D EV E LO P M E N T
Diesel Engines
The New 2.2 l Diesel Engine from Mazda Diesel engines for the European market should deliver impressive performance and fuel economy but also high levels of environmental performance, good noise and vibration suppression. With this in mind, Mazda developed a new diesel engine for the launch of the second-generation Mazda 6 with the aim of realising its product philosophy of sustainability combined with a sportive driving experience. The new MZR-CD 2.2 engine will be available for future models like the all-new Mazda 3 as well.
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1 Development Targets and Key Features Today market expectations are at an alltime high. To meet these expectations, Mazda has developed a new diesel engine, the MZR-CD2.2, which has higher output, higher torque and better fuel economy. At the same time, customers also expect environmental performance as well as noise and vibration levels comparable to those of a gasoline engine. The new engine is substantially more refined [1] and offers a 10 % increase in displacement to 2184 cm3 when compared to its predecessor, Table. The improved specific fuel economy, high power output and meeting Euro 5 emissions regulations have been achieved by employing the following technologies: − 200 Mpa injection pressure − high-response injectors with atomised-spray − optimising the injection system, combustion chamber and swirl − optimising Exhaust Gas Recirculation (EGR) − optimising injection settings by Model-Based Calibration (MBC). The use of an improved, highly-efficient turbocharger with variable geometry results in higher output and response. Furthermore, the MZR-CD 2.2 engine features a unique Diesel Particulate Filter (DPF) with a novel catalytic mechanism that contributes a much higher speed of soot oxidation compared to a conventional DPF. An improvement of Noise, Vibration Harshness (NVH), reliability and durability has been enabled by using balancer shafts, a highly-rigid layout of lower block and a maintenance-free, cam-drive chain. Improving drivability and smooth torque development is realised through the entire range by using torque-based engine control.
2 Measures Used to Achieve Development Targets 2.1 Fuel Economy In the interest of reducing pump loss and combustion temperature, and thereby improving emissions, the compression ratio has been lowered to 16.3. This was further aided by the large intercool-
The Authors
Yasunori Uesugi is Assistant Manager Diesel Engine Control at Mazda Motor Corporation in Hiroshima (Japan).
Shinichi Morinaga is Assistant Manager Diesel Combustion Development at Mazda Motor Corporation in Hiroshima (Japan).
Masashi Kouzuki is Assistant Manager Powertrain Development Promotion at Mazda Motor Corporation in Hiroshima (Japan).
Hiroaki Yasuda is Senior Staff Design Base Engine at Mazda Motor Corporation in Hiroshima (Japan).
Tsunehiro Mori is Assistant Manager Engine-NVH Development at Mazda Motor Corporation in Hiroshima (Japan).
Masahiro Naito is Assistant Manager Design Base Engine at Mazda Motor Corporation in Hiroshima (Japan).
er and water-cooled EGR, which reduce intake air temperature and thereby combustion temperatures. Also contributing to overall improved fuel efficiency is the engine’s upgraded injection system with higher response and injection pressure, increased EGR volume and optimisations of injection patterns. Engineers reduced mechanical resistance to offset any increase resulting from the added balancer shafts and expanded displacement. For example, the tensile force of the piston rings was reduced by
Michihiro Yamauchi is Senior Staff Design Base Engine at Mazda Motor Corporation in Hiroshima (Japan).
Kenji Tanimura is Senior Staff Diesel Cars Driveability Development at Mazda Motor Corporation in Hiroshima (Japan).
Joachim Kunz is Project Leader for Mazda6 at Mazda Motor Europe R&D Centre in Oberursel (Germany).
30 % compared to the MZR-CD 2.0 by adjusting the contact surface around a newly formed oil ring rail. The increase of oil pump capacity has been kept at 8 % (with reduced end flow) and pressure loss was lowered by optimising the oil path architecture. The reduction in supply pump drive loss was another measure for better engine efficiency. The same level of friction mean effective pressure as that of the MZR-CD2.0 was maintained, despite the newly-added chain drive and balancer shafts. MTZ 06I2009 Volume 70
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D EV E LO P M E N T
Diesel Engines
Table: Engine main specifications MZR-CD 2.2 Engine
MZR-CD2.0
New MZR-CD2.2
1998
2184
BorexStroke [mm]
86x86
86x94
Combustion type
Direct Injection
l
Intake shutter valve
DC Motor
l
3
Displacement [cm ]
EGR valve
DC Motor
l
EGR cooler
with
l
EGR cooler by-pass valve
N/A
with
Compression ratio
16,7
16,3
Valve train concept
OHC, belt-driven, 16 valves
DOHC, chain-driven, 16 valves
Open BTDC
6°
l
Close ABDC
30°
l
Open BBDC
41°
40°
Close ATDC
8°
l
Valve lift [mm]
IN: 10 mm, EX 8 mm
IN: 9,5 mm, EX: 9 mm
Fuel injection system
Common rail system DENSO U2-P
Common rail system DENSO U2-P (Improved)
180
200
Variavle geometry turbocharger IHI RHFV4
l + Abradable seal & Camber nozzle IHI RHFV4
Inter-cooler
with
l
Max. torque [Nm]
330
400
Max. power [kW]
103
136
Diesel particulate filter
with
l
Linear O2 sensor
with
l
Stage 4
Stage 5 (Stage 4)
IN
Valvetiming
EX
Max Fuel pressure (MPa)
Supercharger system
EU exhaust gas emission level
The improvement in overall fuel efficiency can be related to three main areas: − efficiency improved by the engine unit
− frequency of DPF regenerations reduced by half with a shorter regeneration time by reduced soot raw emissions
− reduction in driving resistance of the related vehicles. On the New European Driving Cycle (NEDC), this accounts for a 5 % to 7 % improvement in fuel efficiency and environmental performance, despite the engine’s improved driving performance, Figure 1 left.
2.2 Engine Performance The MZR-CD 2.2 delivers power output of 136 kW and torque of 400 Nm, which are one of the segment’s highest, Figure 1 right. High engine output was obtained through several measures, including increased displacement, dual overhead camshaft, a lower compression ratio and improved combustion efficiency (via increased peak injection pressure, among other technologies). The simultaneous achievement of higher engine output, low-speed torque and transient response was mainly achieved by improving the supercharging system. An abradable seal was chosen to minimise the clearance between the compressor impeller and casing, and curved vanes were selected to reduce unsteady emission gas flow in the nozzle, Figure 2. These measures led to improved charging efficiency, charging pressure level and transient behaviour, which is directly reflected by the engine’s combination of higher torque and power output and its improved operating response.
2.3 Exhaust Emissions Performance The MZR-CD 2.2 provides the potential to meet the Euro 5 emissions requirements. As an initial condition, good nitrogen oxide (NOx) raw emission performance has been achieved by means of expanded displacement and reduced compression ratio. Based on this, the combustion chamber was optimised and measures were
Figure 1: left: NEDC fuel consumption; right: torque output
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taken to reduce the intake temperature. In addition to the basic system based on the MZR-CD 2.0, the EGR valve and cooler were enlarged. Also, in order to enable a good balance between NOx, carbon monoxide CO and hydrocarbon HC, an EGR cooler by-pass was chosen. The calibration for injection/EGR-patterns was optimised be means of MBC [2], Figure 3.
3 Engine Layout and Components 3.1 NVH In general, an increase in a displacement by 10 % would lead to an increase in secondary inertia force, resulting in poorer NVH characteristics, especially in low frequency range. To counteract this, the new MZR-CD 2.2 applies various features for improving NVH performance. Balancer shafts were introduced, which help to reduce second order engine vibration by more than 10 dB compared to the MZR-CD 2.0, Figure 4. So despite its increased displacement, a linear engine sound pressure without booming noise has been achieved in the low to high engine speed range. Enclosed in an aluminium housing, this balancer shaft (B/S) system is driven
Figure 2: Turbocharger with abradable seal and vane geometry
by a chain interlocked with the crankshaft. A chain-driven method was adopted because it provides certain advantages including lowering the engine’s overall height, easier adjusting of centre distance to the crankshaft, and offsetting misalignment with the crankshaft when compared to a gear-driven system.
The oil pump is driven by the B/S and integrated at its rear end. An optimal balancing ratio was selected, which reduced second order vibrations and drive-chain noise as well. The chain drives for the valve control and the B/S also feature some special characteristics that help the engine obtain good NVH perform-
Figure 3: left: example of MBC; right: multi-injection pattern map
Figure 4: Balancer shaft and second order vibrations of engine mount
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Diesel Engines
Figure 5: left: balancer shaft chain noise (36th engine order); right: soot oxidation performance of new DPF
ance. When a chain engages with the sprocket, order frequency noise is generated in proportion to the number of teeth. Therefore, a Finite Element Method (FEM) analysis was carried out with the aim of dampening vibration here, and the B/S and its casing were designed with ribs accordingly. Furthermore, by using rubber rings at the sprockets, the excitation force generated when chain and sprocket come in contact is reduced. For this purpose, the sprocket teeth profile, the tensioner, chain length and centre distance had to be adjusted accordingly. Due to these measures, chain noise below 3000 rpm was reduced by more than 10 dB, Figure 5 left. As a result a chain noise level was achieved, which is inaudible for drivers. Because of its better damping characteristics, the cylinder block is still made of cast iron. The lower block is aluminium die-cast and the bearing caps and block skirt are rigidly attached to it. Thickness, structure, and rib arrangement were optimised based on FEM analysis and rig tests, which enabled developers to improve NVH performance and to minimise weight increase compared to its predecessor. To further lower radiated noise levels from the bottom of the engine, the oil pan is made of laminated damping steel sheet and has a noise insulating cover. Due to these measures taken at the base of the engine, engine knock typical for diesel engines was reduced as well. Further improvements were achieved by calibration of injection patterns, in particular in the important engine speed range below 2500 rpm and under high loads during acceleration. The better noise quality due to less engine knock 26
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can be analysed by means of an amplitude modulation factor as a common indicator of knocking levels. Here an improvement of 3 % to 10 % was achieved – reaching a level of running smoothness and noise behaviour that differs only slightly from gasoline engines.
3.2 Engine Layout for High Engine Output 3.2.1 Cylinder Head Due to the new engine’s higher loads, extensive changes at the cylinder head were necessary. Again, thickness and structure were optimised by means of FEM analysis, supported by rig tests and durability tests. The coolant flow inside the cylinder head was optimised. This led to temperature reduction between the valve seats, for example, and assured engine durability despite increased combustion pressure.
3.2.3 Crank Train The con rod is made of a higher strength material (approximately two times higher in fatigue strength), and a lead-free small end bush is used. Resistance against the contact pressure of the small end bush and conrod metal was improved by 20 % and 50 % respectively. Crankshaft fatigue strength has also been increased by induction hardening applied at the fillet of all pins and journals.
3.3 Weight Savings Conventional counterweights at each crankshaft pin are used. Nevertheless, a 1.3 kg weight savings was achieved by the new crankshaft design. The engine oil cooler material was changed from stainless steel to aluminium, achieving a 0.6 kg weight savings, with a stiffening plate added. A plastic head cover is used, which achieves a 1.0 kg weight savings compared to the previous aluminium head cover.
3.2.2 Pistons New piston material with higher fatigue strength at elevated temperature (50 % higher at 350 °C) was utilised to withstand the MZR-CD 2.2’s high combustion pressure and temperature. In addition, thermal load was reduced by achieving a 10 °C temperature reduction at the lip area of the combustion chamber, and 40 °C at the top ring groove (by using a cooled ring carrier). The coating thickness on the piston skirt was also increased by 8 μm to address the scuff issue that results from an increase in thrust force. For the top ring, an improved physical vapour deposition coating is applied that has higher resistance against scuffing and cracking, and a half-keystone ring is used to remove carbon trapped in the ring groove.
4 Diesel Particulate Filter System Soot load of the DPF needs to be automatically regenerated at regular intervals, and under many operating conditions this requires additional fuel injection to generate sufficient exhaust gas temperatures. This, however, causes some adverse effects such as oil dilution by fuel and deterioration of fuel economy. Therefore, a substantial goal in developing the new engine’s DPF system was to make sure filter regeneration required as little fuel as possible. This is especially important during frequent driving at low loads, such as urban operation cycles. Nevertheless, the chain systems were developed with sufficient durability even under deteriorated lubrication conditions.
Figure 6: Optimisation of DPF regeneration
Figure 6 shows the combination of all measures used to improve the DPF system performance, with a focus on lowering fuel consumption and preventing oil dilution. An increase in exhaust gas temperature is very effective for shortening regenerating time, Figure 5 right. The following measures were taken: − increasing the regenerating target temperature by 40 °C by means of a high heat-proof DPF − applying post-injection during deceleration − introducing an all-new catalytic structure for the DPF. Mazda developed a unique new catalytic structure for the MZR-CD 2.2’s DPF [3, 4]. Not only does it have oxygen ions on the
surface, but also those inside the coating are utilised for soot oxidation, Figure 7. This greatly improves soot oxidising performance and at the same time offers excellent heat resistance of the DPF. The new system also realises a reduction in precious metal volume, which achieves a lower piece cost. Fuel consumption during DPF regeneration has been reduced by approximately 60 % compared to the MZR-CD 2.0, lowering oil dilution to negligible levels during customer usage. The operating range available for filter regenerations was expanded in the direction of lower speeds like city driving conditions. In total, the disadvantages of filter regeneration – such as oil dilution and higher fuel consumption – were mini-
mised, which avoids a shortening of oil change intervals.
5 Engine Control System 5.1 System Overview As shown in Figure 8, the engine-control system incorporates the intake/exhaust system, common-rail system, charging pressure control and EGR system. A nozzle-opening sensor was added to the variable-geometry turbocharger to refine charging pressure control during transient operations. In addition, the system also controls EGR and thus allows an optimisation with regard to good driveability. The overall DPF system was also refined in the quest for higher performance. In order to avoid DPF overheating, a simple and highly reliable system for calculating the gas temperatures was developed. The common-rail system also features higher-response injectors and an injection pressure up to 200 Mpa, which is industry-leading for solenoid types.
5.2 Torque-based Engine Control
Figure 7: Details of catalysed DPF with SiC Wall Flow Concept
The previous MZR-CD 2.0 engine utilised conventional pilot injection patterns and quantity-based engine control. This control results in a torque step when the injection pattern is switched over, with negative effects on driveability. Much calibration effort was therefore needed to satisfy both the performance and driveability. For the new MZR-CD 2.2 diesel engine, a split injection is applied for pre-mixed combustion, resulting in a much higher number of parameters available for caliMTZ 06I2009 Volume 70
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Diesel Engines
Figure 8: Engine control concept
bration. In conjunction to this, a torquebased engine control system has been applied. Drivers indicate required torque by their pedal input, and all parameters for the fuel injection are calculated by the torque-based engine control system accordingly. These parameters include, for example, fuel pressure in the common rail, injection intervals and the required fuel quantity needed to achieve the target-torque. Smooth torque characteristics can be assured even when the injection patterns have to be changed during transient operation.
customers. Its high level of refinement makes it a very competitive engine in its segment. During substantial development phases, Mazda Motor Europe’s Research and Development Centre was closely involved. Various activities – such as benchmark testing and target setting, optimisation under actual driving conditions, product validation and durability tests – were carried out in Europe. This way, the engine could be optimally adapted to the needs of European customers, with a focus on subjectively noticeable attributes under European conditions.
7 Future Outlook 6 Conclusion With the MZR-CD2.2 diesel, Mazda has introduced a new engine that meets the expectations of demanding European
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This engine will also be installed in Mazda’s sports utility vehicles with higher weight and driving resistance in the future. For this application, an urea selec-
tive catalytic reduction system is currently under development that will further reduce NOx emissions. This technology will enable the MZR-CD 2.2 engine to meet future demands regarding environmental performance and fuel economy, even when used in heavier vehicles.
References [1] Nakai, and others: Introduction of DI Diesel Engine New MZR-CD for Passenger Car, Mazda Technical Report No. 23, 2005, pp. 98-103 [2] Yoshida, and others: Model Based Calibration for Common Rail Diesel Engine, Fundamental Education Workshop by the Mechanical Engineer Society (26 November 2007) [3] Suzuki, and others: PM Oxidation Catalyst on New Oxidation Mechanism, Mazda Technical Report No. 26, 2008, pp. 88-93 [4] Harada, K.; Suzuki, K.; Okamoto, K.; Yamada, H.; Takami, A.: Development of High Performance Catalysed DPF with New Soot Burning Mechanism, FISITA F2008-06-058 2008
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