16 MTZ 11/2005 Volume 66

Authors:Ulrich Hess, Alexander Mitterer,Stephan Neugebauer, Peter Riegert and Gerald Seider

Das Wärmemanagement des neuen BMW-Reihensechszylindermotors

You will find the figures mentioned in this article in the German issue of MTZ 11/2005 beginning on page 872:

Heat Management of the New BMWIn-Line 6-Cylinder Engine

The cooling system must primarily ensure that the permitted limit temperatures are no exceeded.Furthermore, consumption-optimised engine warm-up and rapid climate control of the vehicle inte-rior must be assured. This article describes a new highly flexible cooling system for new in-line 6-cylinder engine, which matches these very different goals equally.

1 Introduction

The cooling system in a vehicle must meetthe most diverse requirements during oper-ation. Its primary task is to assure the per-missible limit temperatures for the engineand gearbox, while also taking into accountthe competing requirements of consump-tion-optimised engine warm-up and rapidclimate control of the vehicle interior.

These differing objectives can be met on-ly by a highly flexible, all-encompassing sys-tem, Figure 1. BMW has therefore realisedtechnological advances with the develop-ment of the cooling system of the new in-line, 6-cylinder engine. The central compo-nent is the electric coolant pump (EWP).Supported by other electrically operatedcomponents such as the map thermostat(KFT), electric fan and heating valves, a high-ly flexible system was created. Besides therigorous development work on the newcomponents, the new cooling system calledfor considerable feats of engineering in theoverall system, such as the consistent designlayout of the system based on the new mar-ginal conditions, creation of the overall heatmanagement logic, realisation of the systemlogic in the software structure of the enginecontrol and the targeted, electronic net-working between the control units for theengine, transmission and air conditioning.

2 System Layout

Conventional cooling systems are governedby the delivery characteristics of the me-chanical coolant pump. Due to the forcedcoupling with the engine speed, the so-called “hill-climbing values” at high engineload and low vehicle speed are the decisivefactor in the design layout of the coolantpump. The coolant pump must delivercoolant at an adequate volumetric flow rateeven at low engine speeds. At high enginespeeds, such mechanical systems produceextremely high volumetric flow rates thatare generally not required for the purpose ofcarrying off the amount of heat that occursunder these conditions.

The electric coolant pump (EWP) is notcoupled to the engine speed. It can thereforemake available the full delivery capacity atthe low engine speeds of the hill-climbingvalues or, in the extreme case, even whenthe engine is not operating. Consequently, itis now no longer the hill-climbing valuethat is the decisive factor for the design lay-out but rather the nominal power output ofthe engine. Since – as described above – a“surplus” of coolant is available at the pointof rated power output in the case of the me-chanical coolant pump, this pump can ex-ploit a great potential for reducing thepump or delivery capacity. BMW uses an

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electric coolant pump with an electric pow-er intake of only 200 Watt on the new in-line6-cylinder engine. This power intake is amultiple below the maximum drive powerof the mechanical pump on the predecessorengine, Figure 2.

In view of the distinctly reduced deliveryvolume and the low-pressure build-up of theelectric coolant pump, it was necessary to re-design the entire cooling system, includingthe inner-engine cooling. Initially, dethrot-tling of the water circuit to facilitate the useof the electric coolant pump assumed opti-mised hydraulic cross sections. For instance,the map thermostat was designed with com-paratively large valve plates. Nevertheless, itwas possible to achieve excellent controlcharacteristics by means of systematic de-tailed work while also reducing the pressurelosses in the vehicle radiator and in the en-tire hose system.

The component dethrottling measuresrepresent an important prerequisite for theuse of an electrically operated coolantpump. In addition, fundamental re-engi-neering of the circuit architecture is neces-sary for the purpose of securing the re-quired coolant capacity as the hydraulic bot-tleneck in the system, i.e. the basic engine,can be dethrottled only to a limited extent.For this reason, the secondary circuits of thecooling system such as the engine oil-to-wa-ter heat exchanger or the transmission oil-to-water heat exchanger were connected inparallel with respect to the basic engine. Asa result, the entire system is dethrottledwhile ensuring adequate media supply tothe secondary circuits.

In connection with efficient cooling airrouting, the specific optimisation of circuitcomponents ensures outstanding coolingcapacity values in all vehicle operatingranges.

3 Inner-engine Cooling

In addition to the reduced delivery capacityof the electric coolant pump compared to amechanical pump, the parallel connectionof the secondary circuits with respect to theengine contributes to a distinct reduction inthe available volumetric flow of coolantthrough the engine. The thermal design lay-out of the cylinder head therefore paid par-ticular attention to targeted cooling of thecombustion chamber roof and of the ex-haust valve webs for the purpose of achiev-ing nominal power output. As part of the se-lected cross-flow concept, the flow wastherefore determined separately for eachcylinder. The lower delivery capacity of theelectric coolant pump also means lower sys-

The electric coolant pump is thereforestationary during the initial phase of theNEFZ test, thus also representing a saving inthe corresponding drive power. The coolantin the engine is then heated, renderingpump operation necessary. In this process,heat is additionally given off via the coolantheat exchanger to the engine oil as well as tothe transmission fluid on automatic trans-mission vehicles.

The fuel potentials that can be realised bystationary coolant during the warm-upphase depend on the type of engine and ve-hicle and are in the range of 2 % of the testconsumption values.

5 Engine Temperature Control

Decoupling the coolant pump delivery fromthe engine speed raises the question of thedrive logic. BMW uses the fuel mass injectedinto the engine as the guide variable for theelectric coolant pump. This decision is basedon a robust computational model that deter-mines the amount of heat to be carried off,while taking into account the fuel consump-tion and the thermal balance of the engine.

The spread of the coolant temperaturebetween the engine inlet and engine outletis therefore derived from the volumetricflow of coolant and the current thermalload. The engine inlet temperature itself iscontrolled by the map thermostat.

Thus the resulting engine outlet temper-ature can be set for each specific operatingpoint by means of the map thermostat (KFT)and the volumetric flow of the electriccoolant pump (EWP). In addition to the cool-ing, fuel consumption and output objec-tives, the heat management of the newBMW 6-cylinder engine also takes into ac-count marginal conditions such as “heatingrequirement”, “knocking combustion” and“ambient temperature”.

In total, therefore, the coolant tempera-ture can be varied over a wide range. Conse-quently, BMW has divided the various statesof the cooling system into operating modes.Figure 5 shows an overview of the control op-erating modes and the associated engineoutlet temperatures. The highest tempera-tures are set in consumption-oriented mode,thus minimising fuel consumption. In thesubsequent operating modes, the coolanttemperatures are lowered by increasedcoolant flow (EWP) and reduced inlet tem-peratures (KFT). The lowest engine tempera-tures for optimum performance areachieved in the operating mode for maxi-mum dynamics.

Figure 6 shows the different coolant tem-peratures in the stationary engine charac-

tem pressure, resulting in the coolant reach-ing boiling point distinctly earlier. Conse-quently, the improved heat transfer in con-nection with nucleate boiling can be usedspecifically, particularly in areas that are dif-ficult to access while, on the other hand, therisk of film boiling and local componentoverheating increases.

With the aid of directly coupled flow andthermal conductivity calculations, it waspossible to identify the areas in the cylinderhead exposed to the risk of film boiling andoptimise them in terms of flow efficiency.The supportive cooling affect provided by lo-cal nucleate boiling was permitted andutilised. The drop in cooling capacity at lowoutside temperatures, where only a smallproportion of the volumetric flow of coolantpasses through the radiator and the re-minder is pumped via the bypass circuitwith its predominantly throttling effect, is acritical factor in the design layout of the in-ner-engine cooling. In this case, the lowestvolumetric coolant flow levels over the en-gine also occur even during operation atnominal power output. The consistent detailoptimisation of the engine-internal flowand of the bypass circuit with the aid of 3-di-mensional flow models provided the neces-sary dethrottling, Figure 3.

4 Consumption-optimised Warm-up

The aim is to achieve rapid heat-up of the en-gine oil in order to facilitate engine warm-up with favourable fuel consumption levels.This mainly involves reducing friction withthe higher wall temperatures promotingthe mixture formation and combustionprocess. The warm-up phase is of particularsignificance in terms of the fuel consump-tion values in the NEFZ test cycle, as this testis started with a preconditioned, cold vehi-cle.

The physical limit potential that can beachieved maximally through optimum heatmanagement in the NEFZ cycle is derived bycomparing test runs with and withoutwarm-up.

A part of this limit potential is utilised bythe fact that the coolant is stationary duringthe warm-up phase of the new BMW in-line,6-cylinder engine. This ensures that no heatis prematurely carried away from the en-gine. In addition, there is a delay in thetransfer of frictional and combustion heatfrom the basic engine structure to thecoolant as heat transfer coefficients are min-imised in stationary media. Figure 4 showsthe comparison of the engine oil tempera-tures in the NEFZ test cycle as a function ofcoolant delivery.

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teristic map. The consistent use of electrical-ly operated components such as the electriccoolant pump, map thermostat and electricfan facilitates rapid temperature change,thus supporting the dynamic propertiescharacteristic for BMW.

6 Special Operating Modes

Besides the described control modes for theheated vehicle, the cooling system of thenew BMW engine also features operatingmodes for special vehicle situations.

Particular significance is attached towarm-up mode in terms of fuel consump-tion optimisation. As already described, theelectric pump is stationary until the coolantin the cylinder head has reached the open-ing temperature of the main thermostat.Start-up of the electric coolant pump is con-trolled by means of a special warm-up mod-el in the engine electronics.

Further operating modes relevant tocomfort are triggered based on the require-ments of the air conditioning control unit.For example, if the required heating capaci-ty in the vehicle interior is particularly highdue to large deviations between the targettemperature and actual temperature (coldstart in winter), the heat management initi-ates an increase in the available heat givenoff by the engine by way of applicative inter-ventions in the combustion control. In addi-tion, the coolant delivery of the electriccoolant pump is greatly increased in orderto supply the heating system optimally withheat.

Further functions include the residualheat function (interior comfort when the en-gine is stationary) or afterrunning for cool-ing the hot engine after it has been turnedoff.

7 Effects in Full Load Operation

Efficient dynamics characteristic for BMWwas, of course, the focal point of the devel-opment of the new in-line, 6-cylinder en-gine, with the heat management and elec-tric coolant pump playing an innovativerole. The lower power intake of the electriccoolant pump is of obvious advantage.Through this measure alone, the availabledrive power output of the vehicle is in-creased by up to 2 kW at high engine speeds.However, distinct advantages compared tomechanical systems are also gained in thepartial load range as the delivery of such sys-tems – as described above – is proportionalto the engine speed and therefore is exces-sively high in the partial load range.

Flexible heat management also provides

the option of increasing the compression ra-tio in the engine. The engine knocking thatoccurs when starting off at high outdoortemperatures or in connection with un-favourable fuel qualities normally repre-sents a limiting criterion in terms of thecompression layout. With the heat manage-ment of the new BMW engine, the coolanttemperature is substantially reduced undercritical conditions. The engine compressioncan now be fundamentally increased, mak-ing it possible to exploit the associated fuelconsumption potentials.

8 Engine Control Structures

The classic design layout of the cooling sys-tem primarily meets the requirements ofthe combustion engine. However, modernheat management must also take into ac-count the diverse requirements of differentvehicle systems. Among other things, thismeans taking into consideration the air con-ditioning system, temperature managementof the automatic transmission, drive of theair control flaps, power steering cooling andthe permissible limit temperatures in theengine compartment.

The necessary software structures in theengine control system were therefore com-pletely redesigned and organised into mod-ular and hierarchical structures. The basicidea is founded on the classification accord-ing to the operating modes as specified inthe above. The currently required operatingmode is defined by the so-called operatingmode manager, corresponding to the respec-tive driving situation. The manager facili-tates hierarchical selection as well as transi-tion to a new operating mode in line withchanged conditions (e.g. driver accelerates),Figure 7.

Within an operating mode, the electriccomponents are actuated based on modelsand characteristic maps.

Predominantly physical target variablesare used as the basis for calculation. This canbe well illustrated by way of example of theelectric fan, Figure 8. While a concrete en-gine speed is applied for the electric fan inconventional engine control systems, theBMW heat management coordinator oper-ates with the necessary air mass, thus mak-ing it possible to take into considerationmarginal conditions such as ambient tem-perature, driving speed and radiator charac-teristic map.

By consistently implementing this logicit is possible to achieve a robust data applica-tion of the engine control system, which islargely independent of the componentsused. Initially, when replacing a component

(e.g. modified electric fan) only the newphysical characteristic of the fan needs to bedefined in the engine control. New applica-tion of the previous electric fan speeds re-quiring involved testing and trials can nowbe replaced by a specific check.

The heat management coordinator isconsistently implemented in all new enginecontrol systems at BMW. Thanks to the flex-ible, modular structure, this is possible irre-spective of the specific components in thecooling system. Since the engine control sys-tems are provided by different suppliers cor-responding to specific projects, BMW has de-veloped an option for using a “softwareadapter” for the purpose of integrating theBMW-specific software in all engine controlsystems, Figure 9.

9 The Electric Coolant Pump

Undoubtedly, the central component in thecooling system of the new BMW 6-cylinderengine is the electric coolant pump (EWP),developed in cooperation with the companyPierburg.

The specific requirements such as noleaks, low weight, sturdy construction andhigh efficiency are met by the modular de-sign of the pump based on the wet-rotorprinciple with integrated electronics. Figure10 shows the three modules: hydraulics, ECmotor and electronics.

In view of the arrangement of the electriccoolant pump on the basic engine, the devel-opment of the electronics had to take intoaccount high temperatures and the me-chanical loads. Furthermore, the controland diagnostic functions as well as the self-protection functions (e.g. overvoltage) are in-tegrated in the pump electronics. The pumpelectronics also features a series of safetyfunctions that ensure the highest possiblecooling capacity even in the event of systemmalfunctions.

10 Costs

The 6-cylinder engine is the highest volumedrive unit at BMW. Therefore, particular at-tention must be paid to cost aspects. On noaccount should the component price of anelectric coolant pump be compared with theprice of a mechanical coolant pump. Con-clusive values can only be obtained from therespective system costs which must beviewed based on the specific system proper-ties (fuel consumption, power output, com-fort, package). In the overall consideration,there are clear advantages for BMW offeredby the electric coolant pump system.

The result of this overall consideration is

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greatly influenced by the type of vehicle andengine used as the basis so that the decisionin favour of an electric coolant pump systemcan be made only in specific terms.

11 Summary

As the above considerations illustrate, thedevelopment of the heat management sys-tem on the new BMW 6-cylinder engine ex-tended far beyond the traditional design lay-out of the cooling system. The electriccoolant pump represents great advances inthe cooling system. New interfaces withhigh complexity were created for the pur-pose of realising the heat management sys-tem in the overall vehicle.

The need for an interdisciplinary, system-oriented functional principle represents achallenge that should not be underestimat-ed. The reward is the creation of a highlyflexible system that satisfies the diverse re-quirements of modern heat management.This was achieved by the detailed design lay-out of the overall system and the specific in-teraction of all components in one struc-tured logic system.

Simply attempting to replace the me-chanical coolant pump by an electrically op-erated pump in an existing cooling system isdoomed to failure, as it can realise neitherthe cooling capacity nor the advantages of amodern heat management system. The de-scribed advantages in terms of fuel con-sumption, dynamics and comfort areachieved only by targeted development ofthe overall system while ensuring effectivecomponent cooling.

References[1] Landerl, C.; Klüting, M.: Der neue Reihensechszylin-

der-Ottomotor von BMW. Teil 1: Konzept und kon-struktiver Aufbau. In: MTZ 65 (2004), Nr. 11

[2] Klauer, N.; Kiefer, W.; Krauss, M.; Mährle, W.;Schünemann, E.: Der neue Reihensechszylinder-Otto-motor von BMW. Teil 2: Thermodynamik und funk-tionale Eigenschaften. In: MTZ 65 (2004), Nr. 12