At Icon we are privileged to view the engine test results our customers send us on a daily basis. Recently, we have received many messages from chief engineers who are puzzled by the shape of the pressure curves they are seeing. They look back to front! The reason for this is the engine timing has been retarded.
The late firing is implemented to reduce NOx emissions. This is now common and often leads to confusion. In addition, it is now not unusual to have very high boost or scavenge pressures. This is to increase the mass of air in the cylinder and results in high compression pressures which once again reduces NOx emissions. High compression requires late ignition to make sure that the firing pressure doesn’t go too high and damage the cylinder head. On 2-strokes a common limit is 140 bar, and on 4-strokes the limit is often higher but the principle is the same. So, in both these cases, late firing is required.
Most marine operators will be familiar with the ‘classic’ compression-firing-expansion traces as shown in Figure 1 below.
Figure 1: ‘Classic’ Traces
Often two distinct peaks can be seen as on the trace on the first picture. This is typical of a 2-stroke engine with slightly delayed firing. The first peak (Pcomp) is at TDC and the second peak, usually higher and often referred to as Pmax, is a few degrees later. The second trace is typical of a 4-stroke auxiliary engine where earlier firing (~ 7 degrees before TDC) means that the pressure rise is almost continuous past TDC towards Pmax.
In late firing engines, two peaks can again be seen. However, in some cases, the firing peak may be lower than the compression peak and this can be confusing as the graphs can seem to be back to front. They aren’t of course but it does take getting used to. Figure 2 shows a late firing 4-stroke engine with lower firing pressure. The ignition angle is 3 degrees after TDC.
Figure 2: Late Firing 4-stroke
Sometimes firing is closer to the now very high compression peak, and a flatter top to the graphs is the result, and in some cases can be almost flat-topped. Figure 3 below shows a late firing 4-stroke engine (ignition angle is ~ 2 degrees after TDC) where the compression and firing peaks are almost the same, resulting in what appears to be a flat top.
Figure 3: Late Firing 4-stroke
So, if late firing reduces NOx emissions, is this not a good strategy to follow? As is often the case, it is a compromise. The primary downside of late firing is increased fuel consumption i.e. the engine is not operating at its highest efficiency. Late firing can also result in higher exhaust gas temperatures.