One thing that's missing from this discussion about knock suppression is intake port design and head cooling. There is a trade off between charge motion (tumble, swirl) and port flow capability. The high charge motion ports found on most modern direct injection turbo engines (NOT this engine) like the Ford Ecoboost 3.5 improve knock suppression and combustion.
Cylinder head cooling has gotten a lot better over the years as well, due to better computer simulations of flow in cooling passageways. So there's more to it than just the nominal compression ratio.
Quote:
Originally Posted by Coheed
Prius works just like a regular otto cycle. It just has late valve opening like you said. But it still has a throttle valve, as seen in this pic. As you can see, VE is also affected by its small size.
I don't consider this a real "atkinson cycle" because it is far from his designs, but the modern concept works almost identical, and is much simpler. 
Its a beautiful thing. They close the intake valves late to push extra intake air back out of the cylinder. This limits the actual VE of the engine. |
A "real" Atkinson/Miller cycle engine uses the physical geometry of the engine to have a greater expansion stroke than compression stroke. Today's Atkinson/Miller processes on passenger car engines use valve timing tricks.
Of course the Prius engines have throttle valves. So do BMW Valvetronic and Toyota Valvematic engines. De-throttling is a bit of a catch-all term for reducing pumping loss from the throttle valve. Two primary means of de-throttling are from changes in valve timing/lift and load-point shifting.
Valve timing tricks for de-throttling come in various implementations. The most expensive are continuously variable valve lift systems like BMW's Valvetronic, which they use on basically every engine now. Some use cam-phasing, like the Prius engines. Others use a cam profile change, like on the Honda Civic R18 engines which also use Miller Cycle. You can control engine torque output with more than just a throttle valve. The throttle valve is there as a failsafe and as a way to create some vacuum in order to drive the evaporative emission purge system.
Cylinder deactivation is a type of de-throttling. It's load-point shifting--on an 8 cylinder engine, run 4 cylinders at a higher load (BMEP) to get into a more efficient operating condition. This is now being implemented in 4 & 6 cylinder engines.