Journal of Kones. Combustion Engines, VolB, No 1-2,2001
INVESTIGATION OF COLD START EMISSIONS FROM PASSENGER CAR WITH DI DIESEL ENGINE USING THE MODAL ANALYSIS METHOD Piotr Bielaczyc and Piotr Pajdowski BOSMAL Automotive Research and Development Centre ul. Sarni Stok 93, 43-300 Bielsko-Biala, Poland; Phone: +48338130598, Fax: +4833813560, E-mail:
[email protected]
Abstract. The stricter exhaust emission regulation which a new engine construction have to fulfil, causing continuous necessity development of research tools which make possible an analysis of engine operation at an angle of reduction exhaust emission. One of these research tools is the modal analysis method, which with success found application in exhaust emission tests on chassis dyno for testing complete cars and on engine test bed for testing engines as well. The modal analysis method has been used for the analysis of the emissions from passenger car with direct injection (Dr) diesel engine in the initial period of controlled engine operation following start-up. The tests were undertaken in "cold start" mode (temperature of cooling water and lube oil equal to ambient temperature). The paper subject is analysis of cold start emissions of cars powered by DI diesel engine. Exhaust emission tests results presented in the paper were carried out using the modal analysis method at Exhaust Emission Testing Laboratory of BOSMAL Automotive Research and Development Centre in Bielsko-Biala, Poland within research program investigating problem of cold start emissions for SI and CI at normal and low ambient temperature conditions.
1. Introduction The introduction of the 21 st century has seen more stringent exhaust errussion regulations introduced into Europe that will be in force in the near future [1-3]. Specifically, new requirements regarding emissions and fuel quality improvements specified as Euro ill (Stage 2000) and Euro IV (Stage 2005) were approved in the European Union on 30 June 1998 (published in the Directives 98/69IEC and 1999/1 02IEC [4]) and by United Nation Economic Commission for Europe (ECE) - Regulations ECE 83-05 (figure 1)[5]. In these new regulations the limits on carbon monoxide (CO), hydrocarbons (HC) and nitrogen oxides (NOx ) as well as on the particulate matters (PM) emissions of diesel engines are being tightened, and further improvements in emissions performance are required. How to reduce exhaust emissions further in the present century, one approach is to lower the NOx and PM levels is the direct injection (DI) diesel engine, which is advantageous for curbing CO 2 emissions. Figure I illustrates just how much exhaust emission levels would have to be reduced for European regulations for passenger cars. Reduction of the test limits (EURO ill Directives 98/69IEC and 1999/1021EC) corresponding to a drop of about 30-40 percent for diesel passenger cars (PC) complying with EURO II, and further drop of about 50 percent for Euro IV (Stage 2005) limits. This trend clearly shows needs of the analyses of all aspects of the relevant emissions [1-5].
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The main problems to be solved for future direct injection diesel engines are the reduction in NO, and PM emissions. But due to stringent Euro IV regulations, any source of increased emissions should be eliminated or at least limited - particularly in the case of engine cold start and warm-up phases. "Cold start" means here any start-up of the engine when the temperature of oil, coolant and all elements of the engine equal to the ambient temperature [6-8]. It can be also defined as the ability of the engine to start within a specified duration and continue to run without faltering [9]. Diesel engine cold starting is affected by many interacting processes (physical and chemical) during mixture formation inside cylinder that lead to autoignition and combustion [6,9]. Emissions during DI diesel engine cold start and warm-up are significant for overall emissions (however not so significant as in a spark-ignition engine), since the conditions of fuel atomization, mixture formation and combustion are poor, and moreover aftertreatment devices (e.g. oxidizing catalysts) do not work properly. In diesel engines, in addition to an expected increase in HC and CO emissions (as in spark-ignition engines), increases in particulate matter emissions are expected during the start-up phase [6,7,10]. During cold start and warm-up phases, emission control is mainly concerned with URC (unburned He), CO and PM [6-8,11,12]. The high HC-emissions during cold start become visible in the form of blue or white smoke [13]. The main source of this emission is incomplete combustion due to delayed and retarded combustion or - in the extreme - due to misfiring. The common practice of using excess fuel during cold starting amplifies the problem. The basic reason for undesirable harmful emissions is the low temperature that occurs during cold start followed by the initial phase of work, until the normal running temperature is reached [12,13].
2. The modal analysis of emissions The more stringent emission regulations, which a new engine construction have to fulfil, causing also continuous necessity development of research tools which make possible an exact analysis of engine operation at an angle of reduction exhaust emission. One of emission research tools is the modal analysis method, which with success found application in exhaust emission tests on chassis dyno for testing complete cars and on engine test bed for testing engine as well. In exhaust emission researches there are two types of data analysis are possible, bag - necessary for certification tests and modal, which are very useful during engine calibration and research testing. Bag analysis will yield emission values, which are composite for complete test. This kind of analysis is simpler to perform, and is satisfactory for determining whether a vehicle will pass a given test. Therefore, bag analysis is used for surveillance or compliance testing. Whereas modal
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analysis is one of the most important tools of the engine research engineer is the system used to monitor, control and log the data obtained during the testing of an engine on a dynamometer and a car on chassis dyno. It is necessary for the development of emission control system to determine the relations between emissions and driving mode and relations between cause and effect. The cause is the particular engine system at a specific operating point. The effect is the resulting emissions. Any driving cycle can be broken down into arbitrary modes such as idle, acceleration, cruise, and deceleration modes. The figure 2 presents the elementary ECE(UDC) cycle divided into four type of modes. The length of a mode could be several seconds or as short as I second. V [km/hl 60
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At least two methods of modal analysis are available: continuous analysis of diluted vehicle exhaust, and continuous analyses of undiluted exhaust using the CO 2 tracer technique. . The first method is continuous analysis of diluted vehicle exhaust (fig.3). In this method the mass emission of each pollutant HC, CO and NO. are computed in grams per mode. Generally, a computer will be advantageous for performing the large amount of calculation required for continuous modal analysis. The second modal analysis method is modal analysis using CO 2 tracer method (fig.4). This method is used more frequently, because there are many inherent difficulties in continuously analyzing diluted vehicle exhaust, primarily because of the very low diluted concentrations obtained for some modes. These problems can be avoided by continuously measuring the undiluted exhaust concentrations of HC, CO, NOx, and CO 2 . If the undiluted exhaust CO 2 concentration is also measured continuously, it is possible to calculate the vehicle exhaust volume for each mode. From the exhaust volume and the undiluted exhaust concentrations, the modal mass of each pollutant can be calculated. Actually, any constituent of the exhaust can be used as the tracer, but CO 2 is a good choice because it occurs in the largest and most constant concentration and, therefore, is easiest to measure accurately even after dilution. The mass emissions for individual modes can then be summed for the complete test, and these values compared with the mass emissions computed from the bags. Theoretically, the total of the modal masses should be equal to the mass emissions calculated from the bag data. In practice, there will not usually is perfect agreement, but the bags should agree with the modal total for each phase within a few percent.
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3. Test results and discussion The analysis of exhaust emission during cold start of a DI diesel engine for a Light Duty Vehicle was carried out in the Emission Testing Laboratory of the BOSMAL Automotive R&D Centre. The tests presented in this paper were carried out within a research program investigating the effect of cold start and warm-up at normal and low ambient temperatures conditions upon emissions from spark-ignition and compressionignition automotive engines.
Figure 5. A view of the CESAR system for the modal analysis of emissions - Emission Testing Laboratory of the Bosmal Automotive R&D Centre
The experiment was undertaken on a passenger car with a high-speed, four cylinders, turbocharged, direct-injection diesel engine with swept volume of about 2 drrr'. Additional emission control was provided by the use of a diesel catalytic converter for reducing carbon monoxide and hydrocarbons. The vehicle fulfilled the demands of actual European regulations Directive 98/69/EC (Euro ill). The objective of the research presented in this paper was to determine the increase in emissions of CO, HC and PM in exhaust gases from direct injection diesel engine after engine cold start in aspects of technology improvements necessary for the new European regulation Euro IV. The tests were carried utilising an emission chassis dynamometer, gases were collected by CVS system with full dilution tunnel with the bag and the modal analysis methods. AVL Cesar exhaust emission testing system together with CEB 600 emission analysers bench and CEC CVS sampling system with CET dilution tunnel have been used for these tests. Emission tests were undertaken in accordance with current NEDC cycle. Tests were undertaken in order to determine the influence of the first phase of the UDC cycle on emissions as well as the comparison of individual mode difficulty levels with a
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view to determine the necessary modifications that should be applied to the engine in order to satisfy the Euro IV emission limits.
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An example of the modal analysis of, HC, CO, CO2, NO, and 02 emissions in first 120 seconds of NEDC cycle is shown in the Fig.6. Emissions of all measured pollutants increased rapidly under car acceleration illustrated by peaks presented after second acceleration at 50-60 second of the cycle. Figures 7,9 and 11 presents HC, CO and NOx concentration in tailpipe exhaust and Figs 8,10 and 12 presents HC, CO and NOx emissions in g/km during first elementary phase of UDC cycle (carried out to the present NEDC procedure) measured using the modal analysis method in different modes of the emission cycle. It can be noticed that HC and CO emissions are higher at the initial phase of the cycle (first about 100 seconds of engine operation) than in last seconds of this cycle, this is illustrated by peaks presented after the first, second and third accelerations. Also the average emissions during all fourth modes: idle, acceleration, cruise and deceleration are higher at the initial phase than the final phase of first part of UDC cycle. The NOx emissions increased rapidly under car acceleration, this can be seen in figures 11 and 12. E'1600 r - - - - - - - - - - - - - - - - - - - - - - - - - - - - ,
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Figure 13 illustrates the results of emission test for HC, CO and NOx when the measurement were taken in the four elementary 195-second phases of UDC cycle. The figure shows a great increase in CO and HC emissions in the first 195 seconds phase of UDC test. These emissions are some 100-150 percent higher when comparing the emission of CO and HC in the last elementary phase of this cycle. Also NO, emissions is higher in the first UDC phase than 2-4 phases. '[ 400 ,!!,
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