|
An overhead valve engine (OHV engine) is an engine in which the valves are placed over the cylinder head. This was an improvement over the older flathead engine, where the valves were placed in the block next to the piston. Overhead camshaft (OHC) engines, while still overhead valve by definition, are usually categorized apart from other OHV engines. In a piston engine configuration where the valves are overhead but the camshaft is not, informally called pushrod engine or I-head engine, the camshaft is placed within the cylinder block (usually beside and slightly above the crankshaft in a straight engine or directly above the crankshaft in the V of a V engine), and uses pushrods or rods to actuate rocker arms above the cylinder head to actuate the valves. Lifters or ''tappets'' are located in the engine block between the camshaft and pushrods. By contrast, overhead camshaft design avoids the use of pushrods by putting the camshaft directly above the valves in the cylinder head, thus simplifying the valvetrain. ==Developments== In automotive engineering, an overhead valve internal combustion engine is one in which the intake and exhaust valves and ports are contained within the cylinder head. The original overhead valve piston engine was developed by the Scottish-American David Dunbar Buick. It employs pushrod-actuated valves parallel to the pistons, and this is still in use today. This contrasts with previous designs which made use of side valves and sleeve valves. Arthur Chevrolet was awarded US Patent #1,744,526 for an overhead valve engine design. This patent included an adapter that could be applied to an existing engine, thus transforming it into an Overhead Valve Engine. In 1949, Oldsmobile introduced the Rocket V8. It was the first high-compression I-head design, and is the archetype for most modern pushrod engines. General Motors is the world's largest pushrod engine producer, producing both V6 and V8 pushrod engines. Nowadays, automotive use of side-valves has virtually disappeared, and valves are almost all "overhead". However most are now driven more directly by the overhead camshaft system. Few pushrod type engines remain in production outside of the United States market. This is in part a result of some countries passing laws to tax engines based on displacement, because displacement is somewhat related to the emissions and fuel efficiency of an automobile. This has given OHC engines a regulatory advantage in those countries, which resulted in few manufacturers wanting to design both OHV and OHC engines. However, in 2002, Chrysler introduced a new pushrod engine: a 5.7 litre Hemi engine. The new Chrysler Hemi engine presents advanced features such as variable displacement technology and has been a popular option with buyers. The Hemi was on the Ward's 10 Best Engines list for 2003 through 2007. Chrysler also produced the world's first production variable valve OHV engine with independent intake and exhaust phasing. The system is called CamInCam, and was first used in the SRT-10 engine for the 2008 Dodge Viper. Early air-cooled ohv BMW boxer motorcycle engines had long pushrods and a single centrally-mounted camshaft; but the post-1992 BMW R259 "Oilhead" boxer engines had a camshaft in each cylinder head, located between the combustion chamber and the rocker arms. The pushrods were very short, allowing higher rpm and more power. For instance, the BMW R1100S (which had a R259 engine) could achieve an output of at 8,400 rpm, with no risk of valve bounce. Since 2013, BMW flat-twin motorcycle engines have had OHC valve actuation. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Overhead valve engine」の詳細全文を読む スポンサード リンク
|