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Old 02-27-2012, 11:02 AM   #11 (permalink)
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Originally Posted by Ladogaboy View Post
I want to add something to Post #8. What oil pan 4 was saying in regards to diesel turbos "loving their cylinders stuffed with air" would apply to at least some gas turbos. I was doing some more research on my car's tuning, and its AFR range is from 9-11:1 from factory. By eliminating back pressure in the exhaust, the AFR does improve. I'm not sure how much fuel efficiency can by gained by increasing from, say, a 10:1 AFR to a 12:1 AFR, but that seems to be a pretty common shift.
Um, something's wrong with your numbers. If you're running a 9:1 air-fuel ratio you'll be blowing so much smoke that your car will kill mosquitos and various other small animals as you drive down the road. For gasoline, an AFR of at least 14.7:1 is required to have enough oxygen to burn all the fuel.

Most gasoline cars run at almost exactly 14.7:1. The exception is lean-burn engines which go a little higher. Diesels run higher and much more varied AFR's from 16:1 or so all the way up to 50:1 or more in some cases (like idle).

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Old 02-27-2012, 12:23 PM   #12 (permalink)
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My experience is that opening up the exhaust may improve mpg on a vehicle using a carburetor, as the jetting is generally unchanged, allowing the vehicle to run leaner. On a FI vehicle, the computer compensate immediately for a bigger exhaust by increasing fuel to get the correct a/f ratio. Just my $.02.
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Old 02-27-2012, 12:44 PM   #13 (permalink)
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Correct, a non-lean burn fuel injected gasoline engine will adjust the fueling based on the O2 sensor readings, so as to keep the AFR right near 14.7:1.

On a turbodiesel with fixed geometry turbo decreased backpressure will give you a higher AFR. On a turbodiesel with a variable geometry turbo, it depends on how the controls work. In many cases the VG will adjust to keep the AFR the same, albiet at a lower exhaust manifold pressure, so the pumping losses go down.
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Old 02-27-2012, 09:25 PM   #14 (permalink)
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Quote:
Originally Posted by Diesel_Dave View Post
Um, something's wrong with your numbers. If you're running a 9:1 air-fuel ratio you'll be blowing so much smoke that your car will kill mosquitos and various other small animals as you drive down the road. For gasoline, an AFR of at least 14.7:1 is required to have enough oxygen to burn all the fuel.

Most gasoline cars run at almost exactly 14.7:1. The exception is lean-burn engines which go a little higher. Diesels run higher and much more varied AFR's from 16:1 or so all the way up to 50:1 or more in some cases (like idle).
No, I was being quite literal with those figures. Here is just one reference to stock AFR between 9-10:1.

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Old 02-27-2012, 09:36 PM   #15 (permalink)
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I think forced induction gas engines have low AFR because they use the excess fuel to cool down the charge. Maximum power comes at something like 13:1 or so where the excess fuel can consume all of the air, but after that the excess fuel is wasted and you get less power since you're cooling the charge more. But it's a necessary evil when you have very high boost.
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Old 02-27-2012, 09:43 PM   #16 (permalink)
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Originally Posted by serialk11r View Post
I think forced induction gas engines have low AFR because they use the excess fuel to cool down the charge. Maximum power comes at something like 13:1 or so where the excess fuel can consume all of the air, but after that the excess fuel is wasted and you get less power since you're cooling the charge more. But it's a necessary evil when you have very high boost.
Yeah, you're shooting for 12.5:1. That's the magic number that gives you the most power, but under boost, you have to enrich the AFR. Many pistons have been melted with too lean of a AFR under boost.

The nice thing is, with a predictable spool up, you can usually get away with leaning out the AFR in the cruising range.
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Old 02-28-2012, 12:47 PM   #17 (permalink)
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Okay, I realize I'm a diesel guy and I'm not super familiar with tuning gasoline engines, but something doesn't make sense to me.

The soichiometric air-fuel ratio for gasoline is 14.7:1. This means that if you combine 14.7 kg of air with 1 kg of fuel in a perfectly mixed mixture and burn, all the fuel and all the oxygen in the air will be consumed. Greater than 14.7:1 means there will be oxygen left over, lower than 14.7:1 means that there will be unburned fuel left over. 9 to 1 air fuel ratio is equivalent to 14.7 to 1.64. That means 64% extra fuel! Of the 1.64 kgs of fuel sent in only 1 kg has the possibility of burning--even in a perfectly mixed mixture. So a minimum of 40% of the fuel will go unburned.

Now, I realize that sometimes the AFR will go just a little under 14.7:1 under high power situations, because the mixing isn't perfect. I also realize that ethanol and some other fuels have stoich AFR's down close to 9:1, but I don't see how 9:1 with gasoline can be anything but bad.

What am I missing here?
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Old 02-29-2012, 12:26 AM   #18 (permalink)
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Heat. Lots of it. Old-school turbocharged gasoline engines use a lot of gasoline to cool down the intake charge and combustion chambers in order to prevent detonation. In fact, many of them run the risk of incurring detonation due to running way too rich.

It was simply easier at the time to do it this way than to properly tune the engine to prevent grenading. Sending your car to a market that has potentially bad gasoline? Turn the fuel taps up. Turn them waaaaay up. I once saw a naturally aspirated Toyota on the dyno hit 10:1... at idle.

With a better intercooler and more predictable spark, you can lean that out pretty easily. With modern engines running direct injection, you can lean it out a whole lot more.
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Old 02-29-2012, 12:30 AM   #19 (permalink)
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Quote:
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What am I missing here?
Doing so allows you to get 100% more power than you would get out of a naturally aspirated engine with the same displacement. From a mileage and efficiency perspective, it's very bad. As serialk11r stated, the excess fuel is used to cool the ignition chamber internally. Are there better ways of doing so? Yeah, probably. But this is the route most auto manufacturers have chosen for forced-induction engines.

But regardless, you are right. There is quite a bit of excess fuel that goes unburned. The cat is basically the only thing that prevents the car from shooting a fireball out the tailpipe.

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Old 02-29-2012, 12:36 AM   #20 (permalink)
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