Clean Sweep

The 2002 engines are different, but their care and maintenance have changed only slightly.

As every diesel designer will tell you, it’s difficult to get rid of particulate (visible smoke), and equally difficult to get rid of nitrogen oxides, or NOx. To get rid of both at the same time is really difficult. Yet that’s what the industry has been doing for more than 10 years. The problem this creates is a NOx/particulate tradeoff.

In a diesel, particulate occurs because of incomplete burning, and raising the peak cylinder temperature usually helps that. On the other hand, NOx emissions occur because of too high a peak cylinder temperature. Lowering the cylinder temperature to combat NOx tends to raise particulate. For example, advancing injection timing creates higher temperatures and burns off particulate, while retarding timing reduces temperatures and NOx, but tends to make more particulate.

In 1994, diesels got so clean you could hardly see any smoke at all. That occurred as particulate reached its current level of 0.1 gram per horsepower-hour. NOx at that point was at 5 grams per horsepower-hour, less than half the 1988 level – a standard that was implemented in 1991.

In 1998, the industry was required to drop NOx again, this time to 4 grams, keeping particulate at the 1994 level of 0.1 gram. As of 2002, with particulate still at one-sixth of the 1988 level, NOx must drop to only 2 grams.

A successful technology for getting rid of NOx, one that works well with gasoline engines, is exhaust gas recirculation (EGR). When an engine uses EGR, some of its exhaust gases are drawn or forced back into the intake with the fresh air and fuel that the engine is ingesting.

The purpose of EGR is to lower the peak flame temperature inside the combustion chamber. EGR works in two ways. For one thing, forcing some exhaust back into the cylinder slightly reduces the concentration of oxygen. That slows the burning a bit, making things a little cooler. But, exhaust has another quality – it takes a lot of energy to heat it up. So if you mix in a little exhaust, the cylinder peak temperature drops quite a bit, even if you burn the same amount of fuel. That kills NOx.

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A few basics
EGR works well on gas-powered vehicles because the fuel and air are well mixed in the cylinder, which leaves little particulate. But in diesels, the fuel has trouble finding enough air when it first hits the cylinder, and this creates particulate. The reduced oxygen concentration produced by EGR makes the particulate problem even worse. So it’s best to minimize the amount of EGR needed.

This led engineers at most truck diesel manufacturers to try something not done on cars: cooling the exhaust gas prior to putting it back into the cylinder. Exhaust comes out of the turbo at temperatures of 1,000 degrees plus. But, if the exhaust started out cooler, that would reduce peak temperatures even more, and would minimize the amount of recirculated exhaust needed.

How will the exhaust be sent back into the intake and cooled? It would be easy to just bleed exhaust gas out of the exhaust manifold, which operates at high pressure because of the turbo, and throttle it into the intake system right after the air cleaner. It’d then go into the turbo’s compressor, through the aftercooler, and end up in the engine. But exhaust contains sulfuric and other acids because of the sulfur in diesel fuel and incomplete combustion. Acids are OK if they remain vaporized, but if they cool, liquefy and condense out on engine metal parts, corrosion problems quickly result. The air-to-air aftercooler would drop the temperature of that recirculated exhaust so much that the acids would attack the engine.

The ideal cooling medium turns out to be hot engine coolant. So engineers have designed coolers that use jacket water and reduce the temperature of the exhaust to only about 250 degrees because engine coolant is kept warm by the cooling system thermostat. So the heavy truck diesel’s EGR system will have a heat exchanger that carries heat from very hot exhaust gas to warm engine coolant, to avoid overcooling the recirculated exhaust. The heat exchanger will be made of stainless steel so the potentially corrosive exhaust won’t corrode it or its external housing. This means the engine cooling system will have more heat to carry away – at least 25 percent more by one estimate.

The fact that the exhaust cannot be introduced into the intake airflow before the turbocharger poses a real dilemma. An efficient turbo generates intake manifold pressure that’s well above the pressure in the exhaust manifold. The exhaust needs to be forced into the highly pressurized intake manifold. The answer for three of the four EGR users proved to be an existing technology called variable-geometry turbocharging.

Cummins’ EGR system uses a venturi-mixer, mounted on the right side of this ISX engine. The mixer is the red casting located under the black air cleaner. Air from the aftercooler comes in from the left, mixing with exhaust coming in through the insulated tube that connects at the bottom of the unit. The small pipe connected to the top of the venturi-mixer supplies exhaust-free compressed air to the brake system air compressor.

How VGT works
All turbos have a nozzle that throttles the gases coming out of the exhaust manifold and uses that restriction to create a high-velocity stream. Those fast-moving gases collide with the turbine blades and spin the turbo.

The size of that nozzle is a big factor in matching the size of the turbo to the engine. A variable-geometry turbo allows the size of the nozzle to be varied so the turbo match can be changed by the engine’s electronic control module.

There are two types of variable nozzles. One is the sliding type used by Holset and applied to Cummins’ EGR system. The exhaust passes through a ring-shaped passage on the way to the turbocharger drive turbine, and a sliding collar allows the width of that passage to be varied as necessary. With the three other manufacturers, the nozzle consists of rotating vanes arranged in a circle. The vanes are tied together with a rotating mechanism, and rotate uniformly to form wider or narrower openings as the engine ECM calls for changes in the size of the exhaust nozzle.

The purpose of the variable-geometry turbo is to slightly cut down the size of the exhaust nozzle. This will raise the backpressure in the exhaust manifold until it is higher than the intake manifold pressure, forcing the exhaust from the exhaust manifold around and into the intake manifold. And there’s a lucky byproduct: The engine’s ECM can use the variable-geometry nozzle to get rid of turbo lag. The ECM will suddenly open the exhaust nozzle a controlled amount when the driver stomps on the throttle, resulting in almost instant response. As Detroit Diesel’s John Morelli, vice president of the Series 60 2002 engine program, says, “Drivers will see a side benefit. The variable-nozzle turbo will increase boost pressure more rapidly at sudden acceleration.”

Because EGR increases the amount of certain acids in the engine, and may slightly increase oil sooting, there will be a new API CI-4 oil standard to preserve oil change intervals.

The last links
We now have a variable-geometry turbo, a heat exchanger and piping. The piping runs from the exhaust manifold, through the heat exchanger, and around the front or rear of the engine to the intake manifold. It connects there through a mixing device that speeds up the airflow a little like an old-fashioned carburetor. This guarantees the exhaust will mix evenly with the intake air and provides equal amounts to all cylinders.

The exhaust flow needs to be shut off at times, as when the engine coolant is cold and the recirculated exhaust would condense inside the cooler. Enter the EGR valve, a device designed to shut the exhaust out of the engine or, in some systems, help fine-tune the flow. Most will be positioned on the downstream side of the exhaust gas cooler, on or near the exhaust manifold. Some will be on-off valves, and some will throttle exhaust flow (Volvo will use both on-off and throttling EGR valves).

The final problem is measuring the amount of EGR and fine-tuning its flow. Putting the right amount of exhaust in is critical. Too little exhaust would mean NOx, while too much would mean particulate, and maybe even misfire. So there will be a tiny, fixed restriction near where the exhaust flows into the mixer. Pressure will be measured on either side of the restriction and sent to the ECM. Since the pressure drop always changes as the flow changes, the engine computer knows how much exhaust the engine is getting by knowing the pressure drop.

The ECM will then fine-tune the exhaust flow by changing the position of the turbo’s exhaust nozzle and, depending on the system, throttling with the EGR valve or valves, in what Morelli calls a closed-loop process.

Here are the specifics of each engine manufacturer’s system:

The Caterpillar Engine Division will meet 2002 specs with a new combustion system called ACERT, or Advanced Combustion Emissions Reduction Technology. Although the system will require an oxidizing catalytic converter that Cat has already used on medium-duty trucks to eliminate small amounts of particulate and unburnt fuel, it will meet the NOx reduction requirement without EGR. John Campbell, Cat’s director of truck engine products, says, “The simplicity of the system will mean the transition from current engines to ACERT-equipped engines will be almost unnoticeable to the customer.” There will be no EGR cooler mounted on the engine, meaning less hardware under the hood and almost no increase in heat rejection (2 to 3 percent according to Campbell).

At the heart of this is a new-generation Hydraulic Electronic Unit Injector (HEUI) system. The camshaft on the C15/C16 engines will move back down into the block and operate only the valves. A high-pressure oil pump will provide hydraulic pressure via the engine lube oil to operate the injectors. The engine ECM will employ ADEM 2000 electronics with twice the computing power used on present engines. It will stop and start fuel delivery in each injector by starting and interrupting the flow of high-pressure oil that drives each of them. The HEUI system will also operate an enhanced engine brake.

Camshaft-operated injectors lose the ability to produce extreme pressure as the engine lugs down. The advantage of the HEUI system is that the pressure is always there and can be adjusted by the ECM to meet the exact requirement for each operating speed. The piston bowl will be redesigned. The injectors will provide a small shot of fuel, or pilot injection, before the end of the compression stroke. This fuel doesn’t make a lot of heat, but sets the stage so that when the main injection of fuel starts, burning occurs without delay. This all but eliminates the rapid rise in cylinder pressure, which gives diesels their characteristic sound and, as it turns out, also makes a lot of NOx. Along with refinements to engine blocks, Cat says the system will reduce engine noise by 50 percent! While Cat won’t reveal a lot of detail, it’s likely that the system will burn the main charge of fuel faster, but starting a little later than normal – after the piston is on its way down. Cat says fuel economy will equal that of present engines.

Such major changes mean ACERT engines meeting the standard won’t actually be on the market until production is ramped up from April until October 2003. To cover the months in between, Caterpillar says it will cash in emissions credits covered by agreements already in place, and make slight modifications to existing engines.

Cummins is convinced that jacket water-cooled EGR is the best way to meet the 2002 standards. Cummins’ vice president and chief technical officer, John Wall, believes cooled EGR has the least effect on fuel economy while reducing NOx, because using EGR allows combustion to occur at an ideal timing.

More efficient radiators and more powerful fans shed the added heat with no change in hoodline. More efficient water pumps 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 the previously mentioned Holset turbo with a sliding collar to control the size of the turbine inlet nozzle. The turbo features water-cooling and titanium turbine blades. Cummins’ EGR valve cooperates with the variable nozzle to fine-tune EGR flow, though it also shuts off during cold operation. The ECM monitors turbo rpm and constantly adjusts the variable nozzle to produce lightning-fast throttle response (drivers say the turbo sometimes speeds up when least expected to). The ISX engine already has a soft start of injection and shapes the rate 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 2002 engines will need API CI-4 oil. Oil change intervals are still being determined, but will likely not be significantly shorter. Cummins will still offer the Centinel 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 and into the cooler. Exhaust gas passes out of the cooler and around the front of the engine to the intake manifold in the chrome pipe.

Detroit Diesel
John Morelli says that while there have been detail changes in nozzle spray pattern and piston bowl design, Detroit will stick with its present injection system and “doesn’t see any benefit” in going to pilot injection or other engine changes. It will use jacket water-cooled EGR on Series 60 diesels for 2002. It’s no wonder – the company has experience with smaller versions of the basic components of the system, having used it on the year 2000 Series 50-B bus engines. The picture is not bad, according to Morelli. “We had been achieving lower NOx via timing retard. We find that when you introduce cooled exhaust gas, you end up being able to advance injection timing. With advanced timing, the combustion stays in the bowl. There’s an advantage because it reduces sooting of the cylinder liners. 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,” he says.

Final details are still being worked out. Morelli is not sure whether Detroit will actually control flow with the EGR valve, or just use backpressure. The valve may be used to shut off EGR and minimize cold smoke or protect the engine from high heat, depending upon EPA input. He believes fuel economy will likely be at least as good as with present engines. Oil and filter change intervals should not be affected, though the new CI-4 oil is required.

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, pilot injection, will be implemented 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. Most of the change will be in software to open and close the fueling valves in two separate cycles. Detail changes in the valves, to help dampen pressure pulses in the lines and improve their responsiveness to electronic commands, will be the only injection system hardware changes. Pilot-injected engines will be noticeably quieter.

Jacket water-cooled EGR will come along this year and will use an on-off EGR valve so EGR can be cut off when appropriate. 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 variable-geometry turbo. Additional heat rejection will be handled by putting the engine’s radiator in front of the air-to-air aftercooler, a unique design that makes the radiator work better, maintains air-to-air performance and conforms more easily to the sloping hoodline of aero tractors like the Vision. Higher-capacity fans and drives will pull more air through to complete the package.

Responsiveness will be as good as on the 460 with the new wastegate turbo, and will be improve slightly over earlier models. Governed 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 go up seven quarts to help avoid that. The company’s goal is to maintain the interval. The new CI-4 oil will be required.

Volvo will be using 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 variable turbo nozzle and the increase in backpressure. This, says Bio, should mean good things for fuel economy as opposed to raising backpressure.

Naturally, such a system will use throttling-type EGR valves, one in each circuit, to fine-tune flow. An on-off EGR valve located over on the intake manifold side of the engine will stop EGR flow when necessary.

But this is only part of the story. Volvo has also redesigned the fuel control valves in its injectors to allow them to throttle fuel flow rather than just cutting on and off. This enables the injectors to shape the rate at which fuel flows in, keeping it at a low level until after ignition occurs, softening initial pressure rise. This will lower peak cylinder pressure and help with NOx. The control valves also cut off more abruptly when it’s time for injection to end, reducing unburnt fuel. These changes demand a more powerful engine ECM.

Volvo will also require use of the new CI-4 oil, but users can stick with present intervals.

The bottom line
All the engine manufacturers say all their ratings will be preserved. With the new oil and improved filters, oil change intervals should see little change, if any. The manufacturers say their EGR valves and related equipment will not require routine maintenance. Bio recommends a little more care in cleaning radiator fins and ensuring coolant is treated and flushed as recommended – smart practice with all EGR engines. The 2002 changes should not create nightmares or greatly increase costs; in fact, changes in engine noise and responsiveness should please drivers. What’s not to like?

API CI-4 – Oil for Diesels with EGR

With exhaust gas – as much as 15 to 20 percent – being forced back into the cylinder in most 2002 engines, there are a number of new stresses on the engine oil. One threat is corrosion from the increased amount of acid (formed from the sulfur in fuel). In earlier engines, the acids tended to go out the exhaust pipe, but some are now kept in the engine because of the exhaust that goes back in. So the oil will need to neutralize more acid.

With as much as 25 to 35 percent more heat passing into the coolant, oil temperature will increase. Even though injection timing will likely be advanced, with lower oxygen concentrations in the cylinder, increased sooting of the oil remains likely.

The result of this increased stress on oil is the new CI-4 standard, known as PC-9 (Proposed Category-9) during its development. The standard, finalized on Oct. 16, 2001, consists of 15 tests, eight of them performed in engines under operation, and five on a bench.

The battery of tests includes two that measure wear in various parts of the engine. The Cummins M11 EGR test measures pressure drop across the oil filter (a check on the oil’s ability to keep soot suspended so it won’t clog the filter), and wear at the injector screw and rocker bridge, especially sensitive parts. This test includes abuse – operating with retarded timing and low coolant temperatures at first to load the oil up with soot, then advanced timing and low coolant temperatures to accelerate wear. The Mack T-10 test includes running a low coolant temperature and retarded timing to get the oil loaded up with 5 percent soot, followed by 225 hours at peak torque with advanced injection timing and normal coolant temperatures to put maximum stress on the engine.

Other tests ensure the oil will pump with 5 percent soot in it at low temperatures, check for its ability to prevent deposits even on very hot two-piece pistons with steel crowns, and measure the ability of the oil to protect critical roller cam followers at 5 percent soot levels. There’s even a test for control of the foaming that occurs because of the high-pressure pump in the HEUI injection system.

Jim McGeehan, Chevron Lubricants’ manager of engine oil technology, also heads the ASTM heavy-duty-oil classification panel. He says, “We have raised the quality of the oil. It will be better for existing and older engines. It will provide extended drain intervals for EGR engines. Although it may not allow 50,000-mile changes, I don’t believe change intervals will be significantly less than before.”

For more information, contact the following:

Caterpillar Engine Division
Tel. (800) 447-4986

Cummins, Inc.
Tel. (800) 343-7357

Detroit Diesel Corp.
Tel. (313) 592-5000

Mack Trucks, Inc.
Tel. (610) 709-3011

Volvo Trucks North America
Tel. (336) 393-2000

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