Automobile Engine System
Despite challenges from steam, electricity, and more recently the gas turbine engine, the 4-stroke-cycle gasoline-burning piston engine still is considered the most suitable power plant for automobiles. During the 1960's more than half of all United States passenger cars were equipped at the factory with V-8 engines; that is, engines with 8 cylinders arranged in the shape of a V. The shorter crankshaft of the V-8 offers several advantages over the in-line engine: a rigid construction that permits the use of higher compression ratios; less engine vibration and noise from crankshaft "whipping"; and shorter over-all engine length. At the same time, car manufacturers continued to offer a wide choice of other engine configurations: straight 6, slant 6, V-6, and pancake 6; straight 4 and slant 4; front-mounted and rear-mounted; water-cooled and air-cooled. Four-cylinder engines ensure maximum economy in cars of modest size, while the larger and more powerful 6-cylinder engine provides reasonable economy and better performance in standard cars. Horsepower ranged from 80 to 425, and compression ratios hit a high of 13.5:1 with an average standard compression ratio of 9.16:1.
The basic unit of the internal combustion engine commonly used in automobiles is a cylinder, closed at one end, in which a piston is free to move up and down. When a mixture of air and gasoline is burned above the piston, the resulting expansion drives the piston down into the cylinder. This vertical thrust is transmitted by a connecting rod and suitable bearings to an offset crankshaft journal, where it is converted into rotary motion.
All United States automobile engines operate on the 4-stroke cycle, in which each piston travels alternately from the top to the bottom of the cylinder four times as it completes the following cycle of events:
- Intake Stroke: Piston moves down, pulling a mixture of air and gasoline into the cylinder through the open intake valve.
- Compression Stroke: Intake valve closes; piston moves up and compresses the mixture.
- Power (Expansion) Stroke: Near the top of the compression stroke the highly compressed charge of air and gasoline is ignited by the spark plug and, expanding rapidly, forces the piston back down to the bottom of the cylinder.
- Exhaust Stroke: Exhaust valve opens; piston moves up and sweeps the cylinder clear of burned gases.
At the end of the exhaust stroke the piston is at the top of the cylinder, with the exhaust valve closing, ready to begin the cycle again.
Basically, the modern multi-cylinder automotive engine consists of cylinders that have been cast into an integral grouping or "block" and provided with a common crankshaft. Although most automotive engine blocks are made of cast iron, American car producers have been offering lightweight aluminum engines in some models since 1960. The relatively soft aluminum castings are fitted with cast-iron sleeves for moving parts, to provide acceptable wear resistance.
In water-cooled engines, the block is cast with passages in it to accommodate a flow of cooling water around the cylinders, valves, and combustion chambers. Air-cooled engines dissipate their heat via integral fins that permit maximum exposure of the engine's hot exterior surfaces to a stream of blower-driven air.
Piston and Connecting Rod
The piston, together with its connecting rod, transmits the force of combustion-chamber explosions to the engine crankshaft. To provide the tight piston-to-cylinder seal necessary to prevent the escape of high cylinder working pressures, split steel and cast-iron rings are installed in grooves cut into the outer wall of the piston. The upper, or compression, rings are plain; slots are cut into the bottom ring to facilitate the return of excess lubricating oil to the engine crankcase. All pistons in American-made cars are made of lightweight aluminum alloys.
An I-beam-section connecting rod, made of forged or cast steel, links the piston to the crankshaft. Its upper end is connected to the piston by a steel wristpin; its lower end (fitted with precision steel-backed bearing shells of copper-lead, babbitt, or aluminum alloys) is split to permit its installation on the crankshaft.
Bolted to the top of the cylinder block, the cast-iron or aluminum cylinder head provides a separate combustion chamber of specific volume and shape immediately above each piston. In these spaces and the cylinder bore occur the four events that make up the 4-stroke operating cycle of the automotive gasoline engine.
There are two types of cylinder heads: (1) L-head, which contains no valves (intake and exhaust valves are arranged in the cylinder block); and (2) overhead-valve, in which both intake and exhaust valves are installed in the cylinder head. Virtually all modern passenger car engines are of overhead-valve design. The cylinder head for an overhead-valve engine carries not only the spark plugs and cooling water jackets, but also the valves and most of the valve-actuating mechanism. A gasket of copper-clad asbestos or sheet steel provides a tight seal between cylinder head and cylinder block.
The function of the engine crankshaft is to convert the straight-line, reciprocating (back-and-forth) movement of the pistons into a rotary motion. The crankshaft is supported by and free to turn in split precision journal bearings at the bottom of the cylinder block. Journal bearings are steel shells lined with copper-lead, babbitt metal, or aluminum alloys. Those portions of the shaft that run in the bearings are called journals; the U-shaped cranks between journals are -known as crankpins.
Crankshafts may be forged from plain carbon steel or made of special cast iron. Some manufacturers cast crankshafts with hollow journals, a design that permits a double saving in weight and materials because of the reduced counter-weighting requirements. Counterweights are positioned opposite the crankpins to minimize engine vibration and reduce centrifugal-force loads on bearings at high engine speeds. Crankshafts are balanced statically (at rest) and dynamically (in motion) at the factory by drilling precise amounts of metal from the counterweights.
Power impulses transmitted to the offset crankpins actually twist the crankshaft, and when these forces are relieved, the crankshaft has a tendency to unwind. Because of the inertia and elasticity of the shaft, these vibrations tend to become increasingly severe. In fact, if the torsional (twisting) vibrations should match the natural frequency of the shaft, the resulting vibration could be severe enough to break the crankshaft.
In American cars a torsional vibration damper is installed on the forward end of the crankshaft. It consists of a steel-disk inertia weight that is elastically connected to the crankshaft by a rubber mounting ring. Under the stress of crankshaft torsional vibrations, the rubber deforms and permits relative motion between the crankshaft and the inertia weight. By vibrating out of phase with the natural frequency of the crankshaft, this device prevents any destructive buildup of torsional vibrations.
Because each cylinder is producing power during only one stroke of the 4-stroke cycle, some means must be provided to keep the crankshaft rotating during the other three strokes. A flywheel bolted to the rear crankshaft flange provides the necessary inertia by storing enough energy during the power stroke to carry the crankshaft through the rest of the cycle. Since there is an overlapping of power strokes in a multicylinder engine, the weight of the flywheel is reduced as the number of cylinders is increased.
One intake and one exhaust valve are provided for each cylinder. These valves have three distinct functions: (1) the intake valve admits an air-gasoline mixture from the carburetor and intake manifold; (2) both intake and exhaust valves seal off the combustion space during the compression and power strokes; and (3) the exhaust valve opens to permit the discharge of exhaust gases from the cylinder.
The poppet valve, consisting of a circular head and stem in one piece, is universally used in American automotive engines. The underside of the outer edge of each valve is machined to a 30° or 45° angle, and a corresponding seat is cut into the cylinder block or cylinder head.
The valves are opened in a correctly timed sequence by the revolving camshaft; they are returned to their seats, and held tightly against them, by helical steel springs. The gastight seal thus formed prevents any loss of compression or firing pressures from the engine cylinders.
Engines are identified by their valve arrangements as L-head or overhead-valve engines. In the L-head engine the valves are arranged along one side of the cylinder block and actuated by valve lifters that bear directly on the camshaft lobes. In the overhead-valve engine the valves are installed in the cylinder head and operated either by an overhead camshaft or, more commonly, by a rocker-arm, push-rod, and valve-lifter arrangement.
Combustion-chamber temperatures range from 150° to 5000° F (65.5°-2760° C). Although the intake valve is cooled effectively by the incoming charge of air and gasoline, the exhaust valve must transfer its heat to the engine-cooling water jackets through the valve seat and valve guide. To resist warping and subsequent leakage of cylinder pressures, exhaust valves generally are made of age-hardenable high-chromium austenitic steels; intake valves, of medium carbon steels.
Valve seats are machined directly into the cast-iron block or cylinder head, then induction-hardened by electromagnetic heating for long life. Hard-steel inserts are used in both the intake- and exhaust-valve seats of aluminum engines. Hydraulic valve lifters ensure quiet valve action by maintaining zero lash . (no clearance) between valve-stem and rocker-arm faces.