[Gasification] Air/gas ratios (was fireballs)

[AJH]

On Thu, 7 Sep 2006 09:05:30 +1200, Doug Williams wrote:

>
>Hi Andrew,
>
>You compare:
>
>> OK but if you ran the same engine on natural gas and then made that
>> mixture wrong by a similar ratio how would it fair?
>
>Natural gas and LPG are much the same, and can handle a wider gas/air ratio, 
>than producer gas. It has to be said however, that all fuels have their 
>prefered optimum sweet spot.

OK we have a difference of opinion here, Tom Reed suggests the
producer gas has a wide flammability range, we know that CO and H2
have a high flammability range plus they have widely differing flame
speeds. The evidence of difficulty in running the engine I discussed
comes from a chap I know. We do know these engines were designed to
run on town gas, which differs from producer gas largely only in that
it has much less Nitrogen. This makes producer gas also a much lower
calorific value that town gas.

Again it's a hypothesis that I cannot check but the experiment should
be fairly easy to conduct, the town gas and producer gas can be
synthesised by mixtures of H2m N2 and CO.

>When I had my LPG system installed, it had a gas shot for starting (like a 
>choke reaction) but although it was set up "correctly" according to the 
>experts,
>the power was way off when hot. The trouble turned out to be the hot water 
>supply to the vapouriser, making the gas to hot, so I just put a squeeze on 
>the hose to slow the amount of heating to the vapourizer. Believe it or not, 
>the same thing happens to engines on producer gas when you heat up the gas, 
>so the engine can get rid of the condensate from the cooling gas. Check your 
>hose configerations as a first step, you might be in the same circuit as the 
>car heater.


This makes sense, in my case I have an old carbureted engine and the
gas diffusers/venturi are installed just upstream of the carbs. So it
starts on gas fine.

As you so rightly point out if the gas gets hotter the mixture
weakens, because the mass of gas passing through decreases by about
1/273 for every degree K it heats up. So as the engine gets hot it
weakens. I have allowed for this by only tuning it when hot. I can see
the same being the case for producer gas.

As I see it as carburetors evolved they became quite complicated and
just before they were replace by fuel injection (I take it no cars are
available with carburetors in the west now?) they were quite good at
nearly getting the mixture correct over the whole working range. In
comparison open loop lpg mixers are about the level of effectiveness
of a carburetor from the 1920s, which they get away with because lpg
is a relatively simple fuel to burn cleanly, if you can get it to burn
at all!

As I have a lambda sensor fitted to the vehicle the easy solution is
to close the loop, I may do that some time...

AJH