CHARMED LIFE OF A BOOSTED ENGINE

Notice: Double the horsepower, with only 8 Percent Engine stress at 15 Pounds of turbo boost. Excerp from FORCED INDUCTION. Read on!

THE CHARMED LIFE OF A BOOSTED ENGINE

Turbo-boosted power is easier on an engine than you might guess from the power gains. For one thing, most of the gain in power from turbo-boosting occurs by increasing the force exerted against the crankshaft at times of subpeak stress. Under very heavy loading and higher speeds, the charge mixture in the cylinders of a modern piston engine typically ignites at 20-30 degrees before top dead center, with ignition timed to achieve peak cylinder pressure in the range of 14-18 degrees after top dead center. Only about 20 percent of the charge mixture has burned, but at this point the piston begins to accelerate hard away from the cylinder head such that even though gas volume is still increasing from combustion, the size of the combustion chamber is increasing even faster. Inevitably, cylinder pressure is dropping in this portion of the power stroke, and is a decreasing factor when considering stress on the various engine components. The average torque of a turbo engine is heavily enhanced by gains in this portion of the power stroke, but cylinder pressure and mechanical stress are considerably below the peak.

So let's analyze peak cylinder pressure. If an engine is turbo-supercharged to the extent required to deliver double the charge mixture in the combustion chamber, the cylinder pressure from compression will certainly be higher than an equivalent normal-charged engine, as will the component from boosted combustion. But how much higher? Less than you might think. Suppose normal charged cranking compression yields 185 psi, and a turbo compressor adds an additional 15 psi of boost pressure. The total compression component of cylinder pressure would be 200 psi exerted against a 4-inch piston of 12.57 square inch crown area. Multiplying the 200 psi by 12.57 indicates a total compression loading on the connecting rod of 2,514 pounds at top dead center CTDC) for the supercharged engine. But this is only 8 percent higher than the 2,324 pound loading of a similar normal-charged powerplant.

Obviously, this is small compared to combustion loading, which could easily quadruple pressure in the combustion chamber to 740 psi in the normal-charged engine and 800 psi in the boosted engine, resulting in total loading of 9,301 and 10;056 pounds for the two powerplants. Think of it: atmosphere of boost will double the horsepower but the supercharged engine's pressure is nonetheless only 8 percent higher. And 100 percent of the added load is compressive through the connecting rod against the crankshaft.

This last point is significant. Let's compare the increased pressure-based rod loading of turbocharging to the higher rod stresses that result from increasing the engine redline and thus increasing the inertial resistance of the mass in the reciprocating assembly to extremely rapid changes in piston and rod velocity. Calculating the “weakest link” tensile rod loading (when the rod bolts alone must bear the entire load of decelerating the piston-rod assembly when the crankshaft is yanking the piston to a halt toward the end of the exhaust stroke and there is no compression and combustion pressure to offset the tensile loading), we find that the loads generated by reciprocating motion increase as the square of engine rpm. If redline increases from say, 6,000 to 7,000 rpm, loading increases not 17 percent-like engine speed- but 36 percent! Compare this to the 8 percent increased rod loading from 15 psi boost.

Bottom line, considering the nature of the way turbocharging increases cylinder pressure versus the exponential nature of increased rod loading from higher engine speeds, turbocharging is clearly far easier on an engine than increasing the redline.