Due to the increasing cost and uncertainty in the supply of petroleum in recent years, there have been strenuous effects to make spark ignition and compression engines more efficient. One of the most promising methods as far as SI engines are concerned is the lean burn engine.

The concept of internal combustion engine is one, where oxidation of fuel is brought with the aid of a catalyst in the combustion chamber walls offers the advantages of a stratified charge engine without its disadvantages. The benefits, which could be obtained from homogeneous lean mixture burning in a spark-ignited engine, are well known. Equally known are the potential difficulties encountered with lean combustion [1], cyclic variation etc. However, if these above difficulties are overcome, then the benefits that can be obtained are high thermal efficiency and lower exhaust emissions

There are several methods adopted to burn the lean mixture efficiently. Recently use of catalysts in the combustion chamber is being investigated to improve the combustion process by increased chemical activity of the charge prior to ignition.

The catalyst are used in the form of coating on the walls or in any form to initiate combustion and make it burn faster, then the advantage improved efficiency can be obtained in addition to lower exhaust emissions. At first, NO molecules are oxidized to NO₂ on the surface of exposed Pt clusters. In a second step, these NO₂ molecules displace CO₂ molecules in the carbonate resulting in a chemical storage of the NO₂ [5].

This new concept of catalytic activation of the charge for lean combustion has been tried in this work and its effect on the combustion process has been investigated. The influence of various catalysts such as silver and nickel are determined.

Any discussion of catalyst combustion would not be complete without some methods of catalyst. The catalyst must be capable of withstanding continues and cyclic operation at very high temperatures, ideally as high as 1600^oC, without degradation of either catalyst or physical properties. This requires the use of high melting point materials with good thermal shock properties; the catalyst must have a low intrinsic pressure drop. Finally, the catalyst activity must be high enough to permit catalyst ignition at reasonably low temperatures. This facilitates rapid reliable light of and stable operation. The catalytic ignition temperature of course depends not only catalyst activity but on either factor such as fuel concentration and flow velocity.

Combustor catalyst have been operate at temperature over 1650^oC without loss of low- temperature activity long-term testing of high temperature catalysts is needed. With the discovery that heterogeneous catalyst can be used to promote stable ultra lean thermal combustion at heat release comparable to those of conventional combustors. It has become feasible to consider catalytic system for replacement of conventional combustor. In the catalytic combustor, homogeneous thermal combustion occurring in parallel with heterogeneous reactions at the catalyst valve is inducted by a hot boundary layer, fixed in space by the catalytic surface.

This mode of operation gives the catalytic combustor the potential of operational stability far superior to that of any offer other type of combustor. Catalytic surface reactions result in catalyst temperatures approximately equal to the local adiabatic flame temperature [2]. The main aspects of catalytic theory are when a gaseous fluid passé over a catalyst surface and reacts on the surface the heat liberated in transferred back into the gas phase under adiabatic steady- state conditions, the rate of heat transfer catalytic channels, thereby imposing significant pumping losses. These approaches also relieved on complicated scavenging and flow delivery system.

The use of nickel and silver catalyst in the combustion chamber of an internal combustion engine that operates with lean burn concept offers an opportunity to develop an engine emits particularly low level of HC, CO & improved fuel economy.

A catalytic pre-chamber is attractive from the stand –point of controlling all phase of in-cylinder combustion. Such a pre-chamber influences both chemical and gas dynamic processes. It regulates contact between the fresh charge and the catalytic surface it allows activation to take place in a well-defined volume adjacent to the ignition sources. It provides a means of independently adjusting the catalytic surface temperature, and it acts a turbulence generation pot, which intensifies burning o0f the non-activated portions of the charge [1].

A catalytic engine is an internal combustion engine where the heat release is brought about by the use of a catalyst. This definition includes engines where the combustion is commenced sustained or aided by the action of a catalyst on the air/fuel mixture.

The compression ratio of all internal combustion engines directly affects the fuel economy. The compression ratio of the petrol engine cannot be raised too much because of the tendency of the engine to knock. This is a result of premature spontaneous combustion in the unburnt portion of the charge ahead of the flame front and brought about by the use of fuel with too low octane number; the octane number being a measure of the fuel's resistance to knock in the spark ignition engine. Ignition of the charge before the spark timing is also a problem under these circumstances. It has been shown that as the compression ratio of the petrol engine is increase so its economy increases. In our paper compression ratio was slightly increased by the catalyst coating.

“Develop a new type of combustion chamber, called catalytic combustion chamber in order to make the combustion of lean mixture faster and obtain increased efficiency and lower exhaust emissions”.

There is therefore a need for a method of combustion, which would enable very lean mixtures to be efficiently oxidized, and operate in an internal combustion engine at the compression ratio of about 12:1. The concept of the charge is passed through a catalyst; oxidation can occur even at low temperature and very lean mixtures. Thus all the fuel should be oxidized and the engine can run un-throttled which should give economy.

The formation of HC and CO in the combustion chamber is also strongly dependent on the air/fuel ratio, and lean operation gives reduced emissions of these pollutants in the exhaust of the engine [4]. The catalyst enables oxidation of HC at much lower temperature normally possible. So these emissions are also reduced. Another important advantage of the catalytic engine concept is that it is capable of operating on many different liquid fuels.

The various surface properties that can be enhanced by the proper selection of coating materials are,

• Mechanical properties
• Thermo chemical properties.
• Electrochemical properties.
• Optical properties.
• Electrical properties. Etc.,

Selection of a specific coating is largely dependent on the surface properties demanded by a particular application. The selection of the coating is generally not influenced by the base material on which it is to coated. All elements that do not decomposed at high temperatures can be coated by a thermal spraying including metals, ceramics, cements, intermetallic, etc., Given the large variety of sprayable materials, aside choice of feedstock powders is normally available for obtaining a desired functional attribute. The choice specific powder material is governed by the operating conditions of the coated components as well as by the economic constraints.

The Experimental setup is similar to normal testing. First the vehicle is fitted to the multi-utility road test bench. Readings are taken before coating the piston and cylinder head. Now coating is made over the both piston and cylinder head with nickel and silver catalysts by the process of electroplating and reading are taken in the same manner. Special equipment used in this steady are Exhaust gas analyzer& Multi utility roller test bench. For Exhaust Emission Test the engine outlet is fitted to the exhaust emission analyzer and readings are taken before and after coating.

Since the base metal on which the coating applied was alloy aluminum, a pre-conditioning of the surface was necessary and it was done by a process called zinc coating to remove oxide layer [3] to enable the coating to have good bonding with base metal. After conditioning of piston and cylinder head, electroplate coating was done (Figure 2).

The following modifications were made to the engine for the lean combustion of commercially available gasoline with the carburetion and spark ignition.

• The cylinder heads and pistons were nickel and silver coated (Figure 1).
• A modified carburetor for varying air fuel ratio is mounted to engine (Figure 3).

Figure 1. Section of Piston with Coating

Figure 2. Setup for Electroplating of Catalyst

Figure 3. Section View For Modified Carburetor

Conventional carburetor is modified to obtain different A/F ratio. In order to obtain a lean mixture the fuel quantity that is inducted into the mixing chamber should be reduced. For that the main petrol jet should be a variable delivering unit. But in conventional carburetor the quantity of air and fuel is varied simultaneously so that constant air fuel ratio is admitted to the engine. In the conventional carburetor, a slider is used to control the air and a spindle, which is attached to the slider, is used for controlling the fuel flow.

In the modified carburetor, fuel flow rate is controlled by a separate spindle. This spindle is threaded to cap of the carburetor and it as a conical section at the other end. The conical can be moved in the fuel control valve; so that the opening area of the valve can be controlled which in turn controls the quantity of fuel admitting into the mixing chamber. The accelerator cable is attached to the slider, which controls only the air. Thus we obtain a different air-fuel ratio at different positions of the spindle.

The registered vehicle number TN 58F 0730 has fitted with coated piston, was tested for finding performance on mileage and emission in a Government authorized center of pollution control board of India and the result revealed as tabulated below (Table1).

Table 1. Performance Recorded before and after Coating

Mileage has improved in both nickel and silver coatings by nearly 10%. Good combustion characteristics indicate an ability to use lean air –fuel ratio. In case of exhaust temperature, coated engine temperature decreases. The benefits of using lean mixtures in S.I. engines are obvious. Better fuel utilization and lower exhaust emissions are achieved. However, some loss of heat power output has to be accepted. Catalytic treatment of exhaust gases also can pose problems due to the low temperature of exhaust gas. The result indicates reduction of about 70% to 90% of CO and 10% to 20% of HC.

Acknowledgment: The authors thank the Management, Principal, for their continuous financial support for doing this research work.