The propulsion system of modern large-scale civil ships generally uses a two-stroke low-speed diesel engine as the main engine. This type of host has two distinctive features: the first is that the rated speed is very low, and the other is that the volume and weight are very larg, for example, the world's largest diesel engine 14RT-flex96C has a gross installed weight of 2446 tons (net weight 2300 tons). Medium-speed and high-speed diesel engines are often much smaller in volume and weight than two-stroke low-speed diesel engines under the same power. This makes one wonder: why is the volume and weight of the world's largest marine two-stroke low-speed diesel engine so large? Does this have something to do with its low revs?
The significance of low speed of diesel engine for ships
Before solving the problem, let's briefly understand the meaning of low diesel engine speed for ships. First of all, the low speed enables the diesel engine to obtain better combustion efficiency, thereby improving the efficiency of the main engine. In addition, the larger the diameter of the propeller of the ship, the lower the speed and the higher the efficiency, which means that if the speed of the main engine is low enough, low enough to directly match the speed of the propeller. The torque is large enough to directly drive the large diameter propeller, then the propulsion system of the ship can be simplified, eliminating the need for the reduction gear box to directly drive the propeller by the main engine through the transmission shaft, thereby improving the efficiency of the entire propulsion system. For example, for a ship equipped with a 14RT-flex96C marine low-speed diesel engine, the main engine directly drives a propeller with a diameter of nearly ten meters, and at a speed of less than 100 revolutions per minute, it drives a large container ship of nearly 400 meters long to run to 25 knots above.
The propeller of the 14RT-flex96C marine diesel engine weighs 131.4 tons and is the largest marine propeller in the world.
How can a marine diesel engine reduce revs without reducing power?
An important idea for reducing the speed of marine diesel engines is to reduce the speed of the piston. So how to reduce the speed of the piston? The movement of the piston is produced by the high pressure gas produced by the combustion of diesel in the cylinder doing work on it, which means that reducing the pressure of the gas reduces its speed. However, although this can reduce the speed, it also reduces the power and torque of the diesel engine, which is contrary to our original intention of reducing the speed. Therefore, in order to ensure the power and torque of the diesel engine, while reducing the diesel engine gas working pressure, it also It is necessary to increase the bore diameter to ensure the effective area of the piston.
The effective area of the engine piston has a great influence on the torque
In addition to reducing the speed of the piston, another method to reduce the speed of the diesel engine is to increase the stroke of the piston without reducing the speed of the piston.It's like two cars running at the same speed, one runs a small lap of 100 meters, and the other runs a large lap of 200 meters. When the car running a small lap of 100 meters made two laps, the car that ran a large lap of 200 meters only made one lap. In fact, the work done by the two cars is equal, but the number of laps is doubled. In this way, we introduce this idea to the diesel engine. Increasing the stroke of the piston is like increasing the distance of the car, thereby greatly reducing the speed of the diesel engine. This is also one of the mainstream ideas for large low-speed engines.
Will the low speed cause the marine diesel engine to be bulky and heavy?
Influence of Cylinder Diameter on Length and Dimension of Diesel Engine
Take MAN Diesel & Turbo's 7G80ME-C as an example, the low-speed diesel engine has a bore of 800mm. Low-speed diesel engines are all in-line cylinders, that is, the cylinders are arranged in a row. In this way, we imagine that 7 cylinders with a diameter of 800mm are arranged in a row next to each other, and their length will exceed 5 meters. In fact, 800mm is only the inner diameter of the cylinder, the cylinder itself has a certain thickness, and the cylinders cannot be arranged closely, but require a certain distance, so the actual total length of the 7G80ME-C reaches 13.6m.
Influence of stroke on height and width dimensions of diesel engine
Also take MAN Diesel & Turbo's 7G80ME-C as an example, it is a green ultra-long-stroke diesel engine with a stroke-to-bore ratio of more than 4.6. To this end, in the case of an 800mm bore, it has an extra-long piston stroke of 3720mm. The super-long stroke means that the cylinder length of the diesel engine, the stroke of the crosshead, and the radius of rotation of the crankshaft are all very large. As a result, the height of the diesel engine will be very high. Take the 7G80ME-C as an example, the entire diesel engine is as high as 16m and more than four stories high. In addition to the height, in order to ensure the rotation space of the crankshaft, the diameter of the inner cavity should be greater than 3720mm, so the width will also increase accordingly. For example, the width of the 7G80ME-C reaches 9.7m.
Obviously, the low-speed design will make the diesel engine taller and larger, and the weight will increase sharply, so the volume and weight of the world's largest diesel engine have a lot to do with its low speed.
