The delay affects the pressure and temperature of combustion, which both affect the formation of especially NOx. It also affects the engine efficiency and therefore the amount of all pollutants if not the concentratoin of NOx formed. The most efficient engine has the shortest delay (assuming all else is optimized, including injection point). 

Delay period is one of factor in starting of combustion and hence time availble for complete combustion of fuel. As delay period increases , the time for combustion decreases hence will increase chance for incomplete combustion. Which will result in formation more  carbon particle.

But I believe that a longer delay means that more fuel would have been accumulated and the moment combustion starts a very sharp pressure rise will be expected during this premixed phase ans so cylinder temperature and pressure will be higher than expected. Diesel knock is expected of course but what will be the effect on the different pollutants mentioned above /

There is a difference between delay of combustion (longer combustion) and delay of ignition (later ignition). 

Delay of combustion leads to:

- reduced NOx when reduced peak temperature simultaneously (as it often occurs)

- reduced CO because of increased time for CO2 conversion

- increased particulate matter because of increased time for particles building

- low effect on HC, but can lead to a certain increase of HC

Kind regards,

Mario

Delay of ignition leads to:

- high pressure increase and thus high temperatures: high NOx and CO emissions

- low particulates 

- eventually lower HC

It has to be stated, that the areas of ignition and combustion delay, where the emissions can be improved, are rather small. A positive effect also depends on several other parameters, like kinetic energy in cylinder, injection, etc.

Kind regards,

Mario

Thanks Mario

My question is about the effect of the delay period (which is defined as the time lapse between the start of injection and start of ignition) on pollutants formation inside the combustion chamber of a diesel engine. Do you have experimental evidence for your second answer ? Please attach with my best regards.

Here are some publications, which might be of interest:

http://advtechconsultants.com/EngineTutorial/Diesel%20Engine%20Emissions_Hydrocarbons%20%28HC%29___.pdf

https://como.cheng.cam.ac.uk/preprints/c4e-Preprint-142.pdf

http://papers.sae.org/950213/

http://papers.sae.org/2001-01-0657/

http://papers.sae.org/920470/

http://papers.sae.org/960633/

Kind regards,

Mario