Schiffstechnik | Ship
Schiffstechnik | Ship Technology 56 where: HANSA International Maritime Journal 06 | 2019 An engine designer’s take on propulsion Propulsion Design from the Engine Designer’s Point of V Naval architects are experts in In the order process to of optimize optimizing a the vessel propulsive efficiency design, and thereby achieving design’s the lowest carbon power requirement footprint, PPPP DDDD an for a given vessel Equation 1 and Equation 2. engine with the lowest specific fuel oil consumption is desired. ηηηη However, DDDD = ηηηη HHHH ∙ ηηηη decisions OOOO ∙ ηηηη RRRR that lead to the lowest SFOC are not Propulsion Design from always the Engine as clear-cut Designer’s Point of View Propulsion Naval architects are Design experts from in the process the PPPP DDDD Engine = of PPPP EEEE optimizing Designer’s the propulsive Point efficiency of View ηηηη DDDD o = RRRR TTTT ∙ VVVV ηηηη N DDDD ηηηη DDDD design, Naval architects and thereby are achieving experts in the the lowest process power aval architects of optimizing requirement are experts the propulsive PPPP DDDD for a given in the process efficiency vessel of ηηηηspe DDDD o design, Equation and 1 and thereby Equation achieving 2. the lowest optimizing power requirement the propulsive PPPP DDDD efficiency for a given η D of vessel a spe Equation where: 1 and Equation 2. vessel design, and thereby achieving the lowest power ηηηη requirement P D for a given vessel speed. DDDD = ηηηη HHHH ∙ ηηηη OOOO ∙ ηηηη ηηηη RRRR DDDD : Propulsive efficiency ηηηη DDDD = ηηηη HHHH ∙ ηηηη OOOO ∙ ηηηη RRRR Equation 1 ηηηη HHHH : Hull efficiency PPPP DDDD = PPPP EEEE PPPP DDDD = PPPP = RRRR TTTT ∙ VVVV ηηηη OOOO : Open-water propeller efficiency Equation 2 ηηηη EEEE DDDD = RRRR ηηηη TTTT DDDD ∙ VVVV ηηηη RRRR : Relative rotative propeller ηefficiency D: Propulsive ηηηη DDDD ηηηηefficiency DDDD η H: Hull efficiency where: RRRR TTTT : Vessel-towing resistance η O: Open-water propeller efficiency where: η R: Relative rotative propeller efficiency ηηηη DDDD : Propulsive VVVV: Vessel speed efficiency R T: Vessel-towing resistance ηηηη DDDD Propulsive efficiency V: Vessel speed ηηηη HHHH : Hull PPPP DDDD : Propeller efficiencypower requirement P D: (delivered Propeller power power). requirement (delivered power) ηηηη HHHH Hull efficiency ηηηη OOOO : Open-water propeller efficiency Naval architects are also familiar with the impact that engine selection can have, and it is usually with clear the that impact the highest that engine engine selection efficiency can – have, a ηηηη OOOO Open-water propeller efficiency ηηηη RRRR : Relative Naval architects rotative propeller are also efficiency familiar and thereby lowest specific fuel oil consumption ηηηη RRRR Relative that the rotative highest propeller engine efficiency – and thereby lowest specific fuel oil consumptio RRRR TTTT : Vessel-towing resistance (SFOC) – is desired. However, decisions regarding desired. However, decisions regarding the engine that lead to the lowest SFOC ar the engine that lead to the lowest SFOC are not always as clear-cut. RRRR TTTT : Vessel-towing resistance VVVV: Vessel clear-cut. speed A modern and holistic approach to vessel fuel-optimization to vessel VVVV: Vessel A fully speed modern and holistic approach PPPP DDDD : Propeller power requirement (delivered power). is to fuel-optimization apply the equation. is to apply This the fue expresses the power, in terms expresses of the flow the of power, the fuelʼs in terms heating of the value, flow of that the needs to PPPP DDDD : Propeller power requirement (delivered power). main engine to maintain a given fuel’s vessel heating speed; value, see that Equation needs to be 3. supplied to the main engine to maintain a given vessel speed. Naval architects are also familiar with the impact RRRRthat TTTT ∙ VVVVengine selection can have, and i that Naval the architects highest engine are also efficiency familiar with – PPPP ffffffffffffffff and = the thereby impact ηηηη lowest that engine specific selection fuel oil consumption can have, and (S HHHH ∙ ηηηη OOOO ∙ ηηηη RRRR ∙ ηηηη SSSS ∙ ηηηη EEEE i desired. that the where: highest However, engine decisions efficiency regarding – and the thereby engine lowest that lead specific to the fuel lowest oil consumption SFOC are no η S: Shafting efficiency (S clear-cut. desired. However, decisions regarding η E: Engine the engine efficiency that lead to the lowest SFOC are no clear-cut. ηηηη SSSS : Shafting efficiency A fully modern and holistic approach For to a vessel two-stroke fuel-optimization engine installation is to apply where the the fuel equ A expresses fully ηηηη EEEE modern : Engine the power, and efficiency. holistic in terms approach of the main to flow vessel engine of the fuel-optimization is fuelʼs directly heating coupled value, is to to the apply that propeller, flow the speed; shafting of the see fuelʼs efficiency Equation heating can 3. be value, taken that as a needs fixed to be s needs the fuel to be equs main expresses engine the to power, maintain terms a given of vessel the For a two-stroke engine installation number where that is the usually main in engine the range is directly 0.98 to 0.99. coupled to th main engine to maintain a given vessel speed; see Equation 3. shafting efficiency can be taken as a fixed RRRR TTTT ∙number VVVV that is usually in the range 0.98 determination of the overall PPPP ffffffffffffffff efficiency = ηηηη HHHH ∙ ηηηηeffects OOOO RRRR∙ TTTT RRRR ∙ VVVV∙ ηηηηof SSSS ∙changes ηηηη of the engine type and la EEEE straightforward and is the PPPPfocus ffffffffffffffff = of this article. ηηηη HHHH ∙ ηηηη OOOO ∙ ηηηη RRRR ∙ ηηηη SSSS ∙ ηηηη EEEE where: ηηηη SSSS : Shafting efficiency © Selzer
Schiffstechnik | Ship Technology Figure 1: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 150 bar valve is opened. The fuel energy released in this way is most efficiently converted to mechanical energy, which is delivered to the piston. •• Late part of the fuel injection profile: fuel is injected and burned after the piston has left TDC. The cylinder volume is larger at this time and the combustion gasses resulting from burning this portion of the fuel will not expand and cool to the same degree as earlier. The energy in this portion of the fuel is less efficiently converted to mechanical energy. We want to inject and burn the fuel quickly to achieve the highest efficiency and power output. This is limited by the maximum cylinder pressure that the engine can withstand. Thus, it is important to have an engine that is designed for a high maximum cylinder pressure as can be seen in Figures 1 and 2. In Figure 1, combustion – and thereby heat release – continues until the cylinder volume is 0.28 times the maximum cylinder volume. The last portion of fuel that is burned can therefore expand by a ratio of 1.00/0.28 = 3.6. Compare this to the 200bar engine illustrated in Figure 2. In Figure 2, the same amount of fuel has been injected as in Figure 1, but fuel injection and combustion can occur faster than in Figure 1 without exceeding the maximum-allowable engine cylinder pressure. Combustion and heat release finish at a cylinder volume of 0.22 times the maximum cylinder volume, which results in a higher expansion ratio of 1.00/0.22 = 4.5 for the last portion of fuel burned. This means that more of the chemical energy in the fuel is extracted as mechanito achieve high engine efficiency and power, it is important to have an engine that is designed for a high maximum cylinder pressure; see Figure 1 and Figure 2. In Figure 1, combustion – and thereby heat release – continues until the cylinder volume is 0.28 times the maximum cylinder volume. The last portion of fuel that is burned can therefore expand by a ratio of 1.00/0.28 = 3.6. Compare this to the 200 bar engine illustrated in Figure 2. © MAN Pressure as a function of cylinder volume for an engine Figure 1: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 150 bar with a maximum cylinder pressure of 150 bar In Figure 1, combustion – and thereby heat release – continues until the cylinder volume is 0.28 times the maximum cylinder volume. The last portion of fuel that is burned can therefore expand by a ratio of 1.00/0.28 = 3.6. Compare this to the 200 bar engine illustrated in Figure 2. Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 200 bar Figure 2: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 200 bar However, determination of the overall efficiency effects of changes of the engine type and layout is not as straightforward and is the focus of this article. The combustion engine’s basic job is to burn fuel in a small volume and subsequently expand the resulting, hot, high-pressure combustion gases to a larger volume, while delivering power to that part of the combustion chamber wall that provides the expansion – namely the piston. It should be understood that a high expansion ratio is important in order to achieve high engine efficiency. That is, the greater the extent that the hot gasses can be expanded and cooled before the exhaust valve is opened, accordingly the more energy can be extracted from the gases and the higher the engine efficiency and the colder the exhaust gas will be. Figure 2: Pressure as a function of cylinder volume for an engine with a maximum cylinder pressure of 200 bar Derating In principle, it would be ideal to burn the fuel in a volume so small that the hot, high-pressure combustion gasses could expand until the cylinder pressure reached atmospheric pressure before the exhaust valve was opened. This is not feasible for a number of reasons that are outside the scope of this article. However, it is relevant to mention that if such small combustion chambers were used, it would reduce the amount of oxygen in the chamber unless the pressure was very high – higher than possible from an engine-design point of view.Accordingly, a small combustion chamber means a reduced amount of oxygen, which itself means that it is not possible to burn as much fuel. The result is a lower engine torque and thereby lower power-output. Despite this, the chamber volume is often reduced to some extent – this is called derating. A derated, marine two-stroke engine has a combustion chamber that is smaller than a fully-rated version of the same engine type and this is done to increase the expansion ratio, and thereby the efficiency of the engine, at the expense of a lower power-output. High maximum cylinder-pressure In the previous explanation, it was assumed that all the fuel is burned when the cylinder volume is at its smallest, at the position in the engine cycle called TDC (Top Dead Centre). In terms of engine efficiency, it would also be beneficial to burn all the fuel at TDC. In an engine in operation the fuel cannot all be delivered and burned at once. Even if possible, it would result in cylinder pressures that were too high. The fuel is therefore injected following a carefully designed injection profile. This gives a high engine efficiency and power output without exceeding the physical limits of the engine and other restraints such as exhaust-emission requirements. In order to better understand the positive effect on fuel consumption of an engine that can withstand a high maximum cylinder pressure, let’s take a look at the fuel-injection process: •• Early part of the fuel-injection profile: fuel is injected and burned near TDC and will expand and cool to the greatest possible extent before the exhaust HANSA International Maritime Journal 06 | 2019 57
