What is the temperature the grid heaters are trying to get the intake air up to. I thought that cold dense air is good for a combustion, but maby thats only in a warm engine block. just wonding. I am sure glad to not have glow plugs to go bad.

 

Diesel engines basically don't rely on a spark to ignite the fuel air mixture. The process is diesel engines 101. As the air is drawn in, either with the turbo pressurizing the cylinder chamber or naturally aspirated, the engine rotates and as the valves are closed on the compression stroke, it compresses the air and somewhere close to peak cylinder pressure injects the fuel oil into the cylinder. The increased heat from compression causes the fuel to ignite. The grid heaters only help the incoming air to raise in temp. The more air flow (higher RPM) the less the incoming air will heat and the lower RPM, less air flow velocity, the longer the air has to heat and thus raise the "critical combustion temp" in the cylinder.
So...
If the air temp is really down, like 40 degrees and F and below, it gets harder and harder to develop "ignition heat" in the cylinder chamber because it can't compress it any more. Dense or not, it takes heat to ignite diesel since there's no spark. The grid heaters just raise the initial combustion cylinder air temp to better raise the cobustion air temp to help ignite the diesel when it's sprayed or injected into the cylinder. This helps the engine get to operating temp quicker and thus save wear & tear and fuel as well.
Hope this basic explanation helps.