Overhead valve engine
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An overhead valve engine (OHV engine), or pushrod engine, is a reciprocating piston engine whose poppet valves are situated in the cylinder head. An OHV engine's valvetrain operates its valves via a camshaft within the cylinder block, cam followers (or "tappets"), pushrods, and rocker arms.
The OHV engine was an advance over the older flathead engine, whose valves were situated in the cylinder block. Some early "OHV" engines known as "F-heads" used both side-valves and overhead valves. A variation over the OHV design is the overhead camshaft, or "OHC", engine, whose camshaft lies in the cylinder head itself, above the valves. To avoid confusion, OHC engines are not referred to as OHV despite also having their valves in the head.
In early 1894, Rudolf Diesel's second Diesel engine prototype was built with a cylinder head featuring push rods, rocker arms, and poppet valves. Diesel had published this design in 1893. In 1896, U.S. patent 563,140, awarded to William F. Davis, illustrated a gasoline engine with the same head configuration, patenting his solution to the problem of how to cool the head, which problem had made the overhead valve engine difficult before then. Henry Ford's Quadricycle of 1896 had valves in the head, with push rods for exhaust valves only, the intake using suction valves. In 1898, Detroit bicycle manufacturer Walter Lorenzo Marr built a motor-trike with a one-cylinder OHV engine with push rods for both exhaust and intake. In 1900, David Buick hired Marr as chief engineer at the Buick Auto-Vim and Power Company in Detroit, where he worked until 1902. Marr's engine employed pushrod-actuated rocker arms, which in turn pushed valves parallel to the pistons. Marr left Buick briefly to start his own automobile company in 1902, the Marr Auto-Car, and made a handful of cars with overhead valve engines, before coming back to Buick in 1904. The OHV engine was patented in 1902 (awarded 1904) by Buick's second chief engineer Eugene Richard, at the Buick Manufacturing Company, precursor to the Buick Motor Company. The world's first production overhead valve internal combustion engine was put into the first production Buick automobile, the 1904 Model B, which used a 2-cylinder Flat twin engine, with 2 valves in each head. The engine was designed by Marr and David Buick.
Eugene Richard of the Buick Manufacturing Company was awarded US Patent #771,095 in 1904 for the valve in head engine. It included rocker arms and push rods, a water jacket for the head which communicated with the one in the cylinder block, and lifters pushed by a camshaft with a 2-to-1 gearing ratio to the crankshaft. Arthur Chevrolet was awarded US Patent #1,744,526 for an adapter that could be applied to an existing engine, thus transforming it into an Overhead Valve Engine.
The Wright Brothers built their own airplane engines, and starting in 1906, they used overhead valves for both exhaust and intake, with push rods and rocker arms for the exhaust valves only, the intake valves being "automatic suction" valves. They even built a V-8 engine with this valve configuration in 1910. In 1949, Oldsmobile introduced the Rocket V8, the first V-8 engine with OHV's to be produced on a wide scale.
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 600 horsepower (447 kW) 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 98 hp (73 kW) at 8,400 rpm, with no risk of valve bounce. Since 2013, BMW flat-twin motorcycle engines have had OHC valve actuation.
OHV engines have some advantages over OHC engines:
- Smaller overall packaging: because of the camshaft's location inside the engine block, OHV engines are more compact than an overhead cam engine of comparable displacement. For example, Ford's 4.6 L OHC modular V8 is larger than the 5.0 L I-head Windsor V8 it replaced. GM's 4.6 L OHC Northstar V8 is slightly taller and wider than GM's larger displacement 5.7 to 7.0 L I-head LS V8. The Ford Ka uses the Kent Crossflow/Endura-E OHV engine to fit under its low bonnet line. Because of the generally more compact size of an engine of a given displacement, a pushrod engine of given external dimensions can have significantly greater displacement than an OHC engine of the same external size. As a result, the pushrod engine can sometimes produce just as much power as the OHC engine, but with greater torque (contrary to popular belief, this is simply due to the greater displacement of the pushrod engine versus the OHC engine rather than any inherent advantage of the pushrod design for torque production).
- Only a single head casting is needed on vee and flat pushrod engines: Because of the need to drive both cams on one side of the engine, the camshaft orientation on OHC heads must be the same for both heads of a flat and vee engine. This means that the heads must be (more or less) mirror images of each other. And this requires that two different head castings be produced. Pushrod heads can simply be flipped around, which allows a single casting to be used on both cylinder banks.
- Simpler drive system: OHV engines have a less complex drive system for the camshaft when compared with OHC engines. Most OHC engines drive the camshaft or camshafts using a timing belt, a chain, or multiple chains. These systems require the use of tensioners which add complexity. In contrast, an OHV engine has the camshaft positioned close to the crankshaft which may be driven by a much shorter chain or even direct gear connection. However, this is somewhat negated by a more complex valvetrain requiring pushrods.
- Hydraulic lifters: Although RPM capability is limited by the use of hydraulic lifters, the valve lash is self-adjusting for the lifetime of the engine, reducing a significant maintenance requirement. Some OHC engines also use hydraulic lifters/lash adjusters, but the implementation is more complex in OHC designs.
- Simpler lubrication system: Because OHV engines have no camshafts in the heads, the heads have much more modest lubrication requirements than the heads in OHC engines. Therefore, there is no need for oil galleys to supply the heads with oil or oil galleys in the head to provide lubrication for the cam bearings. OHV heads only need lubrication for the rocker arms at the pushrod end, trunnion, and rocker tip. This lubrication to is typically provided through the pushrods themselves rather than a dedicated lubrication system in the head. And lubrication for the camshaft is provided through the same block galleys that provide oil for the main bearings. The more modest lubrication needs of an OHV engine also mean that a smaller, lower capacity oil pump can be used.
Some specific problems that remain with overhead valve (OHV) engines:
- Limited engine speeds or RPM: OHV engines have more valvetrain moving parts. OHV engines also typically use only a single intake and exhaust valve, which results in large (and heavy) valves, valve springs, and retainers. Thus, the valvetrain in an OHV engine has greater inertia and mass. As a result, they suffer more easily from valve "float", and may exhibit a tendency for the pushrods, if improperly designed, to flex or snap at high engine speeds. Therefore, OHV engine designs cannot spin at engine speeds as high as OHC Modern OHV engines are usually limited to about 6,000 to 8,000 revolutions per minute (rpm) in production cars, and 9,000 rpm to 10,500 rpm in racing applications. In contrast, many modern DOHC engines may have rev limits from 6,000 rpm to 9,000 rpm in road car engines, and in excess of 20,000 rpm (though now limited to 15,000 rpm) in current Formula One engines using pneumatic valve springs. High-revving pushrod engines are normally solid (mechanical) lifter designs, flat and roller. In 1969, Chevrolet offered a Corvette and a Camaro model with a solid lifter cam pushrod V8 (the ZL-1) that could rev to 8,000 rpm. The Volvo B18 and B20 engines can rev to more than 7,000 rpm with their solid lifter camshaft. However, the LS7 of the C6 Corvette Z06 is the first production hydraulic roller cam pushrod engine to have a redline of 7,100 rpm. The Honda CX500 motorcycle engine has a 9650rpm redline, well above the usual limits for auto engines, due to the lighter weight of components.
- Limited cylinder head design flexibility: overhead camshaft (OHC) engines benefit substantially from the ability to use multiple valves per cylinder, as well as much greater freedom of component placement, and intake and exhaust port geometry. Most modern OHV engines have two valves per cylinder, while many OHC engines can have three, four or even five valves per cylinder to achieve greater power. Though multi-valve OHV engines exist, their use is somewhat limited due to their complexity and is mostly restricted to low- and medium-speed diesel engines, with a few notable exceptions such as the four valve per cylinder Honda CX500 motorcycle, and the Harley-Davidson Milwaukee-Eight engine. In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods and the need to accommodate them in the head casting. Spark plug placement is also less ideal in pushrod engines. This is important, since a centrally located spark plug improves combustion efficiency and reduces both emissions and tendency to detonate by reducing flame travel distance (which also reduces combustion time). DOHC engines with four valves per cylinder can have a truly centrally located spark plug because this space is free from both valves in the combustion chamber and valvetrain above this central area. Even SOHC engines with four valves per cylinder can usually accommodate a central spark plug. But since pushrod engines almost always have only two valves per cylinder, it is impossible to have a central spark plug.
- Noise and refinement: OHV engines are generally noisier than their OHC counterparts owing to the increased complexity of the valvetrain and the adoption of chain or gear based camshaft drive.
- Maintenance: The location of the camshaft in the cylinder block often necessitates removal of the engine whenever camshaft work is required. This is particularly true for front wheel drive applications with a transversely mounted engine. Longitudinally mounted OHV engines suffer less from this problem as the camshaft can be withdrawn from the front of the engine after removal of the radiator. Additionally, replacement of lifters generally requires removal of the cylinder heads. And cam bearing replacement generally requires the removal and complete teardown of the engine.
- Increased valvetrain friction and wear: Because OHV engines generally use only a single intake and exhaust valve, the valves are larger and heavier than those used in multivalve OHC engines. Furthermore, when the valve is closed, the spring must accelerate not only the valve and rocker arm, but also the pushrod and lifter. Therefore, OHV engines must use heavier valve springs than OHC engines (multivalve or otherwise). As a result, valvetrain friction and wear is increased. This is especially true with high performance OHV engines utilizing high lift/duration/ramp rate cams with heavier than stock valve springs.
- Limited ability to use variable valve timing: Because OHV engines use a single camshaft for all valves, the ability to independently vary intake and exhaust valve timing is limited, although it is possible to vary the phase of one set of valves while the other set's timing stays constant (as in the "CamInCam" or "DuoCam" system). Also, because all cam lobes are on a single camshaft, there is little to no room for the extra high RPM lobes required for two stage systems like Hondas’s VTEC. Because of these factors, variable valve timing on OHV engines is limited to cam phasing systems that change intake and/or exhaust valve timing with no ability to vary lift or duration.
1994 Mercedes/Ilmor Indianapolis 500 engineEdit
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Each year, the Indianapolis 500 bears some vestige of its original purpose as a proving ground for automobile manufacturers, in that it once gave an advantage in engine displacement to engines based on stock production engines, as distinct from out-and-out racing engines designed from scratch. One factor in identifying production engines from racing engines was the use of pushrods, rather than the overhead camshafts used on most modern racing engines; Mercedes-Benz realized before the 1994 race that they could very carefully tailor a purpose-built racing engine using pushrods to meet the requirements of the Indy rules and take advantage of the 'production based' loophole, but still design it to be a state of the art racing engine in all other ways, without any of the drawbacks of a real production-based engine. They entered this engine in 1994, and because of the higher boost pressure and larger displacement that the "loophole" allowed pushrod engines, dominated the race. After the race, the rules were changed in order to reduce the amount of boost pressure supplied by the turbocharger. This amount was still 13% higher than what was allowed for the OHC engines. The engine was also allowed to retain its considerable displacement advantage. The inability of the engine to produce competitive power output after this change caused it to become obsolete after just the one race. Mercedes-Benz knew this beforehand, deciding that the cost of engine development was worth one win at Indianapolis.
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