Aircraft Engine Valve - Modern combustion engines can use a different number of valves per cylinder depending on the design and the application. Smaller engines such as those used in lawnmowers may have only a single intake valve and one exhaust valve.
Larger vehicle engines such as 4-, 6- or 8-cylinder engines may use four valves per cylinder or sometimes five. Unburned byproducts of combustion frequently leave red or orange deposits on the valve face. These colorful buildups tend to be thickest at the center of the valve (where the temperature is relatively cool).
Aircraft Engine Valve
The cooler the valve, and the richer the fuel/air mixture, the thicker the deposits are going to be. Engine valves are common to many types of combustion engines, whether they run off a fuel such as gasoline, diesel, kerosene, natural gas (LNG), or propane (LP).
Engine Valve Specifications
Engine types vary by the number of cylinders which are the combustion chambers that generate power from the ignition of fuel. They also vary by the type of operation (2-cycle or 4-cycle), and by the design placement of the valves within the engine [overhead valve (OHV), overhead cam (OHC), or valve in block (VIB)]
. Aircraft engines always have two separate ignition systems in order to increase redundancy and reduce the risk of an engine failure during a critical phase of flight. Since the dual ignition systems are kept completely separate, each cylinder will have two spark plugs installed – one for each system.
Typical engine valves are specified by the parameters outlined below. Note that this data is intended for information purposes and be aware that variations in the parameters used for specifying engine valves may exist from manufacturer to manufacturer.
By understanding the specifications, buyers are better equipped to engage in discussions of their specific needs with suppliers of engine valves. On aircraft engines the valves are not operated directly through contact with the cam but rather through a pushrod and rocker system that connects the cam to the valve.
Engine Valve Nomenclature
This system allows for a clearance or gap to be present between the rocker arm and the tip of the valve. This clearance is essential as the temperature of the engine varies during operation which causes the valve to expand at higher temperatures.
Without a clearance between the valve tip and the rocker, temperature increases would result in the valves opening late or closing prematurely which would cause the engine to run poorly and lose power. The clearance can be adjusted, usually through the adjustment of a screw on the rocker assembly.
Hollow engine valves are a special bimetallic valve that contains a hollow cavity that is filled with sodium. The sodium liquefies as the valve temperature rises and is circulated by the motion of the valve, which helps dissipate heat from the hotter valve head.
The hollow design facilitates greater heat transfer through the stem than with solid valves because the martensitic stem material is a better conductor of heat than the austenitic head material. Hollow valves are especially suited for use in modern engines that are delivering more power out of smaller, denser engine designs that have higher exhaust gas temperatures which solid valves are not capable of handling.
Reciprocation Into Rotation
These higher exhaust temperatures are the result of several conditions, including: Spark plugs are classified as being either hot or cold plugs. The ceramic insert on a hot plug has a smaller area of contact with the metal part of the plug than a cold plug.
Hot plugs therefore dissipate heat more slowly than a cold plug and are better suited to running in cooler lower compression engines. Conversely cold plugs are better suited to operating in hotter, higher compression engines as they are able to dissipate heat more effectively.
There are several other types of engine valve designs. So-called sleeve valves consist of a tube or sleeve that sits between the cylinder wall and the piston, and which slide or rotate driven off a camshaft as with other engine valves.
The movement of the sleeve valve causes ports that are cut into the sleeve to align with corresponding ports in the cylinder wall at different points in the engine cycle, thus functioning as a simple engine intake and exhaust valve without the complexities of rocker arms and lifters.
Inline Engine
First, the keys to exhaust valve longevity are: Precise contact between the valve face and the valve seat, and a good fit between the valve stem and the valve guide. In flight, the pilot must keep cylinder head temperatures within normal limits to avoid damage.
Radial engines were commonly used on larger aircraft where multiple banks of pistons could be installed to produce an engine with a large power output while maintaining as compact an engine as possible. World War II era aircraft such as the Republic P-47 Thunderbolt, Douglas C 47, and Avro Lancaster all made use of radial engines.
Large radial engines were largely rendered obsolete after the Second World War as jet engines and gas turbine engines could produce greater power, more reliably with a lighter overall mass. Most engine valves are designed as poppet-style valves because of their up and down popping motion and feature a conical profile valve head that fits against a machined valve seat to seal off the passage of fluids or gases.
They are also called mushroom valves because of the distinctive shape of the valve head. Figure 1 shows the nomenclature for the different elements in a typical engine valve. Oil control rings are located below the compression rings.
Connecting Rod
The purpose of these rings is to allow oil to circulate from inside the piston to the cylinder walls. This circulation is accomplished through a set of small oil drain holes. An aircraft in straight and level flight is subjected to four fundamental forces which must be balanced for the aircraft to remain in equilibrium.
The weight of the aircraft is balanced by the lift produced by the wing and horizontal stabilizer in the vertical direction. As the airplane moves through the air, a resistance or drag force is produced which must be counteracted to maintain the forward flying speed.
This counterbalance to drag is called the thrust force and is generated by the engine-propeller combination. In normal operation, a crankshaft in the engine to which the pistons are attached is tied to a camshaft as part of a valve train arrangement for the engine.
The movement of the crankshaft transfers motion to the camshaft through a timing chain, timing belt, or other geared mechanism. The timing and alignment between the position of the crankshaft (which establishes the position of the piston in the cylinder) and the position of the camshaft (which determines the position of the valves for the cylinder) are critical not only for peak engine performance but also
To Learn More Download The Poster Below
to prevent interference between pistons and valves in high compression engines. Oil scraper rings are located near the bottom of the piston and are shaped such that they are able to scrape oil up and down the cylinder during the piston's motion.
Surplus oil is retained during the up-stroke and then returned to the crankcase during the downward stroke. The crankcase is the name given to the housing that contains the crankshaft and the connecting rods which connect the piston to the crankshaft.
The crankcase on an aircraft engine is usually made from a cast or forged aluminum. This provides sufficient strength and rigidity to hold the crankshaft in place while retaining the mass advantages of aluminum over steel.
Pistons are usually cast from aluminum alloys. On higher performance applications (typically racing engines), the piston may be forged rather than cast. The piston does not directly contact the cylinder but a gas seal between the cylinder wall and the piston is maintained through the use of piston rings and oil lubrication.
Engine Valve Motion
These rings are mounted in groves cut into the piston and are manufactured from cast iron. There are generally multiple piston rings installed, located just below the piston crown. Three different types of rings are commonly installed on an aircraft piston: compression rings, oil control rings, and oil scraper rings.
Inline engines are characterized by a vertical cylinder arrangement built in a single line along the crankcase. One advantage of this layout is the low frontal area that the engine presents to the oncoming air. A low frontal area means that the engine cowling can be made smaller which reduces the aircraft's drag signature.
The connecting rod (con rod) is the metallic component that forms the link between a piston and the crankshaft. The connecting rods and the crankshaft convert the reciprocating motion of the piston into rotational motion at the crankshaft which is then used to drive a propeller and produce thrust.
The intake valves on internal combustion engines are subjected to less thermal stress because of the cooling effects of the incoming air/fuel mixture that passes through the valve during the intake cycle. Exhaust valves, by contrast, are exposed to higher levels of thermal stress by being in the path of the exhaust gases during the exhaust cycle of the engine.
Engine Valve Materials
In addition, the fact that the exhaust valve is open during the exhaust cycle and not in contact with the cylinder head means the smaller thermal mass of the combustion face and valve head has a greater potential for a rapid temperature change.
The camshaft is manufactured with a number of cams or lobes where each cam is located above a valve and drives the motion of that valve. The shape of the cam determines how the valve opens and shuts and the orientation of the lobe determines the sequence in which the valve operates.
It is easiest to visualize this motion by referring to the animation below. Normal buildups on the valve face tend to be red, orange, or gray in color. The valve face can turn yellow or green when the temperature becomes abnormally high.
When a portion of the exhaust valve itself turns green, it means the valve is severely burned and weakened, and the valve should be replaced immediately. In this case, "Green means stop." Horizontally opposed engines can be made shorter than an equivalent inline engine as the cylinders are placed in two banks rather than one.
Piston Engine Layouts
However, with this arrangement, the engine is wider and has to be manufactured with two separate cylinder heads rather than one. In a dry sump arrangement, the oil is not stored in the crankcase but rather in a separate external reservoir.
The engine lubrication system is discussed in more detail in a post dedicated to engine lubrication and cooling. When an exhaust valve doesn't seat properly, ultra-hot gases can leak around the thin valve rim and create hot spots.
The first indication of a problem is usually an asymmetric pattern of deposits on the valve face. This article presented a summary of engine valves, including what they are, key nomenclature, how they work, valve operation, materials, types, and specifications.
For information on other topics, consult our additional guides or visit the Thomas Supplier Discovery Platform where you can locate potential sources of supply for over 70,000 different product and service categories. Engine lubricating oil is stored at the bottom of the crankcase in a wet sump engine arrangement.
Radial Engine
The oil is cycled through the engine, lubricating the crankshaft, connecting rod bearings and other metallic components. The oil finds its way onto the cylinder walls, passing through the pistons, before draining back into the crankcase.
Maintaining the proper valve clearance between the valve stem and the rocker arm or cam is extremely important for the proper operation of the valves. Some minimal clearance is needed to allow for the expansion of metal parts as the engine temperature rises during operation.
Specific clearance values vary from engine to engine, and failure to maintain proper clearance can have serious consequences to engine operation and performance. If the valve clearance is too large, then the valves will open later than optimally and will close sooner, which can reduce engine performance and increase engine noise.
If the valve clearance is too small, valves will not close fully, which can result in a loss of compression. Hydraulic valve lifters are self-compensating and can eliminate the need for valve clearance adjustments. Exhaust valves burn when they fail to seat properly and, as a result, can't efficiently transfer heat to the cylinder.
Engine Operation
Ultra-hot gases scorch these valves starting at their thin rims. This damage erodes and weakens the valves, exacerbates the poor seating problem, and dramatically accelerates wear. Without intervention, the exhaust valve can fail with potentially disastrous consequences.
Burnt exhaust valves have long been a leading cause of cylinder failures and power loss in piston aircraft engines. Modern borescopes allow us to look deep inside cylinders--and this guide will help you interpret what you see through the viewfinder.
Finally, two springs are incorporated into each valve to assist in closing the valve quickly and damping out any valve bounce that may occur due to the vibrations inherent in the running of an internal combustion engine.
Valves must be able to retain their strength and shape at high temperatures and so are usually built from high strength steels. Exhaust valves are usually smaller than inlet valves so as to reduce the possibility of pre-ignition or knocking.
V-Type Engine
The exhaust valve is usually the hottest part of the engine and a smaller valve reduces the likelihood that the high temperatures could cause the fuel/air mixture added during the intake stroke to ignite prematurely. Exhaust valves are designed to rotate, and this rotation is essential to valve longevity since it helps prevent deposits from building up around the seat, stem, and guide.
At a typical 2,400-rpm cruise, each exhaust valve opens and closes 1,200 times a minute, and each valve rotates once per minute. Engine valves are one of the components in internal combustion engines that are highly stressed.
The need for reliable engine operation dictates that engine valves be capable of exhibiting resistance to repeated and continuous exposure to high temperature, high pressure from the combustion chamber, and mechanical loads and stresses from the engine dynamics.
Spark Plugs
aircraft engine valve spring compressor, aircraft fuel valve, radial aircraft engine valve train, fuel selector valve aircraft, aircraft schrader valve, aircraft fuel strainer valve, air valves types, aircraft pressure relief valve