The present invention relates to a method for operating a spark ignition engine at extremely lean air to fuel ratios. Such lean air to fuel ratios are beneficial and desirable not only because they are more economical but also because the total combustion involved results in clean exhaust and greater engine efficiency.
Lean mixtures are commonly used in diesel engines and other continuous burning cycles. However, the spark ignition cycle has a tendency to misfire with a lean mixture. This happens because the excess air present cools and finally extinguishes the flame front. One successful method used to eliminate this tendency to misfire is the creation of a rich zone at the sparkplug. This allows the flame front to develop sufficient energy to maintain combustion throughout the lean charge. To accomplish this, a combustion chamber may be branched off the main chamber and fitted with an intake valve and sparkplug. In such an arrangement a special carburetor provides a rich mixture to the pre-combustion chamber allowing a reliable ignition and subsequent burning of the main lean charge.
The design of engines previously proposed using these features, however, have been very complex, requiring a number of additional components which must be crowded into a combustion area. As a result, the flow paths, pre-combustion chamber geometry, and sparkplug placement were all suboptimal. Accordingly, the results have shown a marked decrease in engine efficiency with increased manufacturing cost.
According to the present invention, an arrangement combining the regular intake valve, a rich intake valve, a pre-combustion chamber, and sparkplug into a single coaxial unit solves the packaging problem encountered with previous designs. Valve actuation is virtually unchanged. While the cylinder head gains an additional flow port, the sparkplug boss is eliminated. The coaxial design is compact and easily manufactured. The designer, therefore, can change important variables such as pre-combustion chamber geometry, distance from main charge and flow areas as desired. All of the pieces may be totally machined which allows more control over surface finish, charge flow, burning rate and heat transfer characteristics. These important variables can be optimized to control emissions without lowering engine efficiency. The coaxial configuration provides ideal flow symmetry and this insures quick, complete ignition of the pre-combustion and main charges.