Cooling Systems for Future Powertrains
Rainer Lutz, Matthias Jung, Laurent Art
MAHLE Behr GmbH & Co. KG, Stuttgart

Abstract. Beside optimized diesel powertrains new powertrain options, such as
hybrid electric, battery electric or fuel cell electric, will be required to comply
with CO2 regulations and to reduce the carbon footprint of the transport sector.
Each of these powertrain options has special requirements to its cooling systemin terms of heat rejection, target temperatures of the components involved andrequired coolant properties. Therefore, the solution for each powertrain looks different and has its own challenges. This significantly impacts the cooling systemlayout, the size of the heat exchangers and the fan power required to get the cooling task done.
This paper will describe the cooling requirements for optimized diesel, HEV,BEV and FCV powertrains and highlight the special challenges for each of them.
Dedicated cooling system layouts will be presented, and the main thermal management components discussed.
Keywords: electric powertrain, cooling system, truck.

1

Introduction

In the last decades, heavy duty trucks relied on powertrains with internal combustion
engines. With increasing awareness of climate change the greenhouse gas emissions ofcombustion engines including CO2 need to be reduced. Legislators worldwide have enacted regulations enforcing a significant reduction of greenhouse gases. These targets
are difficult to achieve with optimized combustion engines only. Therefore, alternative
powertrains options get into focus. These options are:
 Hybrid powertrains (HEV) Battery electric powertrains (BEV) Fuel cell powertrains (FCV)
All these new options have their individual needs for thermal management. They differ
significantly in their temperature targets, the amount of heat which needs to be rejected
and the requirements for coolant qualities. All this has a significant impact on the cooling system layout, the hardware used and the controls.
The temperature targets depend on the components used in the individual system. Figure 1 illustrates the range of acceptable temperature for ICE’s, fuel cells, electronics,
batteries and cabins. The combination of heat rejection and temperature target defines
the stringency of the cooling task.

2Fig. 1. Typical acceptable temperature range of powertrain components

In the last years many medium and heavy-duty trucks with electric powertrains had
been presented to the public. When looking at their specifications you can see a significant spread in electric motor power as well of installed battery capacities (Figure 2).

Fig. 2. Battery capacity and electric motor power of published electrified medium and heavyduty trucks.

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2

Powertrain Definition and Assessment for Cooling System
Heat Loads

The different powertrain options are compared at a constant power to the road of 320
kW, which represents the high end of heavy-duty regional delivery and the low end ofheavy-duty long-haul applications. Table 1 gives an overview of the powertrains assumed for this comparison.

ICE
mHEV
PHEV
BEV
FCV

ICE
350 kW
350 kW
350 kW---

E-motor-24 kW
130 kW
350 kW
350 kW

Fuel cell----300 kW

Battery-2 kWh100 kWh400 kWh80 kWhTable 1. Specifications of the different powertrain concepts as used for the cooling system assessment in this paper

Based on these assumptions, an energy balance tool was used to derive the heat rejections of each option. Using the energy balance of the total powertrain is important, as
due to the efficiencies of each component involved the amount of energy changes fromthe energy source to the wheel. As an example, Figure 3 shows the energy flow fromthe battery to the wheel of the BEV concept at PWheel=320 kW. The decreasing energy
flow from the battery to the wheel needs to be considered when calculating the heatrejection of each component. The diagram also illustrates why high efficiencies areimportant and have a high impact on the heat rejection. For example, if the efficiencyof the traction inverter changes from 95 to 96%, the heat rejection of this component
decreases by 20%.

Fig. 3. Example of the energy flow of a BEV powertrain at p Wheel=320 kW

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Beside the heat rejection, each component has its temperature targets as shown in fig.1.
When looking at powertrains at peak power and maximum ambient temperatures (inthis case Tamb=35°C), all heat sources can be allocated to 3 different cooling circuits: High temperature circuit: Tcoolant >60°C
 Low Temperature circuit: Tamb<Tcoolant<60°C
 Refrigerant circuit: Tcoolant<TambSometimes additional heat sinks are used, e.g., the battery may use an additional radiator to get cooled at low and moderate ambient temperatures directly without the detour
via the refrigerant circuit.
Figure 4 shows the heat rejection for the investigated powertrain concepts, based on theenergy flow analysis. The numbers shown reflect steady state conditions. In case of therefrigerant circuit the number represents the maintaining of cabin climate at Tamb=35°C
in addition to the battery cooling needs. The component efficiencies used for the assessment reflect end of life conditions and are therefore a conservative approach.
The different powertrains show very different heat rejection patterns. With an increasing level of electrification, the amount of heat to the low temperature circuit and therefrigerant circuit gets higher.
For the hybrid powertrains, it’s assumed that during ICE operation the electric motor isonly used for boosting and recuperation. Therefore, when the vehicle is driving constantly uphill at peak power, there is no additional heat from the electric part of thepowertrain and the heat rejection is the same as for the ICE. Pure electric operation is
only foreseen for the PHEV concept when the ICE is off, e.g., driving in environmental
zones. The heat rejection of this operation mode is also shown in Figure 4.

Fig. 4. Heat rejection to the cooling circuits of the investigated powertrain concepts at peak power
and Tamb=35°C

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The fuel cell powertrain differs from the other powertrains by its very high heat rejection to the high temperature circuit. Figure 5 compares the energy balance of a fuel cell
powertrain with a conventional diesel engine. This rejects almost 50% of its losses via
exhaust gas or radiation and convection. As the fuel cell works on a significantly lower
temperature level, the heat flux via exhaust is very small. So almost the complete heatlosses must be rejected via the high temperature coolant loop. Due to some additional
fuel cell ancillaries like the fuel cell DC/DC-converter or the fuel cell cathode air compressor, the heat rejection to the low temperature circuit is higher as of the BEV concept, too.

Fig. 5. Comparison of the energy balance of a diesel ICE and a fuel cell powertrain at road loadconditions.

The required cooling air flow rates for each powertrain option can be estimated by
combining the total heat rejection rates and the maximum temperature targets of each
investigated powertrain option. The exact numbers depend on various parameters, suchas exact size and design of the radiators, packaging space, component efficiencies, etc.
The result of can be seen in Figure 6.

Fig. 6. Relative heat rejection and cooling air flow requirement at peak power and Tamb=35°C

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Cooling System Layouts

The cooling circuits shared in this chapter are generic circuits, which are supposed toillustrate the special needs of each powertrain option. The actual cooling system for a
dedicated vehicle may differ in many aspects due to components used, performancelevel, packaging situation, etc. So, the cooling system layout is always an individual
answer on the actual needs of each application.
3.1

Internal Combustion Engines

Internal combustion engines are still the dominant power source for heavy duty trucks
and will stay in this position at least until end of this decade.
The predominant cooling system architecture for HD trucks has not significantly
changed since the introduction of charge air cooling in the 1980’s. Figure 7 shows ageneric cooling system layout as used today for ICE powertrains. The cooling module
is installed in the front end of the truck and includes a HVAC condenser, charge aircooler, radiator and fan. The fan is driven directly by the combustion engine via a fully
modulated Visco clutch for fan speed control.
With increasing stringency of emission legislations worldwide, EGR coolers started to
become part of the trucks cooling systems in 2000 [2].
With the introduction of Euro V, some engine platforms started to use indirect charge
air cooling. In this system the charge air cooler in the front end is replaced by a lowtemperature radiator and the charge air gets cooled by low temperature coolant. Despite
good performance and fuel consumption benefits of such systems [3], complexity andcost prevented indirect cooling system from becoming mainstream. Today they are onlyused on some high performance, two-stage turbo charged applications.

Fig. 7. Generic cooling system layout for internal combustion powertrains, MAHLE Euro VI
cooling module

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To cope with upcoming emission and fuel consumption regulation the ICE will be further optimized. Nevertheless, the basic cooling system architecture is not expected tochange. The focus will be on optimized components like highly efficient charge aircoolers and fans. To reduce cold start emissions, an EGR cooler bypass may be introduced.
3.2

Hybrid Powertrains

A hybrid powertrain combines the traditional diesel engine with an electric motor anda battery system. The battery delivers energy for operation of the electric motor andstores energy generated by the motor during braking. In addition, ancillaries likepumps, fans, HVAC compressors, ... may be electrified, depending on the level ofhybridization. The variety of hybrid powertrain is quite huge. It ranges from so called
mild hybrid systems to solutions that allow the vehicle to operate purely electric over alimited range.
48 V mild hybrid powertrain (mHEV).
The investigated mild hybrid entry system consists out of the 48 V motor, the battery
and a DC/DC converter, connecting the 48 V with the 24 V voltage level of the vehicle.
To keep the complexity of the system low, all components share a common low temperature cooling circuit (Figure 8). The total heat rejection of this circuit is 5,5 kW andthe target for the cooling temperature at the battery inlet is 5K above ambient. Thisrequires a radiator size of 25 dm² which can be placed either on the side of the vehicle
or in the front end. Depending on the ram air support at the actual location, the radiator
may need a fan up to 300 W. The coolant pump power is less than 50 W.

Fig. 8. Cooling system of an entry-level mild hybrid powertrain

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In case the system power level gets increased by a stronger motor or by additional ancillaries, the coolant cooling of the battery may not be sufficient anymore. In this casean optional cooling of the battery with a chiller can be added.
Plug-in-Hybrid with pure electric driving capabilities (PHEV).
Pure electric driving capabilities may become mandatory in environmentally sensitive
urban areas in the near future. A hybrid powertrain with pure electric driving capabilities may be a good combination to operate emission free in the city as well as going
long distance with the diesel engine. This requires a powerful electric motor to conquer
smaller hills and get satisfying accelerations at traffic lights, but there might be some
limitations.
The battery size should be big enough to guarantee at least 50 km electric range. In
addition, many ancillaries need to be electrified to guarantee a safe electric drive (airbrake compressor, power steering pump, …).
The heat rejection of the electric drive components will hardly allow a front-end integration of the low temperature without compromising charge air cooling quality. That’s
why a remote module on the side of the vehicle is more favorable (Figure 9). This
module contains two radiators: The first radiator is used for battery cooling at coolambient conditions up to 15 to 20°C and the second radiator of the same size is used
for electronics, e-motor and ancillaries. This split allows higher operating temperatures
for the electronics loop, which supports heat transfer significantly. The temperature
difference between battery inlet temperature and ambient can also be increased compared with mHEV system as the chiller will take over battery cooling in case battery
inlet temperature gets too high.

Fig. 9. Cooling system layout for a full hybrid powertrain

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3.3

Battery Electric Powertrains

A typical layout for battery electric trucks is shown in Figure 10. It includes two coolant
and one refrigerant circuit. The oil cooling of the traction motor is optional and depends
on the type of traction motor chosen.
Like for the hybrid system, all electronics and traction components are allocated in themain low temperature circuit which guarantees coolant temperatures below 60°C. The
battery has its own coolant circuit, which either can be cooled by a dedicated radiator
or by a chiller. Both radiators are located in the front of the truck to use ram air forcooling and to minimize fan usage.
As the coolant temperature in the LT circuit is too low for cabin heating an additional
high voltage heater is required. It can either be installed in the coolant circuit of theheater core or on the air side in the HVAC module. The air side integration is more
efficient than the coolant side integration but requires changes on the HVAC module.
This may be difficult if the same module is used as platform HVAC module for ICE
and BEV applications.

Fig. 10. Generic cooling system layout for a battery electric truck

Refrigerant Circuit Layout.
The layout shows a common refrigerant circuit for HVAC and battery cooling. Especially for high performance applications, the range from low to high heat loads may
exceed the capabilities of some component as well as the limits of operation and controls of the system. As the vehicle must cover low load conditions like hotel modes in
resting situations as peak power operation at hot ambient conditions the spread of therequirements for the different components might be too large. In these cases, it may beappropriate to separate the HVAC refrigerant loop from the battery cooling loop. This
allows circuits which are more tailored to the actual needs of the individual subsystems.

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Fast Charging.
Another feature, which may require to revise the refrigerant circuit are upcoming fast
charging capabilities of battery electric trucks. The common narrative proposes to recharge the truck for the next 4,5 hours of driving during the mandatory breaks of thedriver. So, depending on the vehicles weight and duty cycle, the battery has to store
300 to 600 kWh within 30 to 45 minutes. This is a charging power between. 600 and
800 kW. Some are actually talking about more than 1 MW [4]. This results in highbattery heat loads which exceed the heat rejection during driving at peak power of morethan 50%. Although the thermal inertia of the battery may help to cope with this challenge, it is expected that major components as chiller, compressor, condenser and cooling fan need to be upscaled significantly.
Braking Systems.
A critical function of a battery electric truck is braking. Today an independent endurance braking system in addition to the normal service brake is mandatory [5]. At least
for smaller trucks with limited payload, regenerative braking is considered to be sufficient to fulfill the legal demands. For more heavy vehicles, especially for long haul
trucks or trucks operated in the mountains, the desired braking performance of thedriver/operator may exceed the possibilities of regenerative braking system, as they areused to powerful engine brakes and driveline retarders today. In this case the additional
installation of brake resistors or even driveline retarder systems could be an option. The
heat rejection from these systems is very high and require a high temperature heat sink,
which needs to be integrated into the thermal management system and impacts radiator
size and installed fan power significantly.
Fans.
BEV trucks need electrical fans for the main cooling module. As these trucks cover awide range from medium duty distribution trucks to heavy duty long-haul, the electrical
fan power required varies significantly depending on the vehicles specifications likemotor power, gross vehicle weight, charging power and braking system installed.
For medium-duty distribution a typical fan power is between 2 and 5 kW. For a BEV
heavy-duty long haul truck, the required fan power can exceed 30 kW.
Heat Pumps.
For battery electric cars, heat pump systems are an appropriate technology to reduce
the negative impact on the vehicle range due to cabin heating under cold ambient conditions. But the gain in range depends on the ratio of traction power to heating power.
From the viewpoint of the heat pump, this gets more and more unfavorable with increasing vehicle weight. Therefore, it needs to be further investigated, which benefit aheat pump can add on the driving range of a specific application.
The benefit may be different for medium duty distribution trucks, which get charged
overnight in a depot. In this case the cooling system and the battery can be preconditioned so they can be used as heat source at least during the first hours of operation. In11a case study for a 16t gross vehicle weight distribution truck with 200 km range therange loss due to heating was significantly reduced (Figure 11).

Fig. 11. Potential benefit of a heat pump system for a medium duty distribution truck (16t
GVWR, 200 km range in urban driving, 200 kW traction motor power, battery as heat storage)

3.4

Fuel Cell Powertrains

Fuel cell powertrains are currently considered to be one solution for future sustainableroad transport. As shown in chapter 2, fuel cell powertrains show a significantly higher
heat rejection than the other options and are therefore a challenge for the design of thecooling system.
Figure 12 shows a generic layout of a fuel cell powertrain. The cooling system includes
again 3 different cooling circuits. Instead of two low temperature circuits, there is now
a low and a high temperature circuit.
The high temperature circuit is responsible for one or multiple fuel cell stacks. In addition, the charge air coolers and the hydrogen heaters of each stack are located in thiscooling circuit.
The low temperature circuit hosts all components known from the battery electric
powertrain and the electronic components from the fuel cell system (FC DC/DC, cathode compressor motor & electronics, …). The battery is incorporated into this circuit
as a three-layer cooling module in the front end would be too difficult to package.
Remote Modules.
Due to the high heat rejection in combination with the high cooling air flow needs, it isnecessary to add more frontal surface to the cooling system. Otherwise, the fan power
will become excessive. Unfortunately, in the typical truck design the space in the frontof the truck is limited and it will hardly be possible to package radiators significantly
bigger than one square meter in this location. So, one or multiple remote modules need
to be added. These additional modules can be located either on the side of the

12Fig. 12. Generic cooling system for a fuel cell powertrain with two fuel cell systems

truck, behind the cabin, or as fender module beside the front module. If these modules
have to serve the low or the high temperature circuit depends on the available packaging
space and the exact heat rejection rates of the actual application. As the remote modules
typically have no or only little ram air support, electric fans in the range of 1 to 5 kW
have to be installed on these modules.
Figure 13 shows the impact of additional remote modules for fuel cell cooling. The
numbers reflect the fan power required for maintaining a coolant inlet temperature of
80°C to the fuel cell at peak power and an ambient temperature of 35°C. Each remotemodule has a size of 50 dm² and carries a 5 kW single fan.

Fig. 13. Impact on fan power of additional remote modules for FC cooling at peak power,
Tamb=35°C

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Main Fan.
The main fan is a key element of fuel cell cooling. Due to its high power consumption,
the efficiency of the fan is very important.
Today’s fans for diesel ICE are optimized for delivering high air flows at low engine
speeds. The power consumption at peak torque (nengine=1100 rpm) is less than 10 kW
and even at peak power (nengine=1500 rpm) the fan power consumption is less than 15
kW. During engine braking, the fan is actuated at higher speeds and consumes thenmore than 40 kW.
In a fuel cell powertrain, the main fan is not connected to the drive train and is therefore
electrically driven. This results in an additional degree of freedom for optimizing the
fan blade. The new MAHLE fan generation provides a significantly higher efficiency.
At the same air flow, the power consumption is reduced by more than 20% (figure 14).

Fig. 14. Fan consumption vs. air flow for a MAHLE serial production and an optimized fan design for fuel cell applications.

Radiator.
An important aspect for fuel cell radiators is coolant-side cleanliness and low emission
rates for ions. For safe operation and maintaining the necessary isolation resistance thefuel cell uses a low conductivity coolant. During operation, an ion exchanger is used tokeep the conductivity low. To ensure a long service interval of this component and to
avoid too high conductivity levels for new vehicles and after longer inactive periods,
all components in the fuel cell cooling circuit must meet stringent specifications for ion
emissions. Therefore, a special cleaning and passivation process was developed at
MAHLE. This process can be used for all kind of heat exchangers inside the fuel cellcoolant circuit.

4

Summary

For meeting future CO2 legislation and making road transport more sustainable, a growing number of electrified trucks will be introduced. These trucks will use new powertrain options like hybrid, battery electric or fuel cell powertrains. With these new options, additional components like electronics, batteries or fuel cells will be added to thepowertrain. Each of these components has individual needs for thermal management,
which partially differ significantly from the still predominant ICE powertrains.

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Therefore, the thermal management and the layout of the cooling system is a crucialpart in the development process of the new powertrain options. To find the best thermal
management system for each application, a thorough assessment of all involved heatsources is recommended. If possible, this should be carried out on an energy flow level.
With these information and packaging envelopes for the heat sinks a cooling system
can be laid out, which meets the thermal targets with the lowest possible power consumption of compressors, pumps and fans.

References
1. Lutz, R. Electrified trucks 2021, MAHLE internal report, 2021.
2. Banzhaf, M., Lutz, R.: Heat Exchanger for Cooled Exhaust Gas Recirculation, VTMS conference, SAE paper 971822, 1997
3. Lutz, R., Art, L. Haar, D.: Fuel Saving Potential of Indirect Charge Air Cooling for Heavy
Duty Trucks, Baden-Baden Engine Congress 2017
4. Fraunhofer ISI: „Hochleistungsschnellladenetz für Elektro-Lkw“, Kurzstudie im Auftragdes Verbandes der Automobilindustrie (VDA), 29.5.2020, http://publica.fraunhofer.de/dokumente/N-608666.html
5. UN/ECE Regulation No. 13, rev. 8

Glossary for Diagrams

Abbreviations
BEV
EGR
FCV
HEV
HT
HVAC
ICE
LT
PHEV

Battery Electric Vehicle
Exhaust Gas Recirculation
Fuel Cell Vehicle
Hybrid Electric Vehicle
High Temperature
Heating, Ventilation, Air Conditioning
Internal Combustion Engine
Low Temperature
Plug-in Hybrid Electric Vehicle