## 1 Introduction
In order to solve the problem of low system efficiency caused by more power generation than the user's electricity demand, three configuration combinations of the combined power supply system were proposed. In addition, the influence of natural gas internal combustion engine operating conditions on the refrigeration of the subsequent absorption chiller was studied. The results showed that the power change of the internal combustion engine and the working condition changes of the cooling water flow would have a greater impact on the operating status of the subsequent lithium-bromide absorption chiller. For the operation strategy of the combined cooling, heating, and power system, scholars at home and abroad have carried out many optimization studies. Most scholars use the method of establishing an optimization function for the combined supply system and use the optimization algorithm to solve the optimal operation control method of the system. [PERSON] et al. [17] studied a co-generation system with new energy as the driving energy. The new energy driving system includes wind power generators, fuel cells, photovoltaic power generation, and tidal steam turbines. [PERSON] [18] developed an optimization software for the economic and environmental benefits of the combined cooling, heating, and power system and improved the software according to the \"heating to determine the electricity\" mode commonly used in practical applications in China. The cogeneration system is analyzed, and the results show that, compared with the traditional energy system, the operation strategy of the cooling, heating, and power cogeneration system still needs to be further studied after the system is installed, and its strategy optimization has a very important impact on the economical and energy-saving operation of the system. The above research shows that the predecessors carried out related research on system cost saving and energy saving from the aspects of system configuration selection, operation strategy optimization, etc., but less research on the working conditions of the system's prime mover. Currently, natural gas is widely used as fuel for PGU and CCHP systems: It is cheap, easy to gain, and clean. Based on the internal combustion engine technology, for a CCHP system with a natural gas-fueled internal combustion engine as its PUG, a novel method is proposed to achieve a variable heat/electricity ratio. The operating principle of the natural gas engine [19; 20] is that the gas mixes with the air before entering the cylinders. The mixture is inducted into the cylinder and is compressed by the up-moving piston. The compressed mixture is ignited before to the top dead center. The ignited mixture burns in the form of flame propagation throughout the cylinder. The burned natural gas increases the temperature and pressure of the gases within the cylinder, which pushes the piston downward during the expansion stroke and output torque through a connecting rod and crankshaft mechanism. Near the piston's bottom dead center, the engine exhaust valves open, and the burned gas is discharged at a high temperature. Generally, the engine is set to run under the best thermal efficiency condition: that is, to output its maximum power to drive the generator to give maximum electricity. The heat balance of the engine indicates that the brake power takes about 30% and the exhaust gas heat energy takes about 30% of the total fuel energy. In this case, obtaining more electricity requires higher engine power output, which results in more exhaust heat energy discharged and vice versa. That is the reason why the PGU CCHP system has a limited heat/electricity rate. It is known that the combustion phase is controlled by the spark timing for the spark ignition (SI) engines. Thus, if the spark timing is postponed, the combustion within the cylinder is postponed, which will result in a reduction in the engine's power output and the increase in the exhaust gas heat energy.