Chapter 6 Engine Types and Classifi cations 131 Copyright by Goodheart-Willcox Co., Inc. To exhaust manifold From intake manifold From intake manifold Exhaust port Intake port Combustion chamber Intake port Exhaust port To exhaust manifold Crossflow Cylinder Head Noncrossflow Cylinder Head Figure 6-10. A cylinder head with noncrossflow (backflow) combustion chambers has intake and exhaust ports on the same side of the head. Intake camshaft Intake air Exhaust camshaft Exhaust gas Exhaust valves Intake valves Figure 6-11. Many modern high-performance engines have four valves per cylinder to allow better airflow and increase engine power. (Toyota) Ram type intake manifold Injectors Intake camshaft Exhaust camshaft Tubular high-flow exhaust headers Two exhaust valves Two intake valves Figure 6-12. This is a dual camshaft, four-valve cylinder head configuration. In this design, the intake runners and exhaust system are tubular to increase airflow through cylinder head at high speeds. (Mercedes-Benz) A two-valve combustion chamber has one intake valve and one exhaust valve per cylinder. One is pictured in Figure 6-9. This is a conventional design that has been used for years. The intake valve is slightly larger than the exhaust valve. A four-valve combustion chamber has two intake valves and two exhaust valves for each engine cylinder. This is a newer design used on several high-performance vehi- cles. A four-valve chamber is in Figure 6-11. A four-valve combustion chamber normally has siamese ports where one large port divides into two smaller ports right before the engine valve. The valves in a four-valve chamber are generally smaller than the valves in a two-valve chamber. However, they expose a larger port area when open and will increase flow into and out of the chamber. This increases engine speed capability and the resulting power. A higher compression ratio can also be obtained with a four-valve combustion chamber. The valves do not have to be opened as wide to obtain the same flow. Therefore, the piston can be designed with less valve relief to compress the air-fuel charger tighter. This also increases combustion efficiency. Twin (dual) camshafts are normally used with a four- valve chamber. Too many rocker arms and push rods would be needed to operate so many valves with a single camshaft. A view of a four-valve, dual camshaft cylinder head is given in Figure 6-12. A mixture jet combustion chamber uses an extra passage running to the intake valve port. The mixture jet is used to aid swirl in the port and combustion chamber to help burning. The mixture jet mainly helps engine efficiency at low speeds. Refer to Figure 6-13. An air jet combustion chamber has a small, extra valve that allows a stream of air to enter the combustion chamber to aid swirl and combustion efficiency. Shown in Figure 6-14, two conventional valves are provided. A third, smaller valve is also used. It opens to admit a gush of air that mixes the air-fuel charge to speed burning. The jet valve only works at idle and low engine speeds. At higher engine speeds, normal air-fuel mixing is adequate for efficient combustion.