# Research on Energy Saving of PHEV Air Conditioning System Based on Reducing Air Backflow in Underhood

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## Abstract

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## 1. Introduction

_{2}emissions [4,5]. The PHEV power sources rely on electric power generated by electric machines and mechanical power generated by internal combustion engines (ICEs) [6]. Due to the high efficiency of electric machines and the quick development of battery technology, the PHEV is a rational choice for a long period in terms of environmental issues and energy saving [7].

- A heat flux marking method is proposed to characterize the mechanism of air backflow and its distribution in the condenser, thereby quantifying the air backflow phenomenon in the underhood of PHEV.
- The performances of the air conditioning system, including the evaporator outlet temperature, cooling capacity, exhaust pressure of the compressor, and COP, are investigated under different ambient temperatures, air backflow ratios, and air backflow distribution.
- An optimization model of the underhood is proposed to eliminate the impact of air backflow on the air conditioning system of PHEV.

## 2. Research Method

#### 2.1. Research Framework of the Air Backflow Effect in the Underhood

#### 2.2. 1D Model

#### 2.2.1. Compressor

#### 2.2.2. Condenser and Evaporator Model

#### 2.2.3. Expansion Valve

#### 2.2.4. Fan and Radiator Model

#### 2.3. 3D Models

#### 2.3.1. Heat Exchanger

_{r}is the heat capacity ratio and $\epsilon $ is the effectiveness.

_{i}is the source term in the momentum equation, $|u|$ is the velocity scalar, $u$ is the velocity vector, $\mu $ is the aerodynamic viscosity, and D and C is the given matrix.

#### 2.3.2. Meshing of the PHEV Underhood

#### 2.3.3. Boundary Condition

- (1)
- RANS steady-state model is used for steady flow in this model;
- (2)
- Pressure outlet and velocity inlet are selected for outlet and inlet boundary conditions, respectively;
- (3)
- The condenser, intercooler, radiator, and electrical radiator are calculated based on the porous media model;
- (4)
- The cooling fan is simulated based on the multiple reference frame (MRF) method.

#### 2.4. Experimental Test System

#### 2.5. Heat Flux Marking Method

## 3. Results and Discussion

#### 3.1. Effect of Backflow Rate under Different Ambient Temperatures

#### 3.2. Effect of Air Backflow Rate under the Different Air Backflow Distribution

#### 3.3. Mechanism Analysis of Backflow Phenomenon

#### 3.4. Mechanism Analysis of Air Backflow Phenomenon

## 4. Conclusions

- (1)
- The heat flux marking method proposed in this paper can accurately quantify the backflow phenomenon in the underhood of PHEV based on the momentum transport equation.
- (2)
- The decrease in the air backflow rate of the underhood helps to improve the refrigeration capacity of the air conditioning system, thereby increasing the COP of the system
- (3)
- When the air backflow ratio cannot be reduced below 10%, the air backflow should be distributed as evenly as possible at the front end of the condenser.
- (4)
- In order to eliminate the impact of air backflow on the PHEV underhood, the gap between the radiator and the bracket is sealed and the gap around the air guide is reduced. Compared with the original structure, the air backflow rate of the optimized structure is reduced from 32.7% to 9.3% and the cabin temperature can be reduced by 3–5 °C.

## Author Contributions

## Funding

## Institutional Review Board Statement

## Informed Consent Statement

## Data Availability Statement

## Conflicts of Interest

## Nomenclature

PHEV | plug-in hybrid electric vehicles |

ACS | air conditioning system |

ICE | internal combustion engine |

UDDS | urban dynamometer driving schedule |

CFD | computational fluid dynamics |

EMS | energy management strategy |

COP | coefficient of performance |

BTMS | battery thermal management system |

MTMS | motor thermal management system |

Acronyms | |

${V}_{th}$ | displacement of the compressor |

${m}_{com}$ | mass flow of the compressor |

${P}_{comp}$ | energy consumption of the compressor |

${h}_{suc}$ | suction enthalpy of the compressor |

${h}_{dis}{{}_{|}}_{s}$ | exhaust enthalpy under the condition of isentropic compression |

$\rho $ | density |

$\eta $ | inertial resistance coefficient |

γ | volume proportion of the hot air |

$\varphi $ | volumetric flow coefficient |

$Nu$ | Nusselt number |

$P{r}_{t}$ | Prandtl number |

${V}_{tip}$ | linear velocity of the fan blade tip |

${r}_{com}$ | rotary speed of the compressor |

${\eta}_{v}$ | volumetric efficiency of the compressor |

${h}_{dis}$ | exhaust enthalpy of the compressor |

${\eta}_{s}$ | isentropic efficiency |

${f}_{Q}$ | heat loss of compressor |

$u$ | velocity vector |

$\frac{1}{\beta}$ | viscous resistance coefficient |

${C}_{r}$ | heat capacity ratio |

$Re$ | Reynold number |

$\dot{Q}$ | volume flow of air |

${A}_{fan}$ | Fan Area |

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**Figure 3.**The 3D model of the underhood for PHEV. (

**a**) A 3D solid model of the underhood; (

**b**) the CFD calculation domain of the underhood for PHEV; (

**c**) the mesh of the underhood for PHEV.

**Figure 4.**The measurement points of temperature sensor in the cabin. (

**a**) Side air outlet; (

**b**) Middle air outlet; (

**c**) Front seat passenger; (

**d**) Rear seat passenger; (

**e**) Dashboard area; (

**f**) Cabin bottom.

**Figure 7.**Influences of backflow rate on air conditioning performances under different ambient temperatures. (

**a**) Evaporator outlet temperature; (

**b**) evaporator heat rejection; (

**c**) compressor discharge pressure; and (

**d**) COP.

**Figure 8.**Influences of the backflow rate on air conditioning performances under different ambient temperatures. (

**a**) Evaporator exit temperature; (

**b**) evaporator heat rejection; (

**c**) discharge pressure; and (

**d**) COP.

**Figure 9.**Backflow air distribution of original structure. (

**a**) Backflow air distribution front view; (

**b**) backflow air distribution side view.

**Figure 10.**Optimized cooling module of PHEV. (

**a**) Optimized structure front view; (

**b**) optimized structure side view.

**Figure 11.**Backflow air distribution of optimized structure. (

**a**) Optimized structure front view; (

**b**) optimized structure side view.

Heat Exchanger | Viscous Resistance Coefficient (m ^{−2}) | Inertial Resistance Coefficient (m ^{−1}) |
---|---|---|

Radiator | 1192.5 | 160.1 |

Intercooler | 813.6 | 74.5 |

Condenser | 851.8 | 126.9 |

Electrical radiator | 808.2 | 114.3 |

**Table 2.**Comparison of the backflow rate of the optimized cooling module and the original structure.

Air Mass Flow (kg/s) | Backflow Ratio (%) | Backflow Air Mass Flow (kg/s) | |
---|---|---|---|

Original Structure | 0.518 | 32.7 | 0.169 |

Optimized structure | 0.518 | 9.3 | 0.048 |

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**MDPI and ACS Style**

Wu, H.; Tang, X.; Xu, S.; Zhou, J.
Research on Energy Saving of PHEV Air Conditioning System Based on Reducing Air Backflow in Underhood. *Energies* **2022**, *15*, 3183.
https://doi.org/10.3390/en15093183

**AMA Style**

Wu H, Tang X, Xu S, Zhou J.
Research on Energy Saving of PHEV Air Conditioning System Based on Reducing Air Backflow in Underhood. *Energies*. 2022; 15(9):3183.
https://doi.org/10.3390/en15093183

**Chicago/Turabian Style**

Wu, Haibo, Xingwang Tang, Sichuan Xu, and Jiangbin Zhou.
2022. "Research on Energy Saving of PHEV Air Conditioning System Based on Reducing Air Backflow in Underhood" *Energies* 15, no. 9: 3183.
https://doi.org/10.3390/en15093183