Lycoming Flyer - Aircrafts sales, engines repair, spare parts

We aren’t attempting to make mechanics out of pilots by writingabout compression ratio in aircraft engines, but we desire to helpboth groups by providing a simplified description as it relates toour engines.In order to gain a reasonable amount of work from an internalcombustion engine, we must compress the fuel/air mixture duringeach power stroke. The fuel/air charge in the cylinder can be comparedto a coil spring in that the more it is compressed, (withinlimits), the more work it is potentially capable of doing.Engineering tells us that the compression ratio of an engine is acomparison of the volume of space in a cylinder when the pistonis at the bottom of the stroke to the volume of space when thepiston is at the top of the stroke. For example, if there are 140cubic inches of space in the cylinder when the piston is at thebottom and 20 cubic inches of space when the piston is at the topof the stroke, the compression ratio would be 140 to 20 or usuallyrepresented at 7:1.Although we can create a more efficient engine by increasing thecompression ratio, there are limits and a compromise is needed.If the pressure is too high, premature ignition will occur andproduce overheating. Compression ratio is a controlling factorin the maximum horsepower developed by an engine, but it islimited by present-day fuel grades and the high engine speedsand manifold pressures required for takeoff.Our normally aspirated engines are generally categorized aseither low-compression or high-compression power plants. Insurveying the complete range of all Lycoming engine models,we note that compression ratios vary all the way from a low6.5:1 to a high of 10:1. Engineering has generally established thelow-compression group as those with a compression ratio of 6.5:1to 7.9:1; and the high-compression group from 8:1 and higher.All Lycoming engines in the high-compression categoryrequire a minimum of Grade 100LL (blue) or 100/130 (green)octane, FAA-approved aviation fuel, and nothing less. Withhigh-compression engines we must stress the importance ofthe manufacturer’s recommendations as outlined in the EngineOperator’s Manual or in the Pilot’s Operating Handbook. Theseengines require not only the correct fuel, but the proper oil, precisetiming and a good air filter. All are very important in orderto protect this high-performance power plant.James Watt, Scottish physicist, had an engine problem evenin 1769. Although steam engines had been invented before hewas born, they were crude, inefficient machines and only a fewwere in use. So he had, after much experimental work, developeda relatively efficient condensing steam engine, the forerunner ofthe present-day type.Being a good businessman, Watt tried to sell his engineto coal mine operators who were then using draft horses to supplypower to drive the pumps which kept the mines free of water.But the mine owners had sales resistance! They insistedon knowing exactly how many horses each engine wouldreplace, or, in other words, the horsepower of the engine. Howmuch work would his steam engine do? This, then, was JamesWatt’s problem.Although simple machines such as sailing vessels, windmills andwaterwheels had been used for centuries, Watt realized that forthe most part, the majority of work in the world had been doneby man and his domesticated animals. Work was measured andpaid for by the day, from “sun to sun.” With the advent of reliableclocks, work was then accounted and paid for by the hour.Evaluating work by this time method, it was assumed that all menand animals could and did perform the same amount of work.This was far from being true.Watt realized that in order to have his steam engine usedby the coal mine operators, he would have to answer theirquestions — “how much work will it do, and how many men andhorses will it replace?” Since the “power” of one horse was a generallyknown and a constant quantity, he would have to determinethe “power” of his engine in order to compare it with the horseswhich it was to replace. His problem then was to define “power.”Power did not mean force. The mine owners cared nothing aboutthe force Watt’s engine might exert. They wanted to know howfast the engine would pump water out of the mine; in other words,how fast will the engine do the work? Simply, that was the definitionof “power.”The methodical physicist experimenting with draft horses usedto operate mine pumps found that, on an average, a horse pullingwith a force equal to a weight of 150 pounds walked 2-½ milesLow CompressionHigh CompressionBy F. F. Rohm, Chief Qualification Engineer (Ret.)This interesting article was written for us by Fred Rohm,who was our Chief Qualification Engineer when he retired.Fred had a career of 44 years in the industry, with a majorityof those years spent at Lycoming. Most of his careerat Lycoming was as Chief Experimental Engineer, whichestablishes his qualifications to author this kind of article.L y c o m i n g F l y e r 2 1