Desktop version

Home arrow Engineering

  • Increase font
  • Decrease font


<<   CONTENTS   >>

Heavy Duty Vehicles

For USA heavy duty vehicles. 90% of fuel consumed was diesel, with the balance being gasoline and a small amount of LPG (Davis et al., 2017). Diesel engines, with their higher compression ratios, have typically been more energy efficient than gasoline engines, although gasoline engines are improving in efficiency as a result of higher compression ratios, turbo-charging, and other factors, as noted earlier. GHG emission reduction strategies for diesel trucks aimed at reducing energy losses typically include improved engines and transmissions, reduction of aerodynamic drag, reduction of rolling resistance, weight reduction for the empty vehicle, reduced energy consumption for auxiliary loads, and idle reduction. Other strategies include hybridization, use of biodiesel, and powertrains for alternative fuels, such as compressed natural gas, liquefied natural gas, batteiy electric, and hydrogen (USDOT, 2010).

The climate impact of diesel trucks can also be reduced in part by the deployment of diesel particulate filters (DPFs) that are highly effective (typically over 99% efficient) in reducing black carbon (BC) emissions (Sims et al., 2014; Frey, 2018). BC has short-term effects on climate forcing. Therefore, reductions in BC emissions can lead to relatively rapid climate change benefits. However. DPF operation typically leads to a small reduction in overall track energy efficiency related to engine backpressure and the process of periodically regenerating the filter to bum off deposited black carbon, thus leading to a possible trade-off with increased vehicle operational CO, emissions.

Based on real-world measurements using an instrumented trailer, on-road tailpipe exhaust GHG emissions were compared for diesel, diesel hybrid, and compressed natural gas long-haul freight tractors. The vehicle sample included diesel vehicle with and without selective catalytic reduction for NOx control. The CNG vehicle had a stoichiometric bum engine with a three-way catalyst. The measured greenhouse gases included CO„ CH4. and N.O. The CO,-equivalent emissions per vehicle-mile were compared among all of the vehicle types for five driving conditions, ranging from a near-dock drayage cycle to interstate highway. There was more variability in the emission rates with respect to driving conditions than there was when comparing the vehicle technologies. The hybrid track had the lowest average emissions rate among the vehicle types under the near-dock drayage cycle, CNG had the lowest average rate for the regional highway driving, and the diesel tracks with SCR had the lowest average rate for highway hill climbing. Thus, the emissions benefit of one technology over the other were highly route-dependent (Quiros et al., 2017).

Electrification and Transportation

Emissions from electric vehicles are highly dependent on how the electricity was generated. Although electric motors are substantially more efficient than gasoline or diesel engines, the overall energy efficiency of a vehicle depends on all of the losses for the energy cycle and vehicle operation, including upstream power generation efficiency, transmission losses, and batteiy losses. From this perspective, electric vehicles are typically not substantially more or less energy efficient than gasoline or diesel vehicles. However, a key benefit of electric vehicles is that their emissions can improve as the energy mix for power generation becomes less carbon intensive over time, without the need to replace the vehicle itself in order to achieve GHG emission reductions.

The global stock of plug-in electric vehicles, which includes batteiy electric vehicles and plug-in hybrid electric vehicles, was over 3 million vehicles in 2017, of which 40% were in China. Nearly all of the electric buses and electric two-wheel vehicles are in China. The global number of electric chargers reached almost 3 million in 2017 (IEA, 2018b). BEVs represented 0.6% of new car registrations in Europe in 2017. PHEVs accounted for 0.8% of European new passenger car registrations in the same year

(EEA, 2018). Globally, plug-in electric vehicle (PEV) sales, including BEVs and PHEYs, are expected to rise, reaching 8% to 26% of new vehicle sales by 2040, as indicated in Figure 6.

The International Energy Agency projects that, as a result of existing and announced policies, global coal-based power generation is expected to increase from 9.86 TWh in 2017 to 10.34 TWli in 2040, although the relative share of coal for power generation will decrease. Global natural gas-based generated power is projected to increase from 5.86 TWh in 2017 to 9.07 TWh in 2040. Under a “sustainable development” scenario, the amount of power from coal would decrease by about 80% and from natural gas would remain approximately the same. World oil demand for transport is projected to increase from 2.56 Mtoe in 2017 to 2.97 MToe in 2040.

As shown in Figure 7, EIA projects that the global electric power generation fuel mix will continue to be dominated by fossil fuels over the coming decades. Although much of the growth in pow'er generation will be based on renewable energy sources, growth is also projected in the amount of powder generated fr ont natural gas as wrell as some growth in the amount of pow'er. although not the share, from coal (EIA, 2017). Even though renewables are likely to capture a major share of power generation, it may be decades before fossil fuel drops below' 50% of total pow'er generation. The projections from IEA and EIA are based on a large number of assumptions and actual trends in the fuel mix used for pow'er generation are unlikely to be identical to any of these projections (Frey, 2018). However, it is possible

Projected trend from 2020 to 2040 in Worldwide Plug-in Electric Vehicles (PEYs) as percent of light-duty vehicle

Figure 6. Projected trend from 2020 to 2040 in Worldwide Plug-in Electric Vehicles (PEYs) as percent of light-duty vehicle

stock. Source: (Lymes, 2017).

Trends from 2010 with projections to 2040 in the global fuel mix for electric power generation. Source

Figure 7. Trends from 2010 with projections to 2040 in the global fuel mix for electric power generation. Source: (EIA,

2017).

that electrification of vehicles has the potential to shift conventional ah pollutant emissions (e.g., nitrogen oxides, carbon monoxide, hydrocarbons, fine particles) from dense high-traffic areas to the downwind regions affected by power plant plumes.

Based on an analysis of the USA’s energy mix, one study concluded that simply increasing the share of electric drive vehicles (EDVs) will not, in itself, achieve GHG emission reductions, but that substantial GHG reductions require reduction in the carbon intensity of power generation. Reductions in the share of coal and increases in the share of natural gas would reduce GHG emissions for electricity used for vehicle charging. Renewable portfolio standards (RPS) require a minimum share of renewable energy in the grid mix and can lead to GHG emission reductions. Factors that affect the market penetration of EDVs include battery price and oil price. High oil costs and low battery prices would motivate a larger penetration of such vehicles into the market. However, a secondary effect of high EDA' deployment is that sustained demand for electricity might lead to continued operation of existing coal plants that might otherwise be retired under a different power demand scenario (Babaee et al., 2014).

However, other studies indicate that electric vehicles might even lead to lower operational emissions of GHG, except in regions that have very high proportions (e.g., 90% or more) of coal-based power generation. For example, in an assessment of BEA' GHG emissions in selected regions of China and the USA, estimated GHG emissions were lower in all cases except for the Beijing-Tianjin region. For example, GHG emissions were estimated to be 20%-40% lower in the Yangtze River and Pearl River Deltas, and were as much as 50%-60% lower in California and the U.S. northeast (Huo et ah, 2015).

Based on a comparison of selected 2017 model year BEA's and conventional gasoline LDA's, as shown in Figure 8, the annual CO, emissions for the BEA's is highly variable, depending on the energy mix used for power generation among states in the USA, but on average would be lower than for the comparable gasoline vehicle. For example, the Bolt, Golf, i3, Clarity, Leaf, and Focus are approximately comparable in size to the Honda Civic, but would have CO, emissions from power generation that average 42-47% lower than the tailpipe exhaust emissions of the Honda Civic. There are some power generation energy mixes, such as for the state of West Virginia, that would lead to higher CO, emissions for the electric vehicles. The larger Tesla S and BYD e6 would, on average, have operational CO, emissions that are 38-54% lower than the Chevrolet Impala, with the Tesla being lower emitting in all states. Thus, except

Annual CO, emissions from vehicle operation for selected 2017 model year battery electric vehicles and comparable conventional gasolme vehicles

Figure 8. Annual CO, emissions from vehicle operation for selected 2017 model year battery electric vehicles and comparable conventional gasolme vehicles. Indirect CO, emissions for battery electnc vehicles from power generation are estimated for each U.S. state. A'alues shown as box and whiskers for each BEr include the median, inter-quartile range, minimum, and maximum among USA states. Source: Alternative Fuel Data Center, U.S. Department of Energy.

for a few exceptional cases among certain states of the USA, the CO, emissions directly attributable to ВЕЛ' operation would be lower. The actual emissions depend on the time of day of vehicle charging and the marginal emission rate associated with dispatching of power generation assets.

Battery electric trucks could reduce operational CO, emissions. A batteiy electric tractor-trailer truck with a 500-mile range would have a tare weight 6,000 kg greater than a diesel truck, resulting in loss of payload. Alternatively, a batteiy electric tractor-trailer truck with no additional weight would har e a range of only 140 miles. Range would be reduced at cold temperatures (Sharpe, 2019).

Lithium-ion batteries are the key batteiy technology used in electric vehicles. Key focus areas for further development of these batteries including chemistry, energy storage capacity, manufacturing scale, and charging speed. Batteiy cost reductions are expected. Plug-in electric vehicle stock is expected to increase substantially by 2030, depending on policy and cost reduction, to between 125 million and 225 million vehicles. There would be concurrent growth in vehicle chargers under these scenarios. Based on crurent batteiy designs, and expected growth in the number of batteries, the demand for cobalt and lithium will increase. Research on batteiy chemistry includes potential reductions in cobalt demand per batteiy, but increases in total demand for cobalt are likely (IEA, 2018b). However, actual shortages of materials for batteiy production are unlikely (Le Petit, 2017).

 
<<   CONTENTS   >>

Related topics