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When running gently, the SCC engine adjusts its valve timing so that the exhaust valve, as well as the inlet valve, is open on the induction stroke. This sucks exhaust back into the engine to give a very diluted mixture that's still able to burn if you give it enough time. Fuel is then injected directly into the cylinder through a combined spark-plug injector. To make the mixture burn, the ignition timing is advanced a long way and the spark is made to jump a huge 4 mm gap (a normal plug has a gap of about 0.7 mm).
When you need more power, the valve timing changes to a more normal setting so the engine can take in clean air (helped by a boosting turbo, of course - this is a Saab, remember) and the ignition timing retards. The cylinder pressures are greater now, so the spark can't jump that big gap. Instead, the spark passes directly to the top of the piston, which has a small peak ready to receive it. When it's time for the spark, the piston is far enough up the cylinder for the gap to be exactly right.
As well as these two extreme modes, the engine can operate at stages in between them. When you move the accelerator pedal, you're adjusting the engine's output at first purely by altering the valve timing, bringing a throttle into play only when the engine's working harder.
As well as being cleaner, the SCC engine is more efficient under light load because it doesn't suffer from the pumping losses found in a normal engine whose throttle is nearly closed. Imagine trying to breathe through a straw: that's something like what's happening when you're trickling along with a normal engine, and it's not very efficient.
Saab, which has patented both SVC and SCC, is now trying to get other car makers interested in the ideas. Other brands within the General Motors empire could be the first to benefit.
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