Thursday, 25 September 2014
MAN 15-litre CGI diesel cuts weight
As German truck maker MAN establishes closer ties with fellow VW stablemate Scania through shareholding and heavy truck transmission technology, it also appears to have moved the goalposts with its newly-developed 15.2-litre, six-cylinder in-line D3876, the latest of the company’s Euro 6 range of engines.
With torque figures of 2500 to 3000Nm and power outputs of 520 and 560bhp for long distance and towing vehicles, or 640bhp for heavy goods vehicles, the D3876 is the company’s top model and offers a 160kg weight-saving over a Euro 5 unit.
MAN has already set a benchmark in its use of compacted graphite iron (CGI) –its previous, smaller truck diesel designs having been taken up by Navistar International in the US to create the 11- and 13-litre MaxxForce engines.
The six-cylinder in-line engine shares the same basic design and high-tensile materials with the D20/D26 engines and is built around a design that has proved itself. This includes use of high-strength cast iron with vermicular graphite (GJV450) for the engine block and the cylinder head together with the solid basic geometry of the engine and the crankshaft main bearing.
By way of example, combined with a 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.
MAN claims the D3876 is one of the lightest engines in its performance class.
A plastic sump and valve rocker cover contribute to the engine’s low weight. The sump material has the advantage that it can be shaped using the MAN patented spider's web structure for the underside. This assists in diffusing sound waves to reduce NVH.
Based on this design, and during the development of the D3876, MAN claims engineers have drawn on all aspects of modern engine construction and achieved numerous technical innovations.
Comparative trials suggest the 560bhp D3876 with Euro 6 emissions standards uses 3 per cent less fuel than an equivalent Euro 5 engine of 540bhp output – and with 200Nm more torque.
Intercooled turbocharging comprises two differently-sized units connected in series. The smaller, high-pressure charger responds at low engine speeds. As engine speeds and loads increase, it receives increasing 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. Turbocharging design allows the engine to gives maximum torque from 930 to 1,350rev/min.
Two-stage air cooling
The D3876 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 turbocharger. This design also increases component reliability and service life.
The engine features a third generation common rail to inject fuel into the cylinders at up to 2,500 bar. The system is configured for pre-, main and post-injection with the engine designed for peak firing pressures of up to 250 bar.
MAN claims that for the first time in a truck diesel engine, it has used a system to pump coolant from top to bottom through the engine. 'Top-down' prioritises cooling regions of the cylinder head subjected to high thermal stress. This provides special cooling, despite the use of CGI material, and hence protection, for the injectors and exhaust valves in particular.
Top-down uses lower temperature coolant and ensures, according to MAN, high cooling capacity evenly for all cylinders. The process aims to prevent localised peak temperatures and thermal stresses. As the coolant flow is managed, the engine achieves a high cooling capacity with a comparatively low coolant flow rate. This is to the benefit of fuel consumption as the optimum circuit for the coolant flow allows the use of a coolant pump that consumes less power.
The D3876 incorporates domed valves for the first time in a truck diesel engine. This aims to prevent inlet and outlet valves (four per cylinder) from deforming by means of the additional strengthening of the valve disk through convex shaping.
Each time valves open and close, they accelerate and decelerate. The convex strengthening of the valve disc reduces the possible distortion of the valve in the area of the valve seating ring from these accelerations to "virtually nil". This increases service life of the valve and the valve seating ring, making the D3876 a more reliable truck engine.
The gas forces in the cylinder are transferred by forged steel pistons offering 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 wall, and wear is reduced. Less wear means not only reduced consumption, but also longer service life for pistons and cylinder liners.
The engine has been designed with eight cylinder head bolts per cylinder to distribute forces evenly across the cylinder liners. The cylinder liners should therefore remain uniformly circular under load, with minimal distortion. This suggests 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.
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, to reduce the material fatiguing under vibrations from long-term use. The cables have a longer service life and are also protected against damage during installation work on the engine.
To comply with Euro 6 limits, the 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 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 per cent and low NOx engine-out emissions. This implies the exhaust gas after-treatment system has to reduce less NOx. The suggests too that MAN's low usage of the AdBlue additive is reduced by approximately a further 60 per cent in comparison to the Euro 5 solution.
It implies also that components in the exhaust silencer are compact for such a large engine. The dimensions of the exhaust silencer for the 15-litre engine are 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.