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Technical Paper

Analysis of the 3rd Generation IC-Stirling Engine

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.
Technical Paper

A First Look into the Regenerative Engine

The regenerative internal combustion engine is a novel concept for increasing the economy and the power output, and decreasing the pollutant emissions of the conventional reciprocating engine. This engine is based on the recovery of heat from the exhaust gases, to be utilized in the engine cycle itself. This is carried out through simple modifications to the conventional engine, affecting only the combustion chamber. In this paper, a theoretical analysis of the cycle is presented first. Recommendations are made concerning the optimum strategy to control the engine, the reduction of pollutants, and the critical design parameters. Some of the possible shortcomings of the new engine are discussed, particularly speed limitations.
Technical Paper

New Experiments and Computations on the Regenerative Engine

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.
Technical Paper

Time-Dependent Analysis of the Regenerative Engine Cycle

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.
Technical Paper

The Adaptive Cycle Engine on Standard Duty Cycles

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.
Technical Paper

The Adaptive Cycle Engines

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.
Technical Paper

Stability of Flowing Combustion in Adaptive Cycle Engines

In an Adaptive Cycle Engine (ACE), thermodynamics favors combustion starting while the compressed, premixed air and fuel are still flowing into the cylinder through the transfer valve. Since the flow velocity is typically high and is predicted to reach sonic conditions by the time the transfer valve closes, the flame might be subjected to extensive stretch, thus leading to aerodynamic quenching. It is also unclear whether a single spark, or even a succession of sparks, will be sufficient to achieve complete combustion. Given that the first ACE prototype is still being built, this issue is addressed by numerical simulation using the G-equation model, which accounts for the effect of flame stretching, over a 3D domain representing a flat-piston ACE cylinder, both with inward- and outward-opening valves. A k-epsilon turbulence model was used for the highly turbulent flow field.