Re: What exactly makes a diesel smoke under a load

Posted by RAB on August 25, 2006 at 01:28:59 from (195.93.21.42):

In Reply to: What exactly makes a diesel smoke under a load posted by carpenter in belmont on August 24, 2006 at 16:43:51:

None of what you suggested and not a lot of what some of the others have said.
Compare a spark ignition engine to a compression ignition engine. The spark ignition has a flame front starting at the spark plug and spreading across the fuel air charge at a high rate of knots.
The volatile fuel/air mixture has already been made pretty uniform, concentation wise, and contact surface (of air and fuel) is on the molecular level. The ratios are fixed to just burn all the fuel in all the oxygen. Some regard the burning as an explosion, but I would consider it to be a controlled high speed flame front. In a properly designed combustion chamber the flame path consumes all the fuel before the piston has travelled too far thus optimising the efficiency and power etc while not over-loading any engine components. It would do this at room temperature as well (Many people have witnessed fuel explosions) given a high enough temperature source(a spark) to trigger the reaction. Too little or too much fuel will not explode.

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

Replies:

• Re: What exactly makes a diesel smoke under a load jdemaris 07:19:12 08/25/06 (0)
• Re: What exactly makes a diesel smoke under a load massey333 05:18:05 08/25/06 (6)
  • Re: What exactly makes a diesel smoke under a load RAB 11:13:42 08/25/06 (0)
  • Re: What exactly makes a diesel smoke under a load buickanddeere 11:01:17 08/25/06 (0)
  • Re: What exactly makes a diesel smoke under a load RodInNS 05:44:48 08/25/06 (3)
    • Re: What exactly makes a diesel smoke under a load massey333 12:34:03 08/25/06 (2)
      • Re: What exactly makes a diesel smoke under a load RodInNS 13:02:24 08/25/06 (1)
        • Re: What exactly makes a diesel smoke under a load massey333 13:44:12 08/25/06 (0)

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