MAN engine D1556 LE5xx

MAN Truck & Bus SE – MAN Engines

Compact and powerful: the 9-liter diesel engine from MAN for off-highway applications

In 2019, MAN launched a newly developed engine with a displacement of 9 liters. In addition to traditional applications for trucks and coaches, the industrial version has been presented at the same time. For its first application in the off-highway segment, the engine is used in agricultural tractors, where SCR-only combustion and exhaust gas aftertreatment have been completely revised and adapted. The low-speed concept and excellent engine dynamics were of particular importance. A large number of field cycles were analyzed during testing and the test program was adapted to the tough conditions in the tractor. Moreover, this report presents activities and results for recording the tractor’s Real Driving Emission in a wide range of applications.

A few years ago, MAN decided to start developing the new D15 engine platform, bridging the gap between the 7-liter and 12-liter displacement class. During the development process, the main objectives included a low power-to-weight ratio and compact dimensions. The straight-six power unit delivers between 205 and 324 kW (279 and 440 HP) and its highest power variant achieves a maximum torque of 1970 Nm between 1150 and 1300 rpm (see Fig. 1, on the left). This makes the MAN D1556 the lightest off-road engine in its displacement class, with a dry weight of just 860 kg. Even at low speeds, it delivers an impressively high torque.

In its first application as an industrial engine, the new D1556 engine is used in tractors where, due to space restrictions and high utilization, it has a maximum output of 305 kW. In order to serve the global market, the engine has been certified for emission stages EU Stage V, EPA Tier 4 Final, and EU Stage IIIa for Low Regulated Countries.

The following figure shows the current state of the power curve in tractors. A competitive comparison (see Fig. 1, on the right) clearly shows that the D15 is in a leading position – particularly at maximum torque.

Fig. 1 Power curve, as well as specific output/effective mean pressure
Fig. 2 CAD representation of the D15 as truck, coach, natural gas and tractor variant

The variants differ, for example, in fan attachments and belt drives, cylinder head covers, and oil sumps made of plastic and aluminum, as well as turbo chargers with wastegate or Variable Turbine Geometry (VTG). In contrast to its diesel counterpart, the E1856 natural gas variant features cooled exhaust gas recirculation (EGR). The coach variant can be supplied with a crankshaft starter alternator as an option.

The following section describes in detail the development (see the following table for technical data) and integration of the D15 into tractors.

Technical data – tractor engine

Engine type D1556 LE5xx
Exhaust gas status EU Stage V, US EPA Tier 4 final, Downgrade UN-ECE
R96, H (complies with EU Stage IIIA)
Number of cylinders/arrangement/valve s per cylinder 6/inline/4
Bore/stroke [mm] 115/145
Displacement [cm³] 9.0
Compression [-] 20:1
Injection/ignition pressure [bar] 2,500/230
Weight (dry) [kg] Approx. 860 (without exhaust gas aftertreatment), approx. 960 (with exhaust gas aftertreatment)
Rated power [kW] at speed [rpm] 217 239 261 283 305
Max. torque [Nm] 1,550 1,650 1,750 1,850 1,970
at speed [rpm] 1,100–1,300 1,100–1,350 1,100–1,400 1,150–1,400 1,200–1,350

SCR-only concept

The emission requirements of EU Stage V and EPA Tier 4 final are fulfilled with SCR-only technology – in other words, without the use of external exhaust gas recirculation (EGR). In addition to the mechanical advantages such as reducing the weight and complexity of the system and relief of the cooling system, there are also considerable advantages for the combustion design. The SCR-only concept makes it possible to optimize the combustion efficiency and thus adjust it to the best possible fuel consumption. Due to the optimum combustion process, particle emissions are at an extremely low level, which in turn has a positive effect on the service life of the particulate filters and minimizes the ingress of soot into the oil. In addition to the advantages of high consumption efficiency and low particulate emissions, the physical properties of a highly efficient combustion process also result in high engine-out NOx emissions. This problem was countered by an adapted combustion chamber geometry that is specifically designed for low nitrogen oxide emissions (see Section 2.2). In conjunction with an optimized AdBlue fluid dosing strategy and a high-performance SCR system, MAN has succeeded in combining optimum fuel consumption with low AdBlue fluid consumption.

Fig. 3 Flame front, combustion chamber temperatures, and NOx emissions in the combustion chamber at 5%, 50%, and 90% combustion progress

Combustion chamber

MAN conducted numerous studies on combustion chamber designs. A large number of different piston recess geometries combined with different injection nozzle variants were studied both in simulations and on combustion test benches, resulting in an entirely redesigned piston recess that is ideal for the SCR-only concept and enables both low fuel consumption and low engine-out emissions.

The piston recess has been designed so that the flame is directed towards the beam splitter on the edge of the piston, splitting the flame into two parts. The lower part of the flame is deflected in the piston recess and runs along the piston towards the piston center. The upper part of the flame is first directed towards the head of the piston via an incline in order for it to spread radially in a further step. This delays the combustion at the incline which means that the peak temperatures are lowered and NOX emissions are reduced (see Fig. 3).

Injection system

The engine is equipped with state-of-the-art common rail injection technology. The injectors feature leakage-free control valves which have a greater hydraulic efficiency than conventional injectors with gap leakage. This reduces the fuel consumption by approx. 1%. Significantly improved hydraulics also increase the accuracy of the injection quantity. Even the smallest pilot quantities can be displayed steadily over the entire service life, which has a positive effect on emissions and combustion noise. A 12-hole injection nozzle is used together with the redesigned piston recess geometry. The maximum system pressure is 2,200 bar.

Turbocharging and engine dynamics

Off-road applications place high demands on torque characteristics and engine dynamics. The deployment profiles range from light, semi-static operation on harvesters, for example, to heavy alternating operation on wheel loaders. Torque and dynamics should accordingly be available over a wide band – even under different climatic conditions and at high altitudes. With the turbo charger with variable turbine geometry (VTG) used, high air masses are provided over the entire engine characteristics range. Even at low engine speeds and small injection quantities, sufficient charging pressure is provided to ensure a fast, transient load response. Load jumps at different engine speeds were run on a highly dynamic engine test bench and the relevant combustion parameters were calibrated in several optimization loops, resulting in dynamic power development over the entire speed range.

Exhaust gas aftertreatment

In the case of tractors, the available installation space of the previous machine model had to be maintained. The previous model’s engine featured exhaust gas recirculation and had a lower max. torque with a lower displacement. The previous model’s engine featured exhaust gas recirculation, low torque, and displacement. The development team was therefore faced with the challenge of improving the uniform flow distribution via the SCR at the one hand side and to optimize the AdBlue fluid preparation with significantly higher exhaust-gas mass flows at higher NOX conversion rates on the other side. At the same time the maximum exhaust-gas back pressure had to been taken into account.

Particulate filters

The diesel oxidation catalyst (DOC) and diesel particulate filter (DPF) geometries were set by the size of the engine compartment lid situated above them. A substrate design with asymmetric channel geometry was implemented to maximize the ash cleaning intervals of the DPF. The field and endurance tests resulted in an ash cleaning interval of at least 8,000 operating hours. The soot loads in the DPF proved uncritical during

the entire test. Due to the high engine-out NOX emission level and the correspondingly high NOX/particulate ratio, it is usually not necessary to regenerate the DPF over and above a fixed regeneration interval of 1000 hours, even in the case of low-load applications. However, in order to validate the quality of the DPF loading model, a vehicle was tested in an extremely low-load application on the MAN plant premises (see Section 4.1). This unilateral and extremely low-load machine application produced DPF regeneration intervals of approx. 250 hours.

Fig. 4 Variants of flow optimization

SCR catalytic converter

The previous model’s dimensions were also adopted for the exhaust piping and AdBlue line.

In the previous vehicle version, the SCR was already dimensioned to 13 inches – the largest standard diameter available on the market for cordierite substrates. It was only by extending the SCR substrates that the higher requirements for exhaust-gas mass flow and NOX conversion were taken into account. The uniform flow distribution at the SCR inlet had to be increased to a uniformity index of at least 0.98. In order to fulfill the requirements regarding uniform distribution and exhaust-gas back pressure, various optimization loops were calculated during a simulation test at different operating points. During validation, the target variant was checked for uniform flow and NH3 distribution on a hot-gas test bench to confirm the simulation results.

Functional test

The functional tests are limited to the parts for use in tractors which differ from the parts for use in on-highway engines. Amongst others, the following parts were subjected to extensive tests: the load on the viscous damper, the alternator, and the air compressor in the heat chamber. To determine their suitability, the components are tested under real installation conditions and high ambient temperatures. Cold-start applications and the legal requirements for the thawing times of the AdBlue system were tested and optimized on two tractors at extremely cold temperatures on the climatic roller test bench at the MAN plant in Munich.

Fig. 5 Testing the boundary conditions on the viscous damper, cold start, belt drive, etc.

The exhaust gas aftertreatment was subjected to targeted low-load testing at the engine plant in Nuremberg. For this purpose, a tractor was operated by the internal Logistics department and equipped with a data logger. At regular intervals, the depositing tendency of the AdBlue® mixer was tested using an endoscope (see Fig. 6) and the load on the DPF was determined by weighing.

Fig. 6 Optimizing the deposits in the AdBlue® preparation (mixer)
Fig. 7 Low-load testing with the help of Logistics

Fig. 7 illustrates the weak utilization of commuter traffic. The application was adapted according to the findings. The fact that the test vehicle is located close to the Development department means that it is much easier to implement the knowledge gained.

Endurance testing

Fig. 8 Test bench – scope of testing

Endurance tests can also be used – and were used – to test the engine in trucks and coaches. In total, the new D15 engine ran for over 200,000 hours on a wide range of test benches. About 10% of this time was spent on off-road applications (see Fig. 8).

Another important validation cornerstone is achieved by testing the engine in the vehicle. This means that the engine is running for a comparable number of test hours in the tractor. During the development phase, more than 20 tractors were validated under the toughest conditions (see Fig. 9).

In order to select the correct endurance program, the actual measured field cycles were compared in advance with the internal endurance cycles.

Fig. 9 Vehicle – scope of testing

While creating the test program, real application cases were specifically examined and evaluated with regard to different criteria.

The following criteria, among others, were used for evaluating the design:

  • Work over the runtime (average engine load)
  • Average speed (number of load cycles)
  • Thermo-mechanical damage using exhaust manifold as an example
  • Switching cycles of mechatronic components

Fig. 10 Comparison of real and test cycles

Fig. 10 compares the measured field cycles with the test cycles. Taking into the account the number of load cycles and the engine load over the runtime (e.g., 8,000 hours), it becomes clear that it is almost impossible to test these factors during an endurance run. In comparison, it is possible to adequately map the thermo-mechanical damage by using generic cycles. The endurance cycles, however, were not optimized with regard to the dynamic load of the actuators, as these were tested during the component tests.

Legal provisions in the EU

For the first time, emissions from internal combustion engines of mobile machines and equipment must be measured in real operating conditions for EU Emissions Stage V within the legal framework of the European Union. Engines in performance categories

NRE-v-5 and NRE-v-6 must be measured with regard to their exhaust tailpipe emissions in real operating conditions. With an output of more than 300 kW, the MAN D1556 LE engine falls into category NRE-v-6.

Attaching the measuring equipment to the exhaust gas aftertreatment

The MAN D1556 LE engine has a modular exhaust gas aftertreatment consisting of DOC/DPF and SCR catalytic converter. Once the exhaust tailpipe has been dismantled, the measuring tube of the PEMS measuring equipment (see Fig. 11) is mounted onto the outlet of the SCR catalytic converter. The exhaust gas is then extracted via a heated line to measure the exhaust gas volume flow. In addition to the gas emissions, data from the engine control unit is also recorded in accordance with the legal framework and output via the measuring equipment.

Fig. 11 PEMS measuring equipment

Challenges and difficulties in off-road use

In contrast to measurements from passenger cars or trucks, the off-road sector presents extensive challenges and difficulties for which the mobile exhaust-gas measuring equipment had to be adapted.

For example, the PEMS equipment had to be designed as a modular system that can be mounted onto every possible machine in the off-road sector. While the design was relatively simple for tractors, additional installation space – within the legal maximum dimensions of a vehicle – had to be created for measuring vehicles such as harvesters to be able to install the mobile equipment.

The power supply was deliberately not supplied by the vehicle or a power unit. This prevented coming into contact with the legal 1% rule as well as cross-influences from the unit’s exhaust gases affecting the emission measurement. The power was suppliedby a battery pack tailored to the needs of measuring equipment. This pack ensures that the measurements are carried out during at least one day and can be recharged overnight.

Further difficulties included enclosing the measuring equipment in order to completely rule out influences from dust (e.g., harvest dust), humidity, and splashing water. Moreover, a measuring instrument that measures the recirculated air independently via synthetic air was used for analyzing the exhaust-gas emissions. The measuring equipment also had to be cooled as the maximum measuring temperature of 40 °C is quickly exceeded, especially during summer harvests.

Fig. 12 Attaching the PEMS measuring equipment onto the front hydraulics of a tractor

Fig. 12 shows how the PEMS measuring equipment is attached onto the front hydraulics of a tractor. All of the aforementioned issues are taken into account to ensure that the exhaust tailpipe emissions are measured correctly.

In addition, the PEMS measuring tube is insulated to prevent excessive surface temperatures and thermal incidents during harvesting.

One of the biggest differences to on-road applications is the dependency on weather conditions and harvest seasons. In Europe, for example, tractors can only be used for agricultural tasks under certain conditions and harvesters can only be operated during harvest seasons. As a result, there are only short time windows in which PEMS measurements can be carried out.

Measurements and results

The length of the measurement varies greatly to achieve the legally required length of 5–7 times the cycle work of a Nonroad Transient Cycle (NRTC). For example, the measurement can take only approx. 1 hour for high-load tasks and up to 3.5 hours for low-load applications. Since the cold start is not yet a fixed component of the ISM measurements, several measurements can be carried out during the day.

ISM measurements that have already been carried out on various vehicles in a wide range of applications have shown that the results of the determined conformity factors vary greatly, depending on the load and what the vehicle is used for. However, it can been proven that results from the engine test bench are reproducible under real operating conditions and that conformity factors smaller than 1 are also possible in highly utilized applications such as ploughing with a tractor.


The new 9-liter D15 engine with its compact dimensions and high torque perfectly complements the MAN engine portfolio. Thanks to the EU Stage V, EPA Tier 4 Final, and EU Stage IIIa certificates, it serves the global market. The SCR-only concept paired with state-of-the-art common rail injection technology and efficiency-optimized VTG turbocharging provides a solid basis for future developments. During testing, the results of real driving cycles were compared with results from the endurance program, thus optimizing the scope of validation and improving the understanding of component stress. Statutory ISM measurements must be carried out for the first time for emission stage EU V. The results vary greatly depending on the load and type of vehicle use. The results from the engine test bench are reproducible under real operating conditions.

Authors: Tobias Herrmann, Vanessa Simon, Markus Fuchs, Marc Winterhoff, Reinhard Lämmermann