Re: GAS-L: Re: Doubt

[Andrew Heggie <andrew.heggie@dtn.ntl.com>]

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On Fri, 14 Apr 2000 19:55:44 +1200, Doug Williams wrote:


 >
 >Yes you can generate electrical power below 50% on dual fuel without
 >creating unstable combustion, and no it is not because the fuel charge is
 >too lean, but because there is insufficient air for the diesel and the
 >combustion is too rich.
This looks at the problem from the opposite to my position. I think
Doug is assuming that the off gas and air are premixed in a fixed
proportion. In this event, with no throttling or restriction of the
inlet then as the diesel needs to be injected into an excess air
environment but the cylinder is full of a mixture of offgas and air
then yes the environment will be too rich to burn the diesel
completely.
 >
 >In answer to your questions:
 >
 >Q1: Before you can answer this question you have to know 50% of what?  Is it
 >the percentage based on maximum output capability on dual fuel at the
 >minimum diesel consumption i.e. 80% of the engines normal output on diesel,
 >whereby the gas replaces about 80% of the diesel normally consumed.  Or, the
 >amount of power available by restricting diesel flow to just achieve
 >ignition, which can be as low as 5% depending on the original size of the
 >injectors.
In this case it is my contention that the unit is not working in the
diesel cycle, the injection of diesel is just initiating the normal
spark ignition cycle. Not necessarily anything wrong in this it may
well be a cheap route.
 >
 >Either choice is determined by the circumstances of whether you are
 >measuring an existing engine or planning a project or just reviewing other
 >people's work, which has many pitfalls due to the lack of fixed
 >specifications of the equipment used.
 >
 >The importance of knowing at what point of output to set the engine, to then
 >set up the gas/air control system is critical particularly if the system is
 >variable load rather than base load.  The reason for this is due to the
 >often forgotten fact that diesel fuel has a changing stortiometric ratio
I shall disagree on the semantics here: unless the fuel composition
changes the stoichiometric ratio of fuel to air remains constant for
perfect combustion, to increase the chance of a fuel molecule
combining completely with oxygen it is necessary to provide excess
air. At low loads the diesel injected is burned in a high excess air
environment, as power increases the fuel to air ratio increases and
exav decreases. One of the amazing things about reciprocating engines
is that they achieve combustion in milliseconds whilst stove builders
have the luxury of seconds to complete combustion. Even the 1:14 fuel
to air ratio I posted earlier appears to represent an excess air of
50%. Below this the reaction does not complete, hence the sooty
exhaust from a car with a choke on or a diesel with the max fuel screw
too open.
 >that varies from about 20:1 on full load and over 100:1 at no load.  I also
Yup this is a feature of the diesel cycle: the air mass remains
constant per cycle (subject to volumetric efficiency at different
speeds) but the fuel is varied. The spark ignition cycle keeps
fuel:air ratio constant power is varied by throttling the intake, this
has repercussions for thermodynamic efficiency at low power output.

<snipped> practical advice of which I have no experience.

 >know how to control the gas/air system and any statements regarding dual
This is a subject that interests me, in the Kalle thread Joacim
pointed out the science of system analysis/feedback was in its infancy
when the Kalle was designed, the simple control of the primary air to
the gasifier by the indication of air mass flow through the engine
(derived from the depression in the intake) could be replaced now by
an air volume meter with air temperature sensor to calculate air mass,
this integrated with engine rpm could be mapped onto the primary air
control by a stepper motor with a lambda sensor in the exhaust for
good measure! A bit beyond the village blacksmith however, still IT is
falling in price so it would not be expensive in volume production.
AJH

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