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Mazda’s New Common Rail DI Diesel Engine
The Mazda 6. Its 2-litre 4-cylinder diesel engine develops 100 kW of power and 310 Nm of torque thanks to the high-pressure common rail system.
The 2-litre 4-cylinder diesel engine develops 100 kW of power and 310 Nm of torque thanks to its high-pressure common rail system. The mid-sized passenger car equipped with this engine meets the Euro 4 exhaust standards and provides excellent NVH performance with multiple injection technology.
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The MZR-CD Installed in the Mazda 6 The engine installed in the Mazda 6 (MZR-CD) has undergone substantial development and has become a next-generation diesel engine that meets customers’ demands with new concepts. Mazda has developed this state-ofthe-art engine by achieving excellent driveability and good fuel economy, as well as environmentally friendly exhaust emissions and superior NVH performance The 2-litre diesel engine equipped with a new Denso ultrahigh-pressure (1800 bar) common rail system and a variable geometry turbocharger accomplishes a class-leading maximum 100 kW of power and a maximum 310 Nm of torque. Moreover, it generates 90 % of the maximum torque over 1750 rpm, which is again at
the top of its class. Thanks to this outstandingly high torque, the Mazda 6 accelerates from 0 to 100 km/h in 10.7 seconds, which is more than 10 % faster than the previous model. Furthermore, a top level driving range of 893 km and 6.5 litres per 100 km (NEDC combined fuel consumption) have been achieved. With the combination of the new combustion technology and the advanced common rail system, the MZR-CD not only complies with Euro 4 exhaust standards with only a diesel oxidation catalyst, it also has excellent NVH properties, allowing it to compete with the gasoline engine, Figure 1.
Technical Features These performance figures have been attained by using various advanced technologies. For examAutoTechnology 3/ 2003
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Figure 1: The MZR-CD not only complies with Euro 4 exhaust standards with only a diesel oxidation catalyst, it also has excellent NVH qualities that compete with the gasoline engine.
ple, in order to improve power and efficiency, the MZR-CD maintains the conventional compact design, but the engine has been upgraded with design features such as resistance against high cylinder pressure and high exhaust gas temperatures. In addition, further improvements were made in order to achieve the low emissions by making the combustion system suitable for the common rail system by such means as modifying the swirl ratio and the bowl shape. In order to achieve further quietness, a series of new technologies was developed, such as an improved engine structure and fuel injection control to reduce combustion noise.
Figure 2: The new MZR-CD common rail DI diesel engine.
Common Rail Fuel Injection System The new engine employs state-ofthe-art common rail fuel injection technologies, including a maximum injection pressure of 1800 bar and an optimised injector nozzle shape, Figure 2. As a result, the engine performance has improved in all driving conditions. With the improvement of the injector design and the injector driver, the multiple injection of various patterns up to five times per cycle became possible.
High-Rigidity Structure The cylinder head has a double tangential port with the same compatibility between the swirl ratio and the intake efficiency as that employed by the previous model, with modifications to the port angle and the valve shape. In addition, the reduced throat AutoTechnology 3/ 2003
diameter enhances the valve bridge rigidity, which provides the cylinder head with much more resistance against high cylinder pressures and heat load. This serves to develop the airflow to provide the high power. The cylinder block structure was reinforced based on analysis data for the structural transfer path. The vibration of the block outer wall was restricted by placing unique ribs at the upper part of the water jacket. This improvement not only results in a drastic improvement in rigidity, it also reduces radiated noise without resulting in extra weight. Furthermore, the application of an aluminium lower block and a small-sized oil pan also reduces noise. The piston has been reinforced against high combustion pressures. The improvement of the porous metal enhances the wear resistance of the top ring groove for the high maximum cylinder pressure, with the result that engine durability is upgrad-
ed. Moreover, the piston skirt has been shortened based on the FEM analysis data, and the shape under the cooling channel has been changed based on the results of the heat conduction analysis.
Drastic improvement in rigidity
New Vertical Vortex Combustion Following the introduction of the common rail system, the entire combustion process, from mixture to combustion, was modified, resulting in a significant reduction in emissions. Compared with the rotary pump system, the common rail system for this diesel engine has greater capability with regard to high-pressure fuel injection, which makes it possible to provide a finely atomised fuel spray. When the finely atomised spray is burned however, a lot of NOx is generated. A general way of reducing NOx is to lower the flame tem-
Figure 3: Formation mechanism of the vertical vortex (EVVC).
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Pilot combustion process
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perature by using EGR, retarded fuel injection timing, intake air cooling, etc. In addition to these technologies, Mazda has developed a new combustion technology for NOx reduction by upgrading the utilization of the airflow in the axial direction. This type of combustion is called Expansive Vertical Vortex Combustion (EVVC), Figure 3. EVVC is activated with the ignition of the air/fuel mixture generated during an ignition delay time, thus creating the strong flow (expansive combustion flow). This flow hits the entrance wall of the combustion bowl and is guided down to the bottom of the combustion bowl. The vertical vortex is forced and mixed into the lower-temperature extra air at the centre of the combustion bowl. Thus, the vertical vortex immediately cools the high-temperature burned gas and restricts NOx generation. In order to reinforce this vertical vortex, the following elements were optimised using our newly developed combustion simulation methods: the effect of the spray cone angle, penetration, protrusion size and the height at the centre of the combustion bowl, bowl lip diameter and bowl lip bottom, Figure 4. By this means, a total NOx reduction of about 30 % was achieved in the NEDC.
Multiple Injection One of the goals in the development of the diesel engine was to reduce the combustion noise without negatively influencing emissions and performance. In order to achieve this, Mazda has
Figure 5: Example of control map for multiple injection.
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Figure 4: Example of study parameter for EVVC.
developed an optimum multiple fuel injection system that provides various injection patterns depending on the engine operating conditions, Figure 5 . Among these multiple injection technologies, premixed pilot injection and multi-stage injection are unique. Mazda has developed a new premixed pilot combustion process to reduce both combustion noise and emissions. It is well known that pilot injection reduces combustion noise. Although reducing the injection interval or increasing the fuel quantity injected during pilot injection also reduce combustion noise, they significantly increase smoke. Premixed combustion at the pilot injection stage solved this problem. Compared with conventional pilot injection, the injection timing of premixed pilot combustion is more advanced, and the pilot injection quantity is increased further. Due to this injection timing advance, the injected fuel is fully mixed with air until ignition, which results in premixed combustion. The average cylinder temperature during the main fuel injection is higher than that of conventional pilot combustion, due to the increased pilot injection quantity. This shortens the ignition delay time of the main combustion and allows a moderate heat release. At the same time, smoke is dramatically reduced. It is con-
sidered that this is because the lean oxygen area is reduced, for the following two reasons: 1) The pilot fuel causes premixed combustion at the piston top side and not inside the bowl, thus raising the average cylinder temperature. As a result, there is enough oxygen for the main combustion present inside the bowl. 2) The fuel quantity for the main injection is diminished, due to the compensation for the increase in the pilot injection quantity, which results in the decrease in diffusion combustion. Thus, premixed pilot combustion can significantly reduce both combustion noise and smoke. As mentioned above, multistage combustion technology based on premixed pilot combustion has successfully reduced combustion noise and emissions at low engine speed in low load conditions. Figure 6 show the needle lift and heat release rate of the multi-stage combustion during idling. In order to improve the pre-mixture formation, fuel is injected twice during the first half of the injection process, so that premixed combustion occurs efficiently. In order to control the heat release peak of the diffusion combustion to reduce noise, the fuel is injected twice during the latter half and diffusion combustion is divided. Using the technology of four-fold injection, threestage combustion (a combination AutoTechnology 3/ 2003
Figure 6: Example of multi-stage combustion at idling.
of premixed combustion and split diffusion combustion) was developed, and combustion noise was reduced by 5 to 10 dB. Multiple injection requires a technique to control the minimum fuel injection quantity accurately during the entire engine life in order to cope with injector variability and deterioration. Mazda has developed a new methodology to satisfy this requirement. The MZR-CD system measures the minimum injection quantity every 10,000 km and automatically adjusts the quantity to the initial value. As a result, the MZR-CD prevents emissions deterioration and combustion noise.
Conclusion By using Mazda direct injection diesel engine technologies and the enhancement of performance over large engine ranges by introducing breakthrough technologies, the MZR-CD has become the competitive power unit as a next-generation engine with well-balanced performance at a high level. The Mazda 6 equipped with this MZR-CD engine achieves high-level dynamic performance that fulfils its development concept.
by Yasuyuki Terazawa, Yoshiaki Enseki, Masami Nakao, Eiji Nakai, Masaru Yamamoto, Motoshi Kataoka, Mazda Motor Corporation. AutoTechnology 3/ 2003
