Low Alcohol Fuel Energy Content but same Engine Power
Low Alcohol Fuel Energy Content but same Engine Power
(OP)
I am studying about the use of alcohols as fuels in CI engines . The energy content (LHV / HHV) of alcohols is almost half as compared to that of petro diesel . My understanding of this difference is that , in order to get the same amount of work from the same engine we would need to put in twice as much alcohol as compared to diesel .The text further goes on to say that the A/F ratio for alcohols is lower as compared to petroleum fuels because of its inherent oxygen molecules .Agreed , makes perfect sense .
What I am finding difficult to grasp is the next sentence where it says "Their(alcohols) air requirement for combustion is lower , and hence, the energy content of the mixture is almost the same........Even with the lower energy content of alcohol, engine power for the given displacement would be the same "
So , the A/F ratio is lower for alcohols . This would mean that if we use a metering system originally designed for petro fuels we would get an alcohol/air mixture which is lean according to the stoichiometric calculations using the chemical equation of alcohol combustion. Can some one please explain how this results in "the energy content of the fuels being almost the same" ? Does it have something to do with the relatively high degree of completion of the combustion process due to the abundance of available air? This has been confusing me the whole morning because we know that the maximum power/MEP for engines is obtained with relatively rich mixtures.
All help would be appreciated.
What I am finding difficult to grasp is the next sentence where it says "Their(alcohols) air requirement for combustion is lower , and hence, the energy content of the mixture is almost the same........Even with the lower energy content of alcohol, engine power for the given displacement would be the same "
So , the A/F ratio is lower for alcohols . This would mean that if we use a metering system originally designed for petro fuels we would get an alcohol/air mixture which is lean according to the stoichiometric calculations using the chemical equation of alcohol combustion. Can some one please explain how this results in "the energy content of the fuels being almost the same" ? Does it have something to do with the relatively high degree of completion of the combustion process due to the abundance of available air? This has been confusing me the whole morning because we know that the maximum power/MEP for engines is obtained with relatively rich mixtures.
All help would be appreciated.





RE: Low Alcohol Fuel Energy Content but same Engine Power
----------------------------------------
The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
In fact when you read the first response in conjunction with the second one, then it all makes sense. So , for a given compression ratio and engine stroke we would be able to get in more 'pounds' of fuel (alcohol) into the combustion chamber and because the fuel carries 'less energy per pound' , effectively we will end up with power equal to a fuel with more energy per pound but less pounds per sucked into the cylinder during the induction stroke.
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
So, I suppose your statement is correct w.r.t efficiency which is the measure of the effectiveness with which the fuel's energy is utilized.
RE: Low Alcohol Fuel Energy Content but same Engine Power
- Steve
RE: Low Alcohol Fuel Energy Content but same Engine Power
You can take the statement as a general statement but not wholly accurate for high oxygenate fuels like methanol.
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
The other solution, that might have been alluded to or mentioned above, is the use of a Cetane number enhancing additive; the particular one employed, by Cummins, I believe, was called Avocet(TM), IIRC.
A third option, of which I have no first-hand knowledge, is on-board reforming the alcohol (or just a portion of it?) to an ether, e.g. Di-methyl ether, which has a decent Cetane number, I understand. I believe Navistar was pursuing this, in a period subsequent to the termination of the methanol truck project.
Since the methanol truck project was publically funded, there are several reports available in the public domain, that might be found on the sites of the various sponsors. If you're interested and need help finding these reports, I'll be glad to assist.
The other developmental hurdle with these engines was injectors, quite understandably. For reasons adequately dealt with above, their capacity needed to be nearly doubled e.g., for methanol versus diesel. The bigger difficulty, at the time, was dealing with the low lubricity of methanol vs. diesel. Given the extremely high pressures of common rail diesel systems today, unknown back then in heavy duty engines, this may yet be big hurdle for adapting the injectors and pumps to a low lubricity fuel such as alcohol.
Other, non-engine-related issues experienced or apprehended with alcohol fuels in the truck project, some of which have been mitigated in the mean time are:
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
In addition to the fairly obvious advantages of greater internal cooling due to the greater mass of alcohol needing to be evaporated vs gasoline, which directly accrue to better antiknock qualities and power density potential, there are also some fairly subtle factors such as the difference in the ratio of specific heats (k or gamma) of the charge between compression and expansion that are in the alcohols' favour, regarding the work of compression vs the work of expansion.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
If an engine is operated on a fuel it wasn't originally designed for, it's tougher to optimize, as you might expect.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: Low Alcohol Fuel Energy Content but same Engine Power
RE: Low Alcohol Fuel Energy Content but same Engine Power
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz