Wayne, thank you for your explanation. First time I saw the subject explained correctly in an easy-to-understand way.
“ Atkinson Cycle engines offer better fuel economy than Otto Cycle engines, but reduced low-speed torque… “ – Jim Kerr
The low-torque nature of Atkinson Cycle engine is due to reduced effective compression stroke compared to Otto Cycle engines of the same geometrical displacement. Shorter compression stroke means less fuel burnt per cycle with weight and friction of a larger displacement engine. Therefore, torque (and power) of Atkinson engines is lower at any rpm (not just at low speed, though possibly more pronounced at low rpm) than the Otto Cycle counterpart. Another reason is its effective compression ratio is lower… if its expansion ratio is 11:1, the compression ratio is something like 8:1, which means the engine is a good candidate for superhcarging. (Turbocharging might not be so effective since exhaust gas does not have much punch left.)
In the case of Mazda Millenia S, as Wayne mentioned, it uses Lysholm type screw compressor to supercharge it. At the time of its introduction technology for variable valve timing control was in its infancy. Also, high-pressure direct-to-cylinder fuel injection for gas engines was not available, which means max boost pressure must be kept at a low level to avoid knocking. Mazda had to use pressure release valve to avoid overfeeding. In 2007 Mazda introduced a 1.3-litre NA Miller Cycle engine for its Demio model. Who knows, Mazda might be working on its supercharged version?
