CFA/DAGA'04, Strasbourg, 22-25/03/2004

Acoustic Diesel Cold-Start

Dr. Norbert Alt, Jakob Nehl, Dr. Stefan Heuer, Rainer Thiele FEV Motorentechnik GmbH, Aachen

injection respectively, injection pressure, etc.). The engine Introduction structure as a transfer system is also of significant Over the past few years, the noise and vibration behavior of importance. Therefore, a careful acoustic optimization in the diesel passenger has been further improved, thereby engine design process is necessary to specifically avoid continuously diminishing the disadvantages of diesel engines structural weaknesses excited by excitation. compared with engines. However, during cold start The influence of injection parameter variations for a and warm-up at low ambient temperatures, many diesel turbocharged 4- was engines still produce an unpleasantly loud knocking sound, investigated on an engine cold test bench. The test bench resulting from a significantly higher combustion excitation. allows the variation of air, oil and coolant water As an example, Figure 1 shows the cylinder pressure curves temperature as well as boost pressure. Timings and and their first derivation versus angle of a diesel quantities of pilot and main injection, rail pressure as well as engine with common rail injection system and standard exhaust recirculation rate were systematically varied. Aside calibration at cold (approx. 0°C) and warm condition from exhaust gas raw emissions (HC, CO, NOx and (approx. 85°C), respectively. At cold state, a considerably particulates), the cylinder pressure was measured to predict higher cylinder pressure gradient results in louder and more the engine noise level by means of FEV-CSL. unpleasant combustion noise. Toil/water[°C] Tintake air [°C] pint. pressure[barabs]BOI[°CABTDC] 0 20406080100-20 0 20 40 60 1,01,11,21,31,41,5 6420-2-4 60 75 cold engine BOI + 6°CA 50 warm engine 74 pRail -70 bar 40 73 30

20 72 10 71 0 Cylinder pressureCylinder [bar] 90 120 150 180 210 240 270 70 6 2500

4 2000 2 1500 CO [bar/°CA] 0 α

-2 1000 dp/d

-4 500 HC 90 120 150 180 210 240 270 Crank angle [° CA] Raw emissions [ppm]0 FEV-CSL [dB(A)] Figure 1: Cylinder pressure trace and gradient for cold and Figure 2: Engine noise and exhaust gas emissions at cold hot condition. conditions.

Based on internal research at FEV Motorentechnik, it could As documented in Figure 2 the variation of coolant water, be shown that even at low ambient temperatures, systematic intake air temperature as well as boost pressure shows no acoustic-related optimization of the calibration significantly significant influence on noise (FEV-CSL), whereas reduces combustion excitation and thus the unpleasant hydrocarbon and carbon-monoxide emissions are knocking sound. It improves interior as well as exterior considerably diminished, especially by increased oil/water vehicle noise. At the same time, disadvantages regarding temperature and intake air temperature. By delaying BOI, startability, combustion stability, visible black or white combustion noise is – as expected – reduced, however, HC smoke and emission behavior (for instance in the MVEG and CO emissions are simultaneously augmented. test) are avoided. FEV-CSL (Combustion Sound Level) is an The minor dependency of combustion excitation on intake important development tool in the acoustic optimization of air and wall temperature (oil/water the calibration which facilitates combustion noise prediction temperature) as well as boost pressure seems to contradict in the optimization process based on a special cylinder literature (/2/). However, it can be explained by the pre- pressure analysis /1/. injection which has become customary today. Especially at low temperatures, ignition delay is drastically reduced by Basic Investigations on the Engine Cold pre-injection. With the start of pre-injection, first pre- Test Bench reactions (for instance radical formation) occur which at the onset of main injection facilitate a quick ignition of the The combustion noise is mainly determined by the cylinder induced fuel with a subsequent "smooth" combustion pressure excitation which in turn is characterized by the process. injection parameters (quantity and timing of pre- and main

609 CFA/DAGA'04, Strasbourg, 22-25/03/2004

Combustion Noise Optimization at Cold in Figure 4. The depicted modulation analysis of the interior noise shows that with optimized calibration the noise Start and Warm-up Phase amplitudes could be considerably lowered in the frequency By using modern measurement and analysis methods range (1 – 3 kHz) that is characteristic for knocking sound. (engine or vehicle cold test bench, noise prediction by FEV- The demonstrated significant optimization potential was CSL, DoE guided measurement programs) calibration verified in principle with a number of other vehicles with adjustment and thus noise quality can be considerably modern common rail diesel engines. improved with respect to cold start behavior. “typical” production calib. Pre-Optimization on the Engine / Vehicle Cold NVH optimized calibration Test Bench with the help of FEV-CSL and DoE Acoustical potential

The investigations on the cold test bench were supported by combustion faults NVH

the Design of Experiments (DoE) method, as exemplarily less Knocking“ „Diesel MVEG Start-ability black-/white smoke shown in Figure 3. Begin of pre- and main injection, pre- exhaust emission limits injection quantity, exhaust gas recirculation rate and rail -40-30-20-100 102030405060 Temperature [°C] pressure served as variation parameters (factors). Optimization parameters (responses) were hydrocarbon, Figure 5: Sketch of calibration NVH-optimisation potential carbon monoxide, nitrogen and particulate emissions as well and limitation demands at cold start. as the predicted engine noise level (FEV-CSL). Figure 5 illustrates the basic interrelations and limitations

factors with regard to the decisive requirements in cold start and

L L L

S S

S warm-up for a typical series application. In the temperature

C C C

- -

> Begin of pilot injection -

V V V

E E

> E range up to about –20 °C, startability is by far the major

F F

Pilot injection quantity F > Begin of main injection requirement. This is superimposed by the requirement for > EGR-Rate the prevention of ignition/combustion faults which is critical f n o o P in ti > ilo g c Rail pressure t inj e je up to approximately 0 °C. Reasonable acoustic optimization ecti n qu on B i ant in ity a starts at about –10 °C while black and white smoke must be m RESPONSES avoided. From 20 °C onwards, MVEG relevant exhaust

>

n n

FEV-CSL n 9

o o

o emission regulations must be achieved.

i i

8.5i

s s

> s

s s

HC emission s 8

i i i

7.5

m m

> m

e e

CO emission e 7

- - -

C C

> C

H H NOX emission H Summary -3 > 2.00 particulate -3.75 P 1.75 Within an internal research project at FEV Motorentechnik, i -4.50 f lo 1.50 o t on in 1.25 -5.25 in B: BOI t i D:q Meng e VEj u ec eg ec it could be demonstrated that the acoustic optimization of the a t 1.00 -6.00 j n ion B in tity n ai m injection parameters has the largest potential for combustion noise reduction during cold start. As a consequence further Figure 3: Use of DoE for cold start optimisation. acoustic improvement in many vehicles is possible without negative drawback onto startability, combustion quality, Application and "Fine-Tuning" in the Vehicle visible smoke or emission behavior, which could be proved for a number of common rail diesel engines. The After the calibration potential for the reduction of diesel methodology developed by FEV – combustion noise knocking was determined on the cold test bench, series prediction by FEV-CSL supported by DoE – as well as state- application into the vehicle was carried out on a conditioned of-the-art engine and vehicle cold test benches play a vital vehicle cold test bench. role in the optimization process. Base - calibration NVH optim. Calibration References [1] Prediction of Combustion Process Induced Vehicle Interior Noise. Alt, N.; Nehl, J.; Heuer, S.; Schlitzer, M., SAE 2003-01-1435

[2] Abschlussbericht zum Sonderforschungsbereich 224 “Motorische Verbrennung”. URL: http://www.vka.rwth- Figure 4: Comparison base /optimised calibration: aachen.de/sfb_224/bericht.htm Modulation analysis interior noise. [3] DoE in der Motorenentwicklung: Multiple Diesel With the optimized calibration, emission targets were Vehicles Calibration Based on Space-Filling DoE and achieved. Aside from a total level reduction in the vehicle Kriging Modelization. Figueres, F.; Raynaud, Y.: HdT interior and exterior noise (idle, –10 °C intake air Essen, Fachbuch Band 26, 2003 temperature) the noise quality was significantly improved with regard to typical diesel engine knocking as can be seen

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