[Gasification] combustion math question (help needed)

[doug.williams]

Hi Jim,

You ask some interesting questions, but I doubt is you can get anything like 
the answers you seek, so my thumb goes up to pass on what has been applied 
for commercial application.  I cannot offer comment on all your questions, 
but here goes:

>the other variable i didn't mention in the first note that seems very
>important is fuel/air "compostion".  or rather, the variance in
>"explosive flammability" as percentage fuel/air varies.  you can have
>the needed temp for auto ignition, but below the % limit fuel/air and
>still not have ignition.

Producer gas has what I call a very narrow range of ignition capability, 
which is "about .9 - 1.1:1 for the gas/air mixture. You will find that it is 
more critical when trying to start the engine, than when running, but once 
operating under load, the air should constantly change to match the 
fluctuations of the gas making process, but only within the range I just 
quoted.

>therefore to recast the question, are there general answers for the 
>following?

> if i double the compression of an unthrottled IC engine, how much
>will my efficiency increase?

This depends on what the compression was increased from, but once you get 
over about 11:1, the frictional forces use up the extra power gained from 
increased compression of producer gas.

>these questions are motivated by a desire to assess whether a
>different type of heat cycle might be possible with H2, CO and CH4
>based fuels.  these gaseous fuels are so detonation resistant, that
>one can run in the diesel realm of compression (high teens to 20:1
>range).

You have a slightly squewed view of the octane rating of producer gas, which 
is only detonation resistant in reliable operation at about 16:1 compression 
ratio. This relates to about 600-620C.  I do not know of any commercial 
gasified engine systems operating at 20;1, but many trials have been 
reported in literature. These engines are those with pre combustion 
chambers, and premature detonation makes for a very unstable operation.


>if you do not have a detonation problem, it is actually preferable to
>run an otto cycle and premix the fuel/air instead of inject it at the
>ingnition event like a diesel.

This is the traditional way of using producer gas.

 >note that diesel has an auto ignition temp nearly identical to
>gasoline.  around 220C or so.  one could not run an otto cycle with
>diesel fuel at diesel engine compressions.  you  would get bad
>detonation.  but you can run H2, CO and CH4 at diesel type
>compressions.

I think you are talking about Cetane rating in comparing diesel with 
Producer gas, but there is a pre history of running Otto cycle engines on 
gasoil, but not sure of their compression ratio. (see History of Massey 
Ferguson Tractors)

>my proposal for H2, CO and CH4 based fuels is then twofold:

>1. can we run such gaseous fuels in a manner that combines the best of
>otto and diesel?  namely, can we run the engine unthrottled (lean
>burn), with premixed fuel/air, and ignite with a spark plug?

Not sure what you mean by unthrottled, and how that relates to lean burn? We 
have operational spark ignition gas engines that do not use a speed 
controlling throttle, but the gas/air must be controlled to maintain it's 
flammability.

>certainly we could do this at higher power amounts, which will have
>close to stoich fuel air mix, and thus reasonable ignition, but will
>the engine still run at lower fuel amounts?  or in formal terms, will
>the fuel/air composition of the charge at low power, say 5% power,
>still be above the fuel concentration explosive limit so ignition will
>still happen?  or will the mixture be too lean to go off?

If you are not controlling the air flow into the gas, then you are unlikely 
to be able to run the engine in a controlled manner. Ignition is likely to 
fail.

>2.  could we run such fuels under an HCCI (homogeneous charge
>compression ignition) scenario, using the turbo and/or supercharger in
>real time to vary the compression, and thus control the ignition
>timing?  if so, such would allow for efficiencies above both typical
>diesel engines, and gasoline based HCCI engines.  the possible
>compression would be about double that of regular gas engine, and the
>fuel/air mix would be much better than diesels, thus better energy
>extraction.

Don't forget producer gas is one of the lowest energy gases, and no matter 
how you get it into the cylinder, you are not going to get more in than can 
be compressed to 16:1. It also comes back to the energy lost to friction, 
which is not an issue for other high energy gases or liquid fuels.


>historic IC engine design was largely in response to the realities and
>opportunities of specific fuels, in combination with machining and
>metalurgical specifics of the day.  if we are now going to start
>running biofuels significantly, we should note the new things they
>allow/disallow in engine design, and alter engine design accordingly.
>we should not just simply be replacing fuels into engines optimized
>for a different fuel.  thus the very high detonation resistance of H2,
>CO and CH4 seem very important positives that deserve more attention.

Engine designs are there to see in almost every configuration we could 
possibly use if a need existed. While the high octane of producer gas is an 
interesting fact, you cannot use that as a basis for new engine design, 
unless you first have a gas system that makes a perfect gas % wise all the 
time.  The variation of the combustible gas component, cannot be made up be 
pushing in more gas, because the combustion loss comes with an increase of 
CO2. If you add to this the variable engine speeds, as for mobile operation, 
then I think more emphasis on developing a new generation of gas making 
systems would be the way to go. It is worth remembering, that engine speed 
is also a factor of combustion behavior, and that favours slower engine 
speeds.

> a gas engine is limited on turbo boost by the premixed
>fuel/air mixture and the resulting auto detonation problems.

We turbo boosted a purpose built gas engine with premixed air/gas, and had 
no auto detonation problems, even though the turbo was running red hot. The 
key again is staying around the 16:1 compression ratio. Unless you can test 
some of this for yourself, literature is full of reports that need 
qualification about engine behaviour on producer gas, and the information 
can be taken completely out of context to its real meaning.

Not sure how much help that was, but it states the known obvious, and it 
might trigger some one else with experience to comment.

Doug Williams,
Fluidyne Gasification.