DE VELO PMENT Heav y- dut y Engines
Variable Compression for Large Engine
AUTHORS
Dr.-Ing. Udo Schlemmer-Kelling is Executive Engineer Technology at FEV Europe GmbH in Aachen (Germany).
Dipl.-Ing. Stefano Ghetti is Senior Project Manager within the Business Unit Diesel Powertrains of FEV Europe GmbH in Aachen (Germany).
Dipl.-Ing. Peter Methfessel is Research Assistant at the RWTH Aachen University (Germany).
© FEV
In order to meet future requirements in the marine sector, different technologies are needed. The most important
Christopher Marten, M. Sc. is Research Assistant at the RWTH Aachen University (Germany).
technologies like two-stage charging, variable valve train, Miller timing and waste heat recovery are explained in the technical literature [1]. The variable compression ratio is explained as follows in more detail.
IMPROVING THE EFFICIENCY OF DUAL-FUEL ENGINES
Global trade continues to rise steadily and continuously reaches new peak levels. This trend can be attributed in particular to container handling, which has increased by 20 % over the past six years [2]. In the maritime sector, the regulations of the International Maritime Organization (IMO), which address
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the reduction of air pollutants from shipping as well as energy-efficiency of vessels, are driving the development of future propulsion systems. One pillar of these regulations was the introduction of so-called Emission Control Areas (ECA) in major coastal areas [3]. The use of dual-fuel engines, capable of diesel and LNG operation, is one promising approach for vessels, which operate most of the time on the high seas but
also enter these ECA’s, to optimise the overall efficiency despite the trade-off between fuel efficiency and emissions. For such engines, a variable compression ratio (VCR) offers the possibility to adjust the base engine geometries to the combustion principle in use. The effects of dual-fuel operation and VCR on combustion efficiencies will be explained and evaluated in detail. Additionally, the commercial point of view, which is
essential for the introduction of innovative technologies, will be addressed. MAIN ENGINE
The engine environment used for the investigation of the VCR system is based on a unit developed by FEV Europe GmbH. The technical equipment reflects a typical marine application and has been further revised. The engine is part of an engine family consisting V- and inline-engines to cover different power demands. TABLE 1 shows the main engine data. The sustainable and modular design extends over the entire engine family. The charging unit consists of a high efficient turbocharger, as well as a twostage intercooler. The VCR system can be optionally integrated into the basic cranktrain configuration of the engine. Crankshaft design is done according to IACS UR M53 guidelines. Rotational mass forces can be completely compensated in the basic cranktrain config uration, as well as in case of using the VCR system. Inside the crankcase the camshaft with overhead valve configuration is located. The valve train enables Miller valve timing in a range of 20 °CA, which leads in combination with VCR technology to a lower emission level of the engine. To fulfill piston cooling demands, pressurised oil flows through an oil bore inside the conrod. This principle is typically for marine applications and will be carried over from the base configuration to the VCR system. As base for an efficient combustion a proper injection equipment is needed. In this case a mechanical injection system with 1800 bar injection pressure is used for heavy fuel oil (HFO) operation.
Engine configuration
[-]
Four stroke, dual fuel
Bore
[dm]
3.9
Stroke
[dm]
5.5
Stroke/bore ratio
[-]
1.4
BMEP
[bar]
Emission level
[-]
26.0
Gas
23.0
Diesel
IMO Tier II
Gas
IMO Tier III
TABLE 1 Main engine data (© FEV)
In gas operation a micro pilot injector is used, which sprays an injection of approximately 1 % of the full load amount of diesel fuel to the combustion chamber to inflame the 99 % gas share. Gas admission is realised into the intake ports of each cylinder, whereas peak fi ring pressure will be adjusted with real-time modules. THERMODYNAMICS
Modern DF engines combine the combustion process of a self-igniting diesel engine with the principle of a spark-ignition engine. TABLE 2 shows the trade-off between both principles and the influences on the efficiency of DF engines. In the previous chapter it was already described, that a pilot injector will be used to inflame the gas at gas operation. The trade-off between diesel and gas mode valve timing will be solved by a variable valve train (VVT). In this case a long valve overlap is possible, as well as a reduced valve overlap for lower methane slip and higher volu metric efficiency.
–
Fuel admission
Injection
Valve timing
Compression ratio
Diesel
Direct injection
Self-ignition
Long valve overlap
16-17 for high efficiency
Gas
Intake port gas admission
Flushed pre-chamber with spark plug
Reduced valve overlap for lower methane slip and higher volumetric efficiency
≈11-13→ Lower compression ratio to avoid knocking at high-load operation
DF
Direct injection (diesel)/ intake port gas admission
Self-ignition / micro pilot injection for self-ignition
Compromise between diesel and gas fired mode
≈11-13→ Low c ompression ratio to realise sufficient power density in gas mode
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Diesel
The following benefits can be carried out for both operating modes, by integrating the VCR system into the DF engine: –– high compression ratio for diesel operation and part load gas operation –– low compression ratio for high load gas operation to avoid knocking tendency. Both operating strategies will be analysed, by using the previous described engine environment and a zero-dimensional two-zone combustion model. FIGURE 1 shows the simulation results for part and full load gas operation in case of an integration of a VCR system into the engine [4]. At full load an air/fuel ratio of λ = 2.2 is used, whereas λ = 1.9 at part load operation is chosen. In addition, the isolines for constant effective efficiency and constant NOx emissions are shown, as well as the NOx emission limit for compliance with the speci fications within the coastal regions (ECAs). The impact of the knocking limit and the peak pressure limitation of 250 bar on the achievable centre of c ombustion points are also shown.
TABLE 2 Trade-off between diesel-/ gas- and DF engines (© FEV)
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DE VELO PMENT Heav y- dut y Engines
FIGURE 1 Strategies for increased efficiency at gas operation of modern DF engines with VCR system (© FEV)
By using FIGURE 1, the following c ontexts can be derived by increasing the compression ratio at constant centre of combustion in partial and full load operation: –– increased effective engine efficiency due to increased thermodynamic efficiency –– at full load slightly increased NOx emissions due to increased process temperature –– strongly increased knock tendency and retardation of knock limit –– at part load increased efficiency and reduced specific NOx emissions –– increased peak firing pressure.
Based on these trends the engine operation is limited to the green operating areas. Within this green area, various layouts for the combustion process can be realised, which are characterised by 1, 2 and 3 for full load, and 1, 2, 2’ and 3 for part load operation. At full load operating point 1 represents an engine configuration, which is designed with respect only to the NOx limit. Therefore the compression ratio is too low for efficient operation. Since the design of modern DF engines is aimed at an optimum gas operation [5], the maximum compression ratio is generally determined by considering the
NOx and knock limit. This context will be illustrated by point 2, with a com pression ratio of ε = 12.3, that reflects a typical maritime application and is also used in the investigated engine. Design point 3 shows a theoretical scenario with a further increase in compression ratio. It can be seen, that an increase of compression ratio leads to retarded combustion position due to knocking. This late adjustment results in an unstable combustion process and an increase in HC emissions at gas operation. The theore tical design points 1 and 3, as well as point 2, which is characterised by a compression ratio of ε = 12.3, are shown for
FIGURE 2 Schematic illustration of the e ccentric operating principle (© FEV)
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FIGURE 3 Benchmark of the piston pin bore ratio (© FEV)
part load operation. By use of the twostage VCR system, an adjustment up to a typical diesel compression ratio of ε = 17, while maintaining the NOx limit (operating point 2’), is possible. This increased compression ratio shifts the operating point close to the knocking limit of the engine, while at the same time the effective efficiency is increased. Based on this range of adjustment (Δε = 4.7) an increase of engines efficiency of ∆ηerel = 4.1 % can be achieved during the gas operation at part load. However,an increase in efficiency of ∆ηerel = 4.2 % is present in diesel operation at full load. This results in a relative consumption improvement of about 4 %. DESIGN
For the design implementation of the VCR system on the above-mentioned large engine, the well-known and proven FEV concept was used, which uses an eccentric adjustment on the small connecting rod eye, which is driven by engines gas- and mass forces [6]. A schematic illustration of the eccentric operating principle is shown in FIGURE 2. At the beginning of the adjustment cycle, the eccentric is located in the top dead centre position (E-TDC) and the support piston (SP), which is attached to the eccentric disc, prevents possible rotation. The dimensioning of the SP is adjusted according to the different magnitude of gas and mass forces (G/M). Based on the high peak firing pressure level in marine MTZ worldwide 07-08|2017
applications the G-SP was implemented as an oval piston with two symmetrically arranged supporting piston rods. The switching event is performed by using a 3/2 directional control valve, whereby the oil from the chamber of the G-SP is forwarded to the chamber of the M-SP or to the oil supply of the piston. This switching operation requires several cycles to move the piston from the E-TDC into the bottom dead centre position (E-BDC), in which only the positive piston forces can be used. Non-return valves allow only one direction of eccentric movement per shift. If the 3/2 directional control valve is switched again, the mass forces ensure that the piston moves again from its E-BDC into the E-TDC position. Due to the integration of the VCR adjusting system just in the small end of the connecting rod, a modular unit replacement of the connecting rod top end is possible. In this case, the lower connecting rod parts are taken over from the conventional connecting rod of the series production. With this concept, it is possible to meet current market requirements for the modularisation of the entire engine and to ensure retrofitting of the VCR system or an optional DF VCR engine version for new acquisition. In the design of the eccentric, several combinations of eccentricity and adjustment angle were examined. The optimum of the parameter variation has been found for the DF application with an eccentricity of 18.5 mm and an
adjustment angle of ± 22.5°. This results in a maximum change in the connecting rod length of 14.2 mm, whereby the already mentioned compression of 12.3 and 17 can be adjusted. A specific challenge in the implementation of the VCR system for large bore engines is the boundary condition of the relatively high stroke-bore ratio (≈1.4) and also high piston pin bore ratio (≈0.45). This results in a reduced package space, which is required for the adjustment kinematics of the VCR mechanism. FIGURE 3 shows the difference between the small end of the current marine VCR connecting rod and the passenger car or commercial vehicle segment. However, to meet future requirements for rising peak firing pressures, further variations of the VCR concept are currently under investigation by FEV Europe GmbH. COSTS
The previous chapters have shown the influence of the VCR connecting rod on the combustion and how the variability has been implemented. However, the following analysis, carried out with the previously defined 15 MW engine as lead engine in a cruise ship, shows the benefits of this technology for the operator. The costs are based on current fuel prices from Rotterdam for heavy fuel oil (≈290 euros/t HFO including transport to the ship = Free on Board) and LNG (≈470 euros/t including Free On Board).
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DE VELO PMENT Heav y- dut y Engines
FIGURE 4 Load profile (left) and TCO over time in ECA (right) (© FEV)
The size of the investment costs refers to the main engine and its additional engine components, such as the VCR system or the variable valve drive, which is implemented to use the variable compression under optimum conditions. For the annual operating costs, the fuel consumption, oil consumption, depreciation and maintenance costs were taken into account. The load profile, FIGURE 4 (left), describes the typical behaviour of an engine installed in a cruise ship with the performances described in TABLE 1 This was followed by a calculation with respect to the total cost of ownership (TCO) on the time in the ECA region, FIGURE 4 (right). Looking at the TCO in a ten-year financing period, with stable costs, the savings will increase with increasing ratio of time in the non-ECA zones. However, the financial result depends heavily on fuel prices, where at the high prices of 2014 the savings are significantly higher than in 2016. It is obvious that for applications, which rarely enter the ECA zones, the HFO operation is the preferred option. If the ECA zone is frequently served or never left, the conventional DF system is recommended. For applications that often leave ECA zones the here presented VCR system is favoured. The total savings potential of a cruise ship with 60 MW of power (four engines with 15 MW each) is about 2.4 % and is about 220,000 euros per year, with a typical ECA share of 70 % at today’s fuel prices.
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SUMMARY AND OUTLOOK
In the presented study a standard DF large bore engine was retrofitted from the FEV Europe GmbH with a VCR and related systems. The following goals were considered: –– highest efficiency and performance with low fuel consumption –– low emissions (IMO Tier III) –– modular engine family concept (inline-/V-engine) –– low costs (production/TCO) –– easy maintenance by integrating the VCR system into the conrod upper part. To achieve these goals the following technical features where taken into account: –– high cylinder pressures –– mechanical injection system for HFO operation –– common rail system with pilot injector for LNG operation –– gas admission for LNG into the inlet ports. The design implementation of the VCR concept was carried out with the support of the FEV Europe GmbH’s existing VCR technology, which had to be adapted according to the requirements of the large bore engine and which allows further optimisation. The calculated efficiency improvements in the diesel and gas combustion have a direct effect on the TCO of the engine operation. The additional investment costs are compen-
sated after one to two years. With the DF equipment and the integration of a VCR system it is possible to allow further emission and fuel reduction in the marine sector. REFERENCES [1] Pischinger, S.: How legislation and customer needs drive innovation. 4 th Rostock Large Engine Symposium, 2016 [2] Maatsch, S.: RWI/ISL-Containerumschlag-Index beendet das Jahr 2016 mit Rekordwert. Online: https://www.isl.org/de/news/rwi-isl-containerumschlag-index-beendet-jahr-2016-mit-rekordwert, access: 24.02.2017 [3] TBA: Abgasgesetzgebung Diesel- und Gas motoren. Frankfurt on the Main: VDMA, 2011 [4] Bergmann, D.; Ghetti, S.; Geiger, J.; Virnich, L.; Methfessel, P.; Marten, C.: VCR – Key technology for high efficient Dual-Fuel-Engines. International FEV Conference Variable Compression Ratio, Garmisch-Partenkirchen, 2017 [5] Merker, G.; Schwarz, S., Teichmann, R. (Ed.): Grundlagen Verbrennungsmotoren: Funktionsweise, Simulation, Messtechnik. Wiesbaden: V ieweg+Teubner, 2011 [6] Wittek, K.: Hubkolbenverbrennungskraftmaschine mit einstellbar veränderbarem Verdichtungsverhältnis. Patent DE102005055199 A1, 24.05.2007 [7] Meyer, M.: China zieht die “ECA-Zügel” an. In: HANSA – International Maritime Journal 153 (2016), No. 4, p. 46
THANKS The authors wish to thank Dipl.-Ing. Holger Schever, Dr.-Ing. Jose Geiger, Stefan Wedowski and Lukas Virnich for their support in creating this article.
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