Starting with the 2019 model year, the Skyactiv-G and Skyactiv-D engines will be replaced with the Skyactiv-X engine. As all our forum members know a gasoline engine is a Spark Controlled Ignition engine (SCI) and a diesel engine is a Compression Controlled Ignition engine (CCI). Mazda's new engine is a Spark Controlled Compression Ignition engine (SPCCI), a radical new design. Using regular gasoline and a compression ratio of ~ 16:1, the new engine is essentially a hybrid with operating elements of both spark and compression ignition types utilized in its operation.
On the intake stroke, an ultra-lean A/F mixture of about 37:1 is introduced into the cylinder. Compare this with the normal stoichiometric A/F ratio of 14.7:1 utilized in a gasoline engine. At the end of the compression stroke, a second shot of fuel is injected right next to the sparkplug dropping the A/F ratio to ~ 29:1. Just before the detonation starts to occur the sparkplug fires igniting the less lean mixture around it creating an expanding flame front. The flame spreads from the plug out and down creating a pressure wave moving opposite the rising piston. This increase in the cylinder's effective pressure ignites the primary A/F mixture and initiates the power stroke as occurs in a diesel engine.
Integral to the functioning of this process is the inclusion of a Roots-type blower used as an air pump (not as a supercharger) to lean out the A/F mixture to improve fuel economy at the same time reducing combustion temperatures, lowering NOx emissions and reducing heat loss to the cylinders.
A number of sophisticated controls are required to obtain finer control over the blending between spark and compression ignition by altering the flames pressure wave using injection and spark-timing adjustments. Changes to the pressure wave equate to changes in the effective cylinder pressures making the engine effectively a variable compression type.
A water-cooled EGR system is also utilized to control cylinder temperatures and prevent destructive overheating.
This has been a bare-bones discussion of this radically new engine. For a more detailed analysis consult the sources available on the web for more information.
How this engine compares with the current Skyactiv-G engine and a description of pre-production driving characteristics will be detailed in the next post.
Happy Motoring!
On the intake stroke, an ultra-lean A/F mixture of about 37:1 is introduced into the cylinder. Compare this with the normal stoichiometric A/F ratio of 14.7:1 utilized in a gasoline engine. At the end of the compression stroke, a second shot of fuel is injected right next to the sparkplug dropping the A/F ratio to ~ 29:1. Just before the detonation starts to occur the sparkplug fires igniting the less lean mixture around it creating an expanding flame front. The flame spreads from the plug out and down creating a pressure wave moving opposite the rising piston. This increase in the cylinder's effective pressure ignites the primary A/F mixture and initiates the power stroke as occurs in a diesel engine.
Integral to the functioning of this process is the inclusion of a Roots-type blower used as an air pump (not as a supercharger) to lean out the A/F mixture to improve fuel economy at the same time reducing combustion temperatures, lowering NOx emissions and reducing heat loss to the cylinders.
A number of sophisticated controls are required to obtain finer control over the blending between spark and compression ignition by altering the flames pressure wave using injection and spark-timing adjustments. Changes to the pressure wave equate to changes in the effective cylinder pressures making the engine effectively a variable compression type.
A water-cooled EGR system is also utilized to control cylinder temperatures and prevent destructive overheating.
This has been a bare-bones discussion of this radically new engine. For a more detailed analysis consult the sources available on the web for more information.
How this engine compares with the current Skyactiv-G engine and a description of pre-production driving characteristics will be detailed in the next post.
Happy Motoring!
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