Research groups at Tokyo Institute of Technology and Keio University have applied in-cylinder water injection to an ultra-lean-burn gasoline engine with an increased air excess rate, achieving the world's highest level of thermal efficiency of 52.6% for a passenger car gasoline engine.
In an ultra-lean-burn gasoline engine, it is expected that the thermal efficiency will be significantly improved as the cooling loss due to low-temperature combustion decreases, but in order to improve the thermal efficiency, it is essential to suppress knocking and reduce the cooling loss in the high load region.Most gasoline engine water injections in the conventional stoichiometric air-fuel ratio inject water from the intake port.In this case, the air-fuel mixture is cooled relatively uniformly, but the combustion rate is greatly reduced by the uniform addition of water to the mixture, so there is a concern that combustion instability will increase in ultra-lean combustion. ..
Therefore, in this research, we propose a "layered steam heat shield" that injects water directly into the cylinder and distributes steam intensively near the piston surface while avoiding the vicinity of the spark plug.As a result, the cooling effect of water can be obtained without deteriorating the combustion even in ultra-lean combustion, and knocking and a large cooling loss from the piston surface to the outside, which often occur in the unburned region near the piston, can be effectively reduced.
As a result, by optimizing various conditions, increasing the excess air ratio to about 2, and finally increasing the compression ratio to 17, the net thermal efficiency of the passenger car engine, which was about 40% so far, has been increased to 51.5%, and the illustrated thermal efficiency. (The value obtained by dividing the work of combustion gas on the upper surface of the piston in the cylinder (shown) by the amount of input heat) was improved to 52.6%, achieving the world's highest level of thermal efficiency for a passenger car gasoline engine.
In the future, the mechanism for improving thermal efficiency will be elucidated by simultaneously measuring the water vapor distribution and heat flux, and the thermal efficiency can be further improved by optimizing the shape and installation position of the water injection injector.
