Smart questions
Smart answers
Smart people
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Member Login




Remember Me
Forgot Password?
Join Us!

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips now!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

Join Eng-Tips
*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Donate Today!

Do you enjoy these
technical forums?
Donate Today! Click Here

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.
Jobs from Indeed

Link To This Forum!

Partner Button
Add Stickiness To Your Site By Linking To This Professionally Managed Technical Forum.
Just copy and paste the
code below into your site.

PhdDave (Chemical)
31 May 07 19:43
What is the highest compression ratio that E85 is able to run at without undue harm? What are the chances that the automotive manufacturers will make a car that can only run on E85, and gain power by using a lighter engine of smaller displacement?
reidh (Automotive)
31 May 07 20:31
Dave,

The maximum CR for E85 depends on so many factors, its probably not worth listing them all.

Regardless, an engine that is designed to run solely on E85 could have a higher CR than a flex-fuel engine.  We have to remember though the increased octane rating of E85 will never make up for the reduced energy density.

Reidh
NickE (Materials)
1 Jun 07 10:14
reidh- you do mean in mileage? Cause with a bit of FI and good fueling you can get more power for sure.
lugnuts67 (Industrial)
1 Jun 07 14:33
there is less energy in E85 than an equal amount of gasoline. period.  However, because you can run higher compression ratios, you can achieve higher cylinder pressures and produce high HP.  You need a good bit more juice though.  A pontiac guy here in the south converted his supercharged drag car and posted his findings at http://hardcorepontiacs.com/forum/169-e85-report.html
He said that he used almost twice the fuel as he would with race gas but it cost 1/3 as much.

Also, if you are going to give it a try I recommend Quickfuel's metering blocks - they make the job much easier even though the E85 is more forgiving as far as jetting for max power goes.
globi5 (Mechanical)
1 Jun 07 19:10
This engine runs with a compression ratio of 28 on E95:
http://www.greencarcongress.com/2007/05/scania_to_unvei.html
(probably not spark ignition though).

But I remember reading of a spark ignition engine with a compression ratio of 19 on pure ethanol.

In order to deal with the higher pressure, an engine will have to be strengthened and therefore gain weight.
A significant power (per weight) increase (to allow for a smaller engine) may or may not be achieved with a naturally aspirated engine. It will work, if the engine was supercharged.
crysta1c1ear (Automotive)
4 Jun 07 19:39
For use in my next posting

Adiabatic compression:
P1V1k=P2V2k

Gas Law:
P1V1÷T1=P2V2÷T2

Divide the first equation by the second:
T1V1(k-1)=T2V2(k-1)

Regroup temperatures and volumes:
T1÷T2=(V2÷V1)(k-1)

Move powers to other side:
(T1÷T2)1÷(k-1)=V2÷V1

I don't really like all this gamma or k stuff - the ratio of the specific heats, so let me simplify it.

Cv is the number of degrees of freedom in the gas, and Cp is two more. So k=(dof+2)/dof and k-1=2/dof. So 1÷(k-1) = dof/2. Before combustion air molecules don't reach the sort of temperatures needed to make them vibrate, so the number of degrees of freedom is say 5 and power is therefore 5/2.

Now if state 1 is autoignition and state 2 is the start of compression with ambient air let's write the equation again.

CR=(Tauto÷Tamb)5÷2
crysta1c1ear (Automotive)
4 Jun 07 20:02

Quote (globi5):

But I remember reading of a spark ignition engine with a compression ratio of 19 on pure ethanol.

When did the intake valves close on the engine with the 19:1 compresstion ratio?

I ask, because I read an autoignition temperature for ethanol of 630 Kelvin and estimate the maximum you could compress air before reaching the autoignition temperature of ethanol as being ...

CODE

      PRINT (630/300)^(5/2)

Maximum compression ~ 6.4 ???

I know that sounds really low. Bear in mind it is from start of compression to autoignition and not from V_Max to V_min so my calculated compression ratio would only start at intake valve closing for example.

I am wondering if my autoignition temperature of 630K for ethanol is wrong, or my theory, or what?

And I am wondering how the 19:1 ethanol engine doesn't autoignite? Was the engine spark ignition, but direct fuel injection, thus preventing the fuel from igniting under pressure, as it cannot light until it enters the engine?
patprimmer (Publican)
4 Jun 07 20:59
Alcohol being methanol or ethanol will run with quite rich mixture. This rich mixture cools the charge by evaporative cooling so offsetting some of the adiabatic temperature rise.

Also the engine may have less than 100%Ve and as you say, late intake valve closing will reduce effective CR.

Heat added from and lost to the block and head surfaces will also effect charge temperature as in the early part of the cycle the charge will gain heat, but at a point in the compression stroke it will start to lose heat.

I have seen over 20:1 on methanol. I have never pushed ethanol to the verge of detonation.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

crysta1c1ear (Automotive)
5 Jun 07 0:10
Thanks Pat
I read 680K for an autoignition temperature for methanol, compared to 630K for ethanol.
globi5 (Mechanical)
5 Jun 07 15:09
I don't know whether the intake valve actually did close late on this engine.

But, if I also remember correctly the latent heat of evaporation of ethanol is at least 3 times higher than that of gasoline.

So, not only does ethanol have a higher octane rating and run richer, it also requires more heat (and takes it from the adiabatically heated air).

If you also calculate the heat required for the ethanol phase change (liquid to vapor), you'll notice that the compression ratio can be significantly higher than 6.4.
(9kg hot air and 1kg liquid, cold ethanol).
dcasto (Chemical)
5 Jun 07 23:52
I'd bet E85 is 85% ethanol octane=100(?) and 15% 6rvp natural gasoline octane = 40, net about 90 octane, so compression ratio could be 11:1 or so.
crysta1c1ear (Automotive)
8 Jun 07 11:38
I did an estimate the other day, which I won't put here as the calculations aren't handy, but from memory, at 9:1 air fuel ratio, ethanol evaporating could take about 130 kelvin out of the air. That sounds a lot. Can it be right? And similar calculations (so similar mistakes if wrong) gave iso-octane removing about 30 kelvin.

The autoignition temperatures I had were 630K for ethanol and 690K for isooctane.

octane  690+30=720
ethanol 630+130=760

So air that might reach 760 due to compression (in the absence of fuel) could reach 630 due to ethanol evaporation (a drop of 130) and just start to autoignite.

What I am saying is at stoichiometric AFRs ethanol looks like at could support significantly higher compression ratios than isooctane due to the large amounts of evaporative cooling.

If you look at the enthalpy of vaporization of ethanol (around 40 kJ/mol but varies depending on source in the literature!) and that of octane, the numbers are broadly similar. But the figures per molecule only tell part of the story, as an ethanol molecule has two carbons and octane has eight. With that in mind, it comes as no surprise then that evaporating the ethanol required for combustion drops the temperature about 4 times as much as evaporating octane.
(You have to evaporate about four times as many ethanol molecules.)

So if the temperature drops are as much as I estimated the other night, 130 kelvin (ethanol) and 30 kelvin (octane), it would explain why low autoignition temperature ethanol can have a higher compression ratio than high autoignition temperature isooctane.
dcasto (Chemical)
8 Jun 07 12:11
crystal1clear, I don't think your analysis is correct. I use pure methane gas as a fuel.  When the methane is injected with the air, the combined temperature is the same.  Methane has an Octane rating of over 100 and because of this we can run up to 11:1 compression ratio.  Maximum compression ratio is more a function of the fuels octane rating which is inturn a function of how the molecules stucture and burning rate.  Look at normal octane, its physical properties are nearly the same as isooctane, yet it has a 0 octane value and if used as a fuel, the compression ratio for an engine would have to be about 4:1

The original OP was for E85, unless you know what the other 15% is, you can't committ to a compression ratio.  Like my original answer, E85 probabily has a net octane of 90.  I haven't see the spec's but I'll bet the new flex fuel cars are less than 10 : 1 compression ratio and the knock sensors retard the timing big time if you put plain ol 85 octane in it!
globi5 (Mechanical)
8 Jun 07 12:54
dcasto,
if a higher latent heat of evaporation wasn't just as effective as using a fuel with a higher octane number, then water injection would not be applied.
spdracer22 (Automotive)
8 Jun 07 15:03
As a reference... In an auto engineering trade mag I picked up a SAE World, there's an article about a prototype Lotus Elise running E85...

If I remember correctly, it's 11-11.5:1 CR, supercharged, with  6 injectors (4 standard location, 2 pre-S/C). They were also running it with the knock control disabled. The two pre-S/C injectors aided charge cooling, and I believe they said it could have run just fine without the air-air intercooler altogether. I think they picked up 60 extra horsepower with just the addition of the two injectors and more advanced timing. Those were the only changes (injectors/timing) over the base car.

From what I've read, E85 has an octane rating of about 100, thus the higher timing that can be used (not to mention the better charge cooling effects). BUT, mixes as low as E70 can be labeled for sale as "E85" for better winter/cold starting performance, so you have to be careful if you're building/tuning an engine based solely on E85.

My thoughts on the flex fuel cars is that they probably run their standard gasoline-only engine compression ratio, but increase/decrease timing and fuel flow based on the fuel mix. Since the change is mostly software-based, and most people will run plain old E10 anyway, it seems to be the most logical choice (to me) for an OEM.
patprimmer (Publican)
8 Jun 07 18:50
Octane rating is simply a measure of knock resistance. The mechanism of the knock is not considered.

There are a whole host of other factors that effect the onset of detonation.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

crysta1c1ear (Automotive)
8 Jun 07 21:09

Quote (dcasto):

Look at normal octane, its physical properties are nearly the same as isooctane, yet it has a 0 octane value and if used as a fuel, the compression ratio for an engine would have to be about 4:1

Autoignition temperatures
n-octane   500
iso-octane 690
methane    850

Compression ratio required to compress 300K air to
500 ->  3.6
690 ->  8.0
850 -> 13.5
calculated using
CR=(Tauto÷Tamb)5÷2
Helpful Member!  PhdDave (Chemical)
9 Jun 07 16:09
To clear up things,if I were to build an engine to run only on E85 rather than on gas and/or E85 and my target was 100HP engine output, would I be able to save weight on the car with the smaler engine and gain miles per gallon?
patprimmer (Publican)
9 Jun 07 21:22
No

The potential decrease in engine size is minimal if any.

Ethanol contains less energy per units weight or volume than petrol and the power increase comes from it requiring a richer mixture as well as higher potential compression ratio.

The percentage increase in fuel required is greater than the power increase, therefore the fuel consumption goes up, not down.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

Mechaniac (Automotive)
10 Jun 07 9:44
If your question is would a smaller engine built specifically for the advantages of E85 generate better fuel efficiency than a larger one built as a gasoline engine but burning E85, both generating a peak output of 100 HP, then yes.  Most certainly the smaller engine would consume less fuel under normal use.  Is it less MPG than a gasoline engine built for the same purpose, no, but closer than the currently produced "FlexFuel" engines that are a compromise on E85.  I've been told on another forum that the current Car & Driver Magazine has an article on some OEM manufacturers doing just what you're considering.  That is designing small E85 only engines with specific outputs much higher than possible on Gasoline but better fuel efficiency than a larger FlexFuel engine.  I'm going to have to pick up a copy and check it out myself as I have converted my '01 Chevy Silverado 3/4 ton pick-up with a 8.1 liter V8 Gasoline engine over to E85.  It's only been just over a week but power has gone up an exciting amount and fuel efficiency isn't that bad compared to gas.  It's at least 85%.  If I were to rebuild with higher compression and optimized ignition timing both could only get better.

Vernon

pipehead (Petroleum)
10 Jun 07 19:55
Crystal1clear, and your point is?  By definition at 100 octane you can run 12.5 : 1 and not detonatein a variable compression engine test stand, so auto iginition doesn't hold water.  Try your numbers out on nitro-methane and methanol too.
After many years of study, catapillar engines and waukesha engines use a modified octane calculations for fuel mixtures, all emperical and not related to autoignition.

spracer2, I'd like to read that article too, I run the same 2zzge engine myself. The engine is a 11.5 :1, Over in England they can run higher net CR (as adjusted for turbos/superchargers) than in the US, just look up a misubishi FQ400.  

Phdave, the 2zzge engine has a high  HP ouput to weight ratio at 100HP/liter and the FQ400 is 400HP for 2 liters with a highly tuned and fuel cooled engine, so a turbocharged 900 cc engine could get you 100 HP.
patprimmer (Publican)
10 Jun 07 20:41
pipehead

The two engines you mention are diesel so they are hardly relevant to this discussion.

crystal1clear's calculations work reasonably well for gaseous fuels, but get fairly well out of whack with liquid fuels due to latent heat of evaporation and variations in burn rate (and therefore rate of pressure rise) due to fuel particle size. As already mentioned, they also obviously ignore a number of other factors which generally only have a moderate to minimal effect.

I should have posted this link a lot earlier as it contains good data to answer the OP and to dispel a lot of myth and garbage that has been presented in this thread to date.

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

pipehead (Petroleum)
10 Jun 07 21:11
WHAT PATPRIMMER?  the  2 liter mitsubishi engine sold in the EVO and the 1.8 liter toyota sold in the celica, vibe, matrix, corolla, elise are gasoline....  These along with the 2.5 liter subi and 2.3 liter ford/mazada are top contenders for HP/CI ratio engines.

Good generalized link for the consumer. It doesn't eplain 400 HP from 2 liters on 95 RON (oh, ineurope they do not post ron + mon / 2 like here only RON)). How, do dragsters work based on all the info.  It's a bigger picture once you step outside the typical consumer stuff.  Don't you remember all the word fights in the 70's, whats better a 454 vette or a 3 liter (190 ci) porche.  Man a VVTL-i ot DI engine changes all the common rules.
patprimmer (Publican)
10 Jun 07 22:32
Umm

These are the diesels you refer to

Quote:

After many years of study, catapillar engines and waukesha engines use a modified octane calculations for fuel mixtures, all emperical and not related to autoignition

This guy asked a pretty general question in the OP

Can we respect the forum rules and stay on that topic and be constructive rather running off on tangents.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

pipehead (Petroleum)
11 Jun 07 12:44
look up this link to the cat spark engines.
http://www.cat.com/cda/files/98944/7/lehw0815_03.pdf

The OP wanted to know the maximum compression ratio and people started talking about ignition temps that dictate it which is wrong.  The term of octane number is the best way to calculate maximum compression ratio.  Put the octane value has to be found emperically from the lab, not by ignition temperature.  To make things even more difficult, there is no set rule for what E85 is othan 85% ethanol, heck, the other other 15% could be jet fuel.  So to make a point that ignition temperature is not a corerelation, I pointed out all kinds of other engines and fuels that do not follow the trend trying to be laid out.
bentwings1 (Mechanical)
11 Jun 07 13:28
Hi,
I just came from a streetrod event Sat.  There was guy who was selling carbs modified for E-85. His 2 demo cars were a 6-71 blown small block Chev in a Nova..quite a large one 427 cu in. This one had a dyno sheet that showed well over 500 hp and 600+ ft lbs torque. It ran about 7 pounds of boost. He said that while it is really a hot rod it actually was pretty mild mannered. He said it could make a lot more power just by upping the boost and changing carb jets.  His other car was a tamer Camaro with a 383 cu in SBC. This one was much more of a "driver".  Both cars are driven to the local streetrod events and on numerous rod runs and are not trailer queens.

To answer the question the blown one had 12.7:1 and the unblown one had 13.7:1 compression ratios.

E-85 is readily available here in Minn so it is an alternative.  It takes high compression to make it work and you probably won't get the greatest MPG as it simply takes more fuel to burn properly. It does work however.

99 Dodge CTD dually.

dcasto (Chemical)
11 Jun 07 14:25
Found this on e85 octane.   This article states e85 will be 100 to 105 octane, but there is specification like we have for gasolines RON + MON being at least 85 octane minimum (83 at high altitude).  Right now the flex fuel vehicles are tuned and have a CR such that you can use 85 octane.  The CR will be from 8.8 to 10.0 to 1 based on other engine and tuning factors. As soon as blenders get it fiquired out, they will reduce octane on E85.


http://www.agriculture.state.ia.us/e85q&a.html
patprimmer (Publican)
11 Jun 07 17:54
pipehead

Thank you for the link.

That is all you need to do to avoid controversy when you throw in comments from left field.

Re your comments to crystal1clear.

Auto ignition point is just one of many factors to consider. That does not mean it is wrong to consider it in detail.

The point that there are many factors involved was already mentioned. See quotes below.

Quote:

patprimmer (Automotive)      
4 Jun 07 20:59
Alcohol being methanol or ethanol will run with quite rich mixture. This rich mixture cools the charge by evaporative cooling so offsetting some of the adiabatic temperature rise.

Also the engine may have less than 100%Ve and as you say, late intake valve closing will reduce effective CR.

Heat added from and lost to the block and head surfaces will also effect charge temperature as in the early part of the cycle the charge will gain heat, but at a point in the compression stroke it will start to lose heat.

I have seen over 20:1 on methanol. I have never pushed ethanol to the verge of detonation.

Regards

patprimmer (Automotive)      
8 Jun 07 18:50
Octane rating is simply a measure of knock resistance. The mechanism of the knock is not considered.

There are a whole host of other factors that effect the onset of detonation.

Regards

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

crysta1c1ear (Automotive)
12 Jun 07 18:31

Quote:

http://blizzard.rwic.und.edu/~nordlie/cars/gasoline.html

6.2 ...
Simply put, the octane rating of the fuel reflects the ability of the unburnt end gases to resist spontaneous autoignition under the engine test conditions used.
...


Quote:

http://www.agriculture.state.ia.us/e85q&a.html

What is the octane rating of E85 compared to gasoline?
Regular unleaded gasoline has an octane rating of 87; E85 has an octane rating ranging from 100-105 making it a high performance fuel. Ford FFVs produce a 5% horsepower gain when using E85.


Quote:


http://www.technologyreview.com/Energy/16727/page1/

The researchers solved the knocking problem by injecting into combustion chambers precisely controlled amounts of ethanol at moments when the engine is working hard enough to cause knock. Compared with gasoline, ethanol has higher octane, a rating of how much a fuel can be compressed before it combusts spontaneously, that is, before it causes knocking. The injected ethanol also cools the mixture, so it effectively increases the octane rating of the fuel mix to about 130 -- as good as high-performance racing fuels, Cohn says.

Ok, here are some more thoughts about ethanol.

Firstly, if octane rating is about ability to resist autoignition, then I think it is just as valid to think about autoignition temperatures as it is to think about octane ratings, since the main subject is neither. The thread is about compression ratios.

Now, does it actually make sense to talk about an octane rating for ethanol? What is on my mind is that evaporation can cause such huge differences in behaviour, depending on whether evaporation occurs in the cylinder and depending on concentrations (ie AFR), that what we'd like to think of as a single property - octane rating - is not going to be something that fixed in practice.

(Of course octane ratings will be defined under precise conditions so in a sense is fixed. But if the conditions in our engines are different from the reference engine, the figure somebody gives us as an octane rating for ethanol are then likely to be meaningless to us.)

Note the quote above about ethanol evaoration being able to push octane ratings of gasoline ethanol mixtures to 130.
Here's why I think ethanol evaporation is so important.

Autoignition temperatures (kelvin)
n-heptane 560
iso-octane 690

There is a 130 kelvin difference between the two fuels, thus causing n-heptane and iso-octane to have very different octane ratings.

Now I'm going to calculate later that evaporation of ethanol can cause an even bigger 140 kelvin temperature drop (or potentially even more in a rich mixture).

With gasoline, the temperature change caused by evaporation
will be very significantly less. So changes in AFR shouldn't affect knock so much, and maybe using an octane rating makes more sense.

If you design a high compression ratio ethanol engine with evaporation in the cylinder, then I would think very carefully about what might happen if it runs a bit leaner, doesn't get the temperature drop from evaporation, and so maybe starts to knock.

The references above give octane ratings of 85 for gasoline and 100 for E85, and yet of 130 for some mixtures. Maybe that sort of variation is a partial cause of the question of compression ratio for E85, or lack of a clear answer. And now I am suspecting that evaporation is the main reason why.

In the next post I'll calculate 140 kelvin or so as a potential temperature drop due to evaporation.

Oh, and how does all this affect compression ratio? Well, as you compress the mixture in the cylinder the temperature rises. As the fuel evaporates the temperature drops. If the temperature rise from some of the cylinder compression and the temperature fall from evaporation cancel each other out, then that bit of compression is in a sense free and hardly contributing to the end gases autoigniting, ie knocking.
crysta1c1ear (Automotive)
12 Jun 07 18:46
Here are my calculations for the possible cooling effect of ethanol (143 kelvin) and gasoline (33 kelvin) in a stoichiometric mixture, together with sources for my data.

My procedure is quite simple. Take the heat required to evaporate a kilo of fuel.
Divide by the AFR to work out the heat taken out of a kilo of air (as the fuel evaporates at constant temperature).
Divide that by the specific heat of air to see how much that kilo of air would cool down.

Heats of vapourization
http://www.michigan.gov/documents/CIS_EO_Control_of_Emissions-E85-Final_AF-E-_87915_7.pdf
Ethanol 920 * kJ/kg
Gasoline 350 * kJ/kg

AFRs
Ethanol 9
Gasoline 14.7

Specific heat of air
http://wright.nasa.gov/airplane/airprop508.html
715 * J/(kg*kelvin)

Calculations to paste into a windows calculator
920000/9/715=
350000/14.7/715=

results
ethanol 143
gasoline 33

dcasto (Chemical)
17 Jul 07 11:36
I just talked witha large gasoline blending and here are some insider things taking place in ethanol.
The gasoline blenders are testing ethanol movements in pipelines with extreme caution, it may take a year or more for the results. A early error occurred in the Alanta market where a small amount of water from the ethanol got into the the tanks at a few gas stations.  The water caused consumer engine problems and over 100 gas stations were shutdown in the area for a week or so.  This cause the other stations to pick up the slack with extra fuel brought in and lines at the stations.
They have also set up a very low octane grade of gasoline to be used as ethanol blending stock.  This low octane, low vapour pressure blend mix can only be used with high (E85) type blends.  It will allow the ethanol blend to be low octane.
The blenders are scrambling to find ways to get the ethanol to other markets, from the midwest.  The southeast will be the next area to place ethanol fuels.
JonB11 (Mechanical)
18 Jul 07 11:43
crysta1c1ear, the information you provided on heat of vaporization is very interesting. If I remember correctly, H vap. is the result of IMF's (intermolecular forces) between the hydrocarbon molecules found in both fuels. Ethanol is a much smaller molecule with less powerful dispersion forces (attractive) between molecules but its polar nature (the hydroxide group "O-H") of the molecule results in much stronger IMF's then the larger but non-polar Octane. This accounts for an increased capacity for cooling during the phase change from liquid to gas as the fuel enters the engine. This however, has nothing to do with the amount of energy contained within the fuel. A greater cooling capacity will fundamentally have greater knock resistance but it is the CHEMICAL reaction with O2 that results in combustion and the powerful explosion caused by ignition. I'm sure that any chemists here can explain that better than I can.  

The fuel that I run in my 60 Chevy Hot Rod and in the plane I fly is a 100 octane low lead gasoline. The increased octane is the result of lead increasing the cooling capacity of water (I believe the principle is similar to adding salt to water to raise it's boiling temp.). This was how so many muscle cars where able to benefit from running both gasoline and high compression. FYI most lycoming all aluminum, air coolled engines run on 100LL and have very high compression ratios (ie. 13-15 range I believe) and these little engines have a VERY high power to weight ratio, although at a much lower RPM than most car engines (they red line under 3K) and they have much different torque requirements because of the systems that they are driving. I hope this is helpful.   
JonB11 (Mechanical)
18 Jul 07 11:43
crysta1c1ear, the information you provided on heat of vaporization is very interesting. If I remember correctly, H vap. is the result of IMF's (intermolecular forces) between the hydrocarbon molecules found in both fuels. Ethanol is a much smaller molecule with less powerful dispersion forces (attractive) between molecules but its polar nature (the hydroxide group "O-H") of the molecule results in much stronger IMF's then the larger but non-polar Octane. This accounts for an increased capacity for cooling during the phase change from liquid to gas as the fuel enters the engine. This however, has nothing to do with the amount of energy contained within the fuel. A greater cooling capacity will fundamentally have greater knock resistance but it is the CHEMICAL reaction with O2 that results in combustion and the powerful explosion caused by ignition. I'm sure that any chemists here can explain that better than I can.  

The fuel that I run in my 60 Chevy Hot Rod and in the plane I fly is a 100 octane low lead gasoline. The increased octane is the result of lead increasing the cooling capacity of water (I believe the principle is similar to adding salt to water to raise it's boiling temp.). This was how so many muscle cars where able to benefit from running both gasoline and high compression. FYI most lycoming all aluminum, air coolled engines run on 100LL and have very high compression ratios (ie. 13-15 range I believe) and these little engines have a VERY high power to weight ratio, although at a much lower RPM than most car engines (they red line under 3K) and they have much different torque requirements because of the systems that they are driving. I hope this is helpful.   

dcasto (Chemical)
18 Jul 07 14:58
http://www.radford.edu/~wkovarik/papers/kettering.html

Here's a great history and some more info on octane.  I can noit seam to get Crysta1clear off the auto ignition as a reason for octane number. Nor the heat of vaporization.  He's even proved my point earlier.  n octane has a low compression ratio about 4 before detonation and has a low auto ignition 650 degrees, iso octane at 100, has a 12 to 1 compression ration and an autoignition higher than n-octane, but he cannot explain that pure methane has the same 100 octane rating and can go to 12 to 1 compression, but it's auto ignition temp is way above that of iso octane.. get it no connection here.

Then the heat of vaporization, pure methane as fuel has no heat of vaporization, get it again, it has nothing to do with it.  What you do get  with the heat of vaporization is more dense air and fuel, more pounds in the cylinder, more explosion, more power.  SEE also CAI, CO@ Chiller, NOs dry injection systems.

I don't think the lead changes chemistry or temperatures, it interfeers with the combustion process and inhibits detonation.
Eric68 (Automotive)
21 Jul 07 10:09
Interesting thread, hope you guys don't mind a newbe commenting on this.  

I have a carbureted 68 Camaro that I recently converted to E85 and thought I would offer some comment from the "practical experience side" of this discussion.  

My engine is a 410" small block Chevy and has a static compression ratio of 11.49:1.  The intake valve closes at 68* after bottom dead center (during compression stroke).  Prior to switching to E85 I ran 93 octane in the engine and under certain circumstances (hot coolant temp, hot ambient air temp) it would show early signs of detonation.  Cranking compression on this engine averages 225 psi.

When I switched to E85 I started with a carburetor calibrated for E85 but due to differences in fuel blends (as stated above) and engine characteristics I needed to do a LOT of tuning to get it right.  During the course of tuning I ran the engine too lean (near stoichiometric) while it was too hot with too much ignition advance.  I even ran nitrous oxide too lean (a MAJOR no-no with gasoline) and never once had any sign of detonation.  

The octane rating of the E85 that I use is 105 however, I believe that the cooling effect mentioned above is a significant factor in this fuels apparent effectiveness in preventing detonation.  That would be disappointing if the suppliers find a way to reduce the octane rating because IMO that is the major advantage to this fuel.

Incidentally, my engine picked up power on the E85 fuel (runs 10.9's in the quarter mile).  I do use a lot more of it than gasoline (approx 25% more at cruise and 35-40% more at WOT) but it seems to be a good alternative for me at this point (it runs 50 cents cheaper per gallon than pump premium gasoline octane and is $2 cheaper than race gasoline).  

As an aside, re: the comment on water contamination in the Atlanta area.  ethanol is known to be less corrosive than methanol which is beneficial for the use of aluminum parts, however, I have read that ethanol forms acids when mixed with >1% water and becomes much more corrosive.  Any comments on this?  

dcasto (Chemical)
23 Jul 07 15:49
Your experience matches what I've set out in my posts.

Your opinion is that E85's major advantage is its high octane. All the E85 cars sold today have nothing done to them to take advantage of the octane and until SAE or someone sets an absolute octane number on E85, they will not.  About the onlything I can see they need to do is change out the fuel system, put in bigger injectors, and do something with the ECU mapping.

Auto manufacturers will not take advantage of higher octane because they have warranties on their engines whereas your 410 CI has none.  In Europe they modified an engine to 400 HP from its 290 Hp generated in the US because the average temperature across the island in way lower than in Las Vegas and there top gasoline has about 3 more octane than here in the US. (BTW it is a 121 CI engine 11.9 1/4 mile stock)
GregLocock (Automotive)
23 Jul 07 21:09
"All the E85 cars sold today have nothing done to them to take advantage of the octane "

really? None of them? not even the ones with adaptive spark? I find that curious.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Eric68 (Automotive)
23 Jul 07 21:58
I think maybe maybe dcasto was referring to compression ratio ??  

In my opinion increasing compression ratio is the most important way to take advantage of ethanol as a fuel and would have way more effect on efficiency then spark management (unless you are pulling timing out for gasoline so it doesn't knock?).  By the way, in my engine E85 likes less total timing than gasoline so advancing timing past what gasoline likes would not make more power.  Although, at part throttle maybe more timing would help with E85 (sorry not used to thinking about part throttle :D )

Here's a thought

Now what if you could make an engine that was capable of altering cylinder pressure (dynamic compression) to take advantage of high octane fuels when available and back off the cylinder pressure when a lower octane fuel is used.  I believe this would be possible with true variable valve timing -- something that would not be limited by a cam lobe (ie: hydraulic or pnuematic operated valves.)  You could build the engine to be capable of 12:1 static compression and in effect reduce volumetric efficiency and/or cylinder pressure for lower octane fuels.

Maybe I'm just dreaming . . .
patprimmer (Publican)
24 Jul 07 0:14
It's easy, just screw the waste gate down a bit. More boost means more total compression.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

patprimmer (Publican)
24 Jul 07 0:17
Or if not already turbocharged, install a turbo and have adjustable waste gate, set at different boost settings for each fuel.

If you wanted to be real tricky, develop a device so that a knock sensor adjusts the waste gate rather than adjusting timing.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

GregLocock (Automotive)
24 Jul 07 1:25
Yes, I was just pondering the customer acceptance for a car that would only run properly on E85.

The other alternative would be to limit the throttle opening on a high compression ratio car that was an NA, if the octane level was too low.

Either way, I can just imagine the reaction.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Eric68 (Automotive)
24 Jul 07 9:14
Maybe I am a little "turbo naive", but how would that help efficiency at part throttle?  (not that part throttle is important to me LOL)  It would solve the WOT problem, especially if you could measure octane prior to combustion and be a little more proactive with the waste gate and spark curve than a knock sensor.

You would be able to pack more air into the cylinder with boost and increase cylinder pressure at WOT, but you would be in the same boat when cruising?  Doesn't higher static compression still squeeze the charge harder at part throttle?
dcasto (Chemical)
24 Jul 07 9:52
I guess we need this http://www.daledetrich.com/variable.htm

It kinda reminds me of the Model A or T Fords.  Because there were no standards for gasoline, the Fords had a lever on the column that would rotate the distributer and change the ignition timing.  You'd start the car and listen for it knocking then make adjustment as you were driving.

Eric68 (Automotive)
24 Jul 07 10:02
There you go . . . now all we need is a V8 version that will stand up to 500-600 HP for 200,000 miles lol
patprimmer (Publican)
24 Jul 07 11:27
Ummm

Throttling the engine reduces manifold pressure, so the same compression sees lower cylinder pressures at part throttle than at WOT. This effectively eliminates knock at part throttle.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

dcasto (Chemical)
24 Jul 07 16:41
I don't think that the partial throttle stops knock.  Don't you remember cars in the late 70's.  They would run for 3 minutes after you closed the throttle and turned off the ignition, and they "dieseled".  You still compress 9 to 1 and the compression ratio makes heat (temperature) in the cylinder AND the engine is holding in some heat, so the fuel auto-ignited.
patprimmer (Publican)
24 Jul 07 19:19
So are you saying that the same heat is generated by compressing air at 5 psi by 9 as is generated by compressing air at 15 psi by 9.

I would have thought the run on situation was purely from residual heat causing a glow plug effect.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

dcasto (Chemical)
25 Jul 07 9:00
The thermal heat of compression is defined as:

Tout = Tin * ratio ^ ((k-1)/k)
T in absolute T, k= Cp/Cv, ratio = Pd/Ps, Ps and Pd are absolute pressues.  

Eric68 (Automotive)
25 Jul 07 21:54
Interesting discussion.  Part throttle certainly has less cylinder pressure due to being throttled, but there is more that changes than just cylinder pressure at part throttle.  

For one, most engines increase ignition advance significantly and lean the mixture at cruise (like 50+ degrees).  Also, probably the most significant factor in my opinion is load.  There is very little load on an engine at cruise -- even my 500 HP street/strip motor is only developing 10-15 HP when cruising flat ground at 55 mph.  It is very difficult to make an engine detonate under these circumstances.  Increase the load though (like pulling up a hill with a trailer), and combine that with a lot of advance and lean mixtures and an engine can certainly detonate at part throttle.

By the way, run-on after shutting an engine off is not detonation.  Detonation occurs when you have secondary flame front in the chamber and the two flame fronts collide.  Run on, is a single (abnormal) ignition source that ignites fuel after the ignition power is turned off.  Sounds nasty when it happens but it is not nearly as destructive as detonation.  (grew up listening to Mom's 72 Valiant run on for several minutes sometimes -- I can still remember my Dad cursing the old slant 6 hoping it would blow up and finally shut off)
dcasto (Chemical)
26 Jul 07 16:01
At low load the throttle is shut and vacuum is high and nearly no fuel is available to ignite, as a matter of fact the cylinder conditions could be so lean the fuel won't ignite.  

A question just poped in my head, is this condition why some (carburated or even FI)vehicles "backfire" when the throttle is closed?  Its a similar or opposite situation with turbo engines that use a blow off valve. When going into curve a there is a sudden closure of the TB, the air is vented after the MAF sensor and the engines thinks it has lots of air so the engine goes rich and some fuel is not burned in the cylinder and hence we get to see flames.
GregLocock (Automotive)
26 Jul 07 21:17
One typical cause of backfire is that unburnt hot rich mixture travels down the exhaust and mixes with fresh air from an exhaust leak. On protos backfire on closed throttle is almost invariably a poor seal at the manifold to downpipe joint.

Cheers

Greg Locock

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

Eric68 (Automotive)
26 Jul 07 22:23
I think Greg nailed it -- one other thing to remember is that a lean misfire in the cylinder can result in a backfire in the exhaust too.  If it won't fire in the cylinder because it is too lean, the charge may still burn in the exhaust and result in a rich smelling exhaust.  I think this is more common with performance engines with big nasty cams that are subject to reversion (like at idle).
payntir (Automotive)
20 Nov 07 1:22
Max Schauk {sp?) used ethanol (neat, e100 i believe) as a replacement for aviation fuel.  crossed the atlantic with it.  Various "maximum" e85 CRs from 13:1 to 15:1 with different engines.  the composition of the 15% petroleum product and the engine tune are the determining factors.  petroleum blenders have found that profit can be increased by using the lowest possible octane gasoline with 85% ethanol to bring the octane up to that of unleaded regular.  octane rating systems seem to be nebulous concerning ethanol.  seems to be too much $ at stake, a lot of research to spec.
as distilled, ethanol is an azeotrope, with 5% water. anhydrous ethanol is needed to blend with gasoline for e85, adding expense, decreasing net energy.  i suggest optimizing engine to operate on hydrous azeotropic ethanol.  water injection has long been used to reduce cylinder temperatures.  compression ratios should approach 18:1.  As regards supercharging as a form of variable compression, ford motor 1997 mustang super stallion utilized supercharging.  power increase about 10% on e-85, highly tuned engine, 5.4 liter, 590 hp on e85 dynamic compression, boost pressure, i dunno
stanlsimon (Mechanical)
31 Mar 08 20:12
A couple of issues which didn't seem to be addressed previously were EtOH's air/fuel ratio, flame front speed, and also NOX emissions of higher compression engines.
If anyone has any information on these subjects I would be interested.

I am interested  in modifications to an existing V8,( high compression heads, pistons, connecting rods, carb revisions, etc. One Web site claims up to 19:1 cr is safe.
This is not all new technology as the gearheads have already been down this track.  The emphasis has been on maximizing HP/CID.  It would be nice for engineers to have a monopoly on this knowledge, unfortunately, such is not always the case.
patprimmer (Publican)
31 Mar 08 23:37
19:1 is just possible with M100 running very rich and not to much advance and a long duration camshaft.

With E85 it is no way possible. Some bench racers only hear the word alcohol and react as though
1) They know what they are talking about.
2) That all alcohol is M100.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

Helpful Member!  TDIMeister (Automotive)
1 Apr 08 3:26
"A couple of issues which didn't seem to be addressed previously were EtOH's air/fuel ratio, flame front speed, and also NOX emissions of higher compression engines."

Stoichiometric AFR of neat EtOH: 9.0.  For blends, you can find out the StAFR by determining the mass fraction of each fuel component and its corresponding StAFR. NOTE: Mass fraction is not the same as volume fraction, by means of which fuels are usually blended.  Laminar burning speed is faster than gasoline (look up if you want exact values).  NOx emission is ambiguous because of additional variables like charge cooling.

Several posts back tried to calculate the heat of compression using isentropic relations.  The value of the specific heat ratio (kappa) used was wrong.  For pure air, it's about 1.4.  In a fuel-air mixture, that value is lower.  1.3 can be used as an approximation for stoichiometric mixtures of most hydrocarbon fuels and air for engines that are not direct injected.  So in the equation (T2/T1)=(v1/v2)^(k-1) the value in the exponent should be around 0.3, not 5/2.  Within any reasonable effective (dynamic) compression ratio used, autoignition is not an issue.

Next, it must be said that it is a very common mistake to conclude compression ratio in a piston engine as a ratio of pressures.  It's NOT.  Although the relation (T2/T1)=(v1/v2)^(k-1)=(P2/P1)^((k-1)/k) holds true, end temperature is INDEPENDENT of throttling, etc.  It does change due to leakage and heat transfer of a real compression process, that is neglected in isentropic (reversible, adiabatic) relations.  Going back to the temperature independence of throttling, if P1 drops, P2 also drops accordingly only as a function of the volumetric compression ratio (v1/V2) raised by k.  In a closed control volume, the compression ratio is a VOLUMETRIC parameter.  Don't EVER use pressure ratios unless you're dealing with a turbine engine or you have specific pressure trace data or you'll look silly.

To the original poster: Your question is similar to that asked in the recent thread asking about maximum CR in a propane engine.  The answer is that there is no single value nor range of values that can adequately cover every variable in a engine.

A factor that also comes into play are the objectives for the engine design.  If the engine is to live almost exclusively at part load, and you want maximum fuel efficiency, there is an upper limit of compression ratio beyond which efficiency drops off again due to a multitude of factors like heat transfer (worsening ratio of combustion chamber surface area to volume); leakage through the rings; friction; increasing role of crevice volumes and quench zones; etc.  For an engine developed for part load, this maximum compression ratio should be strived for, but again, there is no hard number, but from a theoretical derivation of a dual-combustion cycle with a defined peak pressure limit, thermal efficiencies cease to increase significantly past a (dynamic) CR of about 14:1.  The point is that it's not constructive to raise the compression ratio simply for the sake of striving for the biggest number.  However, if the emissions profile permits obviating a three-way catalyst, in addition to an optimally increased compression ratio, a small benefit in BSFC can be achieved by operating the engine slightly lean.

If the engine is to be run mostly at full-load and you are still concerned about fuel efficiency, you must balance increasing the compression ratio and timing advance with necessary fuel enrichment to prevent knocking.  You might save far more fuel by running at a lower compression ratio and degree of spark advance, but maintaining roughly stoichiometric AFR, instead of running a higher CR and forced to enrich the mixture to a lambda of 0.8 that I've seen for some gasoline engines.

Running on E85, short of a major breakthough in engine efficiency (i.e. 45% increase), you will NEVER come out with a case of an engine that has the same or better mileage (read fuel consumption) on E85 as on gasoline, due to the approx. 45% shortfall in LHV.  Run away from anyone who tries to convince you otherwise!!!
BrianPetersen (Mechanical)
3 Apr 08 0:06
tdimeister has the math right, and Eric68 (and payntir) are on the right track for the practical experience.

Although I have not done it myself (no local source for E85 and the particular race engine that I have does not have parts available to raise the compression ratio enough), I have been told by others who build race engines that you can get away with things by running an engine on E85 that you could never do even on the best and highest-octane racing gasolines, nevermind ordinary pump gasoline. They say that "in the engine", E85 acts like roughly 112 octane fuel. Step away from the calculator ... E85 is a great fuel for a race engine, if the rulebook will allow it. Nominal octane ratings for a given compression ratio obtained in a research engine are not necessarily reflective for a "real" engine with a "real" combustion chamber shape and running at "real" (i.e. high) engine speeds, particularly when the fuel chemistry is way, way different from what the octane test apparatus was designed for.

For a daily driver application in which best efficiency is the main objective rather than maximum possible power, if you were to build a dedicated E85 engine, I think you would want to downsize displacement by around 10% - 15% and up compression by several points, probably into the 13:1 - 14:1 range for an engine with cylinders the size of your average auto engine. Or, keep displacement the same, raise geometric compression even higher, and fiddle with the intake cam closure timing to get some Atkinson cycle happening (less power, i.e. back to the same as baseline gasoline engine, but more efficiency). It should be possible for an optimized E85 engine to approach the nominal "fuel economy" of a gasoline engine by taking advantage of the high octane rating, BUT, such an engine will not be capable of running on gasoline.

The variable turbo pressure scenario (Saab) is good for taking advantage of the octane to make more power, but I don't see that approach improving the part-load efficiency. It's still limited by having to be capable of running on gasoline at an acceptable power level.
stanlsimon (Mechanical)
5 Apr 08 0:26
State of Minnesota tested Mileage of vehicles using e85 and conventional fuel, found no significant difference in fuel economy.
patprimmer (Publican)
5 Apr 08 3:15
Then the state of Minnesota must have a very liberal definition of significant as ethanol contains significantly less energy per unit mass or volume than normal pump petrol.

 

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

globi5 (Mechanical)
5 Apr 08 10:19
What you do get with the heat of vaporization is more dense air and fuel, more pounds in the cylinder, more explosion, more power.


This sentence is contradictory.
At a given pressure one cannot increase density without decreasing temperature.

p*V=n*R*T

Heat of vaporization does lower the temperature at TDC.
patprimmer (Publican)
5 Apr 08 19:14
The reduced pressure in the inlet manifold increases air flow into the manifold thereby increasing the mass of oxygen and fuel in the charge when the inlet valve closes on the inlet stroke. The extra mass of fuel and air generates more heat energy when burnt. that aspect has nothing to do with adiabatic heat.

Because of the charge cooling due to fuel evaporation, without ignition, the temperature at the top of the compression stroke will indeed be lower. This allows for a higher compression ratio or more ignition advance without detonation. Modern engines with high compression and knock sensors might gain from this. Optimising compression ratio to suite the fuel will yield gains, but requires engine modification.  

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
 

drwebb (Automotive)
7 Apr 08 9:32
"State of Minnesota tested Mileage of vehicles using e85 and conventional fuel, found no significant difference in fuel economy."
- citations for unexpected data are always appreciated.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close