The efficiency of the split-cycle approach is significantly improved by over-expanding the gas in the Hot-Cylinder (as in the Atkinson cycle for naturally aspirated engines and the Miller cycle for forced induction engines), which has the two-fold benefit of increasing the mechanical work extracted and lowering the average gas temperature at the Hot-Cylinder (Over expansion has an advantage also of lowering the temperature differential driving the heat rejection to the Hot-Cylinder. See “Superior thermal management strategy” above). The Figure below (Panel A) depicts indicated thermal efficiency (ITE) results at a fixed speed of 2400 rpm from GT-Power simulations for three engine configurations: a 2-cylinder baseline 4-stroke engine with 1000 cc (blue, solid line), an Atkinson cycle engine (green, solid line) and various Tour cycle engines (dashed lines). A symmetric Tour engine (red, dashed line) with 1000 cc (500 cc compression/500 cc expansion) has only a slight advantage over the baseline but asymmetric (over-expanded) Tour engines gain a significant advantage up to 1500 cc (500 cc compression/1000 cc expansion). Combustion strategies similar to those used in the Atkinson cycle enabling a 13:1 compression ratio would further increase the efficiency of the Tour cycle (orange, dashed line). The results at 2400 rpm suggest that overexpansion of engines with the above stated displacements can increase indicated thermal efficiency (ITE) relative to the baseline by up to 19%, and relative to the Atkinson cycle by up to 13% depending on the specific engine configuration. In order to simulate the performance maps (shown with ITE contours) for the baseline engine (Panel B) and the over-expanded Tour engine (Panel C), a throttle was added to the models. The performance maps show a higher ITE for the Tour engine and an expanded high efficiency region across a wide range of engine speed, including low engine speeds. This is advantageous for increasing engine durability while maintaining high efficiency.
Indicated Thermal Efficiency (ITE) from GT-Power simulations