DCR and E85

Now that I'm up and running on E85 I am considering a new High compression engine to take full andvantage of the E85. I am looking for information on how high I can safely go with my dynamic compression ratio. I have read people setting these motors up with 14:1 static compression with no issues but have not been able to find what Dynamic compression to run.

Has anyone seen any info on this?

I am interested to know this as well.

+2 on this.

nobody really knows yet. E85 is a relatively new fuel..

nobody really knows yet. E85 is a relatively new fuel..

I really don't want to be the guinea pig here, but I guess I will if needed. Does anyone know what is a good DCR to shoot for with say 100-110 octane racing fuel?

nobody really knows yet. E85 is a relatively new fuel..

Actually some people DO know. The ones I asked speak more of SCR than DCR. One engine guy really likes building for E85 if you use nothing else.

Unfortunately I don't see any of them posting on this forum. Too bad.


Joe Urban

Match your cam to your compression, your fuel is secondary to that. If you begin to cross 240-260psi cranking you are likely going to have problems. Just because you probably can run more compression doesn't mean you have to. It will on the other hand let you run a very racy cam and compression on fuel you can get for cheap all over the place.

I've said it many times, people here make way too much of compression. It is not a magic bullet, it does help you out when it matches everything else though. It does improve part throttle and idle. Which is what a big racey cam usually kills off with pump gas compression ratios.

anyone else?

Actually some people DO know. The ones I asked speak more of SCR than DCR. One engine guy really likes building for E85 if you use nothing else.

Unfortunately I don't see any of them posting on this forum. Too bad.


Joe Urban

Ok, how about this, those that do know, aren't telling ;)

Ok, how about this, those that do know, aren't telling ;)

It depends on who is doing the asking, but they certainly aren't telling on-line.

What you said is true with many (most?) of the people who really make power or fast cars. Some of them will talk to an individual if he keeps the knowledge to himself. Every so often that happens to me. It's fun to see something become general knowledge a couple of years later. Unfortunately you can't even mention that you knew or you'll not get anymore "inside" info. Keeping ears open and piehole closed is the best idea.


Joe Urban

that's very true about a lot of things

ws6t3rror hit the nail on the head, we have a winner.

ws6t3rror hit the nail on the head, we have a winner.

Well, it is one opinion anyway. I believe the OP wants to take full advatage of E85's high octane rating and cooling properties. The peope who are doing that are running really well.

I suggest that if you optimize the engine for E85 (compression, cam, timing, etc) it won't like WOT on straight pump gas.

Just another opinion. Not necessarily a winner.


Joe Urban

Well, it is one opinion anyway. I believe the OP wants to take full advatage of E85's high octane rating and cooling properties. The peope who are doing that are running really well.

I suggest that if you optimize the engine for E85 (compression, cam, timing, etc) it won't like WOT on straight pump gas.

Just another opinion. Not necessarily a winner.


Joe Urban

Exactly right. I have heard that for every compression point raised you can expect to pick up 4%, so I want as many points as possible (within a safe limit! Say E70). Also If E85 is always available in my area I will never run pump gas again. And if for some reason E85 tanks in a few years I can allways get new heads.

Let me ask you this, if you ran the equivilent octane rated fuel in relation to what e85 is would the engine run the same as it did on e85? or run better?

Let me ask you this, if you ran the equivilent octane rated fuel in relation to what e85 is would the engine run the same as it did on e85? or run better?

Correct me if I'm wrong , but according to a few dyno runs I have seen and what I have read you actually would see a slight drop in power (About 5%) when running a high octane gasoline compared to E85. So basicly on E85 you should be able to pick up about 5% power. From what I understand this is due to alcohol being oxygenated and carring extra oxygen into the combustion.

I would assume this would hold true with high octane racing fuel as well.

Ever hear of VP's Q16?

Correct me if I'm wrong , but according to a few dyno runs I have seen and what I have read you actually would see a slight drop in power (About 5%) when running a high octane gasoline compared to E85. Nope. If that's the case, the gas tune is probably less than ideal. E85 has a lower energy density than gasoline which measn more is needed to get the same energy delivery, one of the reasons your E85 equipped injectors are twice the size of gas injectors for the same engine. Essientially you're burning pre-oxygenated fuel... to get the same hp you need to up the volume. If you're able to squeeze out every last pony from upping the CR you could get say 1% more, but no more than you could with any other race gas at that octane rating... the catch of course is the increased fuel required to do this so E85 will require a "wetter" charge which makes all those internal swirl thingys less effective. The up side is your fuel will vaporize easier on the valves/piston, but likewise, droplets will be smaller or even missing depending on delivery. This idea of droplet size and charge density ("wetness" of air/fuel mix entering cylinder) is the stuff of NASCAR and top builders... way more than I could tell you about. Chemicly though, there is no way E85 gets you anywhere NEAR 5% gain if both are tuned well. Chemically, it's a NEGATIVE impact on the energy density. This is why winter gas (with less/no ethanol) can get a touch more hp than summer blends (besides the ambient temp differences).

So basicly on E85 you should be able to pick up about 5% power. From what I understand this is due to alcohol being oxygenated and carring extra oxygen into the combustion. Actually that's the reason it has a lower energy density... ethanol is already partially oxidized by having an oxygen bond. If you ran Ethane and oxygen separate you'd get that energy bond back for combustion, but as ethanol it's reduced the potency of the fuel... it's like throwing a half-burnt log into the fireplace... sure it'll burn, but it doesn't have the same energy content as a similar non-charred log.

I would assume this would hold true with high octane racing fuel as well. Nope... high octane race gas is not from pre-oxygenating the fuel, instead the gasoline is doped with aeromatic compounds like toluene or other high-carbon molecules. The energy density for premium vs regular gas is non-existant... same goes for most race gasolines. Aviation fuel has high octane (usually leaded) fuel as well but due to the density difference between Av fuel and gas you'll get different hp curves unless you adjust the injectors to deliver more fuel.

Without getting into some more detailed O-chem jargon that's about as far as I can explain it... for use in gas engines, octane rating has nothing to do with energy density, which is related to hp. Pre-oxygenating the gasoline is basicly like adding a EGR leak that's always flowing. :)

Well then, let us take the thermodynamics approach.

After a quick search the specific energy of each fuel are as listed below.

Gasoline
47,300 kj/kg

E85
32,340 kj/kg

Now to be easy for everyone to follow let us do our analysis on 1kg of air to be combusted. Let us also use maximum power rich mixture. The values for these are as listed below.

Gasoline
12.5

E85
6.975

Now lets figure out due to the mixtures exactly how many kj's are being released to the air if a complete burn occurs.

Air fuel ratio is defined as mass of air / mass of fuel. We have 1kg of air.

So 1kg air / max power air fuel ratio will give us the mass of the fuel.

Gasoline = 1kga/12.5 kga/kgf = 0.08 kgf

E85 = 1kga / 6.975 kga/kgf = 0.143369176 kgf

Now back to those specific energies which are in kj per kg and we just learned how many kg it takes to do the job so we can solve for how much energy is released for each fuel on 1kg.

E85 = 32,340 kj/kg (0.143369176 kg) = 4637 kj

Gasoline = 47,300 kj/kg (0.08 kg) = 3784 kj

Uh oh, the E85 just slapped down gasoline hard in terms of energy released when given the correct air fuel ratio. Not by a little but by 22%....... in theory only.

In reality you can only move so much volume of fluid at a certain rate Through your engine. If your fuel vaporizes before your get it to the cylinder, your ve goes down and with it your power. Now you also have to pump all the water and spent gasses out as well as the fuel on the intake, this also costs you power. Phase changes pumping losses, heat losses etc etc etc all sorts of things chip away at that margin until there is just a little bit left. It may be as much as 5% by the time you get to the tire or as little as nothing.

Also in my own testing, e85 preffered a much leaner ratio for best power approximately 8:1. Then again I dont run gasoline at 12.5:1 either, more like 12.8-.9, compare your real numbers to your real numbers ;). Its gonna be different for everybody.

I will be back later to post about compression and why not to go insane with it and give reason to match it to the engine.

1) You're assuming complete combustion... not likely, but good enough for our model.

2) Calculating the amount of fuel needed for the combustion of 1 kg air is novel if you had an unlimited air supply (please, no The One That You Love jokes :)), however as shown by the very low a/f ratio needed for EtOH to combust efficiently you see the need for twice as much fuel, meaning you're decreasing the amount of air that can fit into the cylinder with the fuel used. Considering the vaporization of EtOH under normal conditions I'd be shocked if it wasn't fully vaporized by entry. What's this mean? It means that instead of a very small amount of gasoline droplets in suspension, the EtOH vaporizes, becomes gaseous and literally becomes part of the air. This "displacing air" is known to me minor in gasoline due to the high A/F ratios used and the fact that most gasoline enters as fuel (much more condensed than gaseous vapor). This is the same "displacing air" effect used by Nitrous to increase the amount of oxidizer by replacing air (20% O2) with Nitrous (100% N2O... worth about the same as 50% O2), but in the case of alcohol, it REDUCES the amount of O2 available (1 OH group on an alcohol is worth about 50% that of O2) meaning any air displaced by EtOH reduces the oxidative power by half.

So how much air IS removed from vaporized EtOH?

1.000 kg EtOH * (1000 mole EtOH / 46.06 kg EtOH) = 21.7 moles EtOH
6.975 kg air * (1000 mole air / 14.2 kg air) = 491 moles air

(21.7 moles EtOH) / (491 moles air + 21.7 moles EtOH) = ~5% of the mixture.

Hence... there is a 5% drop in oxidizing power of the charge when using EtOH... assuming you haven’t found a way to make the cylinder larger to compensate (ie. using forced induction).

Personally I wouldn't consider a 12.5:1 gas ratio ideal... I've found 11.5:1 to 12 is probably a better solution... which raises your calculated fuel weight per kg air to about 8.6, and hence the kj/kg air to about 4100... almost the same as EtOH once you reject the 5% loss EtOH has due to vaporization.



3) I'd like to point out here that the max kJ/kg is simply not more than about 30% due to technical losses. The reason we can harness this energy is due to thermo-baric expansion and careful valve timing. With comparable gas and E85 set ups generating similar kJ per combustion, the differences in hp and MPG is due primarily due to harnessing this expansion differently. It's known that max-cylinder pressure near ~12.5 to 15 degrees past TDC leads to ideal conversion between combustion and kinetic energy. Turbos and forced induction can lengthen this push a bit, but changing chemistry (such as using propane or Nitrous) changes the flame front speed and the pressure curve per cycle. E85 has a faster flame speed than gasoline and pulls the curve closer to a pulse than a longer "thump"... which means less advance timing is needed to get ideal timing for hp... but that also means it falls off faster after that point than gasoline. On the upside any compression during the advance timing of gas or EtOH is stored and released due to the spring effect of a sealed cylinder, but the loss of a longer expansion can't be made up for. The effect of this is probably minimal, but I suspect it adds to the loss of hp/mpg seen in E85 vehicles.

At full air flow they will likely be similar and you'd hope that not all the E85 would vaporize at high flow rates (less time to vaporize before getting to the chamber), though at higher vacuum near-idle driving the differences are undeniable... EtOH gets lower MPG due to lower energy density and flex vehicles using both are known to get between 6% and 25% less mileage with E85. The proof is there for mileage. HP seems to be a toss up and more a tuning game, but to claim it gets even a 10% gain in hp is umm... "unlikely".

Also in my own testing, e85 preffered a much leaner ratio for best power approximately 8:1. Then again I dont run gasoline at 12.5:1 either, more like 12.8-.9, compare your real numbers to your real numbers ;). Its gonna be different for everybody.

And that's the catch isn't it? at stoich ratios... trying to get max mpg, e85 loses miserably (see any Flex fuel vehicle), and as you said, leaning to 8:1 will help power (much like going from 14.7:1 stoich for gas at idle to 12:1 for power will as well).

Here's the crux of the issue:

8:1 means you have only have 0.125 kg f to 1kg air... reducing the amount of air/O2 the vaporized E85 will displace (cutting your losses to probably only 4%). This of course affects the total kj available at max power... dropping it to 0.125 * 32340 = 4040 kj without ANY O2 displacement. With only a 3% loss you'll get 3921 kj. Even closer to gasoline; less than a engine with peak hp at 12:1 a/f which starts with 3941 kj.

The harnessing differences due to flame front and thermal expansion differences make up the most of the differences seen, but at ideal tuning, it's pretty much a toss up. Definitely not a gain worth chasing for $1000's of dollars to convert and suck up the heavy MPG losses (the government won't subsidize E85 forever... when they cut the handouts prices will spike HARD).



From an "environmental" view, E85 doesn't make much sense for now... using a corn source is inefficient and leads to food shortages/price hikes, but even worse, Ethanol easily vaporizes. Evaporative emissions from gas pumps pumping E85 is MUCH higher than gasoline pumps... the differences are even apparent in E10 verses straight gasoline, it's a matter of chemistry, no dodging it.

As switch grass and other options to make EtOH become feasible E85 will cease to be a burden on tax payers through subsidies and allow corn and other agricultural prices to drop for consumers, but the pump emissions will continue to rise.

I'm convinced heavier fuels like diesel are WAY more responsible from a pollution stand point, though every engine in existence will need to have a turbo system and pricey injectors which will raise car prices for sure.

Best option? Electric motors, Li-Ion or more advanced batteries, and a clean-power-grid (nuke, hydro, wind). Battery and motors are a great place to invest IMO. :)

BTW, a quick search online shows most E85 users treat it jsut like ~100 octane fuel from a boost/DCR stand.

One forum says Saab changes it's boost from "5.8 PSI for pure gasoline to 13.8 PSI for E85" meaning the extra 10 octane points is good for about 8 psi of boost, which sounds about right (maybe a psi or two more than 100 octane gas but that's again, likely just a tuning variance).

If you run SOLELY E85 in the engine (i.e. not a NA flex fuel vehicle) your best bet is to run a turbo/supercharger and play with the boost to an appropriate level... but you could go for NA with high DCR and make fine adjustments with cam swaps... but unless you run a DOHC/OHC cam design or have adjustable valve timing, that's probably more effort than most will invest in.

Pump gas is good for about 8.5 to 9 DCR... 100 octane gas is good for about 2 more SCR over pump gas... so a DCR of 9.5 shouldn't be out of range with E85... but that's a LOT of assumptions (such as VE being similar regardless of the e85 vaporizing effect).

Personally if I was going to try a E85 engine without boost, I'd go for a safe SCR like 12:1 or so and then bump the DCR to about 9.5 using a moderate cam (extreme, high-rpm targeted cams will typically drop your DCR... which is how my 396 with 12.5 SCR runs around 8.9:1 DCR). This allows you to swap cams for future gasoline use, and still gets your E85 octane gains allowing you to increase hp numbers a touch (probably about 1% compared to DCR that you'd used for pump gas).

Lots of variables, and lots of new ground to forge through... is it worth it?

I am going to be honest here, I am not going to read all of those novellas you wrote in any detail in the very near future. I have too much other stuff to do right now, and my retort and addition of information would take me a day to dumb down and make concise.

Yes my model is very simple and straight forward however unlikely. That is very much on purpose. The user base here is not capable of basic physics much less more advanced thermodynamics and 3-4 phases of heat transfer and fluid mechanics all rolled into the same model. If you will notice I said the gain could be from 0-5% I also spoke of the ve reduction due to phase change of the ethanol etc etc etc. I did not take into account gasoline is 10% ethanol either which makes them closer and throws off the max best power ratios and the stoich ratios. I also avoided talking about the wiebe equation for the different parts of a combustion event, its easier to just say you only get 30%.

I am not here to teach anybody upper level energy processes or engineering fundamentals. But, I will nudge them in a direction of correct thinking.

I will try to make a post this weekend regaurding compression ratio. Best just simple straight forward advice is just to match it to the camshaft just as you would with gasoline. Mid to low 9's for dcr is not out of the question. What e85 will enable you do to is to run a nice big racey camshaft and have the compression to make it livable on the street with fuel you can get at the corner and it is cheap (well sort of).

Steve whats your day job, chemist?

Biochemist actually... but I suffered through P-Chem for a minor. :-/

Like I said above, the E95 is really a wash except for small gains by being able to run higher DCR... but the fuel itself is less efficient, vaporizes more and may approach gasoline at max hp a/f... but at stoich and less than WOT conditions it's a serious loser. Flex fuel may be useful in future-proofing your car if gasoline ever get bans... but I have a feeling that's not gonna happen any time soon.

As for CR vs hp gains, David Vizard did a great couple articles on that many years ago that's still floating around the net. Diminishing returns is a known issue, as is the issue that detonation doesn't rely just on DCR, but the cam ramp profiles, the VE of the engine (which varries with rpm), and any changes to the chamber/piston top/spark plug over time. What really matters here is chamber pressure (PSI) near TDC and outside of a basic compression test or an ultra-expensive piezo sensor in the chamber you have to go with what works in gthe pastg and hold your breath when fordging ahead (computers can sim all you want, but the sim is only as accuate and the data put in).

I know your case is that e85 and gasoline will be very close. However you do have to consider that in a fuel injected enginethe droplets are very fine and they are not subject to heat from the manifold really. Also if they are, your manifold temp is going to drop pretty rapidly and achieve somewhat of a steady state. Heat transfer is most generally related to a temperature differential in at least 2 of its modes. So as manifold temp goes down vaporization will be reduced. An 11 second jump down the racetrack is more than enough to do the job of cooling down an intake on an alcohol engine (go put your hand on a methanol cars intake after a run it is quite cold). I do consistantly see a power gain on e85 which I run in the summer, and make it myself inbulk. It has to be denatured to avoid a visit from excited guys in atf jackets though, gasoline works for that. My personal car goes from about 118mph on gas to 121mph on ethanol.

As far as air/fuel ratios for cruising. I do not have the same initiative as oem manufacturers. They run slightly rich of stoich to keep NOx uHC and all of that nonsense under control. We obviously know that at stoich you get the maximum temp and pressure and therefore very likely maximum NOx so they run just a bit fat of it. Maximum temp is not likely going to make your engine/exhuast system very happy. I therefore choose to run as lean as will allow regular combustion at cruise. The idea that engines should be run at stoich is nonsense that people have generated due to oem polution initiatives (monkey see monkey do). Engines could give a rip what the a/f is as long as it allows regular combustion and doesnt foul the plugs or wash the oil off the walls they are happy with it. Less fuel also usually allows for enough energy to be available in the combustion space due to compression and heat transfer to vaporize the mixture and avoid puddling. You do want the fuel to vaporize completely, but only after its in the chamber with the valve shut. Using this I only see about a 20-23% loss. Of course if you do it like the oem's, you are going to lose 30% of your economy that is exactly how they have set it up. But we do not have the epa monkey on our back as individuals at least here.

Steve, I had guessed as much from the view you were looking at it from. I am an ME. I have worked in two different places doing engine R&D but I can't talk about anything I saw there without getting into trouble. All I can share is common knowledge and personal experience and it is really a difficult thing to skirt critical information. I agree about the diesel fuel. Imo tractor trailers should function in the same way that diesel trains do. They should also be equipped with two stroke turbocharged diesels as well instead of as they are. If the truck manufacturers would do that we would eliminate the large black clouds from semi trucks. The cost of a truck would go up but I bet in the life of the truck it would break even might be better even at nearly $5/gal and be much better for everyone. I have personally seen diesel engines run as lean as 35:1 a/f under load and still happy.

You know, the diesel-electric version of a semi has been thrown around for years, but with the proper battery developement and regenerative braking (not just air brakes, but kinetic fly wheels or electric motors per axle) I can see it happening some day.

The use of a low redline and multiple gears has served diesel engines well in keeping near max VE and more efficent tq levels, but I agree, the idea of a load-independant diesel engine using electric motors (which are most efficient at very low rpms and generate very high torque values from a standstill) seems like a perfect fit for semi's. The diesel-electric train will be with us for a VERY long time... espeically with biodiesel developments making fuel selection options more available.

Main problem with a diesel-electric design however is the weight overhead that has to be absorbed to make it work. The upside is that electric motors can be made to work without a transfer case, and if placed per wheel can replace driveshafts and axles, and if Li-Ion takes off for automotive sizes the weight could be decreased as well... the cost issue however is really prohibative.

If diesel-electric takes over semi's as it has trains and ships, you'll probably only see it in long haul over-the-road designs with multiple trailers at first and later in single trailers that haul near major ports and warehouses... maybe even a pure electric option in those cases.

E85 or E100 will not overtake diesel in these "infrastructure" applications, but in consumers which account for a major share of CO2/CO/NOx/CHx emmissions I see a real problem due to the volitility of the fuel in CHx pollution from pumping alone, and in an application reality there simply isn't enough land to support our economy by converting corn... you need more efficent EtOH production (ie. switch grass, algae, etc...) especially considering the reduced milage which makes for even worse efficiency due to consumer demand for a 300 mile tank... bigger tanks = more weight to carry around. In all but the most optimistic scenarios, ethanol is a very long way off from making any sort of change in our energy policy and may actually be doing more damage environmentally than the gas its replacing.

No doubt e85 is crap as far as a replacement, do not fuel your car with your food. It is however a cheap and available alternative to 8-12 dollar a gallon race gas. That is the extent of my interest in it. Making the most of the mileage from it is purely for my own pocket book. The performance increase is rather nice too. This summer it was about 2 dollars here and 93 was 4 on average. Well thats a 50% savings but its 20% less efficient for me, so i save 30% making my fuel cost for the same mileage about 2.80 a gallon. Bad in the big picture... really good for ole number 1 who I like taking care of.

On the diesel trucks it would be a complete redesign keeping likely only the chassis and perhaps the transmission setup. Ideally though you would drive all three axles with one motor per axle with some sort of slip detection. With enough battery capacity you could keep the revs pretty low on the engine and it would last for freaking ever. The key to it is though that you can design the engine to run at one data point optimize everything because its basicly always going to run right there. A two stroke industrial type diesel is a mean machine too as far as efficiency is concerned. Compound turbocharging and intercooling will bring along efficiency quite a bit as well. Under high power conditions perhaps you could pull current from the engine to the motors as well as from the batteries, maybe up the cut off ratio and rpm to a max power situation. That would allow a semi truck to take off in a hurry relatively speaking.

The big gain though would be in air quality around shipping yards and things of that nature where there are alot of diesel trucks running all the time. If you have ever been in an area like that i almost garentee you did not stay long it is highly unpleasant. Part of the reason for that is the big plumes of black uHC that they all puke out when trying to take off. The other thing is those big plumes of smoke is money just floating around in the air, and the exhaust is horrible for you. Funny thing is if the truckers just saw it squirt out liquid diesel in thier mirror, they would have a fit, but as a fog of smoke... whatever :lol:.

Its too expensive right now, I already know. Me and a few dozen other engineers all keep notebooks on ideas for this particular project in our spare time. Updating as things become smaller and sometimes rewriting how its done as we go. Then again in a 2 year span I have seen stuff go from filling a 747 from tip to tail, to fitting inside of a shoebox.

im running 12.8:1 SCR with a LT4 Hotcam (with more lift due to my 1.72:1 rocker arms) and E85... so my DCR should be 9 to 10:1

Looks like GM faced energy content issues with their E85 version of the corvette C6R:

Corvette Racing laps up E85 (http://www.sae.org/mags/AEI/4359)
Running the 7.0-L Small Block V8 engines on E85 racing fuel proved to be a relatively easy technical feat. "The transition from E10 to E85 was seamless and uneventful," noted Allen. "If I had to say what area we had to work on the most, it would have to be fuel economy. With the reduced energy content of E85, much work was done to be as efficient as possible. Calibrating the fuel control to maximize the air/fuel ratio and optimizing areas like deceleration fuel cutoff became important enablers to reduce consumption.

Obviously they did just fine with the change over, but it's interesting they specifically pointed out the fuel economy issue. Seems like carrying MORE weight in fuel to drive the same distance isn't a great idea from a racing standpoint, but I'd imagine it's a moot issue compared to the weight of a full-bodied car... especially when the E10 version of the car was dominating the pack in 2007 with the same chassis. In 2007 their 2 cars finished 1-2 in 11 of 13 races. In 2008 they pulled off the same feat... exactly. Spooky.

Here's the engine spec comparison between the vette/ZO6/C6R in 2008: http://www.corvetteracing.com/cars/c6r/engine_specs.shtml

I wish they had the 2007 C6R engine specs on E10 as well... would have been interesting to see how the torque/hp peaks moved.

it makes sense to switch to a higher oxygenated fuel for an engine that is restricted in airflow.

Look at that torque number! I have to wonder what compression ratio they are using with the E85.

I applaud them for this. They are using their time racing to find the limits of E85 so hopefully it will migrate more into street vehicles in the future.

I do agree that it would be nice if they shared the E10 info with us.

Vizard's data on CR and hp.

http://www.sunyabem.org/images/chart.bmp

The gains are not nearly as much as many people think. However, as another post pointed out, a higher CR allows you to run more cam. If the rest of the combo is up to the task, the higher rpm's allowed by the bigger cam can make substantial hp increases. So, as the other poster said - chose the CR according to the cam (and desired rpm range).

Rich

Agreed... though there's something wonky with that chart (maybe I'm just having trouble interpreting it)... I keep getting a hp gain of 0 going from 10:1 to 12:1... that can't be right.

I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).

Here's (http://www.hotrod.com/techarticles/hrdp_0801_e85_ethanol_alternative_fuel/index.html) an interesting warning about tuning with E85:
The first thing we need to know is that E85, the most common of the ethanol fuel blends, is actually three fuel grades:
Class 1 or "pure" E85 contains 80 to 84 percent ethanol, while the remainder of the blend is commercial-grade (around 85 pump octane) gasoline.
Class 2 or E75 is 75 to 79 percent ethanol, while
Class 3 or E70 is 70 to 74 percent ethanol.

However, all three classes of fuel may be marketed as E85 at various times during the year. While it seems confusing, this is done mainly to offer better cold-starting performance-which is a problem with ethanol fuels. Since straight ethanol has a relatively low Reid vapor pressure (meaning it doesn't like to light off at low temperatures), greater percentages of gasoline are added to the blend for colder weather.

So while E85 is often described as 105 pump octane, its actual rating can vary depending upon the seasonal blend. Naturally, higher gasoline content will tend to lower the pump octane from 105 for "pure" E85 to perhaps 100 for E75...

Tuning with Class 1 E85 and then running on Class 2 or Class 3 could lead to knock/detonation. Be careful what you put in the tank when tuning.

Agreed... though there's something wonky with that chart (maybe I'm just having trouble interpreting it)... I keep getting a hp gain of 0 going from 10:1 to 12:1... that can't be right.

I do recall the diminishing returns though... almost a logarithmic curve beyond 12:1, but I can't seem to find the chart anywhere (maybe its in a book at home).

Here's (http://www.hotrod.com/techarticles/hrdp_0801_e85_ethanol_alternative_fuel/index.html) an interesting warning about tuning with E85:


Tuning with Class 1 E85 and then running on Class 2 or Class 3 could lead to knock/detonation. Be careful what you put in the tank when tuning.

The chart was printed with the axes mislabeled in the book, hence my chicken scratches. Going from 10-12:1 nets a 2.8% hp increase. The numbers printed on the left are new CR, the numbers I scribbled on the right represent the old CR. Ignore the numbers I printed and crossed out on the left.

Nice point about the different blends!

Rich

Yeah, the scary part about the three E85 classes is that, well, they call them all E85. Car nuts would probably prefer E75, E80, E85 instead, but I guess they figure the average consumer looking to fuel their E85 engine would balk at using the lower blends.

Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).

The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.

If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"

Yeah, the scary part about the three E85 classes is that, well, they call them all E85. Car nuts would probably prefer E75, E80, E85 instead, but I guess they figure the average consumer looking to fuel their E85 engine would balk at using the lower blends.

Do you know if the DCR vs hp chart is specific to gasoline engines? or to any combustion event? I have a feeling curves between fuels would follow the same relationship, but that the hp vs. DCR curves may slope more or less depending on energy density of the fuel. I'm temped to belive a lower energy dense fuel would exhibit even less hp gains with a comparable DCR increase on a gasoline engine (the EtOH gains say 6hp/2% for a DCR increased from 8 to 9, while a gasoline engine may see 7.5hp/2.5% gains for the same change). Every time I've seen this chart (or the graph of %hp gains vs DCR, it was solely for gasoline charts).

The only reason I say this may be the case is that diesel shows a similar hp gain vs. DCR but there compression numbers are approaching 22:1 in the pre-turbo area. On most gasoline charts any DCR over 15:1 is nearly pointless so it seems strange the diesel guys would run so high when it would light off at 18:1 and be quieter to boot. Diesel energy density is higher than gasoline so i'm assuming the minimal gains those DCR's show in gasoline actually become more than insignificant when multiplied by diesels energy advantage... but like I sadi that's all conjecture based on 2 or 3 observations.

If that's the case, I'd wager the Chevy guys running E85 don't need to run on the jagged edge of detonation... DCR increases may return less hp gains than in gasoline (and it's nice to have the octane headroom over pump gas for hot days, etc...). This would be an interesting article for SAE or even just Car Craft Mag. "HP vs. DCR... fuel selection implications"

Vizard's data is based on actual testing of gas engines, as I understand it. The testing was done to see if the math was right and again as I understand it, the theories were confirmed. There are equations that describe the relationship between thermodynamic efficiency and CR and they are not fuel specific, but I am not sure how this translates to hp. I have a couple of engineering texts on automotive design, and I got the equation referred to above from one of them. That book also explains why CR over ~16-17:1 are never used in gas engines even when it's possible to do so w/o detonation - as you figured out it's because there are no power gains to justify the increased mechanical stress.

The book I am thinking of also has a big section on diesels. You got me curious and I will dig it out and look for some answers. Problem is, I can understand only bout 50% of the math in the book. When I dig it out I will also post the title - good reading though difficult. Bok is old, but the principles are still true.

Rich

Well I tried running through two engineering texts I have and they really didn't go deep enough into this topic to be much help. So, I did what I REALLY didn't want to... i broke out the Physical Chemistry text. Uggg...

The issue here is primarily that of the compression stroke (a nearly abiabatic compression), followed by the combustion which liberates heat to increase pressure. Basicly its a reversible compression cycle with energy entering as fuel and leaving as heat. The exact details of this combustion are WAY more involved than I'm willing to go. :)

But there are trends to identify that can shed some light on fuel choices.

Compression Factor
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). The smaller the Z value on the compression stroke, the less pumping losses you'll have. Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.

This suggestes that as far as pumping losses on the compression stroke, Diesel<Gasoline<<Ethanol (small weight AND polar dipole). The other benefit here is that unburnt fuel after combustion would make the diesel less compressable during the start of the power stroke, imparting more force on the piston top sooner than gasoline and even more than ethanol.

It should be noted that the total resistance to compression includes a factor for the total number of moles used. A mole of Diesel is 150% the weight of gasoline and about 370% that of EtOH. For a fixed cylinder size with the same weight of fuel (diesel and gas both use ~14#air:1#fuel), diesel would have say 0.66 Moles diesel, or 1 mole gasoline, while EtOH would have 5.5 moles (using a AFR of 6). As you can see, the total resistance (pumping losses) are nearly 6 times greater for EtOH than gasoline. This is just the fuel of course, but the air is the same and dilutes the effect by about 95%... so the differences aren't much, but they are still significant enough to measure based on the chart I'm looking at in this text.

What does all of this have to do with compression ratio? Well, it seems the higher the pre-combustion pressure the lower Z drops, allowing greater gains (or more correctly, fewer pumping losses) from a lower Z. Beyond ~80 to 100 atm (~1400 psi) the benfits start to reverse, but this is way less than you'd see in any engine pre-combustion. The lowest Z value reached is directly related to molecualr weight and neutral charges... diesel can reach very efficient compression compared to gasoline and WAY more than EtOH.

It should be noted, that after complete combustion, all 3 have simlar Z values and molar volumes and the post-combustion pressures make for very high Z values, increasing the expanding exhaust's resistance to compression (and hense moving the piston instead). The high temperatures post combustion also translate to much higher Z values as well.

Kirchhoff's Law (Yes, the same guy as Kirchhoff's Electrical Circuit Laws... smart guy)
This is where things get a bit hazy, and where all the expensive R&D automakers rip into. While most general chemistry courses teach that the heat released by combustion varies by its reactants, they usually reference a standardized table that shows the molar heat of combustion value at a designated temperature. This allows for some reaction math to investigate different combustion reactions, but it that's not the whole story.

See the heat of combustion (enthalpy increases) for a chemical process actually increases with temperature. That means that 1 mole of diesel may have a complete combustion energy of 125,000 BTUs at a starting temp of 220*F, but could have 130,000 BTUs if combusted at 250*F. Those numbers are completely arbitraru though, since finding the exact data would NOT be easy. The two curves showing the enthalpy of products and the enthalpy of reactants can be charted on a graph of enthapy/heat (H) vs Temp (T). When done so they'll look like a parabola centered just above the origin (T=0, H=0) with the products curve being translated vertically some amount and the slope also increasing... this allows the difference between the curves (the net heat released) to get wider and wider apart as temperature rises.

While combustion will rise chamber temps significantly, the compression stroke itself actually does a great job of this as well. The hotter the initial temps, the more efficient the energy produced from combustion. This is likely one reason emmissions has increased engine temps in an attempt to get a cleaner burn. In reality, the temperature increase also brings with it BSFC gains (lower BSFC), but it also increases the likelyhood of detonation... especially at max engine VE (which is about the max torque and max heat generated per ignition event).

Now here's the interesting part. In complete combustion, the products of diesel, gasoline, and EtOH will all be the same... CO2 and H2O. The reactants however will vary by stoichometry and molar heat of combustion. From the values in my text:
EtOH: 5.5 moles x 1400kJ/mol = 7700 kJ
Octane: 1 mole x 5500kJ/mol = 5500kJ
Diesel (C12): 0.66 mole x 5550kJ/mol= 3663 kJ

So in our hypothetical cyclinder volume we'd see a measurable heat of combustion difference (difference between the numbers above and the much higher, yet equal reactants). This gives diesel the heat generation advantage we'd expect with its higher energy density, but it also says that the advantage of increasing the inital temperature (via increasing DCR) would more pronounced in diesel fuel than in gasoline, and even more so than ethanol.

All this points to measurable gains for increasing DCR in diesel and even gasoline to a point, but in EtOH the hp gains vs DCR would be even smaller (less bang for your buck as it may be). Obviously the hp gains from higher DCR is limited in gasoline due to knock, but teh Visard and other NASCAR data shows that even at higher octane, premium pump gas can get you pretty close to ideal.

Increasing temperatures also seems to be good for BSFC in the combustion event, but detrimental in the compression stroke (higher temps increases Z values)... so the temperature increase is more of a balancing act when you look at max VE conditions (but less so at part throttle when compression stroke losses are much less due to starting in a vaccum).

I still believe the C6R running on E85 mentioned earlier is probably running a poitn or two high compression than we could run on premium pump gas, but to think that it would be finding hp gains by upping the DCR from even racegas levels seems unlikely.

I don't think they had to mess with DCR to get E85 to perform.

Disclaimer: I hated P.Chem. I'm probably missing a few points in here but I think that's the overall thermodynamics pertaining to DCR... except for the really gritty combustion changes at high pressures (the stuff engine makers spedn big bucks to investigate). EtOH may behave much differently in the combustion event that longer hydrocarbons (i.e. shockwave propigation), but that's WAY beyond the scope here and unlikely to change any apprecable amount due solely to DCR (once the pressure spike from combustion happens, the few extra psi added by 1 point fo DCR is minimal).

A mole of Diesel is 150% the weight of gasoline and about 370% that of EtOH. For a fixed cylinder size with the same weight of fuel (diesel and gas both use ~14#air:1#fuel), diesel would have say 0.66 Moles diesel, or 1 mole gasoline, while EtOH would have 5.5 moles (using a AFR of 6). As you can see, the total resistance (pumping losses) are nearly 6 times greater for EtOH than gasoline. This is just the fuel of course, but the air is the same and dilutes the effect by about 95%... so the differences aren't much, but they are still significant enough to measure based on the chart I'm looking at in this text.

I need to pointout that the term "pumping losses" here is specifically reffering to the energy needed to compress the intake charge alone.

When we (car nuts) normally refer to pumping losses, we're attempting to single out ALL the sources of lost hp, including friction losses associated with piston movement in an engine (which are the largest issue outside the compression stroke). True "pumping losses" are minimal when one of the valves is actually open and the piston is just push/pulling charge in/out of the cylinder compared to the force needed to compress the chamber. It's this force I referring to in this analysis of DCR changes and fuel selection. The others losses are unaffected. It may SEEM like a lot to say diesel is over 6 times more efficient in compression, but the dilution of this affect by the 90% air in the cylinder, and the fact that these losses are but one componant of the parasitic losses in an engine make the differences much smaller (though still measureable).

Very good post, I need to digest it some more and dig out that text.

Thank you.

Rich

Compression Factor
This factor describes how resistant a gas is to compression. An ideal gas has a Z of 1, while real gases just above 0psi have Z values less than 1, making it easier to compress than ideal, but by ~250+ bar the Z curve becomes >1 meaning it becomes more difficult to compress than an ideal gas (increasing pumping losses). ... Z value curves (Z vs P) vary with each unique substance, but on the whole, the larger the molecule, and the smaller the ionic/dipole charge the smaller the Z value will be intially, but it will transition over Z=1 at a lower pressure at which point the Z is higher for such substances.

I need to point out here that the Z-curve trends I described here are for organic compounds which are essientially non-polar. Any ionic or polar nature to the molecule can cause Z values greater than 1 for ANY pressure. Other compounds that show >1 Z values include hydrogen gas, H2, which behaves as such due to its such small electron field that facilitates close atomic packing... By the time you get to Methane (CH4) and Ammonia (:NH3) this inheritly high Z value is gone. Without finding the Z curve, I'd imagine Helium (He) gas and maybe Nitrogen and Oxygen (N2/O2) would show similar >1 Z values. Likewise, the temperatures of the gas will also increase these inter-molecular forces increasing compression force needed:
http://upload.wikimedia.org/wikipedia/commons/2/2c/Compressibility_Factor_of_Air_250_-_1000_K.png8
This graph is for air, and you can see how going from 250*K (-23*C / -10*F) to 300*K (27*C, 80*F) you start to lose this "ease of compression" advantage... showing how colder intake temps can reduce pumping losses (in addition to increasing total O2/oxidative content in a cylinder).

It's worth nothing that organic compounds like Methane show similar trends, but even a 2-carbon organic like ethane takes a much steeper dip at room temp than you see for air at even 250K. Organics compress easily until ~200 atmospheres (~3000psi) and by ~300 atmospheres becomes more resistant to compression than diatomic gases.

Diesel gets an advantage here, especially since it gets a lower Z value AND lower number of moles per combustion cycle (given the same hp or displacement)... and of course by having a favorable curve can make temperature increases (to increase BSFC/fuel efficiency) less of a penalty.