Continued from Mazda Revamps Diesel Engine for 'CX-8' SUV (1)
Unlike the Skyactiv-D 1.5, which has only one turbocharger, the Skyactiv-D 2.2 is equipped with a "two-stage" turbo system that uses either a large or small turbocharger depending on the rotation speed of the engine (There are two turbochargers in total). The large turbocharger is the variable geometry turbocharger.
With the employment of the variable geometry turbocharger, it becomes possible to use the large turbocharger both at lower and high rotation speeds, expanding the engine rotation speed range that realizes a high supercharging performance. As a result, Mazda not only increased maximum torque and maximum output but also made it possible to linearly increase vehicle speed at the time of wide-open throttle acceleration from a speed of 100km/h (approx 62.1mph).
For the improvement of fuel efficiency, Mazda employed a high-response multi-hole piezo injector that it developed with Denso Corp and realized high-speed multi-stage combustion. As a result, the company shortened combustion period and increased compression ratio from 14.0 to 14.4, improving combustion efficiency.
The Skyactiv-D 2.2 engine of the CX-5 has a low compression ratio to ensure the time it takes for fuel to be mixed with air after injecting fuel. With a low compression ratio, the speed of temperature rise in the cylinder (caused by compression) becomes slow, making it possible to extend the time from fuel injection to ignition and, thus, to mix fuel with air more homogeneously. If fuel is not mixed well with air, nitrogen oxides and soot are easily generated.
The remodeled Skyactiv-D 2.2 uses a high-response injector and carries out multi-stage injection (up to six times) at short intervals near the top dead center. As a result, continuous combustion like a mountain is generated, shortening combustion period. In the past, it was combustion with which each of the pilot/pre-combustion, main combustion and after-combustion forms a mountain, making it impossible to shorten combustion period enough.