WATER INJECTION FOR PETROL COMBUSTION SYSTEMS

WATER INJECTION FOR PETROL COMBUSTION SYSTEMS Further CO 2 emission reduction of passenger cars is mandatory beyond 2020. FEV has developed a concept of condensed water injection, which is ideally combinable with exhaust heat recovery. The condensed water injection concept demonstrates a fuel consumption reduction potential of 3.5 % in the operation area of minimum BSFC. 26

Authors DR.-ING. MATTHIAS THEWES is Department Manager Thermodynamics in the Business Unit Gasoline Engines of FEV GmbH in Aachen (Germany). DIPL.-ING. FABIAN HOPPE is Scientific Assistant at the Institute for Combustion Engines at RWTH Aachen University (Germany). DR.-ING. HENNING BAUMGARTEN is Vice President of the Business Unit Gasoline Engines of FEV GmbH in Aachen (Germany). DR.-ING. JÖRG SEIBEL is Team Manager Thermodynamics in the Business Unit Gasoline Engines of FEV GmbH in Aachen COMBUSTION ENGINE WILL REMAIN DOMINANT One major challenge for today s society is the sustainable satisfaction of its energy demand. Despite the recent improvements of electrical vehicles, a total independence of internal combustion engines cannot be foreseen for the upcoming years. Policy makers reflect this in the CO 2 fleet targets for all new registered passenger cars and their further planned reduction for 2020 and beyond. The European average of new passenger car registrations in 2013 with petrol engines is characterised by CO 2 emissions of 128.6 g/km, which represents an average annual reduction in CO 2 emissions of 4.7 g/km since 2009 [1]. Still significant fuel consumption reduction is required towards 2020 and beyond. The technical solutions implementable in a time frame until 2020 is described in the next section, before discussing the condensed water injection concept as an idea for further fuel consumption reduction beyond 2020. FUEL CONSUMPTION REDUCTION UNTIL 2020 Starting with a conventional downsized turbocharged engine as baseline, several technologies are available today to reduce fuel consumption, which can and will be introduced in the market soon. FRICTION REDUCTION Friction reduction plays an important role as a cost-effective measure for fuel consumption reduction. Crankshaft friction can be reduced by up to 40 % by minimising the bearing diameters using advanced simulation and measurement tools. Piston ring friction can be optimised by very accurate piston ring dynamic measurements and the utilisation of advanced simulation being able to capture even piston ring twisting. FEV has bundled all these efforts to come close to borderline dimensioning of components [2]. DE-THROTTLING Variable valve lift has already been intro- autotechreview August 2015 Volume 4 Issue 8 27

for the performance reduced engine variants within an engine family. COOLED EXTERNAL EGR 1 CO 2 emissions of a 1 l three-cylinder gasoline engine in a C-segment vehicle considering the application of different fuel consumption technologies Further improvements of the efficiency are possible by using external, cooled EGR. Low pressure EGR can in principle be applied in the entire engine map if sufficient cooling capacity can be provided by the vehicle cooling system. In such a case also, the increase of the geometric compression ratio becomes possible. However, similar to Miller cycle, the utilisation of cooled low-pressure EGR in the entire engine map requires advanced boosting or a performance de-rating from today s state-of-the-art. VARIABLE COMPRESSION RATIO duced into the market on downsized engines for very small to big displacements. Even for highly downsized engines, fuel consumption reductions of around 2 % can be realised. Also cylinder deactivation has been introduced for cylinder numbers as low as four with fuel consumption reductions of up to 8.4 % in the VW Golf [3, 4]. MILLER CYCLE Miller cycle seeks an elongated expansion phase compared to the compression 2 Spray targeting of the two injectors used and technical details of the research engine phase via adapted cam timings. By external charging and heat dissipation in the charge air cooler, the in-cylinder temperature decreases. This reduces the knock tendency and enables to increase the compression ratio. This can be a very cost-effective technology if it is intended for engine variants with low specific performance utilising a single or a two-stage intake cam profile with adapted event lengths. Applying Miller cycle to today s performance levels requires two-stage boosting which adds cost, complexity and development effort. Thus the introduction of Miller cycle is expected predominantly Variability in compression ratio (VCR) has always been investigated in the past decades and several approaches have been developed in order to reduce fuel consumption by means of a higher geometric compression ratio at part load, like the two-stage con rod VCR from FEV [5-7]. When combined with Miller cycle and/or cooled external EGR, it can serve as the technology to avoid necessity for twostage boosting by enabling to run on a normal low compression ratio without Miller cycle or cooled EGR at full load. At part load, the entire fuel consumption benefits of Miller cycle and cooled EGR can be exploited. 1 illustrates the potential of the above mentioned technologies exemplarily for different certification procedures for a 1.0 l three-cylinder engine, demonstrating that the achievement of the 2020 fuel consumption legislation is possible even without hybridisation [8]. EXHAUST HEAT RECOVERY FOR FUEL CONSUMPTION REDUCTION BEYOND 2020 3 Influence of the start of the water injection at n = 2,000 rpm and IMEP = 10.5 bar 28 Several solutions have already been proposed to make use of the energy that is still present in the exhaust gases of a combustion engine. These solutions may provide intake manifold temperatures below ambient conditions, electricity via thermo-

4 Influence of the water pressure at n = 2,000 rpm and IMEP = 10.5 bar electric generators or mechanical energy via the Rankine processes. A fuel consumption reduction of 1 to 2 % for the NEDC has, for example, been predicted for a thermoelectric generator with further improvements on the system [9]. Systems using a Rankine process for exhaust heat recovery have been put to demonstration stage for passenger car applications even as the thermoelectric generator [10] and research for passenger cars is still ongoing [11]. Also, the combination of both systems has been proposed to cover a bigger portion of the relevant temperature range of the exhaust gases [12]. As a summary it can be stated that exhaust heat recovery can contribute to further fuel consumption reduction, likely limited to < 5 % for passenger cars. WATER INJECTION In this respect, FEV sought for potential synergies that could be enabled by exhaust heat recovery in order to improve the combustion efficiency even further and developed the condensate injection concept. Additional heat rejection enables to cool down the exhaust gas below the dew point. The condensed exhaust gas is fed back into the engine. This operation principle allows the entire injected condensate to be recycled. The loss in water from the humid exhaust gases leaving the system is compensated by the water formed during the combustion. Investigations regarding the potential benefits of this concept were carried out on a single cylinder engine as already described in previous publications [13, 14] that has been equipped with a dual DI injection system to enable both, condensate and petrol fuel, to be injected directly into the combustion chamber. More technical data and information can be derived from 2. First tests were conducted in order to determine the ideal timing and pressure for the water injection. Fuel was injected via the central injector and water via the side injector. The results of a start of injection (SOI) variation in the operation point of IMEP = 10.5 bar at n = 2,000 rpm are depicted in 3. The 50 % mass fraction burned point can be advanced by nearly 5 CA compared to the MFB 50 without water injection, if the water injection is started around the closing of the intake valve with the best SOI at 120 CA BTDC. Advancing or retarding the start of injection results in reduced gains in MFB 5 Influence of the injected water quantity at n = 2,000 rpm, IMEP = 14.5 bar and n = 3,000 rpm, IMEP = 14.6 bar 50 and increased burn duration. NOx emissions can be reduced by 5.6 % at the optimised SOI. The hydrocarbon emissions increase by up to 11 % due to the occurrence of more quenching as a result of the additional dilution of the cylinder charge and the reduction in combustion temperature. A clear correlation between MFB 50 and water pressure can be derived from 4, such that lower water pressures result in worse MFB 50. The reason for this behaviour is expected to be the time span required for injection and evaporation. This time span worsens with lower pressure and consequently the end gas temperature and knock propensity is not reduced as effectively as with high pressure levels. In the following, a water pressure of 100 bar and a start of the water injection of 120 CA BTDC was chosen for determining the potential of the condensate autotechreview August 2015 Volume 4 Issue 8 29

6 Comparison of side and central water injection in a water/ fuel ratio variation at n = 3,000 rpm, IMEP = 14.6 bar injection concept in combination with Miller cycle and cooled external EGR. The load was increased to IMEP = 14.5 bar at n = 2,000 rpm and the EGR rate was preoptimised. MFB 50 decreases linearly with increased water quantity and reaches the optimum of ~ 8 CA ATDC at a water/ fuel ratio of 50 %, 5. Hydrocarbon emissions also increase linearly with the water quantity and as a consequence the best fuel consumption is achieved at a water/ fuel ratio of 37 %, with a fuel consumption reduction of 3.5 %. Similar results are achieved if the engine speed is increased to n = 3,000 rpm at almost constant load. The maximum gain in fuel consumption is 3 % at a water/fuel ratio of 50 %. The water/fuel ratio variations have also been repeated using the central injector for the water injection and the side injector for the petrol injection, as depicted in 6. Fuel consumption is nearly identical between both configurations with slightly higher knock restriction for petrol injection via the side injector. As a consequence, a higher water/fuel ratio is required to achieve optimal combustion phasing. More detailed CFD analysis will be required to understand if this phenomenon is caused by differences in the petrol fuel mixture preparation or the water mixture preparation. SUMMARY AND OUTLOOK The target of this study was to explore the potential of combustion systems beyond 2020. Exhaust condensate can 30 become available in the context of exhaust heat recovery to realise an engine concept with dual direct injection. The results of the conducted engine investigations with this concept support the following main conclusions: :: In the region of the minimum BSFC, the efficiency of the engine can be increased by ~ 3.5 %; :: Condensate injection will allow a further increased efficiency also at part load for an engine concept with VCR since a higher compression ratio can be used; :: For lean burn engines with NOx storage catalyst, a further fuel consumption benefit can be achieved by the lower NOx raw emission level and thus longer intervals between regenerations; and :: A comparison between side water/central petrol and side petrol/central water injection has revealed no significant differences between both concepts. Improvements of ignition systems could support the concept even further by supporting a fast and safe ignition of the cylinder charge, which is not only diluted by EGR but also by the additional water. REFERENCES [1] European Environment Agency: Monitoring CO 2 emissions from new passenger cars in the EU. Summary of data for 2013 [2] Baumgarten, H. et al.: Borderline Design: CO 2 -Potential of Conventional Technologies for Gasoline and Diesel Engines, 35th International Vienna Engine Symposium, Vienna, 2014 [3] Middendorf, H. et al.: The 1.4-l TSI Gasoline Engine with Cylinder Deactivation In: MTZ 73 (2012), No. 3 [4] N.N.: http://www.volkswagen.de/content/me- dialib/vwd4/de/dialog/pdf/golf-a7/katalog/_jcr_con- tent/renditions/rendition.download_attachment. file/golf_katalog.pdf. Retrieved 17.07.2014 [5] Pischinger, S. et al.: Two-stage Variable Compression Ratio with Eccentric Piston Pin.In: MTZ 70 (2009), No. 2 [6] Pischinger, S. et al.: Two-stage variable compression ratio with eccentric piston pin and exploitation of cranktrain forces. SAE paper 09PFL-0468 [7] Wittek, K.: Variables Verdichtungsverhältnis beim Verbrennungsmotor durch Ausnutzung der im Triebwerk wirksamen Kräfte. Dissertation, RWTH Aachen, 2006 [8] Pischinger, S.: Future Powertrains for Passenger Cars in 2020. FISITA 2014 World Automotive Congress, Maastricht, 2014 [9] Liebl, J. et al.: The Thermoelectric Generator from BMW is Making Use of Waste Heat. In: MTZ 70 (2009), No. 4 [10] Freymann, R. et al.: The Turbosteamer: A System Introducing the Principle of Cogeneration in Automotive Applications. In: MTZ 69 (2008), No. 5 [11] Smague, P. et al.: Integrated Waste Heat Recovery System with Rankine Cycle, 22nd Aachen Colloquium Automobile and Engine Technology, 2013 [12] Neugebauer, S. et al.: Analysieren, Verstehen und Gestalten - ein Gesamtansatz zur konsequenten Vermeidung von Wärmeverlusten [13] Thewes, M. et al.: Analysis of the Impact of 2-Methylfuran on Mixture Formation and Combustion in a Direct-Injection Spark-Ignition Engine. In: Energy & Fuels, doi: 10.1021/ef201021a [14] Thewes, M. et al.: Analysis of the Effect of Bio-Fuels on the Combustion in a Downsized DI SI Engine. SAE paper 2011-01-1991 Read this article on