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Engine, Page 1 of 2

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How the engine works
See Figures 1, 2, 3, 4 and 5

The basic piston engine is a metal block containing a series of chambers. The upper engine block is usually an iron or aluminum alloy casting, consisting of outer walls that form hollow jackets around the cylinder walls. The lower block, which provides a number of rigid mounting points for the bearings that hold the crankshaft in place, is known as the crankcase. The hollow jackets of the upper block add rigidity to the engine and contain the liquid coolant that carries heat away from the cylinders and other engine parts.

An air-cooled engine block consists of a crankcase that provides a rigid mounting for the crankshaft and has studs to hold the cylinders in place. The cylinders are individual, single-wall castings, finned for cooling, and they are usually bolted to the crankcase, rather than cast integrally with the block.

In a water-cooled engine, only the cylinder head is bolted to the block (usually on top). The water pump is mounted directly to the block.

The crankshaft is a long iron or steel shaft (and sometimes aluminum in more high-tech or high performance applications) mounted rigidly at a number of points in the bottom of the crankcase. The crankshaft is free to turn and contains several counterweighted crankpins (one centered under each cylinder) that are offset several inches from the center of the crankshaft and turn in a circle as the crankshaft turns. Pistons are connected to the crankpins by steel connecting rods. The rods connect the pistons at their upper ends with the crankpins at their lower ends. Circular rings seal the small space between the pistons and wall of the cylinders.

When the crankshaft spins, the pistons move up and down in the cylinders, varying the volume of each cylinder, depending on the position of the piston. At least two openings in each cylinder head (above the cylinders) allow the intake of the air/fuel mixture and the exhaust of burned gasses. After intake, the pistons compress the fuel mixture at the top of the cylinder, the fuel is ignited, and, as the pistons are forced downward by the expansion of burning fuel, the connecting rods convert the up and down motion of the pistons into rotary (turning) motion of the crankshaft. A round flywheel at the rear of the crankshaft provides a large, stable mass to smooth out the rotation.

The cylinder heads form tight covers for the tops of the cylinders and contain chambers into which the fuel mixture is forced as it is compressed by the pistons reaching the upper limit of their travel. Each combustion chamber contains at least one intake valve, one exhaust valve, and one spark plug per cylinder (depending on the design). The tips of the spark plugs protrude into the combustion chambers.

The valve in each opening of the cylinder head is opened and closed by the action of the camshaft. The camshaft is driven by the crankshaft through a gear, chain, or belt at 1/2 crankshaft speed (the camshaft gear is twice the size of the crankshaft gear). The valves are operated either through rocker arms and pushrods (overhead valve and some overhead cam engines) or directly by the camshaft using cam followers which usually contain shims for adjustment (overhead cam engine).

Lubricating oil is stored in a pan at the bottom of the engine and is force-fed to all parts of the engine by a gear-type pump, driven from the crankshaft. The oil lubricates the entire engine and seals the piston rings, giving good compression.

Figure 1 Cutaway view of an in-line overhead cam four-cylinder engine.
Cutaway view of an in-line overhead cam four-cylinder engine.

Figure 2 Common automotive cylinder block designs.
Common automotive cylinder block designs.

Figure 3 The four-stroke cycle of a basic two-valve, carbureted, gasoline-fueled, spark-ignition engine.
Click on picture to enlarge view

Figure 4 Cutaway view of a V6 gasoline-powered overhead valve engine.
Cutaway view of a V6 gasoline powered overhead valve engine.

Figure 5 Basic engine cylinder dimensions. The ratio between the total cylinder and clearance volume is the compression ratio.
Basic engine cylinder dimensions. The ratio between the total cylinder and clearance volume is the compression ratio.

The diesel engine
See Figures 6 and 7

Diesel engines, like gasoline powered engines, have a crankshaft, pistons, camshaft, etc. In addition, four-stroke diesels require four piston strokes for the complete combustion cycle, exactly like a gasoline engine. The difference lies in how the fuel mixture is ignited. A diesel engine does not rely on a conventional spark ignition to ignite the fuel mixture. Instead, heat produced by compressed air in the combustion chamber ignites the fuel and produces a power stroke. This is known as a compression-ignition engine.

No fuel enters the cylinder on the intake stroke, only air. Since only air is present on the intake stroke, only air is compressed on the compression stroke. At the end of the compression stroke, fuel is sprayed into the combustion chamber and the mixture ignites.

The fuel/air mixture ignites because of the very high temperatures generated by the high compression ratios used in diesel engines. Typically, the compression ratios used in automotive diesels run anywhere from 16:1-23:1. A typical spark-ignition engine has a ratio of about 8:1-10:1. This is why a spark-ignition engine, which continues to run after you have shut off the engine, is said to be "dieseling." It is running on combustion chamber heat alone. Designing an engine to ignite on its own combustion chamber heat poses certain problems. For instance, although a diesel engine has no need for a coil, spark plugs, or a distributor, it does need what are known as "glow plugs." These look like spark plugs, but are only used to warm the combustion chambers when the engine is cold. Without these plugs, cold starting would be impossible. Also, since fuel timing (rather than spark timing) is critical to a diesel's operation, all diesel engines are fuel-injected rather than carbureted, since the precise fuel metering necessary is not possible with a carburetor.

Figure 6 Cutaway view of an in-line overhead cam four-cylinder diesel engine. Note the similarity to the gasoline engine shown above.
Cutaway view of an in-line overhead cam four-cylinder diesel engine. Note the similarity to the gasoline engine shown earlier.

Figure 7 The four-stroke operating principal applied to a diesel engine.
Cutaway view of an in-line overhead cam four-cylinder diesel engine. Note the similarity to the gasoline engine shown earlier.
Click on picture to enlarge view

The Wankel engine
See Figures 8 and 9

Like a conventional piston engine, the Wankel engine is an internal combustion engine and operates on a four-stroke cycle. Also, it runs on gasoline and the spark is generated by a conventional distributor-coil ignition system. However, the similarities end there.

In a Wankel engine, the cylinders are replaced by chambers, and the pistons are replaced by rotors. The chambers are not circular, but have a curved circumference that is identified as an epitrochoid. An epitrochoid is the curve described by a given point on a circle as the circle rolls around the periphery of another circle which is twice the radius of the generating circle.

The rotor is three-cornered, with curved sides. All three corners are in permanent contact with the epitrochoidal surface as the rotor moves around the chamber. This motion is both orbital and rotational, as the rotor is mounted off center. The crankshaft of a piston engine is replaced by a rotor shaft, and crank throws are replaced by eccentrics. Each rotor is carried on an eccentric. Any number of rotors is possible, but most engines have one or two rotors. The valves of the piston engine are replaced by ports in the Wankel engine housing. They are covered and uncovered by the path of the rotor.

One of the important differences between the Wankel rotary engine and the piston engine is in the operational cycle. In the piston engine, all the events take place at the top end of the cylinder (intake, compression, expansion, and exhaust). The events are spaced out in time only. The Wankel engine is the opposite. The events occur at the same time but at different places around the rotor-housing surface.

The intake phase takes place next to the intake port and overlaps with the area used for compression. Expansion takes place opposite the ports, and the exhaust phase takes place in the area preceding the exhaust port, overlapping with the latter part of the expansion phase. All three rotor faces are engaged in one of the four phases at all times.

Figure 8 Cutaway view of a two rotor Wankel rotary engine.
Cutaway view of a two rotor Wankel rotary engine.

Figure 9 The path of the rotor in the Wankel engine. Note the constantly varying shape of the combustion chamber and the two spark plugs per cylinder.
Click on picture to enlarge view

Turbocharging and supercharging
See Figures 10, 11 and 12

Turbocharging, sometimes called supercharging, has been under investigation practically since the invention of the automobile. Gottlieb Daimler, generally credited with development of the first auto, sought to boost the volumetric efficiency of his first vertical gasoline engine in 1885, but was unable to make it work. It wasn't until 1921 that Mercedes-Benz introduced the first production supercharged vehicles.

Between 1921 and the end of World War II, various companies exploited the use of superchargers and turbochargers for racing, marine, and large truck engines. In 1962, General Motors introduced the Oldsmobile F-85 Jetfire, powered by a turbocharged V8 engine and quickly followed that with a turbocharged version of the Chevrolet Corvair. With gasoline prices going rapidly out of sight, the number of manufacturers offering turbocharging has rapidly increased as manufacturers try to maintain performance, reduce engine size and emissions, and increase fuel economy, all at the same time. Models available run the gamut from the Porsche Turbo and the Volkswagen Turbodiesel to the Supercharged Pontiac.

The word turbocharger is an abbreviation of the word turbo supercharging. Although there is a difference between turbocharging and supercharging, the principle is the same-to drive a small compressor which will increase the quantity of fuel/air mixture going into the combustion chamber as it is needed, increasing the volumetric efficiency of the engine and increasing the power output.

Supercharging accomplishes this by operating the compressor mechanically, through a gear-driven shaft. The supercharger is normally activated on demand, when the accelerator pedal is pushed to the floor. A turbocharger is actually a small turbine, which uses exhaust gasses to spin a turbine wheel mounted on a common shaft with a compressor. As the turbine turns at high speed, it causes the compressor to pack a greater charge of air into the engine's cylinders.

In both systems, air enters through an air intake, passes through an air cleaner, and travels through a duct (usually funnel shaped) to the compressor inlet portion of the turbocharger. From there air is forced through a diffuser into the intake manifold, to the individual cylinders.

The turbocharger itself and the principles involved are extremely simple, but sophisticated engineering problems are created by application. The most critical problem is controlling the manifold or boost pressure. This is the amount of additional boost or pressure created by the turbocharger. The boost must be controlled or the engine will begin to detonate and eventually burn holes in the pistons and self-destruct.

The solution lies in the wastegate or safety valve, which is keyed to intake manifold pressure, exhaust pressure, or a combination of both. At a predetermined pressure, the wastegate valve will open, allowing some of the exhaust gas to pass directly into the exhaust system bypassing the turbocharger. This keeps the intake manifold pressure at a preset maximum.

The second problem with turbocharging is the generally inconsistent quality of gasoline available. If the octane of the fuel is unpredictable, then so is the point at which the engine begins to detonate, making it difficult to set the maximum manifold pressure. The answer to this problem is a knock sensor, a device that detects the harmful pressure waves of detonation in the cylinders and instantly retards the ignition timing to prevent detonation.

The turbocharger itself spins at a maximum speed of about 110,000 rpm at highway speeds and is capable of supplying boost pressure of up to 60-70 psi (413-483kPa) on professional racing engines. However, for the average auto or light truck, 3-9 psi is about the maximum boost pressure expected.

Figure 10 A few of the additional components required for a turbocharged engine. Clockwise from the left: exhaust manifold, exhaust plenum, wastegate assembly, throttle body adapter, and intake manifold.
A few of the additional components required for a turbocharged engine.

Figure 11 Exhaust gas and fresh air paths in a turbocharger.
Exhaust gas and fresh air paths in a Turbocharger.

Figure 12 Air flow through a turbocharged engine.
Air flow through a turbocharged engine.

Two-stroke engines
See Figure 13

Although currently out of production, several vehicles imported into the United States used two-stroke engines. These operated with only a compression stroke and a power stroke. Intake of fuel and air mixture and expulsion of exhaust gases takes place between the power and compression strokes while the piston is near the bottom of its travel. Ports in the cylinder walls replace the cylinder head valves of the four-stroke engine. The crankcase is kept dry of oil, and the entire engine is lubricated by mixing the oil with the fuel so that a fine mist of oil covers all moving parts.

The ports are designed so the fuel and air are trapped in the engine's crankcase during most of the down stroke of the piston. This makes the crankcase into a compression chamber that force-feeds the combustion chambers after the ports are uncovered. The pistons serve as the valves, covering the ports whenever they should be closed.

Figure 13 The two-stroke cycle of a gasoline-powered, spark-ignition engine.
The two-stroke cycle of a gasoline-powered, spark ignition engine.

Engine identification
See Figure 14

It is important for servicing and ordering parts, to know which engine you have. The place to start identifying an engine is with the Vehicle Identification Number (VIN) of the vehicle. The VIN is visible through the windshield on the driver's side of the dash and contains data encoded into a lengthy combination of letters and numbers. A specific letter or number is used to designate the installed engine.

Beginning in 1981, all domestic manufacturers adopted a uniform, 17-digit VIN. The tenth digit of the VIN indicates the model year and the eighth digit indicates the engine code.

Some import manufacturers also follow this rule. Others only use the tenth digit for the model year. In most cases, the engine designation is also located on a tag or a stamped number located on the engine block or bell housing. Check with your dealer or vehicle-specific manual for more information.

Figure 14 The VIN is visible through the driver's side windshield.
The VIN is visible through the driver's side windshield.

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©1998 W. G. Nichols - Chilton's Easy Car Care