28 ATZ 9/2001 Jahrgang 103

Conserving resources and reducing emissions are major goalsin all new Behr products. Three development projects, each ofwhich makes a considerable contribution to these goals, arepresented below. These are: indirect charge-air cooling, exhaustgas cooling for direct injection internal combustion engines andvehicle thermal management.

By Matthias Banzhaf,

Daniel Hendrix,

Michael Schmidt and

Eberhard Pantow

Ökologische Fortschritte

bei der Motorkühlung

You will find the figures mentioned in this article in the German issue of ATZ 9/2001 beginning on page 808.

Ecological Advances in Engine Cooling

29ATZ 9/2001 Jahrgang 103

1 Indirect Charge-Air Cooling

With indirect charge-air cooling, the chargeair is not cooled with air, as is usually thecase, but by using normal engine coolantsin a low temperature circuit. The charge aircan thus be cooled to the same or lowertemperature than with current air cooledsystems, but with a significantly smallerdrop in pressure of the charge air. Thismeans that there is a larger massd of airavailable for combustion in the engine.This primarily performance-enhancing ef-fect can then be used to reduce fuel con-sumption and emissions, since a standardperformance can be obtained using a small-er, more economical engine. The benefit ofindirect charge-air cooling is 40 to 65% lesspressure loss.

Any pressure loss reduces the mass of airintroduced into the cylinder, which resultsin a corresponding reduction in engine per-formance. The lower pressure loss encoun-tered with indirect charge-air cooling re-sults from:■ a charge-air cooler with a better flowcharacteristics, which is possible becauseof more efficient heat transfer by thecoolant, as well as ■ significantly shorter charge-air ducts,since the charge-air cooler can be locatednear the engine.The charge-air cooler operates either with aseparate cooling circuit or one which is partof the main cooling circuit. With an inte-grated circuit, part of the coolant is divertedfrom the engine cooling circuit, and, aftercooling the charge air, returns to it. In bothcases, a small amount of coolant (about 5 to10 % of the througput of the main circuit) iscooled to a temperature close to the ambi-ent air temperature, in order to cool thecharge air down to its low target tempera-ture.

By means of the coolant throughput, it is even possible to warm up the charge airin a cold engine. The high HC emissions as-sociated with cold starts can thus be re-duced.

2 Exhaust Gas Cooler forDirect-Injection Engines

In the cylinder of direct-injection internalcombustion engines, a leaner atmosphereis formed so that the engine is actually op-erated with excess air. However, in a con-ventional 3-way catalytic converter, theNOx emissions can in this case no longer beconverted.

A new technology now promises tosolve this problem: the NOx trap catalyticconverter in conjunction with an exhaustgas cooler. The NOx trap catalytic converter

stores the nitric oxide, and when it is full,the engine management system switchesbriefly to higher fuel consumption mode.The nitric oxide can then be renderedharmless in the 3-way catalytic converter.The engine can then return to fuel-econom-ical lean combustion mode and the storageof NOx starts all over again.

The exhaust gas cooler is required tomaintain the NOx trap catalytic converterwithin the temperature range in which ahigh NOx conversion rate can be achieved,i.e. between 250 and 450°C. The coolant-cooled exhaust gas coolers developed byBehr can be used for this purpose.

The benefits of exhaust gas cooling are: ■ extension of the lean operating range ofthe engine■ in operating conditions with a high ex-haust gas temperature, e.g. at high speeds(on the open road) and uphill under load,the exhaust gas temperature will be main-tained within the range required for highNOx conversion rates ■ the transfer of exhaust heat to thecoolant allows the engine to reach its oper-ating temperature more quickly than be-fore on start up and to switch over to leanoperation sooner.The NOx trap catalytic converter achieveshigh NOx conversion rates only within thetemperature range referred to above. Thus,excessively high and excessively low tem-peratures must be avoided. To maintain thetemperature within this range, an exhaustgas cooler with a bypass, including a valve,must be fitted in the exhaust line ahead ofthe catalytic converter. This bypass allowsinfinitely variable adjustment of the ex-haust temperature ahead of the storagecatalytic converter.

3 Thermal Management

The aim of the thermal management sys-tem is to achieve optimal engine coolingand air conditioning of the car. This is onlypossible if both aspects, cooling and air con-ditioning, and their interaction are consid-ered together. A further important goal ofthermal management is reducing fuel con-sumption and emissions.

Reductions in fuel consumption of be-tween 3% and 5% are achieved by:■ an increase in engine efficiency■ a reduction in engine warm-up time■ a reduction in the energy requirementof auxiliaries ■ a reduction in periods of critical temper-ature operation for engines.The reduction in fuel consumption wasmeasured on a thermal management testvehicle equipped by Behr. The following re-sults were observed.

3.1 Improved CoolingPerformanceThe test vehicle was equipped with acoolant pump operating independently ofengine speed. It enables an increase in radi-ator performance, particularly for uphilland stop-and-go driving. As a result, the en-gine reaches critical temperature rangesless frequently. In addition to lower ther-mal stress on parts, the generation of NOxemissions, which tends to increase at hightemperatures, is inhibited. As a result, theengine can be driven in more favourable ef-ficiency ranges.

3.2 Shorter Warm-Up TimesThis reduction is achieved by restrictingheat losses: a tight-fitting thermostat pre-vents leaks via the radiator, a coolant pumpcontrol limits the volume of coolant in cir-culation, thus reducing heat loss from theengine, and the radiator shutter protectsthe coolant from heat loss, something thatis particularly important when externaltemperatures are low.

With these restrictions placed on heatloss, the engine of the test vehicle reachedoperating temperature much more quicklythan with the engine in normal condition.HC emissions generated by incomplete fuelcombustion in cold engines were substan-tially reduced. The faster attainment of theoperating temperature also resulted inhigher engine efficiency, as less energy waslost by the release of heat to the outside. Fi-nally, the engine oil, and in particular thetransmission oil, reached their optimumoperating temperature more quickly, thusresulting in substantially lower frictionlosses and fuel consumption than would bethe case in a cold powertrain.

3.3 Lower EnergyRequirements for AuxiliariesBy means of a control system linking theradiator fan and compressor, it is possibleto determine the optimum control pointsfor both components in terms of fuel con-sumption, without compromising enginecooling performance. The controllablecoolant pump also contributes to a reduc-tion in fuel consumption, for instance bylowering the flow of coolant at high enginespeed. ■

SUPPLEMENTThermomanagement at Behr