Advanced Thermal Management of Diesel Engines

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Advanced Thermal Managementof Diesel Engines
The potential of thermal management with respect to CO2 reduction is given by
faster warm-up of engine and drivetrain, reduced losses from water pump and fan
and finally the operation of the engine in an optimal temperature range. In a new
approach, BorgWarner applies a variable coolant pump and a fully controlled
coolant valve to a conventional cooling system. Both components, as well as theviscous fan clutch, are controlled by a newly developed controls approach.
By Dr. Wolfgang Wenzel, Program Manager for PassCar Systems, BorgWarner,John Shutty, Chief Engineer of Advanced Controls, BorgWarner, andDr. Michael Becker, Director Advanced R&D, BorgWarner

Motivation

Driven by requests for higher engine efficiency,
thermal management for combustion engine
driven vehicles has received increased attention [1 ]. Standard coolant pumps which are linked to the engine speed provide too high flowsof coolant in some driving conditions. Radiatorflow control with wax thermostats is inflexible
and relatively slow. Variable devices with aproper control can reduce engine warm up times and limit temperature fluctuations in transient conditions.
In a new approach, the combination of a dual
mode coolant pump (DMCP) that can be driven
by an integrated electric motor or the engine
belt and a fully controlled coolant valve which
replaces the wax thermostat, is applied to aconventional cooling system. The newly developed thermal controller activates both components in addition to the viscous fan clutch. To
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support the controller development a detailed
simulation model of the complete test vehicle
was created and accurately calibrated. The development was carried out via simultaneous simulation and testing. The performance wasanalysed in test cycles and the thermal management approach was refined iteratively.

Thermal System Setup

The selected vehicle is a Mercedes-Benz
Sprinter model certified for sale in the U.S. Thevehicle is equipped with a 3.0-l-six-cylinder
Diesel engine (type OM642). The after-treatment system is designed to achieve USEPA/CARB 201 0 emission limits. The seriesproduction vehicle provides a conventionalcooling system with wax thermostat.
The cooling system was modified, for example,
with a DMCP. This pump was described in detail in the MTZ, [2], and is currently in series
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Mercedes-Benz Sprinter in climatic test bench, OM642 cooling systemdevelopment. It replaced the mechanical coolant pump as a drop as a drop-in replacement
and is connected to the accessory drive belt inthe same manner as the stock coolant pump.
The interface to the pump was realised via
CAN to control both electrical pump speed andclutch position. The pump offers a high degreeof freedom in operation: It can stop, run fully
flexible in the range between minimum electricspeed (due to sensor less control) and maximum speed which depends on electric motor
size. The applied pump provides an electricrange between 500 and 2500 rpm which is well
suited for the coolant flow requirements in lowand mid-engine loads.
A prototype coolant control valve was used inplace of the stock wax thermostat. The poppet
style valve was combined with a brushed DC
motor as an actuator. The valve could be controlled to any opening regardless of coolanttemperature.
The engine driven fan with viscous clutch ispart of the original equipment. The fan speed isadjusted to maintain optimal engine temperatures and to minimise parasitic losses. Due tothe characteristics of the viscous clutch, it isspinning with a minimum speed when the engine is running. The top speed is limited by theBorgWarner Knowledge Library 201 4

pulley speed. For the majority of cycle testing,the fan remained at its minimum speed.
To optimally control these devices, the controller has to achieve three main goals: keep theengine in a temperature range that friction isminimized, protect the engine from overheating
and create minimum auxiliary losses.

Simulation

A full vehicle simulation was created to support
the development of the thermal management
strategy [3]. The model was linked to a Simulink based controller to be able to use the same
control software for both, the simulation andthe vehicle testing. This reduced the development effort and improved the correlation between the model and the physical tests. The
simulation was advantageous in different respects:• The simulation with the calibrated model provides a repeatable platform which is able toevaluate small improvements and compare different configurations before experimental analysis is started.
• The simulation allows observing parametersthat were not measureable on the vehicle (forexample FMEP, heat transfer rates).
• It reduces development time and effort andthus cost.
2resistance, tire size, axle and gear ratio andaerodynamic drag. In addition, an algorithm
which approximated the shifting strategy wasdeveloped; although for most cycles, a map ofthe actual time based shift points was used toimprove repeatability. For the cycle simulation,
a 1 D front end sub model was derived from arough 3D discretisation of the front end geometry to reduce simulation time.

The host environment, in which the simulation
ran, was Simulink with the model running as anS-function. Descriptions of the major elementsof the simulation are given below.

The engine model was created based on engine parameters and calibrated with measured
engine data. The 1 D combustion model wasconverted to a mean value model using aneural network in order to significantly reducethe computing time. A comparison between themean value model and the fuel combustion
model showed very good correlation over theengine operating conditions studied. Enginefuelling, boost, EGR and bypass control wereimplemented in in the simulation model.

The vehicle sub model includes weight, rolling

The cooling circuit was modelled in 1 D with

Dual mode coolant pump (DMCP)

Model Structure

Simulation block diagram
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Simulation (dotted curves) vs. vehicle measurement (solid curves), comparison of coolant
temperatures in FTP75 test

coolant pump, thermostat or controlled coolant
valve and the fan with viscous clutch. Thermal
inertia and heat transfers are modelled for theengine block, the transmission, rear axle housing and the complete exhaust line including
the after-treatment system. Thermal efficiency
effects were based on local temperatures suchas block, cylinder wall oil temperature, crankshaft oil temperature, sump oil temperature,transmission oil temperature, axle temperatureand coolant temperature.

Model Validation

The model was compared to vehicle data atsteady state conditions for correlation and during various drive cycles to insure a good transient performance match. For the matching
shown below, the DMCP was operated in aclutched state so that it followed the engine
speed with a fixed gear ratio (mechanical mode). The standard wax thermostat was used forBorgWarner Knowledge Library 201 4

the initial validation tests. The example given
here shows key thermal parameters during aFTP cycle. The dashed lines represent the simulation results while the solid lines represent
the vehicle testing data. Generally, the model issufficiently accurate to use the simulation as atool for controller development and optimization.

Thermal Controller

In order to run the simulation and vehicle
through transient cycles, a model based thermal controller was developed. The controller
was developed using Simulink and was used inboth the PC based simulation environment aswell as within a rapid prototyping ECU on thevehicle.
While preventing engine overheating is still apriority of the cooling system design, the project objective was more focused on how to4

Performance of the thermal management controller, NEDC test results for coolant temperatures,
pump speed and coolant valve position

achieve optimal temperatures with the minimum parasitic losses from the thermal system(that is pump power, fan power). In general,these two objectives could be achieved simultaneously; however, when in conflict, maintaining optimal temperatures had a much bigger
effect on efficiency than reducing auxiliary losses. In addition to the above stated objectives,
ensuring engine protection drove constraints
on system operation. Due to the localised heat
input in the engine and in other heat sources(for example EGR cooler), compromises had tobe found to achieve both objectives. The system needs to heat up quickly but cannot tolerate local hot spots or large temperature
gradients across components.
In the recent development work, the control of
the DMCP speed and clutch state was of primary interest. Of further interest was the impact of adding a fully controllable thermostat(coolant control valve). Also the fan speed was
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actively controlled, but in practice, the controller strove to keep the fan at its minimum speed
while controlling temperatures with the coolant
pump and thermostat. A real-time capable model of the thermal system was also embeddedin the controller. Values which would normallynot be available to the controller in a production ECU were taken from the model and used
for control.
Here, an example shows the controller performance in a NEDC cycle. The pump stop at thebeginning for faster warm-up ends after 1 30 s
to avoid overheating of cylinder head andEGR-cooler. The DMCP can be kept at the minimum speed until 1 035 s after the start of thetest. Only in the extra-urban part of the cycle,
the pump speed is increased to maintain EGR
cooler outlet temperatures; the controlled coolant valve had opened at 1 000 s after test start.
After 11 25 s the pump switches to mechanicalmode since this is the most efficient operating
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DMCP: mechanical versus electric mode, fuel economy results in NEDC

mode at this point. The fan does not need to be
switched on at all during the complete cycle.
For more details and further results refer to [4].

Vehicle Testing

The thermal controller was implemented in aprototyping unit. The DMCP contained its own
electric clutch and brushless motor controller.
Commands for CCV position, pump speed andclutch state were handled via CAN interface.
Power to drive the pump, fan and CCV weretaken from the vehicle electrical system while
power for DAQ and controller logic was takenfrom an auxiliary power source (to allow a goodcomparison). The vehicle was tested on a climatic test chamber and different test cycles
were run (NEDC, US07, FTP75).
The fuel consumption benefit found in theNEDC was 2.1 % and is linked mainly to twoBorgWarner Knowledge Library 201 4

different characteristics: zero or low flow in theengine leads to quicker local heating up, which
reduces friction and improves combustion conditions. Secondly, the controlled pump speed
leads to lower auxiliary losses. The controlled
coolant valve does not provide a significant fuelconsumption reduction in this setup. Its application only seems to make sense in conjunction with additional technologies for fasterengine warm up.

Conclusions

A thermal management development approachand its application to a modern diesel powered
vehicle have been presented. The vehicle wasequipped with a DMCP, a variable coolant valve and an advanced controls approach.
Thermal performance along with associatedfuel economy improvements were shown for
the NEDC test cycle. The DMCP offered signi6

ficant fuel consumption benefits. The controlled
coolant valve did not provide fuel consumption
benefits in the investigated environment. However, the valve was an integral part of a moreadvanced concept investigated in the next project phase, where the cooling system layout
was modified further and the vehicle wasequipped with a heat exchanger for waste heatrecovery.

References[1 ] Chalgren, R.,: Thermal Comfort and Engine
Warm-Up Optimization of a Low-Flow Advanced Thermal Management System; SAE Technical Paper 2004-01 -0047[2] Keller, P.; Wenzel, W.; Becker, M.; Roby, J.:
Hybrid coolant Pump with Electric and Mechanical Drive; Motortechnische Zeitschrift (MTZ),
volume 71 , issue 11 , Wiesbaden 201 0[3] Keller, P.; Wenzel, W.; Becker, M.; Roby, J.:
Coupled Thermal-Engine Simulation for a Light
Duty Application; SAE World Congress 201 0;
SAE Technical paper 201 0-01 -0806[4] Shutty, J.; Wenzel, W.; Becker, M.: Advanced Thermal Management for a light duty Diesel Vehicle; to be published at the SAE World
Congress 201 3Contact

Email: technology@borgwarner.com
For more information, please visit
borgwarner.com
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