Navistar’s ‘Ground Up’ Approach to All-New Diesel Engine
Navistar has unveiled its S13 integrated powertrain, which the company claims will be its final clean-sheet design of an ICE diesel powertrain and is intended specifically for the North American commercial-vehicle market.
Though electrification has taken center stage for the commercial trucking industry, diesel ICE powertrains will continue to serve as the preferred propulsion system of many fleets for decades to come. Navistar has committed to keeping the diesel combustion engine relevant for the near future by revealing the S13 Integrated Powertrain.
SAE Media was on hand for the company’s reveal event in Las Vegas, where Navistar proclaimed that the S13 will be its final clean-sheet design of an ICE diesel powertrain. The S13 is intended specifically for the North American commercial-vehicle market as was developed over the past five years in a global collaboration effort by the Traton Group, which encompasses Scania, MAN, Volkswagen, Navistar, and RIO.
“The path toward a zero-emissions future requires improvements to traditional propulsion technologies parallel to the development of new powertrain technologies,” said Michael Grahe, executive vice president of operations. “To ensure optimal reliability, efficiency, performance and sustainability, we designed the S13 Integrated Powertrain from the ground up.”
Grahe also discussed the extensive validation testing that Navistar performed while developing the S13 powertrain. “Before this integrated powertrain goes into serial production, this product will have logged more than 4 million miles of field testing on North American roads,” he said. “We are testing on all terrains – from the desert of Arizona to the frozen tundra of Alaska.”
Navistar has not given a start date for the S13 to enter production for the North American market (the engine is already in production in Europe, where it is sold as the Super 13). When it does enter serial production for North American customers, the S13 will be manufactured at Navistar’s Powertrain Manufacturing Plant in Huntsville, Alabama.
A clean slate
The S13’s clean-sheet design started with a modular approach. Navistar’s goal was to commonize many of the S13’s systems for mass customization across a wide array of variants. This approach helps to ensure the long-term viability of the S13’s design should emissions regulations for commercial diesel-powered ICE vehicles become increasingly stringent. The engine, transmission and aftertreatment system were designed and developed concurrently to ensure maximized compatibility and integration between systems.
Navistar claims that the S13 is the lightest powertrain available in its size class thanks to a 52-lb (23.5-kg) reduction in weight over the outgoing engine. Total dry weight is listed at 2,284 lbs. (1,036 kg). Other weight-saving measures include a cast aluminum camshaft cover and oil pan as well as a compacted graphite iron block.
The S13 also reportedly offers up to a 15% gain in fuel efficiency when combined with the updated aerodynamics package (see sidebar “Wind cheaters”) that Navistar revealed for the LT Series. These gains were achieved through greater combustion efficiency combined with a reduction of friction and pumping losses. Low friction materials and finishes also were used for the liners, rings and main bearings.
One of the guiding principles for the S13’s operation was to reach higher torque ratings at lower revolutions. According to Navistar, operating the engine at lower speeds requires fewer fuel injections and improves overall fuel consumption. There are a total of seven engine outputs for the S13, ranging from 370 to 515 hp (276 to 384 kW) and 1,250 to 1,850 lb-ft (1,695 to 2,508 Nm). That torque figure is realized at just 900 rpm. Maximum engine speed for all outputs is governed at 2,000 rpm.
All S13 variants are inline-six engines displacing 12.74 liters. The bore and stroke are 5.12 in. x 6.30 in. (130 mm x 160 mm) with a fixed compression ratio of 23:1. The cylinder head geometry features four valves per cylinder with dual overhead camshafts and a common rail fuel system. Navistar’s target service life for the S13 is 1,200,000 miles (1,931,000 km).
The S13 was designed with selective catalytic reduction (SCR) as the primary emissions control technology. However, the S13 has no exhaust gas recirculation (EGR) cooler, so all exhaust output is sent directly through the turbocharger during normal engine operation, which delivers more power and improved engine performance.
Navistar explained that by eliminating the recirculation of exhaust gas, a more complete fuel burn was achieved, which also allows cleaner air to enter the combustion chamber during the intake cycle. The net benefit is that soot buildup is mitigated in the engine and emissions system.
Another one of the overarching goals for the development of the S13 was improved uptime through elimination of subcomponents. A prime example is the fixed geometry turbocharger that was selected for the S13. While variable vane turbos have become a popular design choice thanks to their flexibility and power output across various engine speeds, they do raise cost and serviceability concerns.
By reducing the complexity of the turbo system, Navistar claims the S13 will have improved the reliability and durability over powertrains with more complex turbo systems. The fuel system design also was simplified by eliminating the downstream fuel injector. The high-pressure fuel pump also operates at a lower pressure than comparable systems with a peak output of 1800 bar (26,100 psi).
The dual-stage aftertreatment system was reportedly designed with ease of serviceability in mind thanks to its one-box design. According to Navistar, the cleaner combustion cycle of the S13 allows for extended service intervals and eliminates the need for active regeneration cycle and the diesel oxidation catalyst.
The aftertreatment system also features two DEF injectors that feed the dual-stage catalysts via upstream and midstream DEF injection. The system automatically manages the regeneration process with no driver interaction required. Additionally, the DPF filter does not need to be cleaned or replaced until 650,000 miles (1,046,000 km) in most use cases.
A new shift
The S13 will feature the first commercial transmission offered by International, the T14, which is a 14-speed fully automated manual transmission. The T14 features an electronic clutch actuator, a single countershaft and a planetary gear set that supports a compact design and optional programmable reverse speeds.
The shifting logic and integrated software controls were programmed to maximize efficiency and performance resulting in gear shifting that is perfectly matched to the S13’s low operating speeds. The T14 also has two crawler gears for improved acceleration with heavy loads and better low-speed maneuvering.
Drivers will have shift-on-the-fly capability with Economy, Performance and Performance+ modes. Also included are intelligent shifting capabilities, such as skip shifting and turning radius offsets. Additional modes for low-speed maneuvering, hill hold, and initial vehicle movement will also be available. Like the S13 engine, the T14 transmission will be assembled at Navistar’s powertrain plant in Alabama.
Behind the wheel
Navistar had several variants of its new S13 powertrain on hand at Las Vegas Motor Speedway where SAE Media sampled the driving experience. The demo vehicles were International LT Series semis with an unladen 53-foot trailer.
True to its design briefing, driving the S13 is a low-rev affair with good power delivery immediately off idle. The torque band feels broad and there is no discernible lag from the turbo system. The transmission is busy keeping the engine in its powerband, but it goes about its job quietly and seamlessly.
SAE Media sampled several of the available power outputs but spent the most time in the 370 hp/1250 lb-ft, economy-minded S13 variant. Even in its lowest output trim, the S13 never felt lacking for grunt. SAE Media also sampled several of the various driving modes available from the T14 transmission and found the powertrain very adaptable to a diverse range of customer needs and driving conditions.
Though our time driving the S13-equipped trucks was limited to a few laps on the flat apron of the oval course of Las Vegas Motor Speedway, the capabilities and overall drivability of the S13 powertrain were impressive.
After experiencing the S13 on the track, SAE Media sat down for an interview with Navistar’s Grahe. He discussed some of the engineering decisions that Navistar made to develop the S13 such as lack of active regen on the S13.
“We made that decision in the definition phase of the powertrain,” Grahe said. “We looked at today’s offerings and what our customer’s concerns are, which were uptime, reliability, maintenance, and of course fuel efficiency. We also have the advantage over other OEMs that have already launched their engines. So, we could really do this from the beginning and ask how we can avoid certain things which are really cumbersome to our customers.”
Grahe also explained how Navistar approached customers during the development of the S13 and how they used their feedback to set development targets for the S13. “You have to indirectly ask the customer what it is they are looking for,” Grahe said. “If somebody asks you what you want, you will answer with what you know. What we did is we asked the customers what are they doing? Are you driving from A to B? Are you driving over a mountain? What are you struggling with in the repair shop?”
Grahe went on to explain how this approach helped Navistar focus on what their customers needed from this powertrain and the flexibility they would have to offer their customers within a single displacement. “We have much more flexibility because we have just one displacement” he said. “So we have the ability to do more with less.”
Grahe continued, “We focused strongly on the 13-liter [engine] because we think from a weight-to-performance ratio that this is what you need for the North American market. Over time, with new technologies, we can continue to increase performance without increasing size. So, when customers say they want a 15-liter engine, we can then ask, ‘Why?’ And when you look at torque curves, drivability and horsepower, you see that almost every application can be satisfied with a 13-liter.”
There were several aspects of the S13’s development that differed greatly from Navistar’s approaches of the past, according to Grahe. “We focused much more on overall in-house development,” he stated. “As we had this clean-slate approach, we asked how we want to concurrently develop this transmission and aftertreatment for this integrated powertrain?”
Grahe further explained some of the challenges of the S13’s development such as designing and validating many of the components in-house. “We couldn’t take any off-the-shelf products from sub-suppliers because they didn’t fit to the concept,” he said. “This is both a challenge and an advantage when you develop a powertrain as a group. It can be a more complicated or cumbersome approach, which needs time and money, but as a group when you do this for several brands around the world, then it makes sense.”
The S13 also helped Navistar leverage internal knowledge from the company’s European subsidiaries, specifically in the development and manufacturing of the T14 transmission. “The transmission was one of those challenges where Navistar does not have vast experience in assembling and manufacturing,” Grahe admitted. “However, Scania does, as does MAN in Europe. So, we learned from them and exchanged people between the factories so that we could learn how to do this. We are assembling the entire transmission in Huntsville, Alabama, including portions of machining.”
While the S13 may be the swan song for clean-sheet designs of a diesel powertrain for Navistar, Grahe estimates that the engine series could have a service life for many future decades thanks to updates and advancements in engine technology that the company will continue to invest in for the foreseeable future.
Sidebar: Navistar introduces aero and saftey updates for LT series
In addition to the North American debut of the S13, Navistar also revealed updates to the International LT Series. Navistar’s Class 8 truck now offers multiple safety and performance updates, including an improved aerodynamic package, as well as the option to specify the S13 powertrain.
“The new International LT Series updates will deliver superior operating economy and optimal value to our customers,” said Chet Ciesielski, VP of on-highway heavy-duty truck business. “We ensure the customer is top of mind for any product updates we make. This latest LT Series update is a proof point of listening to our customers, ensuring they can maximize their fleet vehicles’ efficiency and performance.”
Fuel efficiency has reportedly been improved thanks to an updated aerodynamic package that includes closeouts over the DEF and fuel fill ports that allow air to flow smoothly over the chassis skirts. New steer wheel closeouts and a modified lower skirt trim also reduce drag by restricting airflow through the wheel opening, while a new roof extender improves airflow between the tractor and trailer gap. There also has been an update to the front-end design that includes a new bumper and grille.
Navistar also announced updates to the LT’s suite of advanced driver-safety features such as automatic braking and collision mitigation systems. There is a new electronic parking brake that automatically engages the vehicle and trailer parking brakes if the driver forgets to set them.
“The brake system as we know it today is no longer the sum of parts. It is becoming an ecosystem where AI and software play an active role,” Ciesielski said.