If your engine went screech, rattle, ker-thunk in Papua New Guinea, would you be able to fix it yourself? As a sailor, you should be capable of tacking your way out of a jam, but as long as you cruise with an engine, the more you know about the iron genny, then the more self-sufficient youll be in the far corners of the globe.
The first step toward mastering your diesel engine is understanding the basic principles behind it. So it’s only natural that we begin our in-depth series with a fundamental look at these mechanical marvels. We’ll also examine the essential innards that keep them chugging along. In subsequent articles, we’ll delve deeper into the engine and discuss troubleshooting, routine maintenance, and common repairs so that if your engine ever rattles to a stop, you’ll know why and you’ll have half a chance of fixing
The heart of a diesel engine is a piston reciprocating within a cylinder, much as in a piston pump. The trick is turning this reciprocating motion into rotary motion so that the propeller happily spins, pushing the boat along. But before your engine can work this magic, it needs combustion.
Unlike gas engines, which require a spark to ignite the fuel/air mixture in the cylinder, diesel engines are compression-ignition engines, which means the compression of air in the cylinder is alone sufficient to kindle combustion. Because compression in a diesel engine is about twice that of gas engines, diesels must be built of much heavier-duty components than gas engines, which in part explains their high initial cost.
Today, most auxiliary diesels in sailboats are four-stroke-cycle engines with overhead valves (OHV), so we wont discuss the two-stroke variety, like Detroit Diesels. To complete a cycle, the piston of a four-stroke engine–as the name implies–requires four strokes: the intake stroke, the compression stroke, the power stroke, and the exhaust stroke. The design of the crankshaft, to which the pistons are connected, determines the length of each stroke. One half revolution of the crankshaft in a four-stroke engine equals one stroke of a piston. Figure 1 shows what takes place during each piston stroke in a typical OHV four-stroke engine.
The Four Strokes
In the first stroke, the intake stroke, the piston begins near the top of the cylinder. The exhaust valve is closed, and the intake valve is open. The piston moves downward and, like a pump, creates a vacuum that draws air through the intake valve into the combustion chamber. Once the piston reaches the bottom of the cylinder, the intake valve closes, trapping the air inside the cylinder. On the compression stroke, both valves remain closed as the piston moves upward, compressing air inside the cylinder to about 300 to 500 pounds per square inch (psi), depending on the design of the engine. According to the laws of physics, when air is compressed, its temperature increases. In the case of diesel engines, the air must get extremely hot because diesel fuel won’t ignite at temperatures much below 750 F. As the piston gets close to the top of the compression stroke, it approaches what’s known as “top dead center.” At that point, the fuel injector sprays into the cylinder a carefully calibrated amount of fuel that begins to burn, causing the fuel/air mixture to expand rapidly. Both valves remain closed, so the expanding gases force the piston to the bottom of the cylinder, which causes the crankshaft to rotate. This is the power stroke. When the piston reaches the bottom of the power stroke, the rolling inertia of the flywheel carries the crankshaft another half turn back upward in the cylinder for the exhaust stroke. On the exhaust stroke, the exhaust valve opens, and the piston again acts like a pump, pushing gases out of the cylinder through the exhaust valve.
Each cylinder of your engine goes through these four strokes in a carefully engineered sequence, or firing order, that your engines camshaft choreographs. Timed precisely to the crankshaft and the fuel-injection pump, the camshaft determines exactly when the intake and exhaust valves open in relation to the position of the pistons. It also controls the exact moment when each cylinders fuel injector delivers its atomized spray of diesel fuel.
An Inside View
Figure 2, a cutaway view of a typical four-stroke OHV diesel, details the relationships between essential components in the four-stroke cycle. Starting at the top, we see the valve rocker, which controls when the valves open and close. The rocker is activated by the push rod, which the camshaft sets in motion.
Below the valve rocker we see the intake and exhaust valves. The beveled faces of both valves must be evenly mated with their seats in the cylinder head so that the cylinder is tightly sealed during
the compression and power strokes. Next in line are the pistons. To withstand the extreme pressure inside the combustion chamber, diesel-engine pistons must be much tougher than their gas-engine counterparts. Another obvious difference is the doughnut-shaped crown, or top, of the diesel piston, which helps to thoroughly mix the air and atomized fuel for combustion.
Moving below the piston, we find the connecting rod. One end attaches to the bottom of the piston (through what is known as the wrist pin), and the other rides one of the crankshaft-bearing journals. The crankshaft is perhaps the most complex part of any engine because it converts the reciprocating motion of the pistons into rotary motion. Proper balance, extreme strength, and durability must be engineered into every crankshaft. To withstand the extreme loads that the piston transmits through the connecting rod, the crankshaft demands carefully controlled lubrication from the engines oil pump. Any flaw in the lubrication of crankshaft bearings will reduce the engines life. In severe cases, poor lubrication can lead to a nearly instant catastrophic failure of the engine.
Finally, the crankshaft is bolted to the engines flywheel. The flywheel balances the engine and ensures that it runs smoothly. More important, its inertia keeps the engines internal parts rotating between firing pulses in the cylinders. In multicylinder engines, as revolutions per minute increase, this period between firing pulses is measured in fractions of a second.
Small Parts, Big Jobs
We’ve identified the lead actors inside the diesel engine, but what about the bit players? Truth is, without the supporting cast, the whole production would screech to a halt within seconds. The shell-type bearings used in most engines as well as such components as bushings, valve guides, and piston rings all play important roles. Bearings and bushings allow the spinning components to turn smoothly, while the valve guides and piston rings ensure a tight seal in the combustion chamber in each cylinder. Let’s not forget the important pumps found on all engines, particularly the water and oil pumps. The water pump circulates coolant around each cylinder and inside the cylinder head through a water jacket. This cools the upper part of the combustion chamber and keeps the engine operating at peak thermal efficiency. The closed cooling system in a modern engine prevents the buildup of internal rust and scale, which often shortens the life of a raw-water cooled engine. As for the oil pump, it keeps the engine’s lifeblood circulating. The microscopically thin film of oil this pump delivers throughout the engine reduces heat and friction and keeps the heavily stressed parts from coming into direct contact and destroying themselves.
Besides these carefully engineered bits of steel, bronze, iron, copper, aluminum, and magnesium, all engines have gaskets and seals to keep fluids and gases where they belong. On a diesel, the head gasket performs a herculean task. Working under pressure from all sides and with its inside edge exposed to temperatures above 1,000 F, it must keep engine coolant and cylinder gases from escaping their respective chambers. Once a head gasket begins to fail, disaster is imminent unless you catch the problem early and replace the gasket. The photograph on page 82 shows some typical diesel-engine seals and gaskets.
Now you should have a better picture of what your diesel engine looks like on the inside and how the parts interact with each other to make it run. In the next installment, well take a close look at your diesels fuel system, the source of some of the most common–and often easily fixed–problems with diesel engines.
Ed Sherman is Cruising Worlds electronics editor.