DE VELO PMENT Diesel Engines
© BMW
The Next Generation Diesel Engine Family from BMW REVIEW
With the launch of the current Efficient Dynamics engine family, the BMW Group has attached great importance to achieve an extraordinarily favourable relationship across all vehicle segments between driving performance and consumption/emission values. Now, the BMW Group is presenting revised three- and four-cylinder diesel engines. The consistent further development has the aim to strengthen the position of BMW in terms of driving pleasure and environmental sustainability. 36
Since 2013, a modular diesel and gasoline engine system comprising three-, four- and six-cylinder engines has been consistently rolled out in the BMW Group [1], in the form of the current engine family. In the case of diesel engines, this rollout was completed with the launch of the six-cylinder top-end variant in the middle of 2016, fitted in the BMW 750d xDrive [2]. This technology demonstrator had already anticipated many innovations which are now being introduced alongside further new developments as part of the revision of the current modular diesel family.
AUTHORS
Dipl.-Ing. Michael Salmansberger is Head of Project Management for Technical Revision of Diesel Engines at BMW Motoren GmbH in Steyr (Austria).
Dipl.-Ing. Detlef Hiemesch is Head of Diesel Engine Design at BMW Motoren GmbH in Steyr (Austria).
Dipl.-Ing. Wolfgang Stütz is Head of Application, Combustion, Exhaust Aftertreatment, Hydraulic Systems at BMW Motoren GmbH in Steyr (Austria).
FIGURE 1 Technical solutions taking the example of the four-cylinder transverse engine (© BMW)
the modular family forms the basis for reliably fulfilling future emission and legal requirements.
Dipl.-Ing. Thaddaeus Steinmayr is Head of Test Engineering, Mechanical System Development, Testing at BMW Motoren GmbH in Steyr (Austria).
OBJECTIVE
The technical revision includes all threeand four-cylinder transverse and inline engines with different power variants between 70 and 140 kW. The rollout will start at the end of 2017 with the launch of the three-cylinder transverse engines fitted in the Mini derivatives and the four-cylinder transverse engine in the upper performance range fitted in the BMW X1. The objective of the further development was to further reduce fuel consumption and weight, yet improve dynamics and engine acoustics, while retaining the proven modular principle. Furthermore, the second generation of MTZ worldwide 12|2017
CONCEPT
This challenging objective was achieved by applying the following solutions, FIGURE 1: –– cylinder block with reducedfriction cylinder surface (wire arc spraying coating) and shapehoned bearing surface –– increased efficiency in oil separator system –– friction-optimised belt drive with freewheel function in the decoupled pulley –– tolerance-reduced counter balance shaft system –– further development of pressureincreased common rail (CR) injection system with innovative needle closing control (NCC) technology on the injector –– comprehensive use of a two-stage turbocharging system in the fourcylinder engines
–– further development of highpressure exhaust-gas recirculation (HP-EGR) system –– exhaust aftertreatment by means of NOx storage catalyst and diesel particulate filter (DPF) combined with selective catalytic reduction (SCR) system –– consistent further development of the combustion process, thermodynamics and thermal management (e.g. switchable piston cooling). The architecture of the revised threeand four-cylinder engines continues to be based on the proven design of the BMW modular engine system. The cylinder spacing which has been normal for many years was retained, as was the individual cylinder volume of 0.5 dm³, TABLE 1 [3]. This means it is simple to realise derivatives with different output levels based on the same basic engines. The various engine variants can be integrated seamlessly into the global production network of the BMW Group. In the following, the design configuration taking the example of the four-cylinder transverse engine is shown.
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DE VELO PMENT Diesel Engines
CYLINDER BLOCK Characteristic value
The cylinder block is a further development of the proven concept used in the predecessor engines: –– heat-treated solid aluminum cylinder block made of AlSi8Cu3 –– weight-optimised sintered main bearing caps with interlocking teeth –– two-part water jacket for robustness, even at highest output –– closed-deck and deep-skirt design for safely absorbing high loads –– cast pressurised oil duct for using a map-controlled oil pump –– mounting of the counterbalance shaft in a pre-cast tunnel, directly in the cylinder block. In addition to this, the following technologies have been newly implemented: –– cylinder surfaces coated by wire arc spraying, including modification of cylinder surface activation to use a mechanical roughing process which makes efficient use of resources –– first-time series use of a shape-honed cylinder surface to minimise engine friction, FIGURE 1 –– oil circuit designed for switchable piston cooling, FIGURE 1 –– optimised coolant channeling for using two independently operating thermostats. The cylinder block is cast using the core pack method, as a result of which a large number of functions can be integrated into the housing. The use of chill castings in the core pack means that particularly high static and dynamic strengths are achieved in the highly loaded areas. It was possible to considerably further reduce the component weight by using our own raw parts for inline and transverse housing variants.
The counterbalance shafts, FIGURE 1, of the four-cylinder engine are integrated into the cylinder block and installed from the front. The straight-toothed version of the drive unit permits the gearing to be made narrower, thereby reducing the moment of inertia. The counterbalance shafts are driven by a toothed ring on the last crank web. One special feature is a clamping system which uses only the intermediate gear that reverses the direction of rotation of one of the two counterbalance shafts. This reduces the torsional flank clearance
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Three-cylinder 16d
Four-cylinder 18d
20d
Basic dimensions of the engine Displacement
[cm³]
1496
1995
Bore
[mm]
84
84
Stroke
[mm]
90
90
[-]
1.07
1.07
Stroke/bore ratio Individual cylinder volume
[cm³]
499
Conrod length
[mm]
142.5
142.5
138
Conrod ratio
[-]
0.316
0.315
0.326
Block height
[mm]
289
289
[-]
16.5
16.5
[mm]
91
91
Diameter
[mm]
55
55
Width
[mm]
25
25
Diameter
[mm]
45
50
Width
[mm]
24
24
Compression height
[mm]
42.5
42.5
47
Top land height
[mm]
10.5
10.5
9.12
Diameter
[mm]
28
28
32
Length
[mm]
60
60
64
Intake/exhaust diameter
[mm]
27.2/24.6
27.2/24.6
Intake/exhaust valve stroke
[mm]
8.5/8.5
8.5/8.5
Stem diameter
[mm]
5
5
Compression ratio Cylinder spacing
499
Main journal
Conrod bearing
Piston
Piston pin
Valves
TABLE 1 Characteristic values and main dimensions (© BMW)
on the outlet side, leading to significant acoustic benefits. BELT DRIVE
COUNTERBALANCE SHAFT SYSTEM
Unit
In the four-cylinder engines, the L-shaped friction-optimised belt drive familiar from the six-cylinder engines is used. The core of the belt drive is an integral component comprising the torsional vibration damper (TVD) for reducing the torsional vibrations of the crankshaft and decoupled pulley, in order to reduce irregular rotation of the ancillary units, FIGURE 1. The pulley is decoupled from the hub by means of a rubber element. A freewheel connected in series with the decoupled pulley decouples the entire
belt drive in the deceleration phases of the high irregular rotations during the engine start. The belt drive is recoupled in the acceleration phases. This means that the belt drive is steadied and the load is significantly reduced in the decoupled pulley. This decoupling function makes it possible to use a belt with a higher coefficient of friction, without this resulting in the belt squeaking. In addition, the belt tension force can be reduced by up to 40 %, thereby significantly reducing the friction power. In order to achieve additional consumption advantages and increase dynamic properties, the structure of the engine including the belt drive has already been designed for integration of a belt-driven 48-V starter-generator.
OIL SYSTEM
The new diesel engines use a further development of the fully variable rotary vane pump [3]. Switchable piston cooling is integrated as a new feature. An electrical switching valve combined with a hydraulic valve makes it possible to activate and deactivate the piston cooling precisely, FIGURE 1. This allows cooling according to demand across the entire map range of the engine, and additionally reduces consumption. Furthermore, the oil pump can be made more compact compared to the current series-production engine. CYLINDER-HEAD COVER
The cylinder-head cover with integrated oil separator system is manufactured in plastic, with optimum weight and costs. The two-stage passive oil separator system consists of a settling chamber with a pre-separation function and a fine separator. An efficiency gain (increased oil separation performance at comparable pressure loss) is achieved by using a ball impactor in the fine separator. As the blow-by volume flow increases, a greater number of passage openings is released, FIGURE 1, thereby establishing an optimum oil separation level independently of the blow-by gas flow rate and the p ressure loss is limited.
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TWO -STAGE TURBOCHARGING SYSTEM
A two-stage turbocharging system is used for all four-cylinder engines that comprises an integral manifold with a non-adjustable high-pressure stage as well as a low-pressure stage with variable guide vanes on the turbine side, FIGURE 2. The actuation mechanism on the low-pressure stage ensures precise and rapid adjustment of the variable guide vanes, thereby achieving optimum boost control with a low hysteresis. The exhaust gas mass flow is split between the two exhaust turbochargers by a pneumatically activated exhaust gas control valve. Depending on the operating range of the engine, exhaust gas flows through both the high and low-pressure stages, thereby contributing to the generation of charging pressure. Only the low-pressure stage continues to operate in the upper speed range. In this case, the charge air bypasses the high-pressure compressor by means of MTZ worldwide 12|2017
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DE VELO PMENT Diesel Engines
change-over between cooled and uncooled operation is realised by a vacuum-controlled bypass flap, FIGURE 1. In the new BMW diesel engines, the cooler is arranged directly downstream of the valve, and consists of several plate-type heat exchangers; these permit a very high heat transfer rate at comparatively low packaging space. Special elements are used to generate turbulence in the heat exchanger plates. For one thing, this increases the heat discharge while, for another, it reduces the tendency toward soot buildup as a result of condensate deposits. FIGURE 2 Sectional view of two-stage turbocharging system (© BMW)
a pneumatically activated compressor bypass valve. INJECTION SYSTEM
A CR injection system is used with solenoid-valve injectors and 2200 (lower engine output range) or 2500 bar (upper engine output range) of peak pressure. A single-piston pump is used as the high-pressure pump; this features a camshaft modified for stroke and cam profile, in order to increase the delivery volume. A special, asymmetrical cam profile significantly reduces the load on the engine timing drive, in spite of the
FIGURE 3 Exhaust system – interplay of NOx storage catalyst and selective catalytic reduction system (© BMW)
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increased injection pressure level. The injectors have undergone further development on the basis of the familiar 2000 bar injectors. For the first time, an NCC system has been introduced to control the injection duration and thereby significantly increase the metering accuracy. EXHAUST GAS RECIRCULATION
The further-developed HP-EGR module has the task of regulating the exhaust gas flowing in directly from the exhaust manifold by means of a valve and channel ing it into the suction module after conditioning (either cooled or uncooled). The
EXHAUST AFTERTREATMENT SYSTEM
A combined exhaust aftertreatment concept is used, FIGURE 3. A combination unit is installed at engine level, consisting of a NOx storage catalyst (NSC) with a coated particulate filter. This is supplemented by a selective catalytic reduction converter arranged in the underbody. The urea/water solution in the selective catalytic reduction filter is supplied and distributed as effectively as possible by means of a dosing valve and a specially developed mixer. FUNCTION RESULTS MECHANICAL SYSTEMS
In order to optimise the engine friction, a new measuring method for friction measurement was developed with the Institute of Internal Combustion Engines and Thermodynamics (IVT) at Graz University of Technology, focusing in particular on separate recording of the crankshaft drive friction in combustion mode (“fueled operation”) [4]. A further significant improvement in the basic engine friction compared to the predecessor model has been achieved due to the use of this new process. In addition to the measures on the oil system and belt drive already described above, focus was directed at the following measures: –– Shape honing: The cylinder surface is manufactured at a larger diameter in the lower stroke range, resulting in a bell shape. In addition to clear advantages in terms of friction and thus consumption, it has also been possible to achieve a slight reduction in the piston play that is relevant to acoustics in the area of top dead center. –– Low-friction engine oil: In addition, it was possible to reduce the overall
FIGURE 4 Operating ranges and CO2 advantages of the two-stage turbocharger (© BMW)
friction of the engine by using a further-developed low-friction engine oil. COMBUSTION PROCESS
As a result of the internal mixture formation in diesel engines, the combustion process represents a key element for most engine properties. As a result, particular attention has been paid in the new engine to a sustain able architecture based on the main elements of charging, injection tech nology and the exhaust gas recirculation system. Lower untreated emissions because of an EGR system with increased efficiency, better torque and power delivery, low consumption even under real driving conditions, as well as the absence of impulse combustion noises were in the foreground of the development of the combustion process. Based on extensive concept evaluations, the approach selected was “high charging pressures combined with small injection nozzle cross-sections”. For the new four-cylinder engines, this means: –– two-stage turbocharger for all power output levels –– CR systems with 2200 to 2500 bar maximum injection pressure MTZ worldwide 12|2017
–– increased-efficiency HP-EGR, no low-pressure EGR.
as a result of the very low level of untreated emissions within the engine.
FUNCTION RESULTS TWO -STAGE TURBOCHARGING SYSTEM
FUNCTION RESULTS INJECTION SYSTEM
In contrast to the combination of two vari able nozzle turbochargers (VNT) in the currently most powerful four-cylinder engine, the new four-cylinder engines will have their lower and upper engine output ranges configured with non-adjustable high-pressure stages and VNT low-pressures stages. As a result, it is possible to achieve very fast charging pressure buildup, high charging pressures even at low engine speeds, as well as significant consumption advantages across broad operating ranges due to high charging efficiencies, FIGURE 4. As a result of the two-stage design and the low-pressure stage being optimised with regard to the entire operating range (extended surge limit and improved turbine efficiency), a significant efficiency increase was able to be achieved. This is revealed in the part-load range by a significant consumption advantage and in the full-load range by the higher available torque, leading to improved driving dynamics. The economic disadvantages of two-stage turbocharging could be compensated by the omission of low pressure EGR which became possible
In order to implement the principle of “small nozzle holes” for improved fuel preparation without any compromises in nominal power, the common-rail injection system with solenoid-valve injectors was upgraded to a maximum pressure of 2500 bar. Innovative needle-closing control (NCC) technology was developed in cooperation with a supplier in order to meet the challenging future objectives. The core of NCC is a piezoelectric sensor integrated in the solenoid-valve injector, by means of which the system measures the exact timings of the individual injections. The second innovation in NCC concerns the software: The actual injector behaviour during real driving operation is calculated continuously on the basis of the sensor measurements and physical software models. Deviations from the nominal injection timings and volumes are corrected in a closed control loop. This opens up a series of highly valuable possibilities: –– robust implementation of injection patterns with the smallest volumes
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DE VELO PMENT Diesel Engines
two four-cylinder output ranges cover a power and torque range from 110 to 140 kW and 350 to 400 Nm. The port folio has been extended downward with the three-cylinder derivatives (70 to 85 kW and 220 to 270 Nm); a four-cylinder topperformance variant will be introduced at a later date. In spite of unchanged full-load characteristic values, the dynamic response has been improved significantly compared to the predecessor engines, particularly through the introduction of two-stage turbocharging in the four-cylinder engines. FUEL CONSUMPTION OF COMPLETE VEHICLE
FIGURE 5 Functional principle of needle-closing sensor and injection pattern in the characteristic map (schematically) (© BMW)
and very short intervals (digital rate shaping, DRS) –– adaptation irrespective of the drivetrain, and fast control of the smallest injection volumes –– detection and compensation of nozzle hole carbon deposits as a measure to implement very small nozzle holes –– throughout the entire vehicle service life, injection timings and volumes can be kept constant to a large extent during real driving operation. The functional principle of NCC parameter detection is shown in FIGURE 5. The solenoid valve of the injector is configured so that the central armature pin responds sensitively to the change in characteristic injector parameters, such as nozzle needle closing. Integration of a piezoelectric sensor directly above this armature pin makes it possible to detect nozzle needle closing very robustly in spite of the relatively slight complexity.
ciency EGR system, but also very effective exhaust aftertreatment. DeNOx systems are essential in order to meet the RDE NOx limits. The combination system described above is used. This allows a very wide temperature and load spectrum to be covered at high degrees of conversion, FIGURE 3. DYNAMICS AND PERFORMANCE OF COMPLETE VEHICLE
The stationary full-load values have not been increased further compared to the predecessor engines, FIGURE 6. The
FUNCTION RESULTS EXHAUST AFTERTREATMENT
The new engines must have the potential to fulfill the real driving emissions (RDE) legislation, introduced in stages from 2017 onward. To do this, it will be necessary to introduce not only emissions measures within the engine by means of a high-effi-
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FIGURE 6 Full-load curves (© BMW)
The measures described in the previous sections have made it possible to achieve consumption improvements of 4 to 5 % in the NEDC through engine measures alone, depending on the engine variant, FIGURE 7. The technologies used, such as shape honing or staged turbocharging, take effect in a wide range of the characteristic map, FIGURE 4, as a result of which corresponding advantages are also apparent in driving cycles with higher loads. In the vehicle as a whole, this results in the following CO2 reductions and consumption savings compared to the predecessor drivetrains, FIGURE 8. EMISSIONS OF COMPLETE VEHICLE
The vehicles with the new drivetrains have been homologated as per the new WLTP regulations. However, as a result of the technology and applications used, the RDE1 specifications applicable from September 2017 have already been
achieved, and the NOx emissions also already remain significantly below the RDE2 limit values across a very wide range of driving and environmental conditions. The boundary areas of the future RDE2 specifications in particular remain very challenging, however. VEHICLE ACOUSTICS
FIGURE 7 CO2 reduction through thermodynamic and friction power measures, taking the example of a four-cylinder engine (schematically) (© BMW)
FIGURE 8 Consumption and emissions, taking the example of an X1 xDrive 20d AT (© BMW)
A significant development objective for the new engines involved further acoustic improvements with a focus on reducing the pulsation. Significant improvements have been achieved using the following technological approaches in particular: –– optimised design of contact surfaces of cylinders/pistons by shape honing –– tolerance-reduced counterbalance shafts –– combustion process with small nozzle holes and NCC injectors. FIGURE 9 shows, by way of example, the improvements in air-borne sound compared to the predecessor engine. Sub jectively, the pulsation that used to be typical of diesel engines has thus been forced further into the background. SUMMARY
The further development of diesel engines into the so-called Next generation modular system has not only enabled the described functional improvements, but has also optimised the effects on the production network (in-house production and com ponents industry) in terms of the number of variants and flexibility, while at the same time establishing the prerequisites for reliably meeting future emissions requirements. Following the start of the rollout of the new engine generation in the transverse application, additional engine variants with different performance levels and in inline applications will subsequently follow.
FIGURE 9 Acoustic improvement of a four-cylinder engine in air-borne sound compared to the predecessor model (© BMW) MTZ worldwide 12|2017
REFERENCES [1] Steinparzer, F.; Ardey, N.; Mattes, W.; Hiemesch, D.: The new BMW Efficient Dynamics Engine Family. In: MTZworldwide 75 (2014), No. 5 [2] Steinparzer, F.; Nefischer, P.; Hiemesch, D.; Rechberger, E.: The New BMW Six-cylinder Top Engine with Innovative Turbocharging Concept. In: MTZworldwide 77 (2016), No. 10 [3] Ardey, N.; Stütz, W.; Hiemesch, D.; Kaufmann, M.: The New BMW Three- and Four-Cylinder Diesel Engines. In: MTZworldwide 75 (2014), No. 7 [4] Wichtl, R.; Eichlseder, H.; Mallinger, W.; Peterek, R.: Friction Investigations on the Diesel Engine in Combustion Mode
– A New Measuring Method. In: MTZworldwide 78 (2017), No. 12
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