For those of us of a certain age, the word "diesel" evokes images of smelly, soot-spewing vehicles clogging highways and crawling around construction sites. Future generations, though, are unlikely to have those same associations. Thanks to stringent emissions control regulations issued by the U.S. Environmental Protection Agency (EPA), it won't be long before the exhaust from diesel engines is clean and clear.

Many people are familiar with those regulations as they apply to over-the-road trucks. But they also apply to diesel-powered "nonroad" vehicles, including the heavy-duty lift trucks typically used outdoors. The first level of emissions restrictions for diesel-powered lift trucks—designated Tier 1—went into effect in 1997. Over the next decade, Tier 2 and Tier 3 as well as Tier 4 interim rules were introduced.

But even stricter standards are looming. The next iteration, the Tier 4 final standards, mandate that harmful emissions from diesel-powered lift trucks be reduced by more than 90 percent compared with emissions levels before the regulations were first imposed.

Here's a look at what those rules require, how manufacturers are responding, and what they will mean for fleet operators.

RULES OF THE NONROAD
The main components of vehicle emissions are particulate matter (PM), nitrogen oxides (NOx), hydrocarbons (HC), and carbon monoxide (CO). Of these, only PM (soot, or "black smoke") and NOx are currently regulated by the EPA, according to J.B. Mayes, manager, counterbalanced product management for the Hyster Co.

Tier 4 interim regulations require that diesel engine manufacturers reduce PM exhaust emissions by 96 percent and NOx emissions by 76 percent compared with the Tier 1 standards, Mayes says. The Tier 4 final regulations reduce NOx emissions by 94 percent compared with the Tier 1 standards.

That last mandate, as they say, is a doozy: According to the lift truck manufacturers we spoke to for this article, moving from Tier 4 interim to Tier 4 final compliance will be difficult and expensive. "The EPA's objective is that whatever we shoot out of the exhaust should be cleaner than what we take in," says Tim Webb, manager, product development for Hyundai Construction Equipment Americas Inc. "Everybody can get to that, but the problem will be the components and the cost for modification of the diesel engine."

Compliance deadlines are keyed to engine output in kilowatts (also expressed as horsepower). For lift trucks rated up to 75 hp, the Tier 4 final standards went into effect Jan. 1, 2013. The deadline for engines rated between 174 and 750 hp is Jan. 1, 2014, and for those between 75 and 173 hp, the deadline is Jan. 1, 2015. Only engines manufactured on or after those dates are affected.

DESIGN CHALLENGES
For lift truck manufacturers, the challenge is to redesign their products to comply with the regulations but without compromising fuel efficiency, performance, or durability.

Most of the lift truck makers are partnering with engine manufacturers such as Cummins, Perkins, Kubota, and Deutz. (A notable exception is Toyota, which is producing its own Tier 4 final engine.) The engine manufacturers are using a variety of technologies to meet emissions requirements. The most common include:

• Selective catalytic reduction (SCR) systems, which break down NOx into nitrogen and oxygen by mixing a reagent (called diesel exhaust fluid, or DEF) into the exhaust gas flow in a catalytic converter.
• Exhaust gas recirculation (EGR), which dilutes the oxygen in the combustion chamber, lowering the combustion peak temperature and reducing the formation of NOx. The lower temperature increases particulate matter, which must be filtered out.
• Diesel particulate filters (DPF), which trap particulate matter from the exhaust and burn it off to prevent soot from being released into the air.
• Diesel oxidation catalysts (DOC), which convert carbon monoxide and hydrocarbons to water vapor and carbon dioxide.
• Turbochargers, which help small, lower-emission engines generate more horsepower.

Each lift truck manufacturer must decide which combination of technologies will work best with its models and the applications for which they were designed. Hyundai, for example, will use diesel particulate filters in its 1.5- to 3.3-ton trucks and a diesel oxidation catalyst/selective catalytic reduction combination for larger trucks. Mitsubishi Caterpillar Forklift America Inc. (MCFA) chose diesel particulate filters for its Cat lift trucks and Mitsubishi forklift trucks lines. For its largest trucks, Hyster will use exhaust gas recirculation with a diesel particulate filter. Toyota's 8-Series trucks feature a new electronic common-rail fuel injection system that works with an intercooled turbocharger and a diesel oxidation catalyst. And Crown Equipment's Hamech V811 Series trucks are equipped with a diesel oxidation catalyst system. (See sidebar for a list of some of the Tier 4 models that are now available or will be shortly.)

The regulations have created some design challenges for lift truck manufacturers, says Jason Provancher, director of IC (internal combustion) product development and engineering at Crown Equipment Corp., which makes diesel-powered lift trucks under its Hamech brand. Components like the canister-shaped diesel particulate filters and the containers required to hold and burn oxidation catalysts take up space inside a truck, and it can be difficult to make room for them in a small lift truck, he notes. Manufacturers may also have to redesign the inner workings and exterior cowlings and panels to accommodate changes in the size and positioning of exhaust pipes, hoses, cables, and other components, says Lucas Dumdie, a product line manager at MCFA.

Another concern is the effect of overheating on the combustion process and the changes in emissions levels this can bring. Toyota, for one, has introduced an improved cooling system with the ability to reduce power usage automatically to help prevent overheating and maintain the proper emissions levels, according to Mark Faiman, a product manager for Toyota Material Handling, U.S.A., Inc. (TMHU).

Typically, operators must stop the truck and wait while accumulated particulates burn off. That's a productivity-buster, says Dumdie. So some manufacturers have found ways to carry out that process while the truck is running, without overheating. The Perkins-built engines in Mitsubishi's and Cat's Tier 4 final trucks, for example, heat up the DPF and automatically burn off soot while the vehicle is in operation.

Diesel lift trucks are designed for heavy-duty applications, so manufacturers that opt for smaller engines (which use less fuel and therefore, produce lower emissions) can't compromise power or lifting capacity. Toyota addressed that by incorporating a new electronic common-rail fuel injection system and an intercooled turbocharger into its fuel-sipping four-cylinder engine. Thanks to these and other design changes, the new model provides greater torque and as much horsepower as the previous inline six-cylinder engine, says Cesar Jimenez, TMHU's national product planning manager.

Fuel and lubricating oil are another consideration with Tier 4-compliant engines. Because sulfur can severely damage catalytic converters, low-emissions engines require ultra-low-sulfur diesel (ULSD) fuel, Provancher says. Although ULSD is in wide use, it may not be readily available everywhere lift truck fleets operate, he observes.

As for engine oil, because the emissions control systems generate more heat and the ash residue left when lube oil burns off during combustion can foul particulate filters, CJ-4 lubricant is recommended for engines using DPF and EGR systems, says Webb. "CJ-4 is more resistant to heat and produces less ash," he explains, which in most cases prolongs the interval between cleanings.

BUYERS BE AWARE
Initial purchase prices for Tier 4 final lift trucks will be higher than those for their Tier 3 predecessors. That's not surprising considering the additional costs engine and lift truck makers have incurred over the past decade, Provancher says. It's too early to say what Tier 4 final pricing could or should be over the long term, he continues, "but realistically, there's a good substitute with LPG (liquefied petroleum gas), and the market is only going to bear so much when it comes to purchase price." In the short term, there will be opportunities to buy less-expensive, lower-tier trucks, but he expects that supply will dry up in about a year.

Lift truck makers counter that the fuel savings from their Tier 4 final models compared with their previous models should offset the higher prices. Hyundai and Hyster, for instance, expect to boost average fuel efficiency by 10 percent and 15 percent, respectively, in the models they've scheduled for rollout. Mitsubishi and Cat, meanwhile, have both achieved a 22-percent improvement in fuel consumption. But Toyota may be the champion when it comes to fuel savings: The company is citing a 30-percent average reduction in fuel consumption for its 8-Series diesel trucks compared with the previous model.

Those improvements are due not only to hotter-burning, more efficient engines, but also to manufacturers' concerted efforts to cut fuel consumption—and thus, the total cost of ownership—through such innovations as regenerative braking, automatic engine shut-off, idle management, and on-demand hydraulic, cooling, and power steering. But there could be hidden costs. Engines that use selective catalytic reduction technology require a reagent called diesel exhaust fluid (DEF) that is approximately one-third urea and two-thirds water. "Trucks utilizing this technology require separate DEF tanks and regular fill-ups, creating additional costs for the user," Mayes notes.

Maintenance costs should not be much higher, according to the lift truck makers. Depending on the type of emissions equipment, infrequent or even no cleaning (in the case of DOC systems) will be required. Some diesel particulate filters, for instance, could go 4,000 hours or more before they need cleaning. But the Tier 4 final engines are highly electronic and thus "more complex than the diesel engines of yore," says MCFA's Dumdie. "As a result, they are a lot different to troubleshoot."

There's speculation that the advent of ultra-clean diesel engines could cause a shift in the lift truck market. For one thing, when pricing eventually comes down, there may not be as big a cost difference between electric and IC trucks as there is now, Webb suggests. "I think we'll see a more level playing field for the initial cost of a truck, battery, and charger versus the initial cost of a diesel truck."

The Tier 4 final engines, moreover, will be clean enough to use indoors in some areas, making them appropriate for at least a few applications where they've long been barred. Diesels could also become more attractive in California, where the California Air Resources Board (CARB) has imposed fleetwide emissions limits. Instead of having to replace older trucks with electrics or other power sources to keep a fleet's total emissions below the applicable limits, Toyota's Faiman says, buyers could purchase the powerful diesels they want and still be in compliance with the regulations.

That situation could soon spread to other states. The EPA has delayed approving CARB's diesel emissions rules for now, Faiman says, but if and when it does, other states with strict air-quality-attainment goals could follow California's lead and adopt fleet emissions averaging, too.