Four-stroke cycle engine valves

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Modern internal combustion engines using either four-stroke , two-stroke or six-stroke cycle with spark ignition and compression ignition, use poppet valves to allow air to flow through the cylinder head cylinder and exhaust gases out. Very early engines used alternative valve types such as D slide valves, that proved to be unsatisfactory, especially at higher speeds. The actuation of these valves is the subject at hand.

Valves are opened by raising them from their seats by a camshaft acting on a tappet. They self-close under spring pressure, with exception of desmodromic valves, used on Ducati motorcycles, in which a separate cam follower is used to close valves without spring force. F1 racing car engines use pneumatic valve springs with compressed gas rather than metal springs to close their valves because these have faster response through lower mass than metal springs and thereby allowing greater engine speed. Resonance of the valvetrain with the rhythm of the engine at high RPM can cause "float," where two coils of the valve spring wire are forced together, effectively shortening the spring. This leads to loss of contact between the valve and the rocker or cam follower, and thence to either mechanical failure by the piston hitting the valves, or power loss through failure to close. Valves seal against a conical seat in the cylinder head which nearly matches the angle of the valve periphery, to form a knife-edge seal.

Some automobile engines use more than one inlet and/or exhaust valve to reduce individual reciprocating mass for separate valves. For a given valve lift this also increases open passage area, improving power an engine can produce. The diameter of the cylinder limits the circumference of two valves to a smaller total than the circumference of smaller valves that give a larger opening area.

Valves are round for mechanical reasons, among which are self aligning, easy manufacture and ability to rotate without compromising sealing.

Exhaust valves are often the hottest part of the engine, being directly engulfed by exhaust gases and can only lose heat to the cylinder head (and hence the cooling system) through the closed valve seat and to a lesser degree the valve stem. Exhaust valves require special steels, and some are made hollow, are evacuated and carry a small volume of sodium metal that is liquid at engine temperatures. This oscillating liquid carries heat away from the valve face in bucket brigade fashion. Although better heat conductors, aluminum cylinder heads require steel valve seat inserts while cast iron cylinder heads often used integral valve seats in the past. In the 1980's, when unleaded gasoline (petrol) was prescribed, a phenomenon known as valve seat recession occurred without steel inserts for exhaust valves. Leaded fuel had prevented this action, for which the remedy is either a suitable fuel additive with the same protective function as tetraethyl lead, or to install steel inserts.

Because the valve stem extends into lubrication in the cam chamber it must be sealed against blow-by to prevent cylinder gases from escaping into the mechanical part of the engine. A rubber lip-type seal ensures that excessive amounts of oil are not drawn in from the crankcase on the induction stroke and that exhaust gas does not enter the crankcase on the exhaust stroke. Worn valve seals are characterised by a puff of blue smoke from the exhaust when pressing back down on the accelerator pedal after allowing the engine to over-run, such as when changing gears.

[edit] Desmodromic valve drive

Before the days when valve drive dynamics could be analyzed by computer, desmodromic drive seemed to offer solutions for problems that were worsening with increasing engine speed. Famous examples of successful desmodromic engines were Mercedes-Benz W196 and Mercedes-Benz 300 SLR racing cars. Since those days, lift, velocity, acceleration, and jerk curves for cams have been modeled by computer ^[1] to reveal that cam dynamics are not what they seemed. With proper analysis, valve adjustment, hydraulic tappets, push rods, rocker arms, and above all, valve float, became things of the past... without desmodromic drive.

4 Stroke Engine.

Today most automotive engines use overhead cams, as shown in the adjoining dynamic illustration, driving a flat tappet to achieve the shortest and most inelastic path from cam to valve, thereby avoiding elastic elements such as pushrod and rocker arm. Computers enabled accurately designing acceleration profiles, the most important dynamic of valve motion, because it defines forces from F=Ma.

Before numerical computing methods were readily available, acceleration was only attainable by differentiating cam lift profiles twice, once for velocity and again for acceleration. This generates so much hash (noise) that the second derivative (acceleration) was uselessly inaccurate. Computers permitted integration from the jerk curve, the third derivative of lift, that is conveniently a series of contiguous straight lines whose verticies can be adjusted to give any desired lift profile.

Integration of the jerk curves prduces a smooth acceleration curves while the third integral gives an essentially ideal lift curve (cam profile). With such cams, that mostly do not look like the ones "artists" formerly designed, valve noise (lift-off) went away and valve train elasticity came under scrutiny.

Today's cams have mirror image (symmetric) profiles with identical positive and negative acceleration while opening and closing valves. An asymmetric cam either opens or closes valves more slowly than it could, speed being limited by Hertzian contact stress between curved cam and flat tappet from accelerating the mass of valve, tappet and spring.

In contrast, desmodromic drive uses two cams per valve, each with separate rocker arm (lever tappets) whose mass and bending elasticity cancel supposed advantages. Maximum valve acceleration being limited by cam-to-tappet galling stress, is governed by moving mass and cam contact area. Rigidity and contact stress are best achieved with conventional flat tappets and springs whose lift and closure stress is unaffected by spring force, both occurring at the base circle ^[2] where spring load is minimum and contact radius is largest. Curved (lever) tappets ^[3] of desmodromic cams cause higher contact stress than flat tappets for the same lift profile, thereby limiting rate of lift and closure.

With conventional cams, stress is highest at full lift, when turning at zero speed (engine cranking), and diminishes with increasing speed as inertial force of the valve counter spring pressure, while a desmodromic cam has essentially no load at zero speed (in the absence of springs), its load being entirely inertial, and therefore increasing with speed. However, its greatest inertial stress bears on its smallest radius. Acceleration forces for either method increase with the square of velocity resulting from kinetic energy. ^[4]

Desmodromic valve drive was often justified by claims that springs could not close valves reliably at high speed and that the forces caused by suitably strong springs exceeded what cams could withstand. Since then, valve float was analyzed and found to be caused largely by resonance in valve springs that generated oscillating compression waves among coils, much like a Slinky. High speed photography showed that at specific resonant speeds, valve springs were no longer making contact at one or both ends, leaving the valve floating ^[5] before crashing into the cam on closure.

For this reason as many as three concentric valve springs, press fit into each other, were often used, not for more force (the inner ones having no significant spring constant), but to act as snubbers to reduce oscillations in the outer spring.

An early solution to oscillating spring mass was the mousetrap or hairpin spring ^[6] used on Norton Manx ^[7] engines. These avoided resonance but were ungainly to locate inside cylinder heads. Today, formula-one racing engines use gas springs that have no resonant parts, their working parts having an insignificant mass compared to the force of their compressed gas. These springs are expensive and short lived, therefore, offering no benefit for most motors.

Valve springs that do not resonate are progressive, wound with varying pitch or varying diameter called beehive springs^[8] from their shape. The number of active coils in these springs varies during the stroke, the more closely wound coils being on the static end, becoming inactive as the spring compresses or as in the beehive spring, where the small diameter coils at the top are stiffer. Both mechanisms reduce resonance because spring force and its moving mass vary with stroke. This advance in spring design removed valve float, the initial impetus for desmodromic valve drive.

Today, desmodromic valve drive is an anachronism that, with diligence, can be made to work but at significant cost and maintenance effort. Overhead cams using flat tappets and springs offer advantages over desmodromic drive that are apparent in current automotive engines. Only those in Ducati motorcycles use desmodromic drive. Reasons for not using a desmodromic approach are increased maintenance and valve noise which can be uncomfortably loud in engines with four or more

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