Thermal imaging system for internal combustion engines

A thermal imaging system for use in an internal combustion engines employs a removable optical housing that is inserted through a hole in the cylinder wall. The distal end of the housing supports a lens made of a material such as polycrystalline spinel or sapphire to provide a desired field of view within the cylinder and to project an image from the field of view through the internal passageway of the optical housing. A camera receives and records the image provided by the lens and optical housing. In the preferred embodiment, the distal portion of the optical housing is secured to the cylinder wall by means of threads or a breech-mount mechanism to permit quick and easy removal and cleaning of the lens. A thermocouple or heat flux gauge can be mounted to the cylinder wall within the field of view of the thermal imaging system to provide a reference point for temperature measurements.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to the field of thermal imaging 
systems. More specifically, the present invention discloses a thermal 
imaging system for viewing within the cylinder of an internal combustion 
engine. 
2. Statement of the Problem 
In designing and testing internal combustion engines, it is advantageous to 
know the temperature profile of the various surfaces within the cylinder, 
including the cylinder head and valves, while the engine is in operation. 
For example, thermal images from within the engine cylinder can be used to 
detect localized hot spots and to study combustion chamber deposits. 
Ideally, a thermal imaging system should be suitable for use with a wide 
variety of actual production engines, and should not be limited to a 
specially-constructed engine used for testing. In addition, the system 
should not require extensive modifications to the engine to accommodate 
the imaging system. Extensive modification of the engine increases the 
risk that the image data generated by the system does not accurately 
reflect conditions within an actual production engine. 
It is also important to be able to quickly and easily remove those portions 
of the imaging system that are exposed within the engine cylinder for the 
purpose of cleaning. Films and combustion deposits tend to rapidly 
accumulate on the exposed optical elements of the system. In particular, 
it should not be necessary to disassemble the entire engine to clean or 
replace the optical train. 
A number of optical sensors for internal combustion engines have been 
invented in the past, including the following: 
______________________________________ 
Inventor U.S. Pat. No. Issue Date 
______________________________________ 
Linder, et al. 
4,377,086 Mar. 22, 1983 
Linder, et al. 
4,422,323 Dec. 27, 1983 
Franke, et al. 
4,425,788 Jan. 17, 1984 
Haftori, et al. 
4,444,043 Apr. 24, 1984 
Boning, et al. 
4,446,723 May 8, 1984 
Dils 5,052,214 Oct. 1, 1991 
Remboski, et al. 
5,067,463 Nov. 26, 1991 
Dils 5,099,681 Mar. 31, 1992 
Remboski, et al. 
5,113,828 May 19, 1992 
Kubota, et al. 
5,195,359 Mar. 23, 1993 
Kubota, et al. 
Japan 3-A8742 July 18, 1989 
______________________________________ 
Zhao et al., "The Cylinder Head Temperature Measurement by Thermal Imaging 
Technique", SAE Technical Paper Series 912404 (October 1991) 
The patents of Linder et al. show two optical sensors for combustion 
chambers. Both apparently use a photodetector (e.g. a photo diode) to 
monitor the overall luminosity of the combustion process. The patents of 
Linder et al. are representative of a large body of prior art in the field 
of "knock detectors" that are concerned with monitoring overall luminosity 
during the combustion cycle, rather than providing a thermal image of the 
interior of the combustion chamber. The Linder '323 patent shows an 
optical window that can be removed for cleaning. 
Franke et al. disclose a combustion monitoring system for a multi-cylinder 
engine to detect knocking. Each cylinder is equipped with fiber optics to 
direct light from within the cylinder to a photo diode to measure overall 
luminosity. Here again, no image is provided. 
Hattori et al. disclose another example of the knock detector for internal 
combustion engines. An illumination detector 10 is threaded through the 
cylinder head. The illumination detector is made of quartz glass or 
sapphire (column 3, line 16 and column 4, lines 61-65). 
Boning et al. disclose an optical sensor incorporated in a spark plug 
housing for use in a knock sensor. Here again, the optical pickup is a 
quartz glass rod. The end of the rod facing the combustion chamber is 
coated with a layer of graphite 13. 
The Dils patents discuss a knock detector using an optical fiber and a 
black body emitter to sense variations in heat flow within the cylinder. 
The two patents of Remboski et al. disclose an internal combustion engine 
having a luminosity detector. The system controls operation of the engine 
based in part on the luminosity signal. The optical probe 19 is made of 
synthetic sapphire (column 4, lines 43-46 of the '828 patent). 
Kubota et al. disclose another example of an optical system for detecting 
knocking in an internal combustion engine. The system uses a sapphire rod 
1 (or 22) with a black body 2 (or 23) covering the end of the rod within 
the combustion chamber to measure heat flux. 
The SAE paper by Zhao et al. discloses a thermal imaging system for 
measuring the temperature profile of the cylinder head of an internal 
combustion engine. A schematic drawing of this system is provided in FIG. 
3 of the SAE paper. The system requires a specially modified engine 
cylinder and piston having a window in the side wall of the cylinder, a 
window in the side wall of the piston, a 45.degree. mirror within the 
piston, and a silicon window that replaces the head of the piston. The 
cylinder head is painted black to provide a high and uniform emissivity 
across its surface. In addition, a number of thermocouples are mounted 
onto the cylinder head, as shown in FIG. 4 of the SAE paper, to provide 
reference temperature readings. It should also be noted that the camera is 
only able to view the mirror and cylinder head during those portions of 
the combustion cycle when the windows in the cylinder wall and piston wall 
are in vertical alignment. 
3. Solution to the Problem 
None of the prior art references uncovered in the search show a system for 
viewing thermal images from within the combustion chamber (as opposed to 
measuring overall luminosity) using a removable optical probe that is 
inserted through the wall of the cylinder. In addition, none of the prior 
art references disclose: (1) use of polycrystalline spinel in an optical 
probe; or (2) use of a black body/thermocouple within the field of view of 
the optical probe for temperature calibration. 
SUMMARY OF THE INVENTION 
This invention provides a thermal imaging system for use in internal 
combustion engines having a removable optical housing that is inserted 
through a hole in the cylinder wall. The distal end of the housing 
supports a lens made of a material such as polycrystalline spinel or 
sapphire to provide a desired field of view within the cylinder and to 
project an image from the field of view through the internal passageway of 
the optical housing. A camera receives and records the image provided by 
the lens and optical housing. In the preferred embodiment, the distal 
portion of the optical housing is secured to the cylinder wall by means of 
threads or a breech-mount mechanism to permit quick and easy removal and 
cleaning of the lens. A thermocouple or heat flux gauge can be mounted to 
the cylinder wall within the field of view of the thermal imaging system 
to provide a reference point for temperature measurements. 
A primary object of the present invention is to provide a system for 
obtaining thermal images from within virtually any internal combustion 
engine without requiring substantial modification of the engine. 
Another object of the present invention is to provide a thermal imaging 
system having an optical probe that can be easily removed for the engine 
for cleaning. 
Yet another object of the present invention is to provide a thermal imaging 
system in which the field of view within the engine cylinder can be 
readily changing by adjusting the position of the optical housing or by 
substituting different optical housings with various types of optical 
elements. 
These and other advantages, features, and objects of the present invention 
will be more readily understood in view of the following detailed 
description and the drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Turning to FIGS. 1A and 1B, simplified cross-sectional views are provided 
of one cylinder of an internal combustion engine that has been retrofitted 
with an optical probe in accordance with the present invention. The engine 
includes a conventional cylinder wall 12, cylinder head 14, and piston 
defining a combustion chamber. Intake and exhaust valves 17 shown in FIG. 
2 allow a fuel/air mixture to be drawn into the combustion chamber, 
ignited by a spark plug 18, and exhausted in the conventional manner, for 
either a two-stroke or four-stroke engine. 
The present invention permits the engine to be retrofitted with an optical 
probe assembly 20 simply by drilling a hole 15 through cylinder wall 12 or 
cylinder head 14, as illustrated in FIG. 1A. The hole 15 is threaded to 
engage corresponding threads on the exterior of the optical probe housing 
20. However, other equivalent means could be employed for securing the 
optical probe housing in the hole through the cylinder wall. 
For example, FIG. 1B illustrates another embodiment in which the housing 22 
of the optical probe assembly 20 extends through a hole 34 in the cylinder 
wall 12 and is held in place by means of a breech-mount mechanism or an 
indexing fixture that has been secured to the exterior of the engine. For 
example, the breech-mount mechanism can be designed similar to those used 
to removably attach a conventional camera lens to the camera body. The 
distal end 36 of the optical probe housing 22 is initially inserted 
through the breech-mount mechanism 32 and then into the hole 34 in the 
cylinder wall 12. The position of the breech-mount mechanism 32 is fixed 
relative to the engine by a bracket 28 connected to a flange 29 that has 
been bolted 38 to the exterior of the engine as shown in FIG. 1B. The 
shape of the opening through the breech-mount mechanism 32 and the 
exterior surface of the optical probe housing 22 are keyed to one another 
so that the housing 22 can be rotated about its longitudinal axis by a 
predetermined number of degrees relative to the breech-mount mechanism in 
order to lock or unlock the housing 22 in the breech-mount mechanism. For 
example, the optical probe housing 22 can be initially inserted through 
the breech-mount mechanism 32 and then rotated by 90 degrees in the 
clockwise direction to lock the housing 22 in place. The optical probe 
housing 22 then can be removed at any time by rotating it 90 degrees in 
the counterclockwise direction to unlock the housing 22 from the 
breech-mount mechanism 32. 
FIG. 2 is a cross-sectional view corresponding to FIG. 1A showing the 
cylinder head 14 and the field of view of the lens assembly 20 after the 
optical probe 20 has been inserted through the hole 15. In its simplest 
form, the housing 22 of the optical probe 20 has a tubular shape with an 
internal passageway extending from the distal opening to the proximal 
opening of the tube. The exterior surface of the distal end of the housing 
has threads to removably engage corresponding threads in hole 15 in 
cylinder wall 12, as previously mentioned. The distal portion of the 
optical housing 22 extends well into the hole 15 in cylinder wall as shown 
in the drawings. An objective lens 24 is supported by the distal end of 
optical housing. This lens 24 projects an image along the internal 
passageway of the optical housing 22 from a field of view within cylinder, 
as shown by the ray tracings in FIGS. 1A and 2. In the preferred 
embodiment, the objective lens 24 seals the distal opening of the optical 
housing, and the optical housing completely seals the hole 15 in the 
cylinder wall 12, so as not to result in loss of cylinder pressure or 
otherwise interfere with normal operation of the engine. 
It should be noted that the optical housing 20 can be quickly and easily 
removed by unthreading it from the hole 15 in the cylinder wall. This 
feature makes it particularly easy to clean films or deposits that 
inevitably accumulate on the exposed surface of the objective lens 24. 
Different optical assemblies can also be readily interchanged to view 
different regions within the cylinder or to provide different optical 
properties. In the preferred embodiment, the optical housing 20 is 
threaded into a hole 15 in the cylinder wall or cylinder head, as 
previously described. Alternatively, the optical housing can be removably 
secured in the hole by means of a breech-mount mechanism or bayonet-mount 
mechanism, similar to those used for mounting a camera lens to a camera 
body. 
In the preferred embodiment, the objective lens 24 is made of 
polycrystalline spinel, or possibly sapphire. Additional lens 25 and 26 
can be included in the optical probe to project the image along the 
interior or the optical housing, and to properly focus the image for the 
camera 50 at the proximal end of the optical housing. The objective lens 
is directly exposed to the combustion chamber and therefore is subject to 
extreme temperature and pressure conditions while the engine is in 
operation. The lens must also be scratch-resistant and able to withstand 
severe vibration. 
As shown in FIGS. 1A and 1B, a transparent window 21 that at least 
partially conforms to the contour of the combustion chamber wall is placed 
over the distal end of the optical probe housing in front of the objective 
lens 24. The window 21 is preferrably also made of spinel or sapphire. 
Depressions or cavities in the combustion chamber tend to promote fuel 
dropout from the homogeneous fuel-air mixture within the cylinder. This 
results in incomplete combustion which tends to create carbon deposits in 
the depression or cavity. The window 21 is intended to reduce this 
deposition of carbon on the distal end of the optical probe housing. The 
window 21 also helps to protect the objective lens 24 from thermal and 
mechanical stresses. In addition, since the window 21 is merely a 
transparent sheet, it is typically far less costly to replace than the 
objective lens 24 in the event that it is scratched or broken during use. 
Spinel offers exceptionally strong and hard mechanical properties. In 
particular, spinel is very resistant to scratches, pitting, and cracking. 
It has good chemical resistance to compounds typically found in internal 
combustion processes. Spinel has moderate thermal expansion 
characteristics over a wide range of temperatures, that are roughly the 
same as the thermal expansion characters for the cylinder wall. Spinel is 
also able to transmit images over a wide bandwidth from ultraviolet 
(approximately 0.2 .mu.m), through the visible light spectrum, and beyond 
to approximately 6.5 .mu.m in the infrared without absorption peaks. For 
example, still photographs in the visible light spectrum can be used to 
provide spatial references for thermographs in the infrared range. 
Finally, spinel is relatively low cost. 
The term "spinel" is often employed to denote any one of a group of 
materials having analogous chemical compositions that are crystallized in 
an isometric system with an octahedral habit. Some of the more important 
minerals of the spinel group are spinel (MgAl.sub.2 O.sub.4), gahnite, 
zinc spinel (ZnAl.sub.2 O.sub.4), franklinite 
(Zn,Mn.sup.2+,Fe.sup.2+)(Fe.sup.3+,Mn.sup.3+).sub.2 O.sub.4, and chromite 
(FeCr.sub.2 O.sub.4). These minerals also may be thought of as 
combinations of bivalent and trivalent oxides of magnesium, zinc, cerium, 
lanthanum, iron, manganese, aluminum, and chromium, having the general 
formula: R.sup.2+ O.multidot.R.sub.2.sup.3+ O.sub.3. Thus, the bivalent 
oxides may be MgO, ZnO, FeO, and MnO. The trivalent oxides may be Al.sub.2 
O.sub.3, Fe.sub.2 O.sub.3, Mn.sub.2 O.sub.3, La.sub.2 O.sub.3, Ce.sub.2 
O.sub.3, and Cr.sub.2 O.sub.3. In another sense, spinels may be thought of 
as being comprised of a first metal having a first oxidation state and a 
second metal having an oxidation state higher than that of the first 
metal, and wherein each metal is appropriately associated with oxygen in a 
spinel lattice structure. The first and second metals may even be the same 
metal (in two or more different oxidation states). Spinels can also be 
composed of materials having variable ratios of oxides of more than one 
metallic element homogeneously distributed through a crystalline matrix 
which is held together by loose crystalline lattice bonding. The atomic 
ratio of the first metal to the second metal in any given spinel need not 
be consistent with the classical stoichiometric formulas. Thus, in an even 
broader sense, spinels may be thought of as being composed of bivalent and 
trivalent metallic oxides of continuously varying proportions, i.e., 
materials having the general formula: nR.sup.2+ O.multidot.mR.sub.2.sup.3+ 
O.sub.3, wherein the ratio of n to m may vary. Those skilled in the art 
will appreciate that continuously variable ratios of atoms are not at all 
unusual in those materials commonly called "solid solutions". 
Sapphire is also able to transmit images over a relatively wide bandwidth. 
The choice of optical materials for the primary lens can be tied to the 
specific range of optical wavelengths that are of interest. A number of 
materials can be readily substituted for use in narrow bands. However, for 
wide band applications, spinel and sapphire are the clear choices. 
A number of thermocouples or heat flux gauges 30 can be attached to the 
cylinder wall 12 or cylinder head 14 within the field of view of the 
optical assembly. For example, a thin film thermocouple can be used to 
provide a reference point for temperature measurements on the image. This 
reference point provides independent verification and calibration of 
temperature conditions that can be measured from the thermographic images 
produced by the optical assembly. A heat flux gauge not only provides a 
reference temperature, but also measures the heat flux through the 
cylinder wall at that point. This feature can be useful in thermodynamic 
analysis of the engine. 
FIG. 3 is a schematic diagram of the optical assembly 20, CCD camera 50, 
and computer imaging system 40. The objective lens 24 projects an image 
from a field of view within the cylinder 12 along the internal passageway 
within the optical housing to the CCD camera 50. A number of secondary 
optical elements can be included along the optical path within the 
passageway of the optical housing to focus and direct the image. For 
example, these secondary optical elements can include refractive lenses, 
filters, or reflective mirrors. 
Additional detail of the CCD camera 50 is shown in FIG. 4. The image 
received by the camera 50 focused on a CCD (charge couple device) chip 55 
that converts the image into digital data in the form of a two-dimensional 
array of pixels. As shown in FIG. 4, the camera 50 also includes a Dewar 
flask 51 containing liquid nitrogen to cool the CCD chip 55 to its proper 
operating temperature. A CCD camera manufactured by Amber Engineering with 
a 128.times.128 pixel array has been found to be satisfactory for the 
present system. 
A computer 40 periodically reads out the pixel data from the camera 50. 
This data can then be processed and displayed by the computer system 40. 
The pixel data output by the CCD chip can also be recorded on magnetic 
media (e.g., tape or diskettes) for subsequent viewing and/or analysis. 
The computer system normally includes a printer or plotter 45 to generate 
a hard copy of the processed image and related data. FIGS. 5 and 6 are 
graphs showing examples of the resulting thermographic analysis of a 
portion of a cylinder head. 
It should be expressly understood that multiple instruments could be used 
concurrently by addition of an optical beam splitter to the optical path 
behind the primary lens. For example, this would allow simultaneous use of 
infrared imaging and high speed visible light photography. 
A primary advantage of the present invention is that the optical probe can 
be easily and quickly removed and replaced. In testing an engine., it is 
sometimes desired to view the interior the cylinder after the engine has 
been operated for a while to reach a steady state condition, or to view 
the interior of the cylinder after the engine has been subjected to long 
duration operational runs. In either of these situations, a probe with an 
inert or "dummy" lens can be installed while the engine is operated to 
reach the desired state. The probe with the dummy lens is then removed and 
a real optical probe assembly is installed. The dummy lens provides a 
similar structural and thermal environment in the cylinder without the 
costs associated with real optical elements. In addition, this approach 
eliminates the possibility of a buildup of deposits on the primary optical 
lens during the initial operational period for the engine. 
The above disclosure sets forth a number of embodiments of the present 
invention. Other arrangements or embodiments, not precisely set forth, 
could be practiced under the teachings of the present invention and as set 
forth in the following claims.