Competition General diesel engine timing questions...

[GOT-Torque]

Thinking in very general terms, what all affects the timing of a diesel engine? Are any of the following not true?



1)cylinder pressure

affected by:

-compression ratio (the higher the C. R. , the more cylinder pressure you end up with and the more effective timing you have)

-boost (amount of air compressed into the cylinder)??? not sure on this one ???



2)fuel injection event

affected by:

-timing boxes (boxes cause the inj. pump to inject sooner)

-intake air temperature (read by the ECM and can cause the inj. pump to inject sooner or later) to compensate for the density of the air

-injector size (as the holes get bigger, more fuel reaches the cylinder sooner in the injection event, and the more timing you end up with)



In my quest for more power I have progressively gone larger and larger on injector sizes. With each step, the quantity of fuel delivered increases. Also, the effective timing of the engine increases as more fuel is delivered sooner in the injection event that the previous set of injectors.



Will a camshaft change affect the timing in any way? more air in the cylinders...



What options do I have for reducing engine timing besides reducing the compression ratio?

 

[fest3er]

Reasonably pondered. Compression ratio and boost both affect ignition; raising either one will cause the charge air to reach ignition temp sooner. Lower either and ignition will be retarded.

In a sentence, ignition begins to occur when the charge air reaches the stable ignition temperature. That is, it might reach the temp, but some of the fuel evaporates which reduces the temp. But the surrounding air re-heats the fuel-air mix, causing more fuel to evaporate. But this is small scale and might not affect ignition timing much.

Other things: if you cool your fuel, it will take longer to evaporate, thus retarding ignition. If you increase the injector pop-off pressure, the injector will open later, retarding ignition; but some of this is offset by the higher injection pressure which causes better atomization and vaporization and, thus, advances ignition. If you use a much lower cetane fuel, you will retard your ignition timing, because the fuel is that much more resistant to ignition. A cold engine will scavenge heat from the charge air and delay ignition. A hot engine will heat the charge air and advance ignition. A slow-turning engine will allow it more time to affect the charge air temp, so ignition timing at low RPM can be 'variable'. A fast-turning engine should be less affected by engine temp because the engine has much less time to affect the charge air temp.

Also, be sure to read Mr. Cameron's Exhaust Notes in the latest TDR mag. He points out a certain phenomenon that could well have a significant effect on ignition timing. And vice-versa.

 

[GOT-Torque]

Good info, many many variables affect timing.



So if I install a cam (for example a Helix 2), would it make sense that it will allow more air to enter the cylinders for each combustion event therefore increasing the cylinder pressures and raising timing some more? Or does the additional air come easier (less restrictions) and therefore alittle cooler than before (and leave the timing at about the same spot)?

 

[WUnderwood]

I would think if you installed a Helix cam, the cam would hold the valves open a tad bit longer than a stocker, thus resulting in cooler EGTs, a little bit more power, more air into the motor, etc.

I dont think the cylinder pressures would increase.

just my opinion

 

[NoSeeUm]

One effect of my H2 cam lowers overall boost values. So I wondered about that also.



For some reason, my mind, decided for me that it went the other way. The cam allows more air in due to increased duration open of the intake valves during the intake stroke. That made me think the compression pressure would go up. In effect, allowing more air to make it into the cylinder for the same boost.



Anyone know for sure?



Jim

 

[Timebomb]

With this said, it would be nice to see a spec card/sheet on some of the aftermarket cams showing Lift,Duration,overlap and so on.