I suppose a point to keep in mind is that if you're changing timing by upgrading from a mechanical injection system to an electronic one, you may be able to improve BSFC in parts of the operating range where you don't measure NOx, while improving NOx elsewhere.
Reducing NOx basically requires reducing peak combustion temps. Retarding injection timing from MBT will usually result in lower NOx, but higher HC and SFC.
Charge air cooling and EGR are also effective approaches to reducing NOx.
The tradeoff is about 30:1 NOx vs Fuel economy in g/km. I.e. reduce NOx by 30% from retarding timing alone from a nominal setting and you will increase fuel economy by 1%.
PM will also increase with retarded main injection timing (About 20:1 PM:CO2).
in this case the application seems to be a medium-speed diesel engine burning 180 cst heavy fuel oil. This could be in a marine propulstion, marine aux, or land-based power gen application (not clear from post).
Pilot injection and other strategies for injection rate shaping could also reduce NOx by reduce the peak rate of heat release and pressure rise without the disadvantages of retarded start of injection and EGR.
@Tdi, a common problem with such a strategy is that your end-of-injection timing becomes critical due to oil sooting and piston life concerns. If you then swap injectors to give higher delivery rate, you hurt your smoke & pm.
Changing injection strategy and cut-off ratio will of course affect the NOx and PM outputs as well, but as a rule of thumb those are the tradeoffs when retarding/advancing main SOI whilst leaving separation constant for HSDI engines within that timing range (i.e. around an already optimised combustion).
@70btdc: I don't get the impression from the OP post that the engine in question is either HSDI or in an optimised-state.
@ivymike: That's why proper combustion development and any changes to injection strategy should be only one part of a holistic exercise, which may include reshaping of piston bowl geometry. This latter factor in itself also has a huge influence on the aforementioned NOx/PM/NVH/SFC tradeoffs.
Anything you do to reduce NOx will reduce fuel efficiency. After all N2 combined with oxygen to create NOx releases energy, so N2 in this case is fuel. There are a lot of band-aides to try to fix this but the best they can do is get you back where you started at the cost of making things more complicated. Two things that I found that reduce NOx generation and at the same time improve fuel efficiency is to reduce the charge air/ air box temperature but there is a limit to that as you start getting white smoke. The other thing is to raise the jacket water temperature and this has a ignition delay feature that they are trying to acheive with electronic control. I would like to complete the experiment on this idea. Anybody have a locomotive I can borrow?
L4189, thats very interesting. What is the mechanism to reduce NOx by reducing the temperature gradient? I would have thought that promoting higher temps to be kept within the chamber by reducing the gradient between charge and chamber wall would have had the opposite effect - needing less advance and creating higher peak temps?
TDImeister, being already optimised was an assumption, but if we dont assume some constants at least, the OP question is to broad to answer. I only have experience of HSDI engines, from 1000-2500rpm steady state these tradeoff ratios are correct for a good range of engine displacements.
70btdc,
Advance doesn't directly cause NOx, temperature causes NOx. The rate of the NOx generating reaction is exponential with temperature (as are many reactions). The peak temperature within the chamber virtually defines the amount of NOx created, because all the lower temperatures in the combustion chamber (in time or geometry) do not contribute very much to the total NOx generation.
