Hi Offshore snipe,
I think a lot depends on the individual engine. When any machinery cools down or heats up all the metal expands or contracts, this obvious causes the clearances in the engine to change, as well as introducing a lot of thermal stress.
Generally if you want a machine to be able to heat up and cool down rapidly it has to built with larger clearances.
For instance Steam Turbines designed for warships had a much greater clearance between the blade tips and the casing, more steam leaked past, reducing the efficiency, but it meant the turbine could be manoevred from stop to full ahead and back almost instantly. Merchant ship turbines had tighter clearances to reduce fuel consumption, but had to take several hours to "warm through" before starting to manoever the ship and then several more hours to build up power from "manoevring" to "Full Sea Speed". No good for a Destroyer Captain!
When you admit steam to a turbine the rotor is totally surrounded by steam and heats up, and expands, very rapidly. The casing only has steam on the insideand is losing heat to the outside and heats up more slowly. Therefore from the time steam is first admitted until the casing has fully warmed through there is a reduced clearance between the blade tips and the casing, with an increased risk of the tips making contact and destroying the entire turbine.
A similar arguement applies to large slow speed diesels, but in this case many of the components have more complicated shapes and are made of much thicker material so the shapes can be distorted as well. Modern CAM/CAD techniques have improved cooling by using "Bore Cooling" and finite element stress analysis has enabled the thermal stresses to be controlled much better than they used to be. So generally newer engines should be capable of heating up and cooling down more rapidly.
Slow speed engines still require the load to be increased gradually from Manoevring Power to full Sea speed to avoid problems.
One of the Classic problems if the engine was allowed to cool down completely in Port, was the relative expansion between the cylinder liners and entablature, which leads to the lIner "O" rings failing, always much better to keep the preheating on if possible.
With Generator sets, including Diesel Electric Plant, the engines can start automatically, go immediatly to full RPM, go straight onto the switchboard and start generating immediatly, as D. Winsor has said.
In this case we are talking about Medium or High speed engines where the components are smaller and thinner and will reach an equilibrium temperature much more quickly, therefore the dimensions and clearances will stabilise much more quickly.
With preheating for this sort of engine I would generally try to preheat the Jacket Water and L.O. inlet temperatures to what they would be when the engine is running.
With regard to running the engine off load to cool it down, I would do this until the exhaust temperatures and Jacket water outlet temperatures have stabilised . If the pistons, liners and heads have been running at high power the combustion chamber surfaces will be very hot. If the engine is stopped the heat will continue to flow into the cooling water and, if only a small preheating pump is running you could boil your jacket water!
Always remember that while taking care not to load or unload the engine too quickly, Diesel Engines like to be run "Hot and Hard", many manufacurers used to forbid running their engines at less than 2/3 rated output, so always keep the running temperatures as the maker's say. I have known some ships where the Engineers thought they would be "kind" to the engines and run them colder than they should be run, and run 2 generators at 40% load rather than one at 80%, it didn't do the engines any good at all!!
Hope this adds to your onboard discussion, good to hear you are talking about engines.
It is always better to ask a stupid question than to do a stupid thing.