The D2676 marine engine developed by MAN Engines is based on the D26 engine, which has proven its worth in heavy-duty vehicles, and provides an optimal, compact, low-consumption engine with a low performance weight for use in boats. It not only meets the current exhaust gas standards, but is also equipped for future exhaust gas legislation. The engine solutions for the various applications in a variety of boat types are described below. This demonstrates how the targeted use of specialised and standard parts from the vehicle engine could be used to develop a comprehensively validated engine at a feasible cost.
The idea behind developing a new six-cylinder serial engine for marine applications was to replace the D28 engine that has been serially produced since the 1990s, thus also ensuring the future of the entire low-performance segment in all marine applications. The aim was to develop an engine series that would cover the requirements of two fields of application: on the one hand they were to be used in work boats in both a classified and an unclassified version, suitable for full-load operation and year-round use. On the other hand, adapted solutions for yachts with a high specific output are also to be developed. While the planning and design places particular emphasis on life cycle costs due to the use of these engines in the shipping industry, a high-powered engine with an attractive design is to be developed for yachts
The basis of the freshly-introduced engine has already been created in 2003 when the D20 and in 2006 when the D26 engines became available as standard in the TGA utility vehicle ranges. Since then, the larger D26 engine has been introduced in almost all the MAN business segments. This not only involves their use in MAN's own heavy-duty vehicles and buses, but also in almost the entire external engine business for OEM customers, ranging from industrial, rail and agricultural applications to gas and diesel gensets with a performance spectrum from 147 to 440 kW (200 to 600 hp).
As is usual for MAN, the marine engine featured here also covers all the operating modes, from light-duty to medium-duty and heavy-duty, thus meeting the varied customer demands, from light implementations in pleasure boats to heavy-duty implementations in work boats (Table 1).
Due to the wide range of applications for the D26 base engine, a series of well-tested components successfully used in serial production could be employed in low-performance marine applications. The combustion-related components were specifically optimised for more high-powered applications. The long-term tests were carried out both on the test bench and in field tests, mostly using high-performance engines. The standard parts concept described below means that the results of these tests could also be transferred to the low-performance marine-specific components.
In addition to the validation advantages described above, there are also commercial reasons for pursuing a standard parts strategy. In order to be able to offer high-quality products at attractive prices despite comparatively low unit numbers, standard parts were consistently used in the development of the D26 marine engine, as had been the case with the latest V-engine generation . This was based on three strategies: Using components from the D26 vehicle engine series, other D26 industrial applications and other marine engines.
The large-series technology particularly came in handy in the development of the so-called base engine from the vehicle engine. Components such as the crank case, crank shaft, piston rod, cylinder head, oil and coolant pumps, oil filter, flywheel housing, wheel drive, etc. could all be used without adaptation. In some cases minor modifications made the use of complex components possible. Thus the engine performance could be improved simply by changing the transmission of the high-pressure injection pump to the vehicle engine, so that the same pump can be used for the entire series of marine engines. As indicated, other combustion-related components such as pistons and the injection of the vehicle engine can be used for lower-powered marine engines.
In deviation from the standard parts strategy, the base engine makes use of different camshafts and a specially developed turbocharger. Existing semi-finished products could be used in the development of the camshafts. In order to achieve higher engine performance and to reduce fuel consumption, adapted pistons, valves and injectors were used (Fig. 1).
Other components from the wide range of marine engines that had proven their worth in serial production were also used for this engine series, sometimes with slight modifications. When it comes to engine cooling, the intercooler, salt-water pump, the plates of the engine heat exchanger, the coolant connection and the thermostats were taken over from other engines. Apart from the complete engine controls, the alarm system with its various displays and many other electrical and electronic components have made use of existing attachments such as the engine mounts, the air filter, flywheels and classified filter elements (Fig. 2).
With regard to thermodynamics and combustion, the six performance variants are to be created with only two different basic component states (A and B). The main differences are in the camshaft and turbocharger, as well as the piston and injector. The various emission settings in accordance with the strictest EPA Tier 3/EU sports boat regulations that apply from 2016 onwards (5,8 resp. 5,6 g/kWh NOx+HC) and the IMO Tier II/EU inland waterway transport regulations (7,2 g/kWh NOx+HC) could be met by merely adapting the data set (Table 1). As was the case with the V-engine , the timing was adjusted by means of simulation and follow-up tests on the engine test bench.
In the performance range up to 412 kW, the combination of the turbocharger and camshaft could be used together with the Atkinson cycle for the heavy-duty engines and the Miller cycle for the medium-duty engine. Both cycles reduce the effective compression ratio in the cylinder. While the time interval for cylinder filling is much abbreviated with an increase in speed, the gas speed remains relatively constant by comparison. This allows the maximum effective compression ratio to be achieved with ever later "inlet-closing" times, taking the gas dynamics into account and thus permitting a further inflow of combustion air. This in turn results in the Miller and Atkinson cycles counteracting each other at high speeds. At low speed, the Atkinson timing reduces the effective compression ratio more than at high speed. The Miller timing works exactly in the opposite way (Fig. 3). The use of extreme Miller timing would result in higher throttle losses  caused by short valve strokes.
By reducing the effective compression ratio and transferring the compression work to the turbocharger, the combustion temperature can be lowered if the intercooler is adequately dimensioned  to cope with the resulting higher final compression temperatures. Despite the use of Miller and Atkinson cycles, the standardised intercooler designed for 588 kW achieves a charge air temperature of approximately 38°C, even at a sea-water temperature of 32°C. The reduction in the combustion chamber temperature in turn results in a reduction of the thermally formed nitric oxide. In combination with a clever fuel injection strategy, this can significantly reduce consumption.
As previously mentioned, the injector and steel piston are the same as for the vehicle engine. For the injector it was possible to fall back on tried-and-trusted, well-adjusted injection nozzle geometry. The robust steel piston is ideally suited to meet the high operating time and load spectrum requirements in heavy-duty marine operation.
A filling-optimised camshaft is used for engine powers of 478 kW and above; together with the turbocharger, this guarantees a high air turnover. To avoid long injection times and high exhaust gas temperatures, the nominal throughput of the injector was increased by approximately 30% for the high-performance range. The adaptation of the pistons to the higher thermal requirements included the microsections of the liners and detent geometry (Fig. 1).
To optimise combustion, the turbochargers of both configuration variants were specially redesigned and comprehensively validated for the D26 marine engine.
One of the requirements of SOLAS (International Convention for the Safety of Life at Sea) is a maximum surface temperature of 220°C for all engine components, so that potentially leaking fuel does not ignite . This renders it necessary to screen the parts of the engine through which exhaust gas is ducted towards the outside. In order to provide customers with a rugged and durable solution, the D26 has an exhaust gas routing area that is fully encapsulated in a coolant-ducting shell. In addition to the insulating effect, the shell duct all engine coolant to the engine heat exchanger in a flow-optimised manner. The air gap insulation between the turbine resp. the exhaust gas pipes mounted on the cylinder head and the cooling shells, there is little heat loss in the exhaust gas system, so that almost the entire exhaust energy is made available to the turbocharger. Moreover, tension cracks in the components are avoided by maintaining high temperature gradients in comparison with directly cooled systems.
When it comes to work boats in particular, it is important to provide classified engines. This means that the regulations of various boat classification associations (e.g. DNV-GL) must be fulfilled, so that the engines can be used in classified boats. In addition to redundancy in the electrical/electronic systems and the sensors, this includes switchable filter units for fuel and oil, double-walled injection pipes and an encapsulated rail. In the fuel-bearing area in particular, no plastic or aluminium materials may be used. By taking these requirements into account from the very beginning, the component variance could be minimised and the required measures implemented with only a few new, engine-mounted parts (Fig. 4).
Some boat types have an integrated cooling system in the hull, which is also used for engine cooling. This variant, the so-called keel cooling, which does not require an engine-mounted heat exchanger, is provided for all performance classes. In this variant, charge air cooling also takes place with the aid of the boat's cooling system in a closed circuit, for which the sea-water pump is used, that normally operates in an open circuit.
As power take-offs a separate connection for hydraulic pumps (e.g. for a thruster) and a shaft extension at the crankshaft (e.g. for extinguisher pumps) are available. In addition, the customer can choose among other optional equipment offered by MAN. This includes a control lever and the corresponding control system, various displays, a second generator and an oil extraction and filling pump. Two oil sump variants were developed for ideal adaptation to the installation height and the banking requirements of various boats.
The interplay of all components in practice was tested worldwide at an early stage onboard different types of vessels in order to validate the various influences of different operating conditions (climate, fuel quality, load profiles, etc.) and to integrate customer experience into the development (Fig. 5).
The design of the engine is in accordance with functional requirements. As yachts are characterised by extremely limited engine spaces, the engine was designed to be as compact as possible. At the same time, the emphasis was on easy accessibility of the areas to be serviced. The filters, as well as filling and maintenance openings are all positioned so that they are accessible both from the top and the side. The engine can be ideally integrated into the existing engine space by using the serially produced, swivel manifolds used for the exhaust gas outlet and the sea-water inlet. The base of the engine was also designed in such a way that it can be adjusted to various installation widths, thus making the engine ideal for engine replacements. All electrical and electronic components are integrated into the design at an early stage, thus optimally integrating and attaching the cable harnesses to the engine.
For yacht customers, the engine is available with a cover bearing a design typical for MAN that also places great emphasis on functionality. All maintenance areas are easily accessible by simply removing the central section attached by clip-on connections. In 2016, the Yacht engines, named MAN i6-800 and MAN i6-730, have won the international Red Dot award for their excellent design quality.
The D26 marine engine was consistently designed and developed on the basis of the current series of vehicle and industrial engines for use in the shipping industry. Given the high number of variants and the low unit numbers typical for this market segment, targeted combustion solutions and a low parts variance helped to provide a wide range of performances and applications while making use of a comprehensive validation basis, thus fulfilling the most stringent quality requirements.
 Stein, Huneke, Reetz: Derivation of engines for various applications from one baseengine exemplified by the MAN D2868/D2862 V-engine, 7. International MTZ Symposium, Heavy-duty, on- and off-highway engines, Nuremberg, 2012
 Nagler, Huneke: Gas exchange optimization of marine engines during introduction of EPA Tier 3 emission standard, 9. International MTZ Symposium, Heavy-duty, on- and off-highway engines, Saarbrücken, 2014
 Theißl, Kraxner, Seitz, Kislinger: Miller-Steuerzeiten für zukünftige Nutzfahrzeug-Dieselmotoren, MTZ 11/2015
 Schutting, Neureiter, Fuchs, Schatzberger, Klell, Eichsleder, Kammerdiener: Miller- und Atkinson-Zyklus am aufgeladenen Dieselmotor, MTZ 06/2007
 SOLAS II-2 Reg. 184.108.40.206.1
Authors (as at June 2016)
The authors thank their colleague, Thomas Malischewski, for his assistance in compiling this article.