1. INTRODUCTION
Figure 1 : Optical Single Cylinder Engine
Internal
combustion (IC) performance is one of the important aspects in automotive
industry. The higher the IC performance, the powerful the engine car to be,
therefore cause the higher request between the lovers of high performance cars.
However, engineers had faced lots of problem in the way to determine the exact
and more accurate performance of IC engine. These problems happen due to lots
of factor such as the material that IC engine made of from. The IC engine is
made of from all-metal materials and this prevent engineers to gain a detailed
insight of the mixing, combustion and emissions formation processes occurred in
the cylinder of IC engine. Therefore, the Optical Engine has been introduced as
the improvement to the original system. High-speed imaging combined with the
optical access provided by a optical engine offer the ability to directly image
and compare ignition and combustion phenomena of internal combustion engine.
DISCUSSIONS
A example of
real view picture of the optical engine is shown in Figure 1( we take the
optical single cylinder engine as our reference). Unlike the conventional all-metal single cylinder engine, this optical engine features several
particular arrangements. For example, the piston arrangement is the
conventional Bowditch type with the optical piston screwed into the hollowed
extension which allowing a fixed 45°mirror to be mounted on the engine
crankcase.
Figure above shown 3D schematic showing main components of the optical engine including the
extended piston assembly and optical piston. From the figure also we can
identify the common material for every specific parts in this engine. The
extended piston is bolted to the conventional piston in the crankcase connected
by an ordinary connecting rod with a crankshaft. Basically, this optical engine
has a pneumatically operated “drop-down” liner which allowing quick inspection
of the combustion chamber for different purposes such as for inspections or
maintenance of the engine.
There are
lots of study, research and experiment that conducted regarding the optical
engine in internal combustion engine. Therefore, for a further understanding in
how the optical engine functioned, we may refer to the research that conducted
by a group of researcher from University of Michigan based on an optical
research engine that donated by the Ford Motor Company.
Single
cylinder combustion research supports the development of new combustion and
engine design without the investment in multi-cylinder prototype hardware for
the benefit of faster and lower cost validation of new concepts. This is
very useful in automotive industry. An example of automotive company that used
this optical engine is Lotus. Lotus offers the advanced Single Cylinder Optical
Research Engine (SCORE) that has a glass cylinder and piston that enables the
use of laser diagnostic techniques to visualise and measure combustion
phenomena in the running engine. This
SCORE product allows real-time images of various phenomena inside the cylinder
such as fuel spray dispersion and flame propagation to be captured so that
their research teams can develop new understanding of combustion and the
reduction of fuel consumption and emissions. The engine features are mentioned
as below :
·
Extended piston with
stationary 45 degree mirror and sapphire piston crown window
·
Removable glass cylinder
forms the optical bore
·
Upper crankcase with
hydraulic platform for mounting / release of optical components
·
Quick release liner to
enable easy and rapid maintenance
·
Accurate 'real engine'
geometry
·
Lower crankcase containing
crankshaft and primary and secondary balance shafts
·
Operates at real engine
loads and speeds up to 5,000 rpm
There are lots of benefits that the optical engine can
offer such as :
1.
Optical engine allow application of both qualitative
and quantitative measuring of internal combustion engine.
2.
Optical engine
allow us to gain a detailed insight of mixing, combustion and emissions formation
processes occurring in cylinder of internal combustion engine.
3. In automotive industry,
the optical engine supports the development of new
combustion and engine design without the investment in multi-cylinder prototype
hardware for the benefit of faster and lower cost validation of new
concepts.
4.
By referring to
the SCORE that offer by Lotus, various phenomena can be investigated using
optical engine :
(a)
Flow velocity can be measured using Particle
Image Velocimetry (PIV)
(b)
Flow velocity and particle size can be
measured using Phase Doppler Anemometry (PDA)
(c)
Concentrations in spray fields can be
measured using Laser Induced Fluorescence (LIF)
(d)
Combustion, flame growth and propagation can
be measured by high speed imaging
3.
LIMITATIONS
The latest the techology, the more
limitations that it will have. Hence, as the newest technology in internal
combustion engines, the optical engines also have their own limitations that
contribute to a slow research and development (R&D) of optical engine. The
limitations happen due to some factors and in this topic we will focused from
the aspects of material and manufacturing process.
The
optical engine piston are commonly made up from optic-characteristic material
such as sapphire and titanium. Sapphires can be
found naturally, by searching through certain sediments (due to their
resistance to being eroded compared to softer stones), or rock formations, or
they can be manufactured for industrial or decorative purposes in large crystal boules. Because of the
remarkable hardness of sapphires, 9
on the Mohs scale (and of aluminium
oxide in general), sapphires are used in some non-ornamental applications,
including infrared optical components. However, the real problem here is the uses of
titanium as the main material because titanium is commonly used. Due to its
good physical properties which are has the combined properties of being light
and strong, corrosive resistant and can withstand high temperatures, titanium
is the best suitable material that can be used for manufacture optical engine
piston. Titanium is the
fourth most abundant metal making up about 0.62% of the earth's crust. Rarely
found in its pure form, titanium typically exists in minerals such as anatase,
brookite, ilmenite, leucoxene, perovskite, rutile, and sphene. While titanium
is relatively abundant, it continues to be expensive because it is difficult to
isolate. Titanium is particularly expensive to refine, process, and fabricate.
In terms of processing cost per cubic inch, titanium refining is as five times
more expensive as aluminum. Titanium is usually produced using the Kroll
process. The Kroll process functioned by converting titanium dioxide bearing
titanium ore into chloride thus creating titanium chlorides. These are
chemicals separated through a process called fractional distillation, with the
final product being a porous mass of titanium metal mixed with byproducts,
known as titanium sponge. This sponge is then subjected to leaching or heated
vacuum distillation to remove further impurities.
From the study of Kroll process, we
can conclude that the process of manufacturing titanium are very complicated
and costly. Therefore, we can assume overally the cost to produce one unit of
titanium piston is very expensive. Hence, the researcher need to be more extra
carefull in their experiment during handling the variable for optical engine
research for preventing the waste of titanium piston.
The other issue in optical
engine research is the appropriate selection of the material for optical
components, which meet demanded specific requirements. The selection criteria
for the choice of material are not only based on mechanical property
consideration but also on their optical properties. The use of material such as
sapphire and titanium in optical engines has a significant impact on engine
combustion characteristics thus bringing into question the fidelity of optical
engine performances compared with an identical all-metal engine. The reason for
this is that the thermal conductivity of these materials is significantly lower
than the thermal conductivity of aluminium
(aluminium being the base
material used for the manufacture off all-metal pistons). The low conductivity
of these materials will give rise to higher temperature which could lead to
significant thermal expansion. Hence, the high temperature gradient across the
engine wall might lead to excessive thermal-induced stresses and mechanical
failure.
4.
SOLUTIONS
Improved and
cheaper tehnologies of producing titanium are explored currently in titanium
industries, the most advanced being the FFC Cambridge process. The FFC
Cambridge Process is an electrochemical method
in which solid metal
compounds, particularly oxides, are cathodically
reduced to the respective metals or alloys in
molten salts. The FFC Cambridge process is shown in figure below.
We can
compare this FFC
Cambridge process’s figure with the previous Kroll process. From the
comparison, we can identify that this process are less complecated and not
required too much set up means its more cheaper than Kroll process.
We also can
considering to use a fused silica instead of titanium. Fused silica are made from silicon dioxide (SiO2). Silicon
dioxide occurs naturally as sand or rock and when processed the resulting
product is called Fused Quartz. If the silicon dioxide is synthetically
derived, the material is often called fused silica. Hence fused silica is a
high purity synthetic amorphous silicon dioxide. Fused silica as a material for
optical components offers a number of following advantages:
a) excellent optical qualities and wide radiation
transmittance spectral range
b) a low coefficient of thermal expansion providing
stability and resistance to thermal
shock over large temperature excursions without cracking,
c) a wide operating temperature range,
d) high
hardness and resistance to scratching
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