The Intake Stroke

[Admin]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

First, some definitions for those of us with only a rough idea of what’s inside a four-stroke internal combustion engine.

The simplest such engine has one vertical cylinder, containing a close-fitting piston that can move up and down inside it. The top of the cylinder is closed by the head, but the bottom of the cylinder is open. Located in that head are two cam-operated valves: an intake valve which when open can admit fresh air-fuel mixture to the cylinder, and an exhaust valve which when open releases spent combustion gas from the cylinder.

Fuel is added to intake air flowing toward the cylinder in the intake duct, either by a carburetor or by a fuel injection nozzle. Because it becomes tiresome to refer to “air-fuel mixture” every time, I will shorten this to just “air” in what follows.

Valves are in most engines held shut by valve springs and are opened by cams. In this case a cam is a rotating disc with a bump on one side that, acting through a cam follower and maybe other parts, controls the motion of the valve.

A modern four-stroke single valve train laid out about the head in which it sits. (KTM/)

Below the cylinder is the rotating crankshaft, which in concept is no different from the crank on a fishing reel or old-time coffee grinder. The handle of an engine’s crankshaft is called the crankpin. Encircling that crankpin is one end of the connecting rod, whose other end pivots on a steel pivot in the piston, called the wrist pin. As the crankshaft revolves, the piston moves up and down in the cylinder.

The piston at the top of its travel is said to be at Top Dead Center (TDC). At the bottom of its travel (farthest from the head) it is at Bottom Dead Center (BDC). Any piston position between the extremes of TDC and BDC can be defined in terms of crankshaft degrees before or after them (i.e., 36 degrees BTDC, 12 degrees ATDC, etc.).

What the Intake Stroke Does

The purpose of the intake stroke is to refill the cylinder with fresh air-fuel mixture. In the simple version, the intake valve opens at TDC and the piston descends. This, by reducing the pressure inside the cylinder, allows atmospheric pressure to push air into the cylinder through the open intake valve. When the piston reaches BDC, the intake valve closes. The cylinder has now been filled with fresh air and both valves are closed, ready for the compression stroke to begin.

During the intake stroke, the piston descends, pulling a vacuum that is then filled by the air-fuel mixture. (Illustration by Robert Martin and Ralph Hermens/)

Note that the piston does not really “suck” fresh mixture into the cylinder; it is atmospheric pressure outside the cylinder that pushes air into the lower-pressure inside the cylinder.

First Dose of Reality

In reality, the intake valve cannot suddenly snap open at TDC and then instantaneously snap shut at BDC. Such violent motion would quickly break the valve. Valves must be smoothly accelerated up off their seats (the valve seat is the ring-shaped part of the intake duct against which the valve head seals when closed), then decelerated, reversed in direction, and smoothly and without shock returned to the closed position. Valves do not snap shut; they are decelerated and seated by the cam profile. Seating velocity must be limited to 1-1/2 to 2 feet per second to avoid damage or valve bounce.

Valves are smoothly accelerated and decelerated by the cam lobe (here with a finger follower in between the two). (Kawasaki/)

Second Dose of Reality

Because air has mass, it takes time to accelerate it to high speed. Not only that, pressure changes in air travel at the speed of sound, so when the piston begins to descend on its intake stroke, the “message” that this is happening takes time to travel through the intake duct. If our engine is an old-time 500cc single, revving at peak power rpm of 7,200, each turn of the crank takes 0.0083 second (60 ÷ 7,200). If the intake duct is roughly 1 foot long, the “message” will take 1 foot, divided by the speed of sound at sea level: roughly 1,100 feet/second, or 0.00091 second. As a fraction of the time taken for the crank to make one revolution, this is 0.00091 ÷ 0.008 = 0.114 of that, or 0.114 x 360 (degrees in a circle) = 41 crank degrees. This means that not much happens in the intake duct for many degrees after the piston has started its downstroke.

A GM engineer once showed me a cylinder pressure trace taken from one cylinder of a Yamaha 400 engine. In its case, during the first half of the intake stroke, nothing much happened except that the descending piston pulled a pretty strong vacuum in the cylinder. This delay was the time it took to get the mass of air in the intake duct accelerated up to a significant speed.

This means that intake air, once accelerated, moves really fast, i.e., many hundreds of feet per second. This being so, wouldn’t it be foolish to close the intake valve right at bottom center, wasting the energy in that fast-moving intake air? To put it to work, we can leave the intake valve open for some time ABDC, letting high intake velocity keep coasting into the cylinder, making the filling process more complete.

This has to be a compromise, because highest intake velocity occurs only at higher rpm. In the mid- or bottom rpm ranges, intake velocity is lower, so it is unable to continue coasting into the cylinder as long after BDC.

So we must choose: If we want highest possible power at peak rpm, we close the intake valve long after BDC to fill the cylinder as much as possible. But this late valve closing will, at lower rpm and lower intake velocity, allow the piston, rising on its compression stroke, to push back out some of the air it has just taken in. By reducing cylinder filling, this will reduce midrange torque. An engine of this kind, with excellent power at peak rpm but weak in the midrange, is said to have “light-switch power.” It may be useful at Bonneville, but in daily use or around a road course, its lack of midrange acceleration makes it sluggish.

So we must compromise by closing the intake valve at a more moderate number of degrees ABDC. The result is somewhat less peak power, but with a useful gain in midrange acceleration.

Third Dose of Reality

We also have to consider the weight of the valve and other parts that move with it. At some high engine speed, it may become difficult for the valve spring to keep those moving parts following the cam profile. The result, if the valve can no longer follow the profile, is called valve float. This is one reason why modern four-stroke motorcycle engines have rev limiters—to keep rpm from rising high enough to cause valve float. The reason for avoiding float is that there is sharp impact when the floating valve train crashes back into contact with the cam profile. Repeated often enough, this damages or breaks parts.

There is much more to say about the intake process—but we have three other piston strokes yet to describe, so 1,100 words is a good place to stop.

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[Bender]

That's not the content I was expecting

[curlylegend]

What exactly were you expecting ?  Remember this is a family forum !

[Bender]

5 minutes ago, curlylegend said:

What exactly were you expecting ?  Remember this is a family forum !

[RideWithStyles]

Think a open public domain for any viewer capable of operating a computer is able to view is more correct.

[Simon Davey]

Just when I thought I knew the simple stuff

[Bender]

22 minutes ago, Simon Davey said:

Just when I thought I knew the simple stuff

Don't encourage kev