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Aviation History
1991
1991 - 0097.PDF
Tomorrow's light aircraft might well be powered by that long-disdained mechanism, the two-stroke engine. Pressure on the conventional four-stroke to meet looming US emission- control and fuel-mileage legislation has caused a surge in interest in the two-stroke from car manufacturers. (Two-stroke engines have, of course, been used in aircraft since the beginning, notably the Gnome monosoupape (single valve) rotary of World War One and today's ul tralight engines.) Although no specific work has been done on aero-engine adaptations, engineering for lower emission levels has pro duced spin-offs, ac knowledged by the motor industry's increas ing interest in the two- stroke, which developers believe will have extra relevance in future light aircraft. General Motors, Ford, Chrysler, Volkswagen, Honda, Peugeot, Subaru and Toyota all have seri ous two-stroke develop ment programmes under way. Some have set out on their own two-stroke research but most have opted to buy in technol ogy developed by an Australian engineering firm, Perth-based Sarich Technologies. The claimed advan tages of a high-technol ogy two-stroke over con ventional four-strokes in clude fuel-efficiency gains of 25-30%, weight savings in the order of 50%, inherently smoother operation, up to 70% reduction in packaging space (for equivalent power), fewer parts translating to cheaper manufac turing and maintenance, and lower emis sion levels. Several innovations make possible the process of converting the noisy, smoky two-stroke to something that might power upmarket cars within five years. While the two-stroke and some of Sarich's develop ments are nothing new, they combine to produce a vastly different end product. The Sarich Orbital Combustion Process (OCP) engine is so named because most of the individual engineering developments it incorporates were derived from the com pany's now-shelved orbital engine. They have since been found to be uniquely adaptable to two-stroke engines. A key element is the pneumatic/electronic fuel-injection system, which is electroni cally controlled and regulates the shape and the timing of the charge. A conventional two-stroke's fuel/air/oil mixture is compressed in the crankcase by the down (combustion) stroke and passed through transfer ports to the combustion chamber. The OCP engine compresses pure air in the crankcase, passing it through the transfer port where it purges the exhaust before compressing further on the up stroke. Fuel is then injected by air pressure, The 2.8lit OCP X6 is an externally scavenged engine incorporating plain shell hearings after the exhaust ports are closed, into an annulus-shaped chamber in the cylinder head into which air is also introduced at about 5.5 bar (801b/in2). A computer- controlled valve at the bottom of the annular chamber is then opened by a solenoid and a small quantity of air/fuel mixture is blasted into the turbulent com bustion chamber. This creates a dynamic shock wave which atomises the fuel into tiny droplets that vapourise readily for good combustion. Charge stratification is achieved by introducing the charge close to the spark plug so that the rich mixture near the plug burns readily and well, while the mixture further away is leaner and burns more fully. Advanced electronic ignition and an ex haust tract that is electronically adjusted according to load and engine speed also improve torque, burn efficiency, emissions and smooth running through the engine speed range, according to Sarich. Conventional wet-sump pressure lubrica tion is precluded by the two-stroke process. Instead, a fine mist of oil is introduced into the airflow entering the crankcase for lubri cation of cylinder walls and bearings. The oil is contained in an external cartridge and there are no oil changes. Oil consumption is therefore comparable to that of equivalent four-stroke powerplants when oil changes are considered, and the fuekoil consump tion ratio is about 185:1 compared with 25:1 in a conventional two-stroke. Extra engine efficien cies result from the ab sence of valve-train fric tion, lower-friction roller bearings, less rubbing friction because there is no oil-scraper ring, and a reduction of some 60% in losses associated with pumping air through the engine, says Sarich. In contrast to four- strokes, pumping work is also reduced as the throt tle is closed, further im proving light-load operat ing efficiency and smooth running at low power. Smoothness is enhanced by the higher frequency and lower amplitude of the combustion pulses and the balancing of pri mary and secondary forces. Two important advan tages claimed for the OCP in aero-engine applica tions are its small size, which has obvious aero dynamic benefits, and its independence from wet- sump lubrication. The en gines are liquid-cooled but typically require a radiator about half the size of that needed for a conventional four-stroke. With optimum power efficiency normally occurring at around 5,000rpm, a reduction gearbox would be needed for any aircraft application. While this would allow use of a slower-turning propeller, it is feasible to retune the OCP engine to give peak power at lower speeds, says Sarich. Both control-system variants currently in use inherently compensate for altitude, and mixture control has little relevance in the stratified combustion system. There would be no problem developing dual ignition, as a second spark plug could easily be accom modated in the absence of valve gear. Both turbocharging and supercharging are 'practicable, the company claims, and engines currently under development range up to a 150kW (200hp) vee-six displacing 2.8 litres and weighing only 85kg. • FLIGHT INTERNATIONAL 9 - 15 January, 1<W1 35
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