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Efficient drives from MAN: Diesel and CNG engines for manufacturers of coaches and city buses

Efficient drives from MAN: Diesel and CNG engines for manufacturers of coaches and city buses

MAN Truck & Bus offers external customers engines for installation in the on-road, off-road, power generation and marine segments. The engines offered range from 150 to 1,800 hp. MAN common-rail diesel engines are distinguished by their high power density and economy, smooth running and long maintenance intervals. MAN Truck & Bus develops future-proof emission technologies and utilises the resulting synergies for applications in all segments. MAN's OEM customers can put their trust in the more than one century of competence in diesel manufacturing that underpins MAN engines, axles and transfer cases.

The Competence Center for the development and manufacture of all the MAN engines is in Nuremberg. In 2011, around 95,000 engines will come off the production line, some ten percent of them for the OEM business. These engines are in operation all round the world in all segments. This is why it is important to have a world-wide service network providing fast, specialist maintenance and repair. In Europe alone, the MAN Truck & Bus Service network has over 1,200 outlets.

Engines for the on-road segment

In the on-road area, MAN offers engine kits, axles and transfer cases for manufacturers of bus chassis and body builders of complete buses. Typical applications are mini-, midi-buses, city and intercity buses, touring coaches and bus chassis. For installation in buses, MAN has compact diesel engines with outputs ranging from 132 kW (180 hp) to 371 kW (505 hp) in all emission classes from EURO 3 to EURO 5 as well as EEV and, in good time for the introduction of the standard, also EURO 6. For operation with compressed natural gas (CNG), the range of MAN Truck & Bus products includes compact gas-powered engines from 162 kW (220 hp) to 228 kW (310 hp) that meet the requirements of the EEV and future EURO 6 standard. Moreover, MAN provides its customers with advice on selecting and matching important components of the power train, such as intake and cooling systems, which can be obtained from MAN together with the engine. Besides ensuring that the components are quickly and correctly matched, this saves the customer time and effort that would otherwise have to be spent in purchasing, logistics and engineering.

Tailor-made exhaust technology thanks to modular construction

All add-on components relevant to the MAN diesel engine exhaust system are designed in strictly modular form. The exhaust-gas standard required on any particular market decides the exhaust system. The installation conditions for the basic engine thus remain identical. The advantages are clear to see: vehicle and machine manufacturers select the exhaust systems they need for their specific emission standard and their vehicles and machines are then optimally equipped for the most differing of markets.

The following technologies are available:

  • Diesel engines with Pure Diesel Technology - EGR engines (exhaust-gas recirculation) with PM-Kat and oxidation catalytic converter for operation that does not require additives like SCR does.
  • Diesel engines with SCR technology (Selective Catalytic Reduction) in order to meet the most stringent exhaust standards of the future.
  • Engines for operation with natural or bio-gas, which achieve the lowest pollutant emission using a three-way catalytic converter.

MAN engine technology: the common-rail injection system

MAN's range of external engines employs the common-rail injection system throughout. The primary advantage of this technology is that the system pressure, the timing and amount of fuel injected can be ideally tuned to the engine's operating conditions. Selective direct fuel injection under pressures of up to 1,800 bar is the basis for meeting the strictest exhaust-gas standard and in conjunction with state-of-the-art turbocharging, achieves low fuel consumption and powerful acceleration. High injection pressure at low engine speed enables high torque with little smoke, low injection pressure under partial load means optimised NOx figures and optimal fuel consumption. High injection pressure at full load enables high exhaust-gas recirculation rates and low particulate emission and simultaneously, economical fuel consumption. The separation of the injection amount into pilot and main injection results in a significantly smoother combustion process. This in turn results in a further advantage: noise emission is lower, especially when the engine is idling or under partial load.

Two-stage turbocharging with external exhaust-gas recirculation (EGR)

The engines in MAN's D08, D20 and D26 ranges are equipped with two-stage turbocharging and external exhaust-gas recirculation (EGR). The steplessly controlled EGR system has a recirculation rate of up to 30 percent and reliably reduces the nitrogen oxide in the engine to under the statutorily prescribed limits.

Low emissions without loss of performance - that is what two-stage turbocharging can achieve. Its major advantage lies in being able to maintain maximum torque across a wide engine-speed range. In order to do this, the turbochargers use the thermal energy of the exhaust gas to drive the turbines. At low engine speeds, the first or high-pressure stage is pressurised to 100 percent with exhaust gas. This raises the percentage to which the cylinders are filled and torque output increases particularly fast. At high engine speeds, a wastegate valve routes part of the exhaust gas past the high-pressure turbine, thus increasing pressure in the second stage, the so-called low-pressure turbine.

EGR is used to decrease the nitrogen oxides. Here, part of the exhaust gas is routed to an EGR cooler, mixed with charge air in the intake tract and then recirculated to the combustion chamber. This reduces excess oxygen in the cylinder charge and lowers the peak combustion temperature: less nitrogen oxide is formed. Fuel consumption can be reduced further by adjusting the injection timing.

The SCR exhaust-gas aftertreatment system for Euro 5 and EEV

For its D20/D26 series of six-cylinder in-line engines, MAN offers the additive exhaust-gas aftertreatment system SCR (Selective Catalytic Reduction) under the trade name MAN AdBlue®. In order to reduce nitrogen oxide emissions, the additive AdBlue® is sprayed into the exhaust-gas stream. This converts nitrogen oxide into water vapour and elementary nitrogen, which is a natural component of atmospheric air. The technology reduces NOx by over 80 percent.

Engine design and materials

With the aid of the Finite Element Method, a computer simulation that models strengths and component design, MAN is able to match the thicknesses of materials used for engine components precisely to the stresses they are subjected to at those points. The result is a saving in material and thus weight at the same level of strength. In this way, the materials and design of the components make a significant contribution to ensuring the long service life and high availability of MAN engines.

The prerequisites for realising high mean and ignition pressures are extremely resilient materials and a rigidly structured block for the common-rail engines. This is why the crankcases in MAN's D20, D26 and D28 series are made from top-class GJV-450 (vermicular graphite cast iron). This material exhibits considerably greater dynamic strength than grey cast iron.

In addition, MAN separates the bearing caps of the D20 and D26 series crankcases by means of controlled fracturing, in a procedure is known as cracking. In the cracking plant, two diametrically disposed notches are introduced on the inner side of the crankshaft bearings by means of a laser unit and the bearings are then separated by an hydraulic expanding mandrel. Without any further finishing, this process produces exactly positioned joining surfaces that can be assembled with perfect mechanical interlocking. The rough supporting surfaces optimally absorb transverse loads.

When designing the engine, MAN's engineers focused on the rigidity of the block and a reduction in the number of components. The D08, D20 and D26 series have single-piece cylinder heads which provide especially high rigidity. Those engine components where strength is critical, such as pistons and conrods, have been designed with adequate reserves for the necessarily high ignition and mean pressures.

The engineering design of the D20 and D26 series in particular is outstanding in this regard. With overhead camshaft and four valves per cylinder, the number of moving parts has been radically reduced. The sealing system is an eminently simple concept, so that there is no longer any transfer of media between cylinder block and head. This drastically minimises potential sources of failure.

The new MAN E2876 LUH03 natural gas engine features 320 hp and fulfils the EEV standard by far. It will be available from 2010 on in MAN citybuses and chassis.