MAN D26 Fendt engine

The design was based on the fully developed and tested D2676 Euro 6 power train for trucks, which was adapted and revised to meet tractor requirements (Figure 3). MAN demonstrates the performance of the tried and tested D26 power train components using the new off-road application in the field for heavy soil cultivation. Table 1 compiles the fundamental characteristics of the new D2676 LE5xx off-road engine.

More about D2676 for agricultural machinery

Commercial vehicle-specific design

High performance at nominal speed with the best possible fuel efficiency (Area 1 in Figure 4) and high torque at low engine speed are both required for the D2676 power train to be used in tractors. There are two possible solutions to achieve the tractor-specific torque profile on the D26 engine with exhaust gas recirculation:

- Revise the two-stage turbocharger on the Euro 6 truck engine.

- Design a new turbocharger with variable geometry.

In comparison with the two-stage turbocharger, the VGT offers many advantages for use in tractor engines. Firstly, its air system has simpler pipe-work, and the VGT itself is characterised by its extremely compact design. In addition, to help with the demands of off-road work, VGT technology is significantly more robust thanks to its lack of an intercooler.

VGT technology offers optimal dynamic response at any engine speed. The response for heavy power take-off (PTO) work in particular has been optimised (Area 2 in Figure 4). The fuel consumption is not important for the short amount of time spent in this area during PTO shifting.

The newly developed protection functions mean that the engine can operate up to an altitude of 3,000 metres without reduced performance.

For agricultural transport, tractors, with a maximum speed of 60 km/h and equipped with an exhaust brake flap, compete with trucks. The braking performance of the engine is doubled by VGT technology with a sliding ring compared to engines without the use of additional systems such as brake flaps or valve brakes.

Tractor-specific design

The load spectrums for trucks and tractors are completely different. On-road engines largely run at lower speeds under average loads, while a tractor primarily operates under full load at high engine speeds. The full load percentage can reach up to 90%, depending on the mode of operation. To account for these increased stresses, in contrast with on-road engines, the D2676 LE5xx for off-road use is equipped with steel pistons that have been tried and tested in various Euro 5 applications.

To compensate for the increased heat input, modifications to the oil spray nozzles and the oil-circulation system have improved the cooling of the piston crown and the ring belt. Through extensive functional and endurance testing, the piston ring pack and the cylinder liner have been completely re-engineered, with plateau honing for reduced oil consumption and improved durability.

Engines that form part of the vehicle framework and carry some of its load are considered load-bearing structures. This means that the engine not only works as the main drive of the vehicle, but also simultaneously provides the necessary rigidity to the vehicle framework (Figure 5a).

In off-road use, three load cases are most relevant for the structural mechanical design of load-bearing engines. The example of a load case caused by one of the front wheels moving over an incline, (Figure 5b) explains the "torsion" of the load-bearing structure and its effect on the engine components. In this load case, three aspects of the structural mechanical design of the engine components must be considered:

a) A prerequisite for further design of the load-bearing parts is a standard engineering method – a numerical structural analysis using the Finite Element Method (FEM) to identify and evaluate structural weak points [3].

b) In addition, the load-bearing oil sump in agricultural tractors must fulfil another fundamental function: carrying mechanical loads from the chassis. Another task involves the load-bearing oil sump being sufficiently rigid to reduce the amount of deformation of the crankcase to a tolerable maximum. The influence of the deformation on the cylinder liner and bearing channel must be within the permissible limits. Consequently, the oil sump must be designed to be sufficiently resistant in order to prevent the powertrain’s components from deformation. Figure 6a shows the developed oil sump with new design of cross ribs. The raw material GJS-400 with a high modulus of elasticity was selected for low deformation. A tubular shape offers the largest section modulus against torsion using the least material, as determined by FEM topology optimisation. However, this tube shape is interrupted at the flange of the crankcase by the space required for the connecting rod (Figure 6b). The structural weakness that this interruption causes must be compensated for in order to continue to achieve the greatest possible section modulus. To do so, the interrupted component that contributes to the section modulus is arranged vertically. In comparison with state-of-the-art oil sumps, the U-shape of the cross ribs in this construction creates a high section modulus, thus increasing the rigidity of the oil sump.

c) In addition to structural weak points and the rigidity of the oil sump, the relative movement between the load-bearing components caused by distortion from torsion and other load cases, must be considered. The primary task is to guarantee that the joints are leakproof, which requires minimising “gapping” (Figure 7) and “sliding”.

To minimise this relative movement, the pre-load of the threaded connections can be increased. This is achieved by increasing the number of screws, the screw diameter and the screw strength category, and by angle controlled tightening of the screws. On the D2676 LE5xx, all four measures are used in the design and the manufacturing process.

Figure 8a portrays the flange between the oil sump and the crankcase. Using M8 screws to hold the oil sump bay is sufficient for trucks. Figure 8b indicates the increased number of screws and the modified screw connections for load-bearing use in tractors.

This measure requires a different machined version of the crankcase on the flanges where the oil sump and the flywheel housing are attached. A common raw crankcase, machined in two different ways, fulfils the different demands of on-road and off-road applications. The flexible manufacturing processes at MAN support a cost-effective solution for the two variations of the modular crankcase.

Relative movement has also been measured and investigated on the distortion test bench, and a final field test confirmed that the joints of the load-bearing structure are leak proof.

As a minor deformation of the oil sump is transferred to the crankcase, there is also sliding at the sealing point between the cylinder head and cylinder liner. The design of the cylinder head gasket for trucks is largely based on the ignition pressure. However, on tractor engines the seal is subjected to the ignition pressure and to deformations transferred from the chassis.

For this reason, minimising sliding is the primary goal at this sealing as well. This guarantees that the seal is leakproof, although the combustion process can cause additional distortions. As a result, there had to be a compromise when designing the screw connections for the crankcase. On one hand, the pre-loads selected for the oil sump screws could not be so high that the crankcase would be connected too tightly to the twisting oil sump, as the distortion from the oil sump would have been directly and fully transferred to the crankcase. On the other hand, the distortions must not cause any excessive relative movement because this would affect the seal at the joint between the oil sump and the crankcase.

The design of the D2676 LE5xx completely fulfils this compromise for controlled permissible relative movement of the load-bearing parts.

The seal of the cylinder head gasket has been proven on a distortion test bench by using a substitute medium to simulate the maximum ignition pressure (oil compressed to ignition pressure). There were no leaks of the substitute medium when the load-bearing structure was twisted.

In real life use, the manoeuvrability of a tractor is just as important as its performance the crucial factor here is the steering angle, which increases as the engine becomes narrower. The very narrow design of the D2676 LE5xx engine means that, despite its large displacement, the front axle has an extreme 36 degree steering angle (Figure 9).

The narrow engine also offers an optimal base for a narrow engine hood, providing the best possible visibility from the driver's position. To fulfil this demand, auxiliary equipment such as the electric generator has been positioned closer to the crankcase. When routing the piping and the cable harness, care was taken to ensure that the wires run along the crankcase housing rather than above the auxiliaries.

The demand for the 12.4-litre engine to feature a short wheel base and therefore an optimal turning circle led the customer to reject the concept of a sucking fan. On one hand, this means that the pressing fan could have a hydrostatic drive and the radiator system could be located near the belt drive, which shortens the wheel base. On the other hand, this measure requires a robust power take-off at the front end of the crankshaft to run the hydraulic pump. This and other measures related to torsional vibration dampening mean that the power train had to be re-balanced with respect to torsional vibration. Due to re-balancing, this development resulted in another feature that differs from truck engines. An optimised damper with an inertial mass for dampening the vibrations and a new, robust hub with the interface for the hydrostatic fan drive were introduced.

SCR exhaust gas aftertreatment systems have been successfully used in numerous on-road commercial¬vehicle applications since Euro 5. However, the rather cubic design of the fixed-frame EGA for trucks is hard or impossible to integrate in many off-road applications. The main challenge for flexible off-road exhaust gas aftertreatment is to provide a system of inlet and outlet chambers in the smallest possible space, including flexible installation solutions for various off-road and marine applications, but that is also designed to cover a wide range of high performance engine use. Linked to this was the challenge of processing the urea/water solution in a short distances over a wide range of dispensed volume in order to prevent deposits from forming as much as is possible. Another prerequisite to be able to minimise the volume of the exhaust gas aftertreatment was the optimisation of the SCR catalytic converter with respect to catalytic activity and uniform distribution of the reducing agent on the SCR substrate in the input chamber [4]. The solution is a modular EGA for both V-engines and in-line engines with single EDC, as described here. Figure 10 indicates the flexibility of the modular EGA design when arranged in the tight installation space in a tractor. With a small number of tractor-specific components, the core components from the EGA design have been arranged to meet customer and legal requirements as well as possible (such as visibility from the driver's position and the maximal permissible vehicle width).

More about the modular exhaust gas aftertreatment