Due to the fact that diesel engines generally have a high compression ratio, the compression temperature and pressure at piston top dead center (TDC) are extremely high. If fuel is injected under these conditions, ignition will take place before an adequate air-fuel mixture is formed, causing heterogeneous combustion to occur locally. As a result, the formation of NOx and, due to combustion with insufficient oxygen, the formation of soot are brought about(Fig.1).
Under recent strict emissions regulations, this makes it difficult to ignite the mixture at the optimal timing (TDC), leaving no other choice but to delay combustion until the piston begins to descend and lower the cylinder pressure and temperature, although this causes fuel economy to worsen.
When the compression ratio is lowered, compression temperature and pressure at TDC decrease. Consequently, ignition takes longer even when fuel is injected near TDC, enabling better mixture of air and fuel. This alleviates the formation of NOx and soot because the combustion becomes more uniform without localized high-temperature areas and oxygen insufficiencies. Furthermore, injection and combustion close to TDC result in a highly-efficient diesel engine, in which a larger amount of actual work (or, a higher expansion ratio) is obtained than in a high-compression-ratio diesel engine(Fig.2).
Due to its low compression ratio, the maximum in-cylinder combustion pressure for SKYACTIV-D is lower than the current diesel, realizing significant weight reduction through structural optimization.
For example, it became possible to change the cylinder block’s material to aluminum, which saved 25kg (vs.current diesel). The cylinder head became 3kg lighter with thinner walls and an integrated exhaust manifold. As for the reciprocating parts, the weight of the pistons were reduced by 25%.
The crankshaft had its main journal diameter reduced from 60mm to 52mm, achieving a 25% weight reduction. As a result, the mechanical friction was greatly reduced to the same level as an average gasoline engine(Fig.3).
There are two main problems that have been preventing the spread of low-compression-ratio diesels regardless of these merits. The first is the fact that when the compression pressure is reduced, the compression temperature during cold operation is too low to cause combustion, preventing engine-start. The second is the occurrence of misfiring during warm-up operation due to lack of compression temperature and pressure.
The newly adopted multi-hole piezo injectors allow for a wide variety of injection patterns. Precision in injection amount and timing increases the accuracy of mixture concentration control, ensuring cold-start capability. Hardware-wise, the injector is a high-spec type capable of a maximum of 9 injections per combustion. Along with the three basic injections: pre-injection, main injection, and post-injection, different injection patterns will be set according to driving conditions. Definite engine-start even with a low compression ratio is attributable to this precise injection control and also the adoption of ceramic glow plugs.
Any misfiring that may occur during warm-up operation after engine-start is prevented by adopting a VVL (variable valve lift(Fig.4)) system for the exhaust valves. A single combustion cycle is sufficient for the exhaust gas temperature to rise. Given this, the exhaust valves are opened slightly during the intake stroke to regurgitate the hot exhaust gas back into the cylinder, which increases the air temperature. This promotes the elevation of compression temperature, stabilizing ignition.
It goes without saying that turbochargers greatly contribute to the diesel engine’s high torque, but they are also indispensable in reducing emissions and fuel consumption. SKYACTIV-D utilizes a two-stage turbocharger in which one small and one large turbo are selectively operated according to driving conditions. This technology achieves high torque and response at low speeds, and high power at high speeds. Moreover, due to the synergetic effect with the low compression ratio, optimally-timed combustion is realized with low NOx and soot emissions because a sufficient amount of air (oxygen) can be secured.