MAN D26 Fendt engine
From street to field – Development of the first load-bearing 13L tractor engine
Motivation
In recent years, tractor performance has increased due to a rising demand for extra productivity on the field. With the Fendt 1000 Vario and MAN D2676 LE5xx engine, Fendt – the premium brand of the AGCO Group[1] in agricultural engineering – sets a new power class for standard large tractors (Figure 2).
Over the last 20 years and following recent developments by a number of manufacturers, the engine performance of standard tractors has increased from around 200 HP (147 kW) to the high level of approximately 400 HP (294 kW). At the time the engine was developed for Fendt, there were almost no standard tractors between 400 and 500 HP (294 and 368 kW) on the market. Specialised tractors, such as tracked units and tractors with articulated steering, cover the top end of the scale above 500 HP (368 kW). One of the advantages of a standard tractor over a specialised tractor is its manoeuvrability. The crucial factor here is the steering angle, which increases as the engine or chassis becomes narrower. Fendt sacrificed a simple frame construction in favour of the larger steering angle, and decided on a narrow block design, also known as the load-bearing structure. To meet these demands, MAN developed a narrow, load-bearing 12.4-litre engine in four performance classes, 400 HP, 440 HP, 480 HP and 520 HP (294 kW, 323 kW, 353 kW and 383 kW).
- Development
- Specification
- Load-bearing structure
- Manoeuvrability
- Exhaust gas aftertreatment
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 robustness of the power train components by using the D2676 in off-road applications 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
To apply the D2676 engine to the tractor, both high power at nominal speed at the best possible fuel efficiency (Area 1 in Figure 4) and high torque at low speeds are required. To realise the tractor-specific torque curve for the D26 engine with exhaust gas recirculation, two solutions were under discussion:
- A revision of the two-stage turbocharger of the Euro 6 truck engine
- A new turbocharger with Variable Turbine Geometry (VTG)
Compared to two-stage turbocharging, VTG turbocharging in the tractor engine has a number of advantages. On the one hand, the charge air system has simpler piping and the VTG turbocharger itself is characterised by its extremely compact design. On the other hand, the VTG technology is significantly more robust for off-road requirements due to the elimination of an intercooler.
VTG technology provides optimum dynamic response in all speed ranges. In particular, the response behaviour for engaging the heavy PTO devices was optimised (Area 2 in Figure 4). Fuel consumption is secondary due to the short amount of time spent in this area during the shifting process.
The newly developed protection functions allow the engine to be operated up to an altitude of 3,000 metres without power reduction.
For agricultural transport, tractors with a max. driving speed of 60 km/h, equipped with an exhaust brake flap, compete with trucks. By using the VTG technology with sliding ring, the brake output is doubled, compared to the towing power, without the use of additional systems such as brake flaps or valve brakes.
Tractor-specific design
The load spectra of the truck and the tractor are fundamentally different. While the engine in road traffic runs mostly in the lower speed range at medium load, the tractor is primarily operated at higher working speeds pressed to full load. The full-load share can be up to 90 %, depending on the type of application. In order to take these increased stresses into account, the D2676 LE5xx for off-road use is – in contrast to the engine for on-road use – equipped with steel pistons that have already proven themselves in various Euro 5 applications.
To compensate for the increased heat input, the cooling of the piston crown and the ring belt was optimised by making appropriate adjustments to the oil spray nozzles and the oil circuit. After 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.
a) A prerequisite for a 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) Furthermore, agricultural tractors require the supporting oil pan to fulfil an additional fundamental function: the absorption of mechanical loads from the running gear. Another task is to provide sufficient stiffness of the supporting oil pan to transmit minimal deformation into the crankcase. The residual deformation may only affect the cylinder liner and bearing channel to the extent permissible. Consequently, the oil pan must be designed with sufficient bending and torsional stiffness to protect the engine mechanics against deformation. Figure 6a shows the developed oil pan with new cross ribs. GJS-400 with a high modulus of elasticity was chosen as the material 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 crankcase flange by the “connecting rod violins” (Figure 6b). The resulting structural weakness must be compensated for in terms of construction in order to continue to realise the greatest possible section modulus. For this purpose, the interrupted component that contributes to the section modulus was arranged vertically. In comparison with state-of-the-art oil pans, the U-shape of the cross ribs in this construction creates a high section modulus, thus increasing the rigidity of the oil pan.
c) In addition to structural weak points and the stiffness of the oil pan, the relative movements between load-bearing components resulting from deformation due to torsion and other load cases must be taken into account. As a primary task to ensure tightness at the joints, “gapping” (Figure 7) and “sliding” must be minimised.
In order to minimise this relative movement, it is possible to increase the pre-load of the bolted components. This can be achieved by an increased number of bolts, a larger bolt diameter, a higher bolt strength class as well as the angle controlled tightening of the screws. In the D2676 LE5xx, all of the four measures are effective in design and production.
Figure 8a shows the flange between the oil pan and the crankcase. In truck use, it is sufficient if the oil pan yoke is held in place with M8 bolts. Figure 8b shows the increased number of bolts and changed size of the bolting for load-bearing use on tractors. This measure requires a machining variant of the crankcase on the side of the oil pan and the flywheel housing. A common raw part of the crankcase, machined in two different ways, fulfils the different demands of on-road and off-road applications. The flexible processes in MAN production enable a cost-optimised solution for the two variants of the crankcase as a modular system.
The relative movements were also measured and examined on the distortion test bench. A final field test finally confirmed the tightness at the joints of the load-bearing structure.
As any remaining deformation of the oil pan is transferred to the crankcase, sliding also occurs at the sealing point between 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 running gear.
For this reason, the primary goal at this sealing point as well is to minimise sliding and the wear caused by it. This guarantees that the seal is leak-proof, although the combustion process can cause additional deformations. As a result, there had to be a compromise when designing the screw connections for the crankcase. On the one hand, the pre-loads selected for the oil pan screws could not be so high that the crankcase would be connected too tightly to the twisting oil pan. This would transfer the deformations from the oil pan to the crankcase directly and fully. On the other hand, the deformations must not cause any excessive relative movement because this would affect the seal at the joint between the oil pan and the crankcase.
This compromise for a controlled permissible relative movement of the load-bearing parts was fully met with the design in the D2676 LE5xx.
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). No leakage of the replacement medium was detected during torsion of the load-bearing structure.
The requirement for a short wheelbase and thus an optimum turning circle with the 12.4-litre engine led the customer to dispense with the suction fan. On the one hand, this means that it was possible for the pressing fan to be equipped with a hydrostatic drive and for the radiator system to 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 the balancing, another distinguishing feature from the truck engine resulted from this development. This is the introduction of the optimised damper with an inertial mass for vibration damping and a new robust hub with the interface to the hydrostatic fan drive.
SCR exhaust gas aftertreatment (AGN) systems have been successfully used in numerous on-road commercial vehicle applications since Euro 5. However, the mostly cubic design of the fixed-frame AGN 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, that can provide flexible installation space solutions in various off-road and marine applications, and is at the same time designed to cover a high engine performance spectrum. This was associated with the requirement to prepare the aqueous urea solution in a wide range of dosing quantities over the shortest possible distance in such a way that deposits are prevented to a high degree. Another prerequisite for keeping the volume of the exhaust gas aftertreatment system low was the optimisation of the SCR catalytic converter with regard to catalytic activity and uniform distribution of the reducing agent on the SCR substrate in the inlet chamber[4]. The solution is a modular AGN for both V-engines and in-line engines with single EDC described here. Figure 10 indicates the flexibility of the modular AGN design when arranged in the tight installation space in a tractor. With a small number of tractor-specific components, the core components from the AGN modular system were optimally arranged according to customer or legal requirements (such as visibility from the driver’s position and maximum permissible vehicle width).