DefJeff, Gord and a few others here have given you very good answers. Go take a good long look at some of the links posted. I also suggest you take a look at the site "how stuff works" and look at the deisel engine.
To quickly explain about the load a fuel dose relationship however. With a Petrol engine, the throttle governs the amount of Air entering the engine. Carburators work by that air volume sucking fuel in, thus controlling engine speed and power. Modern day engines inject the fuel in and the air and fuel ratio is carefully controlled by the CPU. This is becuase a Petrol engine has a very fine balance between lean and rich fuel mixtures and an extreme either way is harmful to the engine.
A Deisel engine is compleatly different. It has a constant Air supply. The volume of Air is governed purely and simply by the speed of the piston taking in a gulp of fresh air. The speed and power is derived by controlling the fuel delivered. So the throttle in this instance is controlling the fuel pump
and allowing a set amount of fuel to be delivered to each cylinder. OK, so now we have to controll the speed and power. This is simply done by a part in the fuel pump called a Governor. It is actually the Gonernr that is being changed by the throttle. But once the throttle has set the govenor to a postion, the engine RPM then continues to change the Governor, so as the two settle to a constant RPM. If a load is applied to the engine, the govenor alters and opens the fuel rack and allows more fuel to be delivered to the engine and it's speed picks up to maintain RPM.. Load comes off and the reverse happens.
The answers to the "engine under load" argument have been given to you in great detail. They are NOT MYTHS! To not head
them is at the peril of your engine. Most all engines you see started from cold in your marina, that have the large amounts of blue smoke issuing from them, will most likely be as a result of people that allow their engines to warm up under idle or no load. The replies given so far have been extremely accurate. There is one other simpler answer. Back to the Petrol engine, Gasoline is very "dry". It has absolutely NO lubricating properties at all. So the the cyclinders of the petrol engine do not easily lubricate at cold, untill the engine oil
is splashed up around the piston. The other detrimental thing about petrol engines is the heat during combustion is very much hotter than Deisel. So valve seats and other direct contact areas with teh extreme heat must be heated to operating temp much slower. Otherwise they crack.
Desiel does not have these issue to the same extent. Deisel fuel is much cooler and slower burning. Infact it is very poor at burning and requires a lot of compression heat to cause it to burn efficiently through out it's power stoke cycle. This is partly where the load issue comes in, but only partly. The main issue is not producing the heat, but simply getting the engine warm as fast as we can. the hotter the engine, the more efficiently the fuel burns and the less carbon remains in the burn after the combustion cycle. Desiels are high compression, heavy lumps of very high tolerance peices of metal moveing at high speed. No1 impotance on the list is no leakage past rings. This is partly maintained by wear. Unlike a petrol engine at cold start up, a Deisel is actually required to have a little wear at start up. It keeps the rings bedded into the Hard metal bores. If they don't, the high lubricating properies of the Deisel fuel cause the ring to skate on the surface of the bore and the bore will become contaminated and will glaze. This causes the sump oil
to mix into the combustion and create the blue smoke you see from engines.
I am not going to go over others posts. Just add whatr they have said into this and I think you should get a clear picture.
I recomend however, that you follow the Start up procedure of your engine manufacturer, should it be given in the manual. This is very important, as some designs have very specific things taking place to allow the engine to start. For example, one real quirky engine, is the old Volvo
2000 series. You need to pull and release the engine stop before you fire it up. It releases a special "lock" inside the pump that allows an extra dose of fuel to help the engine to cold start. As soon as the engine starts, this "lock" goes back to normal postion. Anyway,
as soon as the engine is started, give it a minute to allow oil pressure to come up and then place it into gear
. this places more load on the engine than a compressor
will. Of course, it also depends on size of engine. A compressor
can take about 5hp. To a 10hp engine, that is 50% of it's power, but to a 50hp engine, it is of little consequence.