Pneumatic valves represent one approach to overcoming some of the disadvantages inherent in the
valve train of an
internal combustion engine. In a conventional
four stroke engine valves open and close in defined set of sequences. For example,
intake valve will open on a downstroke just before the
compression stroke. If the valve opens on upstroke the compression created by the approaching piston will pump the fuel and air out of the
cylinder. Which is why the
exhaust stroke is always an upstroke. This is why engines run poorly, if at all, if even a tiny error in timing is present.
In order to synchronize these motions requires a number of parts. The good old American overhead valve v-8 offers a fairly simple illustration of how a valve train operates. These engines are relatively simple, with only two valves per cylinder, one intake and one exhaust. Millions of these engines have been built and they are very efficient at producing torque at low RPM.
In a V-8 engine like Chrysler's famed 426 Hemi the crankshaft is connected via timing chain to the camshaft. The camshaft is the brains of all automobile engines, with the exception of rotary engines used in the Mazda RX-7. The distributor, which controls spark timing, is driven off the camshaft, and crank, camshaft and distributor all must be precisely adjusted for the motor to run. We call that timing the engine, and spark timing in cars with conventional distributors is accomplished by physically turning the distributor.
Since our concern here is valve train, we will return to it. The camshaft is a fixed arrangement of cams which operate the valves. As the cam turns in our V-8, it will apply direct pressure on a lifter or pushrod. That pressure moves the lifter, transferring the load to a rocker arm which will pivot to apply direct pressure on the valve, opening it. Valves are normally closed, held closed by a valve spring. When the cam pushes on the lifter, the rocker will end up pivoting to push the valve into the cylinder, overcoming the resistance of the spring. As the cam continues to turn and falls off, the spring returns the valve to the closed condition when it stops pushing.
All modern camshafts follow a shape originally developed by Col. E.J. Hall in late 1921 at the request of Frederick Duesenberg who needed a new camshaft for their 183 cubic inch racing engine. The Hall Camshaft ramps its motions smoothly with a low initial angle to overcome inertia then a very steep ramp up to full lift. Camhafts are described with three numbers, cam timing, lift and duration. Timing defines exactly when in the cycle the came will open, and is measured in degrees. Lift tells how far the valve will extend into the combustion chamber and is measured in thousandths of an inch. A half inch of lift is characteristic of engines prepared for racing, 0.375 inches of lift, more typical of a street engine. Duration measures how long the valve is held open, and is also expressed in degrees. Taken together, the sum of these measurements is sometimes referred to as a cam's profile.
One problem from this arrangement is that every fixed camshaft represents a compromise. As any driver knows, a car engine changes RPM constantly as road conditions vary. Yet any fixed camshaft produces maximum horsepower and torque only at a very limited RPM range. Ideally, you would like a different cam for every RPM. Practically, that is impossible. So cams are engineered for different RPM ranges. A cam concerned solely with maximum power, would be optimized for very high RPM, but would give up low RPM power, making it impractical, and possibly undriveable for the street. A camshaft designed for low RPM power can be very driveable, but gives up a lot of power for flexibility.
One answer to this problem has come with Variable valve timing, first pioneered by Honda. Most systems advance or retard the camshafts to adjust for speed. This system is mechanically simple, and offers significant improvements, but affects only one cam parameter, timing, and is thus less than optimal. The Toyota VVTi and BMW systems work like this. Others, notably the Honda V-tech shift the cam fore and aft, shifting between two different cam lobes, of different profile. This system is better in many ways, but is more complex and still only offers two cams when what you really want is a different cam for every rev.
Pneumatic and electric valves represent potential approaches to overcoming this problem. In a pneumatic valve system, the valves are closed by releasing compressed air, produced by a compressor operated off the engine. Theoretically, such a system would free the engine from any of the constraints inherent to mechanical camshafts, with the valve operation defined not only by RPM, but by throttle position. Potentially, such a system can produce very efficient engines.
Such engines do not exist today, and for good reason. The valves must be operated by a compressor. That compressor requires engine power to drive. It is expensive, relative to conventional design. It might fail, which could lead to uncontrolled valve motions inside an operating engine. In some auto engines the valve and piston can may strike each other in the event of a valve train failure. Such engines are referred to as interference engines. Such contact can destroy an engine.
Another disadvantage is that the metering system is itself complex, and also prone to failure. But the biggest disadvantage is the compressor, which returns more power than it consumes only at very high RPM, usually around 13,000. Such high revving is common only in auto racing. Only in racing are pneumatic valves found.
In racing the motivations for moving to pneumatic valves were different. If you recall from our OHV V-8 engine illustration, a lot of parts are involved between cam and valve. I raced a car whose exhaust pushrods were each about a foot in length. At 6000 RPM each pushrod, rocker combo must change direction 6,000 times every minute. The long pushrods bend easily. That's a lot of reciprocating mass to overcome. For this reason OHV engines cannot be revved above 9,000 RPM and live. High performance street engines generally redline at around 6,000 RPM.
Overhead Cam (OHC) design eliminates the pushrods., greatly reducing reciprocating mass and simplifying the valve train.. Overhead Cam engines can rev as high as 14-15,000 RPM before they fail. Failure occurs when the valve spring is moving so quickly that it achieves the spring metal's harmonic frequency. Yet race engine designers want to go even higher.
The reason is simple. Race engines are almost always limited in displacement, the product of cylinder swept volume times the number of cylinders. As you increase swept volume, you increase the amout of fuel and air you can get in the cylinder. Power is, after all, a product of burning fuel and air.
Renault was the first to deploy a pneumatic valve system in its RVS-9 engine. Variants of that engine powered Jacques Villeneuve and Nigel Mansell to the F1 world championship. The system operates on compressed air, and only closes the valves, replacing the valve springs. That permitted the Renault engine to rev as high as 19,000 RPM, a significant advantage. It has been adopted by all F1 engine builders out of competitive necessity. This shifted the weak point from the valve spring to the connecting rod. No one has yet thought of a way to bypass the rods, which tend to fail catastrophically. Most racing series, such as CART have banned them for cost reasons.
Pneumatic valves offer significant advantages, for a price. The price may be too high to pay for most street engines. For racing they dominate when the rules permit, such as in Formula 1. Further development may bring pneumatic systems to the market, but they are unlikely to be used in any but the most expensive cars.