ICCT working paper highlights benefits of current and emerging light-duty diesel technology; “promising pathway for compliance”

ICCT working paper highlights benefits of current and emerging light-duty diesel technology; “promising pathway for compliance”

11 July 2017

The International Council on Clean Transportation (ICCT), one of the organizations at the root of uncovering the Volkswagen diesel emission scandal (earlier post), has published a new analysis of developments and trends in advanced diesel engine technology.

The ICCT team—John German and Aaron Isenstadt—concluded that diesels have and will retain two significant advantages over gasoline engines: significantly better fuel economy and cargo hauling and towing ability. Those attributes make diesels a strong option for customers who put a high priority on towing or fuel economy and manufacturers that want to market high fuel economy, they conclude. “Diesels offer a promising pathway for compliance and another option in manufacturers’ toolboxes.


Diesel vehicles have had a rough time penetrating the US light-duty vehicle market, reflecting outdated negative perceptions, higher fuel prices for diesel than for gasoline, the cost of complying with stringent US emission standards, and rapidly improving gasoline engines. Then, just as light-duty diesels were finally gaining market share, the VW emissions testing scandal set diesels back once again.


Still, improved aftertreatment systems are rapidly reducing the cost of complying with emissions standards, and further cost-reducing advances are expected. While the efficiency advantage of diesels over gasoline engines is narrowing, diesel engine developments are also occurring and will further improve diesel efficiency. The VW scandal will eventually blow over.

… [diesel] use is likely to vary significantly from manufacturer to manufacturer and model to model. This will depend on each automaker’s assessment of diesel’s cost-effectiveness compared with other technologies, such as hybrids, and customer desire for the advantages of diesels. Coming cost reductions will improve the diesel’s competitiveness and likely increase its market share in the future.

—Isenstadt and German (2017)

Background. The combined National Highway Traffic Safety Administration (NHTSA) / US Environmental Protection Agency (EPA) fuel economy and greenhouse gas standards for light duty vehicles require an estimated combined average fuel economy of 34.1 miles per gallon in model year 2016, and 49.1 mpg in model year 2025—equating to 54.5 mpg as measured in terms of carbon dioxide emissions with air-conditioning refrigerant credits factored in. Meeting the 2025 standard will require an average improvement in fuel economy of about 4.1% per year.

A mid-term review will be conducted this year. The original technology assessments informing the NHTSA/EPA rule are now about 5 years old. The ICCT is collaborating with automotive suppliers on a series of working papers evaluating technology progress and new developments in engines, transmissions, vehicle body design and lightweighting, and other measures that have occurred since then. Each paper will evaluate:

  • How the current rate of progress (costs, benefits, market penetration) compares with projections in the rule;

  • Recent technology developments that were not considered in the rule and how they impact costs and benefits; and

  • Customer-acceptance issues—e.g., real-world fuel economy, performance, drivability, reliability, and safety.

The new diesel paper is part of this series. The paper relies on data from publicly available sources, as well as data and information from participating automotive suppliers.

In addition to improvements in basic engine technology—some of which are being applied to gasoline engines—such as advanced friction reduction, higher turbocharger pressures, improved variable geometry turbochargers, downsizing and downspeeding, better thermal management, greater use of high- and low-pressure EGR, and higher-pressure injection systems capable of multiple injections, the ICCT focused on two critical areas of diesel technology development: improved aftertreatment and emissions control; and e-boost and 48V systems.

Emissions. The ICCT team noted that future aftertreatment systems for compliance with Tier 3 emissions standards are expected to cost less than $100 compared with today’s Tier 2 systems, albeit requiring a higher-pressure injection system.

In addition, they found that many systems are capable of NOxconversion over a wide temperature range at rates close to those of three-way catalytic converters on gasoline vehicles. Examples of aftertreatment devices that can be configured to reduce cold-start and operating diesel emissions include NOx storage catalysts, SCR-on-filter catalysts, and ammonia slip catalysts.

High- and low- pressure EGR loops can reduce engine-out NOx and assisted the NOx storage and SCR systems to reduce tailpipe-out NOx to regulatory levels.

Further, as NOx conversion catalysts are improving, so are diesel particulate filters and their methods for regeneration.

E-boost and 48V. Diesel engines will also benefit from e-boosting and 48V mild hybridization. FEV estimated that a 48V hybrid system applied to diesel engines would cost $708 to $712 more than a 12V stop-start system. That cost estimate is in line with estimates from suppliers that 48V systems cost between a third and half as much as full hybridization while reducing fuel consumption by one-third to two-thirds as much.

All of these factors result in lower cost projections for advanced diesel engines from suppliers, compared to the government projections used in the rulemaking.

The International Council on Clean Transportation (ICCT) diesel
Comparison of rulemaking and ICCT/supplier estimates of direct manufacturing cost per percent fuel consumption reduction in 2025. Isenstadt and German (2017). Click to enlarge.

For the increasingly popular larger vehicle classes, diesels provide a uniquely cost-effective alternative to hybrids for achieving deep reductions in fuel consumption. They have plenty of room for emission control systems and benefit from the low-engine-speed performance of diesels.

As a result of all these trends, it seems likely that diesels will have a larger market share in 2025 than predicted by the EPA and NHTSA, unless the diesel defeat device cases brought against VW and FCA cause some manufacturers to abandon diesels. Diesel share will expand further as incremental costs decline.

—Isenstadt and German (2017)

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