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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > The Basics of Fuel



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      05-16-2012, 07:56 AM   #1
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The Basics of Fuel

I want to take a second to talk to you guys about fuel in order to help give you a better understanding of where the actual energy in fuel comes from and the pros and cons of different fuel types. Fuel is very complex subject matter that extends beyond what I have posted here, but there seems to be a lot of interest here on the forums regarding different fuels so I figured this would be helpful to outline some of the basics.

Engine calibration is used in order to create precisely controlled conditions in which we can extract energy from the hydrocarbons in fuel in order to produce power. Air comprised mainly of oxygen and nitrogen is pumped through the engine, compressed by the piston with the fuel and is then ignited by a controlled spark ultimately yielding acceleration as the cylinder pressure builds and pushes the piston down. The energy from fuel comes from the heat that the hydrocarbons give off after being mixed with air and ignited in the cylinder. It is the breaking of the molecular bonds in the fuel under heat and the reforming of new ones that generate the heat energy that builds cylinder pressure and drives the piston. Octane is only a measure of knock resistance, not energy or power output. Fuels with higher octane ratings have more stable molecules that require more heat to break and are thus more resistant to detonation. Fuels that can make good power will have high chemical energy or “heating value” so that more energy can be extracted given the same fuel mass. That does not mean you cannot make good power from a fuel with lower heating value though. Other things to consider when thinking about fuel are the burn rate and heat of vaporization. The burn rate is important to keep in mind because high compression, high revving engines need fuel that burns at a faster rate in order to ensure the fuel burns quickly enough to keep up with the engine speed. The fuel density can give you a hint at the burn rate. Usually denser fuels burn slower while lighter fuels burn more quickly. A fuel’s density is usually shown by a measure of Specific Gravity, or density as a it relates to water being 1.0. Heat of vaporization is the ability of the fuel to cool the intake charge making it more dense packing more energy per volume in the engine.

Here is a cool chart that nicely lays out the energy value in different types of fuel all in one place.



For those of you interested in E85 as your fuel source, it’s interesting to look at the heating value and stoichiometric ratio in comparison to other fuel types. Although it has a lower heating value or thermal energy in comparison to other fuels it also has a high resistance to knock. E85’s knock resistance allows you to extract more power by advancing the ignition timing and increasing the boost for a more complete, powerful burn. Ethanol has a high heat of vaporization that helps it cool down the intake mixture. The downside is many people experience worse gas mileage with ethanol because ethanol has less energy than gasoline and you have to run rich mixtures in order to offset the lower energy per unit volume. 100 octane has a higher resistance to knock than regular 93-octane pump gas and a higher heating value, but as you know it also costs more an availability is an issue. Diesel is also interesting to look at because of its low octane rating and high heating value in comparison to other fuel types. Diesel engines use the heat from compression in order to create ignition. They are very thermally efficient.

An important thing to note is you do not want to use leaded race gas in your BMW! Not only is it illegal for road use, but it is also bad for oxygen sensors and catalytic converters. Fuel manufacturers like VP and Sunoco offer unleaded, oxygenated fuels that will not harm you oxygen sensors or catalytic converters. Also note that simply filling your tank with race fuel will not do much other than waste your money. In order to properly leverage the benefits of different fuel types you should get calibrated to run the specific fuel mixture.

Here are a few helpful links to help you learn about/locate different fuels that are available to us.

http://www.vpracingfuels.com/

http://www.racegas.com/

http://torcoracefuel.net/

-Josh
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      05-16-2012, 08:17 AM   #2
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What would you do different tuning wise on this car with race gas instead of pump? Have you done an all out race gas tune yet on an n54?
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      05-16-2012, 08:33 AM   #3
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Quote:
Originally Posted by jazze90 View Post
What would you do different tuning wise on this car with race gas instead of pump? Have you done an all out race gas tune yet on an n54?
It depends on the specific fuel really. Hopefully we will be tuning my 135i on race gas within the next few weeks. I believe there is a Sunoco station nearby that sells 100 octane unleaded at the pump. Since the car would be more resistant to knock we could tune it more aggressively via adjusting the boost and ignition timing in order to extract more power safely without fear of detonation. Once we do so I will make sure to share results and other information.

We may also be doing some testing soon with E85 on our shop 535i. E85 is pretty available near us and we've had great success on other platforms, but the fuel system on the N54 is a bit fickle so we will have to see how it goes.

-Josh
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      05-16-2012, 08:38 AM   #4
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Great info josh! Thanks for taking the time to contribute your expertise to the n54 and n55 community!
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      05-16-2012, 08:59 AM   #5
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Great info josh! Thanks for taking the time to contribute your expertise to the n54 and n55 community!


-Josh
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      05-16-2012, 09:21 AM   #6
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Another few things to think about are the difficulty of properly mixing a gas and a liquid (air/fuel). The change of rpm's only make this more difficult. At low rpm's you have plenty of time for oxygen to find the right place on the hydrocarbons, but if the rpm's are too low the fuel will go from a fine mist to larger droplets. In the mid rpm range the engine is usually calibrated for this area and everything is working great. As you get into the high rpm ranges, denser fuels begin scavenging for o2 as there is simply not enough time to create a good air/fuel mix. Usually a lot of fuel is wasted here as it is either burning out of time, or out of place. Since at high rpm's there's a lot of heat too, burning is uncontrolled, which is where octane comes in. Octane keeps fuel burning when and (hopefully) where it should be.

Oxygenated fuels address this issue by premixing oxygen and fuel, unfortunately in CA, we are using 5.6% ethanol which as OP says has a lower energy density. If we used chemicals with higher energy density, higher oxygen density, then we would use .1% oxygenation and have the same effect without the drop of power.

As a small side note, it's not the heat of the fuel we necessarily care about, but the rapid expansion of gases that drive the pistons.
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      05-16-2012, 09:38 AM   #7
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Great thread. really enjoy the spreadsheet.
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      05-16-2012, 10:12 AM   #8
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Quote:
Originally Posted by TotalPower View Post
Another few things to think about are the difficulty of properly mixing a gas and a liquid (air/fuel). The change of rpm's only make this more difficult. At low rpm's you have plenty of time for oxygen to find the right place on the hydrocarbons, but if the rpm's are too low the fuel will go from a fine mist to larger droplets. In the mid rpm range the engine is usually calibrated for this area and everything is working great. As you get into the high rpm ranges, denser fuels begin scavenging for o2 as there is simply not enough time to create a good air/fuel mix. Usually a lot of fuel is wasted here as it is either burning out of time, or out of place. Since at high rpm's there's a lot of heat too, burning is uncontrolled, which is where octane comes in. Octane keeps fuel burning when and (hopefully) where it should be.

Oxygenated fuels address this issue by premixing oxygen and fuel, unfortunately in CA, we are using 5.6% ethanol which as OP says has a lower energy density. If we used chemicals with higher energy density, higher oxygen density, then we would use .1% oxygenation and have the same effect without the drop of power.

As a small side note, it's not the heat of the fuel we necessarily care about, but the rapid expansion of gases that drive the pistons.
To build further, it has a lot to do with heat transfer. Heat is transferred from molecule to molecule which breaks the bonds ultimately yielding energy that is leveraged to drive the wheels. In denser fuels, the heat transfer between molecules takes more time because of the density and as the engine speed increases, like you said, the window of time for the heat the transfer to occur decreases. Thus the importance of fuels with a fast burn rate to ensure nothing gets wasted. This will be more important for high compression motors that redline at 8k, 9k, 10k+.

It should be noted that for anyone competing you need to make sure that the fuel you are using or want to use complies with your class regulations.

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Great thread. really enjoy the spreadsheet.
Glad you find it helpful.

-Josh
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      05-16-2012, 10:23 AM   #9
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Quote:
Originally Posted by EFI Logics View Post
It depends on the specific fuel really. Hopefully we will be tuning my 135i on race gas within the next few weeks. I believe there is a Sunoco station nearby that sells 100 octane unleaded at the pump.
the Sunoco on the corner of Kings Highway in Fairfield (near home depot and whole foods) has 100oct unleaded on pump!
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      05-16-2012, 10:29 AM   #10
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Originally Posted by ZTZ1010 View Post
the Sunoco on the corner of Kings Highway in Fairfield (near home depot and whole foods) has 100oct unleaded on pump!
Thank you, Zack. Much appreciated. I was about to start calling around at all of the dealers listed, but I knew there was one nearby.

-Josh
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      05-16-2012, 10:30 AM   #11
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Quote:
Originally Posted by EFI Logics View Post
Thank you, Zack. Much appreciated. I was about to start calling around at all of the dealers listed, but I knew there was one nearby.

-Josh
no problem. I think it was around $8/gal last time I was over there.
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      05-16-2012, 10:37 AM   #12
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Never thought of this but is Diesels high density level a big contributing factor to the comparatively low RPM speeds?
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      05-16-2012, 10:39 AM   #13
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Quote:
Originally Posted by EFI Logics View Post
............. Also note that simply filling your tank with race fuel will not do much other than waste your money. In order to properly leverage the benefits of different fuel types you should get calibrated to run the specific fuel mixture............
If not tuned for it, it still can help will cooling and knock prevention. So there still is a benefit from it and its not just wasting money......just not getting the FULL benefit
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      05-16-2012, 11:09 AM   #14
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Quote:
Originally Posted by dsbj View Post
Never thought of this but is Diesels high density level a big contributing factor to the comparatively low RPM speeds?
Low engine speeds in a diesel really have to do with the long stroke.

Quote:
Originally Posted by BuraQ View Post
If not tuned for it, it still can help will cooling and knock prevention. So there still is a benefit from it and its not just wasting money......just not getting the FULL benefit
I did not detail that in my original post, but you are correct.

-Josh
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      05-16-2012, 11:15 AM   #15
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Nice write up! Question about heat of vaporization and how it relates to power. You state that energy per volume is greater... Yes, but I am guessing that fuel will vaporize during the compression stroke once the intake valve is closed and thus no additional air added.

I think heat of vaporization is mainly part of the octane calculation cooling the mixture reducing detonation.
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      05-16-2012, 11:21 AM   #16
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Quote:
Originally Posted by jazze90 View Post
What would you do different tuning wise on this car with race gas instead of pump? Have you done an all out race gas tune yet on an n54?
Typically with race gas, you can add more boost and ignition timing as well as having a slightly leaner AFR as it has a higher knock threshold than standard pump gas.
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      05-16-2012, 11:55 AM   #17
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Quote:
Originally Posted by Joshboody View Post
Nice write up! Question about heat of vaporization and how it relates to power. You state that energy per volume is greater... Yes, but I am guessing that fuel will vaporize during the compression stroke once the intake valve is closed and thus no additional air added.

I think heat of vaporization is mainly part of the octane calculation cooling the mixture reducing detonation.
Octane and heat of a vaporization are separate ratings, but the cooling effect definitely influences the fuels octane rating. Unfortunately I do not have a chart here, but heat of vaporization is only a measure of the fuel's ability to cool the intake charge. A cooler, denser charge is good for making power and it also helps for keeping engine components cool thus helping them to live a long a healthy life. So yes, the fuels ability to cool the charge will help combat premature ignition. Heat of vaporization is expressed in British Thermal Units per Gallon of fuel with a higher value representing a better capacity to cool. Off the top of my head I think E85 for example is at least 100 BTU/Gal greater than regular pump gasoline. That's part of the reason why you see cars like the Koenigsegg make crazy power on that fuel.

-Josh

Last edited by EFI Logics; 05-16-2012 at 01:25 PM.
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      05-16-2012, 12:44 PM   #18
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Octane rating is measured by a test engine. I thought heat of vaporization would be related to octane since a cooler mixture would result is less detonation combined with ignite temp. Anyway, just trying to gain a better understanding.
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      05-16-2012, 01:22 PM   #19
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Quote:
Originally Posted by Joshboody View Post
Octane rating is measured by a test engine. I thought heat of vaporization would be related to octane since a cooler mixture would result is less detonation combined with ignite temp. Anyway, just trying to gain a better understanding.
I'm sorry. I fixed my post. I realize now that my language was misleading. What I meant to say was that the octane rating and heat of vaporization are simply rated individually, not separate characteristics. A cooler mixture will definitely help combat premature ignition, but the two values are rated separately because they tell you different information about the given fuels characteristics. Detonation occurs from a combination of heat and pressure, not just heat.

-Josh
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      05-16-2012, 01:34 PM   #20
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Here is a nice chart that I just found that outlines the heat of vaporization "Latent heat (Btu/gal)" of a few different fuels.



-Josh
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      05-16-2012, 01:36 PM   #21
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Quote:
Originally Posted by EFI Logics View Post
We may also be doing some testing soon with E85 on our shop 535i. E85 is pretty available near us and we've had great success on other platforms, but the fuel system on the N54 is a bit fickle so we will have to see how it goes.

-Josh
I don't want to de-rail this but, where are you finding E85? If you are going to NY I am going to cry.
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      05-16-2012, 02:16 PM   #22
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I don't want to de-rail this but, where are you finding E85? If you are going to NY I am going to cry.
No problem at all... I will have to ask the rest of the guys in the shop because they know where the stations are. There are some in CT though I know. Our customers get E85 on a frequent basis.

-Josh
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