Turbo FE?

[Biffmeistro]

I know that turbos behave completely different than other cars, at least in the way of fuel milage.

Standard cars, Go easy on the acceleration, but use as high a gear as possible, and depress the throttle as much as possible without revving high. (Wider throttle=less resistance ect...)

Problem is, that turbo cars, if I have the throttle low, and floor it, I start getting into a boost condition, which sucks more gas, right?

I'd heard from some people that it's best to have minor turbo "boost" that will keep the vacuum/boost level at 0.

What is the best way to get fuel milage out of a turbo car? Is it better to go 65 and have the EGR and the turbo spooling the vac down to 0, or is it better to go 55 and have less wind resistance?

[thisisntjared]

the boost only helps with acceleration. that hold true for performance and fuel economy.

also the more throttle for better fuel economy in the low rpm range holds true because it is best to get to the top gear as quickly as possible so that you have the least amount of combustions per rotation of the tire.

these principals hold true whether the car is turbo or not

the advantage of having forced induction is the extra power out of smaller displacement. more power meaning faster acceleration, and from a fuel economy standpoint, less combustions spent in the lower gears.

with that said i will address your last question. it is almost always better to go as slow as reasonable in the cars top gear. there are not any cars out there that get better mpg at 65 than 55.

[Biffmeistro]

The disadvantage of having forced induction is that if you have a throttle that is open any significant amount, you don't just get the extra fuel used from the higher revs you might hit. The turbo shoves more air into the motor, and therefore more fuel.

A turbo that is at WOT at 2500 RPM uses WAY more gas than cruising at 3000rpm. Depending upon boost levels, it could be more than twice the gas.

Therefore, I can't have do the "half throttle at 1500 is better than a quarter throttle at 2000" Because at that half throttle, I'm boosting 5psi, And shoving 30% more gas into the motor than I would if I was at quarter throttle and not boosting at all.

[MetroMPG]

Perhaps Landspeed will chime in. I believe his car has a turbo.

[96hb]

Boost is VERY addicting and will ultimately lead to much less mpg. LOL! :thumbup:

[landspeed]

My car does indeed have a turbo :)

If you have a petrol turbo, you need to make sure you have a boost gauge.

Then, you assume that a tiny little bit below '0' vacuum is equal to full throttle, and halfway between idle and the '0' vacuum is equal to half throttle.

On my car, when I go to boost at low RPM you get a tiny bit more power, but the fuel usage more than doubles (goes from 4.4 to 1.5 on my uncalibrated SuperMID).

If you really want to get good FE on a turbo car, get a wideband lambda sensor, so you can see when the car starts enriching the mixture. The 'half throttle' position I talked about above is where my car starts enriching the mixture, so there is no point going above that.

If you look at my gaslog, you will see that turbo cars can get good FE :). If I drive even a bit of the time on boost, my FE goes to less than half - it just drinks fuel with little benefit (other than the fun of boost). I have just realised that my car has two of the brakes dragging a bit - and have been for ages, and despite this, I am getting good MPG over the EPA!.

The advantages of the turbo are that, you are using 'waste' energy to force the air into the engine (even when at 'half throttle' as described above). This means that you are wasting less energy dragging the air into the engine. Another advantage is that, the turbo makes the air move faster into the engine, so you use less throttle to get the same vacuum. If the air moves faster then it will be more turbulent so might give better fuel mixing. If nothing else, the fact that you get the same vacuum with the throttle plate more closed (because the turbo is pushing air against the throttle plate), means you get the air forced through a narrower opening so it is more turbulent.

On the official tests, my car gets better economy than the 1.8 non-turbo, despite the fact that my car enriches earlier (as it is turbo), and also is lower compression.

[CO ZX2]

Let's hear DRW.

[thisisntjared]

The disadvantage of having forced induction is that if you have a throttle that is open any significant amount, you don't just get the extra fuel used from the higher revs you might hit. The turbo shoves more air into the motor, and therefore more fuel.

A turbo that is at WOT at 2500 RPM uses WAY more gas than cruising at 3000rpm. Depending upon boost levels, it could be more than twice the gas.

Therefore, I can't have do the "half throttle at 1500 is better than a quarter throttle at 2000" Because at that half throttle, I'm boosting 5psi, And shoving 30% more gas into the motor than I would if I was at quarter throttle and not boosting at all.i think you are missing my point. is 2500rpm the lowest shift point??? and if you are going to be burning gas anyway what does it matter how much is burnt in a short period if in the end you end up burning less????

of course you are going to burn less fuel just maintaining speed than you will be accelerating. that is why it is best to accelerate as best as the car allows in the lowest rpms possible. if you had read my post more carefully maybe you could have learned...

[lca13]

>>if you had read my post more carefully maybe you could have learned...

Jeesh, lighten up. And consider the possibility that the corollary is true.

[thisisntjared]

>>if you had read my post more carefully maybe you could have learned...

Jeesh, lighten up. And consider the possibility that the corollary is true.what corollary? we are speaking pretty subjectively... either way there isnt a car that gets better mpg at 65 than 55 given all other variables are made constant. also if you are shifting at 2500 you are definitely not shifting at the lowest rpm. i dont think the corollary implied is able to be proven. although i have no intention of disproving it :)

i didnt mean to make it seem like i was getting heated, i just get annoyed when people disagree with me just because i am not telling them what they want to hear.

[atomicradish]

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.

[lca13]

>>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.

[Biffmeistro]

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.

[lca13]

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.

[Biffmeistro]

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.

[omgwtfbyobbq]

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....

[DRW]

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.

[lca13]

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.

[Snax]

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.

[MetroMPG]

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.

[lca13]

I think a simple explanation would be max torque. Since the amount of work needed to get the car to speed is a constant, running the engine at max torque would be the most efficient.

Not possible of course, but maybe keeping the RPM's in the max torque range yields the best FE on acceleration ????????

[Biffmeistro]

Probably the main thing about CVT's that lets them get better FE than normal automatics.

[Snax]

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.
I believe that efficiency of the motor is more or less the same whether you are accelerating quickly or slowly in closed loop mode. What makes the difference in terms of P&G vs. steady state cruising is rotations per mile, or rpM if you will. In other words, by accelerating more quickly to speed and assuming the same shift points, one can reduce the distance used for acceleration and therefore the number of rotations before going into a low rpm glide and lower net rpM.

Conversely, not gliding nets the exact same rpM whether accerating fast or slow - leaving any improvement in efficiency to whatever differences exist in the BSFC between the two.

[DRW]

I think a simple explanation would be max torque. Since the amount of work needed to get the car to speed is a constant, running the engine at max torque would be the most efficient.

Not possible of course, but maybe keeping the RPM's in the max torque range yields the best FE on acceleration ????????

Most non turbo motors reach peak torque at relatively high rpm, and most cars make an excess of power, so maximum torque isn't needed for normal accelleration. I took a quick look through a few issues of Car and Driver and it looks like many cars/trucks of various displacement make peak torque around 4krpm, even the Smart car makes peak torque at 4500rpm. It seems the Metro XFi makes peak torque at 3200rpm, which is better. www.answers.com/topic/suzuki-g-engine . MetroMPG, what's your favorite shift point?

On a turbo motor max torque is determined by the turbo. Typically most turbo cars reach peak torque at lower rpm, and torque peak is seen a few hundred rpm after the turbo reaches full boost. For example, the non-turbo version of the motor in my car makes peak torque at 5k rpm (135HP@6krpm, 125ft.lbs@5k) The turbocharged version reaches max torque at 3k rpm (195HP@6krpm, 203ft. lbs@3k) It's no coincidence that the stock turbo reaches full boost around 2800rpm. Subie WRX is an exception, reaching peak torque around 4krpm despite a small, quick spooling turbo. VW 1.8 turbo motor has a tiny turbo that spools up quick, so it's peak torque is around 1800rpm.
But since we're not using max torque to accellerate, and not spooling the turbo up to create boost pressure, are we benefiting from the turbo's torque amplifying abilities? Or is it just along for the ride?

[landspeed]

My car has max boost at about 2000rpm :). Although that is with a little T2 turbo. On my car, if I cut the injectors, but leave the boost gauge on, I can get positive boost by going downhill, and flooring the accelerator, at about 3000rpm. This positive boost is simple by the 'air pump' action of the engine.

When driving along normally, (in vacuum just before the enrichment phase), on my car at least, the turbo is spinning quite fast, and I guess it is therefore helping to force the air into the engine, by using 'waste' pressure from the exhaust, and therefore seems to reduce the energy wasted dragging air into the engine. However... I don't know how big an effect this is.

[88HF]

its not pressure that pushes the turbo, its the expanding hot gasses trying to escape.

[Biffmeistro]

The expanding hot gasses create pressure on the turbine, which causes it to spin.

[thisisntjared]

>>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.
ahhhhh i understand now. hahaha i know exactly what you are saying and i was thinking about editing my post but i didnt know thats what you were talking about. looking back at my post a week later, i think you are right. i shouldve eased up.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.
for just cruising, the only way you are going to prove to me that your car gets identical or better mpg at 65 vs 55 is with a scan gauge.

as far as the acceleration argument, i think spoke too soon and did not realize that every car is different. some motors will get best mpg with half throttle and shifting as soon as possible, others will not, and there is no real way to be certain where exactly all motors perform best without a phd.

[Snax]

But don't let a PhD fool you either. That just means they know enough to make it look like they really know what they are talking about - whether they do or not!

[tjts1]

A turbo can be used to make a car more fuel efficient. Check out the B5 1.8t Passat vs the the NA 1.8 Passat only offered in europe. The more powerful 1.8t is more fuel efficient. A turbo allows you to use a smaller engine with taller gearing therefore reducing internal friction. The 1.8t of the late 90s was equivalent in power to a 2.5 liter NA engine of the same era. But again it all depends on how its engineered and how you use it. The original 1.8t was rated at only 150hp by using a very small turbo that was biased for low rpm boost (flat torque curve starting at 1800rpm) instead of peak hp.

[Biffmeistro]

I don't have a scanguage, I have a tripminder. Same diff, pretty much. Came stock with early 80's ford luxury cars.

It reads both speed and fuel consumption, and thus gives a more accurate reading of instant MPG than the scanguage, which has to be calibrated and calculates based on displacement (I think...)

It's a plug and play computer for any ford EEC-IV equipped car.