Chrysler Hemi and Combustion Chamber Design
Chrysler Hemi and Combustion Chamber Design |
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Combustion Chamber - The space within the cylinder when the piston is at the top of its travel. It is formed by the top of the piston and a cavity in the cylinder head. Since most of the air-fuel mixture's combustion takes place in this space, its design and shape can greatly affect the power, fuel efficiency, and emissions of the engine.
Chrysler and Hemi History Hemi is a registered trademark of Chrysler Corporation, and comes from the word hemispherical. The concept of the Hemi engine is a fairly simple one. Instead of having a flat head or a wedge-shaped combustion chamber, the chambers in the Hemi engine’s heads are hemispherically shaped. This design allows for a more efficient engine through better thermal cooling, combustion and compression. The shape allows one to put the intake and exhaust valves in-line, instead of side-by-side, allowing a better air flow through the chamber. It allows for additional smaller valves in some engine designs or larger valves in other designs, greatly enhancing porting. Additionally, the spark plug is typically located in the center of the chamber in a Hemi engine, creating a better fuel/air mixture. Unburned fuel is wasted energy. This design allows for a more efficient burning of the fuel/air mixture, burning more of the fuel in the chamber on each stroke of the engine. The hemispherical design does however have limitations. The same design that gives us advantages in placing valves, also limits us to the number of valves that can be installed. If more than four valves are placed on the cylinder the angle of each valve in relation to the other becomes too complex to effectively manage. New engine designs allow for smaller combustion chambers, cloverleaf and pent roof shaped chambers. The goal with all of these designs, including the Hemi, is combustion chambers that allow for an additional number of valves and spark plugs, better thermal cooling, all of which produces a more efficient and powerful engine. The history of the design is clouded, yet it is clear that Chrysler did not create the first hemispherical engine. Chrysler first built an engine with hemispherical combustion chambers in the World War II era, for fighter aircraft. In 1951 Chrysler produced their first Hemi V8 engine. They were expensive to build, and to purchase, and low sales led to discontinuation after 1959. Hemispherical combustion chambers were commonly used in racing engines built up to the mid-1960s, when the success of the Ford-Cosworth DFV Formula One engine led to a resurgence of four-valve-per-cylinder designs with narrow valve angles incompatible with hemispherical combustion chambers. Additionally, by then, successful racing engines revved up to 10,000 rpm and the large chamber volume of a hemispherical chamber would have caused serious gas-flow and hence power production problems. Performance made a comeback in the 1960s and Chrysler needed an edge on the competition in stock car racing. And so the now-legendary 426 cubic-inch Hemi was born. It won its first race, the 1964 Daytona 500, but it wasn't quite a real production engine. It was banned until 1966, when it became available in street form to anyone with the extra cash ($600-800 option) to purchase one in cars that cost $2,500 to $4,000. In the late 1960s, Hemis were nearly invincible in stock car and drag racing. But after 1971, the Hemi was finished, at least as a street engine; the victim of emissions requirements that were far beyond its design specification. It lived on in drag racing, where it and its derivatives dominated the top classes to the present day. Hemispherical combustion chambers are amenable to supercharging. Chrysler brought the HEMI back in 2003 with the debut of the newest generation of Ram pickups. Now it's in the Chrysler 300C, Dodge Magnum, Durango, Dakota and Jeep Grand Cherokee. Like its predecessors, the new HEMI's two valves per cylinder are operated by pushrods. As in previous Hemis, intake and exhaust valves for each bank of cylinders are at an angle in the head and have their own rockers. In this era, when dual overhead cams and four-valve heads are found in economy cars why did Chrysler go for such a seemingly atavistic design? Although it's more complex than a typical pushrod engine, with its two valves per cylinder in line with the crankshaft and using one set of rockers, it's simpler and more cost-effective than an overhead-cam engine. It produces equivalent power to an overhead cam engine, but costs less to build. Each cylinder in a 5.7-liter V8 is large enough, and the revs are low enough, that the two-valve hemi design works very well. HEMIs can run well on lower octane fuel than some other designs, and Chrysler's newest is happy with less-expensive mid-range gasoline. It's the first variable-displacement engine to make it to production. The ``Multi-Displacement System'' (MDS) is standard equipment on Chrysler 300C and Dodge Magnum, Charger, (2006) Durango and on the HEMI powered 2005 Jeep Grand Cherokee and Commander. The MDS works by deactivating the lifters in four cylinders, keeping their valves closed. No fuel is injected at that time, increasing fuel economy, and no air is pumped, decreasing energy and therefore power that would be lost pumping air. Use of electronic throttle control, electronic fuel injection, sophisticated electronically-controlled hydraulic lifters, and control algorithms makes it all work quickly and seamlessly. It takes only 40 milliseconds (0.040 seconds) to switch between four-cylinder and eight-cylinder modes, and the process can't be felt. Chrysler estimates a 10-20% increase in fuel economy. And if 340 horsepower isn't enough, the 300C SRT-8, will be available in the Spring of 2005. Its HEMI has been enlarged to 6.1 liters and compression increased, to make 425 horsepower and 420 lb-ft of torque. Combustion Chamber Design Hemispherical Combustion Chamber - A round, dome-shaped combustion chamber. This shape permits larger valves and straighter intake and exhaust ports for improved breathing in the combustion chamber. Its small surface area in comparison to volume reduces the amount of heat loss and allows high compression with a minimum of detonation. Because the hemispherical combustion chamber is so efficient, it is often used, even though it costs more to produce. It is used in high performance cars and racing engines. Pent-Roof Combustion Chamber - A combustion chamber whose upper surface resembles a shallow peaked roof. Usually used with four valves per cylinder. A valvetrain with a total of four valves in the combustion chamber, typically two intakes and two exhausts. Compared to the more common two-valve-per-cylinder designs, a four-valve layout offers improved breathing and allows the spark plug to be located closer to center of the combustion chamber. Wedge Combustion Chamber - A combustion chamber using a wedge shape. The combustion chambers are flatter on one end than the other. It is quite efficient and lends itself to mass production and as a result is widely used. The valve is in the longer sloping surface while the spark plug is in the shorter area; the tapered part of the wedge forms a squish zone. "Quench" or "squish" refer to the area in a wedge-shaped chamber designed to create turbulence in the chamber as the piston approaches top dead center (TDC). Spherical Combustion Chamber - The combustion chamber of an internal combustion engine equipped with reciprocating pistons is ideally very compact in design, i.e., without gaps, grooves and edges; the most favorable design would thus be a sphere, but the valves of a four-stroke engine make this impossible. The combustion chamber in diesel engines is located in the piston crown, where it is in fact spherical. Twin Swirl Combustion Chamber - A special design of a four-stroke engine, in which the intake valves are arranged in such a way as to ensure that the gas flow ends in two separate swirls; this design improves swirl and thus enhances combustion of the fuel/air mixture within the cylinder. Popular with motorcycle engine design. | |
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