Evaluation of the combustion noise of passenger car diesel engines

Driven by the increasing demand of customers for comfort, the subject of automotive and engine acoustics is rising in its importance in the process of vehicle development. Along with wind and rolling noise, the combustion noise plays a crucial part in vehicle acoustics, especially in case of direct injection diesel engines. At the IVT at Graz University of Technology the combustion process of several modern passenger car diesel engines with respect to their noise behaviour was analysed.


INTRODUCTION
Regarding the combustion noise, the SI engine can be seen as benchmark based on its smooth cylinder pressure curve. The objective in diesel engine engineering is the best possible approximation of pressure curve and rate of heat release to that of the SI engine while maintaining the typical efficiency advantage inherent to diesel engines. Especially in the acoustically most relevant part-load range, the noise characteristic of current diesel engines exhibits noticeable peaks, which both passengers and passers-by may recognise as diesel-typical combustion noise. Currently, there is no ECU calibration which entirely prevents these peaks without deteriorating consumption and emission behaviour.

METHODOLOGY
Basically, the methodical approach used in this research project can be divided into investigations carried out using complete vehicles and investigations on an engine test bed. Further details relating to the presented topic can be found in [1]. As a first step, three different compact and mid-size cars were investigated on a dynamic acoustic roller test bed. In this setting, cars with four-cylinder common rail injection diesel engines, a unitary displacement of 0.5 l and a power range of 110 to 130 kW were used.
Each car was provided with a considerable measuring equipment. Cylinder one was equipped with high pressure indication, including the measurement of injection timing for determining the cylinder pressure and the rate of heat release. Additionally, lowspeed data, such as boost pressure, rail pressure and EGR rate was recorded. The structure-borne noise characteristic was sensed via several attached acceleration sensors at the engine block, cylinder head and engine suspensions. Furthermore, the airborne noise was measured at several spots, amongst others in the engine compartment and the interior of the vehicle. To serve as basis for stationary behaviour, a characteristic engine map with series calibration was measured in the acoustically relevant operating range from 1250 to 2250 rpm and a brake mean effective pressure from 1 to 10 bar. The dynamic behaviour was described by several runups and load steps. This setup made it possible to calculate the combustion noise based on the cylinder pressure curve, evaluate the transmission path to the outer engine surface using the attached accelerometers and in further consequence illustrate the airborne sound audible in the engine compartment as well as in the vehicle interior and the surrounding. The similar setup used for all three vehicles enabled comparison between them.
To understand the link between combustion process and noise generation, a profound analysis on an engine test bed was conducted, using an engine taken from one of the vehicles mentioned above. Besides the test of the series calibration, the primary focus was on an in-depth sensitivity analysis by varying parameters such as boost pressure, EGR rate, swirl valve position, rail pressure, injection pattern or pilot quantity. This investigation enabled an evaluation of the influence of said parameters on combustion noise, consumption and emission behaviour.

COMBUSTION NOISE
The combustion noise encompasses all sounds caused by cylinder pressure, both directly by excitation of the cylinder walls as well as indirectly out of shock events in components subject to backlash [2,3]. The direct combustion noise dominates the acoustic behaviour particularly in part-load operation at low speeds [4,5]. For calculating the combustion noise the Combustion-Noise-Level (CNL) of AVL List is used. The calculation is based on the cylinder pressure curve, the continuing approach is shown in FIGURE 1. A sharp increase in the part-load area between 2 and 5 bar brake mean effective pressure is clearly recognisable and can also be noticed in the airborne noise during driving. The characteristic of this sharp increase depends on the ECU calibration. In transient operation, the combustion noise behaviour in certain cases deteriorates sharply. FIGURE 3 illustrates that using a comparison of stationary measurement points and a load step. The noise level in dynamic operation is consistently higher with a superelevation up to 4 dB(A) relating to steady-state operation. A closer analysis of the measured data implies two main causes for this noise peak. These are the EGR rate control as well as the delayed boost-pressure build-up.

IGNITION DELAY
Mainly responsible for the combustion noise typical of diesel engines is the premixed amount of fuel, which, due to the already completed mixture preparation, is converted rapidly. The quantity of premixed fuel is determined primarily by the length of the ignition delay. The determination of the ignition delay requires accurate knowledge of the start of injection and the start of combustion. A Bosch tube was used to precisely identify the start of injection. Furthermore, the start of combustion was evaluated manually for all examined measurement points. FIGURE 4 displays the result of this analysis, in which a distinct maximum in the area of 5 bar brake mean effective pressure is perceptible. Ignition delays up to 5 °CA occur.

SENSITIVITY ANALYSIS
FIGURE 5 shows the behaviour of the combustion noise in dependence of a selection of parameters at three different operating points. In the course of the sensitivity analysis, only one parameter at a time was varied to prevent interactions. At all three operating points, an increasing rail pressure leads to a higher combustion noise. The reason can be found in a better fuel atomisation and hence a faster mixture preparation, which on the one hand reduces the ignition delay but on the other hand accelerates the following combustion process.
At low loads an increase of boost pressure yields better ignition conditions and thus a decreasing ignition delay. Simultaneously, the rate of heat release is increased, resulting in an earlier and faster combustion. The effect of the latter dominates, thus increasing combustion noise. The ignition conditions are improved at an operating point of 2000/6, FIGURE 5 (right column), as well, however, the influence on the conversion rate at this point is negligi- ble. As a consequence, the advantage of the reduced ignition delay comes into effect and lessens the combustion noise.
As far as the EGR rate at 2 and 3 bar brake mean effective pressure is concerned, the addition of exhaust gas (up to an amount of 30 to 40 %) has only a minor effect on the combustion noise. Only rather high EGR rates lower the noise level based on a marked slowdown of the combustion process. A later main injection timing shows a similar behaviour, due to deteriorating conversion conditions. At the operating point 2000/6, FIGURE 5 (downright), there is only a negligible impact of the EGR rate and the main injection timing on the combustion noise. An increased ignition delay effects a higher quantity of premixed fuel. An interference of the conversion rate does not occur.
Especially in the low-load range up to approximately 4 bar brake mean effective pressure, the variation of several parameters results in a distinct response of the combustion noise. It is note-worthy that in this load area a decrease in the ignition delay mostly corresponds with a higher rate of heat release and thus with an increase in the combustion noise level. Therefore, lowering the quantity of premixed fuel by decreasing the ignition delay is beneficial only at higher load levels.

TRANSMISSION BEHAVIOUR
The combustion and the related excitation are the origin of noise generation. However, perceptible to the driver and passers-by is the airborne noise radiated from the engine, caused by the transmission of the combustion noise within the structure and its subsequent emission. The graphs in FIGURE 6 compare the results of vehicle 1 and 2. It is obvious that, in terms of combustion noise, vehicle 1 has a better acoustic behaviour in most parts of the considered map area than vehicle 2. The difference can be as high as 9 dB(A). The analysis of the structure-borne noise points out decreasing advantages of vehicle 1. Regarding airborne noise in the engine compartment, vehicle 2 is already less noisy than vehicle 1 at the majority of the evaluated points. The decided advantage of vehicle 1 concerning the combustion noise is diminished by drawbacks in the transmission behaviour, which lead to a higher noise level in the engine compartment.

SUMMARY AND OUTLOOK
The research project analyses the combustion process of several modern passenger car diesel engines with respect to their noise behaviour. The primary focus is put on the combustion noise as the main source of noise excitation. In addition, the transmission behaviour from the combustion chamber to the engine compart- ment is examined, using several structure-borne and airborne measuring points. In terms of the applied methodology, measurements are conducted on an acoustic roller test bed as well as on a conventional engine test bed. In addition to the analysis of the series calibration, the influence of several parameters on the combustion noise is investigated in detail and a method for exactly determining the ignition delay is developed.
The examination of the complete vehicles shows a sharp increase in the noise level in part-load operation, which is typical for modern diesel engines. The particular result is dependent on the used injection strategy and is reflected in the cylinder pressure and the rate of heat release. In steady-state operation, rail and boost pressure, EGR rate and position of the main injection are especially relevant to the combustion noise behaviour, whereas in dynamic operation the delayed boost-pressure build-up is the main cause. Distinct differences are revealed in the transmission behaviour of the vehicles.
In the further course of this research project, an optimisation of the noise behaviour based on the obtained insights should be performed. Conceivable possibilities are adjustments in the ECU calibration as well as modifications of the applied hardware. Especially in case of an alternate calibration, drawbacks in efficiency and emission behaviour have to be considered. Promising changes in hardware concern the charging unit and the injection system, for instance. Finally, obtained improvements should be validated in the overall vehicle.