A Six-Stroke, High-Efficiency Quasiturbine Concept EngineWith Distinct, Thermally-InsulatedCompression and Expansion Components
Abstract: One of the most difficult challenges in engine technology today is the urgentneed to increase engine thermal efficiency. This paper presents a Quasiturbine thermalmanagement strategy in the development of high-efficiency engines for the 21st century.In the concept engine, high-octane fuels are preferred because higher engineefficiencies can be attained with these fuels. Higher efficiencies mean less fuelconsumption and lower atmospheric emissions per unit of work produced by the engine.While the concept engine only takes a step closer to the efficiency principles of Beau deRochas (Otto), it is readily feasible and constitutes the most efficient alternative to theideal efficiencies awaiting the development of the Quasiturbine photo-detonation engine,in which compression pressure and rapidity of ignition are maximized. One of the most difficult challenges in engine technology today is the urgent needto increase engine thermal efficiency. Thermal management strategies and the choice offuels will play crucial roles in the development of high-efficiency engines for the 21stcentury. However, it was during the 19th century that the fundamental principlesgoverning the efficiency of internal combustion engines were first posited.In 1862, Alphonse Beau de Rochas published his theory regarding the idealoperating cycle of the internal combustion engine. He stated that the conditions necessaryfor maximum efficiency were: (1) maximum cylinder volume with minimum coolingsurface; (2) maximum rapidity of expansion; (3) maximum pressure of the ignited chargeand (4) maximum ratio of expansion. Beau de Rochas' engine theory was first applied byNikolaus Otto in 1876 to a four-stroke engine of Otto's own design. The four-strokecombustion cycle later became known as the "Otto cycle". In the Otto cycle, the pistondescends on the intake stroke, during which the inlet valve is held open. The valves in thecylinder head are usually of the poppet type. The fresh fuel/air charge is inducted into thecylinder by the partial vacuum created by the descent of the piston. The piston thenascends on the compression stroke with both valves closed and the charge is ignited by anelectric spark as the end of the stroke is approached. The power stroke follows, with bothvalves still closed and gas pressure acting on the piston crown because of the expansionof the burned charge. The exhaust stroke then completes the cycle with the ascendingpiston forcing the spent products of combustion past the open exhaust valve. The cyclethen repeats itself. Each Otto cycle thereby requires four strokes of the piston- intake,compression, power and exhaust- and two revolutions of the crankshaft. Thedisadvantage of the four-stroke cycle is that only half as many power strokes arecompleted per revolution of the crankshaft as in the two-stroke cycle and only half asmuch power would be expected from an engine of given size at a given operating speed.The four-stroke cycle, however, provides more positive scavenging and charging of thecylinders with less loss of fresh charge to the exhaust than the two-stroke cycle.Modern Otto cycle engines, such as the standard gasoline engine, deviate from theBeau de Rochas principles in many respects, based in large part upon practicalconsiderations related to engine materials and the low-octane fuel used by the engine.The six-stroke Quasiturbine concept engine described in this monograph is designed toovercome many of the limitations inherent in the Otto cycle and bring the engine'soperating cycle closer to Beau de Rochas' ideal efficiency conditions. The preferred fuelfor the concept engine is methanol because of its high-octane rating and its ability to coolthe fuel/air charge during the intake stroke.
Maximum Volume / Minimum Cooling Surface
The first Beau de Rochas principle teaches that the engine should have aminimum cooling surface area while still allowing for maximum charge volume duringintake ("volumetric charge efficiency"). Otto cycle engines generally have coolingsystems.1 The cooling system represents an engineering compromise. Without a coolingsystem, the pre-mixed fuel/air charge could prematurely ignite (or "knock") during thecompression stroke, especially with low-octane fuels like gasoline. Knock reduces theengine’s power because the pressure of the combustion event is not properlysynchronized with the engine’s power stroke. Knock can also seriously damage engineparts. A cooling system also serves to maximize volumetric charge efficiency byreducing the temperature of the charge during intake.Targeting high-efficiency, the proposed concept engine eliminates the enginecooling system. Instead, cooling of the inducted fuel/air charge is achieved through theuse of methanol, a liquid with a high latent heat of vaporization, which is injected into theintake port (port fuel injection or "PFI") during the intake stroke.2 The compressor canthen be thermally insulated in order to minimize the cooling surface, while stillmaintaining volumetric charge efficiency. Similarly, the expander can be thermallyinsulated to minimize the cooling surface and to maximize the pressure of the combustedgases during the power stroke, as discussed below.
Maximum Rapidity of Expansion
Rapidity of expansion in a spark-ignition engine can be achieved by increasingthe engine's compression ratio. A higher compression ratio brings the fuel and oxygenmolecules in closer proximity during ignition and facilitates rapid expansion. In order toincrease engine compression ratio, a high-octane fuel is used. A high-octane fuel is a fuelthat has a high autoignition temperature in air. Because the fuel/air mixture is heatedduring the engine's compression stroke (especially in the thermally insulated compressorcylinder of the concept engine), it is critical to avoid premature ignition or knock duringthat stroke. With high-octane fuels, such as methanol, premature ignition can beprevented while still increasing the engine's compression ratio. Thus, in order to achievehigh compression ratios, lower octane fuels like gasoline (despite anti-knock additives)should be avoided. High-octane fuels are most compatible with the high compressiontemperatures of the present concept engine.The primary limitations on the compression ratio in the concept engine are (1) theautoignition temperature of the selected high-octane fuel and (2) the temperaturetolerance of the oil-free lubricant which is used to coat the piston rings of the compressor.Lubricating graphite surface coatings have a maximum temperature tolerance of about1000F / 540C / 810K.3 Methanol has an autoignition temperature in air of 470C / 740K.Thus, in principle, high compression ratios can be achieved with high-octane fuels whilestill maintaining the temperature in the cylinder during the compression stroke at lessthan the maximum temperature tolerance of the oil-free lubricant.
Maximum Pressure of the Ignited Charge
The pressure of the ignited charge is subject to several conditions: thecompression pressure of the fuel/air charge prior to ignition, the ratio of fuel to air in thecharge itself and the temperature of the combusted gases after ignition. While ideal,maximum pressure cannot be achieved in the concept engine4, the concept engine doesimprove on the Otto cycle engine by eliminating the cooling system and by allowing highcompression pressures with high-octane fuels. The Otto cycle cooling system reducespressure both during the compression stroke and during the expansion stroke. By usingthermal insulation for both the compression function and for the expansion function andby using a near stoichiometric ratio of high-octane fuel and air, the concept engine takes asignificant step closer to Beau de Rochas ideal cycle efficiency.
The fourth Beau de Rochas efficiency principle teaches that the expansion volumeof the combusted fuel/air charge should be maximized. In Otto cycle engines, thecompression volume and the expansion volume are equal because the cylindervolume swept by the piston is the same for both the compression stroke and for the powerstroke. For maximum efficiency, the expansion volume should always exceed thecompression volume. The constant-volume Atkinson cycle has this characteristic.The Atkinson cycle engine is a type of internal combustion engine invented byJames Atkinson in 1882. The Atkinson cycle is designed to provide efficiency at theexpense of power. The Atkinson cycle allows the intake, compression, power and exhauststrokes of the four-stroke cycle to occur in a single turn of the crankshaft. Because of theengine’s novel linkage, the expansion ratio is greater than the compression ratio, whichresults in greater efficiency than a comparable engine operating in the Otto cycle.5Another way to achieve the Atkinson cycle effect is to separate the engine'scompression function, its combustion function and the expansion function. That approachis the one used in the present Quasiturbine concept engine. The stand-alone compressorhas its own set compression volume. Fuel and air would be pre-mixed and compressed inthe compressor. The pre-mixed fuel and air would be combusted in engine combustionchambers. The stand-alone expander would have an expansion volume that is greater thanthe compressor's volume. This can readily be achieved because of the separation offunctions. Unlike an Otto cycle engine, the volume of the expander's expansion chamberdoes not have to equal the compression chamber volume, if the compression andexpansion functions are separated. Instead, the volume of the expansion chamber maybe sized to exceed the compressor's compression chamber volume. The expansionvolume thereby exceeds the compression volume by design.6