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MARINE DIESEL ENGINESMaintenance, Troubleshooting, and Repair
By NIGEL CALDER
McGraw-HillCopyright © 1992 International Marine & 2007 by Nigel Calder
All right reserved.
Chapter OnePRINCIPLES OF OPERATION
In the technical literature, diesel power plants are known as compression ignition (CI) engines. Their gasoline counterparts are of the spark ignition (SI) variety. This idea of compression ignition is central to understanding a diesel engine.
When a given amount of any gas is compressed into a smaller volume, its pressure and temperature rise. The increase in temperature is in direct relation to the rise in pressure, which is directly related to the degree of compression. That is, the rise in temperature is the result of compressing the existing gas into a smaller space, rather than by the addition of extra heat.
For a better understanding, imagine two heaters with exactly the same output. Each has been placed in a separate room. To begin with, both rooms are the exact same temperature, but one is twice the size of the other. Both heaters are turned on. The small room will heat up faster than the large one, even though the output of the heaters is the same. In other words, although the same quantity of heat is being added to both rooms, the temperature of the smaller one rises faster because the heat is concentrated into a smaller space.
This is crudely analogous to what happens when a gas is compressed. At the outset, it has a given volume and contains a certain amount of heat. As the gas is compressed, this quantity of heat is concentrated into a smaller space and the temperature rises, even though no more heat is being added to the gas.
All internal-combustion engines consist of one or more cylinders that are closed off at one end and have a piston driving up the other. In a diesel engine, air enters the cylinder, then the piston is forced up into it, compressing the air.
As the air is compressed, the heat contained in it is concentrated into a smaller and smaller space. The pressure and temperature rise steadily. In a compression ignition engine, this process continues until the air is extremely hot, say around 1,000°F (538°C). This temperature has been attained purely and simply by compression (see Figure 1-1).
Diesel fuel ignites at around 750°F (399°C); therefore, any fuel sprayed into a cylinder filled with air superheated to 1,000°F is going to catch fire. This is exactly what happens: at a precisely controlled moment, fuel is injected into the cylinder and immediately starts to burn. No other form of ignition is needed.
To attain high enough temperatures to ignite diesel fuel, air generally has to be compressed into a space no larger than 1/14 the original size of the cylinder. This is known as a compression ratio of 14:1. The compression ratio is the volume of the cylinder when the piston is at the bottom of its stroke relative to the volume of the cylinder when the piston is at the top of its stroke (see Figure 1-2).
Most diesel engines have compression ratios ranging from 16:1 to 23:1. This is much higher than the average gasoline engine's compression ratio of 7:1 to 9:1. The lower compression ratios of gasoline engines produce lower cylinder pressures and temperatures. As a consequence, the ignition temperature of gasoline is not reached through compression, and the gas-air mixture has to be ignited by an independent source—a spark (hence the designation spark ignition for gasoline engines).
Converting Heat to Power
I've established that when a gas is compressed its temperature rises. It is also true that when a gas is heated in a sealed chamber its pressure rises. In the first instance, no heat is added—the existing gas is merely concentrated into a smaller space, thereby raising its temperature. In the second instance, heat is added to a gas that is trapped in a closed vessel, and this causes the pressure to rise.
This is what happens during ignition in an internalcombustion engine: A body of air is trapped in a cylinder by a piston and compressed. The temperature rises. Fuel is introduced by some means and ignited. The burning fuel raises the temperature in the cylinder even higher, and this raises the pressure of the trapped gases. The increased pressure is used to drive the piston back down the cylinder, resulting in what is termed the piston's power stroke. The engine has converted the heat produced by the burning fuel into usable mechanical power. For this reason, internal-combustion power plants are sometimes known as heat engines.
It is possible to calculate the heat content of the fuel by measuring how many Btu (or joules, in the metric system) are produced by burning a given amount (e.g., 1 gallon or 1 liter). An engine's horsepower (hp) rating (or kilowatt, in the metric system) can also be converted into Btu or joules; e.g., 1 hp = 2,544 Btu. In this way, the heat energy going into an engine (the number of gallons or liters burned per hour x the Btu or joule content of the fuel) can be compared with the power being produced by it. This enables the thermal efficiency of the engine to be determined; i.e., how much of the fuel's heat energy is being converted to usable power.
The average diesel engine has a thermal efficiency of 30% to 40%. In other words, only about one-third of the heat energy contained in the fuel is being converted to usable power. Roughly half of the remaining two-thirds is lost through the exhaust system in the form of hot gases. The rest is dissipated into the atmosphere through the cooling system and by contact with hot engine surfaces (see Figure 1-3). As wasteful as this sounds, diesels are still considerably more efficient than gasoline engines, which have a thermal efficiency of 25% to 35%.
Expansion and Cooling
Just as compressing a gas raises its temperature, so too can reducing the pressure lower the temperature. This is due purely to the expansion of the gas into a larger space, or volume, not to any loss of heat. The greater the reduction in pressure, the lower the resulting temperature of the gas.
As a piston moves down on its power stroke, the volume inside its cylinder increases, causing a fall in pressure and consequently a fall in the temperature of the gases in the cylinder. These declining temperatures reflect the heat of combustion being converted into mechanical power, i.e., the movement of the piston (see Figure 1-4).
The higher the compression ratio of an engine, the greater the expansion of gases on the power stroke. In an engine with a compression ratio of 22:1, for example, the gases will expand into a volume twenty-two times the size of the compression chamber. In an engine with a compression ratio of 7:1, the degree of expansion will only be seven times greater. Since the temperature drops as a gas expands, diesel engines, because of their higher compression ratios, are able to convert more of the heat of combustion into mechanical power than their gasoline counterparts. Hence diesels are more thermally efficient than gasoline engines.
You might well ask, why not increase the compression ratio on the gasoline engine and thereby improve its efficiency?
A gasoline engine draws in fuel with its air supply before compression, either through a carburetor or through fuel injection into the inlet manifold (not the cylinder). A diesel, on the other hand, has the fuel injected after the air has been compressed. Increasing the compression ratio for a gasoline engine would raise its compression temperature beyond the ignition point of gasoline, which would lead to premature combustion of the fuel-air mixture. This would rapidly wreck the engine. To avoid this, the compression ratio on a gasoline engine must be kept low, and the fuel-air mixture must then be set off by a spark at the appropriate moment. This accounts for the need for an electrical ignition system on these engines.
On occasion, gasoline engines can become sufficiently overheated to cause the fuel-air mixture to ignite before it should. This is known as autoignition, or pre-ignition, and most often occurs when the overheated engine is turned off but refuses to quit, even though the ignition has been turned off.
You might next ask, why not jack up the compression ratios on gasoline engines and use fuel injection directly into the cylinders to prevent premature ignition, just as is done on a diesel? Apart from the obvious fact that you now have a diesel engine to all intents and purposes and might as well use the often cheaper diesel fuel, you run up against the nature of gasoline itself, which is far more volatile than diesel.
Even though a diesel engine may be turning over at 3,000 rpm (revolutions per minute), with the power stroke of any one piston lasting no more than 1/100 of a second, the injected diesel fuel burns at a controlled rate, rather than exploding. Indeed, if it fails to burn at the correct rate, ignition problems result and engine damage is likely.
Because of its greater volatility, the same degree of control cannot be maintained over gasoline. Explosive combustion would occur, destroying the high- compression power plant. The gasoline engine, at the current level of technology, is therefore locked into lower compression ratios and decreased thermal efficiency.
Cost And Power-to-Weight
Because diesels feature higher compression ratios than gasoline engines, they are subjected to greater stresses and must be built more ruggedly. To sustain these higher compression ratios and loads, diesels generally have to be machined to closer tolerances. The heavier construction and closer machining tolerances account for the increase in weight and price of diesel engines over gasoline engines of the same power output. In recent years, however, tremendous advances in metallurgy and engine design have achieved drastic weight reductions on many diesels, considerably narrowing this power-to-weight gap.
Types of Diesels
Diesel engines can be four-cycle or two-cycle. The differences will become clear in a moment. Let us first look at a four-cycle engine.
A Four-Cycle Diesel
1. Imagine the piston is at the top of its cylinder. The inlet valve opens as the piston descends to the bottom of its cylinder. The descending piston draws air into the cylinder. When the piston reaches the bottom of its cylinder, the inlet valve closes, trapping the air inside the cylinder (see Figure 1-5). This movement of the piston from the top to the bottom of its cylinder is known as a stroke. It also constitutes one of the four cycles of a four-cycle engine—in this case the suction (induction), or inlet, cycle.
2. The piston now travels up the cylinder, compressing the trapped air. The pressure rises to between 450 and 700 pounds per square inch (psi; as compared to 80 to 150 psi in a gasoline engine) and the temperature to 1,000°F (538°C) or more. This is the compression cycle.
3. Somewhere near the top of the compression stroke, fuel enters the cylinder via the fuel injector and starts to burn. The temperature climbs rapidly—from 2,000°F (1,093°C) to 5,000°F (2,760°C). This increase in temperature causes a rise in pressure to around 850 to 1,000 psi, pushing the piston back down its cylinder. As the piston descends, the cylinder volume increases rapidly, leading to a sharp reduction in the pressure and temperature. This is the third cycle and is known as the power stroke.
4. When the piston nears the bottom of the power stroke, the exhaust valve opens. The cylinder still contains a considerable amount of residual heat and pressure, and most of the gases rush out. The piston then travels back up the cylinder, forcing the rest of the burned gases out of the exhaust valve. This is the fourth, or exhaust, cycle. At the top of the exhaust stroke, the exhaust valve closes and the inlet valve opens, ready to admit a fresh charge of air when the piston descends the cylinder once again. This brings the engine back to the starting point of the four cycles.
A Two-Cycle Diesel
Note: The following is a description of the operation of a Detroit Diesel two-cycle engine, the most common and widely known type. There are engines with other forms of two-cycle diesel operation, but such engines are unlikely to be encountered in small-boat applications.
A Detroit Diesel two-cycle engine operates in essentially the same manner as a four-cycle engine, but condenses the four strokes of the piston into two—once up the cylinder and once down. Here's how:
1. We start with the piston at the top of its cylinder on its compression stroke. The cylinder is filled with pressurized, superheated air. Diesel is injected and ignites. The piston starts down the cylinder on its power stroke. As it descends, the cylinder pressure and temperature fall. When the piston nears the bottom of its power stroke, the exhaust valve opens and most of the burned gases rush out of the cylinder (see Figure 1-6). So far all is the same as for a four-cycle diesel. Now as the piston continues to descend the cylinder, it uncovers a series of holes, or ports, in the cylinder wall. A supercharger or turbocharger blows pressurized air through these ports, pushing the rest of the burned gases out of the cylinder and refilling it with a fresh air charge. The piston has only now reached the bottom of its cylinder and is starting back up again. The exhaust valve closes.
2. As the piston moves back up, it blocks off the inlet ports, trapping the charge of fresh air in the cylinder. Although the piston has only covered a little over one stroke, it has already completed its power stroke, the exhaust process, and the inlet cycle. As the piston comes back up the cylinder on its second stroke, it compresses the fresh air. When it reaches the top of the cylinder, injection and combustion take place. The cycle starts over. The engine has done in two strokes what a four-cycle diesel does in four.
A two-cycle engine has two power strokes for every one power stroke of a four-cycle engine. For a given engine size, a two-cycle engine develops considerably more power than a four-cycle. This leads to lower costs per horsepower and improved power-to-weight ratios.
A two-cycle diesel, however, is less thermally efficient than a four-cycle and has a higher fuel consumption, resulting in higher levels of exhaust pollution. The life of a two-cycle diesel tends to be shorter than that of a four-cycle model because of the higher loads placed on the engine. What's more, for reasons that will become clear later, two-cycle diesels tend to be far noisier in operation than fourcycles, which makes them unsuitable for a wide range of pleasure boat applications.
In recent years, tightening emissions regulations have pretty much put an end to the manufacture of two-cycle diesels.
Principal Engine Components
So far I have talked in a purely abstract fashion of a piston moving up and down its cylinder. To harness a piston to the rest of the engine, and to utilize the mechanical energy developed by its power stroke, this reciprocal motion must be converted to rotary motion. This is done by a crankshaft and connecting rod.
Excerpted from MARINE DIESEL ENGINES by NIGEL CALDER Copyright © 1992 by International Marine & 2007 by Nigel Calder. Excerpted by permission of McGraw-Hill. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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