Four-stroke cycle engine valves

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Internal combustion engines using either four-stroke or two-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 is used to closed 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. Inertia of valve, tappet and spring at high cyclic rate can cause "float" or loss of contact with its camshaft, to cause mechanical failure or power loss through failure to close. Valves seal against a conical seat in the cylinder head that matches the angle of the valve periphery.

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] Why use desmodromic 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 speeds. 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.

Today most engines use overhead cams that drive flat tappets, thereby achieving the shortest and most inelastic path, with no bending elements between cam and valve. [2]. Computer analysis enabled designers to accurately assess acceleration, the most important dynamic of valve motion, because it defines forces from F=Ma. These cams have symmetric profiles with positive and negative acceleration of opening and closing valves a mirror image. An asymmetric cam either opens or closes valves more slowly than it could, speed being limited by cam contact force from accelerating valve, tappet and spring mass.

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 is limited by cam-to-tappet galling stress, which 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 [3] where spring load is minimum and contact radius is largest. Curved (lever) tappets 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, 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 bellows having an insignificant spring constant 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. That overhead cams using flat tappets and springs offer advantages over desmodromic drive is apparent in current automotive engines, none except Ducati motorcycles use desmodromic drive. Reasons for not using desmodromic valve drive besides having no advantage over conventional valve drive, are high maintenance, and for cars, valve drive noise for four or more cylinders becomes uncomfortably loud, especially without a loud exhaust as a mask.