PM Exhaust Characteristics from Diesel Engine with Cooled EGR

Transcription

1 Proceedings of International Symposium on EcoTopia Science 07, ISETS07 (07) PM Exhaust Characteristics from Diesel Engine with Yutaka Tsuruta 1, Tomohiko Furuhata 1 and Masataka Arai 1 1. Department of Mechanical System Engineering, Graduate school of Engineering, Gunma University, Tenjin-cho, Kiryu-shi, Gunma, Japan Abstract: Exhaust Gas Recirculation (EGR) is an effective system for reducing NOx. This research focused the cooled EGR effect on Particulate Matter (PM) exhaust emission. Test engine was air cooled type single cylinder direct injection diesel engine. EGR cooler was placed between exhaust and intake pipes. Excess air ratio was set to 2.3 or 2.8 under without EGR condition. Intake O 2 concentration was measured by O 2 meter and EGR ratio was defined as the reduced ratio of the intake O 2 concentration with EGR. The PM emission was analyzed using a scanning mobility particle sizer and smoke number was measured with Bosch smoke meter. Size distributions of exhaust PM were measured in cooled and hot EGRs. As a result, at =2.8 condition, the peak diameter of PM under cooled EGR condition was larger than that under hot EGR condition. At =2.3 condition, the PM number concentration at peak diameter under cooled EGR condition was higher than that under hot EGR condition. Moreover, the relationships among total number of PM, peak diameter of PM and smoke number were discussed. Keywords: Diesel engine, EGR,, PM, PM number distribution 1. INTRODUCTION CO 2 emission from diesel engine is lower than other internal combustion engines, because its thermal efficiency is superior to the others. On the other hand, diesel engine exhausts much NOx and Particulate Matter (PM). NOx deteriorates environment and PM injures human health directly. For example, the possibilities of getting asthma and lung cancer become high by breathing much PM [1]. To reduce these environmental and health effects, many efforts have been done for diesel engine improvement. Exhaust Gas Recirculation (EGR) system is introduced into diesel engines to reduce NOx, and high pressure fuel injection is used to reduce PM. Further, cooled EGR is applied for more reduction of NOx in recent years. Effects of EGR on NOx reduction have been studied in detail [2-4], but there are a few reports about exhaust PM characteristics under EGR condition. To reduce more PM, it is necessary to study the behavior of PM emission under cooled EGR condition. Therefore the objective of this engine test was to analyze exhaust PM characteristics such as size distributions under both hot EGR and cooled EGR conditions. 2. EXPERIMENTAL APPARATUS AND METHOD Test engine was air cooled type single cylinder direct injection diesel engine and its specifications were listed in Table 1. Experimental apparatus was shown in Fig.1. Test fuel was JIS No.2 diesel fuel. Excess air ratio was set to 2.3 or 2.8 under without EGR condition. The fuel injection quantity was 12.8 mg/cycle at =2.3 and.0 mg/cycle at =2.8. Engine speed was set at 10rpm and fuel injection timing was set at 16deg.BTDC. To measure heat release rate, the cylinder head was equipped with a pressure indicator. EGR cooler was placed between exhaust and intake pipes. To control EGR gas flow rate, two waist valves were placed in exhaust line and at upstream of EGR cooler in EGR line, respectively. A thermocouple was inserted into the EGR line downstream of the EGR cooler to measure EGR gas temperature. Intake air flow was measured by a flow meter and intake O 2 concentration was measured by an O 2 meter. EGR ratio was defined as the reduced ratio of the intake O 2 concentration with EGR. Sampling method of exhaust gas was shown in Fig.2. For smoke number measurement, exhaust gas was directly sampled. Its sampling position was the exhaust line just after the engine and smoke was measured by Bosch smoke meter. Moreover, exhaust gas for PM size distribution measurement was sampled at the same position and was introduced into a sampling bag. The bag was previously filled with N 2 gas and exhaust gas was diluted 00 times in the bag. It was analyzed using a scanning mobility particle sizer (TSI, SMPS-34). Table 1. Engine specifications Engine name Engine type Robin DY41DS 4-stroke OHV 2-valve Number 1 Cylinder Bore 82mm Stroke 78mm Displacement Top clearance Compression ratio 412cc 0.87mm 21 Intake valve Open 16 B.T.D.C Close 126 B.T.D.C Exhaust valve Open 14 A.T.D.C Close 124 A.T.D.C Cooling type Air cooling 231

2 Proceedings of International Symposium on EcoTopia Science 07, ISETS07 (07) Orifice Intake Manometer Water IN Thermocouple Pressure Indicator Valve Surge tank Water OUT EGR cooler Nozzle Thermocouple Valve Exhaust Bosch smoke meter Heat release rate dq/dθ J/deg. 35 λ = % Hot 8% Cooled Engine O2 meter Engine Fig.1 Engine test apparatus Exhaust P N 2 Fig.2 Sampling method of exhaust gas 3. RESULTS AND DISCUSSION 3.1. Effect of EGR on heat release rate Heat release rate and EGR gas temperature at excess air ratio =2.8 are shown in Fig.3. Only the pre-mixed combustion peak was observed because of lean combustion condition at =2.8. At high EGR ratio, O 2 concentration in cylinder was low and therefore ignition delay became long. Then the peak of heat release rate increased with an increase of EGR ratio. Though the EGR gas temperature under cooled EGR condition is lower than that under hot EGR condition, the peak of heat release rate under cooled EGR was lower than that under hot EGR. It was considered that cooled EGR did not promote fuel spray evaporation even if the ignition delay was prolonged. Heat release rate and EGR gas temperature at =2.3 are shown in Fig.4. Since the excess air ratio was in the range of medium load condition, both of pre-mixed combustion peak and diffusion peak were clearly observed in the heat release diagram. With increasing the EGR ratio, pre-mixed peak was increased and cooled EGR effect on the pre-mixed peak was almost similar to P Sampling gas dilution ratio=00 SMPS Bosch smoke meter SMPS EGR gas Gas Temperature temperature TDC Crank angle θ deg. (a) Heat release rate (b) EGR gas temperature Fig.3 Heat release rate and EGR gas temperature ( =2.8) Heat release rate dq/dθ J/deg. EGR gas Gas Temperature temperature λ = % Cooled 4% Hot TDC Crank angle θ deg. (a) Heat release rate (b) EGR gas temperature Fig.4 Heat release rate and EGR gas temperature ( =2.3) 232

3 Proceedings of International Symposium on EcoTopia Science 07, ISETS07 (07) 0 90 λ = dn/dlog(d p ) cm 3 8 Ne= 10 rpm λ= 2.8 9% Hot 8% Cooled 0 Fig.5 Relationship between EGR ratio and smoke number ( =2.8) 4 1 5x 1 2 5x 2 Diameter d p nm Fig.6 PM number distribution ( =2.8) 0 90 λ = dn/dlog(d p ) cm 3 8 Ne= 10 rpm λ= 2.3 4% Cooled 4% Hot Fig.7 Relationship between EGR ratio and smoke number ( =2.3) the case of Fig.3. Owing to the long ignition delay caused by high EGR, the peak of the diffusion combustion was decreased with EGR. However owing to the slow evaporation process before ignition, peak of diffusion combustion under cooled EGR was slightly higher than that under hot EGR EGR effect on PM emission Relationship between EGR ratio and smoke number at =2.8 is shown in Fig.5. At high EGR ratio, smoke number under cooled EGR was lower than that under hot EGR. PM number slightly at =2.8 are shown in Fig.6. With increasing the EGR ratio, peak diameter of PM number distribution was shifted to larger size side. Also, the number concentration in the diameter range larger than 0nm was increased with the EGR condition changed from hot to cooled. The peak number 4 1 5x 1 2 Diameter d p nm Fig.8 PM number distribution ( =2.3) 5x 2 concentration under cooled EGR was higher than that under hot EGR regardless of the excess air ratio. More significant number difference between cooled and hot EGRs was revealed at the high EGR ratio condition. Relationship between EGR ratio and smoke number at =2.3 are shown in Fig.7. The smoke number under cooled EGR was lower than that under hot EGR. PM number distributions at =2.3 are shown in Fig.8. The PM number concentrations at the diameter peaks under cooled and hot EGR conditions were larger than that of without EGR condition. Further, the peak number concentrations under cooled EGR were larger than that under hot EGR. Generally EGR effect on PM emission had different the tendency between smoke number and PM number distribution. Slight decrease of smoke number with the change from hot to cooled did not correspond to the 233

4 Proceedings of International Symposium on EcoTopia Science 07, ISETS07 (07) 85 =2.8 =2.8 0 =2.8 =2.8 ( =2.3) 3 Total Total Number number of of PM 1/cm ( =2.8) ( =2.8) ( =2.3) ( =2.3) Peak Peak Diameter diamter of of PM nm ( =2.8) ( =2.8) ( =2.3) Fig.9 Relationship between smoke and total number of PM Fig. Relationship between smoke number and peak diameter of PM 8 =2.8 = =2.8 =2.8 ( =2.3) =2.3 =2.3 Total Number number of PM cm cm ( =2.8) ( =2.8) ( =2.3) ( =2.3) Peak Diameter of PM nm ( =2.8) ( =2.3) ( =2.8) Intake O 2 % Intake O 2 % Fig.11 Relationship between intake O 2 and total number of PM Fig.12 Relationship between intake O 2 and peak diameter of PM number increase of PM. This evidence was more clearly explained in following section PM characterization Relationship between smoke number and total number of PM is shown in Fig.9. From this figure, it was shown that smoke numbers at low excess air ratio ( =2.3) were higher than those at high excess air ratio ( =2.8), but the total number of PM did not always correspond linearly to the smoke number. At =2.8, the total number of PM increased with an increase of smoke number under both EGR conditions in the smoke number range from % to %. As for the phenomena in the smoke number range over %, the 234

5 Proceedings of International Symposium on EcoTopia Science 07, ISETS07 (07) total number of PM under cooled EGR did not depend on smoke number. Further total number of PM decreased with an increase of smoke number in a range over %. As for the excess air ratio of =2.3, the relationship between smoke number and total number of PM under cooled EGR was similar to that at =2.8. However in the case of hot EGR, the total number increased linearly with increasing smoke number. Relationship between the smoke number and peak diameter of PM is shown in Fig.. There was general tendency of positive correlation between smoke number and peak number diameters of PM. At =2.8, there were clear correlations between smoke number and the peak diameter under both EGR conditions. Namely, the peak diameter increased with an increase of smoke number. Under hot EGR condition, however, the increase rate was low compared with case of cooled EGR. At =2.3, on the other hand, we could not find clear correlation between the smoke number and the peak diameter under both EGR conditions. From Figs.9 and, it was clear that the total number of PM and the peak diameter of PM in cooled EGR did not always agree with those in hot EGR even if the smoke number were similar in both EGR cases. These may be caused by the difference of PM properties such as PM composition between cooled and hot EGR cases. Figures 11 and 12 show the effect of oxygen concentration on total number and peak diameter of PM. It was clear from Fig.11 that total number of PM under cooled EGR was always higher than that of hot EGR. Peak diameter shown in Fig.12 indicates that peak diameter at =2.3 condition was around 0nm and did not change with EGR and with oxygen concentration. However peak diameter at =2.8 was sensitive on oxygen concentration. 4. CONCLUSIONS Characteristics of PM exhausted from diesel engine with hot and cooled EGRs were experimentally investigated. Main results are summarized as follows. 1. The PM number concentration at peak diameter under both EGR conditions were higher than that of without EGR condition. 2. The PM number concentration at peak diameter under cooled EGR was higher than that under hot EGR. 3. At =2.8 condition, the peak diameters of PM under both EGR conditions were larger than that of without EGR condition. However at =2.3, peak diameter change was not clear. 4. The total number of PM under cooled EGR was higher than that under hot EGR. 5. There was general tendency that the peak diameter of PM increased with smoke numbers. 5. REFERENCES 1. Y.Goto, Measurement of nano-particle of emission, Japanese journal of Measurement and living body effect estimation of nano-particle of automobile emission and DEP, NTS, Toyko, (05), p.97 (in Japanese). 2. G.H. Abd-Alla, Energy Conversion and Management, 43 (01), pp Nidal H. Abu-Hamgeh, Energy Conversion and Management, 44 (02), pp Ming Zheng, Graham T.Reader, and J.Gary Hawleya, Energy Conversion and Management, 45 (03), pp