MAN Engines

MAN Engine D1556 LE5xx

MAN Motor D1556 LE5XX
In focus:

Compact and powerful: MAN’s 9-litre diesel engine for off-highway applications

In 2019, MAN launched a newly developed engine with 9 litres displacement. Alongside the classic applications for trucks and buses, the industrial version will be presented at the same time. The use of the engine in a farm tractor is its first application in the off-highway segment. For this usage, the SCR-only combustion and exhaust gas aftertreatment have been completely revised and adapted. A major focus was placed on the low rev concept combined with exceptionally good engine dynamics. During testing, a large number of field cycles were analysed and the test programme was modified for the tough use in the tractor. In addition, measures and results for recording the “Real Driving Emission” of the tractor in a wide variety of applications will be presented.

A few years ago, the development of the new D15 engine platform got the go-ahead. This engine closes the gap between the 7- and 12-litre displacement class. The primary development goals were a low power-to-weight ratio and compact dimensions. The 6-cylinder in-line engine produces between 205 and 324 kW (279 and 440 HP) and, in the most powerful version, generates a maximum torque of 1,970 Nm between 1,150 and 1,300 rpm (see Fig. 1, left). This means the MAN D1556 impresses with a large torque even at low engine speeds and, at the same time, with a dry weight of only 860 kg – the lightest off-road engine in its displacement class.

It is used for the first time as an industrial engine in a tractor. Here, the new D1556 engine has a maximum output of 305 kW due to installation space restrictions and the high load factor. To serve the global market, the engine has been certified for EU Stage V, EPA Tier 4 final and EU Stage IIIa for “Low Regulated Countries”.

The following figure shows the current status of the tractor’s power curve. A comparison with the competition (see Fig. 1, right) clearly shows that the D1556 takes the leading position in terms of maximum torque.

MAN’s 9-litre diesel engine for off-highway applications

Fig. 1 Power curve and specific power / effective medium pressure

Furthermore, great attention was paid to the modularity of the 9-litre engine kit. The following illustration shows the different variants in trucks, buses and as CNG and tractor engines.

MAN’s 9-litre diesel engine for off-highway applications

Fig. 2 CAD representation of the D1556 as truck, CNG/bus and tractor variant

The versions differ, for example, in fan attachments and belt drives, cylinder head covers and oil pans made of plastic and aluminium, and turbochargers with wastegate or VTG. Unlike its diesel counterpart, the E1856 natural gas version is equipped with cooled exhaust gas recirculation (EGR). The bus version can be supplied with an optional crankshaft starter alternator.

The development (see following table for technical data) and integration of the D1556 into the tractor is described in detail below.

Engine type   D1556 LE5xx
Emissions standard   EU Stage V, US EPA Tier 4 final, Downgrade UN-ECE
R96, H (compliant with EU Stage IIIA)
Number of cylinders / arrangement /
valves per cylinder
  6 / in-line / 4
bore / stroke mm 115 / 145
Displacement cm³ 9.0
Compression   20:1
Injection / ignition pressure bar 2500 / 230
Weight (dry) kg approx. 860 (without EGA), approx. 960 (with EGA)
Rated power at speed [1/min] kW [1/min] 217 239 261 283 305
Max. torque Nm 1550 1650 1750 1850 1970
at engine speed rpm 1,100 – 1,300 1,100 –
1,100 –
1150 –
1200 –

Exhaust gas aftertreatment

For the tractor application, the available installation space of the engine’s predecessor model had to be maintained. This model was equipped with EGR and had a significantly lower maximum torque with a smaller displacement. The development team was therefore faced with the challenge of improving flow uniformity across the SCR and optimizing AdBlue treatment to achieve higher NOX conversion rates at significantly higher exhaust gas mass flows. At the same time, the exhaust back pressure had to be taken into account.

Particle filter

The geometries of DOC (diesel oxidation catalyst) and DPF (diesel particulate filter) were set by the size of the bonnet above. To maximise the ash cleaning intervals of the DPF, a substrate design with asymmetric channel geometry was used. The findings of the field and endurance tests result in an ash cleaning interval of at least 8,000 operating hours. The soot loadings in the DPF consistently proved to be uncritical throughout the entire test. Due to the high NOX raw emission level and the correspondingly high NOX particle ratio, it is usually not necessary to regenerate the DPF in addition to a fixed regeneration interval of 1,000 h, even in rather low-load applications. Nevertheless, in order to validate the quality of the DPF loading model, a vehicle was driven in an extreme

low-load application on MAN factory premises (see chapter 4.1). This unilateral and extremely low-load use of the machine showed DPF regeneration intervals of approx. 250 h.

Exhaust gas aftertreatment

Fig. 4 Flow optimisation variants

SCR catalytic converter

The dimensions of the predecessor model also had to be adopted for the exhaust piping and urea treatment.

The SCR was already designed in 13” – the largest commercially available standard diameter for cordierite substrates – in the previous version of the vehicle. Only the extension of the SCR substrates was able to meet the higher requirements for exhaust gas mass flow and NOX conversion. The flow uniformity distribution on the SCR inlet had to be increased to a Uniformity Index of at least 0.98. In order to achieve the requirements regarding uniform distribution and exhaust backpressure, various optimisation loops at different operating points were calculated by simulation. The target variant was checked during validation on the hot gas test bench with regard to flow and NH3 uniform distribution and the simulation results were confirmed.

Functional test

The functional tests for the application in the tractor are limited to the parts that differ from to the on-highway engines. Among other things, the stress on the Visco damper, the alternator and the air compressor in the heating chamber were tested in detail. Here, the components are tested for their suitability under real installation conditions and high ambient temperatures. At the climate roller test bench in the MAN plant in Munich, extremely cold temperatures were used to

test and optimise the cold start application and the legal requirements for thawing times of the AdBlue system on two tractors.


Fig. 5 Checking the boundary conditions, e.g. Visco damper, cold start, belt drive

The exhaust gas aftertreatment system was subjected to targeted low-load testing at the Nuremberg engine plant. For this purpose, a tractor from Internal Logistics was used. The tractor was equipped with a data logger. At regular intervals, the deposit tendency of the AdBlue® mixer was checked by means of an endoscope (see Fig. 6) and the loading of the DPF was determined by weighing.


Fig. 6 Optimisation of deposits in the AdBlue® preparation (mixer)


Fig. 7 Low-load testing with the help of factory logistics

Figure 7 clearly shows the low load of commuter traffic. Based on the findings, it was possible to adapt the application accordingly. The proximity of the test vehicle to the development department substantially facilitated the process.


Fig. 8 Scope of testing at the test bench

Endurance testing

For endurance testing, the testing method is also based on testing the engine in trucks and buses. In total, the new D1556 engine has accumulated over 200,000 hours on a wide variety of test benches. Approx. 10 % of this were used for the off-road application (see Fig. 8).

In-vehicle testing constituted also an important cornerstone for validation. Additionally, the engine accumulated a comparable number of test hours in the tractor. In the course of development, more than 20 tractors were validated for this purpose under the toughest conditions (see Fig. 9).

When selecting the correct continuous operation programme, a preliminary comparison was made between actually measured field cycles and internal continuous operation cycles.


Fig. 9 Scope of testing in vehicles

When creating the test programme, real-life use cases were specifically examined and evaluated with regard to various criteria.

Among others, the following criteria were used for the assessment:

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

Fig. 10 Comparison of real and test cycles

Figure 10 compares the measured field cycles with the test run cycles.

If we look at the number of load cycles and the average engine load over runtime (e.g. 8,000 h), it becomes clear that it is almost impossible to check this within a continuous run. In contrast, the thermo-mechanical damage is sufficiently well represented by generic cycles. In contrast, the endurance cycles were not optimised with regard to the dynamic load of the actuators, as these are checked by component tests.

Legal provisions in the EU

For the first time, mandatory measurements on internal combustion engines of mobile machinery and equipment for EU Stage V emissions within the legal sphere of the European Union must be carried out under real-use conditions. For example, engines in the performance category NRE-v-5 and NRE-v-6 must be measured with regard to their tailpipe emissions under real-use conditions. The MAN D1556 LE engine, with its power output of over 300 kW, falls into the NRE-v-6 category.

Attaching the measuring equipment to the exhaust gas aftertreatment system

The MAN D1556 LE has a modular EGA consisting of DOC/DPF and SCR catalytic converter. After dismantling the exhaust gas tailpipe, the measuring pipe of the PEMS measuring technology (see Fig. 11) is mounted on the outlet of the SCR catalytic converter, through which the exhaust gas extraction takes place via a heated pipe and the exhaust gas volume flow is measured. In accordance with the legal framework, data from the engine control unit is – in addition to the gas emissions – also recorded and output via the measuring equipment.

In-Service Monitoring (ISM)

Fig. 11 PEMS measuring equipment

Challenges and difficulties in off-road use

Unlike measurements of cars or trucks, off-road use poses far-reaching challenges and difficulties for which the mobile exhaust gas measuring equipment had to be prepared.

Thus, the PEMS system had to be designed in a modular way so that it could be mounted individually on every machine in the off-road sector. While the installation of the measuring equipment for tractors is simple, additional space had to be created for the measurement in harvesters within the legal maximum dimensions of a vehicle in order to be able to install the mobile measuring equipment.

In this way, it was possible to prevent the legal 1 % rule from being violated and to avoid cross-influences on the emission measurement caused by exhaust gases from the genset. The power supply was implemented using a battery pack tailored to the needs of the measuring equipment, which guaranteed measurement operation for at least one day and could be recharged overnight. Further difficulties were the encapsulation of the measuring equipment in order to be able to completely exclude influences by dust (e.g. harvest dust) or moisture and splash water. In addition, a measuring device that measures recirculated air independently via synthetic air was used for exhaust gas emission analysis. Appropriate cooling of the measuring technology also had to be ensured, as the maximum measuring device temperature of 40° C can quickly be exceeded, especially during measurements in summer harvesting operations.

In-Service Monitoring (ISM)

Fig. 12 Mounting PEMS measuring technology on the front hydraulics of a tractor

Figure 12 shows the installation of the PEMS measuring technology on the front hydraulics of a tractor, where all the previously mentioned points were taken into account to ensure stable measurement of the tailpipe emissions.

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 dependence on weather and harvest windows. For example, tractors can only carry out their work in arable farming under certain conditions and harvesters work only during the harvest season in Europe. This means that sometimes only short time windows are available to carry out the PEMS measurements.

Measurements and results

The length of the measurement varies greatly to achieve the legally required length of five to seven times the cycle work of an NRTC. For example, the measurement can take only about one hour for high-load work, and up to 3.5 hours for low-load applications. Since the cold start is not yet a fixed part of the ISM measurements, several measurements can be carried out during the day.

In the course of the ISM measurements already carried out on various vehicles during a wide range of operations, it has been shown that there is a large variance in the results of the CF factors determined, depending on the load and type of operation of the vehicle. However, it can be proven that results from the engine test bench are reproducible in operation under real conditions and that CF factors <1 are also possible in high-load applications such as ploughing with a tractor.


The new 9-litre D1556 engine with its compact dimensions and high torque perfectly complements the MAN engine portfolio. Thanks to the certificates for EU Stage V, EPA Tier4 final and EU Stage IIIA, the global market can be served. The SCR-only concept paired with state-of-the-art Common Rail injection technology and efficiency-optimised VTG turbocharging represents a solid starting point for future developments. During testing, the validation scope could be optimised by comparing real driving cycles and the endurance test programme. Also, the understanding of component stress was improved. For the first time, it is legally required to carry out ISM measurements for emission stage EU V. Depending on the load and type of use of the vehicle, there is a high variance in the results. The results from the engine test bench are reproducible under real conditions.

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