## 3 Result and Discussion  
### Correlation between CO2 Emission and Engine Variables  
Air pollutant emissions vary depending on the engine combustion condition, and that condition is greatly affected by the external environment of the vessel and engine operating conditions such as acceleration and deceleration. Pearson correlation analysis was performed on 12 pieces of data with approximately 12,000 rows to identify effective factors associated with CO2 emissions. The data for analysis was collected from SSD 1, the only experimental vessel to acquire various engine operation data, and the data include speed, load, average MIP (mean indication pressure), average pmax (maximum pressure), scavenge air pressure and temperature, turbocharger in/outlet temperature, torque, power, rpm, and exhaust. Excluding the scavenge air temperature and turbocharger outlet temperature, the similarity was relatively high in nine items.  
The data used in correlation analysis are dependent data that fluctuate together by increasing or decreasing rpm. Except for the scavenge air temperature, the correlation coefficient with rpm (yellow box) is over 0.7, showing a very high correlation. In the case of the scavenge air temperature, it appears to have a low correlation with rpm because it is mainly influenced by the surrounding air temperature and the cooling water temperature of the air cooler. As a result of the Pearson correlation analysis, as shown in Figure 4, the correlation coefficient between emissions and rpm is positive at 0.9, showing a very high correlation. Except for commercial ships such as container ships, oil tankers, and LNG carriers engaged in international trade, an engine operation data acquisition system is not installed on ships. However, rpm data are relatively easily accessible on most ships, and data can be acquired by configuring simple data acquisition (DAQ), so this study was conducted on CO2 emission characteristics according to rpm variations.  
In general, ship engine control is performed by setting the rpm and adjusting the fuel injection amount to reach the set rpm. The fuel injection quantity and the rpm variation have a linear relationship. Most of the fuel is composed of carbon compounds, whereas the carbon contained in the fuel is completely burned due to combustion in the engine and emitted as CO2 and water, as shown in Equation (2). However, as the air-fuel ratio  
Figure 3: Experimental design for data analysis.  
decreases, the carbon in the fuel is incompletely burned and included in its exhaust gas in the form of CO or THC. In other words, greenhouse gas emissions by combustion in internal combustion engines are related to the amount of fuel consumption, the conditions of combustion, and the amount of carbon within the fuel [20]:  
\[C_{a}H_{b}+\left(a+\frac{b}{4}\right)\left(O_{2}+3.773N_{2}\right)=aCO_{2}+\frac {b}{2}H_{2}O+3.773\left(a+\frac{b}{4}\right)N_{2} \tag{2}\]  
In the case of a spark ignition engine using gasoline as fuel, the ratio of fuel injected is higher than that of air in the high-load section, resulting in high CO emissions due to incomplete combustion. However, in the case of a compression ignition engine that uses diesel as fuel, CO emissions are very low because the fuel is injected in an appropriate ratio. This study aims to identify emission characteristics and compare them with the results of various studies by measuring CO emissions from the main propulsion engine installed on the ship [21; 22; 23].