What exactly makes a diesel smoke under a load

Now consider the naturally aspirated compression ignition engine. Here we have an engine running with an excess of air at all times (under normal engine settings and with engine in good fettle. Excess air at all times - right? Now we arrange to sqirt in some relatively non volatile (c/f to gas) fuel. However good the atomisation of the nozzle is those droplets will have a dimension much, much, much larger than a molecule. Those droplets have to "find" enough oxygen to burn from the outside in (even if they do evaporate on injection, the concentration of oxygen inside the droplet volume will be zilch) so it needs time to mix and burn completely. Incomplete combustion leads to soot formation (carbon, like as in coal) which then will only burn slowly at high temp.
Turning back to the squirt. Injectors are marvellous little things, operating as they do for long periods in the harsh environment of an engine combustion chamber. They have been designed to work as efficiently as possible over a given range. Excess volume injection is where the problem begins. Those injection nozzles cannot atomise the fuel perfectly and production of any larger droplets will increase the time for heating, evaporation, homogenisation with the air and combustion with the oxygen. Further, any fuel which contacts with any part of the engine will never burn fast enough, as it will not be hot enough and there is even less oxygen available just in those positions. Those injectors just cannot spread the fuel evenly enough in the combustion chamber air charge for all that excess air to come into play, so it forms soot - your black smoke. So it is really down to atomisation of the fuel within the complete combustion chamber.

Notwithstanding all that, lets not blame it all on the injector. Remember a diesel engine also has a carefully designed combustion chamber to induce as much swirl (mixing) as possible. Indirect injection engines had a separate, specially deigned, chamber where the initial combustion was instigated. The first direct injection ("combustion chamber in piston bowl") engines smoked a lot at all loads and needed carefully matched inlet manifolds to get enough swirl to burn the fuel in the alloted time-frame.

So, smoke is caused by too much fuel in a given volume at the least efficient range of the fuel delivery system, ie full injection volume, which is (always?) more than normal fuel volume at rated engine speed, when the governor opens up to maintain engine speed.
Remember, all engine designs are a compromise. Diesels are no different. The big marine engines, turning at a constant couple hundred revs per minute (rpm) are a much different "kettle of fish" than a car engine running up to 5000rpm or a tractor at 2500rpm.
Tractors, which must maintain their rpms at pto speed, might need a much bigger and slower engine to avoid "the heavy load smoke". Different cost, different size tractor (weight), different rear end design (for extra torque at lower speed range). Could be done, but tractor would overall be less efficient? (no point in using more fuel all the time just to lug itself around), and manufactures have to compete for sales.
Like I say - a compromise in design.
Sorry for the long post - but you did ask and there is no simple short answer if you want the reasoning as well.
Regards, RAB

Many of the "newer" tractors, from the 80's and 90's are a lot heavier than the old tractors too, so that takes power to drag around.
However, if you look at the Nebraska tests on many of these tractors, they are more efficient than the old engines. Their specific fuel consumption at rated power, in some cases is probably half what many tractors were during the 50's. There are exceptions, but that would be the general rule.....

Rod

I'll probably start a color war with this, but comparing an old Orange tractor to any green thang isn't really fair to begin with. From what I've seen of most of the Allis engines, they were fairly efficient. From what I've seen of most of the Deere engines, most were not... Mabey others have different experiences, but that's what I've ovserved....

When it comes to truck engines, the electronic engines win hands down in fuel economy, and in so many other respects that it's not even worth comparing them.

Rod

Rod

A piston under load wants to slow down. The operator opens the throttle as a result. That increases the fuel flow and opens the butterfly valve for more air at the same time.

Fuel volume is determined by the fuel pump pressure. Air intake volume is determined by atmospheric pressure (14.7 psi if I remember correctly).

While the fuel pump can happily inject all the fuel the throttle asks for, the atmosphere can only supply as much air as the piston will pull in; no matter how much the throttle opens the butterfly valve.

And a slower-moving piston under load draws in less air (per unit of time).

Resulting in less air for the amount of fuel in the piston, resulting in less complete combustion, resulting in smoke.

That is why, incidentally, turbo diesels have more power. They pump air into the piston at pressures above atmospheric pressure and produce more concentrated combustible material.

If you can concentrate twice the fuel AND twice the air in a piston you get twice the power forcing the piston down -or thereabouts.

Are we talking about Ford Major Diesels here, or where are you getting the "opens the butterfly valve for more air" B.S.????

However, the purpose of this butterfly is generally misunderstood. It does not serve as a throttle so much as it is a governor. Manifold air pressure (vacuum) activates a servo that controls the fuel pump. This system basically maintains constant manifold air pressure: When you open the throttle, the butterfly allows more air into the manifold, reducing vacuum and increasing fuel flow. Increased fuel flow speeds up the engine, increasing vacuum until an equilibrium point is reached.

Like I said, take a look at a Ford Major Diesel, which is ONE exeception to the "no throttle butterfly on a diesel" rule!

And ignore "no matter how much the throttle opens the butterfly valve.

Notice that a non-turbo diesel (IH 806 for example) will smoke way more than a turbo diesel. The newer computer controlled diesels don't smoke much at all, optimized fuel to air ratios.

Most of the "newer" pumps/injectors are higher pressure and normally don't smoke unless someone has been in the pump and turned it up.

My tractor this afternoon was pulling clear up against the nubs. Not a trace of smoke.

Allan

Of course, if it's tubored, the air forced into the engine DOES increase as the turbo "spools up".