Saturday, May 1, 2004
An Advanced Course In Engine Management
When you have to pay for fuel, repairs and overhauls, you’ll want to treat your powerplant to the values of science, not hearsay
Carl Goulet, retired head of Continental Engine Engineering, suspected where the problem was. He challenged Braly to find out what was happening during a combustion inside the cylinder. The only way to determine internal cylinder pressures was to experiment, to directly read internal pressures created by every spark event. To answer that one question, GAMI went right out and built what may be the most sophisticated engine test stand in the world. At the seminar, you’ll get to see this stand in action.
Horsepower production as a function of fuel flow and the resultant EGT and CHTs were well known. What happens inside the cylinder is not. Most aviation piston engines are timed to spark around 20 degrees before the piston reaches top dead center (BTDC). The internal cylinder pressure reaches a peak directly in proportion to the fuel-per-air ratio, and in Braly’s test stand, those pressures are measured from the piston position of top dead center (TDC).
The piston engine is basically an air-cycle machine—hot expanding gas produces energy; in aviation engines, that energy allows the propeller to turn. At takeoff power when the throttle is wide open, the RPM and fuel flow are at a maximum. Any other setting changes the fuel-per-air ratio and the peak internal pressures. At rich mixtures, mass airflow governs horsepower, leaving extra fuel. At lean mixtures, fuel flow governs horsepower, leaving extra air.
The conventional wisdom is that extra fuel cools the process; the test stand shows conclusively that extra fuel delays peak internal cylinder pressures. Reduce the airflow and the fuel-per-air charge burns faster, reaching peak pressures earlier. When peak pressures occur closer to TDC, the actual value of peak pressure increases dramatically.
Bad things happen when cylinder pressures are high. Wear areas, like valves, are pressed harder into their seats and piston rings scrape harder against the cylinder wall. Fatigue is induced in high-stress areas of the piston, the rod, the crankshaft and the bearings. Higher pressures also mean thinner boundary layers between hot gases and the bare metal of the cylinders and pistons. Thinner boundary layers mean more direct heat transfer to the cylinder head, the valve and the valve seat. Higher pressures mean more wear and stress, and hotter temperatures, all of which decrease the engine life.
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