The problems faced by the current PHEV research mainly include the following aspects:
①Performance indicators such as the structure, cost, control strategy, length of pure electric driving range, battery energy and motor power of the vehicle determined according to the needs of the market and users;
② Development of batteries and their management systems to meet the needs of PHEVs;
③ Construction of PHEV charging infrastructure.
The power battery of PHEV needs to have the following performance:
①It is necessary to ensure that the PHEV has the necessary dynamic performance indicators and pure electric driving range without increasing the vehicle mass too much, so the battery must have a sufficiently high energy density and power density;
②Because PHEVs often run in pure electric mode, unlike HEVs, the battery needs to work in deep discharge while still ensuring a long cycle life;
③When the PHEV runs in pure electric mode, when the SOC of the battery drops to a low level, it should still be able to discharge with a large current; when the SOC of the battery is high, it should be able to accept high-power charging to recover braking energy;
④The battery should have a lower cost to reduce the cost of the whole vehicle.
PHEVs use on-board battery packs to reduce emissions when the vehicle's engine is cold-started. The reason for this is that, during a cold start, the vehicle operates in an electric energy-only mode, while using the electric energy of the battery to heat the catalytic converter to a certain temperature. Once the catalytic converter can work normally, the engine can be started as required. However, almost all batteries don't work very well in extremely cold conditions. This is not only due to the reduction of its available electrical energy and the increase of internal impedance (resulting in lower efficiency), but also the service life of the battery will be severely affected when the discharge power is large. The typical operating temperature range of lithium batteries is 0~50℃. In extremely cold conditions, the internal resistance of the battery will increase to a large extent and the output power level is very low. Therefore, in order to increase the range and battery life of PHEVs in extremely cold weather conditions, some necessary measures need to be taken. One approach is to thermally isolate the battery pack to ensure that there is no or very little heat exchange between the battery cells and the surrounding air except for the cooling water outlet. In extremely cold or hot conditions, this thermal isolation method allows the battery to maintain a stable temperature for an extended period of time. Another method is to use an internal heater to heat the battery by draining battery power in extremely cold conditions. In addition, the battery can also be placed in external heating mode by connecting to the grid.
Another problem with PHEVs is that if the owner always drives less than the designed electric range every time, and frequently charges the battery, the engine may never have to start. This condition may result in relatively stale fuel in the tank, and may also cause certain mechanical structures, such as components in the engine, to become stuck. To this end, some PHEV models, such as the Chevrolet Volt, have been designed with a maintenance mode that can be activated either manually by the driver or automatically to keep the engine and generator running on a regular basis to keep these parts running.
In addition, PHEVs are safe under normal driving conditions, but there are certain dangerous safety issues when charging (especially operating in rainy or wet conditions), maintenance or traffic accidents. Like other high-voltage systems, the electrical system of PHEV has certain safety hazards when it is misoperated. Unreasonable operation of the high-voltage system may cause electrical hazards such as electric shock and arc discharge.
For example, under normal circumstances, the high-voltage system of the car is isolated from the ground and the chassis of the car. When the car is being repaired or maintained, if the positive and negative electrodes of the high-voltage system are touched due to misoperation, it will suffer electrical hazards. Using advanced non-contact charging methods (such as inductive wireless charging) can reduce the risk of electric shock during charging, but this method will reduce charging efficiency and increase the cost of the system.
If you use a contact charger to charge the car on rainy days, there may be current leakage, and aging and damaged plugs and wires will cause leakage. So you need to be extra careful when charging your car in the rain. The leaking current can produce electrical shocks that can cause muscle contractions, atrial fibrillation, and tissue damage. Usually 10mA of current can cause muscle contraction. In the event of a traffic accident, high-power, high-voltage connecting devices may short-circuit, causing sparks, overheating, and even battery fires and explosions.
The concept of vehicle to grid (V2G) refers to the bidirectional power and energy exchange capability between the grid and the car battery. With a two-way charger, a plug-in hybrid vehicle can be used as a backup power source for a home or office. The PHEV's battery can also control the stability of the grid (such as a distributed grid) or a renewable energy generation system, managing its frequency and voltage. As thousands of PHEVs will be connected to the grid for the foreseeable future, electric-powered transportation will begin to prosper and gradually achieve its goal of becoming less reliant on fossil fuels. Therefore, it is imperative to study the impact of grid-to-vehicle (G2V) technology on the powertrain, and many factors need to be considered, including battery size, charging process, distribution and efficiency of PHEVs, etc.
Vehicle to grid (V2G)