The makers set the fuel injection timing to the optimum, so that combustion starts fractionally before TDC.
If the the fuel injection timing is advanced, combustion starts earlier, and the the pressure is rising due to combustion AT THE SAME TIME AS THE PISTON IS STILL COMING UP ON THE COMPRESSION STROKE.
Some people misguidedly believe this will lower the exhaust temperature, but that would only happen if the quantity of fuel injected stayed the same.
If some of the energy in the fuel is being wasted opposing the rotation of the engine(i.e. burning during the compression stroke) then extra fuel has to be burnt during the power stroke to keep the engine RPM/Power constant.
If the timing is early, extra fuel is being burnt during both the compression and power stroke to give the same power output, and the engine will be burning far more fuel with higher peak temperatures and exhaust temperatures and a higher volume of exhaust gas being produced than should be for the power output. There will be more energy in the exhaust gas so TC revs will be higher and so will charge air pressure.
If the timing is retarded, combustion starts later, when the piston has already started to travel down the power stroke. Pressure that is available at TDC is expanded, doing work on the piston, until the exhaust valves open and is therefore used most efficiently. Less of the available energy will be used from fuel burnt later in the stroke, so the conversion of Chemical energy to power is less efficient requiring more fuel to do the same work.
In this case peak pressures won't be higher, but exhaust temperatures will be much higher and again there will be more energy available in the exhaust gas so TC revs and charge air pressure will be up.
The theoretical combustion cycle that I was taught when doing my tickets was the "Dual Cycle", initial combustion at "Constant Volume" i.e when the piston is instantaneously stopped at TDC, followed by combustion at "constant Pressure" i.e sufficient fuel is injected to maintain the peak pressure until the desired power outpur/RPM is reached.
The Peak pressure is limited by the mechanical strength of the machinery, and combustion at Constant Volume would only deliver a tiny amount of power, in a practical engine we have to use combustion at constant pressure to "fatten" the power card and produce useful amounts of power, and give good power to weight ratios. In the compromise we accept slightly lower thermal efficiencies.
When I sailed on the Orient Express they had just fitted a blender for the Gen sets instead of running on MDO, the Third decided that running on Blended FO would delay ignition and decided on his own initiative to advance the injection timing on all the gen sets, without telling anyone. As a result we had very high exhaust temperatures. Once we overhauled all the injectors and put the timing backl to the maker's value the engines ran fine on Blended FO.
On the ship with the Pielstick Engines, (Leona 3, ex Lumiere) we limped from Lagos to Malta Drydock and all the cylinder relief valves had to be overhauled for Class survey and the yard reported that one engines valves were all choked solid with carbon. When we refitted all the valves to the engines and ran them up we found all the relief valves on one engne were lifting with the engine clutched out and running at idling RPM. When we checked the timing that was the engine with advanced fuel injection. There are lots of Engineers out there that shouldn't be let loose without a minder, and the problem appears to be getting worse.
That is why I am a great believer in this site as it gives an opportunity for those who want to learn to get a full picture, from people who know what they are talking about.
I hope all this helps you solve the problem.
Last edited by Big Pete
on Tue May 29, 2012 9:32 pm, edited 1 time in total.
It is always better to ask a stupid question than to do a stupid thing.