Low Alcohol Fuel Energy Content but same Engine Power

[AbdullahBajwa]

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.

 

[dgallup]

There is less energy per pound but you can burn more pounds of fuel so the total energy release is similar.

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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.

 

[140Airpower]

This statement, "...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" ?...", is incorrect because it assumes the same metering in both cases while it is essential to use metering appropriate for the Alcohol fuel. The assertion about equal power depends on the metering being correct for the fuel.

 

[AbdullahBajwa]

Thanks dgallup and Airpower .

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.

 

[Compositepro]

The power output of an engine is limited by how much oxygen you can get in the cylinder because oxygen is the lowest density component in the mixture. Adding more fuel than the available oxygen reduces the power output.

 

[AbdullahBajwa]

Yes Compositepro ,theoretically what you are saying is correct because if you have less oxygen than the 'chemically correct' requirement for complete combustion then not all of the fuel burns and some of the fuel's energy goes to waste as the unburnt fuel is exhausted . But actual engine performance curves show that the max power is obtained at a >1 equivalence ratio i.e when your air fuel mixture is slightly rich . However the best fuel conversion efficiency is obtained with leaner fuel mixtures.

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.

 

[SomptingGuy]

"engine power for the given displacement" is the key phrase. It says nothing about fuel flow rate, which might be of interest for economy considerations. Your text is trying to explain how the pros and cons seem to cancel each other for the fuels mentioned, when power density is your measure.

- Steve

 

[azmio3]

From what I read, the author mentioned about the word "almost" which implies that per one induction stroke, there will normally be enough oxygen to optimally burn the originally intended fuel. Once the fuel is switch into alcohol fuel, the burnable fuel with the inducted air will normally have almost the same energy content. In return it will still return about the same amount of power as per the originally intended fuel.

You can take the statement as a general statement but not wholly accurate for high oxygenate fuels like methanol.

 

[tbuelna]

Common alcohol fuels like methanol or ethanol do not work well in a conventional CI engine due to their ignition properties. If you directly injected ethanol or methanol into a typical CI engine with a CR around 17:1 combustion would probably not occur. This is also a problem for CI engines using NG fuel. The way conventional diesel engines are made to run on NG is by pre-mixing the NG with the intake air, compressing the air/fuel charge to a high CR that a diesel engine would use, and then injecting a tiny amount of diesel fuel just before TDC to initiate combustion. This type of engine is called a dual-fuel engine.

 

[Lou Scannon]

By the way, alcohol fueled CI engines were developed to a certain extent of commercial readiness in the late 1980's to early 1990's, in a heavy duty methanol truck demonstration project funded by the California Energy Commission, DOE/NREL, South Coast AQMD, and others. The prevalent solution for achieving compression ignition was continuous, or at least very aggressive use of glow plugs. As a result, glow plug longevity in terms of engine operating hours wasn't quite as desired, but that is understandable considering their derivation from standard glow plugs designed for intermittent use.
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:
[ul]
[li]corrosion and clogging in the on-board and infrastructure fuel supply systems[/li]
[li]health effects due inhalation, skin contact, or ingestion[/li]
[li]invisibility of flame during daylight (i.e. following an accident involving fuel spillage and fire).[/li]
[li]corollary to the injector sizing issue, tankage needs to be increased in inverse proportion to the fuel energy content, to maintain vehicle range.[/li]
[/ul]

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[turbocohen]

In my early experiences with methanol vs gasoline I saw a small but consistently measurable increase in peak torque, presumably because the methanol molecule contains oxygen.. So, in a way it was liquid supercharging more oxygen into the combustion process.

 

[140Airpower]

Methanol gives more power than gasoline unlike ethanol.

 

[Lou Scannon]

Methanol gives more power than gasoline unlike ethanol.

Of course it is not simply a matter of changing which fuel is put in the tank, fuel-flexible vehicles/engines excepted. Assuming that at least the A/F and spark timing are optimized for each fuel, I agree with your statement about methanol but disagree with your statement about ethanol. If CR and/or boost pressure are also optimized, the available power increase is considerable, at least with pump gas as the gasoline reference.
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

 

[140Airpower]

Hemi, I know that the balance of power between gasoline and ethanol is close, but I'm relying on Honda's finding that they got less power in their Indycar turbocharged engines with E85 than with gasoline.

 

[Lou Scannon]

I'm not aware of these Honda Indycar engines; but it's important to clarify, what was the net octane of the E85, and of the gasoline it was compared with? And what optimization did they do... spark timing, A/F, compression ratio?
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

 

[140Airpower]

Hemi, sorry I misremembered the details of the conversion. The conversion was from Methanol to Ethanol.

 

[Lou Scannon]

No worries.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz