# Advanced Engine Technologies for Turbochargers Solutions
## 2 Turbocharging Principles  
If a turbocharger is attached to an internal combustion engine, then the cycle efficiency increases compared to that of the naturally aspirated engine. Figure 1 shows the mixed turbocharged cycle [13].  
In this case, the flue gas energy (area 1-4-5-6-1) is used to drive a compressor. Prolonged expansion (4-5) is achieved in the blade network of the turbine, which drives the compressor. The air is compressed (6-1) to the pressure \(\mathrm{p_{s}}\). So, the pressure and density of the air in the cylinder will increase [15,16]. As a result, a larger amount of air will enter the engine cylinders, compared to the naturally aspirated one and, consequently, more fuel can be introduced into the combustion chamber. The result would be more energy per engine displacement unit [17].  
Figure 1: Mixed turbocharged cycle [14].  
The mechanical losses in the turbocharger are lower than those recorded in the case of prolonged expansion in the engine cylinders. Increasing the air pressure allowed in the supercharged engine cylinders leads to a maximum pressure per cycle of 30-40% [18; 19]. Increasing the system components raises the engine mechanism's requirements (engine block, cylinder head, piston, bolt, connecting rod, crankshaft) [20; 21; 22]. Due to the higher air temperature, which is retained in the cylinders at the end of the intake process, there is also an increase in the thermal regime (maximum temperature per cylinder) and the thermal stresses of the pistons, valves, and vanes of the tubing rotor [23; 24]. The reduction of the thermal regime can be obtained by cooling the forced air from the compressor (intercooler), changing the distribution phases (increasing the crossing angle of the valve opening), and increasing the air flow on the cycle [25; 26].  
A surge occurs when high internal combustion engine speeds are used and may stall the turbocharger [27]. The transient operation of turbocharged diesel engines has also been associated with poor driving as well as gaseous emissions [28]. It is well known that the turbocharger contributes to an optimized combustion process for the engine [29]. Also, the transient operation is mostly optimized in the light of response and provides the demanded torque above the maximum torque generated by a natural aspired engine [30]. The difference in time from the boosted torque and the maximum torque provided by the natural aspired engine is known as turbo-lag; this can be another problem of the turbocharger [31].  
Research showed that power and torque can be improved by 30-50% by turbocharging internal combustion engines compared to those naturally aspirated. Therefore the fuel consumption is reduced by 15-25% [32].  
Since car engines operate in a wide range of speeds and loads, it is necessary to provide the turbocharger with the motor for critical speeds (low speeds and loads) [33]. For the other operating modes (maximum power and speed) it is possible to use a turbine with variable geometry or the solution through which a part of the exhaust gases to bypass the turbine (bay-pass valve), which results in a decrease of the available energy [34]. This involves providing the turbocharger with the engine for the range of speeds characteristic of the maximum torque, and for those specific to the maximum speed the excess energy available in the flue gas to be harnessed by generating electricity using a generator connected in series with the turbocharger shaft [35; 36].  
Another method to determine some turbocharger parameters is the acoustic method [37]. With this method the performance over the axial inflow turbocharger turbine can be evaluated [38]. Some diesel turbocharger engines have failure diagnosis, which can be also analysed with the acoustic method. Therefore, it is very efficient [39]. After obtaining the parameter result, more aerodynamic designs can be optimized [40]. The new optimized design evaluation shows a higher turbocharger efficiency [41].