Went thru this several years ago with engines running fuels from distillate to 240cst HFO. CAT, Mak, Alstrom and a couple Wartsila units from 3 to 8 MW. All were gensets, island and grid paralleled units.
In general timing retard resulted in higher fuel consumption, in the HFO engines actual change was hard to track due to batch to batch fuel variations. Most testing was done with a nominal 4 degree retard. In almost all cases timing was returned to original as NOx reductions were lower than expected and increase in CO, PM, visible smoke and fuel consumption high enough to cause concerns. Exhaust port temps in all cases increased, as did turbo speed. In a few cases we experinced turbo surge.
Are you doing this as a result of regulatory requirements? What are your expected results from the changes you're making?
Mike L.
What are the piston changes? Change to compression ratio? Bowl geometry? Are you changing injection nozzles as well? I'd expect with a piston change that a change in spray angle might be needed to get optimum performance.
I am aware of the mechanisms that create NOx, I was just wondering how having HIGHER combustion temps (due to RAISED coolant temps) would increase ignition delay?
I could understand if the idea was to LOWER coolant temps.
TDI, yes, I would have also thought so, but isn't L1489 suggesting that raising coolant jacket temperature (assuming to the max limit of the design) by a few degrees will not only have an affect on ignition delay, but also that it will actually increase it?
This is meant to be completely constructive criticism and not a personal attack on L1489, but I cannot take somebody's advise seriously, who suggests that "anything you do to reduce NOx will reduce fuel efficiency" because that nitrogen in the charge air is itself fuel and did not combine with the oxygen in the same and release its contribution of energy of oxidation doing so. The ppm order of magnitude of NOx emissions of the worst polluting engines guarantees that this exothermic energy contribution is absolutely insignificant within the amount of actual combusted fuel energy.
What's next to reduce charge air temperature - introduce an air conditioner into the airbox or charge air cooler?
cannot take somebody's advise seriously... The ppm order of magnitude of NOx emissions of the worst polluting engines guarantees that this exothermic energy contribution is absolutely insignificant within the amount of actual combusted fuel energy
@Compositepro - you're right, I goofed. Oxidation of nitrogen in air is (very) slightly endothermic; I verified it running GASEQ for only air as the reactant @ 2000 K, 100 bar, adiabatic and constant volume.
According to GASEQ, at these conditions, the mole fractions of NO and NO2 are 0.00747 and 1.24e-04 moles per mole of air, respectively.
However, that air does not spontaneously combust upon itself and that NOx form only at high temperatures are not in and of themselves factors that make reactions endothermic. Spontaneous oxidation of any oxidizeable material is a function of its autoignition temperature (analog of the energy required to reach and exceed the reaction activation energy). Endothermic reactions can and do also take place at- and below ambient temperatures.
"Anything you do to reduce NOx will reduce fuel efficiency"
I would agree with TDIMeister. EGR will reduce NOx without much effect on SFC, until the EGR rate exceeds about 8% to 10%. Exhaust gas is mostly inert (CO2 and nitrogen), and it reduces combustion temps by absorbing heat during combustion while not contributing oxygen to the combustion event.
Intake charge cooling is also effective at reducing NOx without too much of an SFC penalty. Charge cooling reduces NOx by reducing the (T1) temperature at the start of the combustion cycle. The SFC penalty with charge cooling is only due to the heat rejection loss and intake flow losses at the intercooler.
NOx can also be reduced by improving the injection mixing and combustion rate, which would allow a BOI closer to TDC. Higher injection pressures with a greater number of smaller diameter nozzle holes will give a better fuel spray distribution and mixing rate, leading to less ignition delay and more rapid combustion. But such a modification would be expensive.
opsguy says a piston change is planned and this is a land based engine, so I would recommend reducing CR and increasing boost with lots of charge air cooling. This will keep your peak cycle pressures at acceptable levels while decreasing NOx.
Low SMD fuel particles are key to lowering nox with less specific fuel consumption penalty. Often smd's are too high or too poorly distributed and they transport beyond the desired reaction zone into quench layers. Higher coolant temps can reduce some quench but the higher surface temps radiate heat back into the working fluid.. If the chamber surfaces were less efficient at heat transfer then the coolant temps would need to be higher but that is not the case with any engine ecxept for idling diesels.
