Re: Why is tractor\'s Diesel instead of gas?
</font><font color="blue" class="small">( If that is the case, makes me wonder why more cars aren't diesel.)</font>
A diesel is not normally run through a broad range of rpm, and particularly is not accelerated from low to high rpm at a high rate of speed. Tractors are normally run at a fairly constant rpm. The longer stroke gives good torque, but not a rapid change in rpm over a broad spectrum. A semi has several sets of gear ranges giving, I believe, the equivalent of something like 16 forward gear speeds. Listen to a semi taking off from a stop sometime. For only a couple of seconds you hear a slight increase of rpm, then the shift up and another slight rpm increase as it goes through the next gear, over and over as the truck builds momentum. It takes a loaded semi time to build up to cruising speed. Locomotives take even longer, though the diesel-electric propulsion system is far more efficient than changing gears.
Auto drivers need acceleration in quick bursts. A gas engine is able to wind out from low to high rpm over a broader spectrum and at a quicker rate than a diesel. I used to hate it when my diesel Volksie pickup had to enter freeway traffic; took forever to build up speed. Though I liked the mpg, there were many times I wished I was in a Porsche (or maybe Frank's F-15) and could put a few G-forces behind my rear & hit 60 mph in 6 or 7 seconds.
With its shorter stroke, a gas engine can run at higher rpm. The Rolls Royce Merlin engine used in the P51 Mustang is legendary as a high performance high rpm engine. Dragsters take this ability to race through a wide rpm range very quickly to an extreme with their exotic fuels.
Before big diesels came out for heavy equipment, gas engines just didn't have the high torque needed. The old Caterpillar earth movers often had one bulldozer in front to pull and another one came up behind to push, and they moved quite slowly. Today, while the paddlewheel blades make the physics of breaking the soil much better, the diesel rubber tire tractor on the front moves an earthmoving paddlewheel scraper along at a fast clip, with no booster in back.
