EVALUATION OF DURABILITY OF
AUTOMOTIVE DIESEL ENGINE BASED ON
THE LABORATORY TEST INCLUDING
ENGINE START-UP
Niewczas A., Droździel P., Kordos P.
Lublin University of Technology, Mechanical Faculty, Dept. of IC Engines and Transportation, 36 Nadbystrzycka Str., 20-618 Lublin, Poland
Abstract: The article presents the evaluation of the automotive diesel engine durability on the
basis of a new, shortened laboratory test. The method of laboratory test consists of series of engine start-ups (starting series). This article presents results of measurements of cylinder liners wear according to the new method in comparison with long-term maintenance research methods of the same type of engine in vehicle.
1. Introduction
Test-stand reliability and durability research of automotive engines create an important component of manufacturing quality control systems. During durability tests engine is used on the test bench, and repeatedly undergoes so called basic test cycle. Basic test cycle assumes certain time intervals of engine operation at given loads and rotational speeds. So far, there was no all-purpose test cycle representative for conditions of real on-road operation. In general, each manufacturer uses its own test programs [2]. In Poland a test cycle is used recommended by the Branch Norm BN-79/1374-04.
Characteristic feature of test cycles used so far are constant values of rotational speed of the crankshaft and engine loads. Values of these parameters are selected so as to correspond to “heavy” conditions of engine operation, which, in practice, seldom occur. In laboratory tests used so far, short periods of engine operation with no load were dictated only by the threat of engine overheating and seizure of main tribological pairs. In conditions of real operation a high variability of engine loads takes place. Moreover, in operational practice, pauses in vehicle driving take place with related engine start-ups [3, 5].
Differences in durability research on the test stand in relation to the conditions of on-road operation can result in high interpretation errors of obtained results. Therefore it seems necessary to develop a new methodology of such test-stand research. This paper presents a new methodology of durability research of automotive IC engine, based on the new test cycle which considers condition of engine start-up.
2. Foredesign of engine durability assessment
It is assumed, that engine operation time till the point when the boundary wear of cylinder liner (treated as representative component for the whole engine) is reached, determines its durability. The measure of durability in case of an automotive engine is mostly vehicle mileage. As a wear measure a linear increase of cylinder diameter is accepted.
It was assumed, that course of tribological wear of cylinder liner in a function of engine operation time consists of two independent components. First, representing constant engine operation, and second representing conditions of engine start-up. General scheme of accepted model of operational wear is presented on fig. 1. It can be found in the literature, that “start-up wear” constitutes from 30 to 75% of total operational wear [4]. A new cycle of engine reliability research was developed including engine start-ups. The methodology is based on multiply repeated engine start-up cycle with suitably programmed temperature. Selection of temperatures and number of start-ups was made on the basis of operational research of the engine mounted in a vehicle. After a full series of starts engine was operated continuously for 15 minutes with increased rotational speed. It is necessary to maintain set start-up temperature within the test cycle. This puts certain demands on the engine test stand, related to the temperature control.
Fig. 1. Model of cylinder liner wear of an automotive engine
Experimental research of 4CT90 engine used in LUBLIN delivery truck consisted of following stages [2]:
1. identification of real conditions of operation of the engine,
2. investigation of cylinder wear during long term on-road operation (5 engines),
3. investigation of cylinder wear on the laboratory test stand according to the standard procedure given by the branch norm BN-79/1374-04,
4. research of the cylinder wear on the test stand according to the „high load” test, 5. research of the cylinder wear on the test stand according to the “start-up” test.
Full start-up durability test according to the new methodology consists of 10 repetitions of a single test cycle. Single test cycle included 100 consecutive start-ups: first 1- were done at 10 °C, next 15 at 35 °C, 30 at 55 °C, and last 45 at 75 °C. The number of engine starts at given temperatures was established on the basis of operational research. Wear of the cylinders on the test bench was measured using “artificial bases” method. After completing on-road and laboratory tests, results were compared.
4. Analysis of the results
Wear results obtained during laboratory research were compared to the operational wear. It was assumed, that 1 hour of engine work on the test bench during “high load” test and “standard” test corresponds to the vehicle mileage of 60 km. It was also assumed, that engine start takes place every 7 km during real operation. Comparison was made for the vehicle mileage of 100.000 km [2].
Then statistical analysis of the results was made. In the first step, descriptive statistics were calculated for the obtained wear results (see table 1). Fig. 2 shows histograms of cylinder wear. Evaluation of empirical distributions proved, that both operational and “start-up” wear of the cylinder can be approximated with normal distribution, whereas wear distribution obtained during “standard” and “high load” tests cannot be approximated with this type of distribution [1].
Table 1. Statistical parameters of cylinder wear of the 4CT90 engine
Mean value d [µm] Variance Var (d) [µm2] Standard deviation d [µm] Standard error d [µm] Median me [µm] Coefficient of variability v [%] Maximum value [µm] Minimum value [µm] Operation 19,66 8,53 2,92 0,275 19,45 14,85 25,94 13,89 BN 79/1374-04 20,33 361,66 19,01 1,796 16,32 93,07 85,71 0,04
High load test 21,14 448,19 21,17 2,000 13,15 100,14 83,33 0,04 Start-up test 16,81 59,58 7,71 0,729 17,25 45,86 34,71 2,77 1 3 ,9 1 5 ,1 1 6 ,3 1 7 ,5 1 8 ,7 1 9 ,9 2 1 ,1 2 2 ,3 2 3 ,5 2 4 ,7 2 5 ,9 W e a r [ m ] 0 % 2 % 4 % 5 % 7 % 9 % 1 1 % 1 2 % 1 4 % 1 6 % 1 8 % 1 9 % 2 1 % 2 3 % P e rc e n ta g e fr a c tio n [% ]
2 ,8 6 ,0 9 ,2 1 2 ,4 1 5 ,5W e a r [ m ]1 8 ,7 2 1 ,9 2 5 ,1 2 8 ,3 3 1 ,5 3 4 ,7 0 % 2 % 4 % 5 % 7 % 9 % 1 1 % 1 3 % 1 4 % 1 6 % 1 8 % 2 0 % P e rc e n ta g e fr a c tio n [% ] 0 ,0 1 0 ,0 2 0 ,0 3 0 ,0 4 0 ,0 5 0 ,0 6 0 ,0 7 0 ,0 8 0 ,0 9 0 ,0 W e a r [ m ] 0 % 9 % 1 8 % 2 7 % 3 6 % 4 5 % P e rc e n ta g e fr a c tio n [% ]
0 ,0 1 0 ,0 2 0 ,0 3 0 ,0 4 0 ,0W e a r [ m ]5 0 ,0 6 0 ,0 7 0 ,0 8 0 ,0 9 0 ,0 0 % 9 % 1 8 % 2 7 % 3 6 % 4 5 % 5 4 % P e rc e n ta g e fr a c tio n [% ]
Fig. 2. Bar chart of the wear distribution of the cylinder liner: a) after „long term operation”, b) after „start-up durability test”, c) after „high load test”, d) after „standard durability test”
a
c d
I II III IV Kind of tests -30 -20 -10 0 10 20 30 40 50 60 70 W e ar [µ m ]
Average Average±Std. dev. Average±1,96*Std. dev
Fig. 3. Category frame chart of mean values of cylinder wear. I) after „high load test”, II) after „standard durability test”; III) after „long term operation”, IV) after „start-up durability test”
Wear values obtained in individual durability tests were compared in respect to their mean values. Kruskal-Willis [1] non-parametric test was used. It was stated, that obtained values differ significantly from the operational wear and from the “start-up” wear. Fig. 4 presents schematic results of comparison of mean wear values obtained in different (mentioned above) tests.
On the basis of comparison of wear profiles of the cylinder liner (see fig. 4) it was observed, that both for the “standard” and “high load” tests, highest values of wear were located at top and bottom dead positions of piston rings. However in the new “start-up” test and during on-road operation, more uniform wear profile was obtained. In all three compared laboratory tests, the most close wear profile to the real one was obtained for the new “start-up” test.
6. Conclusions
On the basis of test stand research and on-road measurements, it was possible to design a “new method of durability assessment” of the automotive engine. This method includes engine starts and long term, constant operation. Designed method can be characterized by the simplified research procedure and shortened duration time in comparison to “standard durability tests”. It was also stated that:
1. Start-up wear of the cylinder liners on the test bench after the start-up test amounts to 87% of total wear during engine operation in the vehicle, which made the same number of starts as during the test.
2. Wear profile of the cylinder in the start-up test can be compared to the operational profile, contrary to the profiles obtained for standard durability tests. 3. Conformity in terms of statistical distribution and wear profile indicates
existence of similarities in friction conditions during the new durability test and real operation of the engine.
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