another thing is that any diesel engine is going to pollute les than any gas engine of the same size, example would be 7.3 powerstroke compared to 460 v8
This is a bit of a flawed generalization. Too many factors contribute to pollution. Air to fuel ratio, compression ratio, naturally aspirated vs forced induction, to name a few.
Properly tuned diesel engines generally run slightly below a stoichiometric ratio, meaning there is more oxygen than required to completely burn fuel. This means CO and CO2 emissions are lower with a diesel engine. Flip side of this is that diesel combusts at a much higher temperature than gas, which leads to higher NOx emissions. Modern catalytic convertors do a pretty good job of eliminating NOx, so this is becoming less and less of an issue. Also, even low sulfur diesel contains sulfur, which means SO2 (sulfur dioxides) are higher with diesel.
Bring up the lack of a throttle butterfly which causes diesels to be much more efficient than a gas engine at low rpm. Typical gas engines operate at less than 35% efficiency at idle. Power in a gasoline engine is limited by the throttle plate, meaning airflow is restricted. A diesel limits power by limiting fuel, but maintains airflow regardless.
Diesel contains 128,700 BTU/Gal compared to 115,500 BTU/Gal for gasoline, meaning it is more energy dense than gas. A gallon of diesel is capable of more ‘work’ (force x distance) than gas.
Diesel is also rated in terms of cetane as opposed to octane for gas. The cetane number of a fuel is a measure of it’s ignition delay; basically the time period between the start of injection and the first identifiable pressure increase (combustion). Higher cetane number will have a shorter ignition delay, and vice-versa. Gasoline on the other hand is rated in terms of octane, which evaluates its ability to resist pre-ignition (or compression ignition).
This allows diesel engines to accomplish two things. 1) Run higher compression ratios than gasoline because they are not ‘knock limited’ (18:1 as opposed to a maximum of roughly 10:1) 2) Makes forced induction much easier, which increases efficiency throughout the powerband. Basically a gas engine can be tuned to provide optimal fuel-air ration at low rpm or high rpm, whereas a properly tuned forced induction, high compression diesel maintains the stoichiometric ratio throughout the entire operating range.
Hope all of this makes sense…I’m known to ramble after a long day.