Additional charged air cooling_TUBerlin

Heat2Cool-Engine Operation at Charge Air Cooling below Ambient TemperatureDipl.-Ing. Dipl. Vw. Samir Kadunic, Prof. Dr.-Ing. Roland Baar,Dipl.-Ing. Florian SchererFG Verbrennungskraftmaschinen, TU Berlin, GermanyDipl.-Ing. Tobias Zegenhagen, Prof. Dr.-Ing. Felix ZieglerFG Maschinen- und Energieanlagentechnik, TU Berlin, GermanySummaryCharge air cooling below ambient temperature seems promising for increasing spark engine efficiency at high loads. This paper investigates the potential of an exhaust heat driven cooling device employed for low-temperature inter cooling.The effects on engine efficiency are investigated at various points of operation along thermal and mechanical limits of the test engine. Potentials are quantified and turn out to be significant.Detrimental effects on efficiency of low temperature inter cooling at medium and low engine loads are explored. Effects on exhaust emissions during driving cycles are examined. Negative effects at low and medium loads turn out to be negligible.1 IntroductionGiven currently established fuels and standard of technology a significant reduction of CO2emissions can only be achieved by increasing overall vehicle efficiency. A holistic view at the vehicle can focus the attention on the use of waste heat recovery [1, 2, 3, 4]. The use of exhaust heat offers advantages over the use of heat from the engine coolant due to its higher temperature level.On a different note, it can safely be assumed that future legislation will require the fulfillment of emission standards outside of driving cycles [5]. This calls for a different approach to engine operation and calibration at high loads at least for spark ignition (SI) engines.Fig. 1: System setup of the Heat2Cool conceptThis paper reports about experiments employing an exhaust heat driven cooling device (Fig. 1). The system can provide a cold fluid which can be used to cool the charge air below ambient temperature. This concept makes the temperature of the intake charge widely selectable independent of environmental conditions. The basic operability of the combined system of combustion engine and cooling device was proved in [6]. Major milestones for introducing the concept into mass production are control of both systems and vehicle package integration.2 Opportunities from low-temperature intercooling for SI enginesOptimizing efficiency without crossing mechanical and thermal limits is one goal during design and application of turbocharged SI engines. The most relevant limits are turbocharger turbine inlet temperature, maximum cylinder pressure and self-ignition of unburned parts of the cylinder charge (detonation).Fig. 2 visualizes the effective efficiency of the original sample engine in the investigated area of the engine map. Efficiency decreases from its optimum with deviating load and engine speed. Towards low loads throttling increases chargecycle losses and the mechanical efficiency deteriorates.B r a k e m e a n e f f e c t i v e p r e s s u r e i n b a r0510152025Engine speed in min -1Fig. 2: Effective efficiency of the original sample engineAt high loads and engine speeds efficiency is impaired by other limitations. Fig. 3 shows areas where a rich air fuel ratio is required to keep the turbo charger turbine inlet temperature within its limits. In most of these points of operation the centre of heat release has to be later than desired because of limited cylinder pressure capability although there is no knocking. If earlier ignition timing was possible this would relieve the turbocharger turbine thermally.Fig. 3: Areas of critical temperature at turbocharger turbine intake and cylinder pressureAt high loads and low engine speeds, cylinder pressure during combustion is not critical. Nevertheless, ignition timing has to be unfavorably late to prevent knocking in large areas of the engine map (Fig. 4)Fig. 4: Areas of late centre of heat release and rich air fuel ratioThe temperature of the charge air determines the achievable loads before being limited by any of the aforementioned limits of operation. The lower limit of this temperature is set by the coldest medium available for intercooling. Commonly this is the ambient air, but due to limits in heat changer efficiency the charge air will always be several degrees warmer than that temperature.Turbo cooling and Miller cycle can further decrease charge temperature inside the combustion chamber. Both methods require higher supercharging ratios which causes more back pressure if a turbocharger is used. As a single influence this is detrimental for the gas exchange cycle which decreases engine efficiency and increases residual gas content [7].A cooling device provides a fluid colder than ambient temperature. Used for intercooling, this turns the intake charge temperature into a degree of freedom for engine application. It can be used to increase power density or thermal efficiency. As an alternative engine components can be relieved thermally or mechanically.Extensive experimental studies were conducted to determine the potential (Table 1) to guarantee the applicability of the results to future production engines.Previous investigations [8] show no significant influence of super low charge air temperature on efficiency without adapted ignition timing or air-fuel ratio.The application of very cold intercooling combined with high boost ratios results in air moisture condensing inside the intercooler. To prevent icing charge air temperatures below 0°C were avoided. As a reference an intake temperature of 60°C was used to represent high engine loads combined with low vehicle speeds and warm surroundings.Tab. 1: Reference engine specifications3 Experimental resultsIn the following chapters we investigate the potential improvement of SI engine efficiency with intensive inter cooling. Experiments were conducted at the engine’s operating limits and in various significant areas of the engine map.At first we show effects at n=2500min-1 where high load operation is limited by knocking. This demands delayed ignition timing, resulting in high exhaust temperatures, requiring fuel enrichment to cool the turbo charger turbine. With increasing engine speed, peak cylinder pressure leads to late COHR, causing thermal problems with the turbo charger turbine and resulting in rich air-fuel ratios. Effects applicable to these areas of the engine map are demonstrated at n=4500min-1.Sweeps of ignition timing showed the most efficient COHR to be independent of charge intake temperature between 4° and 8° ATDC. The loads achievable within this COHR range are limited by knocking and peak cylinder pressure. In this paper we show the increase in achievable load at a certain COHR by reduced charge air temperature at COHR=4°ATDC and COHR=8°ATDC.Midway of the engine map operation is not restricted by thermodynamic limitations. Effects of inter cooling on engine efficiency are investigated at NMEP=6bar and NMEP=12bar. Low engine loads and speeds are most relevant for driving cycles. The influence of charge air temperature on effective efficiency and emissions areshown for one significant point of operation.turbine inlet temperature and engine knock Charge enrichment is required in some of the areas of the engine map where ignition timing is later than actually desired. In those points of operation the largest improvement in efficiency can be achieved through inter cooling if ignition timing is advanced and the mixture is simultaneously leaned out along the thermal and mechanical limits of the engine.Charge air temperature is decreased in several steps starting from 60°C at n=2500min -1 for two loads (NMEP=18bar; 20bar). Indicated efficiency can be increased by up to 18.5% at constant load (Fig. 5). This is achieved by advancing the ignition timing and leaning the charge mixture accordingly. Turbine intake temperature is reduced by up to 100K additionally.The intake charge temperature and turbine intake temperature correlate stronger at NMEP=18bar than at 20bar. This is attributable mainly to the lower opportunity for leaning the mixture at the lower load. Leaning causes a reduction in laminar flame velocity resulting in longer combustion, diminishing the reduction of exhaust temperature.A charge air temperature reduction of 10K is sufficient for combustion of a stoichiometric mixture at NMEP=18bar. A temperature reduction of 30K is required to achieve the same at NMEP=20bar.Engine load can be increased by 11% to NMEP=22.2bar referencing to an intake temperature of 40°C, employing stoichiometric combustion and assuming constant efficiency.L a m b d aI n d i c a t e d e f f i c i e n c y i n %T u r b i n e i n l e t t e m p e r a t u r e i n °CCharge air temperature after throttle in °CC O H R i n °C A a . TD C FCharge air temperature after throttle in °C M a x . c y l.p r e s s u r e i n b a rN m e p i n b a rFig. 5:Influence of intake air temperature on efficiency and turbine inlettemperature at n=2500min -1employing optimized COHR and Air fuel ratioturbine inlet temperature and mechanical stress Another trial intake temperature was reduced starting at 60°C and investigating two engine loads (NMEP=16 and 20bar) at 4500min -1.At constant load ignition timing could only be advanced moderately due to the given cylinder pressure limit. For this reason most of the efficiency increase originates from the leaner mixture permitted by the decrease in exhaust gas temperature. At NMEP=20bar efficiency can be increased by up to 16% (Fig. 6).If intake air temperature is reduced from 20°C to 0°C at NMEP=16bar the effect on turbine intake temperature becomes clearly visible. A reduction of charge air temperature by 20K results in a thermal relieve for the turbine of 15K.I n d i c a t e d e f f i c i e n c y i n %2426283032343638L a m b d a0.700.750.800.850.900.951.00T u r b i n e i n l e t t e m p e r a t u r e i n °C925938950963975Charge air temperature after throttle in °C7274767880C O H R i n °C A a . TD C F101214161820222426Charge air temperature after throttle in °CFig. 6:Influence of intake charge air temperature on efficiency and turbine intake temperature at n=4500min -1 employing adjusted COHR and air-fuel ratio3.3 Extension knock boundary at part loadSpark ignition engines show best efficiency if the centre of heat release is between 4° and 8°ATDC. At partial load this can commonly be achieved. The maximum load where COHR can be set as aspired is limited by knocking or detonation. For two engine speeds the potential is investigated for an increas engine load while maintaining ideal COHR using intensified inter cooling.At n=2000min -1 engine load can be increased by 37%, see also Fig. 7. Even an increase of 47% is possible at n=3000min -1 and COHR=4° ATDC. In both cases the feasible increase in load is larger for the earlier centre of heat release. An earlier combustion reduces the cross-influence of heat exchange with the cylinder walls on the effects of intensified inter cooling.B r a k e e f f i c i e n c y i n %Charge air temperature after throttle in °C N M E P i n b a rN M E P i n b a rB r a k e e f f i c i e n c y i n %Charge air temperature after throttle in °CFig. 7:Influence of charge air temperature on engine knock limit at n=2000min -1 and n=3000min -1 with COHR at 4°ATDC and 8°ATDC3.4 Influence of charge air temperature on efficiency and emissions at partload A reduction of intake air temperature increases the charge density keeping all other parameters equal. In throttled operation this means intensified inter cooling results in ascending charge cycle losses from throttling for a given load, which may result in decreased overall efficiency. To investigate this situation sweeps in engine speed were run at NMEP=6bar and NMEP=12bar. COHR was kept constant while intake air temperature was varied in steps between 60°C and 0°C (Fig. 8).The selected test engine achieves a full load of approximately NMEP=12bar when naturally aspirated at 60°C intake air temperature. Intensified charge air cooling requires throttling to achieve retention of air mass flow and engine load.The results show no detrimental effect of a lowered charge air temperature on efficiency at the investigated engine loads and speeds. From the measurements it can be derived that a reduction in wall heat losses compensates for the increasing charge cycle losses.I n d i c a t e d e f f i c i e n c y i n %Engine speed in min -1H e a t t r a n s f e r t o c y l i n d e r w a l l i n % o f t o t a l f u e l e n e r g y101214161820222426H e a t t r a n s f e r t o c y l i n d e r w a l l i n % o f t o t a l f u e l e n e r g yI n d i c a t e d e f f i c i e n c y i n %303132333435363738Engine speed in min -1Fig. 8:Influence of charge air temperature on efficiency at NMEP=6 andNMEP=12barDriving cycles like NEDC are dominated by even lower engine loads. From a vehicle simulation 2000min -1 and NMEP=2,8bar was derived as being highly representative. Reducing the charge air temperature is expected to impair mixture formation when keeping all other influences equal. This could result in increasing HC and CO raw emissions. Fig. 9 shows the influence of intake air temperature on effective efficiency and emissions before and after catalytic converter in the aforementioned point of operation. Inter Cooling does not show any significant influence on exhaust emissions nor on efficiency or exhaust gas temperature.Employing cylinder pressure analysis it was found, that integral wall heat loss diminished with decreasing charge air temperature. This results in the cylinder charge temperature being almost independent of intake temperature during the high pressure cycle.B r a k e e f f i c i e n c y i n %N O X i n g /k W hCharge air temperature after throttle in °C H C i n g /k W h02468C O i n g /k W h8152330Charge air temperature after throttle in °C520530540550560Fig. 9: Influence of charge air temperature on brake efficiency and emissions at 2000min-1 and NMEP=2,8bar4 ConclusionThe potential efficiency increase from charge air cooling was investigated at various limits of engine operation employing selected points of operation.Assuming an intake charge air temperature reduction of 60K engine efficiency could be increased by up to 18%. This applies to engine operation at n=2500min-1 NMEP=20bar where operation was limited by knock and turbine inlet temperature. Advanced ignition timing with correspondingly leaned out mixture was employed. If engine operation is limited by peak cylinder pressure and turbine intake temperature, a reduction in charge air temperature by 60K can raise engine efficiency by up to 16%. This was demonstrated at n=4500min-1, NMEP=20bar. The increase in air-fuel ratio was most influential in this case.Employing intensified charge air cooling, the top load before engine knocking can be increased for a given COHR. At n=3000min -1the highest achievable load while employing a desirable ignition timing could be increased by 47%.It was found out that engine efficiency was not detrimentally affected by low temperature inter cooling at low and medium engine loads. A point of engine operation representative for driving cycles was derived and used for further investigation. Charge air cooling below ambient temperature showed no negative effects on exhaust emissions at this low engine load.5 Definitions, Acronyms, AbbreviationsATDC after top dead centera. afterCOHR Centre of heat releaseTDCF Top dead centre firingNMEP Net mean effective pressure6 References[1] FREYMANN, R.; RINGLER, J.; SEIFERT, M.; TILLMANN, H.Der Turbosteamer der zweiten GenerationMTZ 73, 2012[2] FRIEDRICH, H.; SCHIER, M.; HÄFELE, C.; WEILER, T.Strom aus Abgasen: Fahrzeuggerechte Entwicklung thermoelektrischerGeneratorenATZ 112, 2010[3] DINGEL, O.; SEMPER, T.; AMBROSIUS, V.; SEEBODE, J.Waste heat recovery: What are the alternatives to the thermoelectric generator?Thermoelectrics Goes Automotive IIBerlin, 2013[4] SEHER, D.; LENGENFELDER, T.; Gerhardt, J.; EISENMENGER, N.;HACKNER, M.; KRINN, I.Waste Heat Recovery for Commercial Vehicles with a Rankine Process21st Aachen Colloquium Automobile and Engine Technology 2012Aachen, 2012[5] REULEIN, C.; SCHÜNEMANN, E.; SCHWART, C.; WETZEL, M.Thermodynamik des BMW-DreizylindermotorsMTZ 74, 2013[6] KADUNIC, S.; ZEGENHAGEN, T.; BAAR, R.; ZIEGLER, F.Heat2Cool - Abgaswärmenutzung IIFVV Abschlussbericht, Heft R989, 2013[7] PUCHER, H.; ZINNER, K.Aufladung von VerbrennungsmotorenSpringer ViewegBerlin Heidelberg, 2012[8] KADUNIC, S.; ZEGENHAGEN, T.; SCHERER, F.; BAAR, R.; ZIEGLER, F.Heat2Cool - Abgaswärmenutzung IIFVV Zwischenbericht, Heft R558, 2012。