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The Future Of Diesel Engines

Written By, Mike McGlothlin


With one look at today’s automotive headlines, you would think battery electric vehicles (BEVs) are on the verge of overtaking the internal combustion engine (ICE). While this may eventually be the case in the passenger car and light-duty truck segments, diesel ICEs will remain the backbone of the heavy-duty and industrial sectors for some time to come. That said, ever-tightening emissions standards will require diesel engine manufacturers to implement new technologies in the not-so-distant future. This means advanced combustion techniques, renewable fuels, and diesel-hydrogen or diesel-electric hybrid technology may become more commonplace. Beyond that, when talking all about diesel engines, we could see heavy-duty vehicles with full-on, battery electric propulsion.


But before (and if) we get to full-electric, it’s important to weigh the costs, viability, reliability and practicality of the emissions-curbing technology that’s available right now. Case in point, while the verdict is still out on BEVs, technologies with proven track records such as renewable fuels and advanced combustion strategies are at the ready today. And hybrid configurations (be it diesel-hydrogen or diesel-electric) have already demonstrated they can cut greenhouse gas emissions by considerable margins while also increasing fuel efficiency. So, before the world dives headfirst into BEV technology, let’s take a look at the technologies that can be implemented now to make an immediate difference—and then we can explore the prospect of an all-electric future. We’ll do all of that (and more) below.


Diesel ICEs And Short-Term Emissions Goals

We Should Improve The Modern-Day Diesel ICE, Instead Of Giving Up On It

Just as it has been for decades, looming emissions regulations will continue to shape thefuture of the diesel engine, but despite a particularly stringent set of new emissions standards set to take effect in 2027, engine manufacturers will still be able to meet them. And that will be far from diesel’s last hurrah. In the passenger car and light-duty market alone, it's estimated that the ICE will be around for at least another 25 years, which means they’ll be used in the heavy-duty segment for much longer than that. So why not continue to improve the ICE right now, today?


The Next Tailpipe Hurdle For Diesels To Overcome

The Environmental Protection Agency’s recent finalization of the Clean Trucks Plan calls for a substantial reduction in nitrogen oxide (NOx) emissions by 2027. The new rules when talking all about diesel engines, which the California Air Resources Board (CARB) is in complete alignment with, dictates a NOx standard of 0.020 g/bhp-hr. This is a significantly more stringent standard than the 0.050 g/bhp-hr limit that will be imposed from 2024-2026 model year on-highway heavy-duty diesels. For reference, 30 years ago (1993) NOx standards were 5.0 g/bhp-hr. Accumulatively, the regulations included in the Clean Trucks Plan are more than 80-percent stronger than current standards.


Advanced Combustion Technologies (RCCI)

One method of curbing NOx emissions which is already available is Reactivity Controlled Compression Ignition (RCCI). The only thing left to do is for manufacturers to implement it. RCCI calls for the use of two different fuels within the combustion chamber, one a high octane source (i.e. gasoline) and the other a fuel that’s high in cetane (diesel). In testing, an 87-percent drop in NOx emissions was observed with RCCI employed on a lower compression, heavy-duty engine. As a bonus, the same test yielded a 95-percent reduction in particular matter (PM).


Reducing CO2 In The Immediate Future

Phase 3 of the EPA’s Clean Trucks Plan calls for tighter greenhouse gas emissionsstandards for 2027. Once again, the biggest target is reducing CO2 by a significant margin. But with allowable CO2 limits already a small fraction of what they were just 15 years ago, the internal combustion diesel engine will need some help meeting the next generation of carbon dioxide regulations. The good news here is that the technology to reduce CO2 even further is available today—and it starts with more efficient combustion techniques and carbon neutral fuels.


Opposed Piston Engines

Leave it to a 20th century design to meet 21st century emissions demands, but that’s exactly the kind of potential Achates Power’s opposed-piston, two-stroke diesel engine technology has shown. The company’s 10.6L three-cylinder, six-piston power plant has no cylinder head, which means a remarkable drop in heat losses provides an immediate efficiency advantage. And then not only is combustion and fuel efficiency improved with the company’s design, but the Achates Power engine—which debuted in 2020—is over the target on the 2027 CO2 emissions standard, beating it by an 8-percent margin.


Advanced Combustion Technologies (PCCI)

Being able to precisely control an engine’s ignition point has always been vital in controlling emissions production. Another form of dual-fuel, advanced combustion technology is called Premixed Charge Compression Ignition (PCCI), which has shown promising CO2 results when blending diesel fuel with gasoline. When talking all about diesel engines, the benefit of pulling off more accurately timed ignition events is lower fuel consumption—and reduced fuel usage is the simplest way to begin working toward lower CO2 levels (and most greenhouse gases). PCCI also presents a way to lower NOx production at the source (in-cylinder), with EGR and (later on) SCR further bringing NOx levels close to zero.


Carbon Neutral Fuels

While advanced combustion technology has yet to be embraced with open arms, it would arguably be easier to adopt the use of carbon neutral fuels. The biggest plus here is that many carbon neutral fuels don’t warrant a completely redesigned engine, most can be burned without changing a thing on current diesel engines. Some feasible examples of carbon neutral fuels include biofuels (namely biodiesel produced from vegetable oil or animal fats) and synthetic diesel. When synthetic fuels are burned, they release CO2, but the amount of carbon dioxide used in their production offsets their tailpipe emissions. Another carbon neutral option is also available today: hydrogen.



Why Hydrogen Works

Once you understand the tremendous effect hydrogen has when used in a diesel engine, you begin to see why it’s appeal is so high. Because hydrogen burns roughly 10 times quicker than diesel fuel, introducing it during the engine’s intake stroke makes it available when diesel enters the picture during the power stroke, which provides a much more complete burn. As a result, the increased efficiency of the combustion event often produces very little (if any) PM emissions, eliminates most greenhouse gas emissions (namely carbon dioxide and nitrous oxide), and can improve fuel economy as much as 25-percent.


Cummins: Leading The Way On Hydrogen

One company that is all aboard on the hydrogen train is Cummins. Recognizing the immediate benefit hydrogen can provide, the global engine leader has led the charge in diesel-hydrogen hybrid technology in recent years. Ironically enough, when talking all about diesel engines, they seem to be the only engine manufacturer touting its advantages. Diesel-hydrogen conversions and full-on, hydrogen-fueled engines are part of Cummins’ portfolio. Not coincidentally, its hydrogen-fueled B6.7 and X15H engines are set to debut in 2027—right when the new, more stringent federal emission standards go into effect. And in case you were wondering, yes, both engine’s basic architecture will be based on their 6.7L diesel and X15 diesel predecessors.


Hydrogen: The Better Choice For Low Emissions And Optimum Range

If the choice for meeting emissions standards in the immediate future is between hydrogen or battery-electric propulsion, hydrogen beats batteries in many categories, but the most important one is range. In medium-duty applications, 500 miles worth of range is common, and is made possible due to dual (or more) hydrogen tanks. However, the larger the vehicle the more room there may be for additional storage, which will serve to further improve range. Of course, if future regulations (especially for NOx) make it impossible for hydrogen engines to meet emissions requirements, battery electric may be the only choice (more on that later).


Hydrogen’s Biggest Turnoff(s)

Of course, hydrogen does come with its drawbacks, the first of which is its volatile nature. While believed by many to be no more dangerous than gasoline, the high pressure the gas is stored at in onboard tanks is a turnoff to the general public. In order to increase storage capacity, hydrogen is often pressurized as high as 700 bar (more than 10,000 psi), which optimizes what’s known as storage density. Despite the fact that storage tank safety is a top priority among hydrogen manufacturers, the public’s unfamiliarity with hydrogen has always been the most formidable obstacle to overcome for its proponents. It certainly doesn’t help matters that, outside of California, very few hydrogen refueling stations exist in the U.S.


Diesel-Electric Hybrids

International’s SuperTruck II

At the present time, Navistar’s SuperTruck II concept represents one of the most logical steps in reducing emissions while improving fuel economy in a Class 8 truck application. The project, which was co-funded by the U.S. Department of Energy and that has been labeled as the manufacturer’s first step toward complete electrification, combines a high-voltage battery electric system with one of the company’s advanced diesel ICEs. In testing, it’s shown a remarkable brake thermal efficiency of 55.2-percent (vs. 35 to 40-percent in traditional diesel ICE applications) and has also achieved 16-mpg. Overall, Navistar’s diesel-electric hybrid creation has yielded a 170-percent increase in freight efficiency.


Edison Motors’ Diesel-Electric Hybrid

As an example of what is arguably one of the best concepts available at the present time, Edison Motors of Merritt, British Columbia, Canada is building vocational trucks with diesel-electric powertrains. The company specializes in many things, but retrofitting older Class 8 trucks with a proven, diesel-electric hybridization package seems to be its bread and butter. A small displacement Caterpillar diesel engine is employed to serve as the generator for a bank of batteries that combine with e-Axles (single, tandem, or triple) to propel the truck. The ICE is only operational 25 to 50-percent of the time in most applications, which not only means tailpipe emissions are cut but its longevity is multiplied.


Battery Electric Trucks

Bottom Line Issues (i.e. Cost)

Currently, the prospect of converting existing fleets or replacing fleet vehicles with full-on electric versions is remote. For a lot of reasons, when talking all about diesel engines, fleet owners are yet to be convinced that electric is the most feasible path forward for their freight operations. Their range is questionable, their durability hasn’t yet been proven, their performance in varying weather conditions is yet to be thoroughly tested, and the charging infrastructure is still in its infancy (we’ll elaborate on each of those in a bit). However, one of the biggest deterrents is cost. Margins are already ultra-thing in the over-the-road segment, so spending twice as much on each truck (ICE vs. BEV) is going to take some serious convincing.


Mack’s LR Electric

It’s not surprising that manufacturers offering battery-electric commercial trucks aren’t exactly quick to talk price. Mack, a well-known name in the medium and heavy-duty truck segments, launched the LR Electric refuse truck in 2016, and as of 2021 it ran roughly $500,000—or double the price of a diesel ICE version. Current prices and options can top $600,000. Even with fuel savings factored in, it could take more than a decade of service for a battery-electric commercial vehicle to pay for itself. While taxpayer-funded municipalities might opt for the electric models anyway, privately owned fleets—where the bottom line is everything—will be much slower to embrace battery-electric technology.


The Mack MD Electric

Mack Trucks followed up the LR Electric this year with the unveiling of its MD Electric. As you might’ve already guessed, short of contacting a dealer pricing isn’t easily obtained or readily available. Perhaps it’s because, equipped with certain available options, the price of the MD Electric can exceed $700,000. A Class 6, 25,995-pound GVWR model MD Electric is advertised as offering up to 230 miles worth or range, and makes use of a three-phase, 260 hp motor that’s powered by Nickel Manganese Cobalt Oxide lithium-ion batteries rated at 240 kWh. Time will tell how well the MD Electric is embraced in the vocational sector of the economy.


Kenworth’s T680E

Billed as a true, zero emissions solution, Kenworth’s T680E is the company’s first Class 8 battery electric truck. The T680E features 536 hp continuous power and up to 670 hp peak power, along with 1,623 lb-ft of torque. Meritor’s 14Xe tandem electric powertrain with high/low voltage power electronics provides propulsion. When connected to a DDS1 DC fast charger, complete recharging time spans just over three hours. Unfortunately, at the present time the T680E boasts an estimated range of a mere 150 miles, depending on its specific application. This hardly makes it a threat to overtake current ICE Class 8 tractor trailers which, when 300 gallons of capacity are onboard, can travel up to 2,000 miles between refills.


Volvo VNR Electric

Volvo, which owns Mack Trucks, seems to have spearheaded the range challenges faced by battery-electric truck makers. One of the company’s VNR Electric options entails a six-battery arrangement that can provide up to 275 miles of range, but that can also be charged to 80-percent in 90 minutes. Like Kenworth’s T680E, the VNR Electric is based on the manufacturer’s familiar VNR platform. But Volvo took the subject of battery safety a step further, designing and integrating a side impact protection barrier into the battery box’s mounting system. Volvo’s VNR Electric family includes everything from 4x2 straight trucks to 6x4 Class 8 tractors.


Freightliner eCascadia

Right in line with what everyone else seems to be doing, Freightliner is offering an “E” version of its Class 8 tractor (along with the Class 6-7 eM2). This one is coined the eCascadia and it comes with either one or two Detroit ePowertrain electric axles (e-Axles), which combines the electric motor, transmission and its corresponding electronics into a single unit. Like the VNR Electric, its battery pack can also be recharged up to 80-percent within a 90-minute window. However, even though it’s billed as having a powertrain that compromises on nothing, maximum range on a full charge peaks at 230 miles.


Peterbilt 579EV

Peterbilt has also released its battery-electric option. The company’s 579EV makes use of e-Axles with two-speed gearing to maximize both low-speed take off and highway cruising. The 579EV also makes use of regenerative braking, which keeps the batteries topped off in an effort to optimize range—an ideal system for truck routes that require frequent starts and stops. Unfortunately, fully loaded to 82,000 pounds (GCWR) the 579EV’s estimated daily range checks in at just 150 miles. In place of the ICE, you’ll find an on-board vehicle AC charger, a cab heater, and the truck’s air compressor, as well as a DC-DC and low-voltage battery converter.


Tesla Semi

At the front of the heavy-duty, battery-electric competition you’ll find Tesla. The company’s Class 8 BEV is known simply as “Semi” and it’s ahead of the pack in virtually every way. Semi’s performance is highlighted by a 20-second 0-to-60 mph stat at max GCWR (82,000 pounds), along with an ability to maintain cruising speed on 5-percent uphill grades. As for range, Tesla’s Semi truck is capable of traveling up to 500 miles on a single charge (again, at 82,000 pounds GCWR), with less than 2 kWh being consumed per mile driven. Last but not least, 70-percent range can be had on a 30-minute recharge, so long as the Semi is charged at one of the company’s Semi Chargers.


How Are Battery-Electric Trucks Fairing Out In The Real World?

This is a key question, especially for the Class 8 truck segment, which appears all but poised to transition to full-on, electric propulsion at some point in the future of diesel engines. According to a NACFE (North American Council for Freight Efficiency) test run conducted by a California Pepsi depot, three of its 21 Tesla Semi fleet proved capable of traveling 300 miles or more, and one was shown to have traveled 416 miles on a single charge. A subsequent test returned 794 miles in a single day, but with an unknown amount of charging stops (presumably at least one). So far, and due to essentially still being in their infancy, few concrete data points reflect the long-term costs associated with a fleet of heavy-duty BEVs. In time, we’ll know more about battery degradation, common repair points or pattern problems, and overall energy savings associated with battery-electric trucks.


Range Anxiety

Without question, much has yet to be proven to improve the selling point of a battery-electric truck. Specifically, range anxiety is already a big concern in the passenger car market, but it’s much more worrisome in heavy-duty applications, especially for over-the-road truckers. Even with the availability of fast-charge stations, range limitations say that long-haul OTR drivers will spend at least 3.5 hours per day plugged in, on average. And that charge time estimation presumes the truck is being operated (and recharged) in ideal weather conditions. Extreme weather, hot or cold, is another avenue that is yet to be fully addressed by BEV manufacturers.


Cold-Weather Issues Remain

Cold weather has long been known to decrease the storage capacity of batteries. During frigid conditions, it’s common for battery range to decrease while recharge intervals increase. Case in point, during a one-year pilot program where the New York City Department of Sanitation put a fleet of Mack LR Electric refuse trucks to work both collecting trash and plowing snow (double-duty is required in the Big Apple), the combination of cold temps and increased load took a huge toll on each truck’s range when it was time to move snow. The amount of up-time observed during snow removal was just 33-percent of what it was for trash collection. This test-run serves as yet another reminder that increased load paired with cold weather can pose a big problem for productivity.


Charging Infrastructure

One of the biggest problems facing the battery-electric transition are that not enough plug-ins exist to charge them. This will change in time, but with emissions regulations fast approaching—some of which may even be un-achievable by today’s ICEs—it makes one wonder how long it will take to get charging infrastructure where it needs to be to ensure the transportation industry continues running at full steam ahead. And then comes the problem of ensuring enough “juice” is on tap at charging stations. Some experts believe a large BEV charging station, which supports both passenger vehicles and semitrucks, will call for as much as 19 megawatts of peak power—an amount that matches the demands of a small American town.


Unless Something Changes, Expect BEVs To Be The Future

Despite advanced combustion technology, carbon neutral fuels, and hybridization being readily available options to reduce emissions, engine and truck manufacturers seem to be investing very little (if any) capital, time or effort in them. With BEVs and BEV technology garnering most of the monetary support at the present time (through Department of Energy funding, EPA grants, government subsidies, significant tax incentives, and more) it’s no wonder we’re seeing such a powerful push for battery electric transportation. It may also be why we’re seeing so little public or private support for anything other than battery-electric these days.


Our Take

Over the course of the next two to three years, we expect to see fleets updated with new, ICE vehicles ahead of the 2027 low-NOx deadline. This line of thinking is to be expected, with fleet owners holding onto existing ICE technology as long as regulations allow. However, other fleets may do more of what we’re already seeing: a slow transition toward electric. Beyond that, diesel-hydrogen may make an appearance, but it will likely give way to battery-electric eventually. Our hope is that the range and durability issues that plague today’s battery-electric technology can be extended and ironed out—at least to the extent that they can match today’s ICE-powered vehicles. Either way, the years ahead are going to be very interesting.


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