Emissions guidelines for diesel engines in off-highway equipment continue to get tougher. The introduction this January of EPA Tier 4 interim (Tier 4i) regulations in North America (and E.U. Stage IIIB in Europe) marks a significant tightening from Tier 3 limits which have been in place since 2006.
Although specific limits vary somewhat depending on engine size, they’re all headed toward near-zero emission levels. For example, for engines rated from 175 to 750 hp, nitrogen-oxide (NOx) emissions must be cut in half and particulates by 90%, compared to previous levels. And starting in 2014, Tier 4 final (Tier 4f) takes effect. For engines under 750 hp, this means an additional 80% cut in NOx, compared with Tier 4i.
According to officials from MTU Detroit Diesel, Detroit, the first step in complying with ever-tighter emissions regulations involves upgrading processes inside the engine — for example, by increasing injection pressure, fine-tuning combustion, and using exhaust-gas recirculation (EGR) to keep raw emissions as low as possible.
These measures can significantly reduce the complexity and cost of exhaust aftertreatment and, in some cases, make aftertreatment unnecessary. This, in turn, means simpler designs that require less installation space and maintenance.
Unfortunately, there is an inherent conflict between lowering particulate levels and reducing NOx, say MTU officials. Few in-engine techniques reduce both pollutants simultaneously. More often, an improvement in one area worsens the other.
Where emissions limits cannot be met using in-cylinder techniques, exhaust aftertreatment is needed. These include selective-catalytic reduction (SCR) and dieselparticulate filters (DPF) to reduce NOx and particulates. Here’s how some major engine manufacturers are attacking the problem.
One key feature for many new engines is common-rail fuel injection. For instance, Scania, Södertälje, Sweden, reports its Tier 4i compliant engines ranging from 275 to 700 hp operate at injection pressure above 35,000 psi (2,400 bar). The common-rail system gives engine designers ample freedom in terms of injection timing and pressure, even permitting a series of pulses each cycle, leading to more-efficient and complete fuel combustion.
Electronic control of fuel injection makes timing and duration independent of the camshaft angle. High injection pressures are, thus, available at any time, irrespective of engine speed. This helps keep particulate emissions low and contributes to generous torque at low revs, good fuel economy, and prompt engine response, according to the Perkins feels its Tier 4 engines with NRS reduce both fuel company.
And, in fact, advanced enginemanagement electronics are common among engine manufacturers. They control not only fuel injection, but turbocharging, engine temperature, exhaust aftertreatment, and other performance and emissions parameters.
Another ubiquitous feature of the latest engines is exhaust-gas recirculation. Perkins Engines, Peterborough, U. K., calls its set-up the NOx Reduction System, or NRS. It recirculates exhaust gases to reduce oxygen in the combustion chamber. This slows combustion, lowers peak temperatures, and helps fuel burn more fully which, in turn, reduces NOx formation. When combined with an oxidation catalyst and particulate filter, NRS is cost effective and improves fuel efficiency and performance over its previous Tier 3 engines, says Perkins.
For instance, Perkins’ new 850 Series, rated from 60 to 115 hp, cranks out 40% more power and up to 60% more torque than previous- generation engines. They’re suited for construction, materials handling, and agriculture applications.
The benefit for OEMs, say company officials, is that the 3.4-liter unit does the work of older 4.5-liter engines. With the trend toward downsizing, better power density and fuel economy lets equipment manufacturers opt for an 850 over a larger engine. This frees up valuable space to package the aftertreatment unit, which in the case of the 850 Series can mount in a variety of locations for additional flexibility.
The 850 Series includes commonrail fuel injection, a waste-gated turbocharger, and aftercooler, all electronically regulated. Because of a break in emissions legislation at 75 hp, different engines run different types of aftertreatment. Above 75 hp, the 854E uses high-temperature regeneration that injects additional fuel to burn off soot in the particulate filter. Below 75 hp, it relies on a low-temperature, service-free DPF. Significantly, the sub-75 hp version is already Tier 4f compliant.
While some manufacturers have opted for SCR, Perkins feels its Tier 4 engines with NRS reduce both fuel consumption and emissions over competing SCR engines. This cuts overall engine size, eliminates the need for a second fluid, and lowers operating costs, according to the company. To meet Tier 4f above 75 hp, however, Perkins will embrace SCR.
Selective catalytic reduction
SCR involves injecting a urea-water solution (called diesel-exhaust fluid, or DEF) into the exhaust pipe. The heat of the exhaust converts the solution into ammonia, and it enters a catalytic converter. Here, NOx (NO and NO2) molecules react with ammonia (NH3) and form innocuous nitrogen and water.
SCR requires a tank for the DEF. Consumption is generally around 5% of fuel consumption for Tier 4i engines. DEF is corrosive to some metals such as nonalloyed and zinc-coated steel, copper, and copper alloys. It freezes at 12°F.
MTU relies on SCR for its Tier 4i engines up to 750 hp. They are based on Mercedes-Benz commercial vehicle engines and are used in wheel loaders, excavators, and other construction machines.
High-pressure injection at up to 2,500 bar and tuned combustion let these engines meet Tier 4i specs without particulate filters. The enhancements, say MTU officials, also generate fuel saving of up to 5%, compared with previous models, and better engine-load behavior.
For 2014, MTU will introduce Tier 4f engines equipped with EGR, SCR, and diesel-particulate filters. MTU’s development goal, however, is to meet Tier 4f without particulate filters, which would be a major advantage in terms of weight and space requirements.
According to company officials, this makes MTU one of the first engine manufacturers to offer an engine family for Tier 4f below 750 hp, giving OEMs ample time to adapt equipment for the new engines.
For Tier 4i and 4f engines above 750 hp, MTU’s package includes high-pressure injection, twostage controlled turbocharging, and EGR. This combination reportedly generates low particulates and NOx, making aftertreatment largely unnecessary.
Tier 4 ready
Cummins, Columbus, Ind., reports that the latest generation of its QSL9 engine meets 2014 Tier 4f emissions levels. The 9-liter, six-cylinder engine, rated at up to 400 hp, includes high-pressure injection, a variablegeometry turbocharger, cooled EGR, SCR, and a compact aftertreatment catalyst.
The electronically controlled VGT turbocharger has an inner and outer section, along with a one-piece sliding nozzle that precisely boosts air pressure across the entire engine operating range. At low speeds, the valve closes and exhaust gas flows into the turbo’s inner section to increase boost pressure. At high speeds, the engine controller modulates the valve to let exhaust gas flow into both the inner and outer sections. This varies exhaustgas flow into the turbine wheel to provide rapid boost at low engine speeds and maintain boost pressure at higher rpm.
According to Cummins officials, VGT combines the benefits of both a small and large turbocharger in a single unit, resulting in good transient response and low-end torque. Plus the turbine is less complex and has fewer moving parts than many competing designs, which improves durability.
The engine’s SCR system for Tier 4f is reportedly a step beyond the company’s Tier 4i design. The new package incorporates a copper-zeolite-based catalyst capable of up to 95% NOx conversion, and it operates more efficiently at lower temperatures. Sensors provide closed-loop control that improves DEF mixing and minimizes fluid consumption.
A compact catalyst mounted ahead of the SCR removes particulates by simple, passive oxidation as exhaust flows through. The catalyst is significantly smaller than a particulate filter, so OEMS benefit in space-constrained installations.
Tier 4i QSL9 engines reportedly have up to 5% better fuel efficiency compared to Tier 3 versions. Cummins says fuel efficiency will further increase for Tier 4f by an additional 2 to 3%.
Volvo Construction Equipment, Asheville, N. C, reports its new-generation V-ACT (Volvo advanced-combustion technology) engines combine ultrahigh-pressure variable fuel injection, cooled EGR, controlled turbocharging charging, new engine controls, and exhaust aftertreatment with a particulate filter and thermal regenerator.
Cooled EGR works in tandem with a variable-geometry or wastegated turbocharger, enhancing exhaust recirculation. This lowers NOx compared to previous uncooled versions.
Exhaust aftertreatment is via a DPF and a burner for regeneration. The DPF contains a catalyst-coated ceramic particulate filter that also helps reduce HC and CO emissions.
Exhaust gas flows through the many channels of the filter’s porous walls, and particulates collect inside the pores and on the element’s surface. The collected soot is subsequently burned off in a process called regeneration. This effectively incinerates particulates at temperatures above 700°C, turning them to ash. Regeneration prevents engine backpressure from reaching undesired levels.
Volvo reports the process reduces particulate matter by 90%, compared to previous-series engines, and says regeneration has little impact on fuel consumption. Normally, regeneration will be required once per shift and take about 20 min. The process can take place while the machine is working without affecting performance.
One reason for choosing EGR/DPF rather than SCR at this time, says Volvo, is that the infrastructure for supplying DEF is not yet sufficiently developed for nonroad equipment users. Another reason is that some local regulations require a DPF, such as in California and Switzerland.
Truck meets Tier 4 final
ACERT is Caterpillar’s name for the various techniques it uses to reduce engine emissions. These include, for instance, a high-pressure fuel system that permits multiple injections each combustion cycle, turbochargers that deliver cool air to the combustion chamber, and cross- ow cylinder heads that improve air ow into the engine. In all, the company’s engineers work with about 125 variables in more than 10 million possible combinations to optimize combustion and lower emissions. At the same time, they’re tasked with maintaining or improving engine performance, fuel e ciency, and durability.
To meet Tier 4f limits, the C27 engine also includes EGR NOx reduction and a diesel oxidation catalyst. It requires the use of ultralow-sulfur diesel fuel, but not a diesel particulate lter.
The engine and a new drivetrain are said to increase power 5% over the company’s previous model, letting it hold a higher gear when climbing steep grades. Target gross weight of the truck is 246,500 lb. It goes into commercial production in the fourth quarter of 2011.