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

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The pocket in the piston is one of the Skyactiv engine’s secrets for smooth combustion with a lack of detonation when using poor quality fuel.  It keeps a large enough volume of fuel-air mixture close to the plug to assure stable and smooth ignition.

Next, the physical design of the combustion chamber is designed for high turbulence and a smooth-burning, homogeneous mixture of fuel and air.  The high dome piston has a spherically shaped cup in its dome in close proximity to the fuel injector. This concentrates a volume of the combustible mixture close to the spark plug to help with stable ignition.  The contour of the piston and the angle of the ports are designed to induce a tumble to the air as it enters the cylinder on the intake stroke.  The turbulence, combined with atomizing six hole fuel injectors, also help to produce a homogeneous mixture when the fuel is injected, which burns smoothly without the irregular colliding flame fronts of detonation.

You can see how the injector fires througthis trough in the hot exhaust side of the piston into the pocket near the spark plug.  Exhaust side injection puts the cooling fuel where it’s needed the most and the hot piston helps vaporization and an even distribution of fuel and air.

To further combat detonation, the Skyactiv engine takes full advantage of direct injection’s ability to inject fuel at different parts of the 4-stroke cycle, working closely with the engine’s variable cam timing system.  Mazda’s S-VT variable cam timing system is used on both the intake and exhaust cams to combat detonation as well by giving the engine the ability to run a lot of valve overlap. 

Variable cam timing is super critical to this engine’s strategy to control detonation, reduce pollutants and reduce pumping losses at part throttle.  The engine has a huge amount of cam control authority.  The deep pockets for the valves in the pistons allude to this.  The cam control on the intake is super fast electro-mechanical and the exhaust side is hydraulic.  The cam control is for phasing only.

The intake cam uses electronic control for fast and accurate control of cam phasing over a wide range, while the exhaust cam uses hydraulic control. Overlap is the period of the four stroke cycle at the end of the exhaust stroke and the beginning of the intake stroke when both the intake and exhaust valves are open at the same time.  Normally, on a modern engine the overlap period is cut down because unburnt hydrocarbons from the intake charge can escape out the exhaust port, wasting fuel and increasing emissions.

The oil pump is electronically controlled with a PWM solenoid that controls the oil pump bypass.  When oil is needed for fast cam movement, the solenoid closes off the bypass.  Otherwise it allows oil to just return to the pan so the pump won’t rob power from the crank.