3. FAULT TREE ANALYSIS FOR POWER MEASUREMENT

　

It is confirmed that load index power and shaft horse power have not been corresponded. Then, correctness of the engine output power measurement were investigated with fault tree analysis as shown in Figure 2.

Excepting both of no influencing factor and influenced which can be corrected exactly, the following factors are remained as influenced factor for engine output measurement.

　

(1) Difference of the fuel oil (heavy fuel oil or marine diesel oil)

(2) Difference between engine nominal operation rating and actual operating point

(3) Fluctuation of engine revolution

(4) Instrumental error of power meter

(5) Deviation of power meter zero point

　

The instrumental error and zero point deviation of the power meter are mechanical problem, but others are related to engine efficiency. Then, regard to the pointed factors which are influenced and are not corrected exactly as mentioned (1), (2) and (3), we investigated the difference between engine shop trial and sea trial as shown in Table 1. According to the consideration, it is understood that these difference between engine shop trial and sea trial have an influence on engine efficiency. Power calculation with load index is based on engine efficiency at shop trial, therefore, calculated power is not correspondence to shaft horse power measured by shaft power meter on the sea.

　

Table 1 Comparison shop trial with sea trial

　

4. CHARACTERISTIC OF LOAD INDEX POWER AND SHAFT HORSE POWER

　

In order to verify the result of fault tree analysis, characteristic of load index power and shaft horse power were analyzed as shown in Figure 3.

　

Figure 3 Characteristic of load index power and shalt horse power

　

Load index power is calculated with power estimation curve which shows the relation among the engine output measured by water brake, engine load index and engine speed at the shop trial. In other words, this is a calculation method with engine efficiency at the shop trial and is shown in equation (1). In connection with the equation (1), the load index power is shown in equation (2).

　

Q = LI × α₁ × α₂ × N × γ × H × K　　(1)

　

where

Q : Heat input for main diesel engine

LI : Load index

α₁ : Correction coefficient of fuel injection pump

α₂ : Correction coefficient of fuel voluminal elasticity

N : Engine speed

Y : Specific gravity of fuel oil

H : Calorific value of fuel oil

K : Constant

　

P_LI = Q × η_o　　(2)

　

where

η_o : Base engine efficiency at shop trial

　

On the other hand, shaft horse power is shown in equation (3) which is calculated with heat input for main diesel engine and some efficiencies. The efficiencies are shown in equation (4).

　

P_TM = Q × η　　(3)

　

η= η_o × β_AIR × β_CM × β_FL × β_OP　　(4)

　

where

η : Actual engine efficiency

β_AIR : Correction coefficient of suction air temperature

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