Exhaust valve for internal combustion engine

A low-friction, extended-life exhaust valve has a coating on the valve stem that is protected from hot exhaust gases. The valve stem slides axially in a valve guide pressed into the cylinder head, and the coating on the valve stem is receded away from the end of the valve guide exposed to the hot exhaust gases passing through the exhaust port. Protecting the coating from the hot exhaust gases allows the use of coatings that cannot normally withstand the harsh environment to which the exhaust valve is exposed. The preferred coating is a molybdenum oxide coating, which not only retains lubricating oil because of its porous nature, but also has solid lubricity which is useful in case of hydrostatic breakdown of the lubricant between engine cycles. It is preferred that the non-coated portion of the valve stem, which is located towards the end of the valve guide where the hot exhaust gases are exposed, be recessed so that the bore of the valve guide contacts only the coated portion of the valve stem, thereby further reducing wear. Other embodiments of the invention involve various coatings and configurations for the valve guide as well as the valve stem.

FIELD OF THE INVENTION 
The invention relates to valves for internal combustion engines, and in 
particular to a low-friction exhaust valve in which a solid coating on the 
valve stem is protected from hot exhaust gases. 
BACKGROUND OF THE INVENTION 
The invention is primarily directed to improving the durability of exhaust 
valves and valve guides in internal combustion engines, and is especially 
well suited for use in large industrial combustion engines fueled by 
natural gas. Large natural gas internal combustion engines are typically 
used to produce electrical power or propel ships, etc. Exhaust valves in 
these large engines are expected to last from 15,000 to 20,000 operating 
hours. Some manufacturers have gone to great lengths to provide 
sophisticated lubrication systems to improve longevity of exhaust valves. 
Large internal combustion engines typically have exhaust valves coveting 
the exhaust ports which are opened to allow exhaust from the cylinders to 
escape through the exhaust ports into the engine exhaust manifold. Each 
exhaust port typically passes through the cylinder head. Each exhaust 
valve consists of a valve head which covers the exhaust port, and a 
perpendicular exhaust stem. The exhaust valve stem is supported radially 
within a valve guide that is either cast into or pressed into an opening 
in the cylinder head. There is normally only a small clearance between the 
bore of the valve guide and the valve stem (e.g., 3.2 to 4.3 thousandths 
of an inch clearance). The small clearance allows very little wobble, but 
is enough to maintain an oil film to keep the stem from sticking within 
the bore given manufacturing tolerances and thermal expansion which can 
occur. 
To improve the durability of the interface between the valve stem and the 
valve guide bore, it is common to use a chrome or other hard, high 
temperature coating on the stem. Chrome coatings resist wear well. 
However, it is still desirable to maintain an oil film on the interface of 
the sliding surface between the valve stem and the valve guide to prevent 
metal-to-metal contact which tends to wear both metals. Chrome coatings 
for exhaust valves are not particularly well suited for maintaining a 
sufficient film of lubricating oil in reciprocating internal combustion 
engines. This is primarily due to the fact that exhaust valves are 
stationary almost three quarters of the time that the engine is operating. 
It is therefore difficult to maintain a hydrodynamic oil film because 
there is adequate time for the oil film to break down when the valve is 
not moving. Therefore, lubricating oil is not generally present at all 
contact areas between the valve stem and the bore in the valve guide when 
the valve starts to move. 
Some coatings are more porous than chrome and help retain oil at the 
sliding surface. Nitriding is an example of a porous coating. However, 
even with porous coatings, the oil film can breakdown when the valve stem 
is not moving, thus allowing metal-to-metal contact at least occasionally. 
It is therefore desirable to use porous and even non-porous solid 
lubricant coatings. A solid lubricant coating is a coating having solid 
lubricity, i.e., the ability to reduce friction is inherent within the 
solid coating. 
Molybdenum and molybdenum oxide are examples of solid lubricant coatings, 
however, neither can withstand the high temperatures of the harsh 
environment present in the exhaust port when the exhaust valve opens. The 
exhaust valve leads a particularly severe life because it is open at a 
time in the combustion cycle when exhaust gases are approximately 
1100.degree. F. or higher. In addition, the hot exhaust gases passing 
through the exhaust port pass the exhaust valve at a high velocity. In 
this environment, many coatings are not capable of surviving, including 
molybdenum oxide coatings as well as other solid lubricant coatings. 
Although many of these coatings can survive on intake valves, the 
environment on the exhaust valve is too harsh. These coatings tend to 
erode rapidly and flake into small particles where exposed to exhaust gas. 
The small particles can scratch the valve stem, and can possibly get stuck 
in the exhaust valve seat area which could hold the valve partially open 
and possibly burn the valve. 
Exhaust valve and guide wear has become an important problem, and some 
manufacturers have even produced sophisticated forced lubrication systems 
including channels, etc. to extend the life of the exhaust valves and 
guides. Exhaust valve and guide wear problems are more prominent in 
natural gas engines because the fuel itself contains no lubricity. 
Gasoline and diesel fuel have more lubricity in the liquid states, and 
also form more particulate and ash that can act as a solid lubricant. 
It can therefore be appreciated that it would be desirable to provide a 
cost effective way to reduce the wear of exhaust valves and valve guides, 
especially in large industrial natural gas internal combustion engines. 
SUMMARY OF THE INVENTION 
The invention provides a cost effective way to improve the life of exhaust 
valves and guides in large natural gas internal combustion engines by 
using a solid lubricant at the interface between the valve stem and the 
valve guide, and protecting the solid lubricant coating from the hot 
exhaust gases exiting the exhaust port in the cylinder head. The invention 
preferably uses a molybdenum oxide coating on the valve stem and recedes 
the coating from the end of the valve guide exposed to the hot exhaust 
gases passing through the exhaust port. Molybdenum oxide is the preferred 
coating not only because it is a solid lubricant, but also because it can 
be applied with sufficient porosity to retain liquid lubricant along the 
sliding surface between the valve stem and the valve guide. 
To further improve the wearability of the exhaust valve and guide, it is 
preferred that the non-coated portion of the valve stem be recessed so 
that the valve guide contacts only the coated portion of the valve stem 
when the valve is opened and closed. 
In the preferred embodiment, the invention is an internal combustion engine 
that includes a cylinder head having an exhaust port and a valve guide 
pressed into an opening in the cylinder head. The valve guide has an 
internal bore with one of its ends exposed to the hot exhaust gases 
passing through the exhaust port. The engine has an exhaust valve with a 
head sized to cover the exhaust port, and a stem slidably mounted in the 
bore of the valve guide so that the valve stem can be moved axially within 
the valve guide to open and close the valve head over the exhaust port. 
The valve stem has a coated portion, preferably coated with molybdenum 
oxide, the coated portion is supported laterally by the bore of the valve 
guide. The coated portion is receded from the end of the valve guide 
exposed to the hot exhaust gases passing through the exhaust port when the 
valve is open. In a large natural gas engine, the coating is preferably 
receded so that it is no closer than approximately 4 to 6 millimeters from 
the end of the valve guide exposed to the hot exhaust gases when the valve 
is open. 
To further improve the wearability of the exhaust valve and the valve 
guide, it is preferred that the non-coated portion of the valve stem be 
recessed so that the bore of the valve guide contacts only the coated 
portion of the valve stem when the valve is opened and closed. In a large 
natural gas engine, it is preferred that the recess have a depth between 
0.002 and 0.006 of an inch with respect to the radius of the valve stem. 
Limiting the recess depth protects the coating from the exhaust gas. 
Alternatively, it may be desirable to provide a recess in the bore of the 
valve guide at the end of the valve guide that is exposed to the hot 
exhaust gases passing through the exhaust port. This is another 
configuration in which the bore of the valve guide contacts only the 
coated portion of the valve stem when the valve is opened and closed. 
In another alternative embodiment of the invention, it may be desirable to 
provide a coating on the bore surface of the valve guide in addition to or 
instead of the coating on the valve stem. In this embodiment of the 
invention, the coating on the bore surface should be receded from the end 
of the valve guide exposed to the hot exhaust gases to protect the coating 
from the hot temperatures. In this embodiment, it may be desirable to 
recess the non-coated portion of the bore located towards the end of the 
valve guide exposed to the hot exhaust gases, or alternatively it may be 
desirable to recess the part of the valve stem corresponding to the 
non-coated part of the valve guide bore, thus allowing the valve guide 
stem to contact only the coated portion of the bore of the valve guide 
when the valve moves. 
The primary object of this invention is to improve the wearability of 
exhaust valves and valve guides in internal combustion engines, especially 
large industrial natural gas internal combustion engines in which the 
exhaust valves and guides are expected to last 15,000 to 20,000 hours. 
However, the invention is not limited to use on engine exhaust valves and 
guides, but can also be used on other reciprocating shafts in high 
temperature environments such as compressors, etc. 
Other advantages and features of the invention should be apparent to those 
skilled in the art upon reviewing the following drawings and description 
thereof.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIGS. 1 through 3 illustrate a low-friction, extended-life exhaust valve 10 
in accordance with a first embodiment of the invention. The exhaust valve 
10 is particularly well suited for use in large, natural gas internal 
combustion engines and this is the preferred application for the 
invention. 
The valve 10 includes a stem 12 and an integral valve head 14. The valve 
head 14 has a front circular face 16, which typically can have a diameter 
of about 52 millimeters in a large natural gas engine. The head has a 
shank 18 extending perpendicularly rearward from the front face 16. The 
head 14 is preferably made of stainless steel. A typical diameter for most 
of the length of the shank 18 is about 10.6 millimeters in a large natural 
gas engine. The stem 12 is preferably made of 4140 steel, and typically 
has a diameter of about 11 millimeters in the preferred application if one 
includes the coating depicted by reference numeral 20 when measuring the 
diameter of the stem 12. The stem 12 is welded to the shank 18 to form an 
integral exhaust valve 10. 
Referring in particular to FIG. 1, the exhaust valve 10 is slidably mounted 
within a valve guide 22. The valve guide 22 is typically made of cast 
iron, but could also be made out of powdered metal. Powdered metal guides 
tend to have some amount of solid lubricity. The valve guide 22 is press 
fit into an opening 24 in cylinder head 26 for the internal combustion 
engine. The cylinder head 26 has an exhaust port 28 from an engine 
combustion chamber or cylinder in the area indicated by reference numeral 
30. The valve 10 is opened and closed by actuating the valve 10 in the 
part of the valve 10 by chamfer 32, thus displacing the valve 10 in the 
direction along the axis of the stem 12. The valve guide 22 has an 
internal bore 34 that laterally supports the exhaust valve stem 12. The 
clearance between the bore surface 34 and the valve stem 12 is preferably 
3.2 to 4.3 thousandths of an inch, which is sufficient to maintain an oil 
film given manufacturing tolerances and thermal expansion, yet tight 
enough to allow little wobble. 
When the valve 10 is in the closed position, the backside 36 of the head 14 
mates against a valve head seat 38 to prevent hot exhaust gases in the 
combustion chamber 30 from flowing through the exhaust port 28. When the 
valve 10 is opened, hot exhaust gases in the chamber 30 pass into and 
through the exhaust port 28 as depicted by arrow 40. By way of example, a 
fully opened valve 10 is displaced about 16 millimeters from a fully 
closed valve 10 in the preferred application. 
In accordance with the invention, the stem 12 of the valve 10 has a coated 
portion 20 and a non-coated portion 42. Referring now in particular to 
FIG. 3, the valve stem 12 may include a sharp corner or carbon scraper 44 
at or near the junction between the valve stem 12 and the shank 18 of the 
valve head 14. When the valve 10 is in the closed position, the carbon 
scraper 44 is located within the bore 34 of the valve guide 22. The end 46 
of the valve guide 22, and in particular the region 48 of the bore 34 of 
the valve guide 22, is exposed to hot exhaust gases passing through the 
exhaust port 28. Carbon or other particulates can therefore accumulate 
within the internal bore 34 in the region 48 in front of the carbon 
scraper 44 when the valve is closed due to residual exhaust gases or other 
combustion products in the exhaust port 28. The sharp edge of the carbon 
scraper 44 scrapes away excessive build-up in region 48 as the valve 10 
opens. 
As shown best in FIG. 3, the non-coated portion 42 of the valve stem 12 
extends completely between the coated portion 20 of the valve stem 12 and 
the carbon scraper 44. By way of example, the length of the non-coated 
portion 42 along the axis of the stem 12 is approximately 10-11 
millimeters in the preferred application. Receding the coating 20 from the 
end 46 of the valve guide 22 when the valve 10 is open protects the 
coating 20 from hot exhaust gases passing through the exhaust port 28. The 
exhaust gases in the exhaust port 28 in a large industrial natural gas 
combustion engine are typically about 1050.degree. F. to 1075.degree. F. 
and can exceed 1100.degree. F. However, the preferred coating 20, 
molybdenum oxide, cannot repeatedly withstand temperatures over 
600.degree. F. Receding the coating 20 away from the hot exhaust gases 
protects the coating from the hot exhaust gases and enables the use of low 
temperature coatings such as molybdenum oxide, which would otherwise tend 
to flake and deteriorate due to the high temperatures of the hot exhaust 
gases. 
The molybdenum oxide coating 20 is preferably 0.008 of an inch thick in the 
preferred application. The molybdenum oxide coating 20 not only has the 
advantage that it can be applied with sufficient porosity to retain 
lubricating oil, but also has the advantage of solid lubricity. Thus, the 
molybdenum oxide coating provides roughly a 4/1 benefit (i.e. 1/4 the wear 
rate) over porous, non-solid lubricant coatings such as nitriding. 
The non-coated portion 42 of the valve stem 12 is also preferably slightly 
recessed so that the bore 34 of the valve guide 22 contacts only the 
coated portion 20 of the valve stem 12 when the valve 10 is opened and 
closed. The depth of the recess in the non-coated portion 42 of valve stem 
12 is preferably between 0.002 and 0.006 of an inch with respect to the 
radius of the stem 12. 
One way of manufacturing the valve stem 12 disclosed in FIGS. 1 through 3 
is to grind a typical non-coated valve down approximately 0.008 of an inch 
on the radius of the stem 12 to create a trough for the coating 20. The 
coating 20 can then be applied by placing the valve 12 on a rotating 
fixture and applying the coating 20 with a plasma spray gun. The 
non-coated portion 42 can then be ground down to remove any overspray of 
the coating 20, and also create the 0.002 to 0.006 of an inch recess. 
FIG. 4 shows accelerated endurance test results illustrating the effect the 
invention has on improving the wearability of exhaust valves over time. 
Curve 50 indicates that a prior art valve having a chrome plated stem 
encountered 0.008 of an inch wear after 500 hours of operation under test 
conditions. Curve 52 represents a valve stem having a receded molybdenum 
oxide coating 20. Curve 52 indicates only 0.002 of an inch wear after 500 
hours and 0.004 of an inch wear after 1,000 hours, which is a significant 
improvement. Note that curve 52 represents a valve in which the non-coated 
portion 44 is not recessed. It can thus be appreciated that the solid 
lubricity of the molybdenum oxide coating improves the wearability of the 
valve significantly compared to the conventional chrome plated valve. 
Curve 54 represents a valve stem having a receded molybdenum oxide coating 
20 in which the non-coated portion 44 of the valve stem 12 is recessed. 
Recessing the non-coated portion 44 between the coated portion 20 and the 
carbon scraper 44 on the valve stem 12 further increases the wearability 
as illustrated by curve 54 showing, under the test conditions, a 0.001 
inch wear after 500 hours of operation, and a 0.0024 inch wear after 1,000 
hours of operation. Without recessing the non-coated portion 44 on the 
valve stem 12, the wear rate tends to be higher. 
FIGS. 5 and 6 illustrate another embodiment of the invention in which a 
valve guide 56 is provided with a coating 58 within its internal bore. The 
coating 58 is receded from the end 60 of the guide 56 that is exposed to 
the hot exhaust gases in the exhaust port 28. The bore of the valve guide 
56 thus includes a coated portion 58 and a non-coated portion 62. The 
non-coated portion 62 is preferably recessed so that the valve stem will 
contact only the coated portion 58 of the internal bore of the valve guide 
56 when the valve stem moves. The coating 58 could be applied to the 
internal bore of the valve guide 56 by dipping. In the embodiment shown in 
FIGS. 5 and 6, it is not necessary to provide a coating on the valve stem, 
nor is it necessary to provide a non-coated recessed portion in the valve 
stem. Therefore, a conventional non-coated exhaust valve can be used with 
the coated valve guide 56 shown in FIGS. 5 and 6, and the system should 
provide the advantages of the invention as described with respect to the 
embodiment of the invention shown in FIGS. 1 through 3. 
Other configurations in accordance with the invention are also possible. 
One such configuration would be similar to the embodiment shown in FIGS. 1 
through 3 where the coating 20 on the valve stem 12 is receded but instead 
of recessing the non-coated portion 42 on the valve stem 12 between the 
coated portion 20 and the carbon scraper 44, the bore 34 of the valve 
guide 22 by the end 46 near the hot exhaust gases could be recessed 
similar to that shown by reference numeral 62 in FIG. 5. Likewise, in the 
embodiment shown in FIGS. 5 and 6, it may be desirable to leave the 
non-coated portion 62 of the valve guide bore flush with the coated 
portion 58 of the bore, and recess the corresponding portion on the valve 
stem 12 similar to that shown by reference numeral 42 in FIGS. 1 through 
3. 
Other modifications, alternatives and equivalents may be apparent to those 
skilled in the art. Such modifications, alternatives or equivalents should 
be considered to be within the scope of the following claims.