Quote:
Originally Posted by Sparkeysmall
How does increasing the compression reduce economy? By that logic a prius would be running 4:1. But it uses the atkinson cycle. And from what i found in one min of googleing in the NZ engine, it uses a 13:1 static but with the cams gets dropped to an effective 8:1. So why would they squeeze the 1.5l engine physical size with a smaller effective size because of the atkinson cycle to such a high ratio? Because the greater expansion from spark to exhaust stroke allows better use of the heat generated.
Now. Compairing any atkinson engine to an otto is like apples and oranges.
So what about a typical non-hybrid, otto cycled car. From the multiple modern econoboxes i googled they all where around 10.5:1. Pretty much the max you could use while pumping regular gas. Albiet with a very tiny camshaft.
Anyway. Squeezing a large mixture into a tiny mixture increases heat generated by compression alone. And a hotter mix burns faster, producing more power earlier on the down stroke, as well as a more complete burn (warm and hot air intakes for instance. (Ya ya "smaller" engine, less dense, less parastic losses)). So with higher heat generated from the same amount of air and fuel, more efficiency. Why compress the fuel at all if just igniting it at ambient pressure makes more power than compressing it. Explode a small box vs a gymnaisium with the same amount of fuel and air.
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Lower compression sometimes can actually make sense, since it's safer to run leaner and free of knock with a lower octane rating gas, even if that would be at the expense of some power and torque. There are also fewer pumping losses and heat rejection. Sure, there's some point when decreasing too much the compression ratio is not going to lead to an efficiency improvement, and the same goes for a higher compression ratio. For modern engines fitted with direct injection, Mazda has found the sweet spot to be around 13:1 to 14:1.