COVER STORY
Diesel Engines
The New Daimler Heavy Commercial Vehicle Engine Series As part of the group-wide global standardization of the product portfolio, Daimler AG is developing a new engine series – the Heavy Duty Engine Platform (HDEP) for use in heavy commercial vehicles. With the unveiling of the Detroit Diesel DD15, the first engine in this family, with a capacity of 14.8 l, was introduced on the North American market. Additional variations of this platform will follow with displacements of 15.6 l, 12.8 l and 10.6 l. These engines will then be introduced gradually in all Daimler AG trucks.
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1 Introduction All displacement classes of the Heavy Duty Engine Platform are in-line six-cylinder engines which, in addition to economic considerations such as implementing a maximum shared parts strategy across all brands and capacity classes, stresses technical progress. The technical objectives can be paraphrased simply: Development of a model series that represents a benchmark and meets, or will be able to meet, current and future emissions legislations: – adoption of an exclusive new, pressure-amplified common rail injection system – use of a Turbo Compound technology to utilize exhaust gas energy – design for high peak combustion pressures – maximum flexibility due to two overhead camshafts. This new engine family for heavy commercial vehicle, Figure 1, will make a substantial contribution to the future success and competitiveness of Daimler AG.
2 Project Organization In order to best take individual market requirements into account from the very start, the Heavy Duty Engine Platform project was set up as an international de-
velopment project with development locations in Stuttgart (Germany), Detroit (USA) and Kawasaki (Japan). In order to preserve global commonality, the overall responsibility and central design responsibility was kept in Stuttgart. As an example for all other groups, the breakdown for mechanical validation was made according to competencies and proximity to the customer. For example, all fundamental functional trials took place in Stuttgart, while support for vehicle testing in Portland was handled completely from Detroit. The abilities of the individual locations were integrated optimally into this project thanks to clearly defined responsibilities and competency areas based on a spirit of cooperation and trust.
3 Basic Engine with Drive Train The crankshaft assembly is designed using modern simulation tools to provide optimal rigidity and high loads, Figure 2. The crankshaft assembly was computed using 1D and 3D multi-body simulations in order to achieve low structural height through high peak pressures and low compression height. The crankshaft layout was designed for hydrodynamics with the help of electronic high-pressure diesel injection system and validated for strength using FEM.
Figure 1: Cold and hot side of the engine
The Authors
Dipl.-Ing. Bernhard Heil is Vice President Truck Product Engineering Powertrain at Daimler AG in Stuttgart (Germany).
Dipl.-Ing. Wolfram Schmid is Director Product Engineering Heavy Duty engines at Daimler AG in Stuttgart (Germany). Dr.-Ing. Martin Teigeler is Senior Manager Development Engineering Heavy Duty engines at Daimler AG in Stuttgart (Germany).
Dr.-Ing. Wolfgang Sladek is Senior Manager Component Engineering Heavy Duty engines at Daimler AG in Stuttgart (Germany).
Dr.-Ing. Heinz Öing is Manager Fuel Injection System HDEP at Daimler AG in Stuttgart (Germany).
Dipl.-Ing. Stefan Arndt is Manager Thermodynamic System HDEP at Daimler AG in Stuttgart (Germany).
Dipl.-Ing. Stefanie Melcher is Design Engineer HDEP at Daimler AG in Stuttgart (Germany). MTZ 01I2009 Volume 70
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Diesel Engines
micular graphite (GGV) which makes the component extremely rigid and compact. The cylinder head gasket is a multi-layer sheet metal design. Combustion chamber sealing is achieved by using several beads, and the water/oil passages are sealed by a vulcanized elastomer.
4 Gear and Valve Train with Engine Braking System
Figure 2: Crankshaft calculation
The crankshaft assembly with oilcooled steel pistons, cracked connecting rods and a crankshaft with eight counterweights runs in lead-free bearings and a GG26 (Grey Cast Iron) crankcase which is an open deck design. With external integral oil return passages and a rigid oil pan flange, the cylinder crankcase was designed such that it was not necessary to use a ladder frame to stiffen the side walls of the crankcase. This design detail
Figure 3: Camshaft – cross-sectional drawing 6
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allows the number of components and assembly times to be reduced. Minimal oil consumption is achieved by watercooled “bottom stop” cylinder liners with precision plateau honing. A further reduction in oil consumption is achieved by bolting the cylinder head symmetrically to the cylinder crankcase using eight bolts per cylinder. Because of the unit load, the cylinder head is made from cast iron with ver-
For optimal containment of noise and because of the extremely small irregularities in crankshaft rotary movement, the gear train is located on the output side of the crankshaft. It powers the highpressure pump, the air compressor, the oil pump, two camshafts and the optional PTO. The transmission of the Turbo Compound is incorporated as well. The resulting forces, the width of the gears, smooth operation as well as assembly requirements make it necessary to use spur as well as helical gears. The optional PTO is designed for a maximum output torque of 900 Nm plus 20 %. This gear train plane is therefore dimensioned for correspondingly high torques. The engine family has two overhead camshafts, each driven by one gear. Weight considerations dictate assembled camshafts, each operating four valves per cylinder, Figure 3. The camshafts are running in seven bearings in a die-cast aluminum camshaft frame without additional bushings. For strength reasons, forged aluminum alloy is used for the rocker bearing supports. During engine braking, one exhaust valve per cylinder is hydraulically lifted two additional times by using an additional cam on the exhaust camshaft. The first lift at the commencement of the compression stroke provides additional filling of the cylinder with hot gas from the exhaust manifold and at the same time prevents uncontrolled opening of the exhaust valves by reducing the pressure pulses there; the second lift causes an abrupt decompression of compressed air at the ignition TDC which achieves the actual braking effect. Valve lift and opening times, taking into account the comparatively simple turbocharger with no variable geometry
Figure 4: Engine braking performance, two cylinders, four cylinders and six cylinders
or additional vanes, are designed to provide maximum braking torque of 1780 Nm from 1800 rpm, while simultaneously minimizing exhaust noise and thermal and mechanical loads on the engine. Since the curve for maximum brake performance already surpasses the efficiency factor of the compressors at 1200 rpm and, because of the compression ratio of up to 2.8, the engine brake can be described as a charged system; charge pressure control is managed through the EGR valve. Two electric switching valves allow two, four or six cylinders to operate during braking, thus constituting three discrete braking stages, Figure 4. On and off
delay times of less than 150 ms with an engine at operating temperature, paired with a t90 time of three seconds, characterize the dynamics of the system and allow it to be used to assist shifts with automatic transmissions.
5 The Exhaust System with Turbocharger, Turbocompound and Exhaust Gas Recirculation The turbocharger with the downstream power turbine and the transmission (Turbo Compound) is located on the hot side of the engine, which provides additional effective power of up to 35 kW to
reduce fuel consumption, Figure 5. The power from the turbocompound is transmitted through the compound transmission and the gear train to the crankshaft and is reflected positively in fuel consumption. The die-cast transmission housing contains a hydraulic clutch to disconnect from the turbine wheel and crankshaft, as well as to damp rotational vibration. All HDEP engines have exhaust gas recirculation (EGR). To meet EPA 07 emission requirements, it is linked to an active particulate filter. HDEP engines will meet future emissions legislation, such as Euro 6 and EPA 10, with the additional application of AdBlue technology. The exhaust gas flow through the EGR cooler is regulated by the EGR valve on the hot side, which will be dual-flow in the future. Engine response is positively affected by the low volume and the high pulse of the exhaust stream ahead of the turbocharger turbine. Measuring the EGR mass flow that is directed to the charge air is carried out using a calibration section at the forward end of the engine, in accordance with the Venturi principle using a differential pressure sensor. This exhaust gas, along with the cooled compressed fresh air, is taken to the charge air collector on the cold side of the engine through a mixing section. In addition to the intake system, the control unit and the optionally available singleor twin-cylinder air compressor sit on the cold side of the engine.
6 Oil Circulation
Figure 5: Exhaust system
Depending on the vehicle model, the engine has different oil pans, which are produced optionally from a plastic composite or an aluminum die or sand casting, with a capacity of approximately 45 l. The external gear pump is located in the oil pan on the output side of the engine and is driven directly from the crankshaft gear. In addition to a safety valve to reduce maximum oil pressure, there is also a control valve integrated into the oil pump housing. It regulates constant operating oil pressure and returns the excess oil directly to the suction side to minimize losses. The module MTZ 01I2009 Volume 70
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Diesel Engines
Figure 6: Oil circulation
contains a check valve to ensure rapid buildup of oil pressure when the engine is started. The modular assembling of the suction module allows a combination with only a few components of all versions for oil pans. The pump receives fresh oil through the suction module and transfers it to the oil/water module which is located directly next to the water pump on the front, cold side of the engine. The oil module, which is an aluminum die cast, comprises the eleven-plate cooler, the filter and a by-pass valve. This valve controls the oil flow from the cooler and thus regulates the temperature of the circulating oil. The filtered oil then flows through the cross drillings to the main oil passages in the engine crankcase. The main bearings and the oil spray nozzles for cooling the pistons are supplied from there. The oil blow-by separator, the turbocompound gear and the bearing block for the turbocharger are connected to the pressurized oil circuit. The branch bores to supply the gear train as well as two vertical galleries to supply pressurized oil to the valve train are located at the rear transverse passage. The cams, the camshaft bearings and the rocker arms are supplied with pressurized oil through the hollow-drilled camshafts and rocker shafts. The total volume of oil circulating in the engine is approximately 12.7 l, Figure 6. 8
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7 Water Circulation The HDEP engine is designed with an inlet-side thermostat and fan control, which ensures fast reaction times and optimal temperature conditions in the engine. The coolant pump transfers the coolant past the oil cooler into a collector integral with the engine crankcase. The flow of coolant for the EGR cooler is arranged in parallel. The division of the volumetric flow was developed from numerous tests for optimal cooling, component and operating temperatures and
Figure 7: Water circulation
pressure conditions, in addition to the analytical design. From the collector, the coolant for each cylinder flows into the circular passage between the cylinder liner and the cylinder crankcase. The high thermal energy in the upper area of the cylinder liners is dissipated by a matching high flow velocity. By means of a flange on the cylinder liner, the upper annular gap with a high flow rate is specifically separated from the lower annular gap with a low flow rate. On the hot side of the engine, the cooling medium flows into the lower water jacket of the cylinder head through suitable galleries. Good energy dissipation is provided at the combustion chamber deck by an adjacent coolant flow. The coolant f lows from the lower water jacket through the centrally located injector bore into the upper water jacket provide for cooling the valve guides. The lower water jacket is a cross-flow design; the upper water jacket has an additional front-to-rear flow. On the outlet side, the coolant flows into the water collector and is passed to the radiator at the forward end, Figure 7, where the belt drive is located, which powers the water pump, the A/C compressor and the generator. The drive for the electrically controlled fan clutch is located on the second belt level. The belt drive was designed to be so compact, that the required belt force is achieved on both levels by an integral dual tensioner.
Figure 8: Injection sequence
8 Injection The injection system is a central subsystem of the HDEP engine. When the system was designed, the requirements for flexible multiple injections, high injection pressure, but also the consumptionoptimized injection sequences from the PLN engines had to be met and matched. A pressure-amplified common rail system (Amplified Pressure Common Rail System – APCRS for short) is therefore used in the HDEP engine. Using this system, injection pressures at the injector needle of up to 2300 bar are possible, with a simultaneous open selection of the injection sequence: “Square” – corresponding to a standard common rail system, “Boot”- and “Ramp” sequence, Figure 8. The injector consists of two units: a standard common rail injector with a solenoid valve, flow and cut-off are controlled by the injector needle. The second unit is a hydraulic pressure booster which can be activated through a separate solenoid valve independently of the injector needle. Advanced miniaturization allows both to be integrated into the injector. Rail pressure is present at the injector and, depending on the point in time when the two valves are activated relative to each other, different injection variants of injection at rail pressure from no boost up to maximum boost are possible with the above modes. The high-pressure lines between the injector and the rail are designed to be as short as possible. The rail is the balanc-
ing element which varies in length with the distance between cylinders. Overall injection characteristics, even with multiple injections, are thus independent of engine size. The injection system is supplied through a twin-piston in-line pump which operates at a maximum pressure of 900 bar. The high-pressure pump is entirely suction-regulated and operated synchronously with the engine, i.e. at each injection sequence there is also a pump stroke. The high-pressure pump is lubricated by the fuel in order to keep oil entry as low as possible with regard to the exhaust aftertreatment system.
The multiple potential uses of the HDEP engine require that fuel management take place at the engine itself. Water separation (coalescer) and fine filtration are handled by an integral filter module that is located on the pressure side for long service life. The low-pressure pump is located on the high-pressure pump and is protected by a suction-side pre-filter which is also located in the filter module. The filter module collects all the returning fuel, the greatest proportion of which, the pressure booster return, is cooled by the engine coolant through a fuel cooler also integrated into the module. The filter module takes part of the fuel back to the fuel tank. Most of it continues to circulate close to the engine under controlled conditions so a fuel heater is not necessary. The APCRS system has a potential of 2500 bar to meet future exhaust emission standards.
9 Validation The quality requirements at this new engine series – the goal is lifetime of 1.2 million miles – demand totally new test programs and procedures which have been developed in conjunction with our international development partners in the US and Japan. Besides standard test e.g. focussing at thermomechanical fa-
Figure 9: Wear map and driving cycle and the overall resulting damage MTZ 01I2009 Volume 70
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Diesel Engines
1. The lowest possible over-the-road fuel consumption 2. very good transient torque, both starting torque following transient load response as well as the time to wideopen throttle torque 3. attaining projected power outputs and torques in extreme climatic conditions and at altitude.
Table 1: General engine data Rated power
339 – 418 kW
Rated torque
2109 – 2509 Nm
Stroked volume
14.84 l
Bore x stroke
139 mm x 163 mm
Compression ratio
18
tigue or wear of core components, programs were developed taking validation of the numerous new components into account since their failure rate is relatively unknown at this time. To this end, some engines have been equipped with suitable measuring equipment to determine thermal and mechanical loads in specific operating situations. This combination of load and engine speed was evaluated with respect to its “damaging impact” and compared with driving cycles that were actually measured. The resulting “load profile” then affects how the test bench or test drives are defined. The damaging conditions are being driven intentionally more often. As a consequence, the “damaging impact” that a component would experience in normal operation can be sequenced in time which significantly shortens the overall test time and their respective remedies. In addition, this procedure sharpens awareness of the potential damage root causes that can then be integrated more quickly and more efficiently into the design of the components. For example, Figure 9 shows a wear map for a specific component which was determined as part of a radio-nuclide study. This map is compared with a real drive cycle and from it the overall damage is calculated. In this way, component-dependent “acceleration factors” can be calculated for each test cycle or vehicle test which provide information about which test imposes which load on the particular component. The findings from this load matrix, which has been jointly developed with AVL, had a considerable effect on the following test planning. Validation of the engines was performed on a global basis in all the engineering centers involved. In order to meet all specific market requirements, particular attention was paid in the test 10
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phase that tests are performed right in the target market using the regular local operating liquids such as fuel and coolant and the respective boundary conditions – meaning not only the climatic or topographical conditions, but also drivers’ driving habits, for example. Overall, during the entire development phase well over 100 thousand test bench hours and more than ten million miles were covered. Even more important than defining the relevant test cycles is the systematic compilation and processing of the damage records that occurred during the test. In order to accomplish this, a documentation system has been established which can be used equally by all international development sites. The specific were then discussed in joint meetings and required measures thoroughly tracked and revised.
10 Thermodynamics 10.1 Objectives In addition to the obligatory conformance with the EPA07 emissions requirements, the three most important objectives for thermodynamic development are:
10.2 Systems in the Engine Design to Achieve the Goals The stroke/bore ratio of about 1.17 is a good compromise between the thermodynamic optimum and minimum installation space. The 230 bar maximum peak firing pressure allows a highly efficient combustion even at high charge pressures and compression ratios. Table 1 summarises general engine data. As a logical continuation from the development of the combustion systems of the 500 and 457 series, the HDEP engine has non-rotated, symmetric valve layout that does not produce any swirl. According to internal studies, swirl-free combustion characteristics are better in fuel consumption than those with swirl, if the particulate emissions can be controlled. In addition, a valve arrangement of this kind produces very short intake and exhaust ports, which contributes to reduce the work in the gas cycles exchange. A coolant temperature, which has been raised by 10 K, and the thermostatically controlled oil temperature both contribute to reducing friction compared with the preceding engines. In the selection of the injection system, the focus was, aside from meeting emission standards, on high injection pressure, the capability for multiple injections, flexibility in the
Table 2: Emissions-related technologies and threshold values Exhaust emissions
EPA07 (limits: NOx 1.48 g/kWh, PM 0.013 g/kWh)
Combustion process
six-hole fuel injector with conical spray orifice and conical combustion bowl, with a compression hump and strong air distribution
Injection system
Pressure-amplified common rail system with variably adjustable injection pressures and injection curves in the map, multiple injections
Turbocharging and EGR
Single-stage turbocharger with working turbine downstream from the turbocharger turbine (compound turbocharger), cooled EGR with steplessly adjustable EGR valve, EGR mass flow measurement
Exhaust gas aftertreatment
DPF system (DOC + DPF) with secondary fuel injection and intake throttle valve for active regeneration
rated rpm, but without a power reduction. While the permissible NOx emissions are met internally with EGR, a closed particulate filter is required for particulate emissions.
10.4 Turbo Charging and EGR Transport
Figure 10: Characteristic engine pressure differentials of different turbo systems
injection rate shape in order to create a fuel consumption advantage in high-load regions compared with square injection rate shapes which are generated by standard common rail systems. The basic engine is designed for the application of cooled EGR in the high-pressure path between turbine and compressor, which is superior to low-pressure EGR systems in its basic features: Demonstrable power with single-stage turbocharging, control power speed and packaging A turbine (Turbo Compound) downstream from the turbocharger turbine generates the previously mentioned additional layout power at the crankshaft. The axial turbine supports an easy installation of the engine in the vehicle.
10.3 Boundary Conditions and Derivative Developments The emission level EPA07 is currently the stringent legislation globally for commercial vehicle diesel engines at the time of the launch of the production engine. The critical points are the test limits for nitrous oxides and particulates, Table 2. The test cycles in NAFTA result in the need to not significantly exceed the test threshold at full load, resulting in EGR rates of about 20 % at maximum torque and about 25 % at rated output. Turbo charging and combustion are designed consistently for a low air ratio so that the maximum power level is achieved with single-stage turbocharging. As a result of the existing reserves in turbocharger rpm and compressor outlet temperatures, the engine can be operated up to 45 °C ambient temperature at sea level and up to 3,600 meters above sea level at reduced
With high pressure EGR systems the turbo system has the additional task to produce the needed pressure differential across the engine to drive EGR. Air and EGR path are therefore integral systems, which are directly connected in their function, layout and control. Figure 10 illustrates resulting engine pressure differentials for different turbo systems across several load points at road speed. All curves have same NOx emissions. In principle turbos with variable turbine geometry have negative engine pressure differentials and subsequently a negative gas exchange cycle work. When asymmetric twin scroll turbines are used, only one volute generates the differential pressure for the EGR transport. At high loads the mean pressures of both volutes produce positive pressure differentials and a fuel economy advantage. The 14.8 l charging system with Turbo Compound has been developed to have a negative pressure differential in the engine operation map. The Turbo Compound power distribution overcompensate the gas exchange disadvantage and leads once more to a fuel consumption advantage. As the Turbo Compound wheel is coupled to the crankshaft with a fixed gear ratio, the u/c parameter rises with falling exhaust mass flow and the efficiency drops below zero. This principle effect has been considered through a layout that gives EGR rates of over 40 % in part load. For same target NOx emission this allows advanced injection timings and good efficiencies in the high pressure loop of the combustion. The small turbo charger turbine generates high boost pressures already at low loads. In conjunction with a combustion system designed for a high start torque a step acceleration from motoring to 90 % of the full load torque takes below 2 s at road speed. These values can be derived in spite of driving EGR rates over 10 % in highly transient engine operation, which enables a good NOx control in this mode.
10.5 Fuel Consumption Totaling up the systems that have been developed, their optimization regarding hardware and application, it has been possible to reduce the fuel consumption best point in the map to substantially below 190 g/kWh. In-house and customer over-the-road fuel consumption measurements in the vehicle confirm this excellent result.
11 Looking Ahead This new engine family for heavy commercial vehicles will make a substantial contribution to the future success and competitiveness of Daimler AG. Broad customer acceptance in the American market has proved the superiority of this engine design since its market launch. In addition to the 14.8-liter engine – marketed in the US as the DD15 – the new engine platform will include additional capacity classes in the future: a 15.6-liter, a 12.8-liter and a 10.6-liter engine. Customer benefits at competitive prices can only be achieved by bundling volumes and scaling effects. Over the course of the next few years, these engines will be installed at Mercedes-Benz not only for the NAFTA market, but also for the European market and at Fuso for the Asian market. O
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