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Meaner, Cleaner, Leaner Engines
Last February, Caterpillar president Stu Levenick put impending EPA Tier 4 pollution restrictions in perspective. He said they’d caused his company to take on “the most aggressive and expensive product development initiative in Caterpillar history.”
This comes from the firm that innovated the crawler tractor.
Modifications made to diesels since emissions restrictions took hold in 1996 rival any advance made to machinery since Rudolph Diesel’s introduction of a compression-ignition engine in 1892. “Deere & Company spends about $2.5 million a day on research and development,” says Steve Meinzen of that firm. “In recent years, a significant share of that enormous investment has gone to developing Tier 4 interim engines.”
The environmental payoff is stunning. By EPA estimates, modifications made to diesel to date have cut nitrous oxide (NOx) smog by 1 million tons per year. That is the equivalent of taking 35 million cars off the road.
This is just the beginning. Come January 2011, diesels 175 hp. and larger must meet new Tier 4 interim standards. When the Tier 4 final level is completed in 2015, all diesels, regardless of horsepower, must eject 90% less NOx and 90% less particulate matter (PM). This challenges engineers as never before. In their tightrope walk between the Tier 3 and Tier 4 platforms, designers have a delicate balancing act between NOx and PM. That is where the latest postcombustion treatments now installed on diesels come into play.
One path used to meet Tier 4 interim standards employs exhaust gas recirculation (EGR) to control NOx. This approach still turns out unacceptable levels (by Tier 4 interim levels) of PM. But that soot (unburned fuel) is captured and burned off in a catalytic filter that is part of the engine’s exhaust system.
An alternative technology meeting Tier 4 final rules is selective catalytic reduction (SCR). This approach injects diesel exhaust fluid (DEF) into the engine’s exhaust stream to neutralize excessive NOx. PM output is slashed by tuning the engine to thoroughly combust fuel.
Farmers will literally be able to breathe easier thanks to these advances – but at a cost. Only Caterpillar has estimated its price tag to meet Tier 4, calculating it will add 12% to engine costs over the next three years. Other manufacturers hint at price hikes for whole machines in the 3% to 5% range.
Yet there is a payoff for this extra cost. Cleaner burning engines are more efficient; they drink 15% to 20% less fuel than pre-Tier power plants built 12 years ago. New efficiency records are set at the Nebraska Tractor Test every year. The latest mark breached is 20 hp. hours per gallon. A Massey Ferguson 8680 exceeded that output last year. Indeed, today’s diesels churn out torque levels not possible a decade ago and leap to grow more power in a split second.
The Brain Box
There isn’t a function on today’s diesels that isn’t regulated by the electronic control unit (ECU). Also called the electronic engine control (EEC) or electronic control module (ECM), these brain boxes constantly regulate all aspects of engine performance like injection pressures and timing, turbocharger operation, combustion chamber temperature, levels of nitrous oxide (NOx) or particulate matter (PM), and even engine timing. This breakthrough has made it possible for engines to burn less fuel and eject fewer pollutants while spurring them on to generate surprisingly tall and long torque curves.
A new lexicon of terms has come into use with today’s diesels.
- DEF: Diesel exhaust fluid is a mixture of 32% urea and 68% deionized water.
- DOC: Diesel oxidation catalyst.
- DPF: Diesel particulate filter.
- ECU: Electronic control unit, the diesel engine’s brain box.
- EGR: Exhaust gas recirculation.
- Hp-Hr/gal: Hp. hours per gallon.
- HPCR: High-pressure, common-rail fuel injection.
- NOx: Nitrous oxide (or smog) is formed from unburned oxygen in the combustion process.
- PM: Particulate matter (or soot) is primarily unburned diesel fuel.
- SCR: Selective catalytic reduction.
- Tier: Staged limits of emissions established by the EPA in 1996. There are eight Tier standards to be met in all. In Europe they are called stages.
- VGT: Variable geometry turbocharger.
The need to precisely dole out and then thoroughly atomize fuel has inspired technological breakthroughs on fuel systems. This makes them able to operate at ultrahigh pressures and deliver multiple bursts of fuel in fractions of a second. The advance making this all possible is the high-pressure common rail (HPCR) system.
Gone on large engines is the timed injection pump. Replacing it is a computer- controlled pump that works at pressures in the range of 20,000 to 33,000 psi (depending on engine size and make) regardless of engine rpms.
This high-pressure fuel is sent to a rail, which is, in simplest terms, a tube positioned along the intake manifold. That duct acts as a reservoir that commonly supplies all the injectors with fuel via feed lines.
While other diesel direct-injection systems have to build up the high fuel pressure anew for each injection cycle, HPCR constantly provides pressures matched to the actual operating conditions of the engine at each second even when operating at low engine speeds. The net effect is each injector is instantly supplied with all the fuel it needs. And the higher operating pressures in the rail allow injected fuel to be more thoroughly vaporized in the combustion chamber.
But HPCR’s high-wire act doesn’t end there. Fuel injectors are now also electronically controlled. This enables their nozzles to open and close numerous times in a fraction of a second. So fuel can be injected prior to combustion (called a pilot injection), which eliminates diesel knock for a quieter operating engine, even in cold weather.
This pilot injection is followed by multiple main injection sequences that are ideal for developing power to suit operating conditions.
A third and final injection sequence works to burn off soot particles, thereby cutting emissions.
Even the design of the nozzles on injectors has changed. The holes at the nozzles’ tips are drilled at various angles to suit the size and configuration of the combustion chamber. This helps to thoroughly mix fuel with incoming air prior to combustion.
The end result of all these advances is fuel is more thoroughly combusted, which generates exact power on demand.
In the near future, you may also see piezo injectors in use on diesels. Their ceramic material changes with lightning speed as soon as an electrical voltage is applied. This alters the shape of the crystals, which, in turn, mechanically trigger the opening of the injection nozzle needle.
The advantage with this approach is that injectors can be controlled up to five times faster than the most modern magnetic injectors.
One approach to balancing the yin and yang of Tier 4 emissions – NOx and PM – has inspired some engineers to recirculate exhaust back into the engine’s combustion chamber. Known as exhaust gas recirculation (EGR), this method has proven effective at reducing NOx levels. Meanwhile, PM is captured and burned off in a specialized exhaust filter system.
Here’s how EGR works. A percentage of exhaust gas (20% to 30% on Interim Tier 4 engines) is diverted from the exhaust manifold and piped through a cooler on the side of the engine (shown far right). This cooler drops the temperature of the exhaust gas, which is piped back to the intake manifold. A control valve meters how much cooled exhaust gas is mixed with incoming fresh air.
Cooled exhaust gases have a higher heat capacity and contain less oxygen than fresh air. This lowers the natural oxygen content in the air sent to the cylinders, which also reduces combustion temperatures and subsequently cuts production of NOx during the fuel-air burn-off. But cooled EGR, in use on Caterpillar, Cummins, Deutz, John Deere, Komatsu, and Perkins Tier 4 off-road engines, produces unacceptable levels of PM. To remove PM, manufacturers employ variations of a filtering system. Often these filters have two stages. The first stage (shown above) features a diesel oxidation catalyst (DOC) that has a honeycomb structure called a substrate or catalyst support. There are no moving parts in the DOC, just large amounts of interior surface area. This area in the DOC is coated with catalytic metals. When PM in the exhaust comes in contact with these metals, there is a catalytic (chemical) reaction that oxidizes the PM.
Downstream from the DOC resides the diesel particulate filter (DPF). Its job is to mop up leftover PM. The DPF forces exhaust gases through porous channel walls that trap remaining PM. Trapped PM is then oxidized (burned off) through a self-activating process called passive regeneration, which utilizes exhaust heat. When PM levels can’t be controlled by exhaust heat alone (like when a diesel sits idling a great deal of the time), then the DPF requires active regeneration. And that is accomplished by injecting fuel in the chamber, which instantly burns off boosting temperatures and oxidizing PM. “In field testing, there was little active regeneration needed. Excessive engine idling may require a small amount of diesel fuel injection,” explains Steve Meinzen of John Deere.
“The DOC-DPF system should operate without plugging for the normal life of the vehicle with the use of correct fuels and lubricants. Again, excessive idling can lead to more active filter cleaning, which is why we urge customers not to let an engine warm up or cool down for more than five minutes,” Meinzen says. “Lubricants are far better now, so excessive warm-up and cool-down times are no longer necessary.”
Increasing dependence on pressurized air to improve combustion and to lower emissions has pushed the performance limits of the common turbocharger. This motivated engineers to create a new generation of turbo designs that have the ability to supply air on demand for a wider variety of operating conditions.
This is certainly the case with the modern wastegate turbo. This concept employs a valve that remains closed when the engine is operating at low rpms. This allows the turbo to deliver a full capacity of compressed air at low speeds. As the engine revs up to operating speeds, the wastegate valve opens to release extra exhaust energy. This, in turn, regulates the operating speeds of the turbocharger.
The difference between wastegate turbos from the 1990s (the concept was first introduced by Case IH in 1992) and those in use today is that the wastegate valve is precisely regulated by the engine’s electronic control unit (ECU). This has cut emissions at low engine speeds, while it produces more power on demand at higher rpms.
Another approach to varying charged air can be seen in the variable geometry turbocharger (VGT). The VGT varies air based on operating conditions and throttle adjustments through the use of vanes. These vanes, which regulate the flow of exhaust against the turbo’s turbine blades, infinitely adjust from open to closed as determined by the engine’s electronic control unit.
Closing the vanes (shown above right) accelerates the flow of exhaust gas past turbine blades to make them spin faster at lower engine speeds. This improves the efficiency of the turbocharger because it lets the boost pressure rise faster than a conventional turbo when an engine is throttled up. The result is reduced particulate matter emissions from lean combustion at low engine rpms.
At higher rpms and power levels, the vanes are opened. This reduces the angle of the flow of exhaust against turbines reducing charged air pressure in the combustion chamber.
In operation, a VGT gives higher torque at low engine speeds, faster acceleration, quicker response to varying loads (like pulling through tough spots in the field), improved fuel economy, and increased peak torque to help maintain set speeds.
A unique version of the VGT can be found on the Cummins Holset turbo, which employs a sliding nozzle. With this VGT, the vanes do not pivot; they slide axially to vary turbocharger pressure.
Deere’s PowerTech PSX engines (shown in the illustration below) employ a series of turbochargers. First, fresh air is drawn into a low-pressure turbocharger (a fixed turbine design) where the air pressure is boosted. This pressurized air is then drawn into a high-pressure VGT where air intake pressure is further raised. This air is routed to a charged air cooler where its temperature is lowered. That cooled air is then piped to the intake manifold.
By splitting the compression of charged air between two turbochargers, both can operate at peak efficiency while the engine is running at slower speeds. This design delivers higher power density and improved low-speed torque.
Another variation on using twin turbos can be seen in the illustration at left. It is used on higher horsepower four-wheel-drive tractor models built by Case IH, New Holland, and Detroit Diesel’s Model DD15 diesels.
With turbo compounding (a version of this advance was first used on World War II fighter airplanes), exhaust gases are recycled through a second turbine located downstream from the primary turbocharger. Exhaust gases spin the second turbo, thereby recovering latent exhaust energy. The second turbocharger literally converts this energy into extra horsepower since it is hydrodynamically coupled to the engine’s drive gears. In the case of Case IH and New Holland tractors, the second turbo generates extra driving torque while boosting fuel efficiency up to 3%.
Pistons & Valves
The drive to thoroughly burn fuel has altered the configuration of even the basic pistons and adorned the engine’s combustion chamber with a pair of extra valves. The concept of a piston bowl was first seen in the 1990 Perkins Quadram engine. That concept has blossomed in recent years to include a variety of lobes or complete circular tear-shape indentations complete with a center point (shown right).
The purpose of this depression is to thoroughly mix fuel and air while they both swirl above the piston’s indentation. This turbulence, in turn, also allows combustion to take place across the entire volume of the cylinder and in a shorter time. The feature provides a quick, thorough burn that eliminates combustion hot spots in the chamber.
To boost turbulence in a tornado-like fashion, engineers have added a second intake valve to the combustion chamber. This allows greater volumes of air to be charged into the chamber while helping to swirl the fuel-air mixture getting injected into the cylinder. The ejection of burned fuel is greatly facilitated by a second exhaust valve.
Positioning the injector’s nozzle in the center of the combustion chamber’s roof and between the intake and exhaust valves assists in the mixing action while providing more even piston compression loads.
The result? Cold-starting emissions have been slashed by 50%, while fuel efficiencies climbed 25% and beyond.
Rather than control PM with filtration, engines employing selective catalytic reduction (SCR) are tuned to run at higher combustion temperatures. This produces a hotter, cleaner fire that virtually eliminates PM emissions. But these higher combustion temperatures turn out unacceptable levels of NOx.
So engineers turned to a postcombustion treatment to neutralize NOx. A mixture of 32% urea and 68% deionized water called diesel exhaust fluid (DEF) is injected into a catalyst chamber. But urea first must be converted to ammonia. This transformation occurs instantly in the decomposition reactor (or catalytic chamber) thanks to exhaust temperatures. After urea converts to ammonia, it swirls about the chamber with engine exhaust. The ammonia seizes the NOx molecules and breaks them into nitrogen, carbon dioxide, and water.
The componentry at work includes a tank to store the DEF (ranging in size from 5 to 10 gallons), an electronic control unit, and a catalyst chamber. This chamber resembles a tractor’s exhaust in the case of AGCO’s high-horsepower tractors (SISU engines). Or, it’s a separate cylinder located to the side of the engine, as in the high-horsepower Case IH and New Holland machines (Fiat Powertrain Technologies engines). The two-stage SCR filter illustrated above, featuring two types of filtering channels (shown right), is in use in many on-road diesels.
Some SCR engines also provide for elimination of any remaining PM and initial NOx reduction utilizing a small diesel oxidation catalyst (DOC) chamber. All over-the-road trucks and pickups (except for Navistar and Mack) went to such SCR systems three years ago.
An SCR engine needs to be supplied DEF at approximately 1 gallon for every 25 gallons of diesel consumed. The cost of DEF is generally the same as diesel. “With the vast majority of over-the-road diesels already using SCR, DEF availability is ample,” says Jason Hoult of AGCO.
Both AGCO and Case IH dealers will stock DEF. “And DEF is already sold at many fuel stations and agriculture co-ops,” says Leo Bose of Case IH.
Using DEF does pose some challenges. The liquid, if properly stored, lasts about 36 months. But it can degrade if exposed to temperatures above 90°F. And DEF will freeze at 12°F.
An operating engine warms the DEF tank (which is located next to the engine) in time to meet EPA regulations (the system must function 30 minutes after an engine starts). Manufacturers have equipped their engines with heating systems to defrost DEF, if necessary.
Beyond Tier 4 Interim
EPA restrictions for off-road diesels don’t stop with the Tier 4 interim phase that manufacturers have been scrambling to meet. By 2015, all diesels must be Tier 4 final compliant. “While meeting Tier 4 interim represents a mixture of SCR and EGR use in different engine sizes, it’s generally expected that Tier 4 final will result in a universal embrace of combined EGR-SCR systems,” says Dawn Geske of Diesel Progress (www.dieselprogress.com).
Next up on the Tier trail? Reducing the greenhouse gases targeted by Tiers 5, 6, 7, and 8. “But I don’t expect to see as many sweeping changes being made to off-road diesels beyond 2015 compared to the past decade or so,” says Leo Bose of Case IH