7 Electric and Hybrid Vehicles electronic components for the electric motor drives and the DC/DC converters are presented in Chapter 9. The different types of IC engines and their fundamentals are presented in Chapter 13. The power transmission components, brakes and cooling system are discussed in Chapter 14. 1.4 VEHICLE MASS AND PERFORMANCE The several vehicle mass definitions used in this book are common to those used in the automo- tive industry. The vehicle curb mass m v is the total mass of a vehicle with all standard equipment, components, lubricants, full tank of gas but without any passenger or cargo.
The gross vehicle mass m gv of a vehicle is the curb mass plus the passengers and cargo. The maximum gross vehicle mass is the curb mass plus the maximum number of passengers and the maximum mass of the cargo that the vehicle is designed for. Often the term weight is generally used for vehicles instead of mass, although it is the mass of the vehicle that is being referred. The mass defines the intrinsic physi- cal property of the vehicle, whereas weight is the force of the vehicle due to the gravitational pull. The scientifically correct term mass will be used in this book for the calculations and analysis.
The vehicle curb mass is distinguished between sprung mass and unsprung mass in relation to the location of the components with respect to the vehicle suspension system. The spring mass is the fraction of the vehicle curb mass that is supported by suspension including suspension members that are in motion. The unsprung mass is the remaining fraction of vehicle curb mass that is carried by the wheels and moving with it. The front-to-rear mass distribution is critical for balance and good ride performance of a vehicle. The mass distribution can be defined in terms of axle-to-axle lengths.
Let l = axle-to-axle length, a = front axle to vehicle center of gravity, known as front longitudinal length, b = rear axle to vehicle center of gravity, known as rear longitudinal length. The front vehicle mass is b m vf = m v . l And the rear vehicle mass is a m vr = m v . l The packaging of components for good balance and ride performance is very important, especially for hybrid vehicles that have more powertrain components than a conventional vehicle. The EVs and hybrids with large energy capacity have significant battery mass requiring efficient packaging within the vehicle.
The dynamic effects of braking cause a dynamic mass shift; the front brakes are responsible for about 70% of vehicle braking. A 60:40 or less ratio of front-to-rear vehicle mass distribution is required considering balance, ride performance and dynamic braking. The ratio of sprung-to-unsprung mass also has to be carefully evaluated during the vehicle pack- aging and layout design. A 10:1 ratio of sprung-to-unsprung mass is a desirable target, although a slightly lower ratio can be used for hybrid vehicles that may have more unsprung components. The electric motors mounted on the hub or disk brakes along with the entire caliper assembly add to the unsprung mass of the vehicle.
An equivalent vehicle mass in terms of the curb mass and the number of passengers is used in sizing the powertrain components. The equivalent mass to be used in the design calculations is given by m eq = + k m m v N m p p