[Gasification] ideal wood gas engine

[doug.williams]

Hi Tony and Colleagues

> This may be my first post to this site, I trust you will all not hope it is my last.

As a fellow New Zealander, lets hear more from you, we cover a lot of engine "stuff", and new voices are most welcome. 
 
> Getting the Air/fuel ratio correct is also vital.  Using a "colortune" sparkplug is the best way to really know when you have the correct mixture as you can see the flame color within the combustion chamber.

Fluidyne bought a Colortune 500 kit back in 1974-5, and I used it to teach how exhaust temperatures and engine sound changed across gas/air mixtures, using a single cylinder Iron Horse engine. I sent both Kevin and Arnt a copy of the colour guide out of our kit, seeing as they were interested in this subject.
 
> A turbocharger can be used to increase the volume of mixture which is drawn into engine but whether or not they are practical given the possibility of contaminated gas is something I cannot comment on.

This is a problem for producer gas in most DIY systems. We do better at the commercial level with more sophisticated filtration systems, but it is better to use naturally aspirated engines of larger cylinder capacity of lower RPM, than undersized turbocharged engines relying on high RPM for DIY projects.

> The Mean effective pressure within the engine during the combustion stroke, is largely dependent on the length of stroke of the engine, the compression ratio and the ignition timing.
> The stroke cannot easily be altered but the compression ratio can be changed on some engines by machining the cylinder head.

Generally speaking, this would mainly be applied to very old engines, probably pre-dating around 1949. The literature records a lot of work in this area of compression ratios by Woods in the late 1930's early 40's (from memory), where it was established that around 11:1 was the optimum for producer gas. At this point, the extra friction from compressive forces consumed the "extra energy", and little was gained from higher compression. 

> If a petrol (spark ignited) engine is run on wood gas or any other gas, the Ignition timing has to be altered.  In general the ignition timing will be advanced by several degrees, in order to ensure as high a mean pressure as possible is reached during the combustion stroke.

This is true, but remember that WW2 petrol was of lower octane, and required ignition advancement. Modern engines have that advancement already built in for the higher octane available today. Then, separate charcoal gasifiers away from wood gasifiers, because the H2 content again changes ignition behaviour. Most engines set up to operate on LPG or natural gas, are from 10-12:1 compression ratio (of the smaller sizes), and run without alteration on 110-120 octane producer gas perfectly. Having said that, you can always tweak them if the situation demands that degree of perfection. The engine is the least of your worries if the gas making is unstable (:-)

> The benefit of using a computer controlled ignition system is that most if not all computer controlled systems have a "knock" sensor.  The purpose of this device is to sense when the ignition of the fuel has caused the pressure within the cylinder to rise so high that the remaining un burnt fuel spontaneously explodes.  This results in engine knock, the resulting noise is commonly known as "pinking"   Diesel engines knock a lot of the time because the very design of the engine is to raise the fuel temperature to point when it spontaneously burns.

Speaking "generally", producer gas has no problem in most standard spark ignition engines, as the spontaneous ignition temperature for producer gas in our experience, is around 600C. You find these compression temperatures in diesels around 16:1 ratio, and again from experience, once you go over 17:1, the spontaneous ignition temperature makes the engine very unstable. We worked with Lister (NZ)  to develop dual fuel conversion kits for the Pacific region, converted to gas Ford diesels in the UK, Ford natural gas engines in USA, and purpose built gas engines in Germany. 

In all cases, the operating temperatures around the engine can affect the behaviour of the ignition temperatures, as will the actual CO,H2, and CH4 content. Any uncracked hydrocarbons will also affect the timing behaviour, so be careful how you tinker with the timing. Nothing is written in stone!

> Older engines that use a Distributor lack the anti-knock feature. Commonly distributors have a simple mechanical advise mechanism, to advance the ignition as the engine revs faster, and a >Vacuum Retard mechanism which aids acceleration.  Engines which are subject to varying loads, can benefit from the retard mechanism if there is any kind of control valve /butterfly on the >intake, which would alter the manifold vacuum.

My genset engine is a  1949 Hillman engine, one of the early higher compression engines  (8:1) out of the UK. The vacuum advance and retard is disconnected, but we have not noticed any problems across a wide range of outputs for 1,000's of hours. It is a moot point however, and I will reconnect it next time I play to see if it makes any difference.
> 
> Anyone setting the timing on an engine with a fixed load-speed, needs to be sure the advance/retard mechanisms are either working correctly or have been locked up. As fixed speed engines can "hunt" if there is any faults in or if there is any small changes in the loading or fuel supply.

Gensets have to operate at fixed speed, so use a governor on the throttle butterfly, and as I said, our advance/retard control is disconnected, so cannot in any way affect how the engine hunts on load or gas changes. Producer gas has many surprises as an engine fuel, and we learn more by the day.

Most of the above comments apply to fixed speed (RPM) applications used for electrical power generation, both base and variable loads, from our installation experiences 1978- 2010.

Doug Williams,
Fluidyne Gasification.



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