At a media preview today at the General Motors technical center, we got a few more details on GM's new direct injected four and six cylinder engines that will debut in several 2010 models. Both the 2.4L four cylinder and 3.0L V6 get variable cam phasing on both the intake and exhaust cam-shafts. Aside from the direct injection system, the biggest change to the four cylinder is the engine block. The current engine uses a lost foam aluminum casting. The higher pressures in the more powerful DI engine required a stronger block so a process switch was made to a sand cast block.

Both engines use a split pulse injection strategy for cold starts. Part of the fuel is injected at the end of the compression stroke to provide a rich mixture right around the spark plug. The dished top of the pistons helps contain the fuel and warm up the cylinder. After the initial injection pulse burns the rest of the fuel is injected and ignites in a leaner mixture. This helps with cold start and gets the catalytic converter warmed up faster for lower emissions. Once the engine and catalyst warm up, the control reverts back to a more conventional single pulse strategy.

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