Oil consumption measurement under DOE conditions

DEVELOPMENT Measuring Techniques

24 MTZ worldwide 6/2004 Volume 65

By Paul Werner Jöhren,

Arnim Robota and

Torsten Dörre

Ölverbrauchsmessung unter

der Randbedingung DoE

You will find the figures mentioned in this article in the German issue of MTZ 6/2004 beginning on page 482.

Oil ConsumptionMeasurementunder DOE Conditions

Oil consumption is an increasingly important catchword in the in-ternal combustion engine industry. This article explicitly definesthe term oil consumption and presents a measurement systemfor determining sump oil consumption. Based on the example ofa part-factorial DoE, the procedure and measurement results us-ing the “Movan” measurement system from Federal MogulBurscheid GmbH are demonstrated. The emphasis is placed onthe accuracy attainable with statistical confidence over minimalmeasurement periods.

25MTZ worldwide 6/2004 Volume 65

1 Introduction

Longer oil change intervals and emissionslegislation motivate the internal combus-tion engine industry to optimise engine oilconsumption subject to the boundary con-dition of long life expectancy. Since theterm oil consumption can be misunder-stood, the difference between oil consump-tion and oil emission is explained in Figure1. Oil emission, as the figure shows, is onlyone element of oil consumption, which de-scribes the loss of lube oil from the oilsump. This oil consumption is referred tobelow as sump oil consumption. With re-spect to emissions, oil emission is clearlydecisive, while in terms of service intervalsthe key criterion is sump oil consumption.This article presents a measurement sys-tem for determining sump oil consumptionand its operation in the engine test centreat Federal Mogul Burscheid GmbH. Further-more, based on the quite common levels ofsump oil consumption encountered today,accuracy requirements are discussed fromstatistical perspectives with the support oftest results. Finally, the results of a part-fac-torial DoE are presented. The evaluationand rating of sump oil consumption and oilemission will be addressed in another arti-cle at a later time.

2 Construction of the MovanMeasurement System

2.1 Measurement ProcedureThe measurement procedure is based onweighing the amount of fresh oil put intothe engine. The change in lubricant mass(fresh oil, condensation products, fuel, etc.)is conveniently determined by weighingthe mass in a collection vessel located out-side the engine. Therefore, only the massthat is not in the engine is always weighed.This can be done continuously or discontin-uously. The measurement signal is not di-rectly proportional to the lubricant loss butonly shows the distribution of the amountsof lubricant in the engine and weighingvessel. With changing engine speed, largeredistributions of lubricant occur in a shorttime and should not be interpreted as con-sumption. When the distribution is con-stant or quasi-constant (such as occurs incyclical programmes), the weight signalcan be equated to the consumption.

2.2 MechanicsThe measurement system known as“Movan” (Motoröl-Verbrauchsmessanlage)was developed in the engine test house ofFM Burscheid GmbH. The origins of oil con-sumption measurement using continuousweighing go back to the 1960's. At this time,

engines were changed to dry sump lubrica-tion to avoid the difficulties of determiningthe amounts of oil in the engine. The pre-sent-day measurement system overcomessome of the difficulties by incorporating aspecial stack in the oil sump that preventsany effect from density changes in the oil,such as foaming. The measurement princi-ple involves determining the volumechange, or more precisely, the mass changein the sump by means of a load cell. An il-lustration of the Movan measurement sys-tem and a schematic layout are shown inFigure 2.

The system essentially consists of twocontainers suspended vertically in line, eachattached to a load cell, and a secondary oilcircuit with its own pump having a capacityof about 1 litre/min. The top load cell regis-ters the combined weight of both containers,while the bottom load cell registers only theweight of the bottom container. With thisarrangement, not only can the bottom con-tainer be refilled without interfering withthe total measurement signal, but also a de-fined content can be maintained in the bot-tom container by means of the load cell at-tached to it. The bottom container is con-nected directly to the engine oil sump andtogether with this determines the startingamount of oil in the system. The startingamount of oil can be kept roughly at the dip-stick maximum level, as the stack can be de-signed in such a way that the amount of oilin the engine sump is set slightly below dip-stick minimum (usually about 1 cm below)leaving a sufficiently large residual amountavailable for the primary circuit. The basicstructure of the stack can be seen in Figure 3.This special stack design ensures that no oilcan run directly from above into the over-flow pipe and that the overflow is levelledout with oil from below on the principle ofcommunicating pipes. Additionally, suffi-ciently large ventilation cross-sections areprovided to ensure that no difference inpressure can occur between the overflowedge and oil sump. Through a second pipe atthe oil drain plug, the oil is pumped from thebottom container into the engine oil sumpby the oil pump in the secondary circuit. Thissame connection can also be used to com-pletely empty the engine sump. As will bedescribed below, such a construction allowsdifferent uses (operating modes) for themeasurement equipment. Finally, it shouldbe mentioned that the container sizes andweights were optimised for appropriate en-gine sizes and sump content in order toachieve maximum accuracy and resolution.

Load Cell Accuracy ClassesThe top load cell, whose signal is used to de-termine the sump oil consumption, has an

accuracy class of 0.02, corresponding toOIML R60 C3, and the bottom load cell 0.03,corresponding to C2.

The signal from the load cell is im-pressed onto suitable instrument ampli-fiers in the automation system.

2.3 Digital Resolution of theAutomation System in the En-gine Test LaboratoryThe DCIV automation system supplied bySchenck has a 12-bit analogue-to-digitalconverter. When the binary code is usedand it is assured that no negative valuescan occur (the Highest Significant Bit is notused as a sign), then the resolution of theinstantaneous values can be doubled. Themeasurement process is further improvedstatistically by the use of integral measure-ment (sliding average).

2.4 Calibration Procedure for the Movan MeasurementSystemFor the calibration of the measurement sys-tem, three measurement quantities arecompared.■ The amount of oil actually conveyed in-to the bottom container, measured by de-termining the weight using a calibratedbalance and weights■ The voltage signal from the top load cellafter the instrument amplifier using an ex-ternal digital voltmeter (16-bit resolution)■ The voltage signal after the instrumentamplifier using an internal analogue-to-digital converter in the DCIV automationsystem (utilising the resolution describedabove).

The effective range of the load cell for oilconsumption measurement is divided intothree measurement ranges. Each range isgraduated with 20 points (every additionweighs about 40 g) and is evaluated bymeans of linear regression. The result ofcalibrating a Movan in this way is shown inFigure 4. As the results are practically con-gruent, the axes have been scaled different-ly for illustration purposes.

3 Movan Applications

3.1 Continuous Sump OilConsumption MeasurementIn the case of continuous consumptionmeasurement, the secondary circuit is acti-vated in such a way that the engine oilsump is continuously supplied with lubeoil from the measuring vessel. The engine'sown oil pump supplies the engine in theusual way from the engine sump, the levelof which is regulated by the stack at about 1cm below minimum. The measurementsignal can be used to determine the sump


26 MTZ worldwide 6/2004 Volume 65

oil consumption when steady or quasi-steady distribution states in the enginehave been reached. The change in the mea-surement signal over time is determined bymeans of linear regression, including statis-tics, and converted into corresponding oilconsumption values.

3.2 Draining ProceduresDraining means releasing the oil from thesump into the measurement system (bot-tom container). To determine oil consump-tion and draining quality, however, a setprocedure is followed. This is referred to be-low as the draining procedure.

The draining procedure is divided intothree steps:■ Dynamic draining: In the dynamicdraining mode, the engine operates in amiddle speed and load range; the stack isopened and the secondary circuit oil pumpis operating. After a certain transient time,an equilibrium sets in between the oil levelin the sump and the bottom container ofthe Movan.■ Partial draining: In this mode, the en-gine and oil pump of the measurement sys-tem are turned off. The oil circulating in theengine drains into the sump and the levelin the sump remains at the level of thestack overflow edge.■ Total draining: For total draining, thewhole of the oil is drained from the enginesump into the bottom container of themeasurement system.

The schematic diagrams in Figure 5show the operation of the oil consumptionmeasurement equipment with the engine.Figure 6 shows all of the measured valuesfor a complete draining procedure. The ar-eas used for calculating the oil consump-tion or determining the draining qualityare specially marked. All the measured val-ues are averages over 10 seconds and arelogged on a 10-second grid. This representsa continuous progression of the drainingprocedure and yields the draining curve.The oil consumption is then the differencebetween two draining curves at the begin-ning and end of a test run or oil consump-tion step.

3.3 Determining the Quality ofthe Measurement ResultsIn order to evaluate the quality of the oilconsumption measurement results, it isnecessary to consider the combined systemof the engine and Movan. This involvesrunning a draining series, in other wordsthe procedure is repeated 20 to 30 times. Forthe dynamic, partial and total drainingsteps, the standard deviation is calculatedat a 95 % level. With the aid of the standarddeviation, it is then possible to define the

minimum time that a test programmemust run between two draining proce-dures in order to obtain a certain quality ofmeasurement result. This means, for exam-ple, that if the standard deviation deter-mined from the draining series is ±20 g, thetest programme must run for at least 10hours to obtain a quality of ±2g/h with aconfidence level of 95 % for the oil con-sumption result.

To evaluate the quality of the oil con-sumption measurement result, both thestandard deviation and the path of thedraining curves are considered. If the drain-ing behaviour is similar, then the drainingcurves, as shown in Figure 7, must run vir-tually parallel. To calculate the oil con-sumption, the step (dynamic, partial or to-tal) for which the smallest standard devia-tion was determined from the draining se-ries is chosen. If the curves in the area cho-sen for the evaluation are not parallel afteran oil consumption run, this is an indica-tion that the draining behaviour haschanged. In this case, another area must beused for the calculation, which might alsoentail a poorer quality of the result.

If the quality of dynamic draining is suf-ficiently good in terms of the standard de-viation, dynamic draining can be used as areference point measurement between twoprogramme steps in order to shorten theprogramme run time. The programme cy-cle in the following section is an example ofthe use of reference point measurement.

4 Preparing the DoE

Designs of Experiments are divided intofull-factorial and part-factorial test meth-ods. Using the example of a part-factorialDoE (often the Taguchi method is used), it isproposed to show what measurement ac-curacy is necessary for an L8 plan.

In the present case, five parameters re-lated to rings and liners had to be investi-gated with respect to sump oil consump-tion. For this purpose, an L8 plan on thelines of Taguchi was set up. The resultingcombination of tests can be seen in Figure8. In order to ascertain in which area(range) the oil consumption for the eighttests should be expected, not only wasthere a best case (all parameters level 1) buta test 0 was also added as a worst case (allparameters level 2). The decision as towhich levels have an effect towards bestcase or worst case was established by theexperts before the start of the investiga-tions. From the L8 plan, therefore, an L8+1plan emerged.

To determine the draining quality of thesystem, a dummy engine was rigged upwith a Movan in the test laboratory in

preparation for the investigation. Twodraining series, each with 20 draining pro-cedures, were sufficient to enable the qual-ity of the system to be determined. In thesecond draining series, a 15-minute steadyoperating state was inserted between thedraining procedures. The aim of the seconddraining series was to verify whether an in-tercalated steady operating state affectsthe draining behaviour and how the resultsare statistically distributed.

The evaluation revealed that neither ofthe draining series showed any significantdifferences and the measured values werenormally distributed. To calculate the stan-dard deviation, therefore, all of the 38draining results could be combined.

Test steps with about four hours of oper-ation (depending on the cycle time) andfour repetitions were decided on, subject tothe following peripheral conditions: ■ Testing time of about 100 hours per testvariant■ Differences in oil consumption of lessthan 2g/h cannot be evaluated■ Running-in behaviour with respect to oilconsumption■ Four different test programmes■ Effect of the order of the test pro-grammes on the result

This arrangement could be chosen be-cause the draining quality for dynamicdraining was ± 7g with a confidence level of95 % and hence the required quality of themeasurement result of ±2 g/h could be at-tained at this level. With a view to optimis-ing the operating time only, dynamicdraining was inserted between the four dif-ferent operating programmes as a refer-ence point for determining the oil con-sumption. Figure 9 contains a graphic rep-resentation of the chosen programme flow.The first three steps may be able to supplyinformation on the running-in behaviour.The fourth step may provide informationon the effect of the order of the programmesteps, as the composition of the lube oil, es-pecially in gasoline engines, can be affectedto a greater or lesser extent by the operat-ing level or test programme (e.g. drag-in offuel).

5 Execution and Results of theChosen DoE

In order to ensure that the expected oil con-sumption from level 1 to level 2 has a suffi-ciently large range (difference betweenminimum and maximum sump oil con-sumption value), the DoE was begun withtests 0 and 8. The measured values for therespective runs and operating programmesare set out in the Table.

Viewing the results from the perspec-


27MTZ worldwide 6/2004 Volume 65

tive of total range, as well as range withinthe individual operating programmes andaccuracy, the following can be stated. Evenfor engine operating programme 1, the res-olution of ±2 g/h can, with reservations, beregarded as sufficiently good, especially ifthe changes over the operating time are al-so considered. Differentiation between theindividual parameters of the DoE, however,would be very difficult here. For the othertest programmes, the resolution is clearlynot a problem. The total range and therange of the individual results are so largethat a sufficiently large distinction be-tween the individual test results can be ex-pected and hence an evaluation of the indi-vidual parameters is certainly possible.

In order to underpin the quality of themeasurement results, the average of the oilconsumption values from the individualreference point measurements is comparedbelow with the results from the drainingprocedures. In line with the programmeflow chart in Figure 9, the average of the in-dividual reference point measurements hasbeen calculated for one run.

The results from the draining proce-dures are the sump oil consumption deter-mined in each case over a complete run,Figure 9.

A graphic comparison of the differenceresulting from the averages of the refer-ence point measurements and the sump oilconsumption values from the draining pro-cedures can be seen in Figure 10.

This comparison shows that the resultsof the reference point measurements agreevery well with the results from the drain-ing procedures. Unexpected results from in-dividual reference point measurements aretherefore not false measurements but aregenuine measurement results that requireinterpretation.

The systematic discrepancy between theaverages of the reference point measure-ments and the results from the drainingprocedures is not due to the measurementsystem. It is caused by differently acting re-distribution processes in the primary oil cir-cuit of the engine and by the oil condition-ing as a function of operating time. Refer-ence point measurement and completedraining are carried out at different operat-ing times, and therefore discrepancies canoccur. These complex aspects cannot be ex-plained in detail here. However, as the dis-crepancy is systematic, the evaluation ofthe oil consumption measurement resultsis not affected.

Figure 11 presents the results from thereference point measurements for two se-lected engine operating programmes.

Figure 12 shows the evaluation of theparameters of the DoE using the results

from Figure 11. From the evaluation, it is ap-parent that the effect of the parameters candiffer depending on the engine operatingprogramme.

6 Summary and Outlook

The Movan oil consumption measurementsystem described here is operated withgreat success in the engine test house ofFederal Mogul Burscheid GmbH for sumpoil consumption measurement even overrelatively short measurement times (about4 hours for normal passenger car engines).The equipment is capable of carrying outdraining procedures fully automaticallyand allows good comparability of the re-sults with other engine test laboratories.Furthermore, statistics on the draining be-haviour of the combined system of en-gine/measurement equipment can be sim-ply generated, thus allowing an estimationof the quality of the individual results to bepredicted. Consequently, it was possible to

minimise the operating times for individ-ual operating levels subject to the fringecondition of required accuracy. ■


5 Execution and Results of the Chosen DoE

Table: Summary of the measured values Run 0 and Run 8

4,8 h operating programme 1:

Test 1st Run 2nd Run 3rd Run 4th Run0 -1 g/h -4 g/h -6 g/h -6 g/h8 -7 g/h -9 g/h -6 g/h -10 g/h

difference(absolute)

0 - 8 6 g/h 5 g/h 0 g/h 4 g/h

4,0 h operating programme 2 :

Test 1st Run 2nd Run 3rd Run 4th Run0 69 g/h 77 g/h 64 g/h 58 g/h8 44 g/h 89 g/h 83 g/h 39 g/h


0 - 8 25 g/h 12 g/h 19 g/h 19




0 - 8 31 g/h 23 g/h 21 g/h 19 g/h




0 - 8 124 g/h 76 g/h 37 g/h 24 g/h

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Oil consumption measurement under DOE conditions