Design of Automotive Hood

Objective:

To study and design hood components that satisfy EuroNCAP guidelines for passenger and pedestrian safety and meets the DFM and DFA requirements.

Abstract:

The hood of an automobile is a protective cover that sits on top of the engine with the help of hinges which helps to open and close the hood to enable access to the engine compartment of the automobile.

In this report, we will study about:

1. The car hood and its functions.
2. Various components in an automobile frontal enclosure.
3. Design of Car Hood:
   1. The important aspects considered before designing a hood of an automobile.
   2. Design data
   3. Design for Safety
   4. Design of Reinforcements, and Mastic Data
   5. Placement of Hinges and Striker for the opening, closing, and locking of the car hood.
   6. Outer Panel Details

1. The car Hood and its functions:

Apart from being a protective cover, the hood may serve one or more of the following purpose:

1. Provide a protective cover to the engine compartment from the external environment.
2. Provide access to the engine compartment for repairs and maintenance.
3. During collisions divert the impact to the hinges to provide safety to the occupants.
4. During a collision with a pedestrian enable reduced severity of the head injury.
5. Provide a surface over which air flows such that the drag is minimum.
6. Reduce NVH levels to improve passenger comfort.

Once the shape and look of the hood are finalized by the design team, it is the BIW design engineer\'s task to make the hood manufacturable along with the consideration of all the design aspects that are needed in a car hood design.
Various components of the car hood are designed and analyzed for safety, manufacturability, and assembly until a final design is obtained. Past data of the hood design becomes handy whenever designing a new car hood since the past data has useful information about the design considerations.

Manufacturing Process:

The car hood is mainly a sheet metal part hence it is made by using sheet metal forming processes such as Bending, Hemming, Stamping, etc.
Later the base coat is provided to the component that protects it from the environment and makes the component ready for the main paint coating

Material:

The automobile car hood is generally made of Steel or Aluminium. Sometimes in a performance car or a high premium car, the hood is made up of carbon fiber or fiberglass to reduce the weight and increase the strength of the hood.

2. Various components in an automobile frontal enclosure:

The hood of a car consists of the following main parts:

1. Inner Panel
2. Outer Panel
3. Hinges
4. Latch and striker

Inner Panel:
The inner panel of the car hood serves as the structural part of the hood which with the help of embosses acts as an impact or shocks dissipator. The shocks due to the impact or crashes are diverted to the hinges due to the proper emboss definition of the inner panel. This provides safety to the occupants of the car and also prevents damage to the critical parts of the automobile. The Hinge reinforcements and the striker reinforcements are attached to the inner panel.

Outer Panel:
The outer panel acts more like an external surface that makes the car aesthetically pleasing and also acts as a surface that channels airflow to reduce the aerodynamic drag on the car.

Hinges:
The hinges act as the pivot points where the car hood rotates about while opening and closing. It also acts as a shock-absorbing component during crashes since the shocks are diverted to the hinges when a crash happens.

Latch and striker:
Latch and striker act as a locking mechanism of the car hood. The latch engages with the striker when the hood is closed. To open the hood it must be unlatched either manually by hand or by electronic control

3. Design of Car hood:

Before we start designing the hood of the car there are certain aspects that are needed to be considered so that the final design meets particular legislation. For example the EuroNCAP. The main objective of these legislations is to provide standards for passenger and pedestrian safety during collisions.

a. The important aspects considered before designing a hood of an automobile.

i. Safety for Pedestrian
One of the functions of an automobile hood is to provide safety to the passengers as well as the pedestrian during an event of an impact of the car with a pedestrian. Hence, it is required to consider certain standards that deal with the safety of the car.
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The European New Car Assessment Programme (Euro NCAP) is a European car safety performance assessment program.

They publish safety reports on new cars, and awards \'star ratings\' based on the performance of the vehicles in a variety of crash tests, including front, side and pole impacts, and impacts with pedestrians. The top overall rating is five stars.

The frontal tests are performed at 64 km/h (40 mph) into an offset deformable barrier. This is designed to represent an impact with a vehicle of similar mass and structure as the car itself. The side-impact tests are performed at 50 km/h (31 mph), but the side impact pole test is performed at 32 km/h (20 mph). The pedestrian safety tests are performed at 40 km/h (25 mph).

Source: Wikipedia
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Fig.1: Child and Adult Impact Zones

Fig.2: Child and adult head impact area

The above images show the child and adult impact zones. During an impact, these are the area where the pedestrian head hits the car hood. Hence, the car hood is designed in such a way that the hood absorbs the energy and deforms upon the impact of the pedestrian and dissipates it so that the pedestrian suffers minimum damage.

ii. Safety for Passengers
Another function of the hood is to dissipate the energy in a direction away from the passengers upon impact. The hood should be designed in such a way that the energy upon impact is dissipated away from the passengers to ensure passenger safety.

iii. Hood opening and closing
It is important that the hood of the car closes properly and remains closed at all times until manually or forcefully opened. The position of the striker with respect to the hinge axis is important for the latch to function properly

iv. Corner relief for Outer Panel
The outer panel of the hood is assembled with the inner panel by hemming. Corner reliefs are needed to make the hemming possible.

v. Mastic Sealants

Mastic sealants are rubber adhesives that are glued between the outer and inner panel. They help reduce the NVH levels of the automobile and also helps to bring back the hood outer panel into its original shape after deformation. The Mastic points have a coverage area of up to 80mm, hence, they need to be placed such that it covers a maximum of the hood panel.

b. Design Data

In this report, we will work with the following Design Data.

Hood Thickness Information

• Outer Panel Thickness = 0.75mm
• Inner Panel Thickness = 0.75mm
• Reinforcement Thickness = 1.5mm

Master sketch:

Fig.3: Master Sketch for the hood design

c. Design for Safety

As discussed before passenger and pedestrian safety is an important aspect that is to be considered during designing a car hood.

For pedestrian safety during impact, cut-outs are provided at the head impact area of the pedestrian so that the hood absorbs and dissipates the energy away from the pedestrian.

Fig.4: Cut-outs for head impact area marked in green

For passenger safety the embosses are provided such that they absorb and dissipate the energy to a direction away from the passenger.

Fig.5: Emboss Definition

c. Design of Reinforcements and Mastic Data

The sheet metal of the hood inner and outer panel is very thin, in this case, it is 0.75mm. Therefore, to attach parts such as hinges and striker for the latch, the reinforcements are needed to make the component stronger so that these parts can be attached.

Fig.6: Reinforcements marked in orange

Mastic sealants cover an area of up to 80mm and hence they are to be placed in such a way that they cover almost the entire hood.

Fig.7: Mastic Data

The above image shows the mastic points and the area it covers.

c. Placement of Hinges and Striker for the opening, closing, and locking of the car hood.

Hinges are placed on the emboss provided especially for the hinges with the support of the hinge reinforcement

Fig.8: Hinges marked in orange

Fig.9: Hinge Placement

Fig.10: Hinge assembly placed inline with another hinge assembly

For the latch to function properly thus enabling proper opening and closing of the hood the striker of the latch must be placed such that a tangent drawn from a circle whose center is the hinge axis and the tangent intersects the striker\'s axis should be theoretically perpendicular to a line drawn from the hinge axis to the axis of the striker.

Fig.11: Trajectory circle is drawn from Hinge axis to Striker axis that shows perpendicularity

Fig.12: Striker placement

f. Outer Panel Details

As mentioned before the outer panel of the hood serves the purpose of making the hood look aesthetically pleasing and also to provide an aerodynamic surface that reduces drag on the vehicle.

Therefore this surface should be smooth and streamlined. Hence, no fasteners or weld spots should be visible atop the hood outer panel. For this reason, the Hood outer panel is put together with the inner panel by a hemming operation.

To perform hemming reliefs are required at the front and back of the panel where there is a sharp radius curvature.

Fig.13: Cut-section showing Outer Panel Hem

Fig.14: Relief for hemming

Fig.15: Inner Panel with Outer Panel Hemmed

Fig.16: Final Assembly

Conclusion:

Hence keeping the EuroNCAP, DFM, and DFA requirements in mind, the above hood components were designed using the given outer panel surface and the master sketch.

1. Reinforcements in the form of plates and embosses were provided to ensure safety.
2. Mastic points were provided that covered a wide area of the hood that will act as an adhesive between the hood inner and outer panel and help reduce the NVH levels inside the compartment.
3. Placement of hinge and striker such that the latch closes and remains closed during travel.
4. The outer panel was hemmed with the inner panel to attach them together without using fasteners or welding that would otherwise make the hood aesthetically poor.

The components were designed and assembled using extensive features of the software UG NX