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| A closer side view of the pistons shows just how big the old piston pin was and how much heavier the construction of the old piston was. |
With lower compression and better mixing of air and fuel, combustion is more even with less localized high temperature and lean areas within the combustion space. This results in much lower levels of NOx and soot making the Skyactiv D engine clean by design.
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| Another view of new vs old. |
A major drawback to lower compression ratios in diesel engines is less combustion stability at low speeds and under cold start conditions resulting in misfiring. The Skyactiv engine combats this in several ways.
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| Looking at the piston underside, you can see the holes for the cooling channels built into the piston. The oil squirters shoot oil into these holes to help keep the piston cool. |
Fast acting common rail multi hole piezo injectors precisely control injection timing critical at these times in the engine's operating cycle. Ceramic glow plugs are also fast acting and can be placed to help initiate combustion easily in the chamber to ensure cold start combustion stability.
Perhaps the second most innovative feature of the Skyactiv D engine is its variable lift on the exhaust cam. This allows the exhaust valve to be opened slightly during the intake stroke. This causes some reversion of hot exhaust gas which speeds engine warming from a cold start further improving cold drivability with low compression.
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| Here is a cutaway view of the piston in the engine. Check out the steel reinforced groove for the number one compression ring. This helps resist high combustion pressures and prevents fretting and welding in the groove, cool stuff. Also you can see how the dish shape promotes swirl here. Also look at how the rod end is near net supported in the pocket for maximum support of the piston pin to reduce bending moment. |
With reduced combustion temperature and pressure, improved combustion stability in all operating modes and a high flow EGR system the Skyactiv engine is exceedingly low polluting and green with no expensive and bothersome exhaust post treatment like urea injection to reduce NOx needed. In fact a NOx trap reduction cat is not even needed for the engine. The Skyactiv D engine meets or exceeds the stringent Tier II BIN 5 North American emissions regulations.
