A closer look at the basics of Volkswagen Group’s important new 2.0L diesel MDB engine
22 March 2014
|The EA288 2.0-liter diesel, prominently showing the closely attached exhaust aftertreatment system. Click to enlarge.|
The Volkswagen Group’s new 2.0-liter TDI diesel (EA288) will eventually replace all the 2.0-liter TDI Clean Diesel engines fitted in Audi and Volkswagen TDI Clean Diesel models, and as such, is a strategically important engine. (Earlier post.) The Group was responsible for 79% of all light-duty passenger diesel sales in the US in 2013; worldwide, every fourth car sold by the Group is a diesel, said Dr. Johannes Arning, of Volkswagen’s Powertrain Product Management group.
The new 2.0L TDI is a turbocharged, common-rail, direct-injection four-cylinder engine based on the Volkswagen Group’s modular diesel toolkit (MDB, Modularen Diesel Baukasten). (Earlier post.) Volkswagen also has a modular gasoline engine toolkit, the MOB (Modularen Ottomotor Baukasten), which includes the new EA211 series. Both MDB and MOB play critical roles in the Volkswagen Group’s larger MQB modular toolkit, which is in turn one of the four main modular toolkits (modularen Baukästen) of the Group: the MQB (transverse); the MLB (longitudinal); the MSB (standard drive); and the NSF (New Small Family).
|The MQB strategy of the Volkswagen Group extends from the A0 to the C-segment. There is some overlap at the low-end with the NSF and in larger vehicles with the MLB. Click to enlarge.|
The MQB allows the integration of alternatives, for example, natural gas, hybrid and electric drives, in addition to the conventional gasoline and diesel power units. As one clear example, the new Golf, which is MQB-based, offers gasoline; diesel; natural gas; plug-in hybrid (the GTE, earlier post); and battery-electric (the e-Golf, earlier post) versions, all of which can be manufactured bumper-to-bumper on the same assembly line. Further, the hardware of the plug-in hybrid drive in the GTE is also that of the Audi A3 e-tron plug-in hybrid.
The new MDB EA288 diesel
The 2.0L EA288 diesel engine delivers 150 hp (112 kW)—an increase of 10 hp over the current engine and 236 lb-ft (320 Nm) of torque. Technical development targets for the development of the new MDB-based EA288 diesel engine were CO2 reduction; emissions reduction; performance; comfort; and costs.
|Basic specs of the older 2.0L (left) vs. the new (right). Click to enlarge.|
The new diesel shares only the bore spacing with the previous 2.0-liter diesel. A number of changes have been made to help reduce emissions, such as the use of a complex exhaust gas recirculation system (with high pressure EGR and a cooled low-pressure EGR); integration of the water-cooled intercooler and the EGR valve with the intake manifold (which also improves throttle response); and packaging the exhaust after-treatment components close to the engine by combining the DPF with the SCR Catalyst.
The EA288 is designed to meet the requirements of different markets. For example, depending on the emission requirements, 3 different types of exhaust gas recirculation (EGR ) can be used:
- cooled high-pressure EGR without low-pressure EGR;
- cooled low-pressure EGR without high-pressure EGR; or
- cooled low-pressure EGR and non-cooled high-pressure EGR.
Similarly, the closely-attached after-treatment devices—including an oxidation catalyst; diesel particulate filter; and NOx storage or selective catalytic reduction system (SCR)—can be used singularly or in combination depending upon the needs of the vehicle (e.g., weight) and market regulations.
Crankcase. The crankcase—as in the previous TDI—is made of gray cast iron. The objective was to reduce crankcase weight while integrating further components. Volkswagen engineers achieved this by:
- integrating balancer shafts above the crankshaft;
- a short water jacket for the fast component heating;
- cooling of the area between the cylinders;
- integrating of thermal management measures in oil and water management;
- head bolt threads below the water jacket;
- optimization of oil guides to minimize flow losses; and
- extension of the oil return and blow-by channels to the parting plane of the oil pan.
Design criteria were:
- even cooling of the crankcase;
- cross-flow in the cylinder head from the outlet to the inlet side;
- good flow through the holes to cool the cylinder fins;
- uniform distribution of the flow to the cylinder; and
- optimization of the flow guide with additional consideration of the requirements in the warm-up.
|The dual oil and vacuum pump unit. Click to enlarge.|
Oil and vacuum pump. The oil/vacuum pump is designed as a dual pump, driven directly from the crankshaft by a toothed belt in oil. Both pumps are arranged in a common die-cast aluminum housing below the cylinder crankcase flanges in the oil pan. Pretensioning of the belt is specified during installation by the center distance of the components; this leads to a particularly friction-optimized drive.
The oil supply is realized by a volume-controlled vane pump. A solenoid valve can also be connected depending on the load in a low- or high-pressure stage. Thus, an optimum between lubrication and power consumption can be achieved in engine operation.
The arrangement of the vacuum pump resulted in new design requirements that required a low drive torque at cold start, among others factors. Using a double-reed valve, a sufficiently large cross section for the ejection of the oil is realized in the vacuum chamber. Thus, the drive torque can be kept low even at low temperatures . The connection to the vehicle-side vacuum line is via bores in the vacuum pump and in the cylinder crankcase.
|Cylinder head. Click to enlarge.|
Cylinder head. The cylinder head intake and exhaust valves are arranged one behind the other. This arrangement results in a mixed camshaft, each having an intake and exhaust control. Since the valve assembly has been changed compared to its predecessor, the channels had to be redesigned, with a focus on increasing the maximum flow with good swirl numbers.
A further innovation of the MDB concept is the thermal management system in which the cylinder head plays a central role. A micro-cooling circuit—one of three cooling circuits in the engine—is integrated into the cylinder head.
The outlet is located in the bottom plate with connection to the cylinder block, which takes the return of the water.
To increase the heat dissipation in the region close to the combustion chamber, the water jacket is divided into a lower and an upper water jacket core, each with a cooling channel. The two cooling channels are separated from each other and are guided together only at the outlet.
This enables a more uniform distribution of the cooling capacity between the individual cylinders compared to the predecessor engine.
|Valve train. Click to enlarge.|
Valve train. The valve train of the new MDB 2.0-liter TDI engine differs from its predecessors through the use of an integrated valve drive module ( iVM ). Thus, the camshaft bearing frame can be separated from the cylinder head to prepare it for future emission requirements. In addition, the bearing frame was optimized for friction.
Functional advantages of the iVM include:
Valve train is designed as an independent module with appropriate manufacturing and cost advantages.
Reducing the friction loss of the camshaft through the use of a needle bearing.
Internal oil supply to the bearings with a separate integrated into the bearing frame oil gallery.
Additional supply of oil to the cylinder head.
Regardless of engine capacity, the blank, valve train and cylinder head cover module are always identical. Only the size of the hole and of the valves is different.
|Intercooler. Click to enlarge.|
Intake manifold with integrated charge air intercooler. The MDB TDI features an intake manifold with an integrated charge air intercooler. The predecessor 2.0L TDI had already used indirect water-cooled charge air cooling were used. As a further development, the water-cooled charge air cooler for the MDB engine—as in the 1.4 liter TSI engine—is integrated into the intake manifold.
This constitutes a separate low-temperature coolant circuit with air-water heat exchanger in conjunction with a variable-speed water circulation pump.
The charge-air duct is extremely compact, and the water-cooled module enables a shorter and more compact air intake circuit. The reduced charge air volume improves the transient response of the engine significantly. Further, flow losses are reduced, and icing or condensation in the intercooler be avoided.
|Comparison of the new charge air intercooling system (right) with its predecessor (left). Click to enlarge.|
The integrated intercooler is supplied by Valeo and is made entirely of aluminum. The cooling body, which consists of coolant plates, fins, cover, bottom and side panels, as well as coolant connections,is fully soldered. The inlet and outlet boxes are then welded to the heat sink.
The radiator network consists of 10 pairs of brazed cold plates. The coolant flows through plates are W-shaped counter-currents to force a complete utilization of the radiator network at a reasonable pressure drop.
Through a special geometry of the coolant plates, the cooling agent flow is distributed across the width of the flat tube, and is deflected at the same time. This provides for a low pressure loss for a good heat transfer from the aluminum sheet to the coolant. At the same time the design of the cooling plates offers a high robustness with respect to the change in pressure resistance.
Air side fin thickness and fin spacing were optimized so that the cross-sectional area of the lamella can derive the maximum quantity of heat to the coolant plates and at the same time the pressure loss is minimal. Small punched openings which are arranged alternately like gills provide for a good heat transfer and also allow a flow in the transverse direction.
|The exhaust side systems. Click to enlarge.|
Exhaust-side. The exhaust side of the MDB engine consists of the exhaust manifold module with exhaust gas turbocharger; the exhaust after-treatment system module; and the low-pressure EGR system. As noted earlier, the components within the aftertreatment module vary depending upon the emission standard, as do the three different types of exhaust gas recirculation.
The compact design of the aftertreatment module and its close mounting to the engine enable low heat and pressure losses as well as the fast starting of the oxidizing catalytic converter and speedy heat-up of the diesel particulate filter.
EGR. At high combustion temperatures in the engine result in NOx emissions. The higher the combustion temperature in the cylinder, and the longer its duration, the higher the proportion of NOx in the exhaust gas. Exhaust gas recirculation (EGR ) is used to reduce engine-out NOx, which, in conjunction in come cases with further aftertreatment, results in meeting emissions targets. The rapid oxidation of fuel molecules in the combustion cylinder is inhibited by the presence of the exhaust gas molecules; peak temperature and the resulting NOx emissions are therefore reduced.
Both the forthcoming EU 6 emission standard as well as the even more stringent US EPA Tier 3/ California LEV III standards, call for further significant reductions NOx compared to today. Volkswagen engineered the MDB TDI engine to be able to meet all those coming standards.
The engine features high-pressure and low-pressure EGR loops.
The recirculated exhaust gases of the high-pressure EGR have a high temperature; mixing this exhaust gas to the fresh air in the intake manifold is intended to reduce the air mass, resulting in operation with a lower air-fuel ratio. Also, the average temperature of the fresh charge increases. HP-EGR is used to address dynamic and cold start issues.
The low-pressure EGR system, integrated with the exhaust aftertreatment system, is upstream of the exhaust gas turbo; the recirculated gas is cooler, and low in particulate matter. This allows the reduction of the cylinder air mass without heating the intake air; further, the exhaust gas mass flow upstream of the turbocharger is not reduced, which enables maintaining the high exhaust gas enthalpy. Volkswagen improved the pressure losses in the LP EGR system by some 90%, bringing it down from 200 to 20–25 millibars.
|Exhaust gas recirculation system. Click to enlarge.|
Thermal management. The thermal management of the 2.0L TDI MDB has three cooling circuits which can operate independently.
Micro-circuit. The micro-circuit consists of the cylinder head, the EGR cooler, the heat exchanger and an electric coolant pump.
Main water circuit. The main water circuit includes the crankcase, engine and transmission oil cooler, front radiator and a switchable water pump.
Low-temperature circuit. The low-temperature circuit consists of the integrated intake manifold intercooler, a front radiator and an electric coolant pump.
After cold start only the micro-circuit is operated. With increasing cooling demand, the switchable water pump is switched on. The low-temperature circuit is responsible for the indirect charge air cooling. The high- and low-temperature circuits are operated independently of each other. The aims of the thermal management system are to shorten the warm-up phase after a cold start; bring the emission-reducing components to temperature; and deliver an optimized temperature to the passenger compartment.
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