Integration of Traffic Simulation and Propulsion Modeling to Estimate Energy Consumption for Battery Electric Vehicles - Simulation and Modeling Methodologies, Technologies and Application - page 14

Fig. 7. The effect of gradient on average energy consumption for 125 vehicles for the residen-
tial street
3.4 Comparison across Road Types
Figure 8 shows the impact of accessory loads on the energy consumption across the
three road types. The bars represent the energy usage in W-hours per mile and the
three bold dots represent the average travel time in seconds to travel a distance of one
mile on each of those roads. It can be seen that for a given accessory load the energy
usage is the lowest for a residential road and highest for a freeway, mainly because of
low speeds and stop-go nature of traffic on a residential road. In fact, the energy usage
per mile with 400W accessory load is more than halved from a freeway to a residen-
tial road. But, the travel time on a residential road is almost four times that on a free-
way. This shows a trade-off between travel times and energy usage. Increase in the
accessory loads has very little impact on the energy usage on a freeway where high
speed is the primary driver of energy consumed. On the other hand, the accessory
loads drastically affect the energy usage on a residential road to such an extent that,
the energy used per mile with 2000W accessory load is almost the same as that on an
urban road.
Fig. 8 shows that gradient has a very dominating effect on the energy consumption
of a battery electric vehicle. For a particular road type, the increase in energy con-
sumption with every 2% increase in gradient is about 150W-hrs per mile. High gra-
dients like 4% or -4% are not very common for a freeway.
The three road types each have different speed limits, and for each simulation the
drivers' 'desired speed' is input as a statistical distribution around the speed limit for
the road. Some drivers will drive above the limit and others below, but most will be
close to the speed limit. Fig. 8 shows the energy consumption for vehicles with the
following desired speeds: Freeway - 65 MPH, Urban road -45 MPH, Residential road
- 25 MPH. These values are almost identical to the speed limits on the corresponding
road types. The graph shows that the speed of the vehicles has a strong influence on
the energy usage. Also, the stop-go behavior of vehicles on urban and residential
roads has a significant effect on energy consumption because of regenerative braking.