Big-bore diesels get all the notoriety, but makers of medium-duty engines have taken pioneering strides toward reducing emissions and delivering power. They have to: there are moves afoot to mandate low-emitting vehicles in many urban areas, the bastion of the midrange truck market.

Here’s a quick run-down of recent developments:

International Engine claims its “Green Diesel” engine technology can meet the U.S. Environmental Protection Agency’s strict 2007 emissions limits right now. The company has outfitted a diesel engine with a catalytic converter designed to extract soot and particulates from the exhaust. It’s a two-stage process: the soot particles are trapped in a ceramic filter and are then oxidized through a chemical reaction. The exhaust is mostly carbon dioxide.

To avoid clogging the catalytic converter, the engine needs ultralow-sulphur content fuel: 15 parts per million. Under the Clinton administration, the EPA decided to require ultralow sulphur fuel to be available nationally by 2007. It will be more expensive (around a nickel a gallon more than conventional diesel, according to some estimates) and a bit of energy value will be lost, too, so fuel economy may suffer.

A second challenge for International is the catalytic converter. At least two companies can make the unit, but its production cost, product life, and maintenance costs have yet to be determined. Challenge No. 3 is dealing with NOx emissions. While the ultralow-sulphur fuel and catalytic converter choke off particulate emissions, they do little against NOx. This is not a new problem; NOx and particulates have always been a tradeoff. Some form of NOx aftertreatment device will be needed.

Still, International’s Green Diesel technology looks attractive compared to alternatives such as natural gas. Converting a truck to run on compressed natural gas can be expensive, and the price of a new fueling infrastructure would be huge.

And about that availability issue: California’s South Coast Air Quality Management District has a proposed rule that would require all four counties in the greater Los Angeles area to sell ultraclean (15 ppm) diesel fuel exclusively by 2003.

This month, Caterpillar will start selling its 7.2-litre 3216B “Clean Diesel” low-emission engine, reducing NOx and NMHC emissions by 13% (22% when exhaust aftertreatment is added later this year). Three horsepower ratings-207, 216, and 230 hp-will be certified to meet low-emissions vehicle (LEV) standards. Unlike some other lower-emissions engines that use only ultra-clean diesel fuel available in some parts of the U.S., the 3126B Clean Diesel engine can use conventional diesel blends or ultra-clean and low-sulfur fuels.

The alliance with DaimlerChrysler promises more changes from Cat: the companies will jointly develop medium-duty engines and co-operate on research in areas such as combustion technology, engine electronics, fuel systems, and exhaust aftertreatment.

Hino’s next-generation low-emissions diesels are scheduled to debut with the 2004 model year. They will have common rail fuel systems and EGR to meet 2004 emissions targets.

The engines may also have an intriguing new emissions “filter.” Working with its larger affiliate company, Toyota, Hino says it can cut particulates and NOx emissions by over 80% using a ceramic filter coated with a platinum-based catalyst. First designed for gasoline engines, the filter traps NOx emissions and turns much of the particulate matter into oxygen. Existing vehicles cannot be converted, because it must be used with a new fuel injection system. However, it can use low-sulphur fuel-50 ppm-meaning it could gain market acceptance more rapidly than ultra-low sulphur alternatives.