Valvetronic system

The Valvetronic system is the first variable valve timing system to offer continuously variable timing (on both intake and exhaust camshafts) along with continuously variable intake valve lift, from ~0 to 10 mm, on the intake camshaft only. Valvetronic-equipped engines are unique in that they rely on the amount of valve lift to throttle the engine rather than a butterfly valve in the intake tract. In other words, in normal driving, the "gas pedal" controls the Valvetronic hardware rather than the throttle plate.

First introduced by BMW on the 316ti compact in 2001, Valvetronic has since been added to many of BMW's engines. The Valvetronic system is coupled with BMW's proven double-VANOS, to further enhance both power and efficiency across the engine speed range. Valvetronic will not be coupled to BMW's N53, "High Precision Injection" (gasoline direct injection) technology due to lack of room in the cylinder head, or the N54B30 bi-turbo engine. Cylinder heads with Valvetronic use an extra set of rocker arms, called intermediate arms (lift scaler), positioned between the valve stem and the camshaft. These intermediate arms are able to pivot on a central point, by means of an extra, electronicly actuated camshaft. This movement alone, without any movement of the intake camshaft, can open or close the intake valves.

Because the intake valves now have the ability to move from fully closed to fully open positions, and everywhere in between, the primary means of engine load control is transferred from the throttle plate to the intake valvetrain. By eliminating the throttle plate's "bottleneck" in the intake track, pumping losses are reduced, fuel economy and responsiveness are improved.

It is important to note however, that the throttle plate is not removed, but rather defaults to a fully open position once the engine is running. The throttle will partially close when the engine is first started, to create the initial vacuum needed for certain engine functions, such as emissions control. Once the engine reaches operating speed, a vacuum pump run off the passenger side exhaust camshaft (on the N62 V8 only) provides a vacuum source, much as a diesel engine would, and the throttle plate once again goes to the fully open position.

The throttle plate also doubles as an emergency backup, should the Valvetronic system fail. In this case, the engine would enter a "limp home" program, and engine speed would once again be controlled by the throttle plate.

Valvetronic has so far been limited to BMW's mass-market engines, with no high-performance M-series car using the technology. The Valvetronic hardware adds a great deal of mass to the valvetrain, limiting maximum engine speeds (~7,000 rpm peak rpm in N52) engines and making it unsuitable for the high-revving M engines.

Cylinder head porting

Cylinder head porting is the technology of modifying the intake and exhaust ports of an internal combustion engine to improve the quality and quantity of the gas flow. Cylinder heads as manufactured cannot be optimal due to design and manufacturing constraints. Porting the heads provides the finely detailed attention required to bring the engine to the highest level of efficiency. More than any other single factor porting technology is responsible for the high power output of modern engines.

This process can be applied to a standard racing engine to optimize its power output as well as to a production engine to turn it into a racing engine, to enhance its power output for daily use or to alter its power output characteristics to suit a particular application.

Daily human experience with air gives the impression that air is light and nearly non-existent as we move slowly through it. An engine running at high speed experiences a totally different substance. In that context, air can be thought of as thick, sticky, elastic, gooey and heavy (see viscosity). Pumping it is a major problem for engines running at speed. Porting helps engines deal with this problem.

Valve timing

In a piston engine, the valve timing is the precise timing of the opening and closing of the valves.

In four-stroke cycle engines and some two-stroke cycle engines, the valve timing is controlled by the camshaft. It can be varied by modifying the camshaft, or it can be varied during engine operation by the relatively new technology of variable valve timing. It is also affected by the adjustment of the valve mechanism, and particularly by the tappet clearance; This variation is normally unwanted.

Many two-stroke cycle and all wankel engines do not have a camshaft or valves, and the port timing can only be varied by machining the ports. Some supercharged two-stroke diesel engines do however have a cylinder head and camshaft similar to a four-stroke cycle engine.

In a steam engine, the control of the valve timing is an important part of the operation of the engine. See valve gear, and also Walschaert valve gear, Berry accelerator valve gear, Baker valve gear, Woolf valve gear, Caprotti valve gear, Corliss valve gear.

Multi-valve

In automotive engineering, an engine is referred to as multi-valve (or multivalve) when each cylinder has more than two valves.

All poppet valve, four-stroke internal combustion engines have at least two valves per cylinder — one for intake of air and fuel, and another for exhaust of combustion products. Adding more valves improves the flow of intake and exhaust gases, potentially improving combustion efficiency, power, and performance. It is not practical to simply use two larger valves because of the circular shape of the combustion chamber and the need for valves to also be round, which ensures they can only cover a fraction of the top of each cylinder; three (or more) smaller valves can replace the largest two valves which could be fitted into the space and result in having a greater effective valve area. Adding more valves per cylinder can improve breathing and thus allow an engine to run at a higher RPM, creating more power for a given displacement, though at a greater complexity and cost.

Most multivalve engines use an overhead camshaft to actuate the valves, and many use double overhead camshafts (DOHC). However this is not always the case: Chevrolet recently showed a 3-valve version of its Generation IV V8 which uses pushrods to actuate forked rockers, and Cummins makes a 4-valve pushrod straight-6 Diesel, the Cummins 600.

Starting in 1922, many of Bugattis engines began using 3 valves per cylinder actuated by a single-ovehead-cam (SOHC). Nissan has produced the 1988-96 KA24E engine with 3 valves per cylinder (two intakes, one exhaust) that are also actuated by (SOHC). Mercedes and Ford are currently producing V6 and V8 engines using this configuration. Ford claims an 80% improvement in high RPM breathing without the added cost of a second cam per bank of cylinders. The Ford design uses one spark plug per cylinder located in the center, but the Mercedes design uses two spark plugs per cylinder located on opposite sides, leaving the center free to add a direct-to-cylinder fuel injector at a later date. Thus there are many considerations to deciding how many valves an engine should have besides just the added cost verses adding breathing capability.

Some versions of the Honda D-series 4 cylinder engines and all J-series V6 and R-series 4 cylinder engines actuate 4 valves per cylinder with a single overhead cam.

Volkswagen, Audi, Ferrari and Yamaha have introduced engines in the past that had a double overhead cam operating 5 valves per cylinder (three intakes, two exhaust). Toyota's 1991-98 4A-GE 1.6-liter 4-cylinder engine also uses 5-valves-per-cylinder and was co-designed by Yamaha as well.

Maserati has produced a 2.0L turbo-V6 engine with 6 valves-per-cylinder (three intakes, three exhaust) (http://www.maserati-alfieri.co.uk/alfieri26.htm).

Engines with two or four valves per cylinder are by far the most common configurations. Four valve per cylinder engines are typically actuated by DOHC, and are too numerous to list.

PCV valve

The Positive Crankcase Ventilation valve, or PCV valve, is a one-way valve that ensures continual refreshment of the air inside a gasoline internal combustion engine's crankcase.


Explanation

As an engine runs, the crankcase (containing the crankshaft and other parts) begins to collect combustion chamber gases which leak past the rings surrounding pistons and sealing them to the cylinder walls. These combustion gases are sometimes referred to as "blow by" because the combustion pressure "blows" them "by" the pistons. These gases contain compounds harmful to an engine, particularly hydrocarbons, which are just unburned fuel, as well as carbon dioxide. It also contains a significant amount of water vapor. If allowed to remain in the crankcase, or become too concentrated, the harmful compounds begin to condense out of the air within the crankcase and form corrosive acids and sludge on the engine's interior surfaces. This can harm the engine as it tends to clog small inner passages, causing overheating, poor lubrication, and high emissions levels. To keep the crankcase air as clean as possible, some sort of ventilation system must be present.


PCV system

The PCV valve is only one part of the PCV system, which is essentially a variable and calibrated air leak, whereby the engine returns its crankcase combustion gases. Instead of the gases being vented to the atmosphere, gases are fed back into the intake manifold, to re-enter the combustion chamber as part of a fresh charge of air and fuel. The PCV system is not a classical "vacuum leak." Remember that all the air collected by the air cleaner (and metered by the mass air flow sensor, on a fuel injected engine) goes through the intake manifold anyway. The PCV system just diverts a small percentage of this air via the breather to the crankcase before allowing it to be drawn back in to the intake tract again. It is an "open system" in that fresh exterior air is continuously used to flush contaminants from the crankcase and into the combustion chamber.

The system relies on the fact that, while the engine is running, the intake manifold's air pressure is always less than crankcase air pressure. The lower pressure of the intake manifold draws air towards it, pulling air from the breather through the crankcase (where it dilutes and mixes with combustion gases), through the PCV valve, and into the intake manifold.

The PCV system consists of:
1) The breather tube , and
2) The PCV valve.
The breather tube connects the crankcase to a clean source of fresh air, such as the air cleaner body. Usually, clean air from the air cleaner flows in to this tube and in to the engine after passing through a screen, baffle, or other simple system to arrest a flame front, to prevent a potentially explosive atmosphere within the engine crank case from being ignited from a back-fire in to the intake manifold. The baffle, filter, or screen also traps oil mist, and keeps it inside the engine.

Once inside the engine, the air circulates around the interior of the engine, picking up and clearing away combustion byproduct gases, including a large amount of water vapor, then exits through a simple baffle, screen or mesh to trap oil droplets before being drawn out through the PCV valve, and into the intake manifold.


PCV valve

The PCV valve connects the crankcase to the intake manifold from a location more-or-less opposite the breather connection. Typical locations include the opposite valve cover that the breather tube connects to on a V engine. A typical location is the valve cover(s), although some engines place the valve in locations far from the valve cover. The valve is simple, but actually performs a complicated control function. An internal restrictor (generally a cone or ball) is held in "normal" (engine off, zero vacuum) position with a light spring, exposing the full size of the PCV opening to the intake manifold. With the engine running, the tapered end of the cone is drawn towards the opening in the PCV valve, restricting the opening proportionate to the level of engine vacuum vs. spring tension. At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. Sensing a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway.


Operation

Should the intake manifold's pressure be higher than that of the crankcase (which can happen under certain conditions, such as an intake backfire), the PCV valve closes to prevent reversal of the exhausted air back into the crankcase again. This is where the positive comes from in the name. Positive is basically a synonym for one-way.

It is critical that the parts of the PCV system be kept clean and open, otherwise air flow will not be correct. A plugged or malfunctioning PCV system will eventually damage an engine. PCV problems are primarily due to neglect or poor maintenance, typically engine oil change intervals that are inadequate for the engine's driving conditions. A poorly-maintained engine's PCV system will eventually become contaminated with sludge, causing serious problems. If the engine's lubricating oil is changed with adequate frequency, the PCV system will remain clear practically for the life of the engine. However, since the valve is constantly changing its resistance to flow by opening and closing proportionally as one drives a car, it is subject to eventual wear out over time. Typical maintenance schedules for gasoline engines are to replace the PCV valve whenever spark plugs are replaced. The long life of the valve despite the harsh operating environment is due to the trace amount of oil droplets suspended in the air that flows through the valve. These droplets keep the valve lubricated.

Not all gasoline engines have PCV valves. Engines not subject to emission controls, such as certain off-road engines, retain road draft tubes. Dragsters use a scavenger system and venturi tube in the exhaust to draw out combustion gases and maintain a small amount of vacuum in the crankcase to prevent oil leaks on to the race track. Small gasoline 2-cycle engines use the crank case to compress incoming air. All blow by in these engines is burned in the regular flow of air and fuel through the engine. Many small 4-cycle engines such as lawn mower engines and small gasoline generators, simply use a draft tube connected to the intake, between the air filter and carburetor, to route all blow by back in to the intake combustion air. The higher operating temperature of these small engines has a side effect of preventing large amounts of water vapor and light hydrocarbons from condensing in the lube oil.

Boost gauge

A boost gauge is a pressure gauge that indicates manifold air pressure or turbocharger or supercharger boost pressure in an internal combustion engine. They are commonly mounted on the dashboard, on the driver's side pillar, or in a radio slot.

Turbochargers and superchargers are both engine driven air compressors (exhaust driven and pulley driven, respectively) and provide varying levels of boost according to engine rpm, load etc. Quite often there is a power band within a given range of available boost pressure and it is an aid to performance driving to be aware of when that power band is being approached, in the same way a driver wants to be aware of engine rpm.

A boost gauge is mandatory when boost pressure is being modified to levels higher than OEM standard on a production turbocharged car. Simple methods can be employed to increase factory boost levels, such as bleeding air off the wastegate diaphragm to 'fool' it into staying open longer, or installing a boost controller. To avoid excessive leaning out of the engine (caused by increasing the boost beyond the fuel systems capacity) care must be taken to monitor boost pressure levels, along with oxygen levels in the exhaust gas, using an oxygen sensor.

A boost gauge will measure pressure in either psi or bar and many also measure manifold vacuum pressure also in inches of mercury (in. Hg).

4-stroke power valve system

4-stroke power valve system

A 4-stroke powervalve is a device fitted to 4 stroke engines that constantly adjusts the internal diameter of the exhaust system to perfectly suit engine revolutions.

This ensures superior low to mid-range performance (ca. 12-20% improvement), linear power output and reduced exhaust noise levels while the valve is in its reduced opening position.

* Yamaha EXUP (Exhaust Ultimate Power valve)
* Honda HTEV (Honda Titanium Exhaust Valve)
* Suzuki SET (Suzuki Exhaust Tuning)


EXUP

EXUP (EXhaust Ultimate Power valve) is a device fitted to selected Yamaha motorcycles (FZR,YZF,R series) that constantly adjusts the internal diameter of the exhaust system to suit engine revs. This ensures good low to mid-range performance for a linear power output all the way to the rev limiter. This is achieved by using an internal valve inside the exhaust at the point where the four pipes from the cylinders meet. Due to the high performance of these particular engines at high RPM, the exhaust valve inside the piston tends to open early. This is beneficial to the performance of the engine at high RPM, but detrimental to performance at low RPM. Closing of the valve then creates back pressure inside the exhaust system, forcing down the piston. A servo motor controlled by the Ignitor module opens and shuts the valve. The EXUP valve operation goes from being fully closed at idle speed, through to being fully open at 9000 to 11000 RPM.


HTEV

HTEV (Honda Titanium Exhaust Valve) is a device fitted to Honda engines that constantly adjusts the internal diameter of the exhaust system to suit engine revs. This ensures good low to mid-range performance for a linear power output all the way to the rev limiter.

Blowoff valve

A blowoff valve is a pressure release system present in turbocharged engines, its purpose is to prevent compressor surge and reduce wear on the engine.


Definitions

A compressor bypass valve (CBV) also known as a compressor relief valve is a vacuum-actuated valve designed to release pressure in the intake system of a turbocharged or centrifugally supercharged car when the throttle is lifted or closed. This air pressure is re-circulated back into the non-pressurized end of the intake (before the turbo) but after the mass airflow sensor.

A blowoff valve, (BOV, sometimes hooter valve, dump valve) does basically the same thing, but releases the air to the atmosphere. This creates a very distinctive sound desired by many who own turbocharged sports cars. Some blowoff valves are sold with trumpet shaped exits that amplify the "Psshhhh" sound, these designs are normally marketed towards the keen boy racer. For some owners this is the only reason to fit a BOV. Motor sports governed by the FIA have made it illegal to vent unmuffled blowoff valves to the atmosphere. In the United States, Australia and Europe cars featuring unmuffled blowoff valves are illegal for street use.



Downsides of releasing air to atmosphere

This unique sound sometimes comes at a price. On a car with a mass airflow sensor, doing this confuses the engine control unit (ECU) of the car. The ECU is told it has a specific amount of air in the intake system, and injects fuel accordingly. The amount of air released by the blowoff valve is not taken into consideration and the engine runs rich for a period of time.*

Typically this isn't a major issue, but sometimes it can lead to hesitation or stalling of the engine when the throttle is closed. This situation worsens with higher boost pressures. Eventually this can foul spark plugs and destroy the catalytic converter (when running rich, not all the fuel is burned which can heat up on and melt the converter).

* Note that engines using a MAP (manifold absolute pressure) system are not affected.


Purpose of Relief and Blow Off Valves

Blowoff valves are used to prevent compressor surge. Compressor surge is a phenomenon that occurs when lifting off the throttle of a turbocharged car (with a non-existent or faulty bypass valve). When the throttle plate on a turbocharged engine running boost closes, high pressure in the intake system has nowhere to go. It is forced to travel back to the turbocharger in the form of a pressure wave. This results in the wheel rapidly decreasing speed and stalling. The driver will notice a fluttering air sound. In extreme cases the compressor wheel will stop completely or even go backwards. Compressor surge is very hard on the bearings in the turbocharger and can significantly decrease its lifespan. In addition, the now slower moving compressor wheel takes longer to spool (speed up) when throttle is applied. This is known as turbo lag.

With the implementation of either a bypass valve or a blowoff valve the pressurized air escapes, allowing the turbo to continue spinning. This allows the turbocharger to have less turbo lag when power is demanded next.



How it works




A blow-off-valve is connected by a vacuum hose to the intake manifold after the throttle plate. When the throttle is closed, underpressure develops in the intake manifold after the throttle plate and "sucks" the blowoff valve open. The excess pressure from the turbocharger is vented into the atmosphere or recirculated into the intake upstream of the compressor inlet.



Tuning adjustable valves

Most aftermarket valves are adjustable leaving customers curious on how to set them properly for their vehicle. Typically the adjustment lies in the spring preload. Here is how to set it.

You want the spring as soft as possible without leaking boost at peak pressure. If the spring is set too soft then the valve will not close fully resulting in a boost leak and idle problems. If you set it too hard then the valve will not fully open, close too early, and have compressor surge.

Trial and error with an accurate boost gauge is the perfect way to find the right setting for your vehicle....

Overhead valve

In automotive engineering, an overhead valve internal combustion engine is one in which the intake and exhaust valves and ports are contained in the cylinder head.

The original overhead valve or OHV piston engine was developed by the Scottish-American David Dunbar Buick. It employed pushrod-actuated valves parallel to the pistons and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves.

Today the technology is widespread, and the term, "OHV", is generally used to differentiate a pushrod engine from one which uses overhead cams, although both types employ overhead valves and so are both OHV engines.

Sleeve valve

The sleeve valve is a type of valve mechanism for internal combustion piston engines which have traditionally relied on the more common poppet valve.

A sleeve valve consists of one or more machined sleeves that fit within a piston engine's cylinders and are designed so as to move so that their openings align with the cylinder's inlet and exhaust ports at the appropriate stages in the engine's cycle.

Sleeve valves saw use in some pre-World War II luxury, sports cars and the Willys-Knight car and light truck, and saw substantial use in 1940s aircraft engines such as the Napier Sabre and Bristol Hercules and Centaurus, but they subsequently fell from use due to advances in poppet-valve technology (sodium cooling) and to their tendency to burn considerable amounts of lubricating oil or to seize due to lack of it.



Disadvantages of poppet valves

In a standard internal combustion engine, the poppet valves are opened by the a shaped cam acting on the top of the valve, while the valves are closed by a spring wrapped around the valve stem.

The main problem with this system is that as the RPM of the engine increases, the speed at which the valve moves also increases, increasing the loads involved due to the inertia of the valve, which has to be opened quickly, brought to a stop, then reversed in direction and closed and brought to a stop again. Large valves that allow good air-flow have considerable mass and require a strong spring to overcome the opening inertia. At some point, the valve inertia overwhelms the spring and stops following the cam profile, closing well after the cam lobe has moved away. This "valve float" can eventually cause the valve to not close at all before the cam comes around to open it again and in some engines the piston may even collide with the valve.

The desmodromic system as used by Ducati in some of its motorcycle engines uses mechanical methods to close the valve, but this system requires precision engineering and is markedly more expensive than spring-closed valves.


Sleeve valve description

As its name implies, the sleeve valve is constructed as one or more sleeves that fit around the piston inside the cylinder wall. Ports (holes) in the side of the cylinder replace the more normal intake and exhaust ports on the head, and similar apertures in the sleeve(s) open and close the ports by being rotated into position.

In some engines each sleeve has a gear ring on the bottom that runs in a channel and a small cut in the cylinder wall exposes the gear so that the sleeve can be turned, alternatively the sleeves are operated by a crank driven from the crankshaft, with the sleeve moving in a circular path opening the cylinder ports in the upper part of the circle.

The advantage of the sleeve valve is that very large port openings can be arranged that increase the volumetric efficiency of the cylinder and the combustion chamber formed with the sleeve at the top of its stroke is almost perfect for complete, and detonation-free, combustion of the charge.

Another design involves a reduced height sleeve placed beneath the cylinder head. This has the advantage of being easier to construct, as it does not need to be strong enough to withstand the forces generated by a piston moving within it.


Advantages

No springs are involved in the sleeve valve system, therefore the power needed to operate the valve remains largely constant with the engine's RPM meaning that the system can be used at very high speeds with no penalty for doing so. In addition, the camshaft, pushrods, or rockers can be dispensed with, as the sleeve valves are generally driven by a single gear running directly off the driveshaft. For an aircraft engine this produced desirable reductions in weight and complexity.

An additional advantage of the system is that the size of the ports can be readily controlled. This is of importance when an engine runs over a wide range of RPM, as the speed at which air can enter and exit the cylinder is defined by the size of the duct leading to the cylinder and varies according to the cube of the RPM. In other words, at higher RPM the engine typically requires larger ports that remain open for a greater proportion of the cycle, something that is fairly easy to arrange with sleeve valves, but prohibitively difficult in a poppet valve system.

A minor advantage includes the fact that the cylinder head is not required to house valves, therefore allowing the sparkplug to be placed in the best possible location for efficient ignition of the combustion mixture.


Disadvantages

The sleeve valve has one major disadvantage, in that perfect sealing is difficult. In a poppet valve engine the piston possesses piston rings (often at least 3 and sometimes as many as 8) which form a seal with the cylinder bore, and during the "breaking in" period any imperfections in one are scraped into the other resulting in a good fit. This type of "breaking in" is not possible on a sleeve valve engine however, because the piston and sleeve move in different directions and in some systems even rotate in relation to one another. In the 1940s this was not a major concern because the poppet valves of the time typically leaked appreciably more.


Modern usage

The sleeve valve has begun to make something of a comeback, due to modern materials and newer and dramatically better engineering tolerances and construction techniques which produce a sleeve valve that leaks very little oil. However, most advanced engine research is concentrated on entirely different designs of internal combustion engine such as the rotary engine, as opposed to improvements to existing engines such as the sleeve valve.


History

The sleeve valve principle was invented in 1903 by the American inventor Charles Yale Knight. Although he was initially unable to sell his Knight Engine in the US, a trip to Europe secured several luxury car firms as customers willing to pay his expensive premiums. He first patented the design in Britain in 1908.

Among the companies using Knight's technology were Gabriel Voisin (in his Avions Voisin cars), Daimler (in their V-12 'Double Six'), and Belgium's Minerva company.

Upon Knight's return to America he was able to get some firms to use his design; here his brand name was Silent Knight (1905-1907) — the selling point was that his engines were quieter than those with standard poppet valves. The best known of these were the Stearns Company of Cleveland, which sold a car named the Stearns-Knight, and the Willys firm which offered a car called the Willys-Knight.

A number of sleeve valve aircraft engines were developed following a seminal 1927 research paper from the RAE by Harry Ricardo. This paper outlined the advantages of the sleeve valve, and suggested that poppet valve engines would not be able to offer power outputs much beyond 1500 hp (1,100 kW). Napier and Bristol began the development of sleeve valve engines that would eventually result in two of the most powerful piston engines in the world, the Napier Sabre and Bristol Centaurus.

Following World War II the sleeve valve disappeared from use, as the previous problems with sealing and wear on poppet valves had been remedied by the use of better materials, and the inertia problems with the use of large valves were reduced by using several smaller valves instead, giving increased flow area and reduced mass.

Poppet valve

A poppet valve is a valve consisting of a hole, usually round or oval, and a tapered plug, usually a disk shape on the end of a shaft also called a valve stem. The shaft guides the plug portion by sliding through a valve guide. In most applications a pressure differential helps to seal the valve and in some applications also open it.

Presta and Schrader valves used on tires are examples of poppet valves. The Presta valve has no spring and relies on a pressure differential for opening and closing while being inflated.

Poppet valves are used in many industrial process from controlling the flow of rocket fuel to controlling the flow of milk.


Internal combustion engine

Poppet valves are used in most piston engines to open and close the intake and exhaust ports in the cylinder head. The valve is usually a flat disk of metal with a long rod known as the valve stem out one end. The stem is used to push down on the valve and open it, with a spring generally used to close it when the stem is not being pushed on. Desmodromic valves are closed by positive mechanical action instead of by a spring, and are used in some high speed motorcycle and auto racing engines, eliminating 'valve float' at high RPM.

For certain applications the valve stem and disk are made of different steel alloys, or the valve stems may be hollow and filled with sodium to improve heat transport and transfer.

The engine normally operates the valves by pushing on the stems with cams and cam followers. The shape and position of the cam determines the valve lift and when and how quickly (or slowly) the valve is opened. The cams are normally placed on a fixed camshaft which is then geared to the crankshaft, running at half crankshaft speed in a four-stroke engine. On high performance engines e.g. used in Ferrari cars, the camshaft is moveable and the cams have a varying height, so by axially moving the camshaft in relation with the engine RPM, also the valve lift varies. See variable valve timing.




Components of a typical, four stroke cycle, DOHC piston engine. (E) Exhaust camshaft, (I) Intake camshaft, (S) Spark plug, (V) Valves, (P) Piston, (R) Connecting rod, (C) Crankshaft, (W) Water jacket for coolant flow.



Valve position

In very early engine designs the valves were 'upside down' in the block, parallel to the cylinders - the so called L-head engine because of the shape of the cylinder and combustion chamber, also called 'flathead engine' as the top of the cylinder head is flat. Although this design makes for simplified and cheap construction, it has two major drawbacks; the tortuous path followed by the intake charge limits air flow and effectively prevents speeds greater than 2,000-2,500 RPM, and the travels of the exhaust through the block lead to excessive overheating under sustained heavy load. This design therefore evolved into 'Intake Over Exhaust', IOE or F-head, where the intake valve was in the block and the exhaust valve was in the head; later both valves moved to the head.

In most such designs the camshaft remained relatively near the crankshaft and the valves were operated through pushrods and rocker arms. This led to significant energy losses in the engine, but was simpler, especially in a V engine where one camshaft can actuate the valves for both cylinder banks; for this reason, pushrod engine designs persisted longer in these configurations than others.

More modern designs have the camshaft on top of the cylinder head, pushing directly on the valve stem (again through cam followers), a system known as overhead camshaft; if there is just one camshaft, this is a single overhead cam or SOHC engine. Often there are two camshafts, one for the intake and one for exhaust valves, creating the dual overhead cam, or DOHC. The camshaft is driven by the crankshaft - through gears, a chain or in modern engines with a rubber belt.



Valve wear

In the early days of engine building, the poppet valve was a major problem. Metallurgy was not what it is today, the rapid opening and closing of the valves against the cylinder heads led to rapid wear. They would need to be re-ground every two years or so, in an expensive and time consuming process known as a valve job. Adding tetra-ethyl lead to the petrol reduced this problem to some degree as the lead would coat the valve seats, hardening the metal. Valve seats made of improved alloys such as stellite have generally made this problem disappear completely and making leaded fuel unnecessary.

Gnome Monosoupape

The Monosoupape, French for single-valve, was a particular engine design used by Gnome et Rhône's later rotary engines. It used a clever arrangement of internal ports and a single valve to replace a large number of parts normally found on a conventional arrangement, and made the Monosoupape engines some of the most reliable of the era.

Earlier Gnome (as opposed to Le Rhône) designs used a unique arrangement of valves in order to avoid needing pushrods and other complex devices to operate the engine cycle. Instead a single exhaust valve on the cylinder head was operated by a counterweight that opened the valve when the pressure dropped at the end of the power stroke. The intake valve was operated similarly, but placed right in the middle of the piston head, where it opened to allow the charge to enter through a hollow crank from the center of the engine. Although clever, the system had several drawbacks. One was that maintaining the intake valve, which could easily become jammed, required the cylinder heads to be removed. Another was that in order to get the timing and pressures right for the rod-less operation, the valves opened at times that were not all that efficient; the Gnome's had even poorer fuel economy than other rotaries, which were bad enough.

Beginning with the power stroke, the four-stroke engine operated normally until the piston was just about to reach the bottom of its stroke (bottom dead center, or BDC), when the exhaust valve was opened "early". This let the still-hot fuel "pop" out of the engine while the piston was still moving down, relieving exhaust pressure and preventing exhaust gases from entering the crankcase. After a small additional amount of travel, the piston uncovered 36 small ports around the base of the cylinder, leading to the crankcase which held additional fuel/air mixture (the charge). No transfer took place at this point since there was no pressure differential, the cylinder was still open to the air and thus at ambient pressure. The overhead valve exhausted directly into the slipstream since there was no exhaust manifold in order to save weight.

The piston completed its exhaust stroke until top dead center (TDC) was reached, but the valve did not close. By being open to the slipstream, total scavenging occurred as the air moving past the cylinder created a partial vacuum inside. The piston began to move down on its intake stroke with the valve still open, pulling fresh (presumably un-filtered) air into the cylinder. It remained open until it was two-thirds of the way down, at which point the valve closed and the remainder of the intake stroke caused a partial vacuum to form in the cylinder. When the piston uncovered the transfer ports it sucked the balance of the charge as a result of the partial vacuum in the cylinder and the atmospheric pressure in the crankcase.

The charge was an overly rich mixture of fuel and air, which was acquired through the hollow crankshaft, and fuel that was continuously injected by a fuel nozzle on the end of a fuel line, entering the crankcase through the hollow crankshaft. The nozzle was in the proximity of, and aimed at, the inside base of the cylinder where the transfer ports were located. The fuel nozzle was stationary with the crankshaft, and the cylinders rotated into position in turn. The compression stroke was conventional.

The spark plug was installed horizontally into the rear of the cylinder at the top but had no connecting high-voltage wire. An internal-tooth ring gear mounted on the engine drove a stationary magneto mounted to the firewall, whose high-voltage output terminal passed in close proximity by the spark plug terminals. This arrangement eliminated the need for points, distributor, high-voltage wiring and capacitors. This ring gear also drove the oil pump, which supplied oil to all bearings, and through hollow push rods to the rockers and valves. This ring gear also drove the air pump that pressurized the fuel tank as an early form of fuel injection. There was no carburetor, saving more weight.

With no carburetor or throttle, and constant fuel pressure, there were only two power settings: full throttle or none; the engine did not even have the ability to idle. Like most rotaries, the Monosoupape's were equipped with a "blip switch" that could cut the ignition. This had to be used sparingly, as the engine would continue to pull fuel into the crank and cylinders, so turning the ignition back on after too long a period could cause the engine to explode.

Because the entire engine rotated, it had to be precisely balanced. So castings and forgings could not be used, instead, precision machining of all parts was made necessary. As a result, Monosoupapes were extremely expensive to build, the 100 hp models costing $4,000 in 1916, about $65,000 in year 2000 dollars.


Motorcycle use

From 1921 to 1924, the German Megola motorcycle was produced that featured a monosoupape rotary engine mounted within the front wheel.