Chapter 6 Engine Types and Classifi cations 133 Copyright by Goodheart-Willcox Co., Inc. Twin Plugs Two diagonally opposed spark plugs per cylinder Exhaust-side plug Intake port Intake-side plug Exhaust port Swirl Compact combustion chamber Figure 6-16. By using two spark plugs per cylinder, horse- power can be increased and emissions decreased. Lash adjustment Rocker arm Valve spring Valve guide Valve head Cylinder head Cylinder block Camshaft Lifter Push rod A B Tappet adjusting screw Cylinder head Tappet Camshaft Valve Figure 6-17. An I-head engine has the valves in the cylin- der head. A—Note how in this design the camshaft is in the block and a push rod transfers the camshaft motion to a rocker arm and then the valve. B—This is an I-head engine with an overhead camshaft. Notice how the camshaft acts directly on the valve. (Chrysler) in each combustion chamber. Two spark plugs per cylinder increases engine horsepower slightly while also decreasing exhaust emissions. Refer to Figure 6-16. Valve Location The location of the engine valves is still another way to describe the type of automotive engine. This classification is described in terms of the cylinder head. In an I-head engine, intake and exhaust valves are in the cylinder head. Another name for this design is overhead valve engine (OHV), Figure 6-17. The OHV engine is the most common design. Numerous variations are now in use. An L-head engine has both the intake and exhaust valves in the block. See Figure 6-18. Also called a flat head engine, its cylinder head simply forms a cover over the cyl- inders and valves. This type of head is no longer used. Camshaft Drive and Location Classifications There are two basic locations for the engine camshaft. It can be located in the block or in the cylinder head. Both locations are common. Also, there are three types of cam- shaft drive—belt, chain, and gear drive. The location of the camshaft and the type of drive mechanism are two addi- tional ways in which an engine can be classified. Cam-in-Block Engine A cam-in-block engine uses push rods to transfer cam- shaft motion to the rocker arms and valves, Figure 6-19A. The term overhead valve (OHV) is sometimes used to refer to this design instead of the term cam-in-block. A cam-in-block valve train requires a short timing chain and has less mass to rotate than an OHC valve train. This dependable design is used in many larger-displacement V-8 engines. Overhead Camshaft Engine In an overhead camshaft engine (OHC), the camshaft is located in the cylinder head. Refer to Figure 6-19B. Push rods are not needed to operate the rockers and valves. This