The 5-door HAZUMI hatchback is also equipped with the SKYACTIV-DRIVE (6AT) transmission; the i-stop idling stop system; and the i-ELOOP regenerative braking system.

SKYACTIV-D 1.5. Click to enlarge.

Conventional diesel engines generally have a high compression ratio; hence, the compression temperature and pressure at piston top dead center (TDC) are extremely high. When fuel is injected under these conditions, ignition takes places before an adequate air-fuel mixture forms, causing localized heterogeneous combustion, resulting in the formation of NO_x and soot, Mazda says.

The advent of stricter emissions regulations leads to a delay in combustion until after TDC (with a lower cylinder pressure and temperature), although this causes fuel economy to worsen.

The SKYACTIV diesels feature a low compression ratio of 14.0:1 (low for a diesel); reducing the compression ratio in the diesel decreases the compression temperature and pressure at TDC. Consequently, ignition takes longer even when fuel is injected near TDC, enabling a better mixture of air and fuel.

Top: High compression diesel ignition occurs before the fuel is sufficiently mixed. Local hot spots cause NOx and over-rich spots cause soot. Bottom: Lower SKYACTIV-D compression gives more time to mix before ignition. The result is cleaner combustion with low NOx and soot emissions. Source: Mazda. Click to enlarge.

To realize ideal combustion timing and duration as well as clean emissions at the same time, Mazda uses this premixed compression ignition combustion by controlling ignition timing based on model-based prediction of ignition delay in the low compression ratio engine.

SKYACTIV-D Technology Concept. Source: Mazda. Click to enlarge.

The formation of NO_x and soot is alleviated since combustion becomes more uniform without localized high-temperature areas and oxygen insufficiencies, Mazda says. Furthermore, injection and combustion close to TDC make a diesel engine highly efficient. The expansion ratio (or amount of actual work done) is greater than in a high-compression diesel engine.

Also due to the low compression ratio, the SKYACTIV-D diesel engine also burns cleaner, discharging far fewer nitrous oxides while producing virtually no soot. It can thus do without NO_x aftertreatment and still meet emissions standards globally.

Furthermore, the lower compression ratio enables a reduction in weight through structural optimization and in mechanical friction in the engine.

Two main problems have limited the spread of low-compression-ratio diesels: the first is starting during cold operation, and the second is misfiring during warm-up due to the lack of compression temperature and pressure.

To address these issues, Mazda uses new 12-hole piezo injectors which allow for a wide variety of injection patterns; precise injection amounts and timing increases the accuracy of mixture concentration control, providing cold start capability. The SKYACTIV-D injectors are capable of a maximum of 9 injections per combustion.

Along with three basic injections—pre-injection, main injection, and post-injection—different injection patterns will be set according to driving conditions.

Misfiring is avoided by adopting a variable valve lift (VVL) system for the exhaust valves. The exhaust valves are opened slightly during the intake stroke to recycle host exhaust gas back into the cylinder, increasing the air temperature. The elevated temperature stabilizes ignition.

SKYACTIV-D also uses a two-stage turbo in which one small and one large turbo are selectively operated according to driving conditions. This is intended to achieve high torque and response at low speeds, and high power and high speeds.

However, Mazda North American Operations is delaying the introduction of the SKYACTIV-D 2.2 in North America from the original targeted Spring 2014. Mazda said that the although the SKYACTIV-D can meet emission regulation requirements without the use of a NO_x after-treatment system, it decided that further development is required to deliver the right balance between fuel economy and “Mazda-appropriate” driving performance. (Earlier post.)