At its Innovation Days 2016 event in Munich, BMW unveiled new versions of its three and four- cylinder gasoline and diesel engines. Like their predecessors, the new power units are based on the modular system that enables the application of consistent design principles, a shared architecture and matching components.
The key elements of the standardized concept include the in-line engine’s basic design principle; an aluminium crankcase with uniform positioning of the intake and exhaust sides; a cylinder displacement of around 500 cubic centimeters per combustion chamber; as well as the arrangement of timing chains and ancillary units. In addition to this, the full line-up of gasoline and diesel engines feature BMW TwinPower Turbo technology.
|BMW TwinPower Turbo 3-cylinder gasoline engine. Click to enlarge.|
This commonality generates significant synergies in engine development and manufacture that have a positive impact on both environmental and economical sustainability.
The evolution of the Efficient Dynamics engine family has focused on further reducing fuel consumption and emissions at the same time as optimizing performance characteristics. A number of individual measures have been implemented to make the drive units even more efficient, and the resulting drop in consumption is evident on both the EU test cycle and in real-world driving. The improvements that have been incorporated to minimize emissions are furthermore designed to lower levels not just of emissions but also other exhaust gas components.
The advances made in the new-generation Efficient Dynamics engine family also herald further improvements to the engines’ smoothness and acoustic comfort while reducing weight. The new gasoline and diesel units are suitable for both longitudinal and transverse installation, meaning that they can be fitted in a wide variety of BMW and MINI models.
Gasoline engines. For the gasoline engines, the BMW TwinPower Turbo technology comprises a turbocharging system; direct gasoline injection; variable control of intake valve lift (VALVETRONIC); and continuously variable opening times for the intake and exhaust valves (Double-VANOS).
The new engines deliver a further cut in fuel consumption and emissions of up to 5%, and an increase in the power units’ output and maximum torque of 5 kW/7 hp and 20 Newton meters (15 lb-ft) respectively.
The turbocharging system, consisting of a turbocharger integrated into the exhaust manifold that enables the flow dynamics of the recirculated exhaust gases to be utilized to particularly positive effect, has undergone further development as part of the engine family’s overhaul. The exhaust manifold and turbocharger are now housed together in the cylinder head. The turbocharger casing for the three- cylinder engines is made from either aluminium or steel depending on the output variant, while the four-cylinder units all feature steel casings.
A reworked version of the direct injection system provides for added efficiency in the new gasoline engines. The injectors positioned centrally between the valves are fed from a new fuel pump via a modified system of fuel lines, and will in future operate at an increased maximum pressure of 350 bar. The higher injection pressure enables even more precise metering of the fuel and has the additional effect of helping to improve emissions quality over wide load ranges.
The more advanced cooling system fitted in the new generation of engines likewise serves to optimize the combustion process with the aim of reducing both CO2 output and other pollutant emissions. The new coolant pump now has separate outlets for the flow of coolant to the cylinder head and engine block, which results in far more effective thermal management.
Balancer shafts ensure both the three and four-cylinder gasoline engines display smooth operation. These shafts iron out the vibrations that occur when power is transmitted to the crankshaft. Three-cylinder engines will in future benefit from a new balancer shaft complete with a modified drive mechanism that results in a weight saving, improved excitation and further enhanced acoustic properties.
Other modifications that have a positive impact on engine efficiency include the use of a single-piece timing chain drive, which has the additional effect of optimizing acoustics. Plus, the revised engines are fitted with a new belt drive that is now the same on all variants. An L-shaped belt arrangement is used for driving the alternator, water pump, torsional vibration damper and air conditioning compressor.
|BMW TwinPower Turbo 3-cylinder diesel engine. Click to enlarge.|
Diesel engines. The BMW TwinPower Turbo technology for diesel engines consists of a turbocharging system with one or more turbochargers and common-rail direct injection. These two key elements have undergone substantial development for the modified engines, while enhancements to the basic engine’s construction have also been implemented along with numerous other detail refinements.
Again, these serve to both boost engine efficiency and minimize emissions. The newly enhanced versions of the diesel units in the Efficient Dynamics engine family burn up to 5% less fuel on average, which in turn means they also emit up to 5% less CO2. Internal engine modifications and improved exhaust gas aftertreatment result in a considerable reduction in other emissions too.
|Solenoid valve-/piezoelectric injector for diesel engines. Click to enlarge.|
To ensure even sharper throttle response while also increasing engine efficiency, all four-cylinder diesel units will in future benefit from the multi-stage turbocharging system that was previously only found on the most powerful four-cylinder engines of this type. This principle involves using two turbochargers of different sizes whose precision interaction is designed to put pulling power on tap early and keep it constant over a broad rev range.
The multi-stage turbocharging on the new four- cylinder units features a low-pressure stage with variable inlet geometry and a high- pressure stage. To further enhance responsiveness and acoustics, both turbochargers are equipped with the latest slide bearing technology. The new turbocharging system’s high-pressure stage is fully integrated into the exhaust manifold.
The system is controlled by means of the low-pressure stage’s electrically adjustable charger vanes as well as the wastegate valve for the high-pressure stage and a compressor bypass, both of which are actuated pneumatically. This allows the supply of compacted air to the combustion chambers to be precisely adjusted at all times to suit the load requirements and the driving situation. In future, switchable cooling for the low-pressure stage housing will further improve the most powerful four-cylinder diesel engine’s efficiency.
A redesigned exhaust gas recirculation (EGR) system also helps to both reduce fuel consumption and minimize emissions. A single-stage version of this system is fitted on the new four-cylinder diesel engines and a two-stage version on the three- cylinder units, ensuring particularly effective reduction of NOx. The EGR high-pressure module employed in all next-generation engines diverts the exhaust gases straight out of the manifold via an infinitely adjustable valve before directing them to the intake system, either in a cooled or a non-cooled state, as required.
The new three-cylinder diesel units are additionally equipped with a low-pressure EGR module that likewise includes a cooling facility. This captures exhaust gas that has already flowed through the diesel particulate filter (where it is stripped of its soot particles) and reroutes it to the clean air line. The low-pressure exhaust gas recirculation can also be used in engine running conditions where the pressure difference in the turbocharging system is insufficient to activate the high-pressure EGR.
The common-rail direct injection system for the new three and four-cylinder diesel engines has also undergone further improvement and now operates at a higher pressure and with greater precision. Its revised injectors feature an upgraded system of sensors that enables extremely exact metering of the injected fuel. In multiple injections within a stroke sequence, the intervals between the individual injections can also be shortened as a result. The finer atomization of the fuel brought about by the further increase in maximum injection pressure leads to exceptionally clean combustion with reduced residue in the exhaust gas. The injection systems on the three-cylinder engines will in future work with up to 2,200 bar of pressure, while the maximum pressure for the four-cylinder diesel units will increase to 2,500 bar or 2,700 bar in the most powerful variant.
Manufacture of both the three and four-cylinder diesel engines will in future rely on a process known as “form honing” for machining the cylinder bores in the aluminium crankcase, with their twin-wire arc-sprayed coating.
The standard procedure used to date involves giving the cylinder bores a perfectly cylindrical shape, meaning they have an identical diameter from top to bottom. The influence of thermal and dynamic forces causes expansion in the top section of the cylinder bores, which starts during assembly of the cylinder head but is particularly noticeable during engine operation.
Depending on the piston design, this either causes play at the top of the cylinder bores that has a negative effect on the engine acoustics or increased friction low down that is detrimental to efficiency. The new manufacturing technique being used for the first time for production engines makes allowance for these subsequent alterations. In order to compensate for them, the cylinder bores are now flared slightly towards the bottom. The desired geometry is obtained by means of an axial lifting motion with overlapping rotation. When the engine is operating, the expansion in the top section therefore creates a largely uniform diameter throughout the cylinder bores, allowing the pistons’ friction losses to be reduced without any negative impact on the engine’s acoustics.
A newly improved oil circuit with switchable piston cooling for on-demand operation and a modified belt drive for the ancillary units are two more innovations that help to increase efficiency, while a new tensioning technique for the balancer shafts serves to further enhance the acoustic properties of the four-cylinder diesel engines.
The internal engine measures will be complemented by a system of exhaust gas aftertreatment in the future three- and four-cylinder diesel units. Besides the close-coupled diesel particulate filter and trap, all next- generation diesel engines can also be equipped with a Selective Catalytic Reduction (SCR) system.
With this form of emission control, a water-based urea solution known as AdBlue is added to the exhaust flow. A water-cooled dispensing module injects precise quantities of the solution, which transforms into ammonia in the exhaust pipe before reacting with the nitrogen oxides inside the SCR catalytic converter to produce nitrogen and water. The effectiveness of this exhaust gas aftertreatment is permanently monitored by another sensor positioned downstream from the SCR unit.
The AdBlue solution—carried in a separate reservoir—is injected into the exhaust pipe in precisely calculated doses without the driver noticing. Together with the optimized combustion process and all the other emission control measures, this ensures that all drive unit variants worldwide from the upcoming generation of the Efficient Dynamics engine family will again comply with both current and future legislation governing emissions reduction.