Technology is changing in a very rapid speed. Many of us are not aware of the things currently going on. Here i want to break some myths of idling.
Diesel Engines :
The diesel engine named after Rudolf Diesel, is an internal combustion engine in which ignition of the fuel is caused by the elevated temperature of the air in the cylinder due to the mechanical compression . This contrasts with spark-ignition engines such as a petrol engine, which use a spark plug to ignite an air-fuel mixture.
Diesel engines work by compressing only the air. This increases the air temperature inside the cylinder to such a high degree that atomised diesel fuel injected into the combustion chamber ignites spontaneously. With the fuel being injected into the air just before combustion, the dispersion of the fuel is uneven; this is called a heterogeneous air-fuel mixture. The torque a diesel engine produces is controlled by manipulating the air-fuel ratio (λ); instead of throttling the intake air, the diesel engine relies on altering the amount of fuel that is injected, and the air-fuel ratio is usually high.
It is good to idle your engine in the morning or whenever car is turned on.
With the modernization of engines it is no longer advisable to let your engine in idle position for more than 3 minutes. It can cause more wear and tear to engine than driving it on a highway.
I know many people will argue on this so a link is given below of Volkswagen .You can clear your doubts there.