Beginning in October, the exhaust from newly manufactured diesel engines must produce half the oxides of nitrogen (NOx) of engines produced today with no increase in particulate matter (PM). This is hardly news to anyone in the trucking industry. The accelerated requirement was the fruit of a settlement the Environmental Protection Agency extorted from diesel engine manufacturers in late 1998 upon threat of protracted litigation.
Not settled in 1998, however, were how engine manufacturers will accomplish this feat and, more importantly, how changes in engine design will affect performance and the operating cost of new trucks. The method most engine manufacturers are using to meet the EPA mandate – cooled exhaust gas recirculation (EGR) – has been known for some time. (Caterpillar is the exception. See “Cat still ACERTS it won’t need EGR,” page 32.) But the nuts and bolts of how and why EGR works are still cloudy for many fleet owners and managers. And the financial and operating effects of EGR on fleets remain a subject of discussion and concern.
The NOx/particulate seesaw
As any diesel engineer will tell you, it’s difficult to eliminate PM and NOx at the same time. Historically, efforts to reduce one have tended to increase the other.
For example, advancing injection timing creates higher peak cylinder temperatures and burns off PM, which consists largely of unburned fuel. But higher cylinder temperatures are what cause NOx. Retarding timing reduces temperatures and NOx, but those reduced temperatures result in less complete combustion and increase PM.
EGR has been successful in reducing NOx in gasoline engines. When an engine uses EGR, a percentage of the exhaust gases are drawn or forced back into the intake, at the direction of the engine’s electronic control module (ECM), and mixed with the fresh air and fuel the engine is ingesting.
The purpose of using EGR is to lower the peak flame temperatures inside the combustion chamber. Forcing some exhaust back into the cylinder slightly reduces the concentration of oxygen and slows the burning a bit, making things a little cooler. EGR also absorbs heat, further cooling the process.
Tougher in diesel
EGR works well on passenger cars to reduce NOx because the fuel and air are mixed to a preset ratio in the cylinder, which yields little PM. But diesel fuel wants to burn in as much air as possible, and has trouble finding enough when it first hits the cylinder. This creates PM, and the reduced oxygen concentration produced by EGR makes the PM problem worse. So, it’s ideal to minimize the amount of EGR.
To resolve these conflicting concerns, diesel engineers tried cooling the exhaust gas before putting it back into the cylinders. Exhaust leaves a turbo at temperatures above 1,000 degrees F. But if the exhaust starts out cooler, it reduces peak temperatures even more and minimizes the amount of recirculated exhaust needed to lower NOx.
Engineers settled on using engine coolant as a cooling medium. By using an EGR-to-coolant heat exchanger (jacket-water cooling), they found they could reduce the temperature of the exhaust to about 250 degrees F. But that means the engine cooling system will have more heat to carry away – at least 25 percent more, by one estimate – so radiators may have to be larger.
Expect the heat exchanger, its plumbing, and cooling-system changes to add some degree of cost and complexity to EGR engines. Also look for more complex – read: more expensive – variable-geometry turbochargers (VGTs) on most engines to push just the right amount of air and EGR into the intake stream. And there’ll be an ECM-operated control valve (and related plumbing) that shuts off EGR under cold start-up conditions, to prevent incomplete combustion, minimize white smoke and improve driveability.
Still, with all the added componentry, the end-user premium for an EGR-equipped truck “will probably be only a few percentage points,” according to Tom Freiwald, senior vice president, marketing, Detroit Diesel.
So, problem solved, right? There’s more. Because EGR reduces peak cylinder temperatures, it increases condensation of water and acids in the engine, and may increase oil sooting. Therefore, a new, API CI-4 oil standard has been established specifically for EGR diesels and must be used in them. (See “Oil suppliers board the CI-4 train,” page 30).
Drain intervals, engineers say, will not drop significantly, but you certainly should not expect them to get longer. Moreover, EPA will allow blending drain oil with engine fuel only if truck owners use factory systems – like Cummins’ Centinel – that are certified for the new engines.
Finally, say engine manufacturers, EGR valves and related equipment will not require routine maintenance. Nor will EGR, they say, significantly affect mpg or engine service life.
Following are some manufacturer-specific observations about EGR engines.
Of all technologies available to meet the NOx-reduction mandate, cooled EGR will have the least effect on fuel economy because it allows combustion to occur at an ideal timing, says Cummins’ vice president and chief technical officer John Wall.
Wall adds that more efficient radiators and more powerful fans will shed heat with no change in hood lines, while more efficient water pumps will minimize power loss.
Cummins will use a cooler mounted on the right side of the engine, an EGR valve that controls flow into the cooler, and a Holset VGT turbo, which features water cooling and titanium turbine blades.
Cummins’ EGR valve cooperates with the VGT to fine-tune EGR flow, though the ECM shuts off EGR during cold start-up. The electronic control module (ECM) monitors turbo rpm and constantly adjusts output for optimum throttle response. (Drivers say the turbo sometimes speeds up when least expected to.)
The ISX engine has a soft start of injection, pumping more slowly at the beginning of each cycle to help slow pressure rise and keep NOx down.
Cummins’ executive director of marketing, Tom Kieffer, says the company’s 2002 engines will require API CI-4 oil. The company is still analyzing oil change intervals, but it doesn’t expect them to be significantly shorter. Cummins will still offer its Centinel onboard oil/fuel blending system.
Detroit Diesel’s EGR cooler sits below and forward of the turbo compressor. The EGR valve is visible just below the turbo, and controls flow from the exhaust manifold into the cooler. Exhaust gas passes from the cooler, around the front of the engine to the intake manifold via the chrome pipe.
Detroit Diesel Corp. has made some detail changes in nozzle spray pattern and piston bowl design, but it is sticking with its current injection system. DDC “doesn’t see any benefit” in going to pilot injection, says John Morelli, vice president, Series 60 2000 engine program. The company already has experience with jacket-water EGR cooling, having used basic components of the system on year 2000 Series 50-B bus engines.
The outlook for EGR is not bad and may even bring some benefits, Morelli says. “We had been achieving lower NOx via timing retard.” Detroit found that introducing cooled exhaust gas allowed engineers to advance injection timing, so that combustion stays in the bowl. “There’s an advantage because it reduces sooting of the cylinder liners,” Morelli says. “While there was worry about sooting, we don’t see it becoming an issue. So, the new oils with increased soot dispersancy should be a plus in terms of maintenance.”
Morelli is not yet certain whether Detroit will actually control flow with an EGR valve or just use backpressure. The valve may be used to shut off EGR and minimize cold smoke, or to protect the engine from high heat, depending upon EPA input. Morelli believes fuel economy will likely be at least as good as with present engines.
Oil and filter change intervals should not be affected, although the new CI-4 oil will be required, Morelli says. He expects that those who have extended changes with oil analysis and Detroit approval will not be forced to shorten them and that oil filters will improve as part of the move to EGR.
Because EPA’s settlement with engine makers concerned perceived problems with steady-state – i.e., over-the-road – emissions, the 2002 mandate is an issue primarily for big-bore, heavy-duty engines.
“Our responsibilities under the EPA’s consent decree requirements are a little different from those of linehaul engine manufacturers,” notes Warren Slodowske, International’s manager, environmental staff.
Consequently, the company’s I6 engines will remain virtually unchanged from current models until 2004. However, International will have to cut NOx to 3.2 g/hp-hr (from 4.0 g/hp-hr) in its new VT 365 engines, as opposed to the 2 grams required of heavy-duty engine manufacturers. “Since we’re medium-duty, our NOx debt is not as great as that of linehaul engine builders,” explains Slodowske.
Starting in April, look for the International VT 365 to incorporate cooled EGR and a “performance-enhancing,” electronic, variable-response turbocharger (EVRT), as well as a Generation-2 electro-hydraulic fuel injection system. Also on board will be International’s now-familiar oxidation catalyst to reduce PM.
Although the changes are not required until October, “the engine is undergoing EPA certification now, and the VT 365 will be offered in the International 4200 medium truck in April.” according to Slodowske.
Tim Shick, director of marketing, indicates that EGR is being designed into the product from a clean sheet of paper, rather than being added on, which should favorably impact operating costs. The company adds that maintenance requirements have actually been reduced, with a longer interval for fuel-filter replacement.
Steve Homcha, executive vice president of Class 8 programs, reveals that Mack’s handling of 2002 will crank up in two stages. The first change will be pilot injection, which will be implemented before October 2002, in order to improve “driveability, responsiveness, and fuel economy.” The unit pump injector system will inject a small shot of fuel prior to each main injection cycle to ensure immediate ignition of the main charge and soften the pressure rise.
In addition, Mack will, in this first stage, modify software to open and close the fueling valves in two separate cycles, Homcha says. Subtle changes in the valves to help dampen pressure pulses in the lines, and improve their responsiveness to the electronic commands, will be the only injection system hardware changes. The engine will be noticeably quieter because of pilot injection.
Mack will introduce jacket-water-cooled EGR in October 2002. This first generation will feature an on/off EGR valve, so EGR can be cut off under appropriate conditions (e.g., during cold start-up). An array of sensors to measure flow and temperature will be integrated with a new engine ECM. Exhaust will be driven into the intake, and its flow fine-tuned with a VGT.
Additional heat rejection will be handled in a unique way – by putting the engine’s radiator in front of the air-to-air aftercooler. This design should make the radiator work better, maintain air-to-air performance and conform more easily to the sloping hood line of aero tractors like the Vision. Higher-capacity fans and drives will pull more air through to complete the package.
Responsiveness reportedly will be as good as on the 460 with the new wastegate turbo, and will be improved slightly over earlier models. Governed engine speeds may be 50 rpm higher.
Mack’s long 50,000-mile change interval might have to be shortened as much as 10 to 20 percent, but sump capacity will increase 7 quarts to help avoid that. The company’s goal is to maintain the current interval. CI-4 oil will be required, and Mack and other approved filters will see improvements that should prevent any kind of basic change in filter requirements. As with the other manufacturers, Mack sees no reduction in life to overhaul.
Volvo’s unique V-Pulse system will divide exhaust flow into two separate circuits in order to take advantage of exhaust pulses. The pulses will force the exhaust through the EGR cooler into the intake manifold with no increase in backpressure.
Volvo will use jacket-water-cooled EGR but has found a unique way to make the exhaust gas flow – with pulses. The system is called “V-Pulse,” reports Frank Bio, director of marketing for Volvo power.
The cylinder pressure when the exhaust valve opens is much higher than the average pressure in the exhaust manifold – higher even than what’s in the intake. Volvo has managed to combine cylinders 1-3-5 and 2-4-6 into nests of tubes that take advantage of the resulting pulses.
EGR will enter the cooler in two separate flows. Flapper valves located at the cooler outlet will keep the exhaust from flowing backward once each pulse passes through. This eliminates the VGT and its attendant increase in back-pressure – resulting in a plus for fuel economy, says Bio.
Volvo’s system will use throttling-type EGR valves, one in each circuit, to fine-tune flow. An on-off EGR valve located on the intake manifold side of the engine will stop EGR flow under appropriate circumstances.
Volvo has also redesigned the fuel control valves in its injectors to allow them to throttle fuel flow rather than just cutting it on and off. The throttling capability lets the injectors shape the rate at which fuel flows in, keeping it at a low level until after ignition occurs and softening initial pressure rise. Slowing initial pressure will lower peak cylinder pressure and help reduce NOx. The control valves also cut off more abruptly when it’s time for injection to end, reducing unburned fuel. These changes will require a more powerful engine ECM. The 460-hp engine will also get a wastegate turbo, improving its responsiveness.
Volvo will require the use of CI-4 oil, but users can stick with present drain intervals. However, the engine will be “less forgiving,” if you accidentally extend a change, Bio says.
All in all, engine makers don’t believe EGR will be the nightmare many fleet owners have feared. Soon, experience will tell if they are right.
Caterpillar still ACERTs it won’t need EGR
Caterpillar plans to meet truck diesel emission standards, at least until 2007 and, likely, beyond, without cooled exhaust gas recirculation (EGR). Its approach, announced last spring, is Advanced Combustion Emissions Reduction Technology, or ACERT.
Cat will combine a smarter version of its hydraulic-electronic unit injection (HEUI) system, which would minimize emissions in the cylinder, with a simple oxidizing catalytic converter to polish off particulates and hydrocarbons. This type of catalyst does not require the use of low-sulfur fuel – although emissions would be lower with such fuel – and, says Cat, it has already proven effective and reliable on smaller trucks using Caterpillar 3126 engines.
The HEUI system produces very high injection pressures – Cat won’t reveal specific numbers – with volume and timing controlled independently of rpm and crankshaft position. This capability is part of what allows the system to control NOx in the cylinder without high levels of particulate and hydrocarbons.
Claimed advantages of the system include efficient packaging, lower noise and enhanced engine-brake performance. Cat projects fuel economy to be as good as with current engines, and engine ratings would remain the same.
The ACERT system is slated to become standard on Caterpillar’s offerings in 2003 – too late to comply with the October 2002 deadline. Moreover, the Environmental Protection Agency last spring reaffirmed its intention to hold engine makers to the terms of the 1998 settlement. So what will happen? Cat could opt to pay nonconformance penalties (NCPs) – if available -for the privilege of selling its current-technology engines after October 2002.
Next month: The broader implications of the EPA mandate.