13:1 Compression and 40 mpg on 87 Octane fuel? Introducing Mazda’s Skyactiv Technology

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 skyactiv mazda engine
Most oil pumps use a spring loaded relief valve that simply dumps excess oil pump capacity.  The Skyactiv G engine uses this solenoid for more accurate matching of oil demand to pump capacity.  This wastes less power.

Conversely, a large overlap period in degrees of crank rotation can help volumetric efficiency and cylinder filling particularly in the mid range during times of high power demand.  By timing the injection so no fuel is present in the cylinder during overlap means that the optimal overlap for torque and power can be applied over a wide range using the variable cam timing without an emissions or fuel economy penalty! 

 mazda skyactive engine
High volume oil squirters help keep pistons cool to minimize detonation as well.  Another high oil demand part that the electronically controlled oil pump helps manage.

To further reduce pumping losses, the Skyactiv engine has an unusual part-throttle cam control strategy.  A lot of part throttle pumping loss is due to the engine fighting against a vacuum in the intake manifold caused by the partially closed throttle.  At part throttle, the Skyactiv engine goes to a late closing intake valve mode so that the intake is still open on a large part of the compression stroke.  As the piston travels upward, just enough air is forced out of the open intake valve to eliminate the vacuum in the intake manifold.  Thus, at part throttle the cam timing controls engine output, not the throttle plate.  The engine’s super high compression ratio ensures stable combustion even when some of the cylinder pressure is leaked out during part throttle operation.  This also helps reduce the chances of detonation if the engine is lugged in this mode. This strategy helps reduce overall pumping losses by 20 percent in the Skyactiv engine.

 mazda skyactive engine
The tuned Tr-Y  header is a critical part of the Skyactiv engine’s detonation control system.  It is tuned to over-scavenge the cylinder to help keep the combustion chamber cool internally.  Direct injection helps to keep this from becoming a pollution and fuel economy nightmare as no fuel is introduced into the cylinder during overlap!

The ability to run a large overlap period is also a critical part of Skyactiv technology that we have been alluding to: the ability to reduce detonation with low octane gasoline and high compression.  The engine runs a lot of overlap over large parts of its higher load operating cycle and has a specially tuned, header-like, Tri-Y exhaust manifold to really scavenge the combustion space of hot burnt exhaust gasses with a lot of cool intake air.  This Tri-Y 4-2-1 exhaust manifold is a huge improvement in VE over the previous – and typical for most engines nowadays – close coupled cat with log manifold.

 mazda skyactive engine
A longer exhaust runner like the one found in the Skyactiv engine loses heat and delays cat light off from a cold start.  This is a huge issue as a modern ULEV engine typically makes 90% of its total running pollution during cold start.  Mazda combats this problem by delaying ignition during cold start to increase EGT as well as using direct injection fuel injection scheduling tricks to increase EGT too.

 

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