03-16-2009, 06:56 AM #1
Propper a/f ratio for boosted engines
I thought that this could be in a new thread so that it can be found later. rather than in the one on installing an a/f ratio guage. It would be hard to search.
I have read that 14.7 a/f ratio is ideal for gasoline N/A Engines.
How does adding boost (pressure) affect the ratio. Does the added pressure change the way that it combusts?
What would be the propper a/f ratio for premium gasoline? How does the grade affect the way that is combusts?
Its been a long time since I have had any chemistry so I thought it was time for a refresh.
03-16-2009, 09:25 AM #2
I have been doing research on other sites not dedicated to marine applications and this is what I have come up with:
When discussing engine tuning the 'Air/Fuel Ratio' (AFR) is one of the main topics. Proper AFR calibration is critical to performance and durability of the engine and it's components. The AFR defines the ratio of the amount of air consumed by the engine compared to the amount of fuel.
A 'Stoichiometric' AFR has the correct amount of air and fuel to produce a chemically complete combustion event. For gasoline engines, the stoichiometric , A/F ratio is 14.7:1, which means 14.7 parts of air to one part of fuel. The stoichiometric AFR depends on fuel type-- for alcohol it is 6.4:1 and 14.5:1 for diesel.
So what is meant by a rich or lean AFR? A lower AFR number contains less air than the 14.7:1 stoichiometric AFR, therefore it is a richer mixture. Conversely, a higher AFR number contains more air and therefore it is a leaner mixture.
15.0:1 = Lean
14.7:1 = Stoichiometric
13.0:1 = Rich
Leaner AFR results in higher temperatures as the mixture is combusted. Generally, normally-aspirated spark-ignition (SI) gasoline engines produce maximum power just slightly rich of stoichiometric. However, in practice it is kept between 12:1 and 13:1 in order to keep exhaust gas temperatures in check and to account for variances in fuel quality. This is a realistic full-load AFR on a normally-aspirated engine but can be dangerously lean with a highly-boosted engine.
Let's take a closer look. As the air-fuel mixture is ignited by the spark plug, a flame front propagates from the spark plug. The now-burning mixture raises the cylinder pressure and temperature, peaking at some point in the combustion process.
The turbocharger increases the density of the air resulting in a denser mixture. The denser mixture raises the peak cylinder pressure, therefore increasing the probability of knock. As the AFR is leaned out, the temperature of the burning gases increases, which also increases the probability of knock. This is why it is imperative to run richer AFR on a boosted engine at full load. Doing so will reduce the likelihood of knock, and will also keep temperatures under control.
There are actually three ways to reduce the probability of knock at full load on a turbocharged engine: reduce boost, adjust the AFR to richer mixture, and retard ignition timing. These three parameters need to be optimized together to yield the highest reliable power.
For further in-depth calculations of pressure ratio, mass flow, and turbocharger selection, please read Turbo Systems 103 Expert tutorial.
03-16-2009, 10:06 AM #3
Lots of info in the Seadoo 4-tech section..
03-16-2009, 01:00 PM #4
03-16-2009, 03:01 PM #5
- Join Date
- Jun 2008
depends on how much ( timing ) u can run ,verses, power u make
on my turbo car i ran (11 to 1) with alot of timing just depends
on mods and compression ratio
03-17-2009, 06:08 AM #6
There has been some good info so far but thought I would add my 2 cents.
First I thought I would address the Lambda vs AFR subject.
The sensor used to measure the left over oxygen in the exhaust is called a lambda sensor.
For tuning people use a wide band lambda sensor.
As such most of the generic motor sport population use lambda values to express the AFR of the engine.
Reasons for this as follows
AFR readings are only relevant for that of a known fuel.
As correctly stated above in the case of petrol the stoichiometric air fuel ratio is 14.7
For Alcohol its 6.4
For LPG its 15.5
For Diesel its 14.5
Joe Blogs race gas fuel blend (who knows)
For Lambda on any fuel stoichiometric is always the same “1”
So if you get used to using lambda as your measurement value it will be the same for all fuels that you use.
If you are not familiar with it and would like to learn, here is how you do the conversion.
Divide your current reading say (12.5) into the stoichiometric ratio for your current fuel
( for petrol its 14.7) answer = 0.85
So if you change fuel to another type you don’t need to learn the new air fuel ratio numbers you just use the same lambda numbers that your used to.
I can hear you guys going but we use AFR and only use petrol so I will stick to those values from here on.
So whats the right AFR to use.
There is no correct answer here.
The AFR a tuner might choose to run for a given amount of boost is dependant on a lot of different factors. Some of these are-
- How long does it have to last, 5 seconds on a drag track or 1000 hrs for a standard production engine.
- Will it be raced on a tight short course track or compete in a long distance race involving an hour of wide open throttle at a time.
- Does it require to have every last HP rung out of it like when you are racing against people with all the same engine. ( Stock Class)
- Does it have even mixture distribution, good or bad intake manifold
- What is the compression ratio?
- How reliable is the fuel system.
- Does the ECU have all the relevant temp and battery voltage compensations
- Will the lambda readings be logged and checked regularly or is this it for life.
The list goes on and on but you can see where I am heading here. Lean is mean, but it comes at a price, high exhaust temps and the potential for detonation if careful mapping techniques are not utilized.
So what are the numbers you say?
Ok what I would tune (again just my 2 cent here) for a recreational rider who has some supercharger and intercooler upgrades and wants more power but doesn’t want to buy a new engine any time soon.
Idle and cruise areas = 13.5 (lambda 0.91) (You can go leaner here with care)
Full throttle but no boost yet = 12.8 to 13 (lambda 0.87-0.89)
8 lbs boost = 12.2 – 12.6 (lambda 0.82 - 0.86)
14 lbs boost = 11.5 – 12 ( lambda 0.78- 0.81)
22 lbs boost = 11-11.5 ( lambda 0.75 – 0.7
These are safe mixtures for the average engine.
Maximum power will be made around 13 to 1 but with boost this will have exhaust temps well up and detonation knocking on the door if you have any sort of good ign numbers in the map.
You don’t lose much power at all by dropping back to 12.5 but mixtures under 11.5 will start to effect HP a fair bit and also hurt the acceleration as well.
If you are going to run lean and mean you need to make sure that your lambda meter is telling you the truth, its difficult to know if it is inaccurate by even large percentages, the risk is obvious and it is one of the other main reasons some people run a richer mixture.
Hope some of that’s a help
03-17-2009, 06:21 AM #7
03-19-2009, 01:07 AM #8
- Join Date
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- NE Ohio
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