Automobile headlamps design is an important part of Renault brand, and "C-shaped" LED headlamps are an important part of automobile attraction. The cost of the car headlight assembly is about 30% per 70% in the mechanical and electronic equipment. Thus, the cost savings of electronics will have a far-reaching impact on the overall cost. This paper mainly introduces how to control the head lamp cost and how to use the thermal analysis tool to optimize the design of headlamps.
First generation design
In the first generation full LED headlamps assembly, Renault studied six types C and D models from Espace to Koleos. First, all the components are standardized as follows: a height sensor, an electrostatic leveling device, a DRL (daytime traffic light)/near light/far light driver, a central connector, low and high beam modules, each with two suppliers. In a year, by checking the breakdown of costs, the 60% standardization of the headlight component (Figure 1) to achieve cost savings.
Figure 1. The overall cost division of the first generation of standardized headlamps assemblies and headlamps.
As seen from Figure 1, plastic accounts for only about 30% of the entire price. The volume effect is the main cost factor of the big lamp part price. However, from 2012 halogen headlights to 2014 LED lights, the overall cost increased four times times. This gives us the impetus to see if we can cut the cost of the second generation of headlamps.
Second generation design
The second generation car headlamps focus on our hot-selling B-class car (small car)-Renocrio (Renault Clio). The car was undergoing a change, and in style Renault wanted to transfer it to led C-shaped DRL lighting.
Renault's second-generation strategy has four core:
▲ become the first general-purpose automotive OEM with full LED headlamps in this B-class sedan;
▲ the price of large lamp parts between the first and second generations is reduced by half;
▲ Better LED lighting performance than Clio Initiale (25W xenon lamp);
▲ reduce the depth of the whole headlight assembly by 50mm.
Renault has standardized the Clio: an LED electronic control unit (ECU), a height sensor and a leveling device. We then focus on led near-light, by reducing the number of LEDs and the size of the heatsink 30%, lowering its price to 30% and improving the optical system. By doing all this work (see table), Renault increased the LED luminous flux by 33% and reduced the component from 8 to 5 LEDs.
Renault Clio headlamps from 1 generation to 2 generation led comparison
Renault also increased optical efficiency by 25% and reduced the overall size by 50 millimeters. With the thermal improvement of the LED, we have increased the LED current, maximum junction temperature use and low ambient temperature flux drop, as shown in the table. Similarly, through the associated radiator design, a detailed thermal simulation can be used to better manage the junction reduction (Figure 2).
Figure 2. The comparison chart shows the radiator weight of the first and second generation Renocrio headlamps.
As for the overall headlamps package size, due to better design and assembly, the first generation of halogen headlamps and the second generation of LED headlamps reduced the depth of 50mm. Fig. 3 is a typical CFD (computational fluid dynamics) simulation of halogen headlamps on the floefd of fluid mechanics, which shows the complex airflow and thermal effect found on the surface of the headlamps.
Figure 3. FLOEFD thermal prediction reflects the performance of the Renault halogen headlamps assembly.
Next, the CfD thermal analysis method, which is usually used to optimize the design of headlamps, is discussed in depth. Usually the ambient air outside the head lamp is predicted to be 23°c, and the temperature of the LED outer edge can be up to 70°c to detect the illumination performance. In order to verify the simulation, a number of experiments were carried out to fix the ambient temperature of the headlamps under 23°c conditions and to install eight thermocouples outside the assembly when the engine was turned on and off.
It is obvious that when the engine idling and the headlamps are open for a long time, the temperature in the headlights can reach more than 50 ℃. In addition, under certain idling conditions, the surface temperature of the headlamps may rise to 65°c.
Other tests have found that the low beam of light for an hour, the temperature inside the headlights reached 20°c, if the low beam and high beam at the same time one hours, then measured to increase 5°c.
At the same time, a series of tests found that for 70°c ambient temperature, low beam and high beam open, the engine is on, the LED junction temperature is very close to the worst of 150°c.
It is concluded that it is impossible to design an LED system if all the use is considered. Therefore, OEMs must determine the best compromise. For example, when the engine is idling for an hour, the illumination performance is shown at 23 ℃ 100%, but if the ambient temperature rises to 50 ℃ in the same case, the lighting performance will fall to 80%. In order to respect this specification, it is concluded that a thermal sensor must be added to the PCB (printed circuit board) so that if the temperature of the LED is higher than the defined threshold, the current will be reduced. Then, the full LED headlamps can be heat drop and flux drop.
Steering Production Design
Looking ahead, an action plan is being developed to simulate and test lighting in the transient driving cycle mode. As an OEM, Renault would like to be able to simulate the impact of car speed on the thermal performance of its lighting, especially as a result of the thermal changes caused by the speed of each engine.
It also needs to be able to simulate the thermal behavior of a parallel engine compartment with headlamps because they interact with each other. Also, if there is a heat sensor appears, of course, you will also see the need for the light inside the heat management. In short, Renault believes that OEMs should be responsible for the entire cooling system associated with headlamps design. Doing so will make OEMs the leader in the system market.
The direct objectives of the third generation Renault lighting team are: by 2018-2020, the luminous flux of headlamps will be increased from 270 to 320 lumens (Figure 4), with new LED drivers being introduced to handle higher power, turning indicator and adaptive headlights (AFS), and overall size reduction. Finally, we want to introduce a high sensor adjustment. Our ultimate goal is to reduce the price of our entire headlight assembly by 50% so that it reaches the level of halogen headlamps five years ago. (Compiled: LED Web James)
Figure 4. From 2012 to 2016, Renocrio the evolution of the headlamps: from the No. 0 generation (halogen headlamps and led DRL) to the 1th generation (full LED headlamps), followed by Xenon headlamps, and then to the 2nd generation (full LED headlamps), and finally reduce the cost to the 3rd LED headlights.