Transcription
How a ship engine works. We're looking at a two-stroke marine diesel engine which runs massive cargo ships across oceans. It's called a two-stroke engine because it completes a full power cycle in just two piston strokes.
First, after combustion occurs, the piston is forced downward. That's the first stroke. Then the piston moves back up, compressing air. That's the second stroke. Combustion happens again. Each time the piston goes down and back up, the crankshaft completes one full revolution.
Unlike car engines that operate at thousands of RPM, large ship engines are incredibly slow. They typically run between 60 to 120 RPM and rarely above 200 RPM. Pretty slow, right? These engines are not built to be fast and furious. They are built to be steady and strong.
Bringing the car engine down to its true size, we can compare and see how massive the ship engine is. The engine you see here stands 42 feet tall. That's roughly the height of a four-story building. Let's break it down. This is an engine cylinder. Inside is a piston. Piston rod connected to the connecting rod or conrod. This is the crosshead which guides the piston's motion in a linear direction along the rails. The crankshaft. This engine model has six cylinders, but the largest engine can have 14 cylinders.
To start the engine, combustion doesn't happen right away. Instead, the engine uses starting air from the starting air valve, a highly compressed air that's blown into the cylinder. This blast of air pushes the piston downward to begin rotation. The piston itself travels about 8 ft from top dead center to bottom dead center.
Fresh air is drawn in through these side openings called scavenge ports. As the air is compressed, its temperature rises sharply, reaching above 900° Fahrenheit, hot enough to ignite fuel. At the right moment, fuel is injected into the combustion chamber as a fine mist. Combustion begins.
After combustion, the cylinder is filled with hot burnt exhaust gas. Fresh oxygen-rich air is drawn in and pushes the exhaust gas out through the exhaust valve. The exhaust gas exits the engine and passes through a scrubber system which reduces pollution before it is released into the atmosphere. This is the exhaust gas manifold, the turbocharger, the air cooler, and the charging air manifold.
A turbocharger has two sides, a turbine and a compressor. On the turbine side, hot exhaust gas spins the turbine wheel. That spinning motion powers the compressor side, which draws in fresh air from the inside of the engine room through the air filter. Fresh air is continuously drawn from outside the ship into the engine room. The air is compressed, which makes it hot. So it passes through an air cooler filled with water-cooled tubes, reducing its temperature to around 104° F. Next, the air flows through a mist catcher which removes water droplets to prevent corrosion. This cool, clean air enters the charging air manifold, ready to flow into the cylinder when the scavenge ports open, and the cycle of replacing exhaust gas with fresh air repeats.
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These are cooling water constantly flowing to absorb the heat and cool down these cylinders. On the exhaust valve stem, there is a rotator. Each time the valve opens and the exhaust gas passes through, it rotates the valve a little. The purpose is to maintain even wear on the valve disc every time it makes contact. The exhaust valve stays closed by default. This is the air piston. Below it is a chamber filled with compressed air known as the air spring. The air pressure pushes the piston upward to keep the valve closed. To open the valve, hydraulic oil is pumped in. Its pressure must be greater than the air spring's to push the piston down and open the valve. When the oil drains, the air spring pushes the piston back up, closing the valve.
There's a whole jungle of pipes and lines running across a ship engine. But let's just focus on the few of the main ones. These are the hydraulic oil line and the fuel injection lines. There are also plenty more for cooling water, compressed air, and lube, but we'll keep it simple for now. On the cylinder head, we have the fuel injectors and the starting air valve. Some hybrid engines also have LNG injectors. Each cylinder has a hydraulic cylinder unit. It includes a fuel booster that raises fuel pressure for injection and an exhaust valve actuator to control hydraulic oil flow. This whole unit is managed by an electronic control system or ECS. It precisely controls fuel and hydraulic oil timing to match the piston's motion. Before ECS, engines used a mechanical camshaft to do the same job. The camshaft is driven by a chain gear connected to the crankshaft. Cam lobes are precisely shaped and timed to control the flow of hydraulic oil and fuel injection. They are positioned to match the exact timing of the piston's movement. Not all engines have an electronic control system. Many ships still use a camshaft.
To move the ship astern, the engine must be reversed. That means the crankshaft changes its direction of rotation. In large cargo ships with low-speed engines, there's no gearbox. The propeller shaft is directly connected to the engine and spins with it. So before reversing, the engine must come to a complete stop. Then starting air is sent to the cylinders, beginning with the one that has its crank web on the opposite side, one cylinder at a time. The engine starts rotating in the reverse direction. Once rotation is established, the normal combustion cycle resumes just in the opposite direction. Now the propeller turns astern and the ship begins moving backward.
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The engine is massive. So massive, it's built with walking platforms to allow crew members to access and service different parts.
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Inside the crankcase, the structure is supported by bedplates and A-frames which form the backbone of the engine's frame. This is the stuffing box, a sealing component that prevents oil and gas from leaking between the scavenge space and the crankcase. Everything down here is slippery; it's all coated in lubricant. There are crankcase access doors and ladders that lead inside. Yes, people actually have to go in there for inspections. Entering the crankcase is a high-risk job and it requires a lot of safety procedures.
On this side, there are explosion relief valves. This is a super important component. During operation, heat causes lubricant to vaporize and pressure can build up inside. If not released, the pressure could lead to a crankcase explosion. These valves are designed to release internal pressures but not let outside air in. If an explosion occurs and fresh air is drawn in, it can trigger a secondary, more powerful explosion. To prevent that, each valve is assembled with a spring and a flame arrestor. When pressure gets too high, the valve opens briefly to release it. The flame arrestor prevents any flame from escaping to the outside. Once pressure drops, the spring immediately closes the valve, sealing it again.
At the very bottom of the crankcase is the oil sump, a reservoir where all the used lubricating oil is collected. From the sump, the oil is pumped out, cooled down, filtered, and sent back into the engine for another cycle of lubrication. Lube flows through a network of lubrication lines, reaching every critical mechanical joint, making sure everything runs smoothly.
At the start of this video, you saw this large wheel with gear teeth. This is the flywheel. Its job is to reduce vibration and help the crankshaft rotate smoothly by using its momentum and weight. Next to it is a turning gear. When the engine is shut down, the turning gear engages with the flywheel and slowly turns it. This low rotation is used during maintenance and inspection, letting engineers carefully position the crankshaft. Of course, the turning gear must be disengaged before the engine starts, or serious damage could happen. And don't confuse the turning gear with the steering gear. The steering gear controls the ship's rudder's direction, while the turning gear is simply for rotating the engine slowly when the engine is not operating.
This marine diesel engine runs on heavy fuel oil. Despite the name, it's not the same as the diesel you find at the gas station. Large cargo ships use heavy fuel oil because it's much cheaper and provides high energy output compared to diesel. But heavy fuel oil is so thick and dense inside the fuel tank. Heating coils warm the oil enough so it can be pumped out. Then it's heated again, this time to an even higher temperature. Only when it's hot enough, the fuel can be injected into the combustion chamber as a fine mist.
The engine's cooling system uses water circulating through the cylinders to absorb heat. This heated water flows to the heat exchanger where it cools down before returning to the engine. Sea water is pumped in through the sea chest, flowing through these tubes inside the heat exchanger. The cooling water flows around these tubes. The cooling water and sea water are completely separated. They never mix or touch each other. Heat transfers from the cooling water to the sea water through the walls of the tubes by conduction. The sea water then carries this heat away and it's discharged back into the ocean.
The engine runs at low speed, but it can produce the power of tens of thousands of horsepower. That explains how the engine can run a fully loaded ship with hundreds of thousands of tons deadweight.
The history of this massive two-stroke marine diesel engine traces back to Rudolph Diesel, a German inventor who invented the diesel engine in the 1890s. But it was Burmeister and Wain, later becoming MAN B&W, that turned it into the giant engine we see in ships today.
Thank you for watching. If you know anything more about this topic, please leave a comment below. My name is Lucius. I'll see you in the next video.
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