Fundamentals of Compression-Ignition Engines (Part 2)
A big advantage of CI engines over gasoline spark-ignited is thermal efficiency, which is due to the higher compression ratio. Highway diesels today achieve peak efficiencies on the order of 35-45 percent, whereas a gasoline engine will do well to achieve 25 percent (and that is only reached at wide-open throttle with lowest pumping losses). The very largest CI engines (ship propulsion tall “cathedral” diesels with up to 115,000 hp, turning at 60-200 RPM [direct drive to the propeller, with reversible rotation] and burning heavy bunker C fuel oil) can actually exceed 50 percent thermal efficiency. One byproduct of the high thermal efficiency of CI engines is a lower exhaust gas temperature (EGT) compared to spark-ignited gasoline engines (thus, less waste heat).
Besides the thermal efficiency advantage over gasoline spark-ignited engines, CI engines also attain highway fuel economy (mpg) benefits by burning heavier fuels with higher volumetric energy density (more BTUs or calories per gallon or liter). That superior fuel economy can yield lower CO2 emissions per mile or km compared to gasoline, but diesels produce more CO2 per gallon or liter of fuel (because the fuel is so carbon-rich). An additional efficiency gain is due to lower air pumping losses: diesels are designed to run unthrottled, unlike gasoline engines.
CI engines, despite advantages in efficiency, durability and torque delivery do have a major downside regarding excessive exhaust emissions, which are overly rich in carbon soot and nitrous oxides (NOx). The highway diesel engine emissions regulations (such as the US EPA targets for 2010 and beyond, are among the toughest in the world) have dominated engine industry engineering efforts for many years. Rather costly measures are required to render CI engine exhaust as clean as gasoline engines. There are two main technical paths to deal with emissions: within the combustion chamber and via downstream exhaust aftertreatments. Manipulation of fuel injection can be helpful, by: raising injection pressures (finer atomization), retarding timing of injection (which reduces NOx formation but raises soot output), introducing multiple injection events per combustion cycle (3-7 injections: pre-injections, main injections, post-injections), and adjusting spray patterns (via injector exit hole size/number—typically 4-12). The rich soot output can be handled by a downstream diesel particulate filter (DPF), periodically regenerated (or burned off) as back pressure rises due to soot loading. A well-timed periodic late injection, enrichening the exhaust with unburned fuel can initiate the DPF regeneration (exothermic) process.
Compression ratios can be reduced slightly to bring down both combustion temperatures and NOx formation, and that has been a recent trend. A very key strategy for NOx control at the source 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 there is not too high), which dilutes and cools the combustion charge, albeit at the expense of power output (exhaust gas displaces air/oxygen) 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 external cooled EGR.
To get back fuel economy, most highway diesel producers today reduce extreme flow rates of cooled external EGR, and apply selective catalytic reduction (SCR) aftertreatments, which is up to 90 percent effective in reducing NOx. However, both strategies are still needed to meet the most stringent NOx regulations. SCR requires injection of an aqueous urea solution (carried on the vehicle, temperature controlled to forestall freezing), which typically flows at the rate of 1-3 percent of fuel consumption. With the aid of a catalyst, the urea solution converts to nitrogen-rich ammonia gas, and the subsequent chemical reactions ultimately yield harmless gaseous oxygen and nitrogen. To guarantee zero ammonia slip out to the atmosphere, a downstream catalytic converter may be needed. One other exhaust aftertreatment is standard today for highway CI engines, just ahead of the DPF: the diesel oxidation catalyst (DOC) converter. The DOC converter, typically contains Pt or Pd catalysts in small amounts on a porous substrate that oxidizes any unburned hydrocarbons, CO (which is converted to CO2), and some particulates.
It is worth mentioning the Miller modification to the diesel engine as another means to reduce NOx. It was invented in the 1940s by Ralph Miller in the U.S. and first applied to large-bore 4-stroke supercharged diesels in marine and stationary applications to improve BMEP, power and efficiency (not emissions). It is likely part of Caterpillar’s ACERT diesel emissions reduction strategy, first on-highway and now off-highway. The Miller principle for CAT ACERT diesels involves late closing of the intake valve resulting in part of the intake air charge being expelled. The results in the subsequent compression and power stroke is a reduction of air pressure and temperature that yields reduced air compression and combustion temperatures and thus lower NOx formation on the order of 10 to 30% at full load. However, less air will be compressed than in a normal diesel cycle. So, to restore the lost power, intake air manifold pressure must be raised by aggressive turbo charging. In some cases 2-stage turbocharging is applied (as seen in Cat ACERT and MAN diesels) with overall air pressure ratios up to 7:1.The intake valve timing may be varied with load to optimize results.
Highly stressed HD diesel engines 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.
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.