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Old 06-25-2007, 07:39 PM   #11
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I was attempting to hypermile the family car. It's a Volvo S80 T6, which is only rated at 19 mpg combined. Just using P&G, and intelligent use of the gear lock-up feature I was able to pull 33 hwy and 30 combined. It can be done in a turbo.

As far as the original question... in terms of speed, I am completely unsure. The speed limits here do not exceed 55 so I usually just keep around that speed.
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Old 06-25-2007, 10:56 PM   #12
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>>what corollary?

I was just trying to lighten it up a little. The corrolary was, instead of "If you would have read more closely" how about "If I had written it more clearly...."

Just teasing a little.... often times when I am trying to communicate and people don't get it, sometimes I need to look in the mirror. For what it's worth, I thought your post was very clear.
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Old 06-27-2007, 05:26 AM   #13
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My lowest shift point is around 2000 RPM. At that at full throttle, I'm boosting about 10psi. That means I've almost doubled my "displacement" not good for FE.

2500 is my car's favorite shift point. It operates the smoothest 2000 and up (for accelerating, at least I can cruise at 1500, but it stutters a bit if I try to accelerate out of 1500) and I've got some pretty wide gear ratio gaps.

And from checking my tripminder computer, I DO get better FE cruising at 65 than at 55 Or rather, I get identical. At 65, I'm around 2300rpm and my computer hits the ultra-lean in cruise, and also the EGR kicks in, raising my FE that much more. Plus I've got a very streamlined car as is.
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Old 06-27-2007, 07:42 AM   #14
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Why is a turbo car any different?

1st off, I don't necessarily buy the "lots of throttle during acceleration phase". One theory is to get to cruising speed quickly with as little time spent in the fuel rich acceleration RPM range. For one, it is indeed in the fuel rich range.... the harder you accelerate the richer the fuel mixture (typically). Second it is counter to standard advice. Lastly I think one can make a simple physics argument that harder acceleration will burn more fuel than slow smooth acceleration (think small increments of acceleration on top of constant speed, with little additional stress on the engine and no fuel richening, versus WOT with hard torque on the parts, thus more friction).

10 lbs of boost does not mean double the air displacement. In fact, the throttle pretty much guarantees you will never fill the cylinders, and the throttle argument suggests to me that if you slowly accelerate, the behavior and FE between turbo and normally aspirated engines would basically be the same. For hard acceleration, there may be a difference, but firstly it is balanced out by the fact that it takes the same amount of energy to get the car to cruising speed in both cases.... any differences in expenditures is due to losses.... I can see the hard turbo acceleration case yielding more losses, though.

I try to accelerate nice and slowly in my VX. But it is because I can do this in lean burn mode, versus rich with hard acceleration. I monitor my injector pulses with a DMM and smooth slow acceleration yields injector utilizations in the 3-8% range with cruising in the 6-8% range depending on speed. Dropping out of lean burn into even a moderate acceleration bumps the fuel utilization up into the teens, and WOT practically dumps fuel into the cylinders with a utlization as high as 40%. In my car, slow acceleration doesn't even show up as acceleration in the way fuel is consumed, and I think that is the best case.
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Old 06-27-2007, 01:46 PM   #15
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I think I have finally understood enough to counter that argument.

First, it would be quite nice to be able to accelerate without going rich, but being as I have a 3500+lb car, it would take me far too long to get to cruising speed (If I could do it at all)

Second, I know that 10psi isn't doubling, 14.7psi is doubling displacement. That's why I said almost. And with a forced induction engine the cylinders would be filled, because the cylinder isn't sucking air in, the air is being shoved in by the turbo.

Third, with the turbo, if I have the boost barely there at all, I'm getting negligible amounts of air being added over running vacuum, but the piston doesn't have to work to pull the air in at all. Therefore, though I am burning more fuel than if I were running very little throttle, I'm getting more efficiency out of that fuel.

If I accelerate slowly, I get around 20-25mpg, then when I hit cruise, I get 30-35.
But when I use just barely any boost, I get 15-20mpg during acceleration, but I hit the 30-35mpg cruise FAR sooner than if accelerating slowly.

During that time that I'm barely boosting, I'm getting more HP out of each ounce of fuel because the engine doesn't have to work to bring it into the cylinder. I'm willing to bet that the average FE of boosting a tad to cruise, and cruising a long time is less than the average FE of slowly inching up to cruise, and cruising a short time. (Over the same total distance)

Actually, I'm planning on doing a test to see just that. Choose a set 5 or so mile stretch of highway, start from a stop, and accelerate quickly to say 60mph. Cruise the total length and see average MPG. Go back to start, reset average MPG, and slowly work up to 60mph, cruise total length, and see average MPG. I'll get back with you guys in a few days.
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Old 06-27-2007, 01:54 PM   #16
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A turbo would move around the BSFC ovals depending on the compressor map, but not really change much in the grand scheme. It really depends on where you cruise/accelerate at....
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Old 06-27-2007, 09:56 PM   #17
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Oh hello, I didn't see this thread earlier. Nice to see another turbo 4 banger on the forums.

Ica13 wrote: "10 lbs of boost does not mean double the air displacement. In fact, the throttle pretty much guarantees you will never fill the cylinders,"

Boost is read from the intake manifold, after the throttle plate, not before the throttle.

Since all cars are different, I can only speak for my car here. When I accellerate I like to use enough throttle to keep in vacuum, about 5in/hg, while shifting around 1500 to 1800rpm. My car still accellerates at a decent rate this way because I've lightened it by about 100 pounds and installed a light flywheel. Here's part of the reason why shifting early and using more throttle improves FE http://www.gassavers.org/showthread.php?t=3140&page=2 scroll down to post #23 and begin reading there. It's interesting, if I do say so myself.

I'm lucky enough to have nearly a full code disassembly from my cars ECU. In it I've found the threshold between open and closed loop based on throttle position (around 35% to 55% depending on rpm) so I can use that info to stay in closed loop and avoid the extra rich fuel tables that are used in open loop. The fuel map shows a 12.4:1 A/F ratio where it just goes into open loop, and it gets richer as the throttle is opened more.

On my car the turbo reaches full boost (20psi) at 3500rpm. At that point the A/F ratio is 10.2:1 It gets richer as the rpms go up, peaking at 9.2:1 at 4500rpm. The A/F ratio is this rich not for power, but to help cool the compressed intake charge which reduces knock. It's sort of like having water injection, or methanol injection, except it's gas. Which means the extra fuel is not used to propel the car, it's used for knock suppresion. It's just one of the strategies used to make max power on a small displacement engine.
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Old 06-27-2007, 10:38 PM   #18
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Makes sense... many people do claim best results with quick acceleration.

Boost or not... the throttle is the governor. The biggest difference between the turbo and non-turbo case is at WOT, where the cylinders can fill as much as they can pull in.... assisted by boost in the turbo case. But at closed throttle, the volume of air passing through the throttle is minimized.... is it bigger in the boost case? Sure, but in either case the engine is being starved of air to the point that it simply idles or accelerates slowly.

The point is to not make the mistake of thinking, say a 1500 cc engine is actually pulling in 1500 cc's of normal density at closed throttle positions. And similarly, a boosted intake, even twice normal air pressure will not deliver 3000 cc's of air, even at WOT, all other things being equal.

Turbos are great for FE, but really if and only if you have a smaller displacement engine in the 1st place. Since you can get more HP out of a turbo engine when needed, the decreased size helps during constant speed running. Maintaining 60 mph on my Civic only takes about 13 HP, but it has an engine displacement for 96 HP (and torque) for acceleration. A turbo version could drop displacement and get a closer match between cruising need and supply availability.
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Old 06-28-2007, 07:07 AM   #19
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IMO and in general terms under closed loop operation, accelerating quickly or slowly makes virtually no difference if one intends to just hold a steady cruising speed. Where the faster acceleration shows greater benefit is when P&G tactics are being used.

If you don't intend to P&G, then slower acceleration is likely to net the greatest economy no matter what you are driving. P&G with a turbocharged motor however requires a bit more dilligence on the acceleration front, as the computer switches to open loop mode with far less throttle and starts dumping in large quantities of fuel and retarding timing primarily to keep detonation under control. Full throttle likely ends up being a net loss under that condition.

So in a nutshell, I think one will get the best economy out of a turbo by making sure to not go open loop. Let the oxygen sensor do it's job and the same general rule for fuel efficient driving will apply.
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Old 06-28-2007, 07:10 AM   #20
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Quote:
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IMO and in general terms under closed loop operation, accelerating quickly or slowly makes virtually no difference if one intends to just hold a steady cruising speed. Where the faster acceleration shows greater benefit is when P&G tactics are being used.
I believe this is true too, but it's counter-intuitive isn't it?

Can you explain why you think this is true? I have some ideas, but I'd like to hear yours.

I may split the topic off into its own thread.
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