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.
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
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.
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