1. Matching of diesel engine to fixed pitch propeller:
(1) Basic principles of matching
In order to make the diesel engine fully function, and at the same time obtain high economy, reliability and long life, a suitable propeller must be selected for a designated diesel engine, so that the power of the diesel engine can be fully utilized. Moreover, the power of the diesel engine does not exceed the allowable range within the entire operating speed range.
When selecting a propeller, in principle, the diesel engine absorbs the power of the designed propeller at the rated speed exactly equal to the rated power of the diesel engine. The combination of the propeller and the diesel engine can be used to fully utilize the power of the diesel engine under the calibration conditions, and the power of the diesel engine does not exceed the allowable range within the entire operating speed range.
If the propeller is too heavy, it will cause the diesel engine to be overloaded (this is not allowed), or the diesel engine's power is insufficient and the diesel engine's capacity cannot be fully utilized.
If the propeller is too light, it will cause the diesel engine to overspeed (this is not allowed), or the diesel engine power is insufficient, and the diesel engine's capacity cannot be fully utilized.
The above is only the basic principle of matching diesel engines with propellers. In fact, there must be a certain speed or power reserve when the diesel engine and the propeller are matched. This is because in actual operation, some factors such as the sloping bottom of the hull, the wind and waves on the sea surface will change, etc., which will cause the propeller to be overweight. For modern marine diesel engines, this consideration is particularly important due to increased boost and reduced overload capacity.
2. Power reduction matching between diesel engine and propeller and new power calibration method for marine diesel engine:
In order to expand the scope of use and meet the user's requirements for fuel economy, it provides greater flexibility in configuring the propeller. In recent years, the calibration method for marine diesel engine power has been updated several times. This is because modern diesel engines generally use the power reduction method.
(1) Matching power reduction between diesel engine and propeller:
The shipowner and the shipyard select the diesel engine model and its optional working condition point according to the ship's characteristics and propeller characteristics (also known as the maximum continuous operating point of the contract, the CMCR point, and the power and speed of the optional working point are often lower than the diesel engine. Maximum calibration power and speed). Then adjust the diesel engine and supercharger to keep the maximum burst pressure of the optional point at 100% design value, and optimize the working parameters of the matching point to obtain better economic indicators. This is the derating method. In order to avoid confusion, we refer to the maximum continuous power that a certain model may output and its corresponding speed operating point as the rated operating point MCR point.
The power reduction method can be roughly divided into four types:
1) Reduce power according to propulsion characteristics. Since the average effective pressure is lowered, the ratio of the highest burst pressure to the average effective pressure becomes larger, so that the combustion is optimized, the effective fuel consumption rate is lowered, and the economy is improved. If necessary (such as when the oil price is low), the diesel engine and the supercharger can be readjusted without replacing the propeller, so that the power generated by the diesel engine can be increased to the rated MCR value, and the diesel engine power can be fully utilized.
2) Reduce the speed and reduce the power. Suitable for ships with large diameter, low speed propellers (eg oil tankers). Due to the configuration of the propeller with low speed and high propeller efficiency, the overall economic indicators of the diesel engine power unit are improved. However, in order to have a sufficient excess air ratio, a suitable supercharger should be re-selected.
3) Reduce power according to load characteristics, applicable to ships with limited propeller diameter or equipped with adjustable pitch propeller. This approach focuses on reducing the average effective pressure and increasing the ratio of the highest burst pressure to the average effective pressure to reduce the fuel consumption of the diesel engine.
4) Select the propeller with lower speed and higher efficiency; reduce the average effective pressure of the diesel engine, increase the ratio of the highest burst pressure and the average effective pressure, and reduce the fuel consumption rate of the diesel engine itself. This will give the best overall economic indicators.
The power reduction method (whether it is to reduce the average effective pressure or reduce the speed) can improve the economy, but in essence it is at the expense of the power of the diesel engine. That is to increase the cost in exchange for a reduction in fuel costs. For example, the selection of the optimal speed must consider the balance between fuel cost and investment cost. The lower the selected speed, the higher the efficiency of the propeller and the lower the fuel consumption rate. However, the construction cost is higher. Therefore, the optimum speed is not necessarily the minimum speed of the diesel engine, but may be higher.
(3) Optimized adjustment of diesel engine at the power reduction matching point:
When the diesel engine is matched with the propeller, if the power of the optional point is lower than the rated power, the diesel engine must be optimized at the optional point to perform a series of adjustments, which generally include:
1) injection timing;
2) Fuel injection device specifications;
3) Supercharger optimization including boost pressure;
4) exhaust valve timing;
5) compression ratio;
6) Scavenging timing;
7) The flow of coolant in a component and the capacity of the pump and cooler should also be adjusted and replaced accordingly.