Diesel Engine Emissions Controls
Diesel engines have high efficiency, high torque, and outstanding longevity. Not so favorable is excessive exhaust emission , especially regarding carbon soot particulates and NOx. Thus, measures are needed to meet ever-tightening emissions regulations that are being enforced in developed countries. Certain measures can be taken internally to minimize ?engine out? emissions, however, additional added external controls are now required to meet the most stringent emissions regulations.. Diesel emissions controls raise purchase cost, tend to reduce fuel economy, and present new maintenance burdens. We will touch briefly on the key controls strategies below.
There are two main technical paths to deal with diesel emissions: first within the combustion chamber and second via a downstream exhaust after-treatment system.. Manipulation of fuel injection can be helpful by raising injection pressures to the 2,000-2,500 bar class (yielding finer atomization), by retarding the injection timing (which reduces NOx formation but raises soot output), by introducing multiple injection events per combustion cycle (3-7 injections: pre-injections, main injections, post-injections), and by adjusting spray patterns (via injector exit hole size/number?typically 4-12 holes).
The subsequent rich soot exhaust t can be trapped by a downstream diesel particulate filter (DPF), which is periodically regenerated (or burned off) by adding diesel fuel into the exhaust as backpressure rises due to soot loading. The wall-flow DPF has a porous honeycomb ceramic substrate (such as SiC) that is coated with precious metal catalysts and tightly packaged in a can. When excess backpressure is sensed, a periodic late diesel fuel injection (perhaps every few hours) that enriches the exhaust with unburned fuel can initiate the DPF regeneration (exothermic) process. However, thermal runaway that can destroy the DPF must be avoided. Active regeneration control strategies (involving heaters, sensors, and smart electronics) are preferred over passive strategies for highway truck diesels that may experience highly variable loads (at low loads/idling, the catalysts may not be hot enough to light off). After perhaps 200,000 miles, the DPF is removed, opened, and the inorganic ash is cleaned out.
In-cylinder control measures include slightly reducing compression ratios to bring down both combustion temperatures and NOx formation. This has been a recent trend. A key strategy for NOx control is external exhaust gas recirculation (EGR). A portion of the pressurized exhaust is diverted, cooled, and introduced to the intake manifold (provided that the intake air pressure is not too high), which dilutes and cools the combustion charge, albeit at the expense of power output (exhaust gas displaces the oxygen/air) and reduced fuel economy.
Internal EGR is also available to engine designers, which is attained through clever variable valve timing. This allows some exhaust gas to be retained in the combustion chamber and carried over to the intake stroke. Internal EGR is much more common in the gasoline engine community than in the CI engine world today because internal EGR is not cooled and, thus, much less effective than cooled external EGR for diesel NOx control.
To retain fuel economy, most highway diesel producers today reduce extreme flow rates of cooled external EGR and apply selective catalytic reduction (SCR) after-teatment, which is up to 90 or 95 percent effective in reducing NOx. However, both strategies (EGR + SCR) are still needed to meet the most stringent NOx regulations of 2010 and beyond. SCR involves injection of an aqueous urea solution (carried on the vehicle and temperature controlled to forestall freezing), which typically flows at the rate of one to three percent of fuel consumption. The urea solution sprayed into a reactor, transforms (with the aid of a catalyst, such as nickel) to nitrogen-rich ammonia gas. The subsequent chemical reactions ultimately yield harmless nitrogen gas and water vapor. To guarantee zero ammonia slip out to the atmosphere, a downstream catalytic converter may be needed. Another technology competing with SCR is the catalytic lean NOx trap (LNT). Diesel fuel (via periodic rich exhaust) is used as a reductant for trap regeneration (which is frequent). As a result, fuel economy suffers. LNTs are mainly applied to light diesels and not medium-heavy diesel.
The diesel oxidation catalyst (DOC) can is one other exhaust after-treatment that is standard today for highway diesel engines, placed just ahead of the DPF can. The DOC catalytic converter typically contains Pt or Pd catalysts in small amounts on a highly porous glass-ceramic (cordierite) substrate that oxidizes any unburned hydrocarbons, CO (which is converted to CO2), and any soluble organics. See the example below of DOC + DPF for the Duramax 6.6 liter V-8 diesel used in medium-duty trucks prior to 2010. Subsequently, SCR has been added.
The Miller modification to the diesel engine is another means of reducing NOx and is believed to be used by Caterpillar for its ACERT diesel. The Miller principle for diesels involves early closing of the intake valve (via variable valve actuation technology), before bottom dead center (BDC). Expanding the volume of a fixed mass of air trapped in the cylinder (piston drops while intake valve closed) reduces both the pressure and temperature of the air (as dictated by the ideal gas law). That yields benefits in the subsequent compression and power strokes, reducing air compression heat, combustion temperature, and, thus, NOx formation on the order of 10 to 30 percent at full load. However, less air will be compressed than in a normal diesel cycle so intake air manifold pressure must be raised (as via 2-stage turbocharging). The intake valve timing may be varied with the load to optimize results.
Highly stressed HD diesel engines loaded with emissions controls have very demanding lubrication oil requirements. Lubrizol additives in that oil can help mitigate soot-related wear and viscosity increases while allowing the OEMs to meet stringent emissions regulations. Lubrizol advanced dispersant systems keep carbon soot particles separated in suspension, thus limiting abrasive wear. Lubrizol detergents and anti-wear chemistry control wear and deposits in both the modern and heritage engines. Low sulfated ash, phophosrus, and sulfur (SAPS) heavy duty diesel engine oils protect modern emissions system.
Selective catalytic reduction (SCR) is a widely popular means to reduce harmful smog-forming oxides of nitrogen (NOx) in diesel engine exhaust. NOx is formed at the high-temperature flame front commonly found in compression ignition engines. Nitrogen and oxygen are natural constituents of the engine intake air, but given sufficient heat and time will transform into unwelcome compounds (such as nitrous oxide (NO), and nitrous dioxide (NO2) the most prevalent). SCR is a well-proven technology known to reduce NOx by more than 90%. Click here to read more about SCR.