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Originally Posted by dcb

Who says the exhaust is solely (or even partly) responsible?

Higher compression ratio brings higher combustion efficiency hence power. That's why automotive engineers want to raise compression as high as possible. However, a compression too high will lead to early explosion of fuel-air mixture, or what we call "knocking". Knocking is bad to engines, not only because it causes NVH but also it reduces output. When I started reading about cars, most engines in the world ran at lower than 10:1 compression. As engine management and valve-timing technology improves, nowadays the figure can be higher than 11:1. Direct injection engine may even lift that figure to 12:1 or so thanks to its cooling effect, but anything higher than that remains a dream. However, Mazda made a breakthrough with its Skyactiv-G engine in 2010. It works at an incredible 14:1 compression !

How can Mazda avoid knocking ? A crucial factor causing knocking is the high temperature of combustion chambers. Temperature in the chamber rises during compression stroke. It peaks when the piston reaches the top dead center (TDC, i.e. the highest position). At this point, knocking is most likely to occur. Obviously, if we want to reduce the risk of knocking, we had better to lower the combustion chamber temperature.

Then why is the combustion chamber so hot ? One of the reasons is the existence of residual exhaust gas, i.e. the exhaust gas that flows back into the combustion chamber during the intake stroke just before the exhaust valves close. No one can completely get rid of residual exhaust gas, because for high breathing efficiency engines always need to run with a certain level of valve overlapping (overlapping between the opening period of intake and exhaust valves). Suppose exhaust gas is 750degC and the fresh intake air is 25degC, and their mixture ratio is 1 to 10, you can see the residual exhaust gas can raise the combustion chamber temperature a lot. The more the amount of residual exhaust gas, the higher the combustion chamber temperature is. In other words, if we want to reduce temperature, we can reduce the amount of residual exhaust gas in the combustion chamber.

The graphs above show that a 14:1 compression engine always has higher comnbustion chamber temperature than a 10:1 engine on a given residual exhaust gas level. However, if the amount of residual exhaust gas is reduced to 4 percent, combustion chamber temperature will be about the same as a 10:1 engine running with 8 percent of residual exhaust gas. Now the question is: how to lower the percentage of residual exhaust gas ?

Surprisingly, Mazda uses a very conventional method to do that: a long, 4-to-2-to-1 exhaust manifold. On a typical inline-4 cylinder engine with short, 4-to-1 exhaust manifolds (the first picture below), once the exhaust valve of Cylinder 3 opens, its exhaust pressure waves (grey area) flows through the short manifolds to the exhaust valve of Cylinder 1, which is at the end of its exhaust phase. This pumps some exhaust gas back into Cylinder 1 and becomes residual exhaust gas. When the engine is running at low speed (2000 rpm in the picture below), the exhaust pressure wave arrives Cylinder 1 early enough to cause high percentage of residual exhaust gas. As engine rev rises, the opening and closing of valves speeds up as well, thus the exhaust pressure waves of Cylinder 3 reaches Cylinder 1 at later stage, causing lower percentage of residual exhaust gas. In short, from low to mid-range engine speed the level of residual exhaust gas is pretty high for this engine configuration.

In the case of Skyactiv-G's 4-2-1 exhaust manifolds (the second picture above), exhaust pressure waves from Cylinder 3 has to travel a long way to reach Cylinder 1, by the time Cylinder 1 has already, or nearly completed its exhaust phase. Therefore the level of residual exhaust gas is much lower than the previous case, especially for low to mid-range rpm. As a result, the Skyactiv-G engine attains lower temperature in its combustion chambers and allows a higher compression ratio to be used.

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