Re: What exactly makes a diesel smoke under a load

Posted by jdemaris on August 24, 2006 at 19:00:17 from (66.218.19.28):

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:

The smoke is from more fuel entering the combustion chamber than the engine can burn
efficiently. A gas engine, when using a carburetor, draws air and fuel that is already
mixed together - via a venturi mixer - and tends not to run rich unless it is jetted wrong.
And, a gas engine with electronic fuel injection - while not using a venturi to mix fuel
with air to the proper ratio, still is protected from being overfueled by a computer that
monitors the air-fuel ratio.
With a mechanical diesel injection system - particulary a rotary type pump e.g.
Roosamaster or CAV, the volume of the fuel delivered is determined by a simple valve -
NOT pressure as someone suggested. The fuel injection pump will have a maximum
limit that it is capable of pumping - and then a valve that restricts that flow of fuel to the
injectors. That valve is hooked to the accelerator, governor, torque screw, or whatever.
When the valve is wide open - the injection pump will pump that absolute maximum that
it is calibrated for - whether the fuel can be used, or not. So, take a gas engine with
carburetor, load it down and give it full throttle - and it will not smoke (if it’s running
right). That is because it won’t take in much more fuel-air mix than it can use - unless
the carburetor is jetted wrong. Take a diesel, and load it down, and give it full throttle,
and the injection pump (in most cases) will deliver as much diesel fuel as it can -
regardless if the fuel can be burned or not. Thus the smoke. I’m being a bit simplistic
here, but close enough. Many tractors new did not smoke all that bad under load. But,
we’d often get complaints from farmers who wanted to see some smoke when working
the tractor. Also, pumps like the Roosamaster DB or C series are affected in two ways by
the maximum fuel setting. If you turn the max. delivery down a bit, it will often make
the tractor start hard. So, to the converse, on many high-hours tractors, we’d sometimes
turn the pump up a bit just to make it start better. This, of course, also resulted in more
smoke under load.
The Roosamaster/Stanadyne that was mentioned, depending on the model, has several
controls for timing advance, load sensing, etc. And, if an engine is turbocharged, there
are usually even more control for fuel delivery. A turbocharged engine is less likely to
smoke as bad as a naturally aspirated engine - as stated earlier - because of the extra air
entering the combustion chamber, and also - the compression ratio being effectively
raised - thus the term “efffective compression ratio” versus “mechanical compression
ratio.” Even so, a turbo-engine making high boost can be made to smoke quite a bit if
enough fuel is being pumped.
The newer diesels are not running cleaner just because of higher pressures. Higher
pump and injector pressures do not have a major effect on the visible smoke. Newer
systems are just designed better and deliver more proper amounts of fuel throughout the
working range of the engine -and hold back fuel when it cannot be burned. When I was
working for a Deere dealer in the mid-70s, and we got our first Japanese John Deeres -
850s as I recall. We were amazed that they started and ran so well, and also -did NOT
smoke when lugged down. And, that was 30 years ago. Those Yanmar engines had very
simple injection systems - but - they were constructed and calibrated more carefully than
their US built counter-parts.
One of my sons lives in Colorado and he’s taking on of my Chevy diesel Suburbans back
home with him - an 87. Come to find out that in Colorado, there is a visible smoke test
for all diesels on the road -and it is tested under load on a dyno - the smoke density
measured electronically. But - the 6.2 Detroit-Diesel built, with simple Roosamaster
rotary pump - just like many tractors use, barely makes any smoke at all when adjusted
properly - even under full load. So, hopefully, there will be no problem. But, seems you
don’t want to turn your pumps up in Colorado.

Replies:

• Re: What exactly makes a diesel smoke under a load RodInNS 19:24:20 08/24/06 (0)

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