Continued from Mazda Sees 'Super-lean-burn' Engine in 2019 (1)
Mazda expects to use a mechanism that switches to combustion with a theoretical air-fuel ratio (stoichiometry, excess air ratio: 1) when the excess air ratio becomes lower than about 2 at the time of compression ignition.
One of the reasons why a compression ignition engine cannot be easily commercialized is that it is difficult to smoothly switch to the stoichiometric combustion of spark ignition in the high-load range. Mazda plans to use the SPCCI technology to "completely control" the switching between the two types of combustions that use compression ignition and spark ignition, respectively, said Kiyoshi Fujiwara, director of the company.
The super lean burn can increase thermal efficiency mainly because it can drastically lower combustion temperature. It reduces the temperature difference between in-cylinder gas and cylinder wall, reducing heat loss.
In addition, it prevents NOx from being generated. NOx is generated at a combustion temperature of 2,000K or higher. With a combustion temperature lower than that (low-temperature combustion), NOx is hardly generated.
Suppose that the in-cylinder temperature (compression end temperature) is 800K when the piston is at the top dead point and the temperature becomes about 2,500K due to stoichiometric combustion. When the super lean burn is realized with an excess air ratio higher than 2 (2x amount of air), the amount of increase from the compression end temperature becomes half in a simple calculation. The combustion temperature is kept at about 1,800K, which is lower than the temperature for NOx generation.