Reliability in detail: technology highlights of the new MAN D3876 engine
- Up to 640 HP and 3000 Newton metres from 15.2 litres
- Full torque from 930 rpm
- New design concepts in the cylinder head, cooling system and combustion maximise reliability
With the newly developed 15.2 litre, six-cylinder in-line D3876, MAN presents a powerful, highly efficient and extremely reliable truck diesel engine for demanding transport tasks. Thanks to powerful torque of 2500 to 3000 Newton metres and power outputs of 520 and 560 HP for long distance and towing vehicles, and 640 HP for heavy goods vehicles, the D3876 is the new top model in the MAN Euro 6 range of engines.
As a six-cylinder in-line engine, the new D3876 shares the same basic design and high-tensile materials with the D20/D26 engines and is therefore built around a design that has proved itself over millions of miles. This includes the use of cast iron with vermicular graphite (GJV450) for the engine block and the cylinder head and the solid basic geometry of the engine and the crankshaft main bearing.
Based on this design, in the development of the D3876, the MAN engineers have drawn on all aspects of modern engine construction and have achieved numerous technical innovations. They have made the D3876 an outstandingly reliable yet highly fuel efficient power unit: comparative trials show that the D3876 with 560 HP and Euro 6 emissions standard uses up to 3% less fuel than an equivalent Euro 5 engine with 540 HP – and this with 200 Nm of extra torque. This clearly demonstrates how efficient the new D38 is.
Powerful torque even at low RPM and high starting dynamics for the MAN D3876 are achieved through two stage exhaust turbo charging with intermediate cooling. The turbocharging system comprises two differently sized exhaust-gas turbochargers connected in series. The smaller, high-pressure charger responds even at low engine speeds. As engine speeds and loads increase, it receives more and more assistance from the larger, low-pressure charger. If a high level of output is required of the engine, the low pressure charger takes on the majority of the pressurisation. This turbo charging design has enabled MAN to achieve the ideal supply of combustion air to the engine and hence the optimum torque band over a wide range of engine speeds. The engine gives its maximum torque from 930 rpm to 1350 rpm. This engine characteristic enables low engine speed and economical driving with high drive ratios to be achieved at motorway speeds. This also means that high gears can be engaged for longer when negotiating slopes without the need to change down.
The two stage turbocharging design is based on proven technology and means longer service life. It reduces the load on turbo charger components as the pressurisation is split between two turbo chargers, each of which only needs to produce a portion of the overall pressure.
The temperature of the charge air has a significant effect on combustion and hence on the efficiency of the engine, as cooler air contains more oxygen in relation to volume. The MAN D3876 therefore has a two stage charge air cooling system with its own low-temperature cooling circuit. With this, the main charge air cooler can cool the pressurised combustion air to temperatures that are lower than the engine coolant. Indirect charge air cooling with two charge air coolers and two coolant circuits also enables greater overall cooling performance.
A further advantage of two stage charge air cooling is reduced heat load on the high-pressure turbo charger. This design also means increased reliability and service life of the components.
The MAN D3876 features a third generation common rail system that injects fuel into the cylinders at up to 2500 bar, thereby achieving finer atomization of the fuel. The system enables the injection to be precisely configured with pre-, main and post-injection. The engine is designed for ignition pressures of up to 250 bar. In conjunction with the outstanding properties of the new common rail system, the combustion reaches a particularly high degree of efficiency, which is critical for low consumption. Additionally, the combustion produces particularly low particulate levels.
MAN has used the high-strength material GJV450 (cast iron with vermicular graphite) for the engine block and cylinder head. By way of example, combined with weight optimised design, the aluminium flywheel housing with cast-on engine mounts achieves a weight saving of around 160 kg, compared with the most powerful Euro 5 engine, the D28 V8. The D3876 is therefore one of the lightest engines in its performance class. Plastic sump and valve rocker cover also contribute to the low weight of the D3876 engine.
The sump material also has the advantage that it can be shaped using the MAN patented spider's web structure for the underside. This assists in noise reduction, as it radiates the sound waves diffusely and thus makes the running noise quieter and more comfortable on the ear.
For the first time in a truck diesel engine, MAN has used a cooling system in the D3876 that pumps the coolant from top to bottom through the engine. The concept of 'top-down' cooling is a priority cooling system for the areas of the cylinder head subjected to high thermal stress. This provides special cooling, and hence protection, for the injectors and exhaust valves in particular.
Top-down cooling also guarantees high cooling capacity evenly for all cylinders. The process therefore prevents localised peak temperatures and thermal stresses. As the coolant flow is managed, the engine achieves a high cooling capacity with a comparatively low coolant volume flow. This also benefits fuel consumption as the optimum circuit for the coolant flow allows the use of a coolant pump that uses less power.
The MAN D3876 incorporates domed valves for the first time in a truck diesel engine. This design feature prevents the inlet and outlet valves (four per cylinder) from becoming deformed by additional strengthening of the valve disk through convex shaping. Each time the valves open and close – that is, countless times per kilometre – they are accelerated and then stopped again. The convex strengthening of the valve disk reduces the possible distortion of the valve in the area of the valve seating ring from these accelerations to virtually nil. This increases the service life of the valve and the valve seating ring, making the D3876 a highly reliable truck engine.
In the MAN D38, the gas force in the cylinder is transferred by forged steel pistons. This provides several advantages: due to the high strength of the steel alloy, pistons with a lower compression height can be used. The transfer of force from a shorter piston through a longer connecting rod to the crankshaft is closer to the ideal of vertical force transfer. Furthermore, the compact piston construction reduces surface contact between piston and cylinder well, and wear is reduced. Less wear means not only reduced consumption, but also longer service life for the pistons and cylinder liner.
The MAN D3876 high-performance engine has been designed with eight cylinder head screws per cylinder so that the resultant forces are distributed absolutely evenly across the cylinder liners. The advantages are: the cylinder liners remain uniformly circular under load, with minimal distortion. This means the seal between liner and piston ring is especially effective. This has the benefit of lower oil consumption and longer lifetime of the particulate filter. Load on the cylinder head gasket is also reduced by the even pressure. This achieves a durability of the cylinder head gasket throughout the service life.
An additional design characteristic of the D38 contributes to long engine service life: At the upper end of the cylinder liner, fire rings are inserted into a right-angled recess. This protective ring at the upper end of the cylinder liner prevents the ingress of combustion gases between the piston and the liner and so minimises unwanted oil carbon deposits on the piston lands. Oil carbon has erosive properties, and by the use of fire rings, MAN is able to minimise the wear on the cylinder liners.
For the first time, the cables of the D38 engine have been routed in foam-filled cable harnesses. The cables are held in place by the foam, so as to greatly reduce the material fatiguing vibrations caused by long-term use. Therefore the cables have a longer service life and are also protected against damage during installation work on the engine. The bundles also provide greater protection against rodent gnawing.
With the EfficientLine models, MAN has introduced demand control of auxiliary units for fuel economy. In the flagship TGX D38, MAN has also introduced this technology and optimised APM for the higher air requirements of vehicle applications in this performance class. For the D3876, MAN offers a disconnectable, 2 cylinder air compressor with a cubic capacity of 476 cc, which is an increase of 40% compared with that of the 1 cylinder APM fitted to the smaller engines.
The Air Pressure Management (APM) system only switches the compressor on when compressed air is required – when travelling long distances, the compressor can be disconnected for 90% of the time. This reduces the auxiliary power requirement of the engine and contributes to a reduction in fuel consumption.
For greater air requirements, such as several air-sprung axles or for heavy load use, customers can opt for a more powerful, permanent 2 cylinder air compressor of 720 cc.
At the top of its range, the D3876 is available with either the EVB (Exhaust Valve Brake) engine brake, or the even more powerful Turbo EVB that is being used in heavy-duty vehicles for the first time. As primary continuous braking systems, both provide powerful braking capacity even at low driving speeds. The EVB on the D3876 is particularly effective at mid-range engine speeds and supplies a maximum braking capacity of 340 kW at 2400 rpm.
With the Turbo EVB, a significant increase in the braking capacity to 600 kW at 2400 rpm is achieved by controlled turbo charging. It provides extremely high braking power of 600 kW, with continuous use possible because of its marginal demands on the cooling system – an important additional safety feature. Initially, it will be available from 2015 for heavy-duty vehicles, but subsequently also for vehicles for normal road usage.
The development of the D3876 was monitored at an early stage by the experts from the Service Engineering department, with the aim of achieving the most maintenance and repair friendly design possible. For example, the valve train is accessible without prior removal of the air filter. Also, the extended diagnostic capability of the injection system provides more detail and hence reduces workshop effort.
During the development of the D3876, in certain assemblies MAN used proven components from the D20/D26 engine series. This includes the fuel filter, the oil filter module and oil separator, the coolant pump, the throttle valve, the control cylinder for the exhaust gas recirculation system, the fan drive, the auxiliary drive, the alternator, the starter and the air conditioning compressor. This has enabled MAN to incorporate the servicing and repair friendliness of the smaller engines into the 15.2 litre engine. This is also a sound basis for straightforward parts provision for the most powerful engine.
To comply with the highly stringent Euro 6 emissions limits, the MAN D3876 uses a combination of cooled external high-pressure exhaust gas recirculation (EGR), an SCR system for selective catalytic reduction and a CRT closed particulate filter system. Unlike the smaller MAN engines the recirculated exhaust gas is cooled in two stages. Here, the exhaust gas is initially directed into a high temperature EGR cooler. This is integrated into the engine coolant circuit. Then it is further cooled by a low temperature EGR cooler that is part of the low-temperature cooling circuit of the charge air cooling system.
This design enables exhaust gas cooling levels of up to 40% and very low NOx engine-out emissions. This means that the exhaust gas after-treatment system has to reduce less NOx. The outcome is that MAN's already low usage of the AdBlue additive is reduced by approximately a further 60% in comparison to the Euro 5 solution – a useful TCO cost advantage. Also, this means the components in the exhaust silencer turnout to be very compact for such a large engine. Because of its cubic capacity, the 15.2 litre engine in the TGX D38 produces a large volume of exhaust gas. The dimensions of the exhaust silencer are, however, the same as in vehicles with the D20 or the D26 engine. Therefore the same tank volume can be accommodated on the frame of the TGX D38.
Because of the optimised common rail injection system and consequent low-smoke combustion, the diesel particulate filter has to filter out less soot particulate. This produces low exhaust gas back pressure and therefore has a positive effect on fuel consumption. In addition, active regeneration of the particulate filter is normally not required, so the D3876 does not require downstream fuel injection. For servicing tasks, the diesel particulate filter is easily accessible via a maintenance flap and need only be replaced in the workshop for cleaning after 500,000 kilometres.