Explode the compression ratio! The rebirth of naturally aspirated

    As emission regulations become increasingly stringent, automakers are scrambling to launch hybrid and electric vehicles to avoid huge fines. Traditional internal combustion engine vehicles are also increasingly inclined to small displacement and turbocharging. In such an environment, naturally aspirated engines are becoming less and less common. If there is anyone who is still a loyal fan of naturally aspirated engines, then Mazda, which started with rotary engines, is definitely one of them.

    In 2010, Mazda proposed Skyactiv Technology, which is based on naturally aspirated engines and focuses on improving fuel efficiency, supplemented by optimizing the gearbox, chassis and body structure to reduce energy loss during transmission.

One Center: Naturally Aspirated  

    For naturally aspirated engines, if you want to reduce carbon emissions and improve fuel efficiency, there are only two ways. One is to improve the combustion efficiency of the engine itself, so that more kinetic energy output can be squeezed out of the same amount of fuel; the other is to reduce various unnecessary energy consumption, such as optimizing the transmission system, improving the actual energy conversion rate, or lightweighting the body structure.

    Mazda's Skyactiv technology is based on these two perspectives, improving the combustion efficiency of the engine, and reducing the energy consumption in the transmission process of the engine-gearbox-chassis transmission system, and other aspects such as the lightweight design of the body. The final result is also quite amazing. For example, the localized CX-5 two-wheel drive version and the first 2.0-liter automatic ATENZA in China that uses Skyactiv technology last year both achieved a comprehensive fuel consumption of 6.6 liters per 100 kilometers, far exceeding the same level of cars.

Next-generation Skyactiv technology

    In the article "The New Power Trump Card Series 7: Mazda Skyactiv Technology", Cheyunjun introduced in detail the secret of the Skyactiv engine to achieve such high efficiency without turbocharging. The compression ratio of up to 14 and the unusual 4-2-1 exhaust system, although there are some sacrifices in performance, the improvement in economy is quite obvious with the piston, ignition timing and the lightweight design of the engine itself.

    However, even so, it is difficult to meet the average carbon emission target for new cars set by the European Union - 95 grams per kilometer in 2020 and 65 grams per kilometer in 2025. Therefore, Mazda engineers are also brewing the development direction of the next generations of Skyactiv technology.

Two basic points: high compression ratio

    Shortly after the beginning of 2014, Mazda's power development director Mitsuo Hitomi revealed the research and development plan for the next generation of technology: "The second-generation Skyactiv engine is expected to be launched in 2020. Compared with the first generation, the fuel economy of the second-generation engine will be improved by at least 30%."

    We all know that Mazda engineers made the first generation of Skyactiv-X the gasoline engine with the highest compression ratio in the world, up to 14. Even though the compression ratio of the domestically produced ATENZA and CX-5 has been reduced to 13 due to China's fuel quality issues, it is still higher than the compression ratio of about 12 on SUVs. In general, the compression ratio of car engines is only 8-10 depending on the displacement.

    Obviously, the engineers were not satisfied with this data. Mitsuo Hitomi said that the compression ratio target for the second-generation Skyactiv engine was 18.

    Increasing the compression ratio means that after the compression process of the gasoline engine is completed, the temperature and pressure of the mixture will be higher, and more heat energy can be released during the combustion process, that is, more energy is generated when the same amount of fuel is consumed. In this way, even if the conversion ratio from thermal energy to mechanical energy remains unchanged, the power generated by the engine will increase. Therefore, increasing the compression ratio is equivalent to increasing the base of energy, fundamentally increasing the power output of the engine, that is, improving fuel economy.

    But we also know that gasoline's ability to withstand pressure is limited. High-grade gasoline has high pressure resistance, and its combustion propagation speed is also slower at high temperatures, and it can withstand higher temperatures and pressures; the addition of direct injection in the cylinder allows gasoline to be mixed more evenly and reduces the possibility of knocking, but this does not allow the compression ratio to increase indefinitely. The first generation was able to achieve a compression ratio of 14, but it was achieved at the expense of some performance.

    It is also unrealistic to continue to increase the compression ratio to 18, if you continue to sacrifice performance. If a large displacement, such as a 2.5-liter engine, has no other characteristics except carbon emissions and is particularly sluggish, then it is better to choose a small displacement turbocharged engine.

    So how do Mazda engineers pull off this magic?

Two basic points: HCCI

    The answer is HCCI. When talking about improving fuel economy, Mitsuo Hitomi said: "If we want to significantly improve fuel efficiency, the only way is lean combustion." The so-called lean combustion is to reduce the proportion of gasoline in the mixture of gasoline and air, so that the unit volume of gasoline can produce higher energy and improve fuel efficiency.

    HCCI, homogeneous charge compression ignition, is the compression ignition technology of homogeneous mixture. After using this technology, the combustion method of gasoline engine becomes the same as that of diesel engine - compression ignition. The core of HCCI technology is homogeneous mixture and compression ignition, and in order to achieve homogeneous mixture and compression ignition, high compression ratio is indispensable.

    Only when the pressure in the cylinder is high enough can the mixture have enough time to form an evenly distributed mixture at the end of the compression stroke after it is injected; of course, only when the pressure and temperature in the cylinder are high enough can the ignition point of gasoline be reached and it can be burned without using a spark plug.

    Since the mixture in the cylinder is lean, it is equivalent to improving the pressure resistance of gasoline in disguise, and no knock will occur under high compression ratio. Therefore, HCCI technology and high compression ratio can be said to complement each other. Mitsuo Hitomi also said in the announcement that the main technology of the second-generation Skyactiv is HCCI, and the high compression ratio is just a hidden benefit of HCCI, incidentally.

Three different combustion methods

    GM conducted research on HCCI technology from 2007 to 2009. Mercedes-Benz also launched an engine called DiesOtto (a combination of the names of the creators of diesel and gasoline engines) in 2007, which also used HCCI technology and was equipped with a turbocharger system. The maximum output power was 238 horsepower and the maximum torque could reach 400 Nm at a displacement of 1.8 liters. Honda and Hyundai, as well as parts supplier Bosch, have all reportedly studied HCCI, but this technology has never been mass-produced.

    This is because some key technical difficulties must be resolved to achieve HCCI:

    The first is the control of combustion timing. HCCI makes the mixture self-ignite by increasing the pressure and temperature of the cylinder, so the injection process of the mixture, the temperature and pressure in the cylinder must be accurately detected and controlled to ensure that combustion starts at the end of the compression stroke. If it is too early, it will cause energy waste, and if it is delayed, it will cause power lag and affect the output stability. Therefore, the engine ECU management program must be carefully strengthened.

    Second, in order to avoid knocking, the HCCI mixture is a lean mixture. Compared with conventional gasoline engines, the exhaust temperature of engines using HCCI technology will be relatively low, and the maximum load of the engine will be limited. When the engine is running at high speed under high load, it may still have to rely on spark plugs for ignition. If spark plugs are used for ignition, the compression ratio of the engine cannot be too high to avoid knocking, which requires the compression ratio of the engine to be variable. To achieve a variable compression ratio technically, the structure of the engine will be more complicated, the cost will increase accordingly, and under current technology, stability cannot be guaranteed. In addition, low exhaust temperature is also a problem for the catalytic converter because a fairly high temperature is required for the oxidation/reduction reaction.

    Third, HCCI is a simultaneous compression ignition and heat release process, which puts strong instantaneous pressure on the cylinder and piston, which places higher demands on the strength and weight of the engine. If the weight of the engine increases too much, it will offset some of the efficiency that has been improved with great difficulty. Therefore, how to find a balance between the two is also a challenge.

summary:

    It seems that there are still many challenges facing Mazda engineers. Before 2020, engineers need to not only solve these problems, but also solve them with lower-cost methods. Only in this way can mass production be possible.

    After the second-generation Skyactiv-T, Mazda plans to launch the third-generation Skyactiv-T engine in 2025. The third-generation technology will focus on limiting the changes and losses of heat energy in the combustion chamber caused by the exhaust and cooling systems. The ultimate goal is to achieve carbon dioxide emissions similar to those of hybrid vehicles.

    After the first generation of Skyactiv engine was introduced into China, it was discounted due to the problem of oil quality. Perhaps for those who are eager to see the second generation of domestic products, engineers can find a high-quality and low-cost technical means within six years, but it is difficult for domestic oil quality to be improved in six years.