Automotive Design ME 524 PDF

1 11 INTRODUCTION TO AUTOMOTIVE DESIGN ME 524 CHAPTER Textbook: Automotive Chassis Engineering-Barton, D. C. and Fieldhouse, J. D. 2nd -2024 2...

1 11 INTRODUCTION TO AUTOMOTIVE DESIGN ME 524 CHAPTER Textbook: Automotive Chassis Engineering-Barton, D. C. and Fieldhouse, J. D. 2nd -2024 2 Vehicle Dynamics Dr. Mohanad Khodier Mechanical Engineering Department Yarmouk University Learning Objectives: 2 Understand the general automotive dynamics. Calculate the external forces on the automotive. Calculate tractive effect force. Calculate aerodynamic force. Calculate rolling resistance. Calculate tractive resistance. Tyre design. 2.1. Modelling Philosophy 3  Most of the analyses of vehicle performance rely on the idea of representing the real vehicle by mathematical equations. This process of mathematical modelling is the cornerstone of the majority of engineering analyses. The accuracy of the resulting analysis depends on how well the equations (the mathematical model) represent the real engineering system and what assumptions were necessary in deriving the equations.  Motion or its generalised version when rotations are involved which are usually called the Rigid Body Laws. The approach, which is the preferred method for tackling the majority of dynamics problems is: (a) Define an axis system (b) Draw the Free Body Diagram (FBD) (c) Apply the Rigid Body Laws (d) Write down any kinematic constraints (e) Express forces as functions of the system variables (f) The governing set of equations then come from combining (c), (d), and (e). 2.2. Co-ordinate Systems There is a standard definition embodied in an SAE standard (SAE J670 Vehicle Dynamics Terminology) for a co-ordinate system fixed in the vehicle and centred on the vehicle centre of gravity (C.G.) as shown in Fig. 1.1. 2.2.1. Tractive Force or Tractive Effort (TE) 4 Fig. 2.1. Vehicle co-ordinate system as detailed in SAE J670 vehicle dynamics terminology  The tractive effort (TE), provided by an internal combustion engine (ICE) or electric drivetrain, is the force available at the driven axle tyre/road interface to propel and accelerate the vehicle. 2.2.1. Tractive Force or Tractive Effort (TE) 5  For a conventional ICE vehicle, the vehicle speed (v) is given by:  A typical internal combustion engine power and torque characteristic plotted against engine speed is shown in Fig. 1.2. Note that the maximum torque and maximum power occur at different engine speeds. Fig. 1.2. Typical internal combustion engine characteristics 2.2.2. Tractive Resistances (TR) 6  A vehicle’s resistance to motion is due to three fundamental parameters: 1) Gradient resistance. 2) Aerodynamic drag. 3) Rolling resistance (with slow speed manoeuvres, turning resistance is also important). 1) Gradient Resistance (GR): If the vehicle is progressing up a gradient, GR is the proportion of a vehicle’s weight acting down a gradient, the mgsinθ component as indicated in Fig. 1.3. If the vehicle is progressing down the gradient, the component would be assisting the vehicle in which case the force would be termed “gradient assistance”. GR can be represented as a single force acting at the C.G. of the vehicle and parallel to the road surface: Fig. 1.3. Gradient resistance and assistance 2.2.3. Aerodynamic Resistance or Drag Force (D) 7 2) Aerodynamic Resistance or Drag Force (D):  The aerodynamic drag force is a measure of the resistance of a vehicle to progress through air. It can be represented as a single force acting at the centre of pressure at some distance above road level. This distance would normally be determined, initially, by Computational Fluid Dynamics (CFD) analysis and confirmed by wind tunnel testing.  The drag coefficient is ultimately determined experimentally from wind tunnel tests. The drag coefficient is clearly an important vehicle design parameter from an energy efficiency, and hence fuel economy, point of view.  The best passenger cars now have a CD of around 0.3. Typical values for other vehicles are shown in Table 1.1. Fig. 1.1. Typical values for CD. 2.2.4. Aerodynamic Resistance or Drag Force (D) 8  Although for full aerodynamic analysis, the compressibility of the air must be taken into account, it is instructive to consider the incompressible flow equation, a common form of Bernoulli’s equation, which is valid at any arbitrary point along a streamline:  As air speed increases, the local pressure will fall and may in fact become negative to ambient. This explains why papers will be “sucked” through an open sunroof at high speed. Fig. 1.4. Visualisation of streamlines in a wind tunnel 2.2.4. Aerodynamic Resistance or Drag Force (D) 9  The earlier the air separates from the vehicle, the greater the “wake” (the area directly behind the vehicle), resulting in an increase in negative pressure at the rear of the vehicle and an increase in aerodynamic drag.  Aerodynamic flow over a vehicle will also typically generate vertically downward forces (negative lift). This down force will aid cornering but will effectively add to the vehicle tyre/road interface force and increase rolling resistance forces during straight line driving. The aim of formula racing cars fitted with wings is to balance the increased down force required for cornering with the accompanying increased wing drag developed on the high speed straight. 3) Rolling Resistance(RR):  Defined as the force that must be overcome to cause the vehicle to move at constant speed over a horizontal surface, assuming no vehicle body aerodynamic forces are present, the vehicle is travelling in a straight line and that the road surface is reasonably smooth.  The rolling resistance arises from two main sources: 1) continuous deformation of the tyres during rolling. 2) frictional effects in the mechanical driveline components. 2.2.5. Rolling Resistance(RR): 10  Rolling tyres undergo a continual cyclical deformation as the tyre passes continuously through the contact region area. This causes deformation of the side-walls and tread area and, because it is not a perfectly elastic process, some energy is lost through hysteresis (see Fig. 1.5).  This lost elastic energy appears as heat.  which may be confirmed by “feeling” the tyre temperature after a period of high- speed driving.  If the tyre is under-inflated then sidewall deformation increases along with the temperatures. If the vehicle continues to be driven with excessive deflation, then sidewall delamination (pealing) may occur. Table 2.2. Typical Values for cR. Fig.1.5. Hysteresis loss within a tyre during loading and unloading 2.3. Tractive Resistances (TR) 11  Size—increase in width of tyre results in lower rolling resistance due to lower tyre wall deflections (see Fig. 1.6).  If a normal tyre is inflated to the correct pressure, the rolling resistance reduces and vehicle economy increases.  If the tyre is under-inflated, the rolling resistance will increase due to excessive tyre wall deformations.  If excessive over-inflation occurs, then vehicle handling is affected. 2.3. Tractive Resistances (TR) 12  In general, the cornering resistance depends on: 1) The steered wheel angle (which depends on the cornering radius). 2) Steered wheel load. 3) Tyre/road interface friction level and drive configuration (front, rear or all-wheel drive). where μ is the static tyre/road interface friction coefficient 2.4. Effect of TR and TE on Vehicle Performance 13  The total Tractive Resistance (TR) is the net sum of the rolling resistance, aerodynamic drag and gradient resistance (assistance): (1.7) Figure 1.9 shows typical variations of TE and TR against vehicle speed for an ICE vehicle. 2.4. Effect of TR and TE on Vehicle Performance 14 2.5. Tyre Properties and Performance  A tyre is a means of transmitting the torque developed by the drivetrain to the road such that the tractive effort available may be used to propel the vehicle.  The tyre must also play its part in slowing the vehicle down when the brakes are applied.  It also has to ensure safe manoeuvring, such as cornering. Because of this it must have a sufficiently high coefficient of adhesion with the road surface to avoid wheel slip during acceleration and braking and also to prevent instability during cornering. 2.5.2. Tyre Construction 15  There are two types of tyre construction, radial ply (most common) and cross (bias) ply.  The primary advantage of radial ply tyres is that the side walls are more flexible and so more tread remains in contact with the road during cornering. 450 900 Advantage of Radial Disadvantage  Longer Tread Life.  Poor transport handling, since low lateral stiffness  Cooler Running. causes the tyre sway to increase as the speed of the  Lower Rolling Resistance. vehicle increases.  Enhanced Comfort.  Increased vulnerability to abuse when overloaded or  Increased Impact Resistance. under-inflated. The sidewall tends to bulge which could  Greater Puncture Resistance. cause damage and puncture.  Better Wet Traction.  Lower Running Costs.  Reduced Sidewall Damage. 2.5.3. Tyre Designation 16  Consider the designation of Figure 1.13 is 215/65R15 95 H where. 2.5.4. The Friction Circle 17  The tyre/road interface force necessary for acceleration, braking and cornering is the product of the tyre/road interface adhesion coefficient μ and the normal force on the tyre upper N.(Fig.1.14 and Fig. 1.15)  The Friction Circle diameter is determined by the product of the total vertical load (N) and the tyre/road interface friction coefficient (μ). The minimum tyre/road interface adhesion coefficient (μ) to avoid gross tyre slip for a particular resultant force is given by: 2.5.4. The Friction Circle 18  The longitudinal slip, s, during braking is defined as:  It can be seen that maximum adhesion coefficient occurs at a slip of around 10%. Example: 19 A car has the following prosperities: Engine Torque (Te) = 100 N.m, Gear Ration (ng) = 0.95, Final drive ratio (nd) = 1.0, Transmission efficiency (μ) = 0.95, Effective rolling radius of the tyre (r) = 0.5 m, Engine speed (Ne) = 17 rev/s Rotational speed of the wheel (ω) = 20 rad/s, Assume that the front velocity of type = vehicle speed. Calculate tractive effect force (TE), vehicle speed (v), and % longitudinal slip (s)? 2.6. Rigid Body Load Transfer Effects for Straight Line Motion 20  As shown in Fig. 1.17, the axle loads can now be found by taking moments about any point. e.g. the rear wheel contact patch: This enables N1 to be calculated. Then resolving perpendicular to the slope gives N2 : 2.7. Vehicle Accelerating/decelerating on Level Ground  The free body diagram for a vehicle accelerating on level ground is shown in Fig. 1.18 where: N1 and N2 are the interface loads between tyre and road F is the tractive effort (TE) available at the tyre road interface R1 and R2 are the rolling resistances at each axle D is the aerodynamic drag mg is the vehicle weight acting through its C.G. 2.7. Vehicle Accelerating/decelerating on Level Ground 21  As shown in Fig. 1.18, and by applying Newton’s second law and the take the moment about the C.G: Rear Wheel Drive Vehicle: Front Wheel Drive Vehicle: Four Wheel Drive Vehicle Simplify  2.7. Vehicle Accelerating/decelerating on Level Ground 22  For Caravans and Trailers, consider the following example. 2.7. Vehicle Accelerating/decelerating on Level Ground 23 2.7. Vehicle Accelerating/decelerating on Level Ground 24 2.7. Vehicle Accelerating/decelerating on Level Ground 25 2.7. Vehicle Accelerating/decelerating on Level Ground 26 Now consider car only (see Fig. 1.26) This level of friction demand (adhesion coefficient) should be  exceeded under most road conditions. Therefore rear wheel drive car is suitable for this application. 2.8. Rigid Body Load Transfer Effects During Cornering 27  Longitudinal load transfer effect (LTE) due to acceleration/ deceleration is given as:  Due to aerodynamic drag is given as: The positive sign in the above equations relates to the rear axle and negative sign to the front axle.  During cornering there is a lateral load transfer effect due to the cornering (i.e. centrifugal) forces (See Fig. 1.7). where m is vehicle mass, v is forward velocity of vehicle and R is the radius of the turn. Lateral load transfer effect: where h is the height of the C.G. and T is the wheel track width. More precise, for front axle: For rear axle: 2.8. Rigid Body Load Transfer Effects During Cornering 28  Table 1.3 shows the individual wheel LTE terms for a vehicle braking during a left-hand turn. Note that summing all the individual loads for each wheel leads to the vehicle weight mg as expected. The resultant force (R) in the case of lateral force (LF) and braking force (BF)is as follows: Example 1.4: 29 Assuming both normal wheel loads and lateral cornering forces are proportioned according to the longitudinal position of the centre of gravity, calculate: (a) The total longitudinal load transfer due to deceleration and aerodynamic drag. (b) Proportioned front and rear lateral load transfer during cornering. (c) The road/tyre interface load acting at each wheel during this manoeuvre. (d) The minimum coefficient of adhesion necessary for each wheel considering both braking and cornering. Example 1.4: 30 Example 1.4: 31 Wheelbase = a + b = 1.1+1.2 = 2.3 m Example 1.4: 32 Example 1.4: 33 END