. . . So in a four-stroke gasoline engine, fuel is burned in the combustion chamber every other upstroke. The resulting explosion is what propels the piston back down, which is what makes your car move. During the alternate upstroke, the remnants of combustion (otherwise known as exhaust gases) are pushed out of the combustion chamber by the rising piston, then as the piston falls again it draws unburned fuel-air mixture back into the piston, rises, burns, and there you go.

So there have to be holes in the combustion chamber for fuel-air mixture to come in through, and different holes for exhaust to leave by. And they have to be able to be opened and closed, very fast, because gasoline engines turn over at thousands of revolutions per minute.

These valves are trumpet-bell-shaped pieces that fit into holes in the top of the combustion chamber. When they're pushed down, the holes are open. When they're lifted up, the holes are closed.

So these little trumpet-shaped bits have to get rattled up and down very fast, very precisely, for everything to work. To do this, a camshaft is used. The camshaft is a rod of metal punctuated with oval protrusions (cams) up and down its length. This is hard to describe, but if you can find a picture of one, all will become clear. A camshaft is a work of art.

Anyway, those little oval bits are the key. They are cams. Cams are eccentrically-shaped rotors that convert rotational movement into linear movement. The rotation is the spinning crankshaft of the motor; the linear movement is what actuates the valves. As the cams rotate, they push against whatever's actuating the valves, closing them. The valve springs snap the valves back into the open position. Desmodromic engines have no valve springs, but that's outside the scope of this discussion.

So cams open valves. But how? And where do you put the camshaft? There are essentially three camshaft configurations used in modern engines:

  • Pushrod
  • Single Overhead Cam (SOHC)
  • Double Overhead Cam (DOHC)
In a pushrod motor, the camshaft is right next to the crankshaft, making it easy to drive. Usually the drive mechanism is a toothed gear. The crankshaft spins the camshaft, whose cams act on pushrods running up the side of each cylinder. These pushrods actuate rocker arms at the cylinder head, opening the valves. Simple, yes, but the number of oscillating parts is a bit highter than one would like, so pushrod engines can't rev as high as their SOHC and DOHC counterparts.

In a single overhead cam engine, there's a (no, really) single cam above the cylinder head. Its cams directly actuate the rocker arms to open valves. This is a compromise between pushrod and DOHC configurations; you only have to go to the trouble of spinning one camshaft. This is accomplished by a timing belt or chain that couples the camshaft to the crankshaft.

The configuration most commonly associated with high performance is double overhead cams. In this layout there are TWO camshafts - one for intake valves, and one for exhaust. The cams act directly on the valves themselves, with no intervening pushrods or rocker arms. Because of this, DOHC engines can rev quite a bit faster than the alternatives, but you still have to figure out how to spin twice the number of camshafts. Usually, the solution is two timing belts, one for each camshaft, but occasinally gears are used.

Now you know. Also, here's something to ponder. If the timing chain, belt, whatever slips, the valves will spring into the open position, pushed down into the cylinder, but the piston (traveling several hundred feet per second) will keep going, colliding with the valves. Can you say catastrophic engine failure? Sure you can. Timing failures are nearly the worst thing that can happen to an engine.

As a postscript, the electromagnetic actuation that The Custodian mentions below is a very intruiging idea. By doing away with the camshaft, it eliminates a significant amount of rotating and oscillating mass, allowing for greater efficiency (no need to expend energy spinning those camshafts around) and more revolutions per minute. This, in addition to the benefits of variable virtual cam profiles (through the software he describes) would mean significant performance gain, not to mention serious geek drool potential.

One problem with camshafts is that the profile of the cam, that is, the complete set of valve timings which it produces, is fixed relative. In other words, whether the engine is revving fast or slow, the timings that separate the valve actions remain the same. The problem here is that the cam's designed profile is only maximally efficient within a narrow speed range.

One solution to this is called VTEC, or Valve Timing Electronically Controlled. In a VTEC engine, the timings between valve actions can be modified by the engine computer. The most common method of achieving this is to have a camshaft with various sets of lobes on it which the engine can displace laterally so as to choose which lobe set to use. If you have a DOHC engine, you can tweak the exhaust and intake timings separately, allowing a better match with a profile of optimum efficiency across angine speeds. VTEC is a Honda technology, and as far as I can tell involves shifting the cam profiles electromechanically between a high-rpm and low-rpm profile.

An even cooler method is lurking over the horizon. It is possible to do away with the camshaft entirely, and to move the valves by hooking them up to electromagnetic actuators. Then, you can trigger the valves individually simply by feeding power to the electromagnet; if you want, you can even modify the opening and closing time of the valve. Since this is done by controlling electric power rather than physical motion, it is quite easy to have the valves actuated by a computer, and simply change parameters in the computer's program to change the engine's tuning. In addition, it drastically reduces the number of rubbing moving parts, which increases engine reliability and means that the engine is less likely to suffer a catastrophic failure.

The nice side effect, of course, is that eventually you'll be able to tune your car, live, with your PalmPilot.

As an addendum to TheCustodian's writeup, above, electromagnetic valve actuation is a reality. Used by Renault in certain of their Formula 1 engines, this technology is also under consideration for regular road cars.

One mag has this to say:

One interesting example of engine control is electromagnetic valve actuation (EMV), which can in principle increase torque by 50%, improve fuel consumption by 25%, or reduce exhaust emissions by 40-90%. An EMV system can reduce mechanical complexity and weight by eliminating the camshaft, timing belt, and sprockets. Adding an EMV to an engine control unit increases I/O and software requirements, and may even require a digital signal processor function.

Source: http://www.sensorsmag.com/articles/1298/rol1298/main.shtml

Produce 1.5x the torque? That's a pretty brave statement. The decreased fuel consumption is a lot more believeable. They'll need to use some hella strong/durable solenoids!

Formula 1 racing engines have done away with valve springs by using compressed gas to return the valves. This was pioneered by Renault in the 1986 season and has enabled F1 engines to go at rpms hitherto unbeknownst to man.

The idea of the pneumatic valve return system is pretty simple: a cushion of compressed air or nitrogen is maintained under a ring around the valve stem. The system comprises a gas cylinder charged initially to something like 15 MPa, a main gas line, one-way valves, pressure regulators and an oil scavenging system. The main advantage is reducing the reciprocating mass in the valve assembly and eliminating the spring as a vulnerable element.

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