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The regenerative engine cycle, in which part of the thermal energy of the exhaust gases is stored internally, for use in the following engine cycle, is analyzed as a function of time and several design parameters: compression ratio, regeneration timing, equivalence ratio, regenerator design Reynolds number and engine speed. The effects of fluid friction and heat transfer in the regenerator are taken into account in the model. Calculations show that the regenerative engine maintains a substantial efficiency advantage over the conventional Otto cycle, even after fluid friction losses. The effects of the different design parameters are pointed out, as well as ways to optimize the performance of a regenerative engine.

Traditionally, internal combustion engines follow thermodynamic cycles comprising a fixed number of crank revolutions, in order to accommodate compression of the incoming air as well as expansion of the combustion products. With the advent of computer-controlled valve trains, we now have the possibility of detaching compression from expansion events, thus achieving an “adaptive cycle” molded to the performance required of the engine at any given time. The adaptive cycle engine differs from split-cycle engines in that all phases of the cycle take place within the same cylinder, so that in an extreme case the gas contained in all cylinders can be undergoing expansion events, resulting in a large increase in power density over the conventional four-stroke and two-stroke cycles. Key to the adaptive cycle is the addition of a variable-timing “transfer” valve to each cylinder, plus a space for air storage between compression and expansion events.

The results of experiments and computations over a new two-cylinder regenerative cycle engine are reported. Heat regeneration by means of a reticulated ceramic matrix placed inside the combustion chamber was found to be very efficient, with transient, open throttle surface temperatures in excess of 1150°C. In most cases, the matrix caused a premature ignition of the premixed fuel and air. A time-dependent thermodynamic computation of the cycle shows that the cycle cannot produce shaft power as long as premature ignition is present. Different alternatives for engine design and operation are discussed, with basis on the computations. The highest efficiencies can be achieved by cycles where the compression phase is performed by an external compressor. The predicted performance of regenerative engines with direct fuel injection is similar to that of engines burning a premixed fuel-air mixture.

The Stirling cycle can be approximated in an internal combustion engine by means of regeneration of internal heat. This article shows computational results from a zero-dimensional thermodynamic analysis where a variety of parameters are studied. Results show that the IC-Stirling cycle offers a significantly better thermal efficiency over a conventional IC engine if some effects, such as the tendency for the cylinder air to “hide” inside the regenerator, are solved.

Continuing research introduced at the 2018 WCX conference, this paper shows the result of simulations where a midsize sedan (1700 kg) fitted with an adaptive cycle engine and a CVT is operated over three standard duty cycles: US06, UDDS, and HWFET, and compared with the results obtained from other engine cycles installed on the same vehicle. Four different engine cycles are compared: conventional 4-stroke, 6-stroke cycle with no air storage, 6-stroke cycle with air storage, and fully adaptive cycle with air storage and a number of strokes determined by instantaneous demand and state of charge of the storage tank. Results show that the fully adaptive engine achieves a better mileage in all scenarios, closely followed by the partially adaptive 6-stroke cycle with storage. Gains over a conventional 4-stroke powerplant range from 3.4 mpg on the HWFET cycle, to 7.6 mpg on the UDDS cycle.

Adaptive Cycle Engines, where compression and expansion events do not follow a fixed sequence but rather take place depending on demand, are competitive against electric motors because of their higher power density, lower carbon footprint with current energy sources, and predicted ability to use any kind of renewable fuel. The advantage of Adaptive Cycle Engines is greater whenever the powerplant has at least two distinct operating modes: one for high output, and one for high energy economy. This paper compares the well-to-wheels CO2 emissions and pre-tax costs when operating powerplants based on Adaptive Cycle Engines and on electric motors under several scenarios: passenger car, on-road heavy-duty vehicle, and light aircraft.