All, Valve timing gains are most realised with high rpm engines. to make the discussion short and sweet, great gains in volumetric effieciency can be reachieved for a naturally aspirated engine with proper valve timing"overlap" at specific target rpms. If set up properly the exhaust valve remains open well after the intake valve begins opening while passing tdc. this will allow some pre filling of intake air before the piston even starts going down, as the inertia of the exiting exhaust gas leaves a "hollow" or a momentary low pressure behind its exit. to realise the hollows energy the intake valve is opened premature, thus overlap. the number of degrees overlap, the shape of the cam ramps, and which side of tdc the center of overlap is, and how far are all factors that affect the end result. this is a narrow window to reuse some of the engines inherently lost energy of chugging along, but if you get it right, your dyno resuslts will show the increased volumetric efficiency with power gains in a select band of rpms. the down side of the overlap is that to really realise this feature the engine will not enjoy low rpms whatsoever, and will Idle like a disturbed bowl of jellow if at all. boosted intake engines will see gains from this as well just less pronounced. Luke ----- Original Message ----- From: Mark Ludlow To: 'Discussion of biomass pyrolysis and gasification' Sent: Monday, February 28, 2011 10:43 PM Subject: Re: [Gasification] Benefits of boosting compression ratiowith producer gas
Geoff, Of course you are right, but compression ratio is an artifact of geometry alone, whereas actual compression is affected by piston blow-by, valve/seat condition and intake valve open duration, since the intake valve cannot close precisely (suddenly) at the bottom of the intake stroke. An intake valve can remain partially open for, say, 60-degrees past bottom dead-center (BDC). This has some effect on the volumetric efficiency, and consequently the pressure in the engine at TDC (ignoring the fact that combustion ignition precedes TDC). So compression ratio is not exactly proportional to actual combustion chamber pressures at TDC. The effect of higher absolute manifold pressures on the amount of mass to be combusted can be estimated by the Ideal Gas Law, pV=nRT. I say estimated, because it is impossible to assume adiabatic compression. But observe that the equation is linear in all variables. This was implied earlier by Daniel Chisholm’s excellent Rule of Thumb. Go to Wikipedia for a good explanation of the Ideal Gas Law. Best, Mark From: [email protected] [mailto:[email protected]] On Behalf Of GF Sent: Monday, February 28, 2011 9:14 PM To: [email protected] Subject: Re: [Gasification] Benefits of boosting compression ratio with producer gas Mark , Now you have me confused on this subject. I thought the compression ratio of an ICE, was decided by the BDC cylinder volume divided by the TDC cylinder volume. The rate at which a gas mixture is induced into this swept volume per cycle is dependant upon the inlet pressure. So raising this pressure will raise the amount of gas mixture entering the engine per cycle? The effect of compressing a larger quantity of gas in this defined space raises the temperature adiabatically to a point where "compression Ignition" will occur if an explosive gas mixture is being compressed? I have always pondered on the idea of reducing the amount of nitrogen in the mixture to raise the output of the ICE. What about laughing gas? GF -----Original Message----- From: Mark E Ludlow <[email protected]> To: 'Discussion of biomass pyrolysis and gasification' <[email protected]> Sent: Sun, Feb 27, 2011 7:45 pm Subject: Re: [Gasification] Benefits of boosting compression ratio with producer gas Hi Toby, There is no difference, really, between pressure and vacuum. If we just start to think of anything that has no pressure at all as Zero Pressure; normal Atmospheric Pressure as 14.69 PSIA (PSI absolute) and so forth, then we won’t get into trouble. Pump sizing always considers the Absolute Pressure (e.g. PSIA) at the inlet and outlet, not the Gauge pressure (e.g. PSIG). A third concept is Differential Pressure (e.g. PSID). A pump increases the pressure (measured across the Inlet and Discharge Ports) as a differential pressure gain, more-or-less irrespective of the Absolute Pressure environment that it operates in. There is really no such thing as “Vacuum” conveying. This only refers to the fact the transport network operates at or below the ambient pressure and is operated this way to help reduce fugitive losses of transported materials. A vacuum cannot move anything; it is the force of pressurized gas behind the transported material that provides the propulsive energy. In the case of a conveying system connected to a positive-displacement “vacuum” blower, the force is a maximum of 14.69 PSI, enough to “lift” a water column 33.9 feet, hardly any stretch at all for a pump, though the work done will be the same regardless of the method used. When someone says: “Gravity doesn’t exist; the world sucks!”, this is a very special case of the above that awaits the practical manipulation of anti-gravity. Best, Mark From: [email protected] [mailto:[email protected]] On Behalf Of Toby Seiler Sent: Sunday, February 27, 2011 3:42 PM To: [email protected] Subject: [Gasification] Benefits of boosting compression ratio with producer gas Sorry Tom, I didn't ask the question correctly (or I may be off in left field). This comes from some time spent in a place called Flow Dynamics Laboratory where inlet design of blowers was being tested. My application was sawdust material moving. Dan, the owner, explained how pressure was much more effective than vacuum for moving mass. He explained that drag in a vacuum is hard to overcome, while pressure fills the space with molecules and can push materials great distances. When talking engines, for example on my 74 Ford f600, I watch the vacuum gage and see that it is around 18-22" of vacuum, warmed up 1200 rpm or so (driving hydraulic pump). So what goes to the cylinders is not atmospheric pressure, it is around half, perhaps less. I'm trying to understand how engine efficiency is related to both air and gas pressure and density. If producer gas is operating with 18/1 in a normally aspirated engine, due to high octane, how will the ratio be affected if one has positive pressure at one or two pounds? Seemingly this would involve air flow across an orifice, similar to a butterfly valve (throttle), but I am at a loss for a good start point. Given a large quantity of air and gas to make comparable power (not large derated), the flow of air/gas in a large displacement engine would seem to be a much more critical factor than just saying the cylinder is seeing atmosphere pressure and forgetting the drag that manifolds, throttle valves and pulling gas from a gasifier creates (in a suction system). I can hardly see a direct linear relationship. My work is a low pressure system, so this is not an academic or theoretical discussion that I will never act on and I'm at a point of determining engine for a CHP and what internal modifications to plan, if any. I have several engines, a 345 International, a 460 Ford, a Cumins 5.7 (with needed repair), a 7.3 International (in a ford truck also needing repair). Sorry if this is confusing or not cogent. I'm trying. Toby seilertechco _______________________________________________Gasification mailing list to Send a Message to the list, use the email [email protected] to UNSUBSCRIBE or Change your List Settings use the web pagehttp://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site:http://gasifiers.bioenergylists.org/ ------------------------------------------------------------------------------ _______________________________________________ Gasification mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/gasification_lists.bioenergylists.org for more Gasifiers, News and Information see our web site: http://gasifiers.bioenergylists.org/
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