Cars and utility vehicles are getting greener, but they're far from the only vehicles out there.
To reduce the environmental impact of internal-combustion engines in a meaningful way, the emissions of commercial vehicles have to be curtailed as well.
Manufacturers of tractors and bulldozers have begun to adopt emission-reducing technologies--such as particulate filters and efficiency-focused engine management software--to meet new U.S. Environmental Protection Agency (EPA) standards.
These so-called Tier 4 standards call for a 90-percent reduction in particulate matter and nitrogen oxides in 2014-model-year vehicles, compared to those from 1996.
The Diesel Technology Forum--a trade group that represents diesel manufacturers--says the industry has gone even further, reducing particulate and nitrogen oxide emissions by 99 percent, and "significantly" reducing carbon-dioxide emissions.
Agricultural and construction vehicles also benefit from the "Ultra-Low-Sulfur Diesel" fuel phased in by the EPA between 2006 and 2010.
The fuel has a sulfur content of no more than 15 parts per million, against several times that previously, meaning that it will not poison more advanced emissions aftertreatment systems fitted to modern diesel engines.
However, there remains plenty of room for improvement.
While fleet emissions of passenger cars and commercial trucks tend to decline as new vehicles replace older ones, the process takes longer and can be more convoluted for tractors, bulldozers, and other non-road diesels.
The replacement rate for farm and construction equipment is much slower than that of road-going cars and trucks. Farmers typically wait as long as possible before buying new equipment--and since the new, cleaner tractors are expected to cost more than older, dirtier ones, that probably won't change.
The wider variety of applications make it harder to apply a given emission-reducing solution across the board as well, and the volumes are far lower than for vehicle engines built in the hundreds of thousands or millions annually.
Still, stricter regulations, and the efforts manufacturers have made to meet them, mean that the next generation of agricultural and construction equipment will pollute less.
That means they will finally be subject to the same forces applied to passenger vehicles--which have been required to emit steadily fewer pollutants for four decades now.