The main difference between the evolutionary and advanced passenger car vehicle body is the extent to which more radical new technologies are used to reduce vehicle weight. Table 3.2 reports our projections of vehicle mass by component for all vehicles examined. The estimated mass distribution of the 1996 baseline vehicle is based on the mass distribution
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of a 1990 Ford Taurus (OTA, 1995) and a study by the Ultra-light Steel Autobody (ULSAB) Consortium that especially examined the mass of vehicle components for a range of recent passenger cars (ULSAB-AVC Consortium, 1999).
Based on the vehicle mass distribution of the 1996 baseline vehicle, the distribution of all other vehicles was projected, using the following simple approach. The 2020 baseline vehicle distribution was derived by multiplying the mass of the body structure, other body parts, and steering and brakes by 0.85 to reflect the approximately 15% mass reduction potential of high-strength steel compared to mild steel. For all advanced vehicles, the 1996 baseline vehicle mass of these same components was multiplied by 0.65 to simulate the 35% mass reduction due to aluminum substitution. The mass change of the propulsion system resulted largely through the vehicle propulsion system modeling described in this section and exogenously specific power-to-mass ratios of the major components, determined at 0.9 kW/kg for an advanced gasoline engine, 0.6 kW/kg for an advanced diesel engine, 1.5 kW/kg for an electric motor, and 0.4 kW/kg for a fuel cell system.
The mass of suspension and frame of any 2020 vehicle was estimated by multiplying the chassis mass of the 1996 baseline vehicle by the ratio of the projected mass of vehicle body, propulsion system, and interior of the 2020 vehicle and the mass of these components of the 1996 baseline vehicle. This simple approach ensures that suspension and frame of all projected 2020 vehicles is sufficiently strong to carry the mass of the projected body, propulsion system, and interior through eventually adding mass to the chassis. The maximum extra support mass is 59 kg for the gasoline fuel cell vehicle, where propulsion system mass increases by 100% compared to the 2020 baseline vehicle.
Other notable changes in component mass from the 1996 baseline vehicle include the transition from automatic transmission to auto-clutch and continuous variable transmission, and a reduction in wheel and seat mass due to a larger use of magnesium.
In Table 3.2, the total vehicle mass is subdivided into four subsystems for comparison: chassis and body, propulsion, battery, and fuel. The chassis and body system mass include everything for an un-powered free-rolling vehicle, including the fuel system without the fuel as well as all structural reinforcement for extra mass on the vehicle. The propulsion system mass include the engine, scaled according to power output for ICEs, electric motors, fuel cell systems and reformer systems, and the transmission, allocated a mixed mass for automatic manual, continuously variable, and direct gear.
The battery and fuel mass are also separated for ease of reference. The battery pack size is determined by the maximum power required by the electric motor in a particular vehicle, resulting directly in a specific battery mass and volume. This sizing assumption does not take into account the voltage and current balance that may affect the motor selection and
performance.
The amount of energy the battery pack can store is thus also constrained; this limit has less impact for hybrid systems because the battery pack can be recharged while driving, although care must be taken in the case of sustained peak power supplement to ensure that safe passing and hill climbing are possible. Towing capacity requirements also would impact the battery
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Table 3.2 Mass Distribution (kg) by Component for All Vehicles Examined.*
Technology current baseline advanced advanced advanced advanced advanced advanced advanced advanced advanced
Propulsion System SI ICE SI ICE SI ICE CI ICE SI Hybrid CI Hybrid SI Hybrid FC Hybrid FC Hybrid FC Hybrid Electric
Fuel gasoline gasoline gasoline diesel gasoline diesel CNG gasoline methanol hydrogen electr.
Transmission auto auto-
clutch
auto- clutch
auto- clutch
CVT CVT CVT direct direct direct direct
Body 383 326 249 249 249 249 249 249 249 249 249 Glazing 35 33 33 33 33 33 33 33 33 33 33 Chassis 273 229 216 219 216 219 216 275 25459 244 243 Propulsion System 392 263 252 303 267 303 283 536 475 416 414 Engine 164 103 95 149 64 99 67 0 0 0 0 Electric Motor 19 20 20 73 69 66 66
Fuel Cell System & Reformer
351 278 193
Battery 12 12 12 12 36 37 37 46 43 41 328
Transmission 90 50 50 50 50 50 50 20 20 20 20
Liquids and Storage 64 45 42 39 34 31 46 33 53 84
Other (Accessories, Electronics, etc.)
62 53 53 53 64 64 64 14 13 12
Interior & Exterior 195 214 214 214 214 214 214 194 194 194 194
Other 44 44 44 44 44 44 44 44 44 44 44
TOTAL VEHICLE 1322 1108 1007 1062 1023 1060 1039 1330 1253 1179 1176
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system size and weight. For the pure electric vehicle, extra batteries may be added to increase energy storage capacity and hence extend vehicle range. These add to the vehicle weight and thus require additional batteries (and weight) to maintain performance.
The pack size or volume occupied by the battery system is of concern because of space limitations on board the vehicle. A volumetric analysis should be performed to determine if the battery pack will fit in the vehicle, and if not, the appropriate penalty in aerodynamic drag
factor (CdA) should be taken into account.
The fuel mass is two-thirds of the amount of fuel needed to achieve approximately a range of 600 km in the combined cycle. Except for the pure electric vehicle, whose special case will be discussed in section 3.3, all vehicles meet the 600 km range criteria.
An occupant and cargo mass is added to the total raw vehicle mass. It is the standard FTP test procedure occupant and cargo mass of 300 lb. This estimated average load for a vehicle, is held constant for all vehicles in this study at 300 lb/136 kg, (e.g., the mass of 1.5 adults at 75 kg per person, with some 20 kg of cargo). Therefore, the total operating vehicle mass is the summation of the chassis and body system mass, the propulsion system mass, the battery mass, the fuel mass, and the occupant and cargo mass. Other key simulation variables for the vehicle and transmission, with their assumptions and descriptions are listed below.
Aerodynamic Drag Coefficient
Cd Aerodynamic drag coefficient is a dimensionless
number describing the drag induced by a body traveling in a fluid at a known relative velocity. For this study, the
current vehicle has an estimated Cd of 0.33, improving to
0.27 in the evolutionary vehicle and 0.224 in the advanced
vehicle, both in 2020.
Cross-Sectional Area
Ax Vehicle Cross-sectional area is the largest area in a
plane perpendicular to the direction of vehicle motion.
When multiplied by Cd, air density, and the square of the
relative velocity, the product is the aerodynamic drag force that must be overcome for the vehicle to move at that speed. Note that in this study, it is assumed there is no wind and; the air is still.
Fdrag = 1/2ρCdAxV2
4
Ford and GM (ref) have already built prototypes that achieve below 0.22 for the PNGV program. (National Research Council, 2000).
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Rolling Resistance Coefficient
Crr Rolling resistance coefficient is a dimensionless number
used to characterize the energy dissipated due to friction between the road and the tires. It is multiplied by the total vehicle weight to obtain the tire resistance force.
Froll = CrrMtotg
Transmission Efficiency
ηtrans Transmissions are modeled with a constant efficiency
during all modes of operation, although in practice the efficiency varies among gears. Idling in neutral or in drive (where friction is about double that in neutral) is taken into account, but shifting losses are not. More details on transmission performance could be added in the future; assuming an overall constant efficiency adequately incorporates the power losses in the transmission at this stage.
Five different transmissions are used for this study. For today's vehicle, a 5-speed manual at 94 % efficiency, and a 4-speed automatic at 70 % efficiency city and 80 % efficiency highway are used to verify the accuracy of the model. The future evolutionary and radical gasoline and diesel vehicles use 5-speed automatically-shifting clutched transmissions at 88 % efficiency, while future radical gasoline and diesel hybrids use continuously variable transmissions also at 88 % (Kluger and Long, 1999). An additional benefit from the CVT is that it enables improved engine efficiency by selecting the higher efficiency regions of the engine performance map. Finally, all the electric-drive vehicles, the fuel cell and battery electric vehicles, operative on single ratio direct drive at a speed and power dependent efficiency that averages out to about 93% over the combined cycle.
Auxiliary Load Paux Auxiliary load is assumed to be constant at 400 W for the
current vehicle, and at 1000 W for all 2020 vehicles, during all times of vehicle operation. While future vehicles may be more efficient in power electronics, they are expected to have more on-board electrically driven systems, drawing even more power. The auxiliary load is held constant for all vehicles. Since all vehicles have similar on-board systems, this study has not focused on determining the auxiliary load more precisely.
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