Highest Compression Ratio for E85
Highest Compression Ratio for E85
(OP)
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?
RE: Highest Compression Ratio for E85
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
RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
http://www
(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.
RE: Highest Compression Ratio for E85
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
RE: Highest Compression Ratio for E85
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 ...
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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?
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
I read 680K for an autoignition temperature for methanol, compared to 630K for ethanol.
RE: Highest Compression Ratio for E85
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).
RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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!
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for 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.
RE: Highest Compression Ratio for E85
There are a whole host of other factors that effect the onset of detonation.
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RE: Highest Compression Ratio for E85
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
RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
Vernon
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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:/
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RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
These are the diesels you refer to
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.
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RE: Highest Compression Ratio for E85
ht
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.
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
http://www.agriculture.state.ia.us/e85q&a.html
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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://ww
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
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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?
RE: Highest Compression Ratio for E85
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)
RE: Highest Compression Ratio for E85
really? None of them? not even the ones with adaptive spark? I find that curious.
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Greg Locock
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RE: Highest Compression Ratio for E85
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 . . .
RE: Highest Compression Ratio for E85
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RE: Highest Compression Ratio for E85
If you wanted to be real tricky, develop a device so that a knock sensor adjusts the waste gate rather than adjusting timing.
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RE: Highest Compression Ratio for 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.
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RE: Highest Compression Ratio for E85
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?
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
I would have thought the run on situation was purely from residual heat causing a glow plug effect.
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RE: Highest Compression Ratio for E85
Tout = Tin * ratio ^ ((k-1)/k)
T in absolute T, k= Cp/Cv, ratio = Pd/Ps, Ps and Pd are absolute pressues.
RE: Highest Compression Ratio for E85
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)
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
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!!!
RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
RE: Highest Compression Ratio for E85
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RE: Highest Compression Ratio for E85
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.
RE: Highest Compression Ratio for E85
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.
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RE: Highest Compression Ratio for E85
- citations for unexpected data are always appreciated.