## 2 Combustion Performance Optimization in Variable Speed Genset
### Discussion  
We expected that the constant specific fuel oil consumption (or constant combustion efficiency) circles would be flatter and wider horizontally, as shown in Figure 4. Accordingly, the combustion efficiency does not change much for constant torque. The best SFOC operating line is almost horizontal as a fixed torque line. But in case of the diesel engine in this experiment, there are differences in the measurement result due to the following limitations:  
* Like any other engine equipped with turbocharger, lower engine speed reduces the pressure of the scavenging air.
* The injection pump is driven by camshaft, the injection quality varies with speed.
* The injection timing is not variable.  
Figure 3: The measured specific fuel consumption of experimental variable speed diesel generator set under various operating conditions.  
Figure 2: Schematic diagram of experimental variable speed diesel generator set.  
To achieve much more improved combustion efficiency at low speeds, an auxiliary blower installation is essentially required to provide sufficient air at low engine speeds. Besides, it is needed to equip an electronically controlled system, which can fully control the fuel injection. For the engine with mechanical control as in this experiment, an effective solution consists in equipping the system of variable injection timing (VIT). The VIT injection pump elements are interchangeable with the existing injection pump elements and can be installed during maintenance without any further modifications [15; 16; 17]. Moreover, the amount of fuel injected in each cycle is necessary to remain within the optimized range corresponding to the specific size of combustion chamber despite the variable speed with load. The best performance is always obtained. The engine can achieve maximum combustion pressure at a lower load to improve the fuel consumption. In addition, it reduces the engine's energy expenditure, and therefore more efficient combustion.  
In fact, it is difficult to keep all the combustion factors unchanged. To hold the compatibility between all the operating factors, easy to control and not to overload the other systems, the torque for variable speed is not barely fixed but maintained in the optimal range. The torque-speed operating line of the engine is slightly inclined rather than horizontal as in the case where the torque is hardly fixed. Afterall, in case of the full optimized engine as illustrated in Figure 4, the following operating procedure is recommended for the best combustion efficiency:  
1. Maintain the generator torque corresponding to the optimized range of combustion performance (this is the key point).
2. Determine the appropriate engine speed corresponding to the required generator power using Equation (1).
3. Adjust the engine speed with the governor. The amount of fuel injected will be in the optimized range. In the case of the system, such as the experimental genset, it is necessary to establish the best SFOC operation line as shown in Figure 3. Therefore, the following operating procedure is recommended:  
1. From required load, determine the operating point, including torque and speed, for the best fuel efficiency.
2. Adjust the generator torque.
3. Adjust the engine speed by the governor. The amount of fuel injected will be in the optimized range for the best combustion efficiency.  
Figure 4: Illustration of torque-speed operating characteristics comparison between constant speed and variable speed generators (courtesy of ABB [10]).