For the "CX-8" seven-seat SUV (sport utility vehicle), Mazda Motor Corp made improvements to its 2.2L diesel engine to prevent noise (knocking sound) while increasing thermal efficiency.
Denso Corp's latest injector greatly contributed to this. It can inject fuel six times in a row in a short period of time. Mazda is the first company that has employed the injector. The CX-8 was released in Japan Dec 14, 2017 (See related article).
If fuel is combusted fast when the piston is located near the top dead center, thermal efficiency improves. On the other hand, it is possible to reduce the large knocking sound of a diesel engine by slowly combusting fuel.
To solve this trade-off, Mazda considered injecting fuel up to six times in a row in a short period of time so that combustion starts before the preceding combustion ends. In terms of heat generation rate (speed), it becomes one-time combustion that slowly starts up, reducing knocking sound.
On the other other hand, the actual interval between combustions becomes short, meaning that combustion period is shortened and, therefore, thermal efficiency can be improved.