What is the upper limit for the compression ratio without getting auto-ignition?
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#2
jimmason
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Posted 31 May 2010 - 11:28 PM
alpha,
this is a question that we all go round and round on. the answer depends on who you ask, and the detail specifics of how one is running woodgas under compression.
in general when dual fuelling a diesel with woodgas one wants a lower compression, non-turbo, and direct injection type if possible. indirect inject engines tend to have hot spots that will facilitate detonation.
on 100% wood gas in an otto cycle it seems the max compression is 15-17:1 or so. in a diesel you typically are going higher than this. the amount higher will impact the percentage substitution of woodgas you can achieve. lesser amounts of woodgas substitution will reduce detonation, or in other words, tolerate higher compression.
i have yet to see a good chart of substitition levels achieved across different diesel compression ratios. we should do the tests to generate one at some point.
anecdotally, it seems the 18-20:1 diesels do just fine. people seem to get into problems with turbo diesels, or some idi diesels with hot precombustion chambers. turbo diesels can have dynamic compression ratios well above 40:1. this will cause problems.
but mostly, the specifics are vague at this point.
jim
this is a question that we all go round and round on. the answer depends on who you ask, and the detail specifics of how one is running woodgas under compression.
in general when dual fuelling a diesel with woodgas one wants a lower compression, non-turbo, and direct injection type if possible. indirect inject engines tend to have hot spots that will facilitate detonation.
on 100% wood gas in an otto cycle it seems the max compression is 15-17:1 or so. in a diesel you typically are going higher than this. the amount higher will impact the percentage substitution of woodgas you can achieve. lesser amounts of woodgas substitution will reduce detonation, or in other words, tolerate higher compression.
i have yet to see a good chart of substitition levels achieved across different diesel compression ratios. we should do the tests to generate one at some point.
anecdotally, it seems the 18-20:1 diesels do just fine. people seem to get into problems with turbo diesels, or some idi diesels with hot precombustion chambers. turbo diesels can have dynamic compression ratios well above 40:1. this will cause problems.
but mostly, the specifics are vague at this point.
jim
#3
Alpha Ag
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Posted 09 June 2010 - 03:49 PM
I can imagine any tar left in the wood gas would cause problems with gumming up a turbo. Is that the main problem with a turbo or just the typically higher compression with turbo use?
I have a 7.3L Power stroke with a turbo / 21:1 compression and a turbo powered Cummins M11 with 27:1 compression. Any amount of wood gas to help the fuel economy on these rigs would be welcome.
Funny thing about the Power Stroke which I drive in Canada. On USA fuel I get 14 mpg with no load to haul. On Canadian fuel I get 18 mpg.
I have a 7.3L Power stroke with a turbo / 21:1 compression and a turbo powered Cummins M11 with 27:1 compression. Any amount of wood gas to help the fuel economy on these rigs would be welcome.
Funny thing about the Power Stroke which I drive in Canada. On USA fuel I get 14 mpg with no load to haul. On Canadian fuel I get 18 mpg.
#4
amishmafia00
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Posted 21 November 2011 - 04:44 PM
Why do you want a non turbo engine?
#5
Ben Goldberg
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Posted 28 January 2012 - 01:45 AM
A turbo increases the mass, pressure, density, and temperature of the air that enters the chamber with each stroke.
The increase in mass, of course, increases engine power, which is why engines are built with turbos.
The increase in temperature, however, can result in the fuel/air mix reaching auto-ignition temperature before the spark is fired. This is pre-ignition. It's a bad thing, since it reduces engine power and efficiency.
Furthermore, even if preignition doesn't occur, ignition of the fuel/air creates high pressure sound waves, which move faster than the flame front, and the high temperatures produced by those sound waves can potentially cause auto-ignition ahead of the flame front. This is knocking (or pinging). It's a bad thing, since it can quickly cause damage to the engine.
Knocking can be detected acoustically... when it occurs, it should be stopped my doing one of the following: reduce the amount of wood gas added to the engine's intake, reduce turbo boost pressure, recirculate some of the exhaust, or to add a mist of water to the fuel air mix.
Reducing the amount of wood gas will require increasing the amount of diesel fuel used.
Reducing turbo boost pressure will reduce engine power.
Recirculating exhaust can reduce engine power.
Adding water to the fuel/air mix requires having a source of it... if we're using the right kind of wood gasifier design, the steam that's produced when drying the feedstock is separately condensed into water; if this condensate is clean enough, it can be used for water injection.
The increase in mass, of course, increases engine power, which is why engines are built with turbos.
The increase in temperature, however, can result in the fuel/air mix reaching auto-ignition temperature before the spark is fired. This is pre-ignition. It's a bad thing, since it reduces engine power and efficiency.
Furthermore, even if preignition doesn't occur, ignition of the fuel/air creates high pressure sound waves, which move faster than the flame front, and the high temperatures produced by those sound waves can potentially cause auto-ignition ahead of the flame front. This is knocking (or pinging). It's a bad thing, since it can quickly cause damage to the engine.
Knocking can be detected acoustically... when it occurs, it should be stopped my doing one of the following: reduce the amount of wood gas added to the engine's intake, reduce turbo boost pressure, recirculate some of the exhaust, or to add a mist of water to the fuel air mix.
Reducing the amount of wood gas will require increasing the amount of diesel fuel used.
Reducing turbo boost pressure will reduce engine power.
Recirculating exhaust can reduce engine power.
Adding water to the fuel/air mix requires having a source of it... if we're using the right kind of wood gasifier design, the steam that's produced when drying the feedstock is separately condensed into water; if this condensate is clean enough, it can be used for water injection.
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