MAN gave the first public showing of its new flagship truck engine at the recent IAA Hanover CV Show. The engine, identified by its code number D3876, is an in-line six-cylinder diesel displacing 15.2 L. It will replace the D2868 16.2-L V8 as MAN’s largest-capacity truck diesel engine.
The D3876 is effectively a scaled-up version of the smaller-capacity D20 10.5-L and D26 12.4-L inline six-cylinder engines, sharing the same basic design and construction materials but with a cylinder bore of 138 mm (5.43 in) and stroke of 178 mm (7.01 in), compared with 120 mm (4.72 in)/155 mm (6.10 in) for the D20 and 126 mm (4.96 in)/166 mm (6.53 in) for the D26. Both engine block and cylinder head are made from GJV450 compacted graphite iron (CGI)—cast iron with vermicular graphite, which contribute to the engine’s dry weight of 1345 kg (2965 lb).
The D3876 engine will offer power outputs of 520 hp (390 kW), 560 hp (420 kW), and 640 hp (470 kW) while peak torque, which is produced between 930 and 1350 rpm, is said to cover a range from 2500 to 3000 N·m (1800 to 2200 lb·ft) range.
Like the smaller-capacity D20 and D26 engines, MAN uses twin sequential turbochargers with intermediate cooling to help ensure compliance with Euro-VI exhaust emissions limits. The smaller-diameter turbocharger provides boost at lower engine speeds, while the larger-diameter unit delivers boost at higher engine speeds.
Charge air cooling is handled by a two-stage system. The intermediate cooler reduces the charge air temperature leaving the first turbocharger and is cooled again by the main charge air cooler after leaving the second turbocharger. MAN says that the system can reduce charge air temperatures below that of the engine coolant. The system also helps to reduce temperatures in the second turbocharger, offering a claimed increase in reliability and service life of the components.
The common-rail fuel-injection system operates at pressures up to 2500 bar (36 ksi). It can deliver pre-, main-, and post-injection pulses. The engine is designed to handle ignition pressures of up to 250 bar (360 psi). This is in part due to the CGI construction, which offers high strength with relatively low weight.
Weight saving has been achieved in several ways. The aluminum flywheel housing, for instance, weighs about 160 kg (73 lb) less than that in the outgoing D28 V8, owing not only to the aluminum construction but also to a weight-optimized design. Other weight-saving measures include a plastic sump and valve rocker cover. The sump also contributes to noise reduction. It uses MAN’s patented spider’s web structure in the underside, which helps to diffuse engine sound radiation.
MAN claims a first for a truck with a cooling system that pumps refrigerant from the top down to the bottom of the engine. The company describes this as a priority cooling system for the areas of the cylinder head subjected to high thermal stress—in particular the injectors and exhaust valves. This cooling method is also said to guarantee high cooling capacity evenly spread for all cylinders, which helps to counter localized peak temperatures and thermal stresses. MAN also claims a high cooling capacity with a relatively low coolant volume. This in turn requires a less powerful coolant pump, which helps to reduce energy losses in the ancillary systems.
A new valve design for the four-valves-per-cylinder engine is also credited with being a first in a truck diesel engine. The D3876 engine is equipped with domed inlet and exhaust valves. This provides additional strength via a convex valve surface, reducing the likelihood of distortion of the valve in the valve seat, MAN says.
In common with other heavy-duty diesel engines experiencing higher ignition pressures, the MAN D3876 is fitted with forged steel alloy pistons to withstand those higher pressures. This also allows for the design of pistons with a lower compression height—the distance between the piston pin centerline or “small end” (where the piston is connected to the connecting rod) and the piston crown—and opens up several possibilities. MAN has chosen to fit longer connecting rods, which helps to reduce friction between piston and cylinder wall.
To handle the high peak cylinder pressures, each cylinder is equipped with eight cylinder head bolts around the circumference. This ensures that cylinder liners remain uniformly circular under load, minimizing distortion and improving the seal between cylinder and piston ring.
The top end of the cylinder walls are fitted with what MAN describes as fire rings, which help provide additional sealing to prevent unwanted oil carbon deposits on the piston lands, which in turn helps to minimize cylinder wall wear.
MAN claims another first in using foam-filled cable harnesses around the engine to reduce fatigue induced by vibration, extend service life, and protect against damage during servicing operations. It also helps to make them more resistant to rodent damage, the company says.
The twin-cylinder air compressor providing air pressure for the brake system, clutch system, and suspension is fitted with a clutch to ensure that it is disconnected when not needed, reducing frictional losses.
MAN is introducing its turbo exhaust valve brake (Turbo EVB) for the first time, capable of producing up to 600 kW of braking power at 2400 rpm by controlling the high-pressure turbocharger to enhance exhaust backpressure. MAN claims that the system can be used continuously because it does not have a significant impact on the cooling system. This system will be made available in 2015.
The engine uses a combination of cooled exhaust gas recirculation (EGR) and selective catalytic reduction to control exhaust emissions. The two-stage EGR cooler uses both the engine cooling system and the low-temperature charge air cooler.Continue reading »