Talking about Compression Ratio

High performance engines usually are turbo, so less compression ratio

 

Remember that the Prius engines run on an Atkinson cycle, which requires computer-controlled valve and spark timing. That's how it can use regular fuel without preignition issues.

 

It would be more accurate to say the expansion ratio is 13 to 1. We are used to Otto cycle engines where compression and expansion ratios are equal.

The compression ratio is about 8 to 1 as the intake valves stay open for about 1/3 of the compression stroke, so Fuel/Air can escape back into the intake manifold.

 

In an Otto cycle engine, the exhaust gases are still expanding even after leaving the cylinder since the compression ratio and expansion ratio are the same and the mixture is much hotter.

In an Atkinson cycle engine, with a compression ratio of 8 to 1 and an expansion ratio of 13 to 1, more expansion happens in the cylinder. So a big, loud exhaust is less about performance, and more about annoying the neighbors, as gas pressure at the end of the power stroke is almost atmospheric.

 

And .... As the efficiently of a motor increases, you can get away with higher compression ratios even with lower octane fuels. Both intake track and exhaust savaging play into it as well.

 

Back when my 2010 was released, Toyota's marketing materials even correctly listed this as "Expansion Ratio." But by the time I upgraded with a 2012, marketing had reverted to the less accurate but consumer-familiar "Compression Ratio".