Comparison of Advanced Waste Heat Recovery Systems with a Novel Oil Heating System

Transcription

1 F. Will/International Journal of Automotive Engineering 4 (2013) Technicai Paper Comparison of Advanced Waste Heat Recovery Systems with a Novel Oil Heating System Frank Will 1) 1) Deakin University, School of Engineering, Waurn Ponds, 3216 Victoria, Australia ( frank.will@deakin.edu.au) Received on July 3, 2012 Presented at the JSAE Annual Congress on May 25, 2012 ABSTRACT: Latest trends in waste heat recovery include systems like Thermo Electric Generation (TEG), Rankine cycle, and active warm up systems. The advantages and disadvantages of different approaches are critically discussed and compared with a novel and effective oil heating system that can deliver between 7% and 12% reductions of CO2 emissions and fuel consumption. The comparison includes the expected CO2 and fuel saving potential related to the legal drive cycle as well as real world driving, effects on regulated exhaust emissions, utilisation of resources, maintenance and service, vehicle performance, comfort, noise, and durability. KEY WORDS: Engine, waste heat recovery, 1. INTRODUCTION system, heat transfer [A1] reasons are different operating conditions and different ambient Climate change resulted in the introduction of regulations temperatures, which limit passenger car CO2 emissions in Europe from 2015 (1) lubrication for example lower temperatures, higher temperatures and higher vehicle speeds and -loads. Lower are ambient temperatures are very relevant in most areas as the currently being discussed, ranging from 80 % reductions average temperature over a year is typically around 10 C lower onwards. For 2050, different CO2 limit scenarios (2). compared to the conditions for the legal fuel economy drive Even though up to 85 % of the fuel s chemical energy used in a cycles. This results in higher losses through friction. compared to the status from 2005 to up to 94 % reduction (3, 4), these new CO2 limits will require Furthermore, higher temperatures are important as they require quantum loops in vehicle technology and it is unknown if these the operation of air conditioning systems which also increase the targets will be feasible at all. engine load. Higher vehicle speeds are mostly relevant for car is wasted as heat Utilising some of that wasted heat to improve the car s countries like Germany where a certain percentage of the efficiency seems to be a logical consequence. During the last Motorways do not have a general speed limit. That leads to years several types of waste heat recovery systems have been higher engine loads but also higher heat transfer to the ambient studied due to the higher relative velocity of the cooling air through the extensively to evaluate their potential for engine compartment. implementation into mass production. The most promising systems are Thermo Electric Generation (TEG), Rankine cycle, The different advanced waste heat recovery systems are and active warm up systems. Some waste heat recovery systems evaluated regarding these 12 criteria and compared in a matrix have already been introduced to warm up the passenger to find the most attractive solution. The ratings range from 5 compartment faster for very efficient powertrains like Diesels very positive effect to 0 very negative effect. and Hybrids. 2. COMPARISON Besides the potential to reduce fuel consumption and CO2 emissions, there are many other criteria that are important and 2.1 Rankine cycle need to be considered when decisions are made about The Rankine cycle is the standard process typically used in production implementation. Some of the most important of these include safety, durability/reliability, performance, power plants to generate electricity from fluids with high regulated emissions, cost, noise vibration harshness (NVH), temperatures, typically steam. It seems to be obvious to study package space, cabin warm up performance, maintenance this process as an option to also recover waste heat from cars. requirements, The cycle consists of four different main process changes, that criteria weight and finally real world customer all require a separate dedicated component: A pump increases representative fuel economy. The real world fuel economy is often worse compared to the pressure of a liquid fluid so that the fluid starts to flow into a the results established with the various legal drive cycles. The heat exchanger, in this case an exhaust gas heat exchanger, 33

2 where the working fluid is evaporated and superheated. This is followed by expansion in a turbine where the heat is transferred into mechanical energy. The mechanical energy of the turbine is converted into electrical energy to reduce the alternator load. Finally the condenser converts the steam into the saturated liquid phase to feed the pump. Recently a paper was published about a project where the first Rankine cycle exhaust gas recovery system with a relatively compact size was developed that could actually be fitted into a car (5). This paper probably gives the best overview of the potential and the challenges of such a system in a car. The system is the next generation of a more complicated system that was published in (6) where the ultimate possible benefit of a Rankine type exhaust heat recovery system was analysed on an engine dynamometer without considering package restrictions. The authors expect a fuel economy improvement of approximately 6% over the average operating range of the system from between 70 km/h and 150 km/h vehicle speed. The nominal power output of the system is around 200 W at 70 km/h and over 2000 W at 150 km/h. For the New European Drive Cycle (NEDC) with an average speed of only 33.6 km/h such a system would only deliver some benefits in sections of the part two cycle where speeds of 70 km/h and above are included for about 50% of the cycle. Considering a power output of 300 W at 90km/h that would result in a fuel consumption saving of ca l/100km for the second part of the drive cycle assuming it takes around 1 l/100km to produce 1 kw of electrical power. For the combined NEDC this would mean a fuel economy improvement of 0.1 l/100km. Unfortunately the base engine configuration was not mentioned in this article so it is assumed it could be the standard engine of the quoted BMW 5 Series with a fuel consumption of 6.8 l/100km which would result in an improvement of 1.5 %. If the effects of the additional weight of 15 kg were not included in the analysis - which is almost 1 % of the vehicle weight this would result in a 0.4 % increase in fuel consumption so that the net benefit of approximately 1.1 % can be estimated. For the Japanese drive cycle the benefit will be even smaller due to the lower speed profile. With the four additional components that are required - similar to a downsized turbo charged engine - the additional costs can be expected to be in a similar range. The most important question regarding safety relates to the working fluid. In (7) water was used in the high temperature cycle and ethanol in the low temperature cycle where in (5) the fluid type was not mentioned. Water alone gives a higher efficiency and allows smaller heat exchanger system sizes (7) but unfortunately water can t be used as a working fluid if the vehicle is operated below 0 C. Ethanol has a very low critical flash point of only 16.6 C and therefore needs explosion protection. This is even more critical in case the evaporator would leak and the ethanol would run onto the hot exhaust which immediately would cause a fire. The reliability typically reduces with the increase of complexity. However, if this system fails, no significant effect on the vehicle operation would be expected other than the safety risks in case of a leak, as discussed before. Special efforts would be required to ensure condensed water in the exhaust is drained to avoid freezing of the water at temperatures below zero degrees, such specific design requirements to deal with excessive water condensate need to be considered for all waste heat recovery systems that use some sort of heat exchangers to remove the heat from the exhaust. The effects on the vehicles performance are expected to be very small, similar to the fuel economy: the alternator has to work less hard but the additional weight would partially compensate that. No significant effects for the regulated emissions are expected, the higher weight can help to shorten catalyst light off but on the other hand slightly increases the exhaust mass flow during low vehicle speeds. Regarding noise the steam turbine could lead to noise problems similar to those know from turbo chargers, but they can be managed in a similar way. One of the biggest challenges is the packaging of the additional components which might restrict the application only to larger cars. The condenser needs to be located in front of the normal radiator so that the radiator size needs to be increased as well; a repositioning might also be possible. No significant changes are expected for the cabin warm and finally the maintenance requirements will become a bit more complex due to the additional operating fluid that needs to be serviced. Real world fuel economy should improve under higher loads a little through the higher exhaust temperatures, although during low temperatures and high vehicle speeds some of that benefit will be offset by increased heat losses to the ambient. 2.2 Thermo electric generators The basic physical principle that enables the generation of electricity from heat through Thermo Electric Generators (TEG) is the Seebeck-effect: a conducting material A is in contact with two different parts made from another conducting material B, an electrical voltage is generated between the two different material B parts if each of the two junctions is exposed to a different temperature. 34

3 The main components of a TEG in a car are the thermo electrical modules, one or more exhaust gas heat exchangers and one or several coolant heat exchangers (8). A valve-controlled bypass is required to prevent overheating of the TEG materials and to avoid an increase of exhaust gas back pressure at high operating loads, which in turn would result in a reduction of engine power (8, 9). The exhaust heat which is transferred into the TEG needs to be dissipated either with a separate cooling circuit (8), or through integration into the vehicle cooling circuit (10), in which case a significantly larger radiator size would be required. First fuel economy results of vehicles fitted with TEG s have been reported in (8) and (10). At a constant vehicle speed of 130km/h an electrical TEG power of 200 W could be produced (8). Applying the same rational as for the fuel economy analysis related to the Rankine cycle, this could lead to a maximum fuel economy improvement of about 0.06 l/100km in part 2 of the NEDC test and 0.04 l/100km in total. However, the authors claim that the initial tests were done with a material combination of Bismuth and Telluride which has a relatively low thermoelectric effectiveness. The effectiveness of a thermo-electric material at a certain temperature is characterised by the ZT-number - a dimensional measure for the effectiveness of a TEG material combination as a function of the temperature. In this case the ZT number was only around 0.4 in the operating point tested. Because other material combinations are available like Lead/Telluride or Silicon/Germanium which have higher ZT numbers of up to 0.85, the authors conducted simulations that predicted the drive cycle fuel economy with such more effective materials. A maximum NEDC fuel economy improvement of 1% was predicted, the insulation of the exhaust system would further increase fuel economy by 0.2 %. Even more efficient material combinations with a ZT-value of 2 would lead to maximum fuel economy benefits of up to 2 % (8). However, the authors did not mention the types of material combinations that could achieve such high efficiencies, so the potential implications on other relevant criteria cannot be assessed. The simulations for the combined US drive cycle which has a higher average speed profile - resulted in an approximately 50 % higher fuel economy improvement. That means that the benefits during real world driving are also expected to be more substantial. But on the other hand the fuel economy improvements during the Japanese drive cycle are expected to be lower than the NEDC results. Another somewhat unknown factor is the transient power output of a TEG and the resulting challenges for the integration into the vehicles electrical system (8). The simulation benefits for the US drive cycle were recently verified by Mori et al. who tested a novel combination of low temperature and high temperature material combinations with an insulated exhaust of a hybrid vehicle and achieved a maximum of 2.75 % fuel economy improvement for the average between the LA city/highway and US06 tests (10). Unfortunately the type of material combination that was tested was also not mentioned in this publication. The US06 test represents more aggressive driving with higher vehicle speeds and faster accelerations. These conditions are more favourable for TEG systems. The costs of such TEG systems are expected to be similar to the Rankine cycle system because a similar number of components are required. The costs as well as the safety impacts will be largely influenced by the type of materials that will be used. Many of the materials studied are heavy metals that are quite safety critical. Lead, Tellurium and Bismuth are toxic, so special precautions are required. The comment that the TEG modules should not exceed certain temperatures (8) indicates that the effectiveness of the modules could also deteriorate over time; deposits in the exhaust gas heat exchanger could further reduce efficiency over time, that s a common problem for all heat recovery systems with exhaust gas heat exchangers. The weight of the TEG system tested in (10) was 10 kg, it was hoped that the weight could be reduced down to 5kg without the need for intensive instrumentation in a production unit. Nevertheless, this is a very aggressive target compared to the 5.4 kg weight of a simple coolant exhaust gas heat exchanger (11). The performance impacts are expected to be smaller compared to a Rankine system due to the lower weight. No changes to the regulated emissions are expected or have been reported. The advantage of a TEG compared to the Rankine cycle is that no rotating parts are required and that no additional working fluid is required which will minimise the maintenance requirements and NVH effects. A volume of between 1.7 l (estimated for production) and 3.6 l (measured for a research prototype) (10) should be relatively easy to package. Finally the cabin is expected to warm up faster because the heat transferred into the cooling system should help to warm up the coolant faster. 2.3 Active coolant warm up systems The theory behind active coolant warm up systems is to warm up the coolant faster because the coolant temperature is also one of the driving forces that determines the oil temperature during warm up. This means warming up the coolant faster should also warm up the oil faster so that friction can be reduced. The benefit of this approach has been tested in various passive warm 35

4 up technologies that include, for example, split cooling systems where the cooling system is separated into a low temperature circuit for the cylinder head and a higher temperature cooling circuit for the cylinder block (12, 13), electric water pumps that provide coolant flow on demand (14), or electric thermostats that change the thermostat opening temperature to achieve higher cylinder liner temperatures under moderate loads (15). With such approaches fuel economy benefits between 0.5 % and a maximum of 2 % have been reported for the NEDC test. Systems with exhaust gas heat exchangers that warm up the coolant appear to be very attractive due to their simplicity. Different approaches have been reported with different detailed designs for heat exchanger and exhaust flap valve (16, 17, and 18). These active coolant warm up systems have a heat exchanger installed after the catalyst. A three way exhaust flap valve controls the exhaust flow between an exhaust bypass and the heat exchanger: during low coolant temperatures the exhaust gas warms up the coolant. Once the coolant reaches a maximum threshold temperature the flap valve redirects the exhaust gas away from the heat exchanger through a bypass. On the coolant side the heat exchanger can be installed in different locations, for example between the engine and the cabin heater. Diehl et al. (16) performed a simulation that included two consecutive part 1 cycles of the NEDC test at a temperature of -7 C that only showed a minor improvement in fuel economy. The authors also performed physical tests with a prototype system at a lower temperature of -20 C at constant speed of 2000 RPM and a constant load of 17 Nm, the conditions were chosen to represent a speed of 50km/h in third gear for a middle class vehicle. The test result showed even a slight deterioration of the fuel economy. Chiew et al (11) cited this paper and mistakenly interpreted that a fuel economy improvement of 10.6 % was achieved for the NEDC test, started at 20 C and 4.3 % after the warm up. However, the 10.6 % fuel consumption reduction that was reported in (16) was for the difference between the NEDC tests when started at 20 C and when started after the warm up at 90 C. The 4.3 % fuel economy improvement referred to the difference between a hot test started at 90 C and a hot test started at 110 C. These were results of simulations that were performed to show the ultimate physical potential of warming up the engine faster. In (17) a hybrid vehicle was tested with a similar exhaust gas heat exchanger but fuel economy improvements were also only reported for winter conditions. A similar configuration was described in (18) where a 9 % fuel economy improvement was reported for the LA4 drive cycle at -5 C. However, in none of these reports any fuel economy improvement was reported for test conditions at higher ambient temperatures between 20 C and 30 C as specified for the NEDC and most other main fuel economy test cycles. The reasons why these coolant exhaust gas heat exchangers deliver only marginal improvements at normal ambient temperatures have been described in (19, 20) : The main reason is the very inefficient heat transfer from the exhaust gas to the engine oil. Because the coolant system is designed to protect the mechanical engine components by transferring heat from the combustion chamber walls to the ambient, most of the additional heat that is recovered in a coolant exhaust gas heat exchanger is transferred to the ambient air rather than to the engine oil. Increased thermal masses through the additional heat exchanger are another negative effect. Similarly the larger coolant volume that has to be heated up reduces some of the warm up advantages. These coolant heat exchangers can only help to improve engine efficiency during warm up until the thermostat is open; no fuel economy improvements are possible when the engine is already warm. Another report includes the analysis of a more complex system that warms up the engine and the automatic transmission of a hybrid vehicle (21). Different configurations were tested over the EPA Urban Dynamometer Drive Cycle UDDS which is only the first part of the EPA fuel economy drive cycle. In a first approach the engine and the transmission were heated up by the coolant from the exhaust gas heat exchanger. This resulted in a fuel economy benefit of 0.3 %. A maximum fuel economy enhancement of 2.5 % was achieved with an integrated exhaust gas heat exchanger which warmed up the coolant and additionally the transmission oil directly. The improvements over the complete drive cycle are expected to be smaller, more in the range of 1 %, because only the city cycle was tested. The same applies for real world driving at higher vehicle speeds, only very marginal fuel economy improvement may be possible. Regarding the other attributes the situation is quite mixed for coolant exhaust gas heat exchangers, the main benefit is to improve heater performance, this is the reason why they have been introduced but in most instances only for markets with cold climates. The costs are expected to be moderate with only one or two additional heat exchangers and some valves that can be integrated. An additional weight of 5.4kg has been reported in 36

5 (11) which are also moderate compared to the TEG and Rankine systems. The system is also easier to package although it could be difficult to find the right space for vehicles with high performance engines. For safety, reliability, performance, regulated emissions and NVH the performance is expected to be similar to the baseline configuration. Some systems have already been introduced into mass production, which means that all these requirements can be met. For maintenance the same might apply, although the system performance could deteriorate through contamination with soot on the exhaust gas side of the heat exchanger, and special design considerations are required to deal with the larger amount of condensed water that can freeze below zero C. 2.4 Exhaust gas oil heat exchanger The principle of a system with an exhaust gas oil heat exchanger is very similar as the previously described coolant exhaust gas heat exchangers with the only difference being that the exhaust gas heat is directly transferred to the engine oil instead into the coolant. Test results of such a system have been reported in (19) and (20). The same heat exchanger as described in (11) was tested; it was connected to the engine oil system between the engine block and the oil filter by using a sandwich oil cooler adapter (figure 1). The specific heat capacity of the oil is smaller compared to the coolant so the thermal inertia is reduced. The higher oil temperatures resulted in lower oil pressures and therefore lower power requirements of the oil pump. The result of an average of five NEDC tests with and without the exhaust gas oil heat exchanger was a 7.3 % fuel economy improvement (figure 2). Interestingly the fuel economy improvement during the extra urban part two of the drive cycle was 6.9 % and therefore in the same range as the benefits in the urban drive cycle (part one) with 7.8 %. This can be explained by several reasons: The oil temperature increase in part 2 was higher, up to a maximum of 25 C, because in part 2 much more exhaust heat is available. The engine speeds in part 2 are higher which normally results in higher friction leading to a higher reduction potential. In some instances in part 1 an increased oil temperature can increase engine friction at low engine speeds when moving from hydrodynamic lubrication to mixed lubrication. In part 1 most exhaust heat is used firstly to warm up the catalyst so the potential to warm up the oil is limited. 10.0% 8.0% 6.0% 4.0% 2.0% 0.0% 7.8% 6.9% 7.3% Urban Extra Urban NEDC Overall Fig. 1 Waste heat recovery system with exhaust gas oil heat exchanger (19) The benefits of warming up the oil directly are The heat loss to the ambient is reduced so that most of the exhaust heat can be transferred directly into the engine oil where the friction is reduced. The oil can also be warmed up to much higher temperatures than the coolant without the risk of local boiling. Fig. 2 Fuel economy improvements with oil/exhaust gas heat exchanger (vehicle measurements averaged over 5 tests in each configuration) (19) The real world fuel economy benefits at lower temperatures are expected to be even larger due to the higher viscosity of the oil at lower temperatures. At operating conditions with higher vehicle speeds, under additional loads through air conditions, and at higher ambient temperatures the benefits are expected to shrink, although there should still be a positive effect according to the positive results for the part 2 test. Exhaust emissions were also reduced significantly. Carbon Monoxide emissions (CO) were reduced by 27 % and Nitrogen 37

6 Oxides (NOx) were reduced by 19 % (figure 3). Particularly the NOx reductions are important in light of modern Diesels and stratified direct injection systems where the reduction of NO x emissions can be a big challenge that has led to sophisticated after treatment systems with NO x traps and Selective Catalytic Converters (SRC) that require the maintenance of additional operating fluids. CO emissions are reduced by higher wall temperatures that reduce the flame quenching in the combustion chamber. NOx emissions are reduced because of lower combustion temperatures that are a result of much lower engine loads. Although particulates emissions have not been tested as the tested engine was a petrol with port fuel injection, based on experience, it can be expected that particulate emissions can be reduced by a similar magnitude as NO x emissions. For cost, weight, NVH, and packaging the same applies as for the coolant exhaust gas heat exchangers. However, to ensure a safe system more precaution is required for the detailed design. For instance, in a case where the heat exchanger could leak (for example due to fatigue or corrosion) oil could run onto the hot exhaust where it could catch fire. Because it is installed in the pressure side of the lubrication system the heat exchanger needs higher safety margins similar as the oil passages of a turbo charger. This may affect the effectiveness, cost and weight. operating conditions, particularly during hot idle. The higher oil temperatures may require a special oil formation that is more resistant to higher average oil temperatures. On the positive side the maintenance requirements of an engine can be reduced as the oil quality will improve. If the engine runs at higher oil temperatures, even during city driving, the condensation from water that comes from the blow by gas can be reduced. That reduces engine wear and the oil change intervals can be extended (22). 2.5 Cylinder head bypass with exhaust gas oil heat exchanger A further variation of the exhaust gas oil heat exchanger is described in (23). Here the exhaust gas heat exchanger is installed in an oil bypass between the cylinder head oil gallery and the oil pump (figure 4). With that arrangement the heat exchanger is moved from the high pressure side of the lubrication system to the suction side. That makes such an arrangement much safer as the chances for oil to leak from the heat exchanger onto the exhaust system are significantly reduced. The flow through the bypass is controlled by a valve to ensure an optimum and reliable oil pressure under all operating conditions, which eliminates another key weakness of the previous system. Due to the lower pressure on the liquid (oil-) side - compared to the previous system and even compared with the coolant exhaust gas heat exchanger the required safety factors for potential worst case conditions can be reduced. There are also a number of benefits related to fuel economy: Fig. 3 Emission reductions with oil/exhaust gas heat exchanger (vehicle measurements averaged over 5 tests in each configuration) (19) The engine performance is expected to improve during the warm up phase due to the higher oil temperatures. The cabin warm performance is predicted to be better compared to a baseline configuration although it is expected that the improvements are not as significant as for a coolant exhaust gas heat exchanger because not all of the heat transferred into the oil can be transferred into the coolant during warm up, the oil pan transfers some of that heat into the ambient. Reliability could be affected negatively due to the reduced oil pressure in the engine, so an increase in the performance of the oil pump might be required to ensure safe oil pressure under all The bypass from the cylinder head to the oil pump already reduces the oil pressure during the cold start, similar as a variable oil pump, so there is no need for the oil pump pressure relieve valve to open. The thermal inertias of the oil in the engine oil galleries are partially separated from the oil in the oil sump. That means that the oil in the galleries warms up faster and the oil in the oil pan warms up slower, but the combined oil flow warms up faster. The heat loss from the oil pan is reduced due to lower oil sump temperatures. The oil flow rate through the cylinder head oil galleries is increased and consequently the heat transfer from the combustion gas to the oil increases, leading to a faster warm up. The diameter of the oil by-pass hoses can be much smaller compared to a heat exchanger between oil filter 38

7 and engine block. This reduces the thermal inertia of the oil in the bypass and heat exchanger. the only disadvantage compared to the other systems, so it must be managed carefully during the design process. The second best option is an oil exhaust gas heat exchanger Even though such a system was not tested, a further fuel economy benefit compared to the previous system can be expected. The same applies for colder ambient temperatures. At higher temperatures, at higher vehicle speeds, and higher loads there should also be a slightly higher improvement possible compared to the normal exhaust gas oil heat exchanger as the oil temperature can reach a higher level. Only when the maximum between oil filter and engine block without the cylinder head bypass. The improvements for fuel economy and other attributes are smaller but still significant. However, the safety challenges are much more severe. The coolant exhaust gas heat exchanger can only provide marginal fuel economy benefits during the NEDC test. Their biggest advantage is improved cabin warm up performance. specified oil temperature is reached, no further fuel economy benefits can be achieved. The costs are expected to be slightly higher compared to the previous system. The cost for the Table 1 Comparison and evaluation of different advanced waste heat recovery systems additional valve can be partially offset by smaller hoses, pipes and heat exchanger. Emissions, NVH, package, maintenance and weight are also expected to be similar. Performance and cabin warm-up are expected to be slightly better due to the same effects that helped to improve fuel efficiency. EGR Valve Cylinder head consumers Turbo charger Catalyst Bypass Muffler Tail pipe Intake air Bypass flap valve By-pass valve Exhaust (Blow By) Exhaust in Exhaust out Thermo-electric generators and Rankine cycle systems only Heat exchanger deliver minor fuel economy benefits over the NEDC test. Oil pressure Crank case Cylinder block consumer However, the potential for real world fuel economy relieve valve Thermometer Oil pressure improvements is larger with higher vehicle speeds and loads but the biggest challenges are their costs. Strainer Pump Oil gallery Oil sump Oil filter REFERENCES Fig. 4 Cylinder head oil bypass with exhaust gas/oil heat exchanger (23) 3. CONCLUSION Five different advanced waste heat recovery systems have been compared related to a set of 12 important evaluation criteria as displayed in table 1. An oil bypass from the cylinder head to the pump with an integrated exhaust gas heat exchanger and oil flow control valve has the potential for the largest percentage fuel economy improvements during the NEDC test as well as during real world driving. Further benefits are significant emission reductions, moderate costs, and longer oil change intervals. The connection to the suction side of the pump minimises the risk of oil leaking onto the hot exhaust, which is (1) Dings, J. Reducing CO2 Emissions from New Cars: A Study of Major Car Manufacturers Progress in 2008, European Federation for Transport and Environment (2009) (2) Bruenglinghaus, C. & Winterhagen, J.CO2 limits determine future direction. ATZ 113, 4-7 (2011) (3) Kehn, U. Variables Epsilon Mittel zur Wirkungsgraderhöhung bei hochaufgeladenen Ottomotoren. (Expert Verlag, 2007) (German) (4) Trabesinger, A. Power Games. Nature 447, (2007) (5) Freymann R. et al. The second generation turbo steamer, MTZ 73, (2012) (6) Freymann R. et al. The Turbosteamer: A system introducing the principle of cogeneration in automotive applications, MTZ 69, (2008) 39

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