The ABC s Of Exhaust Gas Recirculation (EGR)

Transcription

1 The ABC s Of Exhaust Gas Recirculation (EGR)

2 Air Is A Mixture Of Gases, Mostly Nitrogen & Oxygen When air is compressed inside the cylinder of a diesel engine, the temperature of the air is increased enough to ignite diesel fuel after it is injected into the cylinder. When the diesel fuel ignites, the temperature of the air increases to more than 1500 F and the air expands, pushing the piston down and rotating the crankshaft. In General, The Higher The Temperature, The More Efficient The Engine Good Performance Good Fuel Economy 1

3 High Cylinder Temperature Low Cylinder Temperature High NOx Formation Zone But, when peak temperatures are high enough for long enough periods of time, the nitrogen and oxygen in the air combine to form new compounds, primarily NO and NO 2. These are normally referred to collectively as NOx. Too Much NOx In The Atmosphere Contributes To The Production Of Smog. Since 1979, NOx emissions from diesel engines have been regulated by the EPA. In October of 2002, new NOx standards required the diesel engine industry to introduce additional technology to meet the new standards. The EPA has regulated heavyduty diesel engines since the 1970s. The following chart shows the trend to ever-lower emissions. Understanding the details of the chart is not of interest to most truckers. Even though the emissions standards become increasingly more difficult to meet, the diesel engine industry has always been able to continue to improve engine durability, reliability, performance, and fuel economy. A quick look at the bottom right hand side of the chart also shows that emissions from diesel engines built in 2007 and beyond will approach zero. Oxides of Nitrogen (g/bhp-hr) EPA Heavy-Duty Engine Emission Standards Steady State Test NOx (Unregulated) NOx + HC NOx PM (Unregulated) Transient Test NOx PM Model Year CD Transient & + Steady State PM NOx Consent Decree g NOx avg 0.01 g PM g NOx avg 0.01 g PM 290K Useful Life CD Oct-02 Jan K NOx + NMHC Particulate Matter (g/bhp-hr) 2

4 How Can NOx Be Reduced? Since high cylinder temperatures cause NOx, NOx can be reduced by lowering cylinder temperatures. Charge air coolers are already commonly used for this reason. Reduced cylinder temperatures can be achieved in two ways: Retarding ignition timing and/or Reducing the amount of oxygen in the cylinder, which inhibits the combustion process Retarded ignition timing makes the combustion process start at a less-than-optimum point and reduces the efficiency of combustion. This technique lowers cylinder temperature, reducing NOx, But it also reduces fuel economy and performance, and creates excess soot, which results in more frequent oil changes. 3

5 Reducing Oxygen In The Cylinder The best way to reduce NOx is to limit the amount of oxygen in the cylinder. Reduced oxygen results in lower cylinder temperatures. This is done by recirculating some exhaust gas (Exhaust Gas Recirculation) into the cylinder. The effect is as follows: High Cylinder Temperature High NOx Formation Zone aximum Cylinder Temperature with Low Cylinder Temperature Since the whole idea is to reduce cylinder temperatures, it s usually wise to cool the exhaust gas before putting it back into the cylinder. This approach is called Cooled EGR and is the technique used on the Detroit Diesel Series 50 and Series 60 engines. This technique will be used on the MBE 4000 and MBE 900 after 1/1/04. DDC has been using EGR technology on the Series 50 since 2000 and has produced a total of 40,000 Series 60 s with EGR since October EGR is so effective that it allows ignition timing to remain advanced, improving performance and fuel economy and reducing soot. 4

6 How Does EGR Work? EGR Is Required Only Under Certain Conditions. Turbocharger & EGR Valve EGR Transfer Pipe (Hot) When EGR is required, the engine s electronic controls open the EGR valve. The exhaust gases flow through this pipe to the EGR cooler. 5

7 EGR Cooler EGR Transfer Pipe (Cool) The exhaust gases are cooled by water from the truck cooling system. The cooled exhaust gases then flow through the EGR transfer pipe into the intake manifold. 6

8 Benefits Of Variable Geometry Turbocharging (VGT) The turbocharger is equipped with moveable vanes that allow the engines electronic controls to adjust the exhaust pressure in the system, ensuring that there is always enough pressure for sufficient EGR flow. (This type of turbocharger is called a Variable Geometry Turbocharger, or VGT). Boost Pressure (kpa) Boost Pressure With VGT Turbocharger urbocharger pical urbocharger As a side benefit, engine performance is improved and the Jake Brake performance is improved when a VGT is used. 0 Idle Speed Maximum Engine Speed Faster turbo response and quicker acceleration A VGT is to a normal turbocharger as a 21-speed bike is to a single speed bike. 7

9 There Are Other Approaches To EGR Internal EGR (Non-Cooled EGR) This is accomplished by changes in valve timing that allow exhaust gas to simply remain in the cylinder and mix with the incoming air. Internal EGR is inefficient and reduces fuel economy. Reed-Valve Technology This technology does not use a VGT and relies on normal exhaust gas pulses in the exhaust manifold to push exhaust gases through the EGR cooler and into the intake manifold. Reed-Valve is the name of the one-way valve in the EGR transfer pipe that allows exhaust gases to enter the intake manifold, but keeps the incoming fresh air from entering the exit manifold. Simple Reed-Valves Are Used To Control The Flow Of EGR How A Reed-Valve Works The Reed-Valve is a one-way door that allows exhaust gas to flow into the intake manifold but prevents air in the intake manifold from flowing back into the exhaust system. Pressure peaks in the exhaust manifold open the Reed-Valves. Reed-Valve Open Reed-Valve Closed 8

10 2004 Approach To EGR Comparison Cooled EGR VGT Reed Internal EGR Other DDC Series 60 MBE CAT Cummins Volvo Mack (On-Highway) (On-Highway) (Off-Highway) Reed Lower Cost, Lighter Weight (Turbo Brake) VGT Better Control, Better Response, Better Braking, Above 450 HP, Higher EGR Rates, Altitude, More Flexibility to Adapt to Custom Ratings. 9

11 Conclusions EGR is Simple Proven Reliable Fuel Efficient Reed-Valve Simple, Cost Effective VGT More Adaptable Improves Driveability Improves Engine Brake Performance DDC EGR Engines Carry The Same Warranty And Have The Same Maintenance Intervals As Non-EGR Engines. 10

12 13400 Outer Drive, West, Detroit, Michigan Telephone Copyright 2003 Detroit Diesel Corporation. All rights reserved. Detroit Diesel, the spinning arrows design, DDC, DDEC and Series 60 are registered trademarks of Detroit Diesel Corporation. 06SA As technical advancements continue, specifications may change. Printed in U.S.A.