Warm-Up Optimisation by means of
Exhaust Heat Recovery

Knowledge LibraryKnowledge LibraryWarm-Up Optimisation by means of
Exhaust Heat Recovery
Driven by higher requests for engine efficiency, thermal management for
combustion engines is receiving increasing attention. The engine efficiency
potential is the result of improved combustion in the warm-up phase and adecrease in friction loss. BorgWarner evaluates the heating up behaviour of amodern vehicle in several testing cycles, including the Worldwide HarmonizedLight-Duty Test Cycle (WLTC).
By Dr. Wolfgang Wenzel, Program Manager for PassCar Systems, BorgWarner,
Iago Gonzalez Tabares, R&D product Engineer, BorgWarner, andDr. Michael Becker, Director Advanced R&D, BorgWarner

Basic Conditions

Due to the high efficiency of modern engines,
the heat from the cooling system is limited inoperating conditions with low engine load suchas in the New European Driving Cycle (NEDC).
For faster warm-up additional heat input may
be used from other sources. Exhaust heat isfavourable due to the high energy level. Twodirections are possible: direct use of the exhaust heat via heat exchanger or indirect useof the exhaust heat with heat storage.

Basis Configuration

The selected vehicle is a VW Golf 1 .2 TFSI
with a seven-gear dual-clutch transmission.
The vehicle is equipped with a turbocharged
1 .2-l four-cylinder gasoline engine andstop/start system [1 ]. The temperature control
for the engine cooling circuit is realised by acooling module which consists of the mechanical coolant pump, the wax thermostat and anadditional wax shut-off valve for the cylinder liBorgWarner Knowledge Library 201 4

ner sub-circuit. In addition, the engine is equipped with an oil-to-coolant heat exchanger
which also supports warm-up by transferring
heat from the coolant to the engine oil.

Warm-Up Improvement

Considering fuel consumption, the main benefits from improved vehicle warm-up are reduced engine friction and cylinder wall heatlosses. Different options for the reduction offuel consumption in the warm-up phase areavailable. Warm-up of engine block and fluids
is improved by stopping coolant flow in thecomplete engine or the engine block due to thefact that the piston group is responsible for themajor part of the overall friction [2]. Ideally, thisis realised by a separation of the cooling loop
through the liner and the cylinder head, a socalled split-cooling layout. This more complex
setup is chosen to allow to stop the flow in theliner while keeping the coolant flow through thecylinder head for protection from overheating.
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Cooling system layout with the high-temperature circuit

As previous investigations have shown, the reduction of auxiliary losses by adjusting thecoolant flow to the actual demand helps to reduce fuel consumption. This can be realised bya variable electric or mechanical pump or thecombination of both [3].
Another way to achieve an improvement is touse available heat from other heat sources, forexample exhaust gas via exhaust gas recirculation (EGR) cooler or exhaust heat exchanger,
and directing the heat to a target fluid to achieve maximal friction reduction. This can be coolant, but potentially also engine oil or rear axle
oil.
Using stored heat from previous engine operation is another option. Different ways of heatstorage are known, important criteria for vehicle application are high storage density and fast
discharging capability. The simplest way is tostore heated coolant in an isolated device but
BorgWarner Knowledge Library 201 4

the benefits are limited. Another option is offered by phase change materials which can store
a significantly higher amount of heat.

Exhaust Heat Recovery

Exhaust heat recovery (EHR) for engine warmup is known since years [4]. Today only a few
applications are on the market since the relatively low exhaust temperatures in the current
test cycles, for example the urban part of theNEDC, lead to a limited fuel consumption be-Integration of the exhaust heat exchanger inhigh-temperature cooling circuit
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Overview of the test cycles investigatednefit for the OEMs. In general, EHR is possible
with different heat sources: by a heat exchanger in the exhaust line or – if applied – in theEGR system. As mentioned above, the heatcan be used in different ways: for engine,transmission or even for rear axle heating. The
benefit versus cost ratio determines the layout
chosen. It might be different for different applications. It is expected that the upcoming CO 2legislation will be a driver for exhaust heat recovery systems.
The application of an exhaust heat exchanger
is relatively straight forward. The heat exchanger can be easily derived from EGR cooler type heat exchangers. Only an additional bypass
valve is required on the exhaust gas side to
deactivate the heat exchanger when the engine is warm. The ideal location in the exhaust line is the position directly downstream of thecatalyst, which also keeps the coolant hoses ata minimum length. To improve heat transfer into the engine oil, the exhaust heat exchanger isapplied upstream of the oil-to-coolant heat exchanger in the heating sub-loop. A small auxiliary pump, which is fed by the board net,compensates the additional pressure drop andBorgWarner Knowledge Library 201 4

allows maintaining the flow during stop/start
events.

Vehicle Testing and Results

The vehicle evaluation was carried out by running different test cycles on a climatic test
bench. To find the theoretically obtainable benefits test were run under standardised test
conditions at 20 °C and also with the warm vehicle with all liquids at their operating temperature. After that a heat exchanger was applied tothe vehicle as described above. However, due
to packaging reasons the heat exchange had
to be installed under the vehicle and higher
benefits can be expected with a closer arrangement.
The monitoring of the engine state variables
was realised by both external sensors and thecommunication with the vehicle CAN bus. An
overall repeatability of +/- 0.5 % of the overallvalue for the cumulated fuel consumption wasachieved. The base evaluation was carried out
in three different cycles: NEDC, WLTC (Worldwide Harmonized Light-Duty Test Cycle) andthe FTP75 (American Federal Test Procedure).

3Fuel consumption for cold (20 °C) and warm
start

Compared to the NEDC, FTP75 and WLTC are
significant longer in duration and driving distance. Average speed and maximum acceleration are highest for the WLTC. Compared tothe WLTC the FTP75 shows significantly higher
engine load after 1 80 s which results in theshortest duration for engine warm-up.

Base Evaluation

The maximum theoretical fuel efficiency potential for improved warm-up is given by the fuelconsumption difference between a cold start at20 °C and a warm start. The highest differencein fuel consumption was found for the NEDC
with approximately 1 0 %. This is due to theslow warm-up and short duration of the cycle
which results in a high importance of the warmup phase for the overall fuel consumption. The
exhaust gas temperature downstream of thecatalyst as well as coolant and engine oil temperatures are plotted for the first 1 000 s of thedifferent test cycles (starting temperature at 20°C). The coolant temperature was measured at
the outlet of the engine, the oil temperature at
the outlet of the oil-to-coolant heat exchanger.
The NEDC shows the slowest coolant and oiltemperature rise, the exhaust heat temperature
also increases very slowly and reaches 200 °C
only after approximately 5 min. So adding heat
could bring the biggest fuel consumption beneBorgWarner Knowledge Library 201 4

Engine oil, coolant and exhaust gas
temperatures for different cycles

fit in the NEDC, but the available heat in theexhaust gas is also limited. In the WLTC an
exhaust temperature of 400 °C is already reached after approximately 3 min and the warmup is much faster. This is similarly true for theFTP75 cycle. So the WLTP and the FTP75 offer a lower theoretical potential of 5.8 and
4.6 % respectively. However, taking into account there is seem to be still potential for
EHR.

Exhaust Heat Recovery

The testing with the heat exchanger in the exhaust was carried out at 20 °C and -7 °C. The-7 °C test temperature was chosen to show thepotential of EHR for cold temperatures which
are expected to be part of the upcoming WLTP
(Worldwide Harmonized Light-Duty Test Procedure) legislation.
For the NEDC at 20 °C exhaust heat recovery
did not lead to a significant increase of coolant
and engine oil temperatures, thus no fuel consumption benefit was found – which is the reason why EHR has no significant volumes in themarket today. For the FTP75 a small increaseof the coolant temperature could be achieved.
However, the temperature increase did notlead to a measurable fuel economy improve4

the potential of an exhaust recovery system forcoolant heating with exhaust gas was investigated.

Engine oil temperatures with and without EHR
system in the WLTC

ment. For the WLTC at 20 °C a fuel economy
benefit of 1 .2 % could be observed which wasachieved by a more than 4 % improvement in
the first phase (589 s) of the cycle. This improvement is due to the combination of the highrelevance of this first phase for fuel consumption and the high amount of available energy toimprove friction in that phase.
The tests at -7 °C were carried out for FTP75and WLTC and fuel consumption benefits of
2.1 % for the WLTC and even 3.9 % for theFTP75 were found. The engine oil temperaturein the WLTC could be increased by up to 7 K.
Because engines run with very high friction at-7 °C, a faster warm-up phase leads to a significant friction benefit. That explains the large
fuel consumption reduction for cold temperatures. The higher benefit of the FTP75 can beexplained by the higher amount of available
heat in the early phase which seems to be thedominating effect for the benefits found.

Even for a very fuel efficient powertrain with anadvanced thermal management system clear
potential could be shown with exhaust heat recovery, in particular for testing at low temperatures which the upcoming WLTP legislation willinclude. Especially with a close coupled EHR
system featuring an exhaust gas to coolant heat exchanger – potentially combined with a LPEGR system – a significant benefit can be expected which makes it a promising approachfor future powertrains. In the next step a heatstorage system will be applied to the test vehicle to investigate the benefits for the different
test cycles.

References[1 ] Szengel, R.; Middendorf, H.; Moeller, N.;
Bennecke, H.: The new Modular Engine Platform from Volkswagen. In: MTZ 73 (201 2), No.
6, pp. 26-32[2] Beykirch, R. et al.: Das ganzheitliche FEVThermomanagementmodell für Antriebe vonheute und morgen. MTZ Conference „Der Antrieb von Morgen“, Wolfsburg, 201 3[3] Wenzel, W.; Shutty, J.; Becker, M.: Advanced Thermal Management of Diesel Engines.
In: MTZ 74 (201 3), No. 5, pp. 38-42[4] Diehl, P.; Haubner, F.; Klopstein, S.; Koch,F.: Exhaust Heat Recovery System for Modern
Cars. SAE Paper 2001 -01 -1 020, SAE World
Congress, Detroit, 2001

Summary and Conclusions

In this paper the analysis of advanced thermal
management in a passenger car with a modern
downsized engine has been presented. The
warm-up behaviour for different test cycles including the upcoming WLTC was analysed andBorgWarner Knowledge Library 201 4

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