28-01-2011, 11:54 AM
The radial engine is an internal combustion engine configuration in which the cylinders point outward from a central crankshaft like the spokes on a wheel. This configuration was very commonly used in aircraft engines before being superseded by turboshaft and turbojet engines. It is a reciprocating engine.
The cylinders are connected to the crankshaft with a master-and-articulating-rod assembly. One cylinder has a master rod with a direct attachment to the crankshaft. The remaining cylinders pin their connecting rods attachments to rings around the edge of the master rod (see animation). Four-stroke radials almost always have an odd number of cylinders, so that a consistent every-other-piston firing order can be maintained, providing smooth running.
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DEBATE OF USE
The debate about the merits of the radial vs. the inline continued throughout the 1930s, with both types seeing some use. The radial was more popular largely due to its simplicity, and most navy air arms had dedicated themselves to the radial because of its improved reliability for over-water flights and better power/weight ratio for aircraft carrier takeoffs. Although inline engines offer smaller frontal area than radials, inline engines require the added weight and complexity of cooling systems and are generally more vulnerable to battle damage.
The vast majority of radial-engined aircraft designed since the 1930s were also were fitted with NACA cowlings to reduce drag & to also enhance forward thrust by virtue of its airfoil effect.
MULTI-ROW RADIALS
Originally radial engines had but one row of cylinders, but as engine sizes increased it became necessary to add extra rows. Most did not exceed two rows, but the largest radial engine ever built in quantity, the Pratt & Whitney Wasp Major, was a 28-cylinder 4-row radial engine used in many large aircraft designs in the post-World War II period. The USSR also built a limited number of Zvezda 42-cylinder diesel boat engines featuring 6 rows with 7 banks of cylinders, bore of 160 mm (6.3 in), stroke of 170 mm (6.7 in), and total displacement of 143.5 liters (8,756 in³). The engine produced 4,500 kW (6,000 hp) at 2,500 rpm.
INSIDE A RADIAL ENGINE
The radial engine idea is very simple -- it takes the pistons and arranges them in a circle around the crankshaft
-- radial engines typically have anywhere from three to nine cylinders. The radial engine has the same sort of pistons, valves and spark plugs that any four-stroke engine has. The big difference is in the crankshaft.
Instead of the long shaft that's used in a multi-cylinder car engine, there is a single hub -- all of the piston's connecting rods connect to this hub. One rod is fixed, and it is generally known as the master rod. The others are called articulating rods. They mount on pins that allow them to rotate as the crankshaft and the pistons move.
PISTON
In general, a piston is a sliding plug that fits closely inside the bore of a cylinder.
Its purpose is either to change the volume enclosed by the cylinder, or to exert a force on a fluid inside the cylinder
There are two ways that a piston engine can make power. These are the two-stroke cycle and the four-stroke cycle. A single cylinder two-stroke engine produces power every crankshaft revolution, while a single cylinder four-stroke engine produces power every other crankshaft revolution. Older designs of small two-stroke engines produced more pollution than four stroke engines, however modern two-stroke designs, like the Vespa ET2 Injection utilise fuel-injection and are as clean as four-strokes. Large diesel two-stroke engines, as used in ships and locomotives, have always used fuel injection and produce low emissions. One of the biggest internal combustion engines in the world, the Wärtsilä-Sulzer RTA96-C is a two-stroke; it is bigger than most two-story houses, has pistons nearly 1 metre in diameter and is one of the most efficient mobile engines in existence. In theory, a four stroke engine has to be larger than a two stroke engine to produce an equivalent amount of power. Two stroke engines are becoming less common in developed countries these days, mainly due to manufacturer reluctance to invest in reducing two-stroke emissions. Traditionally, two stroke engines needed more maintenance, even though they have fewer moving parts and tended to wear out faster than four stroke engines, however fuel-injected two-strokes achieve better engine lubrication and cooling and reliability should improve considerably.
Master rod
In a reciprocating piston engine, the connecting rod or conrod connects the piston to the crank or crankshaft.
In modern automotive internal combustion engines, the connecting rods are most usually made of steel for production engines, but can be made of aluminium (for lightness and the ability to absorb high impact at the expense of durability) or titanium (for a combination of strength and lightness at the expense of affordability) for high performance engines, or of cast iron for applications such as motor scooters. They are not rigidly fixed at either end, so that the angle between the con rod and the piston can change as the rod moves up and down and rotates around the crankshaft.
PISTON RING
Is an open-ended ring that fits into a groove on the outer diameter of a piston in an internal combustion engine.
Piston Rings The three main functions of piston rings in internal combustion engines are:
1. Sealing the combustion chamber.
2. Supporting heat transfer from the piston to the cylinder wall.
3. Regulating motor oil consumption.
The gap in the piston ring compresses to a few thousandths of an inch when inside the cylinder head. Most automotive pistons have three rings: The top two whilst also controlling oil are primarily for compression sealing (compression rings); the lower ring is for controlling the supply of oil to the liner which lubricates the piston skirt and the compression rings (oil control rings). Typical compression ring designs will have an essentially rectangular cross section or a keystone cross section. The periphery will then have either a barrel profile (top compression rings) or a taper or taper napier form (second compression rings. There are some taper faced top rings and on some old engines simple plain faced rings were used. Oil control rings typically are of 3 types. Single piece cast iron, helicoil spring backed cast iron or steel, multipiece steel. The spring backed oil rings and the cast iron oil rings have essentially the same range of peripheral forms which consist of 2 scraping lands of various detailed form. The multipiece oil control rings usually consist of 2 rails or segments (these are thin steel rings) with a spacer expander spring which keeps the two rails apart and provides the radial load.
APPLICATIONS
Radial engines have several advantages for airplanes:
• They can produce a lot of power. A typical radial engine in a B-17 has nine cylinders, displaces 1,800 cubic inches (29.5 liters) and produces 1,200 horsepower.
• Radial engines have a relatively low maximum rpm (rotations per minute) rate, so they can often drive propellers without any sort of reduction gearing.
• Because all of the pistons are in the same plane, they all get even cooling and normally can be air-cooled. That saves the weight of water-cooling.
