IJER Editorial: The Future of the Internal Combustion Engine

November 01, 2019

Print This Article Share it With Friends

by R D Reitz, H Ogawa, R Payri, T Fansler, S Kokjohn, Y Moriyoshi, AK Agarwal, D Arcoumanis, D Assanis, C Bae, K Boulouchos, M Canakci, S Curran, I Denbratt, M Gavaises, M Guenthner, C Hasse, Z Huang, T Ishiyama, B Johansson, TV Johnson, G Kalghatgi, M Koike, SC Kong, A Leipertz, P Miles, R Novella, A Onorati, M Richter, S Shuai, D Siebers, W Su, M Trujillo, N Uchida, B M Vaglieco, RM Wagner, H Zhao (International Journal of Engine Research) Internal combustion (IC) engines operating on fossil fuel oil provide about 25% of the world’s power (about 3000 out of 13,000 million tons oil equivalent per year—see Figure 1), and in doing so, they produce about 10% of the world’s greenhouse gas (GHG) emissions (Figure 2). Reducing fuel consumption and emissions has been the goal of engine researchers and manufacturers for years, as can be seen in the two decades of ground-breaking peer-reviewed articles published in this International Journal of Engine Research (IJER). Indeed, major advances have been made, making today’s IC engine a technological marvel. However, recently, the reputation of IC engines has been dealt a severe blow by emission scandals that threaten the ability of this technology to make significant and further contributions to the reduction of transportation sector emissions. In response, there have been proposals to replace vehicle IC engines with electric-drives with the intended goals of further reducing fuel consumption and emissions, and to decrease vehicle GHG emissions.

...

It is likely that future mobility will be characterized by a mix of solutions, involving battery electric and hybrid electric vehicles (BEV and HEV), fuel cell electric vehicles (FCEVs) and conventional vehicles, depending on consumer acceptance (e.g. cost), the country considered and the specific application (city, country, personal, freight, etc.). Thus, the combustion engine will still play a central role, whether used for power generation or for powering the vehicle itself, even in strongly electrified powertrain configurations. Because of this, there is great interest in improving the thermal efficiency of IC engines without significant increases in purchase and operating costs in the short-to-medium term. These goals can be achieved through improvements in combustion, after-treatment and control systems, and by partial electrification in the form of hybridization, together with vehicle weight reduction and more efficient ancillary systems.

Although there is great current interest in the electrification of transport, only BEVs eliminate the need for an IC engine. However, life-cycle analyses⁷ of the GHG impact of BEVs that consider the energy used in electricity generation and battery manufacture show that their true benefit is significantly less than is apparent at first sight. Many analyses ignore the upstream CO₂ in fuel extraction, refining and transportation, as well as in the production and distribution of electricity. Large amounts of energy are required to extract the critical raw materials needed for batteries and electric motors (cobalt, lithium, rare earths, etc.), together with huge amounts of water. End-of-life disposal—toxicity in particular—also needs to be factored into life-cycle analyses. (Many of these considerations also apply to equipment for wind and solar photovoltaic power generation.) Moreover, the construction of a new electricity infrastructure, capable of recharging millions of BEV’s, will require further raw materials and energy consumption (with consequent CO₂ emission), and may be limited by the availability of critical materials.

The high cost of BEV’s, as compared to IC-engine-powered vehicles (conventional or hybrid), is also driving the development of effective, but previously deemed uneconomical, methods to increase the IC engine’s efficiency with advanced combustion modes, and to further reduce pollutant emissions. In this sense, the competition between electric motors and IC engines is stimulating beneficial evolution of the thermal engine itself.

“Zero emissions”

It has been estimated that the fuel consumption in spark-ignition (SI) vehicles could be reduced by as much as 50% in the United States compared to the current average,⁴ and tailpipe CO₂ emitted will be reduced correspondingly. With existing catalysts and control systems—and these continue to be improved—particulates, NOx, uHCs and CO could also be reduced to negligible levels from both SI and diesel engines. Frequently, pollutant emissions and CO₂ emissions from combustion are presented as being entirely equivalent, so that even engines with exceedingly low criteria pollutant emissions (NOx, CO, uHCs, and particulates) are also regarded as polluting. Technically and practically, there is an important distinction. CO₂ emissions necessarily accompany any hydrocarbon combustion or chemical oxidation process, including human and animal life. The CO₂ emitted from an engine is directly proportional to the hydrocarbon fuel consumed, which is continually being reduced by technological improvements.

In terms of the criteria pollutants, the goal to achieve “zero impact emission vehicles” is very close, thanks to advanced combustion modes and innovative after-treatment systems, including extensive use of catalysts and high-filtration-efficiency diesel and gasoline particulate filters (D/GPF) in the after-treatment system, while the use of urea injections and selective catalytic reduction (SCR) is leading to extremely low NOx emissions (e.g. 0.02 g/bhp-h or 15–20 mg/km). Indeed, there are even examples of vehicles having tailpipe unburned HC emissions below those in the ambient air at the engine’s intake, so-called negative emission vehicles!

Indeed, the pollutant emissions discharged at the tailpipe outlet will be so low as to be hardly measurable, and their practical impact on air quality will be negligible. In terms of particulate matter emission, the impact of tire and brake wearing is already much higher than that due to the IC engine (tire wear produces around 50 mg/km of particulates), reaching values around 10 times the emission from the engine (5 mg/km).⁸ This implies that today’s conventional IC engine-powered-car is equivalent to fully electric and hybrid cars with regard to particulate emissions, when tire and brake and other contributions (e.g. road dust) are accounted for.

There are routes to short-term CO₂ reduction that are viable more quickly. First, a switch from gasoline to diesel ICE reduces CO₂ emissions by an estimated 11% at the tailpipe, and a diesel mild hybrid delivers a further 6% reduction. The final swap to full hybrid delivers another 16%. Note, however, that there is a public misconception, based on obsolete technologies and the recent emission scandals, that the diesel engine is a high-pollution engine. As discussed above, this ignores the major advances that have been made in the last several decades in diesel engines and exhaust emission after-treatment.

Significant improvements to gasoline engines are also available with vehicle electrification. A direct switch from gasoline to gasoline-mild-hybrid can deliver 11%, and a further 23% in moving to full hybrid.⁴ As these numbers demonstrate, there are immediate-term options for significant fuel efficiency improvement and, hence, CO₂ reduction of the order of 30% or more, for both gasoline and diesel.

It is also clear that “zero emissions” BEVs will not replace IC engines in commercial transport to any significant degree because of the weight, size and cost of the batteries required.⁷ Short of a major breakthrough in battery technologies, for the foreseeable future combustion engines, running on petroleum-based liquid fuels, will largely continue to power transport of the world’s goods and services. A transition from the gasoline or diesel ICE to a full gasoline/diesel hybrid can significantly reduce emissions.⁴ But, due to the long turn-over and replacement time of vehicles, it will take a long time (decades) for full hybrids—even if they become common-place and affordable options—to become a major fraction of the world’s vehicle population. The sustainability of transport in terms of GHG and other environmental impacts, affordability and energy security can certainly be ensured by improving combustion engines, and this requires renewed emphasis on engine research and development.

Fuels

In the medium-to-long term, there is even greater scope for improving engines by co-designing fuel/engine systems for optimal performance.⁹ Single- and dual-fuel technologies, such as homogeneous charge compression ignition (HCCI), premixed controlled compression ignition (PCCI), and reactivity controlled compression ignition (RCCI)^10,11 offer significant promise for improving efficiency and reducing unwanted exhaust emissions. These advanced combustion modes can also benefit from available fuels or fuels whose composition is optimized for each application. To also reduce dependence on fossil fuels and for a decarbonization transition, progress is being made in the introduction of CO₂-neutral biofuels and synthetic fuels. Often, criticism of the ICE is not about the engine, but about the source of the fuel, and the use of bio or synthetic fuels can mitigate total carbon emissions. Indeed, some marketed biodiesels are more than 70% net-carbon neutral today. Some countries and states have even implemented a low-carbon fuel standard (LCFS) and provide monetary incentives to encourage the biofuel market.

Hydro-treated vegetable oil (HVO) is a promising renewable drop-in fuel for diesel engines with very low CO₂ impact. Another emerging technology produces liquid transportation fuels from solid ligno-cellulosic, non-food biomass via fast pyrolysis, which is a thermal decomposition process that breaks down materials by heat in the absence of oxygen, producing syngas, bio-oil and biochar. Bio-oil can be upgraded catalytically to liquid fuels. As a result, fast pyrolysis of waste biomass can produce biofuels and could enable a reduced carbon economy.

The use of alternative, synthetic fuels derived from waste biomass and renewable electric energy has also been proposed to produce an electrofuel (e-fuel) with net zero CO₂ emission (i.e. carbon neutral). This approach is currently being investigated as a smart way to store renewable electric energy when a production peak occurs, thanks to a chemical process to generate hydrocarbons from H₂ (produced by electrolysis of water) and CO₂ captured directly from the atmosphere or from other industrial- or bio-sources. Longer term, carbon capture technologies have been demonstrated to be able to collect and then dispose of or sequester CO₂ from vehicle tailpipes, and are projected to be cost effective.¹²

Energy sources and the future

For electrification, electricity has to be produced, either by the IC engine (in the case of a hybrid vehicle), or from a power generating station and the grid. For the latter, it is currently mainly produced fromnon-renewable energy sources (with about ∼40%–50% losses, although these can be substantially higher for older coal-fired power plants which are still prominent in much of the world). In addition, the transport of electricity to the end user, together with corresponding charging/discharging losses at the battery, and the role of low or high operating temperatures in reducing battery performance accounts for another ∼5%–20% loss, resulting in an overall efficiency that is actually comparable to that of hybrid vehicles powered with IC engines and fossil fuels. Perhaps there is a political advantage to drawing power from the grid in that unwanted emissions are “not in my back yard.” The problem is flushed away to less visible areas, but with substantially less—and sometimes no—reduction in global carbon footprint. Indeed, a BEV charged from coal-fired electricity can easily have a larger carbon footprint than a comparably sized non-hybrid IC-engine-powered vehicle.

Renewable sources (including hydroelectricity) currently constitute about 10% of the global energy mix. The BP review of World Energy¹ forecasts that the fraction of total energy production from renewables will only reach about 14% by 2040, and in many regions, fossil fuels, including coal, will remain the greatest source of energy. It is, therefore, clear that in the medium term, the alternative of BEV transportation may modestly reduce, but will by no means eliminate, global CO₂ emissions. Of course, with a reduction of coal-fired electricity and transition CO₂-neutral technologies, this situation could change.

Furthermore, much faster charging will be needed for broad market acceptance of plug-in and BEVs—note that essentially all scenarios currently involve substantial taxpayer or consumer subsidies for such chargers. In addition, mass electrification will require dramatic alterations to the entire electrical distribution system, from the power plant to the charging point. Given these issues, even the more aggressive mainstream market forecasts show IC engines still being in most cars in 2040,¹³ and representing an even higher portion of the truck market.

Replacement of IC engines in heavy-duty transportation faces even greater difficulties in these respects. For example, a heavy-duty Class 8 truck in the United States with a 500-mile range, operating as an electric truck requires a battery with energy of ∼1000 kW h. Assuming a battery-to-motor efficiency of 95%, the appropriate battery weighs at least 5.5 metric tons (compared to about 1.3 metric tons for its diesel engine), and it consumes a significant part of the allowable payload. With the 120 kW Tesla Supercharger the battery takes around 12 h to charge.⁷ In addition, there is little discussion on replacing train and ship engines, a testimony to the extreme power requirements and unacceptably long battery charging times needed for these applications.⁷

A sustainable mobility future will require a diverse portfolio to ensure the right technologies for the right applications, and it will span IC engines, fuel cells, pure EVs and hybrid-driven propulsion systems. Like-for-like (“apples-to-apples”) comparisons are critical for accurate technology assessments of social, economic and environmental impacts. More specifically, successful technologies must be market competitive in cost, user requirements, lifecycle emissions and lifecycle efficiency; must ensure domestic energy security; and must consider societal impacts related to manufacturing and the acquisition and recycling of critical materials. To this end, the ICE and supporting infrastructure are well established, and innovations associated with technology developments continue to improve the overall efficiency and emissions signature of combustion-based technologies. READ MORE

Is it really the end of internal combustion engines and petroleum in transport? (Applied Energy)

Plug-in Hybrids without Behavioral Compliance Risk Failure (Future Fuel Strategies)

Excerpt from Applied Energy: However the greenhouse gas (GHG) impact of battery electric vehicles (BEVs) would be worse than that of conventional vehicles if electricity generation and the energy used for battery production are not sufficiently decarbonized. If coal continues to be a part of the energy mix, as it will in China and India, and if power generation is near urban centers, even local urban air quality in terms of particulates, nitrogen oxides and sulfur dioxide would get worse. The human toxicity impacts associated with the mining of metals needed for batteries are very serious and will have to be addressed. Large prior investments in charging infrastructure and electricity generation will be needed for widespread forced adoption of BEVs to occur. READ MORE