Decarbonization potential of future sustainable propulsion—A review of road transportation

Modern automotive propulsion technologies must achieve the highest CO2 reduction potential quickly to abide by the requirements of the Paris Climate Agreement. A collective utilization of renewable fuels, e‐fuels, hydrogen, and electrical energy will be able to meet different mobility and transport requirements in an optimal and CO2‐neutral approach. The well‐to‐wheel greenhouse gas emissions of a propulsion system are determined by two factors, that is, the energy efficiency of the system and the carbon intensity of the energy source. Regardless of the CO2 emission generated during the battery manufacturing and recycling process, the carbon intensity of the battery electric vehicles during operation is mainly decided by the carbon intensity of the electricity being consumed. The relatively low fleet ratios of battery electric and hydrogen‐powered vehicles and the massive remaining useful life of current internal combustion engine vehicle stock limit their impact on decarbonization in the near term. The expansion of charging infrastructure requires significant acceleration for the success of large‐scale and rapid electric vehicle adoption. For internal combustion engines, the focus is to further improve energy efficiency and the adoption of low‐to‐zero carbon renewable fuels. Hybrid and plug‐in hybrid vehicles are demonstrating the advantages of combining state‐of‐the‐art technologies to reduce both energy consumption and carbon emissions. In this review, the present status of propulsion systems is reviewed in detail, considering both the market penetration and well‐to‐wheel carbon emissions. The decarbonization potentials of various propulsion systems are then discussed.


| INTRODUCTION
In the near term, automotive propulsion technology must be able to achieve the highest CO 2 reduction potential quickly, so that the requirements of the Paris Climate Agreement 1 can be adhered to.In 2020, US passenger vehicles (cars and light trucks) and heavy-duty freight trucks accounted for 15% and 7% of total greenhouse gas (GHG) emissions, respectively. 2][10] Alongside electrification, the energy transition will require more renewable fuels to add to the liquid and gas fuels as part of our affordable and reliable energy mix over the generations to come. 11The high energy density of hydrocarbon fuels makes them the most suitable fuel for heavy-duty applications, such as long-haul airplanes and heavy-duty freight trucks, 9,12 as shown in Figure 1.
Of the three types of energy sources shown in Figure 1, that is, battery, hydrocarbon fuels, and nuclear, battery and hydrocarbon fuels use the energy released via a redox reaction.Present nuclear reactors utilize the energy released during the fission of the fuel atoms, which has significantly higher gravimetric energy density than chemical-reaction-based energy sources.
The major reason for hydrocarbon fuels to have significantly higher energy density than that of batteries (~40 times higher than the gravimetric density of a typical lithium-ion battery cell) is that only part of the reactants (fuel) is on-board.The oxidizer ("O 2 " to support combustion) and the working fluids ("N 2 " to convert heat into pressure), which are ~15 times the weight of fuel, are obtained from the air during operation, without any penalty to the cost, weight, or volume of the propulsion system.
Battery, on the other hand, generates electric energy via a redox reaction between the anode and cathode.A full battery cell needs to carry all the reactants on board, which caps the energy density of a theoretical battery cell.The cathode of a battery is normally in the form of a compound, containing elements other than lithium to support the storage and transportation of lithium ions during battery operation.The presence of nonchemical reactive elements in the battery further decreases the energy density of a battery cell.Other battery components, such as separators, electrolytes, and packaging, are also essential for the proper operation of a practical battery cell, but further reduce the energy density of the battery. 13The present battery technology shows its suitability for light-duty vehicle (LDV) applications, reflected by a substantial market penetration of light-duty passenger vehicles around the globe.
There are two technical pathways to reduce GHG emissions from the transportation sector: • To increase the energy efficiency of the propulsion system.• To reduce the carbon emissions of the energy sources.
First, the improvement of energy efficiency can decrease energy consumption, which directly reduces GHG emissions.Electric motors are more energy F I G U R E 1 Energy sources for transportation, adapted from Yu et al. 9 YU ET AL.
| 439 efficient for transportation use compared with internal combustion engines (ICEs), because of their high efficiency within a wide range of rotation speeds. 14,15ybrid and plug-in hybrid vehicles, battery electric vehicles, and fuel cell vehicles are all utilizing this advantage to reduce the GHG emissions from the propulsion systems during operation.
Second, the carbon intensity of the energy sources also plays an important role in the decarbonization process.Clean electricity from renewable sources, renewable hydrocarbon fuels, and renewable hydrogen are all considered low-carbon or zerocarbon fuels when considering their well-to-wheel carbon emissions. 16Propulsion systems utilizing renewable fuels are also considered low-carbon propulsion systems, which have low or zero carbon emissions.It should be noted that ICEs, like other prime movers, are not the root cause of CO 2 emissions, but the fuels that are burnt with it.The replacement of fossil fuels with low-carbon fuels has the potential to significantly reduce CO 2 emissions from the transportation sector in a progressive way.
In this review, the present status of road propulsion systems is summarized with a special focus on both the market penetrations and fleet ratios, including ICE vehicles (ICEVs), conventional hybrid electric vehicles (HEVs), plug-in HEVs (PHEVs), battery electric vehicles (BEVs), and fuel-cell electric vehicles (FCEVs).Then, the carbon intensity of electricity under the present energy mix is presented, and its impacts on the CO 2 intensity of BEVs are calculated.The primary energy consumption and CO 2 emission of various powertrains are compared, to estimate the decarbonization potential of various powertrains.
The major purpose of this review is to summarize and discuss potential decarbonization pathways of road transportation based on the present powertrain technologies.The analysis integrates carbon and energy estimations on a well-to-wheel basis for comparison among vehicle propulsion systems.Intrinsically, the results were deemed well-to-wheel values as refinement processes of fuels 17 and primary energy efficiency of the electrical grid was considered 18 from government statistics.To simplify the calculation scenario, GHG generated during battery manufacturing and recycling is not considered.Further, the energy losses during electricity transmission and battery charging were omitted and electricity generated from renewable sources, that is, wind, solar, biomass, geothermal, hydro, and nuclear, was assumed carbon-free.The supporting document further details the methodology used for the analysis.

| THE PRESENT STATE OF ROAD PROPULSION SYSTEMS
The market penetration and fleet ratio of various types of propulsion systems have changed significantly in the past 2-3 years, mainly driven by the government's effort toward net-zero carbon emission in the coming decades.The net zero-emission vehicle targets announced by different nations around the world are summarized in Table 1.In this section, both fleet ratio and market penetration of propulsion systems are presented, together with technology improvements, specifically ICEVs, HEVs, PHEVs, BEVs, and FCEVs.
In the past decade, the energy mix of the transportation sector, especially for light-duty applications, has been diversified significantly, following the decarbonization efforts of various Original Equipment Manufacturers (OEMs) following the emission mandates.Among the leading contenders, BEVs have gained the most traction for light-duty passenger vehicles.FCVs have recently started getting significant mainstream attention, mainly contributed by Toyota and Hyundai. 21or heavy-duty applications, hydrocarbon fuels remain the only commercially viable energy source, with the GHG reduction primarily achieved because of engine efficiency improvements.
The efforts toward vehicle electrification date back to the early 1980s but have picked up significant momentum since 2012 with ICEV hybridization.The fleet of battery electric vehicles has increased significantly since the year 2015 and reached over 7 million units in 2020, occupying around 0.7% of the global vehicle fleet, as shown in Figure 2. [22][23][24][25][26]30,31 With the rapid development and further increase in market share, the fleet ratio of BEVs is going to increase further. Chinaaccounted for around 4.5 million of the total on-road EVs, comprising slightly over 1% of the total vehicle stock share in China.The EV stock share in European Union (EU) and the United States in the year 2020 was 1.1% and 0.5%, F I G U R E 3 Fleet ratio of battery electric vehicle (EV).EU, European Union, US, United States.24,27,30 respectively.23,[26][27][28] Of the total EV share, PHEVs contributed approx.Thirty to fifty percent of the electrical vehicle stock.25,32 The current adoption of BEVs is mainly limited to light-duty passenger vehicles.Regardless of the outstanding performance of current electric motors, the lower energy density of the battery poses restrictions on the range and the towing capacity.9 These constraints are further amplified for heavy-duty applications.Batterymotor propulsion systems may be deemed as the best solution to realize zero-carbon transportation, considering the apparent elimination of tailpipe emissions.However, technical challenges and infrastructure constraints remain to be solved within the coming decades.
During the transition HEV and PHEV adoption into the medium-duty sector, mainly intracity buses, has been slowly increasing. 25,27,33For heavy-duty applications, the battery-motor powertrain is currently limited to the demonstration stage and feasibility studies.Because of the gross vehicle weight regulations, the increased battery weight directly impacts the payload capacity of heavy-duty trucks, and the long charging time could lead to economic and logistical considerations.Recently, the strategies for partially mitigating this with efficient packaging and cooling design of battery cells and faster-charging technologies are being researched and developed extensively. 34,35However, a major leap in the material level energy density is required to effectively combat these challenges and extend battery utilization beyond the light-duty sector.
Furthermore, in the light-duty section, hybrid technology has realized noticeable growth, even without direct financial support from the government, as shown in Figure 3. 24,27,30 The bar chart marks the total sales of each type of vehicle since 2015, and the percentage number labeled on each bar shows the market share of each type of powertrain.For the United States and Europe, the market penetration of HEV is higher than PHEV and BEV combined, which indicates the choice of the free market. 27,30For Japan, where energy supply relies heavily on imports, reduction of energy consumption remains to be the most important task.The market share of HEVs reached 25.5% in 2015 and rises to 34.8% in the year 2020, while both PHEV and BEV market shares remain under 0.5%. 24nder steady-state operation conditions, high engine efficiencies are preferred to decrease energy consumption and GHG emissions.7][38][39] Lean burn combustion demonstrates the capability to further increase engine-indicated efficiency up to 52%. 40,41owever, the engine load/speed window for high fuel efficiency operation conditions is narrow, which decreases the fuel economy significantly when only ICE is used to cover the complex driving conditions.
A hybrid electrification powertrain overcomes some of the limitations of ICEVs and realizes high fuel economy and low emission for vehicular applications. 42,43As shown in Figure 4, the reduction in average CO 2 emissions has been sustained over the last few decades.For vehicles burning traditional hydrocarbon fuel, CO 2 emission is directly related to fuel economy.The progressive advances in engine thermal efficiency and the latest inclusion of electrification in the form of growth of HEVs, PHEVs, and BEVs sales associate well with the trends observed in Figure 4. Compared to BEV, hybrid propulsion systems are a more cost-effective technical path to reduce GHG gas emissions, because of the relatively simple structure, matured technology, and existing manufacturing systems. 47In stark contrast, HEVs and PHEVs are limited to complete emissions abatement potential unless combined with net-zero fuels.Therefore, BEVs present a more direct path toward zerocarbon propulsion.
As the other, lesser used, zero local emission technology, fuel cells have also seen considerable growth over the last few years and have reached the commercialization stage.In 2020, the worldwide FCEV market included around 70,000 vehicles on road and in the utility sector (mostly forklifts in the United States and EU), as shown in Table 2. 21,25,48 The growth of the FCEV sector is impeded by the challenges associated with fueling infrastructure and manufacturing costs. 49Additionally, since fossil fuels, primarily natural gas, account for almost all the current hydrogen production 50 until renewable hydrogen constitutes a major portion of the available hydrogen for fueling, the environmental impact of FCEV adoption will remain severely diminished.
For heavy-duty, marine, and aviation applications, hydrocarbon fuels have been the only viable choice, as shown in Table 3.The major energy sources for heavyduty applications include diesel and natural gas for medium to heavy-duty road and rail freight transportation, heavy oil for marine shipping, and aviation gasoline and kerosene for aviation applications.
The application of alternative fuels such as hydrogen (compressed or liquified) 9 and ammonia are being investigated, but principally lack sustainability.Fossil fuels remain the predominant energy source for hydrogen production and pure hydrogen produced based on renewable energy sources accounts for around 0.14% of the total production amount. 66Pure hydrogen is mainly used for refining and ammonia production.The quantity used for transportation purposes is negligible compared with the total consumption.However, hydrogen produced from renewable sources is still considered a promising path to decreasing GHG emissions in the transportation sector.
The carbon intensity reduction of these sectors has mainly been a consequence of the enhancement in the thermal efficiency of the powertrain.Over the last few decades, the efficiency of IC engines has improved significantly.Modern IC engines can achieve >50% thermal efficiency with ultra-low emissions under steady conditions. 6,67,68Because of the high energy density of hydrocarbon fuels, a small increase in powertrain efficiency can translate into significantly higher energy conversion and lower GHG emissions.Further improvements to engine efficiency and hybridization can reduce the environmental impact of ICEVs and HEVs to match those of grid-powered BEVs while countering the energy F I G U R E 4 Current CO 2 emission regulations and future trends.[46] T A B L E 2 Present scenario of FCEVs around the world (as of 2020).| 443 density, recharging rate, and safety challenges of full electrification.

| CARBON INTENSITY OF THE PRESENT ELECTRIC GRID
With the increased adoption of EVs in transportation, both the energy consumption and GHG emissions from the power generation sector will be coupled with the light-duty transportation demands.The power generation mix and carbon intensity of electricity in various regions of the world are presented in Figure 5. 69 Converting energy sources from coal to natural gas can cut the carbon intensity of electricity by around 50%.Further increases in the renewable electricity portion can further decrease the carbon intensity of electricity.
Figure 6 presents the carbon intensity of BEVs in different regions of the world. 69,70The average carbon intensity of ICEVs is indicated by the dashed line (240 g CO 2 /km).It can be observed that the carbon intensity of electricity is highly sensitive to the power generation mix for each region.India and China derive approximately 60%-70% of their power generation from coal as opposed to the United States and EU, where the contribution of coal drops to less than 20%.
Consequently, little to no reduction is apparent with BEVs in China and India, two of the most populous economies.On the other hand, replacing gasoline-or diesel-powered ICEVs in regions like the EU can reduce GHG emissions by as much as 40%.Albeit with the current battery technology, this reduction is potentially limited to the light-duty sector.This further emphasizes the necessity of T A B L E 3 Summary of decarbonization effort of various transportation applications.

Transportation sector
Present scenario Developmental efforts toward decarbonization

Light-duty road transportation
Total of 1100 million passenger cars and light duty vehicles on road in the year 2020. 29,30,51versified powertrains including BEVs, FCEV, and ICEVs with renewable fuels in use.About 6.8 million EVs and 42,000 FCEVs in operation as of the year 2020. 25avy-duty road transportation ~407,000 heavy-duty trucks and buses on road in the year 2019. 30Battery electric hybrid buses being widely used for transit and intracity public services.~425,000 electric transit buses in operation worldwide in 2019.98% in China and remaining in North America and Europe. 33,52tensive ongoing research on high-efficiency engine prototypes for freight trucks.Heavy-duty electrification is limited to the feasibility studies and demonstrations.

Merchant vessels (marine)
~52,275 merchant vessels operational as of 2020. 53ternative powertrains including battery-motor, ammonia/hydrogen-powered engines limited to the feasibility studies and demonstrations for small vessels. 54,55ilways 1.6 million km of railway infrastructure with ~25% using hydrocarbon fuels. 564 partially battery-powered small railcars or small trains in operation in Japan (EV-E301: 4 two-car trains, 57 BEC819: 18 total two-car trains, 58 EV-E801: 2 two-car trains 59 ).N700S bullet train introduced a battery-powered propulsion system in 2020 to power the bullet train at a reduced speed to the nearest station in case of a power outage. 60Siemens prototype tested in Austria in 2018. 61maining powered from the grid by overhead electric lines.
Limited battery-only range (less than 50 km away from power lines) is restricted to short routes and major energy is supplemented from the grid by the overhead power lines.
energy transition in the power generation sector in addition to BEV adoption for effective GHG reduction.Currently, power generation constitutes about 35%-40% of global energy consumption and is another large producer of GHG emissions. 71Even though a gradual decline in the use of coal and petroleum oil in the electricity generation sector is seen since 1985, nonrenewable energy sources account for approximately 60% of total power generation in the year 2020. 71Simultaneously, the penetration of wind and solar power has been steadily increasing since years 2005 and 2010, respectively.This penetration is further driven by economic factors such as the reduced cost of solar photovoltaic cells, and the production and installation capital costs of wind turbines. 72ovided that this trend continues, wind and solar power, alongside hydro and nuclear power, will have to contend with the ever-increasing energy demand and transition from traditional fossil energy sources.Electrical energy storage can ease the intermittencies of renewable, but the present energy storage capacity lacks diversity and sufficient capacity. 73Over 96% of global energy storage is provided by pumped hydro, which is restrained to particular locations. 74Several technological remedies to mitigate intermittent power generation are being researched, specifically electrochemical energy storage via stationary batteries.One strategy integrates the onboard energy storage of plug-in electric vehicles to the grid (V2G) for near-term grid stability. 75To effectively apply the V2G strategy for large-scale renewables, Kempton and Tomić 76 estimated a minimum F I G U R E 5 (A) Power generation mix and (B) carbon intensity of electricity by country.EU, European Union; USA, United States of America. 69rid-linked BEV fleet size of 33.5 million would be necessary to alleviate the intermittency challenges of 700 GW wind production.The integration of renewable fuels into the power generation mix to supplement wind and solar power can limit the intermittency challenges while matching the low carbon intensities of renewable power generation.
Concurrently, power generation infrastructure will have to be upgraded with the measures to mitigate the intermittencies associated with wind and solar power because of the inherent irregularities in both the availability and the available intensity of wind and sunlight. 77The compensation of power using ICEs and small-sized turbines (most prominently fueled by natural gas) is the most widely accepted and utilized mitigative strategy against wind or solar intermittency. 78,79owever, this requires the traditional power plants to operate with increased flexibility, usually in an ondemand ramping or start-stop manner.This suboptimal operation reduces the efficiency of power generation.Consequently, the fuel consumption and CO 2 emissions from these power generation sources increase, which cuts down the expected effective carbon intensity reduction of the power grid. 80dditionally, start-up time is an important parameter to benchmark the transient performance of the power backup systems.For stationary engines rated at 4 MW, it takes less than 1 min for the engine to start up and reach full load within 5 min. 81Small-sized gas turbines can start up from 25 to 45 min. 81Nuclear power stations, despite the low carbon intensity, take days to adjust the output power and weeks to start or stop. 82With engine break efficiency of around 55% and system energy efficiency of over 90%, cogeneration plants show great potential to support future sustainable power generation systems. 83

| ENERGY DEMAND TO ELECTRIFY THE TRANSPORTATION SECTOR
Presently, transportation consumes 24% of the total primary energy used around the globe as shown in Figure 7. 84 Within the transportation sector, road transportation accounts for ~80% of energy consumption, and fossil fuel dominates the energy source. 84It is commonly agreed that converting fossil-fueled ICEVs to EVs can significantly reduce GHG gas emissions and F I G U R E 6 Carbon intensity of battery electric vehicle (BEVs) by country.EU, European Union; ICEV, internal combustion engine vehicle; US, United States. 69,70I G U R E 7 Transportation sector energy consumption by mode and source. 84ther pollutants.In this section, the energy consumption of transportation is compared with electricity generation to demonstrate the energy demand under a fully electrified transportation scenario.In this discussion, the analysis of electricity networks among countries is equivalent to the energy consumed by an electric vehicle.
The power generation and energy consumption in the year 2019 is presented in Figure 8. 71 In the transportation sector, 33,604 TWh of energy is consumed.Compared with that, the total electricity generation is 27,044 TWh to meet the global demand, with around 37% of electricity generated from nonfossil sources.Under the scenario of establishing a fully electrified transportation sector, extra electricity generation is needed.Assuming motor efficiency (100%) to be three times higher than that of combustion engines (33%), the energy demand will decrease from 33,604 to 11,201 TWh per year.
The evolution of global electricity demand and electricity generation from nonfossil resources is shown in Figure 9. 71 An obvious increasing trend in renewable electricity generation is observed from 1985 to 2021, owing to the joint effort from both government support and technological improvement.However, the global electricity demand outpaces the growth rate of renewable electricity.From 1990 to 2021, the energy gap between global electricity demand and renewable electricity generation increases from 7614 17,482 TWh, 71 indicating a major challenge in renewable energy supply.More investment and support are needed to further increase the capacity of renewable electricity generation at a faster pace.
The evolution of electricity generation by various nonfossil energy sources is shown in Figure 10A, and it is observed that wind, solar, and hydropower are the three main contributors to increase the in electricity generation.However, apart from China, worldwide hydropower development is slow, as shown in Figure 10B.The most viable path to increase renewable electricity generation is via wind and solar.
Wind and solar offer an expandable source of lowcarbon power generation.Both sources of renewable energy can be installed in a range of different environmental regions, from on-land to offshore.Intermittency as previously discussed can be limited through optimal installation locations by prioritizing a higher regional solar flux or mean wind speed, both effectively increasing the operating capacity factor.
Currently, solar and wind power generation (2894 TWh, 2021) continues to grow at a rate of 452 TWh/year at an energy split of 64% and 36%, respectively. 71An analysis of the solar and wind generation history indicates an accelerating growth of 19.7 TWh/year each year.Considering the current global electricity demand (28,466 TWh,  2021), it is estimated that the demand must be increased by over eight times its current generation capacity (330 GW, 2021).Such a low carbon grid would comprise ~1,847,000 wind turbines (2.4 MW rated capacity at 40% capacity | 447 factor 85 ) and ~123,000 km 2 of solar panels (8 W/m 2 ).In the hypothetical case of a fully electrified transportation fleet, the energy consumption at the point-of-use can have ~3 times higher efficiency significantly lowering the downstream energy consumption.Considering an electricity supply from wind and solar only, a simplified calculation following current trends predicts a 100% electrified transportation sector could be matched by wind and solar with a three-fold increase in power generation, or roughly 14 years of continued growth.

| ENERGY CONSUMPTION AND CARBON INTENSITY OF VARIOUS PROPULSION SYSTEMS
In the past decade, the fuel efficiency of light-duty ICEVs has improved by about 27%. 45,46This improvement has translated into a ~25% reduction in GHG emissions for each ICEV unit.Renewable fuels and hybrid electrification can further accelerate GHG emission reduction in the coming years.A further 25% reduction can bring passenger cars and light-duty ICEVs on par with the GHG emission from typical BEVs.
The evolution of specific energy consumption of various propulsion systems for LDVs is shown in Figure 11.The specific driving energy of ICE and HEV is calculated based on fuel consumption regulations provided by US Environmental Protection Agency (EPA) in the year 2019, which represents the average fuel consumption of various types of vehicles on the road. 86he energy consumption of BEV considers the yearly grid energy efficiency.The results of PHEV and BEV in hollow markers are calculated neglecting the energy transfer efficiency, which can be considered as 100% "free" electricity, such as wind and solar.Detailed calculation methods for specific driving energy and specific CO 2 emissions are presented in the supporting files.
F I G U R E 10 Evolution of (A) global electricity generation by source and (B) region-specific hydropower electricity production.CAN, Canada; CN, China; EU, European Union; IND, India, RUS, Russia. 71or traditional ICEVs, major improvements in engine efficiencies directly reduce energy consumption over the past 30 years.However, because of the unavoidable operation in low-efficiency regions in the engine map, the potential is limited.By electrifying the engine with the hybrid system, the fuel efficiency can be improved significantly, reaching similar specific energy consumption compared with BEVs.The energy efficiency of the present power grid is similar compared with state-of-theart hybrid electric powertrains 71,86 (~42%).Therefore, whether to generate the electricity at 42% efficiency power grid to charge up the battery to drive the motor, or to direct burn hydrocarbon fuel to drive the wheel at 42% efficiency does not affect the total primary energy consumption (assuming no losses during electricity transmission, battery charging, and electric motor).More favorably, when renewable electricity is used to power BEVs, the advantage is enlarged because of the much higher efficiency of the electric motor compared to ICEs, followed by PHEV, which also partially take the advantage of the battery-motor system.
The specific CO 2 intensity of various propulsion systems is also calculated based on the same scenario, and the results are shown in Figure 12.Because of the lower carbon intensity of the energy used in the grid, BEV still shows obvious benefits in GHG emissions during vehicle operation.
The relationship between GHG emission and energy consumption of various propulsion systems is presented in Figure 13.The blue diamond at the top right corner is a typical light-duty passenger vehicle according to present EPA regulation, which is used as a reference to benchmark the energy consumption and CO 2 intensity of other propulsion systems.As for BEVs, the present technology can already achieve 50% energy reduction and 70% CO 2 reduction compared with the baseline ICEV.Because of the high but stable motor efficiency, the only path to further reduce GHG emissions and improve energy efficiency relies on the progress of electricity generation.Potential battery technology breakthroughs may help reduce vehicle weight, which contributes to further energy efficiency improvement.
For a propulsion system driven by hydrocarbon fuel, the increase in fuel efficiency can be directly reflected in the GHG emissions.ICEVs, state-ofthe-art turbocharged spark-ignition engines can already decrease fuel consumption by 23%, reducing the same ratio of GHG emissions at the same time.
With engine hybridization, a typical HEV can further reduce energy consumption by 48%, 86 which is already on the same level as that of PHEV and BEV.HEVs with even higher fuel efficiencies are available on the market, reaching 59 mpg fuel efficiency, which already has a similar CO 2 emission level as a typical PHEV and consumes 10% less primary energy.OEMs such as Mazda and Toyota also demonstrate the possibility of further decreasing the fuel consumption of the vehicle to 69 and 100 mpg, respectively, which can reduce almost 80% of fuel consumption and GHG emissions, reaching the ideal case for BEV, where electricity is considered to be carbon-free.
PHEV is a flexible solution between HEV and BEV, which allows the vehicle to take advantage of both the high energy density of hydrocarbon fuel and highefficiency battery-motor systems.The sophisticated powertrain system and high production cost is the major barrier to deeper market penetration.

| RENEWABLE FUEL
ICE is an efficient energy converter at a reasonable cost and still has a high potential for further improvement.Figure 14 shows carbon cycles in the transportation sector using fossil and renewable energy sources.It should be noted that the ICE is not the root cause of CO 2 emissions, but the fuels that are burnt with it.In the meantime, CO 2 also contributes to the food to support human beings.Relying on solar energy, water, and CO 2 , plants 13 Energy reduction rate versus CO 2 reduction rate of various propulsion systems.BEV, battery electric vehicle; HEV, hybrid electric vehicle; ICEV, internal combustion engine vehicle; LDV, light-duty vehicle; PHEV, plug-in HEV. 86I G U R E 14 Carbon cycles in the transportation sector using fossil and renewable energy sources.
generate carbohydrates via photosynthesis.After millions of years, the carbohydrates sealed under the earth turn into fossil fuels that supply vehicles after being refined.The concerns about the fossil fuel shortage might be because of this long-term carbon cycle.The rate of fossil fuel usage is considered much faster than natural production.
In this manner, a speed-up carbon cycle is demanded.The replacement of fossil fuel-based liquid hydrocarbon fuels with renewable fuels within the natural carbon cycle would be the quickest and most progressive path to significantly reduce CO 2 emissions from road transport manner.In this cycle, it is presumed that the carbon released at the ICEV tailpipe is matched by the feedstock carbon absorption during the photosynthesis of the biomass-derived fuel.Analysis from co-optima reveals renewable fuel blend stock can potentially be produced at a competitive price and reduce light-duty GHG emissions by up to 89%, and heavy-duty GHG emissions by up to 81% compared to petroleum-based fuel. 7In Canada, ethanol and biodiesel are considered to have well-towheel carbon intensities of 44.1 and 8 g CO 2 e/MJ, respectively. 87This represents a 52% reduction in GHG emissions intensity for ethanol compared to gasoline fuel and a 92% reduction for biodiesel compared to diesel fuel.
While the advantages of biofuels are apparent, largescale deployment is challenging. 88,89The feedstocks for biofuels are generally resource-limited.Biomass for fuels competes with biomass as food for humans and animals implying a direct challenge toward using current foodsupplying crops for energy.Second, the production capacity of crops is limited to locations of suitable weather and soil quality rendering the production regions fixed.Last, the land-use effectiveness and energy efficiency vary greatly on the feedstock, further shrinking the production capacity limits of biofuels.For example, producing bioethanol from sugar cane consumes two to three times less energy and demands less arable land than corn. 90Such considerations imply biofuel as a blend with other fuels for a progressive carbon intensity reduction.
2][93][94][95] The switch from the fossil fuel carbon cycle to the renewable fuel carbon cycle, especially in the case of ethanol and biodiesel, can translate into a significant GHG reduction potential for future clean transportation and power generation sectors.Synthetic fuels such as ammonia and hydrogen, while being carbon-free fuels with no tailpipe emissions, have a huge GHG penalty during production stages.About 96%-98% of hydrogen is generated from fossil fuels, through steam methane reforming of natural gas and from coal gasification. 96The carbon intensity of "gray hydrogen" is ~153 g CO 2 e/MJ, about 60% higher than the conventional gasoline and diesel fuels.Switching the production to "blue hydrogen," which uses similar hydrogen production methodologies as the gray hydrogen but includes the carbon capture and storage technologies marginally reduces the carbon intensity to ~135 g CO 2 e/ MJ. 94 In practice, the true potential of hydrogen as a carbon-neutral energy carrier can only be realized with renewable production.Similarly, ammonia production accounts for about 2% of global total final energy consumption, 97 virtually all of it from fossil fuels, resulting in a carbon intensity varying between 58 and 117 gCO 2 e/MJ primarily, because of the electricity mix and methodologies used for ammonia production. 95I G U R E 15 Greenhouse gas emission intensity for different fuels in the automotive and energy sectors.This variation further emphasizes the need for GHG reduction from the power generation mix and a switch toward renewability, both on the power generation and fuel fronts to achieve a smooth transition toward decarbonizing the transportation and energy sectors.
Besides, considering the electricity generation from sufficient renewable energy sources, for example, nuclear, hydro, solar, and wind, green hydrogen can be generated via the electrolysis of water.Subsequently, there are two paths for hydrogen to be utilized in automotive applications, hydrogen fuel cells and synthetic hydrocarbon fuels (e-fuels).Renewable natural gas and renewable hydrogen are considered to have zero carbon, allowing net-zero emission propulsions using ICEs.Fischer-Tropsch products can further increase the energy density of e-fuels. 98,990][101] Furthermore, the location and time of the manufacturing process are independent of the use, which offers a decisive advantage. 8,10If the consumption of cleaner fuels increases and, subsequently, carbon intensities are improved, ICEs have the great potential to become green engines and keep benefiting mankind.

| CONCLUSIONS
In this review, the present status of propulsion systems is reviewed, considering both the market penetration and well-to-wheel carbon emissions.The decarbonization potentials of various road transportation propulsion systems are then discussed.Considering the highly unevenly distributed energy sources around the globe, energy efficiency improvement, and low carbon energy source adoption are equally important to meet the GHG gas reduction target smoothly and rapidly.Major conclusions are given below: 1. Battery electric vehicles are developing rapidly in recent years, reaching a 0.5% fleet ratio in the transportation sector, with around 2.5% market penetration in the passenger vehicle market.2. ICE-involved high-efficiency powertrains, including HEV and PHEV, have higher market penetration than BEV, because of the low cost and less dependency on infrastructure and battery technology improvements.3. Wind and solar energy prove to be the most viable path toward carbon-neutral electricity generation; more investment is needed to generate enough green electricity to support further electrification in the transportation sector.
4. Under the present energy mix in the power grid, averaged HEVs and PHEVs can achieve similar primary energy efficiencies compared with BEV, but with 10%-20% higher GHG emissions during operation; the technology exists on the market to further reduce the GHG emission of HEV to similar level compared with BEV, with 20% less primary energy consumption.

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I G U R E 8 Energy consumption versus electricity generation in 2019.F I G U R E 9 Evolution of global electricity demand and electricity generation from nonfossil resources.71YU ET AL.

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I G U R E 11 Evolution of specific driving energy of various propulsion systems.BEV, battery electric vehicle; HEV, hybrid electric vehicle; ICEV, internal combustion engine vehicle; PHEV, plug-in HEV.F I G U R E 12 Evolution of specific CO 2 intensity of various propulsion systems.BEV, battery electric vehicle; HEV, hybrid electric vehicle; ICEV, internal combustion engine vehicle; PHEV, plug-in HEV. 21,25,48 Abbreviations: EU, European Union; FCEV, fuel-cell electric vehicle.YU ET AL.