Tony,
you discussed:
"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."
I beg to differ, the obective is to hit the " hit the nail on the head" for
each combustion stroke with correct ignition timing.
the stroke does come into it as the flame front is chasing the piston down
through the combustion stroke, so the longer the stroke the further it
travels through 90 degrees crank rotation. thus lengthening its burn to peak
push time.
The two really important factors to consider are RPM and fuel burn rate.
different fuels have different burn rates, commonly linked to its octane.
due to the simple geometry of the crank and rod there is a "sweet spot"
during a combustion stroke that provides the greatest mechanical advanage
for a piston to create mechanical energy from pressure above it. It is at
this instant that you want the peak effective pressure.
without going into too many details I would have to dig up, this is the
jist,
at 1800 rpm the crank will turn 90 degrees in .0083 seconds
at 3600 rpm the crank will turn 90 degrees in .0042 seconds
Some fuels burn slow and some fast, the fuels if uniform to themselves will
burn at a calculable speed measured in time.
whatever that time is it takes to go from ignition to peak push it would be
desirable to back the crankshaft up that amount of time (which is further
advanced the higher the rpm, more degrees of "back up") and strike that
match in anticipation of hitting that sweet spot with peak push.
swing to early and you hit the ball,, its just a foul, too early and the
piston loads improperly and cocks in the bore, only to violently shift to it
is loaded properly position once the rod swings through tdc..... and you
have piston clatter, or preignition, or pinging, whatever you want to call
it, its the same thing. Put a set of pistons in backwards and you will
have it all the time unless you advance the timing too far and it will
preignite and the constant clatter will go away,,,, if you got lucky enough
to have your valves clear it.
swing to late and you hit the ball,, its just a foul the other way,, this
would be "detonation" and from what i understand, it is a silent killer of
engines something to do with the combine forces of the mass of the
piston/rod and it still loaded with explosion pressure destroys the rod
bearings.
proper ignition timing control is manditory for optimising engine
preformance and life.
for a fixed rpm engine it can be fixed, but it sure would fire up nicer if
it had some flex for low rpm cranking speeds.
Luke Gardner
----- Original Message -----
From: "Tony.Batchelor" <[email protected]>
To: "'Discussion of biomass pyrolysis and gasification'"
<[email protected]>
Sent: Sunday, February 20, 2011 6:47 PM
Subject: Re: [Gasification] ideal wood gas engine
Dear Kelvin and members.
This may be my first post to this site, I trust you will all not hope it
is my last.
Engine power output is a complex issue, factors such as the energy
density of the fuel, the air/fuel ratio that enters the engine, the volume
of air/fuel which is able to enter the engine during the induction stroke,
and the compression pressure reached prior to ignition and the mean
pressure reached during the combustion stroke are just some of the most
important factors. Besides the detailed design of intake manifold and
exhaust pipes which influence how well an engine can breathe.
Where an engine is to be run on wood gas alone, it would be better to do
away with as many obstructions in the intake manifolds as possible. The
key point being to get as much of the gas/air mixture into the engine.
This is one reason why Diesel engines are good as they only have suck in
air, as fuel is added internally. Fuel injected petrol engines come close
behind, carburetor models are generally more restricted as air passes
through the carburetor.
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.
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.
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.
Altering the intake air pressure, using a turbo or other methods. Such as
cooling the intake air/fuel temperature.
And by changing the ignition timing.
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.
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.
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.
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.
Tony Batchelor, ex, road transport engineer, now teaching physics.
Wellington, New Zealand.
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of Kevin
Sent: Monday, 21 February 2011 1:06 p.m.
To: Discussion of biomass pyrolysis and gasification
Subject: Re: [Gasification] ideal wood gas engine
Dear Charles
Your stated need is for "about 20 HP" at 1,800 RPM
You should be able to get about 21.4 HP with an engine of 2500 CC (153
Cubic
Inches), burning about 19 kG/Hr (42 Lbs/Hr)
It would be very helpful if others could comment on the other aspects of
an
engine.... in particular, the Ignition system, whether Distributor or
"Computer Controlled", and the implications of using a Fuel Injected
engine,
rather than a carburetor engine.
Best wishes,
Kevin
----- Original Message -----
From: <[email protected]>
To: <[email protected]>
Sent: Sunday, February 20, 2011 11:32 AM
Subject: [Gasification] ideal wood gas engine
Content analysis details: (0.0 points)
pts rule name description
---- ---------------------- --------------------------------------------------
_SUMMARY_
Hi All,
I have several engines to chose from here for my next wood gas project.
Going to go ahead with it and just hope the stink has settled form my
insurance company ripping hair out of their heads. ( hope they are not on
here.. bugger )
Besides this is not a heating device so should not count.
What out of the junk yard specials would be considered ideal for wood gas
give I Only need this time to come up with 20 hp at 1,800 rpm?
I have everything in the shop for a Mike clone.
Spending a few bucks for the right engine is going to be real cheep just
now.
I am not keen on stuck valves through pistons. ( it that ever happens...
never mind )
Regards all,
Charles
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