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International automotive researchers emphasize the importance of continued development of the internal combustion engine

Thirty-seven globally prominent scientists representing the International Journal of Engine Research have published an open-access editorial addressing the future of the Internal Combustion Engine, and stressing the importance for continued development of more efficient and even lower-emitting technologies.

The article provides an assessment of the state of power generation in the world today, and provides analyses of productive directions for the future. The editorial addresses important issues in the current politically charged discussions of global warming and climate-change alarm.

… the ICE, and IC engine research have a bright future, in contrast with some widely distributed media reports… The power generation and the vehicle and fuel industries are huge, representing trillions of dollars (US) per year in turnover, with a massive infrastructure. We are certainly in revolutionary times, but it is clear that power generation sources will not become fully renewable and transport will not become fully electric for several decades, if ever. However, research to improve efficiency and methods to reduce dependence on fossil fuels are exciting directions for future IC engine research.

It is very likely that highly efficient “fully flexible” engines with hybridized solutions will be a big part of sought-after efficiency improvements, as well as emission/GHG reductions. Finally, it must be acknowledged that, in practice, people select their choice of powertrain based on numerous factors, including cost.

Consumer preference is not decided by politicians, nor by car-makers, nor academia. Policy unilaterally favoring one technology solution may be deeply inefficient and perhaps even the wrong eventual solution. A better approach is to use real-world data to allow competing technologies to flourish; if they evidence efficiency improvements and emission reductions, and they then need to be delivered as soon as possible. Continued progress requires that we recruit the brightest young minds to engage in this effort to deliver a vibrant and sustainable future for the ICE.

—Reitz et al.

In terms of 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).

… 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). 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.

—Reitz et al.

In the editorial, the researchers note that internal combustion engines operating on fossil fuel oil provide about 25% of the world’s power (about 3,000 out of 13,000 million tons oil equivalent per year), and in doing so, they produce about 10% of the world’s greenhouse gas (GHG) emissions.


Global warming potential (GWP) in CO2 equivalent tons by sector. Transportation contributes about 10%. Reitz et al.

The authors assert 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. 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, they write.

A transition from the gasoline or diesel ICE to a full gasoline/diesel hybrid can significantly reduce emissions. 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.

—Reitz et al.

The authors include a list of potentially fruitful research topics that would “certainly benefit from worldwide collaborations between researchers in industries, government laboratories and academia.”

Top of the list is engine efficiency, with a wide range of technical opportunities:

  • Combustion system. Novel combustion systems, including the use of ultra-high fuel injection pressures and new mechanical layouts—possibly beyond the slider crank—should be encouraged. This could be paired with combustion technologies with highly diluted combustion (stoichiometric with exhaust gas recirculation (EGR) as well as lean burn with excess-air ratios above 2). For this combustion improvement, mixture formation and charge motion, and ignition technologies including installation of pre-chambers need to be investigated.

  • Gas exchange. Improvements in engine breathing are of interest, potentially via exhaust gas turbochargers to realize fast response and low temperature combustion with ultra-high-pressure supercharging, large quantities of EGR, and further improvements in the Miller cycle with variable valve systems, while maintaining the required oxygen levels. Further development of exhaust gas energy recovery systems with turbo-compounding and possibly chemical reforming should be encouraged.

  • Electrification. Electrification offers significant improvements in system efficiencies, as well as GHG control, possibly leading to thermal efficiencies beyond 50%. The development of more efficient engines specifically for hybrid and range-extender systems (which enable the engine to run over a limited speed-load range) would also be helpful.

  • Engine lubrication. Reduction in mechanical loss should be achieved by improving lubrication systems with less oil consumption, especially for new engines with restricted operational areas in loads or speeds.

  • Engine thermal and energy management. Not only reducing IC engine heat losses, but also improved thermal systems that include exhaust heat recovery systems, after-treatment systems, and their optimal control will be key technologies for the future.

Other areas of importance include:

  • Engine after-treatment. The establishment of improved and low-cost after-treatment systems to remove uHC, particulate matter and NOx emissions under low temperature and excess oxygen exhaust gas conditions without sacrificing thermal efficiency is needed. Methods to reduce RDEs from gasoline engine vehicles at full load in enriched combustion or cold starting (which generate much particulate matter) with less expensive after-treatment devices should be explored.

  • Fuels. The efficient utilization of dual-fuel combustion, and combustion of diesel/natural gas should be researched. In addition to ultra-dilute burn and development of direct gaseous fuel injection systems, research is needed to reduce methane slip and to improve thermal efficiency and exhaust gas emissions on natural gas engines, especially for large ships and co-generation. The use of surplus low octane number fuels will become an important topic in the near future, and intensified research on bio- and e-fuels for GHG mitigation would be helpful. “Designer” fuels offer the potential for efficiency improvements and near-zero pollutant emission. These could include admixtures of variable H2-quantities to hydrocarbons, oxygenated components and even quite new chemical components (e.g. NH3).

Research tools needed for engine development include:

  • Engine simulations. Computational fluid dynamics (CFD) modeling of combustion processes has made great advances. Simulation tools are now heavily used by most engine OEMs to help design and optimize engines, benefiting from the vast computational power available to both industry and academia. With the rapid development of AI, various automatic predictions and optimizations are also being put into practical use.

    However, the optimization of engine combustion relies on accurate sub-models, many of which need further development to increase their predictive capability, as well as to reduce the need for empirical calibration. This is an active area of research utilizing Direct Numerical Simulations with an imminent introduction of machine learning and data science technologies.

    In addition, engine combustion includes transient phenomena such as cycle-to-cycle variations that are not well understood or analyzed. Development of vehicle simulation models that include the power source together with its system components, transmission, peripheral devices, battery, motor, inverter and driving drag is needed.

  • Engine and vehicle control. Real-time combustion control to reduce control margins and cycle-to-cycle variations requires calibration and control software innovation, possibly with on-board physical/statistical model-based control using AI. On-board optimization of multi-input/multi-output systems with model predictive control is needed. Control of efficient fuel injection systems to optimize mixture formation spatially and temporally in the combustion chamber, and methods to ensure stable ignition in very lean or dilute mixtures in SI engines, possibly using pre-chambers, and low-temperature plasmas would be of interest. Also, the use V2X to reduce fuel consumption of vehicles in real driving conditions needs to be analyzed.

The vast majority of automotive engineers, including IJER editorial board members, are optimistic about the continuing importance of the IC engine to meet the world’s mobility and power generation needs. Certainly, exploring new and competing engine technologies, as well as new fuels, is important for a sustainable future for our planet. The inescapable conclusion reached in this editorial is that, for the foreseeable future, road and off-road transport will be characterized by a mix of solutions involving internal combustion engines (ICEs), battery and hybrid powertrains, as well as conventional vehicles powered by IC engines. Thus, there is a pressing need for recruiting the brightest young minds to engage in this effort.

—Reitz et al.


  • Reitz, R. D., Ogawa, H., Payri, R., Fansler, T., Kokjohn, S., Moriyoshi, Y., … Zhao, H. (2019). IJER editorial: “The future of the internal combustion engine.” International Journal of Engine Research. doi: 10.1177/1468087419877990



Better horses and buggies are coming?


Hard to beat the energy density of liquid hydrocarbon fuels.


An ICE range extender means you can get by with much smaller batteries and so have more electrified vehicles than if you had 60 KwH BEVs. What you need is one that can operate with a low volatility fuel so you can fill it every few months and not have it evaporate or degrade.
Or just a nice hybrid, of whatever degree.

OP> “ The authors assert 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. ”

After buying a Tesla Model S (265 mile range) Model 3 (315 mi), Jaguar I-Pace (220 mi), Hyundai Kona Electric (258 mi) I can confidently say that I will never purchase another petroleum burning vehicle.

I’ve driven long distances on both coasts in all these vehicles.

Pure electrics work just fine, and the TCO is lower than comparable gas vehicles. Now it’s just a matter of awareness and education (and volume production).


Harder to beat the efficiency of an electric motor, and battery density is set to increase dramatically. I'm willing to take the bet that those 37 are dead wrong. They obviously have a stake in the ICE game and can't see beyond.


37 people representing the International Journal of Engine Research...can't be any bias there.


My daily driver is a Chevy Bolt. So far, I have driven over 10,000 miles and averaged 4.3 miles per kWhr and am definitely not hyper-miling. I have also let more than 20 people test drive the Bolt and it never fails to put a smile on their face.

However, we will probably have IC engines for a while longer. I believe that there is no reason not to have battery electric transit and school buses now. I also believe that most urban delivery trucks including the class 8 delivery trucks can go battery electric as the speeds are relatively low, the average required range is typically not more than 100 miles. Having said that, it will be considerably harder to use batteries in the near future for long haul trucking and construction and agriculture equipment where high continuous power is required. Maybe when the change rates are high enough so that the batteries can be charged in 10 minutes or less.

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The lead author Rolf Reitz has been a leader in the research of ICE efficiency and he is correct that in certain areas in particular "commercial transport",(e.g. long range trucks, ships, and airplanes) will still require combustion engines for some time. Combustion engine efficiency and vehicle electrification will go a long way to reducing liquid fuel consumption and correspondingly reduce CO2 emissions.
Battery energy density needs to improve at least 700% to compete effectively with liquid hydrocarbons in long range transport. There are technologies out there, however it will take many years before they progress to commercial use.


Energy densities MJ/L
Gasoline 34.2
lithium-ion battery 0.9–2.63

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You bring up what everyone looks at when comparing ICE vs BEV - the Energy Density of Gasoline vs current battery tech. This looks like 13x, but remember BEV are twice as efficient as the best ICE (so 6.5x close to 700%).
Though for LDV that do not require daily long distance travel, current BEV tech is more than adequate. Check another metric on Wikipedia - "Fuel Fraction", which is s the weight of the fuel or propellant divided by the gross take-off weight of the craft.
The Fuel Fraction of a Boeing 787 is 40% (an automobile is less than 4%). So the Energy Density of the fuel is critical to the requirement that the airplane travel 8,000 miles without refueling.


I'm guessing none of these guys owns a Tesla.
I can't see myself buying another ICE vehicle again.
2015 model S 65,000 miles (no oil changes, no brake pad replacements, ...)


It seems that the die-hards are attempting a revival. For my part, I'll never buy an ICE again. The latent potential in batteries will let them improve in size, weight, and power density in due time.


Really surprised PeterXX hasn't weighed in on this.

Those who tout electric transport as the bees' knees are correct as far as they go... which isn't as far as acknowledging that current battery tech has some serious resource limits for things like lithium and cobalt required for the high energy density cells that pure BEVs use.  PHEVs and hybrids can use cobalt-free formulations like LFP and lithium titanate and even NiMH, and much less of them.

The immediate future can't be an all-BEV future, so if we want to hit both criteria emissions and GHG emissions we are going to have to accept that the ICE is going to be around for quite a while and make it the best it can be.  A lot of this is going to involve patching the ICE's weaknesses with EV drivetrain strengths.  For instance, emissions during transient responses can be eliminated by handling the power transients on the electric side, varying the ICE power slowly and smoothly.  ICE weight is minimized by turbocharging, efficiency is maximized using the Miller cycle.  Turbo lag is eliminated by a combination of battery buffering and electric turbo boost.  GHG emissions are addressed using advanced biofuels.

In a world where a BEV requires 60 kWh to compete but we can only make 6 kWh for each vehicle sold, we're going to have to find ways to make the most of both the batteries and the ICEs which supply the remainder of motive power.  The whole will be greater than the sum of its parts.


The world will probably stop making lithium batteries by 2030 or so, not for lack of lithium but from the arrival of much lower cost superior technologies. The NMC811 is one of many possibilities. Solid State high performance batteries will be another contender.

BEVs with 200 KWh lighter, lower cost, longer lasting, very quick charge storage units will put most ICEVs out of competition by 2030.


Specific Energy MJ/kg
Gasoline 46.4
lithium-ion battery 0.36–0.875


Some people just don't get it. Making specific energy comparisons of fossils with batteries is for the birds. Most of the energy contained in fossils is waste: W-A-S-T -E. Battery efficiency is not only 4-times higher but the waste is nearly nil. So what is the point in making such comparisons?

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When using Gravimetric Metrics, i.e. MJ/kg you must look at the total weight of the propulsion system. This includes all components not only fuel (which is a minor component of automobiles).
One example. looking at the total weight of a Toyota Camry V6 (3572 lbs) vs a Tesla Model 3 Long Range RWD (3814 lbs). The Tesla has much better performance and even lower TCO.

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