Gasoline cars have the same fuel efficiency as hybrid cars. Mazda's SKYACTIV technology overview (Part 1)

Mazda has released the next-generation vehicle technology "SKYACTIV". This is a huge project that completely renews the basic parts of the car, including the engine, transmission, body and chassis. It can achieve a fuel efficiency of 30km/L without motor assist, a compression ratio of 14.0 for both gasoline and diesel engines, and a body weight reduction of 100Kg. This article will conduct a comprehensive analysis of the SKYACTIV technology that achieves the above-mentioned epoch-making performance.

Figure A Key parts of Mazda's next-generation SKYACTIV technology. (Upper left) SKYACTIV-G, a next-generation high-efficiency direct-injection gasoline engine that achieves a compression ratio of up to 14.0, (Upper right) SKYACTIV-D, a next-generation diesel engine that achieves a low compression ratio of 14.0, (Lower left) SKYACTIV-Drive, a next-generation automatic transmission, (Lower right) SKYACTIV-Body, a next-generation lightweight, high-rigidity body

Mazda will launch the "Demio" equipped with a new generation engine in the first half of 2011, which can achieve a 10·15 mode fuel efficiency of 30km/L without motor assistance. Mazda's new generation vehicle technology "SKYACTIV" released on October 20 has caused a huge response in the automotive industry. This is because the fuel efficiency of 30km/L is the same as the "Fit Hybrid" released by Honda on October 8 before Mazda's release.

Promote corporate strength

Figure 1 The new manual transmission "SKYACTIV-MT" achieves a light shift feel and is significantly lighter and smaller.

Figure 2 New chassis "SKYACTIV-Chassis" (a) Front suspension system and steering system, (b) Rear suspension system. Mazda said that the system combines accurate steering function with comfortable riding experience.

Mazda's next-generation vehicle technology "SKYACTIV" consists of the following six key technologies.

(1) The high-efficiency direct injection gasoline engine "SKYACTIV-G" (see Figure A), which has achieved the world's highest compression ratio of

14.0 for gasoline engines (2) The green diesel engine "SKYACTIVD" (see Figure A), which has achieved the world's lowest compression ratio of 14.0 for diesel engines

(3) The automatic transmission "SKYACTIV-Drive" (see Figure A), which has improved transmission efficiency

(4) The manual transmission "SKYACTIV-MT", which has achieved a light shift feel and a significant reduction in weight and size (see Figure 1)

(5) The lightweight body "SKYACTIV-Body" (see Figure A), which has achieved high rigidity and the highest level of collision safety

(6) The high-performance lightweight chassis "SKYACTIV-Chassis", which has both accurate steering function and comfortable ride (see Figure 2)

These key technologies will be introduced in detail step by step in this article. In fact, the biggest feature of Mazda SKYACTIV is not just the improvement of the engine or body, but the unified update of the entire vehicle.

For example, the new gasoline engine. Because it can improve exhaust efficiency, the exhaust manifold is longer than before. This is the biggest feature of this new gasoline engine. Because it takes up a larger space, the exhaust manifold cannot be installed in the engine room of previous vehicles. This time, by updating the frame and engine at the same time, it is possible to use this exhaust manifold. The same is true for the transmission and suspension system.

For manufacturers with larger production scales, it is quite difficult to update the frame and engine at the same time. "It is precisely because our company is a relatively small manufacturer that we can do it" (Mazda).

In addition, in terms of various key technologies, Mazda uses the same basic structure to transcend the differences in displacement and vehicle size, thereby achieving "flexible" production under the same equipment, and improving the efficiency of research and development by 30%, and reducing production investment by 20-60%. Mazda's goal is to link the update of key technologies with the enhancement of corporate strength.

Increase the compression ratio to 14.0

The biggest goal of Mazda's development of SKYACTIV is to improve fuel efficiency. Unlike Toyota and Honda, which position hybrid technology as the core of fuel efficiency improvement technology, Mazda's basic starting point is to first comprehensively improve basic technologies such as engines and transmissions, and then introduce the "building block" strategy of electrification technologies such as brake energy regeneration and hybrid power on this basis.

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Mazda's policy is to improve the average fuel efficiency of Mazda vehicles sold worldwide by 30% by 2015 compared to 2008 by not only improving the engine and transmission, but also reducing the weight of the vehicle by about 100kg. The company aims to improve the fuel efficiency of Mazda vehicles sold worldwide by 15-20% by improving the engine, 4-7% by improving the transmission, 3-5% by reducing the weight of the vehicle, and 30% by adding the idle stop mechanism.

The most distinctive feature is that the new gasoline engine has a compression ratio of 14.0, which greatly improves efficiency and increases fuel efficiency and torque by 15% compared to the past (Figures 3 and 4). The compression ratio of gasoline engines in the past was about 10.0 under standard gasoline specifications. Mazda said that if the compression ratio is increased from 10.0 to 15.0, it is expected to achieve a thermal efficiency improvement of about 9%.

Figure 3 Torque characteristics of SKYACTIV-G Compared with existing gasoline engines, it is improved by about 15%.

Figure 4 SKYACTIV-G fuel efficiency characteristics Compared with existing gasoline engines, it has been improved by about 15%. Fuel consumption is lower than that of existing diesel engines.

The reason why the compression ratio has been kept at around 10 before is that knocking (abnormal combustion) cannot be avoided. To avoid knocking, the compression top dead center temperature must be reduced. Mazda focuses on the high-temperature residual gas that cannot be discharged from the combustion chamber. If we think about it according to the graph, even if the piston on an engine with a compression ratio of 10.0 reaches the top dead center, 10% of the combustion gas will still remain in the cylinder.

Mazda's calculation results show that when the residual gas temperature is 750°C and the fresh air temperature is 25°C, if the residual gas accounts for 10%, the compression top dead center temperature will rise by 160°C. On the other hand, if the amount of residual gas is reduced by half from 8% to 4%, the compression top dead center will not rise even if the compression ratio is increased from 11.0 to 14.0.

Lengthening the exhaust manifold

Previously, the obstacle to reducing residual gas was the exhaust manifold. In recent years, since the catalyst can be activated in advance, most engines have adopted a structure that shortens the exhaust manifold as much as possible and places the catalyst at the rear of the engine. However, if the distance to the exhaust manifold gathering point is too short, as shown in the upper part of Figure 5 (a), the high exhaust pressure generated by the opening of the exhaust valve of the third cylinder will reach the first cylinder in the overlap state where both the exhaust valve and the intake valve are open. The exhaust gas that was once discharged is blown back into the combustion chamber, and the high-temperature residual gas will continue to increase.

Figure 5 4-2-1 exhaust system (a) If a shorter exhaust manifold is used (above), the higher exhaust pressure generated by the opening of the exhaust valve of the third cylinder will enter the first cylinder in the overlapping state where both the exhaust valve and the intake valve are open. As a result, the exhaust gas once discharged is blown back into the combustion chamber, and the high-temperature residual gas continues to increase. If a 4-2-1 exhaust system is used (below), this phenomenon can be avoided, thereby reducing the residual gas. (b) The 4-2-1 exhaust system is reduced in size after being rolled into a ring.

To avoid this phenomenon, Mazda adopted a 4-2-1 exhaust system that increases the length of the exhaust manifold. By first combining the four flow paths into two and then into one, the distance between the cylinders is kept the same.

In order to reduce residual gas and increase the torque in the effective section, a pipe length of 600mm was previously required. Mazda has achieved space saving by adopting a "Loop-type exhaust pipe" rolled into a ring (Figure 5 (b)), but even so, as mentioned earlier, it is an indisputable fact that the exhaust manifold takes up more space than before.

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The problem with this exhaust system is that the temperature of the exhaust gas drops due to the long distance to the catalyst, and the activation of the catalyst is delayed. Although the exhaust gas temperature can be increased by delaying the ignition time, the combustion will become unstable.

For this reason, the new gasoline engine has a cavity on the upper surface of the piston, and by forming a richer mixture around the spark plug, it has successfully achieved stable combustion performance even if the ignition time is delayed. In addition to the above-mentioned improvements in combustion, Mazda also worked on reducing the friction loss of various engine components, and ultimately reduced the friction loss by about 20% overall.

No post-treatment is required to reduce NOx

The new diesel engine SKYACTIV-D features a compression ratio of 14.0, which eliminates the need for expensive NOx post-treatment devices such as urea SCR (selective reduction catalyst) and NOx (nitrogen oxide) absorption reduction catalyst (LNT), and can meet global exhaust gas regulations such as Euro6 in Europe, Tier2Bin5 in North America, and the "Post-New Long-Term Regulations" in Japan. Fuel efficiency is also improved by about 20% compared to the past (Figure 6).

Figure 6 SKY-D fuel efficiency characteristics Compared with previous Mazda diesel engines and engines of similar competing vehicles, fuel efficiency has been improved by about 20%.

There are two reasons why the fuel efficiency was successfully improved by overturning the common sense of engine design that "increasing the compression ratio can increase thermal efficiency". One reason is that the combustion conditions are improved by reducing the compression ratio; the other reason is that the friction loss is reduced compared to the past.

The combustion conditions can be improved by reducing the compression ratio because the uniform mixing of fuel and air promotes combustion. In the previous engine with a high compression ratio, the temperature and pressure in the piston top dead center are very high. If the fuel is injected into the combustion chamber under this state, it will ignite before the fuel and air are fully mixed. A large part of the fuel will become high temperature due to combustion, which will increase the generation of NOx (nitrogen oxides). Since it is burned under insufficient oxygen, the amount of soot generated will also increase.

In order to avoid this phenomenon, the previous diesel engine will wait until the piston is slightly lowered and the pressure and temperature of the combustion chamber are lowered before injecting fuel. However, since the fuel combustion starts after the piston starts to descend, the stroke for generating driving force will be shortened (Figure 7).

Figure 7 The reason why fuel efficiency is improved despite lowering the compression ratio In the old diesel engine with a high compression ratio (above), the piston is slightly lowered before injecting fuel to avoid injecting fuel into the high-temperature and high-pressure combustion chamber. As a result, the piston movement stroke for generating driving force becomes shorter. However, in the SKYACTIV-D (below), the piston movement stroke for generating driving force is increased because the fuel is injected near the top dead center.

The new diesel has a lower compression ratio, so the temperature and pressure at the top dead center can be reduced. Even if the fuel is injected at the top dead center, the time from ignition is increased, so the fuel and air can be fully mixed, thus achieving uniform combustion. (To be continued)