[Christ]

Yep, that's one way that you get knock. It's the same thing that happens when you advance the ignition timing in a gasser. It can seriously re-format your pistons and rods, and could break your crank/main caps.

The other thing that I was talking about is waht happens when the coolant can't work fast enough to take heat away from the upper part of the cylinders in the block, and the heat causes expansion, leaving the pistons loose while they're at the top of the cylinder. As the piston reaches the top of the compression stroke, it "slaps" the cylinder wall, and as the initial injection of fuel occurs, the pressure rise on the top of the cylinder causes it to slap in the other direction, against the opposite wall. Extended operation under this condition will cause out-of-round in the cylinders, and can cause piston/wrist pin damage.

I don't have too much data on cold/hot air MPG's, because most of my diesel experience comes from utility engines, such as farm equipment, and it's limited at that. I"m not the 100% diesel guy on this forum, by any means. I just know what happens when you don't keep them happy. (boom)

Regardless of what you've been told, any engine that runs on uncompressed air (has to suck in it's own air supply) has pumping losses, and even those which run on compressed air and fuel have pumping losses. They still have to compress the mixture in the cylinder, and then exhaust the burnt mixture, which requires work. Diesels are no exception to this rule.

My best experience with efficiency (not necessarily economy) with a Diesel is just tuning it until the exhaust is mostly clear. When that happens, you're getting a clean (haha) burn, and as little particulate emissions as you can. This might mean turning up/down the fuel, heating/cooling the intake air, etc. It all depends on the situation, but again, this is dealing with utility engines, and efficiency, not economy. (There is a difference.)