Recent efforts to reduce the exhaust emissions of internal combustion engines, particularly those of the reciprocating type, have involved staging the combustion for each power cycle of the engine.
It has been found desirable to use a lean mixture in the main combustion chamber in order to provide fewer undesirable by-products of combustion or harmful emissions as they are generally known as, for example, oxides of nitrogen and unburned hydrocarbons.
The most critical period of operation of an internal combustion engine with respect to exhaust emissions is that period following first firing when the engine is started from a cold condition. The cold portions of the cylinder wall and cylinder head cause condensation of the fuel mixture, and further cause undesirable quenching of the burning of the hot gases. When the engine has been operating for a period of time and/or under a sufficient load that it is brought to its normal operating temperature, the combustion of hot gases becomes more efficient and the problem of exhaust emissions is greatly reduced. Therefore it has been found desirable to provide some means of accelerating the initial heating or warm-up of the combustion chamber of the engine when it is started from the cold condition. It has been found that providing two-stage ignition of the fuel charge produces a rapid warm-up effect in a cold combustion chamber and minimizes the exhaust emissions during engine warm up.
More particularly, with regard to reciprocating engines of the spark-ignition type, it has been discovered and it is known in the art that, if a separate, auxiliary, compartment is provided within the combustion chamber and a rich fuel mixture provided within the separate chamber, ignition of the rich mixture may be used to ignite a lean mixture in the main compartment of the combustion chamber. This particular type of two-stage fuel burning is referred to as "pre-chamber" or "stratified-charge" combustion. See, for example, the published German patent specifications No. 2,301,066, 2,259,286 and 2,302,051 published July 26, Aug. 2, and Aug. 29, 1973, respectively.
In the stratified-charge type combustion, a separate auxiliary compartment or pre-chamber is charged at the appropriate time during each cycle, with a very rich mixture of fuel which is then ignited by a conventional spark ignition device. The hot gases from the burning of the rich mixture in the pre-chamber are channeled through passages in the pre-chamber to the main compartment of the combustion chamber, and these hot gases serve as a torch to ignite a lean mixture in the main chamber. This manner of combustion has been a satisfactory solution to the problem of directly spark-igniting a lean mixture in the main combustion chamber.
Previous attempts to provide an auxiliary compartment or pre-chamber in the main combustion chamber of an internal combustion engine have generally used the technique of providing a cup-shaped or hollow tubular insert extending into the main combustion chamber from the cylinder head of the engine.
The pre-chamber is thus formed by the interior of the cup-shaped or tubular member and desired ports are provided in the wall of the cup-shaped member for communication of the interior of the pre-chamber with the spark-igniting means and for communication of the hot gases from the pre-chamber with the fuel mixture in the main combustion chamber. The cup-shaped member is thus usually attached by the open end thereof to the wall of the combustion chamber as, for example, to a portion of the cylinder head of the engine. The rich fuel charge for the pre-chamber may then be supplied to the pre-chamber through the open end of the cup-shaped member attached to the cylinder head by porting the cylinder head to the cup-shaped member. This arrangement has been found desirable because it permits the addition of a pre-chamber to existing cylinder heads without complete redesign on the cylinder head, and also lends itself to economical manufacturing techniques.
In order for the cup to serve its function of rapidly heating the gases in the main combustion chamber upon cold starting, it has been proposed that the cup be made of a material having a high thermal conductivity. However, the pre-chamber cup is exposed to the hot gases of the main combustion chamber, and therefore must have very good oxidation resistance at elevated temperatures and particularly temperatures of up to 850.degree. C. Unfortunately, materials having a desirable high coefficient of thermal conductivity do not provide sufficient oxidation resistance at these elevated temperatures. Furthermore, the materials that have the desired oxidation resistance do not have sufficient thermal conductivity to permit cantilever mounting of the cup from a mounting flange provided on the open end. If such a configuration for a pre-chamber cup is employed using any of these materials, upon the engine's reaching operating temperature, the pre-chamber cup is unable to transfer heat along its length and through the mounting flange to the cylinder head at a sufficiently rapid rate, and the cup suffers destructive overheating in the region of the closed end.
It has therefore been desirable to provide an article for defining a pre-chamber for an internal combustion engine which initially heats rapidly from the burning of the mixture in the pre-chamber yet one which, upon reaching normal operating temperature, maintains a sufficient rate of heat transfer to the engine to prevent overheating of the cup. Such a cup must also have sufficient oxidation resistance when exposed to the gaseous combustion products at normal combustion chamber operating temperatures, to withstand the corrosive attack of the hot combustion gases.