Clean Sweep

| February 01, 2002

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.

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.

Clean Sweep

| February 01, 2002

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.

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.

Comments are closed.