From the August 2017 issue
When it comes to the many variables of combustion inside an engine, engineers measure the timing of key events in degrees of crankshaft rotation, a relative frame of reference that remains constant without needing to compensate for the engine?s changing rpm. Absent a familiar, conventional time scale, it?s easy to underestimate just how quickly everything moves in an internal-combustion engine. Add in the capabilities of ­modern electronics and controls that optimize valve events, fuel injection, and spark ignition for power or efficiency, and firing on all cylinders depends on millisecond precision.
As just one example, BMW?s N55 turbocharged inline-six engine combines adjustable phasing of both the intake and exhaust cams with variable intake-valve lift. At the engine?s 725-rpm idle, the intake, compression, power, and exhaust strokes, together, happen in just 0.2 second, literally the blink of an eye. The events that define that combustion, such as how long the valves remain open, occur over even smaller fractions of a second. And as the engine approaches its 7000-rpm climax, the entire process is compressed into a window that lasts roughly one-tenth as long as it does at idle. To give you an idea of just how quickly today?s engines move, here?s a glimpse into the N55?s operating strategy:
Intake-valve timing: The BMW inline-six?s intake-camshaft phaser can shift the cam profile by up to 70 degrees, but the opening duration of 255 degrees is ...