An overhead valve (OHV) engine, also informally called pushrod engine or I-head engine, is a type of piston engine that places the camshaft 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. The more modern overhead camshaft (OHC) design (still literally overhead valve) avoids the use of pushrods by putting the camshaft in the cylinder head.

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.

Few pushrod type engines remain in production outside of the United States market, and even American manufacturer Ford no longer offers pushrod engines in new vehicles. 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.

History

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 or OHV 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.

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, and these are designated OHC instead - either single overhead camshaft (SOHC) or double overhead camshaft (DOHC).

Advantages

Overhead valve (OHV) engines have specific advantages:

''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 venerable Kent Crossflow OHV engine to fit under its low bonnet line.

''Less complex drive system'' — OHV engines have a less complex drive system to time 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 some complexity to the engine. In contrast a OHV engine has the camshaft positioned just above crankshaft and can be run with a much smaller chain or even direct gear connection.

Limitations

Some specific problems that remain with overhead valve (OHV) engines: ''Limited engine speeds or RPM'' — OHV engines have more valvetrain moving parts, thus more valvetrain 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 revolve ("rev") 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 18,000 rpm) in current Formula One race 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.

''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. In OHV engines, the size and shape of the intake ports as well as the position of the valves are limited by the pushrods.

1994 Mercedes/Ilmor Indianapolis 500 engine

The Indy 500 race in Indianapolis each year 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.

See also

Valvetrain

Overhead camshaft

Cam-in-block

Flathead engine

References

External links

Pushrod (OHV), SOHC and DOHC engine animated diagrams

Ferrari Enzo: The Engine

The Pushrod Engine Finally Gets its Due

Category:Cam-in-block valvetrain configurations Category:Engine valvetrain configurations Category:Motorcycle engines Category:Scottish inventions

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:''For the band (previously known as Black Eyed Sceva), see Model Engine.''

In radio-controlled modeling, a model engine is an internal combustion engine used to power a radio-controlled aircraft, radio-controlled car, radio-controlled boat, free flight and control line aircraft, and tether car models also use these engines. Because of the square-cube law, the behaviour of many machines does not always scale up or down at the same rate as the machine's size (and often not even in a linear way), usually at best causing a dramatic loss of power or efficiency, and at worst causing them not to work at all. Methanol and nitromethane are proven solutions to enhance the power of an automobile engine (Top Fuel) and they can even get these small engines running.

Overview

The fully functional, albeit small, engines vary from the most common single-cylinder two-stroke to the exotic single and multiple-cylinder four-stroke, the latter taking shape in boxer, v-twin, inline and radial form, a few Wankel engine designs are also used. Most model engines run on a blend of methanol, nitromethane, and lubricant (either castor or synthetic oil). Two-stroke model engines, most often designed since 1970 with Schnuerle porting for best performance, range in typical size from .12 cubic inches (2 cubic centimeters) to 1.2 ci (19.6 cc) and generate between .5 horsepower (370 watts) to 5 hp (3.7 kW), can get as small as .010 ci (.16 cc) and as large as 3-4 ci (49–66 cc). Four-stroke model engines have been made in sizes as small as 0.20 in3 (3.3 cm3) for the smallest single-cylinder models, all the way up to 3.05 in3 (50 cm3) for the largest size for single-cylinder units, with twin- and multi-cylinder engines on the market being as small as 10 cm3 for opposed-cylinder twins, while going somewhat larger in size than 50 cm3, and even upwards to well above 200 cm3 for some model radial engines. While the methanol and nitromethane blended "glow fuel" engines are the most common, many larger (especially above 15 cm3/0.90 cu.in. displacement) model engines, both two-stroke and a few four-stroke examples, are spark ignition, and are fueled with gasoline. This article concerns itself with the methanol engines; gasoline-powered model engines are similar to those built for use in string trimmers, chainsaws, and other yard equipment.

The majority of model engines have used, and continue to use, the two-stroke cycle principle to avoid needing valves in the combustion chamber, but a growing number of model engines use the four-stroke cycle design instead. Both reed valve and rotary valve-type two-strokes are common, with four-stroke model engines using either conventional poppet valve, and rotary valve formats for induction and exhaust.

The engine shown to the right has its carburetor in the center of the zinc alloy casting to the left. (It uses a flow restriction, like the choke on an old car engine, because the venturi effect is not effective on such a small scale.) The valve reed, cross shaped above its retainer spring, is still beryllium copper alloy, in this old engine. The glow plug is built into the cylinder head. Large production volume makes it possible to use a machined cylinder and an extruded crank case (cut away by hand in the example shown). These Cox Bee reed valve engines are notable for their low cost and ability to survive crashes. The components of the engine shown come from several different engines.

Glowplug engines

Glow plugs are used for starting as well as continuing the power cycle. The glow plug consists of a durable, mostly platinum, helically wound wire filament, within a cylindrical pocket in the plug body, exposed to the combustion chamber. A small direct current voltage (around 1.5 volts) is applied to the glow plug, the engine is then started, and the voltage is removed. The burning of the fuel/air mixture in a glow-plug model engine, which requires methanol for the glow plug to work in the first place, and sometimes with the use of nitromethane for greater power output and steadier idle, occurs due to the catalytic reaction of the methanol vapor to the presence of the platinum in the filament, thus causing the ignition. This keeps the plug's filament glowing hot, and allows it to ignite the next charge. Since the ignition timing is not controlled electrically, as in a spark ignition engine or by fuel injection, as in an ordinary diesel, it must be adjusted by the richness of the mixture, the ratio of nitromethane to methanol, the compression ratio, the cooling of the cylinder head, the type of glow plug, etc. A richer mixture will tend to cool the filament and so retard ignition, slowing the engine, and a rich mixture also eases starting. After starting the engine can easily be leaned (by adjusting a needle valve in the spraybar) to obtain maximum power. Glowplug engines are also known as nitro engines. Nitro engines require a 1.5 volt ignitor to light the glow plug in the heat sink. Once primed, pulling the starter with the ignitor in will start the engine.

Diesel engines

Diesel engines are an alternative to methanol glow plug engines. These "diesels" run on a mixture of kerosene, ether, castor oil or vegetable oil, and Amsoil cetane or amyl nitrate booster. Despite their name, their use of compression ignition, and the use of a kerosene fuel that is similar to diesel, model diesels share very little with full-size diesel engines.

Full-size diesel engines, such as those found in a truck, are fuel injected and either two-stroke or four-stroke. They use compression ignition to ignite the mixture: the compression within the cylinder heats the inlet charge sufficiently to cause ignition, without requiring an applied ignition source. A fundamental feature of such engines, unlike petrol (gasoline) engines, is that they draw in air alone and the fuel is only mixed by being injected into the combustion chamber separately. Model diesel engines are instead a carbureted two-stroke using the crankcase for compression. Their carburettor supplies a ''mixture'' of fuel and air into the engine, with the proportions kept fairly constant and their total volume throttled to control the engine power. Apart from sharing the diesel's use of compression ignition, their construction has more in common with a small two-stroke motorcycle or lawnmower engine. In addition to this, model diesels have variable compression ratios. This variable compression is achieved by a "contra-piston," at the top of the cylinder, which can be adjusted by a screwed "T-bar". The swept volume of the engine remains the same, but as the volume of the combustion chamber at top dead centre is changed by adjusting the contra-piston, the compression ratio (swept volume + combustion chamber / combustion chamber) changes accordingly.

Model diesels are found to produce more torque than glow engines of the same displacement, and are thought to get better fuel efficiency, because the same power is produced at a lower RPM, and in a smaller displacement engine. However, the specific power may not be significantly superior to a glow engine, due to overbuilding, to assure that the engine can withstand the much higher compression, sometimes reaching ratios of as high as 30:1. Diesels also run significantly quieter, due to the more rapid combustion, unlike glow engines, in which combustion may still be occurring when the exhaust ports are uncovered, causing a significant amount of noise.

See also

Cox Model Engines

Glow plug

Glow fuel

O.S. Engines

Category:Engine technology Category:Model engines Category:Radio control Category:Scale modeling

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