The billions of ICEs have served humanity for >1.5 centuries ushering in modern civilization. They are of 2-stroke/4-stroke cycles, each having compression-/spart ignitions called ‘diesel/gasoline’, respectively. Each of these 4 has pro-&-con and serves in different applications. The “heart” of ICEs is the piston which reciprocates inside the combustion chamber to turn fuel into work. Current pistons are a one-piece design having a ring-section sealing the combustion chamber and a skirt section supporting the piston in the chamber.

Simply splitting the piston ring-section from the skirt (as shown in Fig. 1s) to operate via a (valvetrain-like) piston-train can “retrofit” existing (and future) ICEs in 2 advanced stages – called D-cycle and Diesoline “hybrid” engines - for greater performance and efficiency.

The D-Cycle's piston-train utilizes existing valvetrain technologies synchronized with crankshaft to operate the ring-section in the lighter exhaust/intake gas-exchanges and reuniting with its skirt to do the heavier compressing/power strokes. Thus, this split piston design completes the 4-stroke thermal cycle, different in both displacements and periods, in every revolution (like a 2-cycle) - a 2-/4-cycles hybrid called a differential stroke cycle (D-cycle).

Most ICEs operate mostly under 1/2 load and 1/3 peak rpm region most of the time. During these operations, the piston-train can operate the ring-section to have 1/2 intake with extended power strokes to 3/4 crank-strokes (an Atkinson Cycle). The 50% extended power stroke over intake is the optimal fuel efficiency gain with further extension having diminishing returns. An Engine-Controlled Unit can operate the piston-train for continued operations. Tests have shown >20% fuel efficiency and >2.5 times torque (power) gains - which greatly regains the power output of the baseline Otter cycle. The torque ‘loss” in the lower ¼ of the power stroke is not significant, since its crankarm and combustion pressure are greatly reduced. (It’s worth noting that doubled pressure charging intake can regain the lower ½ cylinder volume intake and would double the baseline engine outputs.)

The Diesoline engine has gasoline-engine's (Stoichiometric) air/fuel ratio intake “mixed  with extremely high (>30%) retained burnt gases called BGR (burnt gas retention)” tto be ignited under the diesel-engine's high-pressure – the combustion-ignition operation.

The high BGR rate is beyond the Otto cycle’s EGR (exhaust-gas-recirculation) range.  The BGR retains the exhaust (instead of the ERG’s exhaust/intake operations) saving system complicity. The premixed air and fuel eliminated the “soot” formation from the (liquid) fuel injection, while the high exhaust-gases content avoids engine knocks, retarding the ignition rate (with quieter combustion) and lower the combustion temperature to eliminate NOx formation – saves diesel engines’ after-treatment equipment and cost – and an additional efficiency gain of 15% for diesel to 30% for gasoline baseline in addition to D-cycle engines retrofits.

These retrofit technologies can readily apply to existing or new ICEs with half or smaller  cylinders using existing facilities and personnels, as demonstrated in the D-cycle concept. Of course, new engines can maximize these hybrid ICEs’ benefits. These retrofit technologies can continue to serve humanity for the next century and beyond.