Intake-valve lift: BMW’s Valvetronic system effectively plays the role of a throttle, metering air into the cylinders primarily as a function of accelerator-pedal position. The dwell translates to a total opening of 0.006 second for a single intake stroke at 7000 rpm. 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 fixed. To give you an idea of just how quickly today’s engines move, here’s a glimpse into the N55’s operating strategy: 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. The events that define that combustion, such as how long the valves remain open, occur over even smaller fractions of a second. 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. 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.Īs 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. Absent a familiar, conventional time scale, it’s easy to underestimate just how quickly everything moves in an internal-combustion engine. 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.
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