Comfort thanks to compact roof engine

In classic railcars, the drive units are usually installed beneath the floor. Compared to separate drive modules, this arrangement offers the advantage of not losing any further space for passengers. A major disadvantage, however, is that this engineering design entails the use of steps to enter the compartments because an end-to-end low floor section cannot be realised.

The specification of the French manufacturer Alstom to MAN as the supplier of the engine for the power pack was to enable the realisation of this end-to-end low-floor section in its railway vehicle Régiolis. What is special about this installation is that the four or six drive systems, each comprising a diesel engine with flanged-on generator, cooling system and controls, are arranged on the roof of the vehicle.

Strict regulations

The operator’s specifications call for an engine that is as compact and lightweight as possible. Compliance with the emission regulation IIIB valid from the beginning of 2012 until the end of 2020 without the use of a second fuel can be realised with the same or even improved fuel consumption compared to engines from

preliminary or comparative projects. Furthermore, in addition to the topic of noise prevention, fire protection is of central importance – not least against the background of the planned installation location on the vehicle roof. Acceptance of the engines in accordance with UIC 623 is required.

The Régiolis has been used by SNCF in France since 2013 as a suburban train on lines with high traffic volumes and short distances between stops. Adapting an engine originally intended for road application to be used as a railway engine means fulfilling a different set of requirements: The load spectrum of a railway engine differs significantly from that of a truck engine. Likewise, a quick and frequent change between operating points can be made. In addition, different laws with regard to exhaust emissions apply in the rail sector.

Due to the roof installation, an almost continuous low-floor section can be realised in the interior of the vehicle. A service hatch accessible from the passenger compartment, which is a requirement in case of underfloor drive systems, can be completely dispensed with. In order to be able to install the drive systems on the roof and not raise the centre of gravity too far, all components have to be as light as possible. With a weight of 1,125 kg, the D2676 LE621 engine meets this requirement. A drive package weighs only 3,600 kg.

The realization

Since the space available on the vehicle roof is limited, all components must be as compact as possible. The PM-Kat® catalytic converter integrated in the drive package requires no more installation space than a conventional exhaust silencer as used in truck applications.

Due to the diesel-electric drive, the engine has to react very quickly, even to transient load changes, as well as to alternating loads. The control parameters in the engine control unit have to be adjusted accordingly for this type of operation in order to ensure both favourable fuel consumption and suitably dynamic power delivery. High average and ignition pressures necessitate a rigid crankcase and cylinder-head design.

The crankcase must withstand ignition pressures of 190 bar. To ensure this, the crankcase is made of high-quality GJV material (cast iron with vermicular graphite). GJV combines the good casting properties of lamellar graphite cast (GJL) iron with the very good mechanical-physical properties of spheroidal graphite cast iron (GJS) [2]. The cylinder liners are replaceable, enabling a straightforward, economical overhaul of the engine. The MAN D20/D26 series implement cracked main bearing shells and connecting rods. This cracking allows MAN to design the crankcase as a closed box shape. Moreover, the positive-locking fracture faces offer outstanding absorption of transverse loads.

A solid crankshaft is driven by the pistons via forged connecting rods made from high-strength steel. Cast counterweights balance the crankshaft and ensure that the engine runs smoothly and harmoniously. Just like the main bearing caps, the connecting rods are cracked. Due to the demanding use as a railway engine, the pistons are designed as one-piece forged steel pistons. This design enables high pressure stability and resistance to thermal cyclic stress. The pistons are cooled by a ring-shaped cooling duct in the area of the ring carriers. Engine oil is injected into this in a targeted manner via oil nozzles.

The D26 employs a continuous cylinder head made from GJV with an overhead camshaft that operates the four valves per cylinder via roller rocker arms. The air-manifold housing for distributing charge air to the cylinders is integrated into the cylinder head. This results in a simple sealing of the charge air supply at the cylinder head. The latter is separated from the crankcase in terms of coolant and oil supply, so that no engine operating fluids need to penetrate the cylinder head gasket. This makes it possible to use a simple beaded metal gasket for the cylinder head.

The engine cooling system consists of two separate coolant circuits. Here, the larger coolant pump supplies the engine cooling circuit with high-temperature coolant while the smaller supplies the charge-air cooling circuit and the electrical EGR servomotor with low-temperature coolant.

This enables the cooling circuits to be adjusted optimally to the changing boundary conditions of the customer’s cooling system. The coolant pump for the engine cooling circuit is implemented in two parts. The impeller is located in the coolant-pump connector housing. The coolant pump itself contains only the impeller and the integrated low-temperature pump. The design allows it to be realised with a comparatively low weight. This simplifies disassembly and re-assembly of the pump in the event of an inspection.

For optimal utilisation of installation space, the oil cooler and filter as well as the coarse and fine oil separators of the crankcase breather (blow-by separator) are located together in a single housing – the oil module. The oil is cooled by means of an oil-water heat exchanger. The cooled and cleaned oil is divided into two separate circuits: cylinder head and crankcase. The oil separation of the blow-by gas is carried out by a rotating disc stack. The oil is separated from the air by centrifugal force, the particles are thrown against the inner wall of the housing and flow back into the oil sump. The purified air is fed into the charge air circuit. The separator is equipped with a hydraulic drive; this oil also flows back into the oil circuit. The efficiency of the separator is >98 % at a flow rate of 160 l/min.

Diesel fuel is injected into the D2676 LE621 by a BOSCH common-rail system at a maximum injection pressure of 1,800 bar. The fuel passes from the tank to the pre-supply pump via the Fuel Service Centre (KSC), where it is filtered, and then continues to the high-pressure pump. The high pressure is generated by the oil-lubricated 3-piston high-pressure pump CP 3.4H+. The high-pressure pump supplies the rail with fuel. The rail acts as a pressure accumulator for injection. The fuel passes from the rail via injector lines and pressure pipe sockets to the individual injectors. The injectors are controlled by the EDC in accordance with an operating map. There are up to three injections per combustion cycle.

Two in-line exhaust-gas turbochargers are responsible for the pressure-controlled supply of combustion air to the engine. At low engine speeds, the high-pressure stage is charged solely with exhaust gas. This increases the filling level in the cylinders and achieves a very strong increase in torque. At high engine speeds, the exhaust gas is partly routed past the high-pressure turbine via a wastegate. This results in a greater charge in the low-pressure stage. The consistent use of the exhaust gas energy results in favourable fuel consumption values and low-particle combustion.

This railway application employs an electronically controlled EGR with a lambda probe. The recirculation of the cooled exhaust gases enables a lower peak temperature during combustion, so that less nitrogen oxides are formed. The EGR control sets an optimum EGR rate under dynamic conditions for each operating point of the engine. This guarantees particularly high efficiency and economical fuel consumption.

The engine is controlled by a BOSCH EDC7C32. The MAN D2676 LE621 complies with emission standard Stage 3B in accordance with Directive 97/68/EC in Cycle C1 and F and is certified as per UIC 623 and UIC 624. The motor is speed-controlled and drives a permanently excited magnetic generator. Both the monitoring of the engine as well as cold starting are adapted to the requirements of railway operation in order to cope adequately with the additional mass in the form of the generator coupled to the engine during the start process. Diagram 1 shows the full-load curves for torque and power. Fuel consumption at rated power is 217 g/kWh, at best point it is a mere 190 g/kWh.

There are various strategies for reducing the exhaust emissions of a diesel engine. Due to the customer’s specifications, the choice in this case fell on an engine with two-stage turbocharging and EGR in conjunction with a PM catalytic converter filter. The PM catalytic converter is a continuously operating, self-regenerating separator system with open channels; the exhaust gas first flows through a catalytic converter part. There, the oxidation of nitrogen monoxide NO to nitrogen dioxide NO2 takes place.

In the second stage, the soot particles are separated in a sintered metal fleece. The trapped soot particles are oxidised using the NO2 formed in the first stage and converted to carbon dioxide CO₂ and water H₂O. Maintenance of the PM catalytic converter filter is not required. The components of the filter system are completely integrated in a standard exhaust silencer.

With the D2676 LE621, an engine specifically for use in Alstom’s Régiolis has been developed. MAN meets the requirements of an engine installed in the roof through a sensible combination of existing technologies and the development of new ones: By using the extremely high-quality material GJV, the crankcase and cylinder head can be designed to be compact and light. Well thought-out arrangement of attachments such as the generator, water pump and PTO enables efficient utilisation of the installation space available.

By dispensing with the second operating fluid, there is no need to provide installation space for the SCR tank. An additional weight advantage arises from the use of the PM catalytic converter already integrated in the silencer as part of the selected exhaust technology. For this purpose, existing series technologies and their components are used and adapted accordingly. This makes it possible to develop engines economically, even for applications with comparatively low quantities.