is combined
Pulse components of all kinds can be identified with this method
Motor cycle

4. Cycle Spectrogram Bands
Different Bands
are evaluated
Main Band
100 Engines overlaid
with Limit Curve
High Band
Order 260
Main Band
Cycle Spectrogram bands allow to overlay different engines. Limit curves can be self-learned

5. Idle speed noise in vehicles
At idle speed, some engines produce an intermittent knocking noise at idle speed
A vehicle with such an engine and a series vehicle with a good engine have been measured.
With the Cycle spectrogram, the bad engine can be identified both in engine vibration and in vehicle cabin microphone measurements.
The bad engine is 15 dB louder at the nocking frequency.

6. In-vehicle measurements
TAS08 Measurement Frontend
Ignition Signal Cyl 1
Microphone in the cabin
Vibration Sensor
on the engine

7. Bad engine nocking noise in vehicle at idle speed
Jetta NF VW 253CNC4048-0S. Idle speed 720 RPM

8. Good engine in series vehicle
Series Car Jetta CPDG 39440

9. Bad and good engine cylinder head vibration
Bad engine is 15 dB louder
at the nocking noise on cylinder head vibration
Bad Engine
Good Engine

10. Bad and good engine microphone signal in cabin
Bad engine is 15 dB louder
at the nocking noise in microphone signal
Good Engine
Bad Engine

11. Hot test measurements Hot Test Application Time Signals Cycle
Spectrograms
Order Spectra
Speed Instrument

12. Good engines These two engines are both good.
Note the different height of the valve action noise.
Good Engine 739
Good Engine 30614
Good Engine 41371

13. Bad engine rock arm roller
The rollers of rock arms have been damaged by cutting into the roller surface.
This produces a coming and going nock noise as the roller advances and produces good and bad contact points between camshaft and rock arm.
This strange noise can be found by maximizing the Cycle Spectrogram together with no averaging.
For this, a special test step with such parameters can be created.

14. Bad engine rock arm roller
Above a good engine for comparison.
Good Engine 739
In this bad engine all rock arm rollers have been scratched.
The noise is higher but does not change as in the example before.
No clear difference to good engines
Bad Engine 6406

15. Conclusion
The noise problem in the car can be found both in the engine via vibration measurement as in the cabin via Cycle Spectrograms.
The vehicle responds to knocking noise in the order range 25 and 50, corresponding to 300 and 600 Hz resonance of the transfer path.
The hot test shows variability in the high order amplitudes of the cylinder head (100th order and more, corresponding to frequencies > 1.2 kHz).
These high orders are not a large concern in the vehicle as they are blocked by the cabin.
The system can be used both for engine and for cabin noise analysis in a mobile version.