132 Auto Engine Repair Copyright by Goodheart-Willcox Co., Inc. Primary barrel Mixture jet Mixing action Mixture passage Throttle body or carburetor Figure 6-13. A mixture jet injects a charge into the intake port right before the intake valve. This increases the mixing action and helps burning. (General Motors) Jet valve Air-fuel mixture Intake Stroke Compression Stroke Power Stroke Jetai Figure 6-14. An air jet chamber uses a small, third valve to inject an airstream directly into the combustion chamber. This helps mix- ing and burning at low speeds. (Chrysler) Auxiliary valve Conventional intake valve Lean mixture Conventional main chamber Prechamber Rich mixture Figure 6-15. A stratified charge combustion chamber has a small chamber to hold a rich air-fuel mixture. A lean air-fuel mixture enters the main combustion chamber. Then, the rich mixture is ignited by the spark plug the flame blows into the lean mixture and ignites it. (Ford) A passage runs from the carburetor to the combus- tion chamber and jet valve. During the intake stroke at lower engine speeds, the engine camshaft opens both the conventional intake valve and the air jet valve. This allows fuel mixture to flow into the cylinder past the conventional intake valve. At the same time, a stream of air flows into the cylinder through the jet valve. A stratified charge combustion chamber uses a flame in a small combustion prechamber to ignite and burn the fuel in the main, large combustion chamber, Figure 6-15. A stratified charge chamber allows the engine to operate on a lean, high-efficiency air-fuel ratio. Fuel economy is increased and exhaust emissions are reduced. A very lean mixture (high ratio of air-to-fuel) is admitted into the main combustion chamber. The mixture is so lean that it will not easily ignite and burn. A richer mixture (high ratio of fuel-to-air) is admitted into the small prechamber by an extra valve. When the fuel mixture in the small prechamber is ignited, flames blow into and ignite the hard-to-burn lean fuel mixture in the main combustion chamber. Spark Plugs Per Cylinder The number of spark plugs per cylinder is another way in which a combustion chamber may be classified. Most engines use one spark plug per cylinder. However, some high-performance, high-efficiency engines use two spark plugs per cylinder. Dual electric arcs (sparks) igniting the air-fuel mixture ensure more complete combustion of the fuel charge. Combustion is actually started in two locations