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What's the connection between compression ratio and fuel economy?

Static and Dynamic Compression Ratios

Like we learned on the previous page, static engine compression is measured when the air intake valve of an engine is fully closed. However, in real operation, that almost never happens. The engine is running so fast, that the air intake valve may need to open again before the piston has finished its full up and down stroke. When that happens, some of the pressure within the cylinder leeches out, which reduces efficiency. In essence, there's more space for the air, so the engine loses some of the power from the air-fuel combustion.

Dynamic compression ratios take the motion of the air intake valve into account. Engineers can tune an engine to have the air intake valve close earlier, which helps cylinder pressure build. The engine can also be tuned to have the valve close later, but that lets some air out and reduces how efficiently the engine uses fuel.

Calculating the dynamic compression ratio is actually pretty tough. To do it, you use the stroke length and the connecting rod length to determine the position of the piston when the valve is fully closed. Because this ratio is found when the piston is in the middle of its stroke, it's always lower than the static compression ratio. Like static compression, a higher compression ratio means more efficient fuel use and better fuel economy.

Today's the high-efficiency engines on many of today's cars owe a lot of their fuel economy to their high compression ratios. But, a high compression engine has its drawbacks, too. To keep it running in tip-top shape, you need to use high-octane gas, which is more expensive than regular unleaded gas. If you skip the premium gas, over time, the engine can develop a knock. An engine knock is when the air-fuel combustion doesn't happen at the optimal time in the piston's stroke. Using low octane fuel in a high-compression engine can make engine knocks more likely, so if you get a new, fuel-efficient, high-compression car, make sure you use the type of gas that's recommended in your owner's manual to get the most out of it.

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