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Variable Valve Timing Explained: An Appreciation of How Quickly Engines Operate

By: · July 20, 2017

Variable Valve Timing Explained: An Appreciation of How Quickly Engines Operate

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From the August 2017 issue

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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.

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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.

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To give you an idea of just how quickly today’s engines move, here’s a glimpse into the N55’s operating strategy:

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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. The dwell translates to a total opening of 0.006 second for a single intake stroke at 7000 rpm.

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N55 Valve-Timing Adjustability
-N55 Valve-Timing Adjustability

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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. It can adjust intake-valve lift between 0.008 inch—equal to the thickness of four pages of the magazine you’re holding—at low loads and 0.4 inch for full load via a fast-acting DC motor that controls how the cam-roller followers pivot.

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Exhaust-valve timing: By controlling the camshaft timing independently, the engine controller can adjust the amount of overlap—the period when both the exhaust and intake valves are open. At low-load, steady-speed cruising, this overlap is increased to allow a portion of the inert exhaust gas to flow back into the cylinder during the intake stroke, reducing combustion temperatures and the formation of oxides of nitrogen. At a steady 50 mph with the engine turning at 1500 rpm, the N55’s maximum overlap lasts 0.2 second. For maximum power at redline, fully minimized valve overlap lasts just 0.0005 second—the amount of time it takes sound to travel just seven inches at room temperature.

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Ignition timing: Ignition timing generally advances during light-load operation to prevent lean air-fuel mixtures from knocking. During both idle and redline runs in the N55, spark occurs around six to eight degrees before the piston reaches top dead center, but the gulf in engine speeds is the difference between spark occurring 0.002 second and 0.0002 second before the piston peaks. That’s 10 and 100 times faster than the single flap of a hummingbird’s wings. The system will also retard ignition timing when the engine is cold, working in conjunction with late fuel injection and earlier exhaust-valve opening to bring the catalytic converters to operating temperature more quickly.

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Filed under: Engine,News

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