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I have heard that when a turbocharger is being used it will cause the fuel economy of your car to drop.

Without thinking about techniques to improve fuel economy in general, how does a driver attempt to prevent their turbocharger from spooling up too much? From my limited observation it seems that if I accelerate slowly, even after reaching a high RPM the turbo will not produce a lot of boost (based on the sound and feel) but I could be wrong about this.

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    which car do you have – Shobin P Jun 30 '15 at 13:44
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    Your question suggests an underlying doubt about the working of a turbo charger. Please search the forum for the same and rephrase your question. For eg. your doubt could then be - Is there a relationship between boost and fuel economy, if so how do adjust it to suit my economy requirements , or something like that. – chilljeet Jun 30 '15 at 22:15
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Does the use of a turbocharger decrease fuel economy?

In my application, yes. But only because the turbo boost is so damn addicting! I find that my foot gets heavier than normal in order to achieve maximum boost in each gear. Common sense tells us that more acceleration = more fuel consumption. This answer provides more detail on the above subject: Does accelerating faster worsen fuel efficiency?

So the short answer is; it depends. Your fuel consumption will vary according to the size of the turbo, size of the engine, level of acceleration, aerodynamic drag, and the list goes on.

Without getting too detailed about the inner workings of a turbo, it's basic function is to provide on-demand power to the engine by forcing more air (and thus, more fuel) into the combustion chamber. The key here is on-demand; instead of providing constant consumption of fuel like naturally aspirated engines, turbos can actually be seen as a fuel saving tool.

How can I limit the use of the turbocharger as to improve fuel economy?

Every turbo has a boost threshold, which is the the period at which the turbo starts producing usable boost. This usable boost is affected by the rate of flow of the exhaust gases, which is what is needed for the turbo to spool up. This limits the turbo boost at a particular rpm, which is referred to as boost threshold rpm.

So, you are correct in thinking that less acceleration = less boost produced by the turbo. The turbo isn't receiving enough exhaust gas (created by acceleration) to create usable boost. Once you discover what your boost threshold is (which is based on your vehicle and other factors listed above), you can then determine the optimal rpm's to shift at in order to stay below the boost threshold rpm.

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tl;dr Yes. More boost => more air at higher temperatures => more fuel.

Your instincts are correct. If you drive around without loading the engine, the turbo won't spin up nearly so much. More importantly, it won't produce much boost and won't require much more fuel.

At a super basic level, there are four things that happen when your turbo starts generating significant boost:

  1. More air per unit volume is being forced into the intake (thus the name forced induction).

  2. The engine computer has to add more fuel to balance. The absolute best efficiency that we could ever achieve at this point in the process is the stochiometric ratio of approximately 14:1 air to fuel ratio.

  3. As the air is forced into the intake tract, however, it's also heating up. The ideal gas law is a reasonable approximation for about how much it's heating up but the point is it's quite a bit.

  4. Now the engine computer has to add even more fuel, except that this time, the fuel acts as a coolant to the mixture. The goal of this step is to keep the engine from literally detonating due to premature ignition from the increased temperatures.

As you can imagine, we turbo people would like to maximize step 1 and we'll accept step 2 as a natural consequence. However, we'd also like to minimize step 3 and avoid step 4 entirely if possible. This is where components like cool air intakes (to start with the lowest possible starting point) and intercoolers (to try to lower the temperatures of the intake charge after the turbo) become important. If you're really feeling excitable, you can replace a small turbo with a larger, higher efficiency one. This can get you to higher boost levels at lower temperatures but will also delay the onset of boost.

As always, everything is a trade-off in this sort of engineering problem.

All that said, you're still right. If you use less throttle and keep the boost levels down, you'll use less fuel.

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It depends. If you floor the car, the air/fuel ratio is dropped from 14.7:1 to 13:1 or even as low as 10:1 depending on the make and model and the boost level. the lower the ratio, the more fuel is wasted in an attempt to keep temperatures under control.

As long as you don't floor the car (sending the car into Wide Open Throttle / Open Loop mode), being "on boost" won't negatively effect fuel consumption. On the contrary, because a turbo forces air into your engine, it actually artificially increases the volumetric efficiency of the engine, making for a more optimal, complete combustion stroke. Which is why a 200HP turbo car uses less fuel than a 200HP non-turbo.

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You cannot limit the usage of the turbo , it will be very difficult to notice at what time you need to shift in order to not have the turbo spool up which is a distraction.

Solution:

There is something called a waste gate in most turbocharged engines(that is why I asked what car you use in the comment) If your turbo has a waste gate you can mechanically let it be open all the time, this will prevent the turbo from creating any boost at all, it will be like you are not have a turbo on your car.

When you feel you need the boost you can revert the waste gate settings.

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    Or get an electronic boost controller... – chilljeet Jul 1 '15 at 10:10
  • @chilljeet Great point.. – Shobin P Jul 3 '15 at 7:21

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