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BMW 3-Series (E90 E92) Forum
|01-29-2013, 12:30 PM||#1|
Join Date: Jan 2013
Location: Noblesville, IN
Not sure if any of you have read this before, but here you go:
E85 optimized engine builds
We all know that E85 is rapidly gaining acceptance as a high performance fuel, but many people wonder how to build an engine to best utilize the strengths of E85 and to accommodate some of its differences compared to gasoline.
First we need to look at how E85 differs from Gasoline as a fuel.
On a gallon for gallon basis E85 has less energy than gasoline, typically this is about 72% of the energy in a comparable gallon of pure hydrocarbon gasoline.
On a pound for pound basis of available air to support combustion E85 will release up to 127% the energy of gasoline (with gasoline considered to be 100%)
What people forget is that they assume available power and performance is determined by the energy in the fuel on a volume basis (ie gallon for gallon), but they forget the fact that that is irrelevant in total engine power output. This is easily illustrated with nitromethane fuel used by AA fuel dragsters. This fuel only contains about 25% of the fuel energy of gasoline on a gallon for gallon basis.
Nitromethane fuel energy per pound = 5000 btu/lb
fuel density = 1.13
weight per gallon = 9.42 pounds/gallon
Max power fuel air ratio = 2.0
Thermal energy available per pound of air available = 2500 btu/pound of air
Percent of available power compared to gasoline 100% = 164.5%
E85 fuel energy per pound = 13475 btu/lb
fuel density = 0.783
weight per gallon = 6.52
Max power fuel air ratio = 7.97 to 6.975
Thermal energy available per pound of air available = 1691 to 1932 btu/pound of air
Percent of available power compared to gasoline 100% = 111 to 127%
gasoline fuel energy = 19,000 btu/lb
fuel density = 0.74
weight per gallon = 6.17 lbs/gallon
Max power fuel air ratio = 12.5
Thermal energy available per pound of air available = 1520 btu/pound of air
Percent of available power compared to gasoline 100% = 100%
Since spark ignition four cycle engines are air limited for power, you can make more power in an engine that is using a fuel that at max power rich mixtures releases the maximum energy. In the case of nitromethane you can burn two times the fuel weight of the air you can cram in the cylinder. Even though the fuel has a low energy density on a volume basis (gallon for gallon) compared to gasoline, you can successfully burn so much of it using the available air that the total power produced is 1.65 times the power you could make on gasoline.
Likewise E85 will burn cleanly at very rich mixtures, when you factor in this large amount of fuel burned the total energy you can release burning a limited amount of intake air mixture is between 1.1 and 1.27 times the power you could make on gasoline. Most high performance E85 fueled engines run best at mixtures that produce 10% to 15% more power than gasoline in the same engine.
E85 also has much higher cooling power than gasoline as it evaporates. This high latent heat of evaporation dramatically cools the intake air charge which directly increases the charge density, allowing the engine to pack more fuel air mixture in the cylinder. In addition during the compression cycle the high latent heat of evaporation of E85 holds compressional heating of the fuel air charge in check until most all of the ethanol is evaporated. This means that peak cylinder pressures during compression are lower for a given physical compression ratio and the engine has to do less work fighting the pressure of the intake air charge as the piston is moving up the cylinder on compression.
Another characteristic of E85 is that at rich fuel air mixtures it burns faster than most gasoline blends. This allows the engine to use less ignition advance to achieve peak mechanical efficiency by having the peak combustion pressures occur at the ideal time after top dead center. Many gasoline fuels will not allow ideal ignition timing for peak mechanical efficiency due to detonation. This is called knock limited performance as the onset of engine knock (detonation) occurs before the proper ignition timing for ideal combustion pressure peak timing can be achieved.
The high cooling power of E85 also reduces the heat load on the engine at high power settings, keeping valve heads and piston crowns at lower temperatures than would be possible at the same power level on gasoline.
In some engine designs the thermal heat load on the engine limits peak engine power in all but very brief periods of max power. This is why many drag racers lose pistons and valves as they are going through the lights,and folks out at Bonneville melt pistons at the 2 mile post on a dyno tune that was just fine for 60 seconds under load. The power load was just a bit too high for too long, and engine temperatures got too high. This high engine heat can lead to detonation or pre-ignition damage that would not occur if the engine was kept just a few degrees cooler.
This also makes E85 an ideal fuel for engines and packaging situations where, they have chronic over heating issues. In fact, some racers have trouble getting enough heat in their engine to get it to run at its best. Some end up putting thermostats back in after switching to E85. This allows the engine to heat up properly and helps build enough heat in the oil to allow it to cook off condensation.
In my WRX E85 conversion, I had to go to a 190 deg F thermostat to get it to warm up properly in sub zero winter weather, even though the stock thermostat was 172 deg F. I could drive 18 miles on the highway and not get above 185 deg F coolant temps in sub zero temperatures. This 20 degree drop in engine temperatures seems to be common in racing as well. Drag racers report that they see about a 20 deg F drop in coolant temp through the lights from what they ran with gasoline, and some also report that they can now warm the engine a bit more at the line, staging about 10 deg F warmer without over heating the engine on the big end. Bracket racers like the stable engine temperatures and by using a thermostat can get very consistent performance throughout eliminations on E85.
One of the key features of E85 is that it allows higher compression ratios. This increases the engines throttle response at low rpm, improves thermal efficiency so you can squeeze more work out of the fuel and helps to get back some of the loss in miles per gallon many people see on E85. Several sources like to sell the story that fuel economy tracks closely with the fuel energy per gallon. This is nonsense, miles per gallon are a measure of the useful work you can successfully extract from the fuel, not the total energy in the fuel.
E85 is a more efficient fuel than gasoline for several reasons. As mentioned above, it makes more power at lower throttle settings, it runs cooler so less fuel energy is lost to the cooling system, and it allows ideal ignition timing due to its high detonation resistance. It also allows the driver to avoid up shifts under heavy load that would force the same engine on gasoline to stumble due to its higher tolerance for heavy load.
That raises the question how high should the engine builder go on compression ratio, just because some is good does not necessarily mean more is better. Like all things in an engine you are looking for the best compromise that nets the best performance in all conditions.
Many who have built E85 engines favor compression ratios (naturally aspirated) of from 12.5 - 13.2. Best efficiency in most modern engines appears to be in the low 13:1 range, with diminishing returns as you get above 13.5. This of course varies depending on other build factors, like your dynamic cam timing, your ignition timing, duration of load etc. but high 12's and low 13's seem to be preferred numbers. A few tuners are running engines built for methanol with 14:1 or slightly higher compression ratios on E85 but to do that they must run only the summer blend with the full 85% ethanol content and have an engine with a stable tune that keeps detonation and pre-ignition in check.
In the boosted import market most engines run with a base compression of 8:1 or near there, and from the factory come boost limits near 14 psi. On gasoline you begin to run into detonation issues rapidly as boost climbs into the low 20 psi range and up. Higher boost levels typically require top dollar racing gasoline to avoid killing the engine with detonation. The same engine on E85 with a proper tune, could very easily handle boost levels in the high 20 low 30 psi range. Many of them hardly have to think about detonation on E85 until they get to boost levels near 35 psi and up.
I found this on the net a few years ago and thought I would pass it along. I know there are a lot of myths about running e85. I have had two cars on e85. My first one was a 2000 gtp running 500whp (e85 and nitrous). On each tear down the engine was so clean with e85. I also have a 2005 mustang on e85. Both cars are boosted and very fun to drive. I have never had a corrosive issue while running e85 in either of my cars. There are a few aftermarket fuel pumps that don't like it, but other than that on all the boards and personal experience with e85 it is safe all late model cars...
Last edited by suprchrgd; 01-29-2013 at 03:31 PM.
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