To Compress or Not Compress?

i usually don't need more than about 10 seconds of compression to start my d4 in cold weather. after cranking just until i get oil pressure.

compressed gases always are warmer than uncompressed---simple thermodynamics where the work done energy is transformed into heat energy--PV=NRT

The workload on the pony motor increases when I flip the decompression off/run, so that convinces me that more heat is being produced not only in the diesel cylinders from the heat of compression, but also going out the pony exhaust as waste heat. For those engines that have the heat exchanger in the intake manifold, it’s almost certainly heating up both the incoming air and the combustion chambers more than with the decompression on.

There is also all the heat generated in the common cooling system.

How cold is your engine?
Below some temp, depending on what type of oil you use, warming the whole engine will make a big difference in actually getting it to run. Not to mention wear on bearings etc until it gets warmed up and oil is flowing well.

I have wondered if the occasional attempts by Caterpillar to have the pony motor share lube oil with the main engine wasn’t to get the oil warmed up faster. Of course, later models with oil to coolant heat exchangers warm the oil without the oil cross contamination problem.

A cheap no touch thermometer should tell the story of which way the head heats faster.

I am not sure it is reasonable to expect the pony to warm up the coolant enough to make a significant difference when starting. You'd probably have to run the pony for an hour just to get a significant temp increase in the coolant.

My thoughts are that the air doesn't all escape through the exhaust and instead may cycle back through the intake manifold with each stroke of the pistons. Thus, the warmer air continues to get warmer in the heat exchange tube instead of just completely fresh and cool air being sucked in and then blown out the exhaust.

A common oil system would be nice, but I think that would involve adding an oil pump to the pony.

My pony has a water pump, but I actually do question if the water pump is necessary considering that once you engage the starter pinion the diesel water pump starts pumping. So a person might be able to create a new oil line that drains into the pony sump and have an exit line that comes out of the pony oil drain and back into the diesel sump.

It's all interesting to think about.