The next step in engine emissions is targeting greenhouse gas emissions in heavy trucks.

Because burning less fossil fuel means lower GHG emissions, truck makers soon must meet North America's first fuel economy standards for medium- and heavy-duty trucks
, finalized by the Environmental Protection Agency and the National Highway Traffic Safety Administration last August.

The first round of emissions reduction targets kicks in for the 2014 model year, which means trucks hitting the street in calendar year 2013 will have to comply.

Fuel economy attributed directly to the engine will have to improve by 6% over a MY 2010 baseline by MY 2018 (3% by MY 2014 and another 3% by MY 2018).

Compliance with the rule will be up to the truck and engine makers. They can meet standards literally or comply using credits. Credits can be earned across the full range of trucks, so those that are more efficient than required make up for some that don't. Thus individual models' mpg numbers might or might not be compliant.

Although there is a lot of interesting technology in development, commercialization is still a few years off. What is expected to materialize for the 2014 round is equipment that is already on the market -- though perhaps not widely accepted -- coupled with the normal advancements OEs are making in existing engine platforms.

For instance, one of the customer-programmable features currently available at Mack Trucks is the ability to disable a timed engine shutdown program for extended idling. For 2014, Mack will enable the shutdown on most highway vehicles. It still can be disabled, but the default will be to a short shutdown period.

Tim Tindall, director of component sales at Detroit, indicated that Daimler expects to see a 1 mpg improvement overall in its Freightliner Cascadia by CY 2015. Only a portion of that will be directly attributable to the engine, but Tindall talked about using lighter-grade motor oils to reduce internal friction, variable-speed fans to reduce parasitic loss, predictive cruise control to counteract inefficient driving habits, and unnamed "further engine technology changes."

Mack and others are looking at everything from clutched air compressors to the electrification of various engine systems that currently are mechanically driven.

There are some recently introduced technologies out there that may play a part as well.

For example, Bendix says its PBS Air Injection Booster, Electronic Air Control (EAC) Air Dryer and Turbo-Clutch Air Compressor can realize more than 5% in fuel savings.

For another example, take a look at Volvo's recently announced XE13 Fuel Efficiency package, which is based on the principle of reducing engine revs at cruise speed -- called down-speeding. It uses a very low-ratio axle, the iShift automated transmission, and software tweaking to broaden the torque band sweet spot from 1,050 to 1,500 rpm. Volvo says for every 100-rpm reduction in engine speed while traveling at the same road speed, you save about 1.5% in fuel.

The heavy artillery

While engine makers are not saying a lot about their future "advanced technologies," several admit they are working on improving the mechanical and thermal efficiency of the engine -- possibly for roll-out in MY 2018.

This could include alternative ways to drive engine systems such as air compressors, A/C compressors, fuel injectors, fans, coolant and power steering pumps. In addition to, and possibly in conjunction with the above, engine makers are looking at using waste heat to power engine subsystems or to feed energy back into the powertrain to minimize engine load.

One challenge, says Dave McKenna, director of powertrain sales and marketing at Mack, is the potential cost of some of these technologies.

"They will certainly meet the GHG emission regulation, but they could pose what I call 'commercially viable' technological challenges," he says. "The added weight and complexity plus the increased acquisition cost may not be offset by the fuel savings."

Some engine makers are looking at engine designs that are radically different.

In the short term, they're likely to be of opposed-piston design. OPs, where two pistons share a single cylinder, are not new. Fairbanks-Morse built hundreds for U.S Navy submarines in World War II, then for its own diesel-electric locomotives in the 1940s and '50s.

A principle advantage of an OP is simplicity: A pair of pistons shares a combustion chamber at the center of the cylinder and fire away from each other, with fuel-air mixture introduced through injectors and ports in the wall, so there are no valves or valve train. Each piston spins a separate crankshaft at opposite ends of the block.

In the past few years, two new companies have been designing modern truck- and auto-sized engines using the OP layout.

Achates Power in San Diego says its OP engine, now in dyno testing, has achieved fuel efficiency that's 21% better than a benchmark V-8, the latest 6.7-liter PowerStroke from Ford (not its Navistar-built predecessor), and meets 2010 emissions limits. President David Johnson says they're "on the steep part of the learning curve for the opposed-piston engine and efficiency should get even better."

In suburban Detroit, EcoMotors International continues work on a variant, an opposed-piston, opposed-cylinder, or OPOC, engine. The basic EcoMotors engine is a module consisting of a single cylinder with four pistons, in two sets of two pistons, one firing against the other, and all linked to a single, central crank.

Similar to the Achates, the EcoMotors OPOC has no heads or valves, and instead uses internal ports to draw in fresh air and expel exhaust gases. That and its frequent power delivery make it a two-stroke engine. It's fuel injected and turbocharged, with the turbo moving the air through circumferential ports.

The OPOC engine is smaller, lighter and more fuel-efficient than anything currently out there, says Don Runkle, EcoMotors president. Its small size would make it easy to package in many vehicle types. That intrigues Navistar International, which has inked an agreement with EcoMotors and hopes to have OPOC-powered trucks for sale in two to three years.

Then there's the Scuderi Split Cycle motor from the Scuderi Group in West Springfield, Mass. It divides the four strokes of a combustion cycle between two paired cylinders. The left cylinder functions as an air compressor, handling intake and compression, while the right cylinder handles combustion and exhaust.

The key to Scuderi's split-cycle design is that it compresses the air before it fires. By optimizing the split-cycle concept, the engine, when fully developed, will reduce NOx emissions up to 80% and improve fuel efficiency by as much 50% compared to a conventional gasoline engine.

What about fuel?

It's pretty safe to say that diesel engines will power most American trucks for some time to come. But can other fuels be used effectively in place of diesel? Is natural gas, for instance, really the answer? Is biodiesel, whether natural or synthetic, a solution for the long term?

"We believe the path toward diesel alternatives is a process," says Brian Daniels, product manager, powertrain, at Daimler Trucks North America. "While we do not see diesel being displaced as the primary fuel for the foreseeable future, we do believe that in the short term, natural gas, hybrid and fully electric vehicles may provide alternatives for specific applications," he says. "These platforms will provide a stepping stone to longer-term solutions, such as zero-emissions hydrogen technology."

Natural gas

Natural gas for trucks used to be seen as a peripheral player. The conversion was expensive, the range of natural gas vehicles (NGVs) was quite short, and the infrastructure to support them was thin on th