Variable Valve Timing: What Is It? Who Needs It?
And no We Didn't Copy This From The Internet.
Valvetrain Basics
Before you can appreciate the benefits of variable valvetrain technology, you have to understand the limitations of traditional valvetrain systems. So let's go back to Auto Shop 101 for a bit.
Designing a camshaft to properly time valve operation is a delicate act. The engines in our cars operate at anywhere from around 800 rpm at idle to, well, you name it on the top end. Even at a relatively moderate 3,000 rpm, the valves are opening and closing two dozen times per second. That doesn't allow a lot of time for air to flow in and out of the cylinders.
The characteristics of a high-performance engine compound the problem. It operates at higher engine speeds than a "normal" engine does, so the valvetrain's opening and closing events are happening even more quickly. Yet at the same time the performance engine requires more air and fuel to create more power. So the typical high-performance camshaft has lobe profiles that provide more valve lift, allowing a greater volume of air to pass through the port. The lobe profiles also open the valves for a longer period (called duration), giving the air more time to pass into or out of the chamber.
But that high-lift/long-duration cam won't work in a normal engine. It would hold the valves open too long for low and normal operating speeds. On the intake side, for example, if the valve stayed open too far into the compression stroke, the piston would push fresh air and fuel back out the intake port. Or if the exhaust valve stayed open into the start of the intake stroke, waste gases could be pulled back into the cylinder and dilute the fresh charge. At racing speeds, that sort of valve timing and overlap works to efficiently move air through the cylinders. But have you ever noticed that racing engines don't idle worth a damn? That's cam timing, folks.
Conversely, the relatively low valve lift and short duration that work so well in a normal engine would strangle the high-power motor. Maybe not at idle or initial throttle tip-in but certainly at high revs where it needs to work.
And there's the limitation of the traditional cam and valve setup. Cam timing, since it's governed by the shape of the cam lobes and the position of the cam relative to crankshaft rotation, is fixed. The cam works most efficiently at one engine speed. You can vary the timing of the events by advancing or retarding the camshaft, which will improve bottom- or top-end power, depending on which way you move the cam. But you can't have it both ways. Getting power and performance at low and high revs is impossible--unless you could somehow vary the cam's timing and lift based on engine speed...
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