Kingtal0n

short version:
Kinetic energy and internal energy of waste exhaust gas is absorbed by the turbine to spin the turbine of a turbo.

Part of that kinetic and internal energy is then imparted to air which increases that air's speed, and this air is fed through a tube to our intake manifold.

As the valve opens, whether the piston is moving or not, this increased internal/kinetic energy is responsible for forcing its way into the cylinder. Thus mitigating some cost of generating an induction event on behalf of the piston/mechanical energy of the rotating engine.

NSFW

Not in cruise it won't, because the throttle plate will be closed to compensate for that increase in pressure. Let's go back to the with/without turbo example I posted earlier: if the engine needs 30 g/s in cruise, and the naturally aspirated one has the throttle plate opened just enough to achieve that, then the turbocharged engine (with higher pressure air in front of the throttle plate) will have the throttle plate opened correspondingly less, so that the engine still breathes in 30 grams per second.

If the engine breathed in air at a higher rate, it would be accelerating, not cruising at a constant speed, and the comparison would cease to be interesting from a mile-per-gallon standpoint.

A turbocharger lets an engine make more power with a given throttle opening, but that doesn't create better gas mileage. That just means that it will cruise with a smaller throttle opening.

Kingtal0n

If an engine needs 30g/sec without a turbo to cruise, it will use 30g/sec - X with a turbo. where X is the amount of air proportional to fuel ratio that represents the reduced cost of breathing due to the turbo. X can be close to 0 if the engine isn't setup to accept the advantage of incoming air with increased energy.

If we leave the engine at 30g/sec and connect a turbo, it will accelerate instead of cruise.

This can be demonstrated by opening the wastegate on a cruising turbo car. If you cruise with the gate connected, then disconnect it while driving (I've had the actuator arm fall off so this is easy to reproduce) the vehicle speed will decrease. If you then meet this decrease in speed with an increase in throttle position, injector duty cycle will increase, even to maintain the same speed, as air density losses and friction of moving air through a tube via vacuum generated by the piston instead of pressure due to a compressor wheel, take their toll on the descending piston work.

Likewise, if you start off cruising without the wastegate arm attached, reach a steady cruise speed, then connect the arm, the vehicle will accelerate at the same throttle positioning. And if we reduce throttle position to accomodate, so does injector duty cycle decrease as well. Thanks to the additional air density, lessening the work of a descending piston.


Energy comes from the fuel, not the air. So you must keep in mind the changing air/fuel ratio that accompanies such events as changing pressure ratios.

Kingtal0n

Another couple ways to see this

If you are looking at a MAP based fuel map it becomes obvious that higher pressure values, send more fuel to the engine.

Thus when connecting a turbine actuator arm and engaging the compressor, we expect to see the fuel map move to a lower pressure (more vacuum) region where less fuel is required.

Since the engine is tuned FOR a turbo, or NOT for a turbo, (but not both) it is impossible to replicate both scenarios using the same fuel map. If closed loop were enabled it would create opposing fuel trims for example. This in and of itself proves that there is a distinct difference in fuel use between the two situations.[/COLOR] It is therefore important that we carefully observe air fuel ratio during these changes, as you can change pressure all day long and not have any idea how much fuel is required or used. Furthermore, injector on-time by itself is useless because changing conditions of vacuum between the two examples alter injector flow rate, for vacuum referenced regulators and even without such, as pressure below the injector at the time of opening affects injector flow rate.

So this whole theory, in theory, as a theory, is extremely difficult to show with actual maths, too many variables, etc... in a running engine, as a derived equation of sorts.

However, it is easy to see, and MEASURE, as an observer who wishes only to view the exchanges of energy as I have done above quite a few times, both using thermodynamics basic principles, and using simple observation of actual engines in operation. You may not neglect kinetic energy of air moving in a tube; as those with headers VS manifolds will attest: moving air contains an energy all by itself that we may harness. Indeed without expansion chambers on 2-stroke engines, and without properly tuned "race" config header exhaust systems, cylinder fill of piston engines will suffer dramatically. And if turbos did not "do this better" than tuned header exhaust systems, well the cylinder wouldn't be any fuller for the cause.

NSFW

Coincidentally, my other car provides counterexamples. The 2005 Subaru Legacy was available in two versions:

2.5 liter engine, turbocharged, 243hp, 19 mpg city, 25mpg highway
2.5 liter engine, non turbo, 168hp, 22 mpg city, 30mpg highway

Kingtal0n

Ah this brings back memories. I've seen this and other "examples" in the past.

The reason these vehicles have different economies is based in their drivetrains and the rotating mass of their engines. The turbo version has heavier parts, not sure exactly what all is heavier but I assure you, they do not install the same weight-HP-capacity parts in the vehicle with no turbo as the turbo model. It may also be reflected in their respective gear ratios; perhaps the turbo model has a more "performance" gear ratios with higher cruise RPMS.

I too can take a turbo off my car, then adjust rear gearing to achieve perhaps similar or better economy.
Depends how heavy the engine rotating weight is. A V8 surely, an 4-cylinder not so much.

And of course compression ratio is lower in the turbo engine I presume. A lower CR, even just 1 point, is a lot of economy on the table given up.

NSFW

Yes, turbo engines generally use lower compression ratios than their NA counterparts, and stronger/heavier pistons, rods, etc - all the more reason I have trouble believing your claim that manufacturers add turbochargers to improve fuel economy.

And then you go on to say that the compressed air is pushing down on the pistons.... apparently oblivious to the fact that, in cruise, the throttle plate's job is to keep the intake manifold pressure low enough to prevent the cylinders from drawing in more air than is required to cruise at constant speed.

It's not that you're wrong about any particular piece of your analysis, it's just that you're missing some really fundamental pieces that change everything: the throttle plate, its effect on intake manifold pressure, and the airflow into the cylinders that follows from the intake manifold pressure.

Kingtal0n

https://www.sae.org/publications/tec...ontent/730633/

https://ieeexplore.ieee.org/abstract/document/1470694/


http://citeseerx.ist.psu.edu/viewdoc...=rep1&type=pdf


And one with no real 'refs':
http://www.autospeed.com/cms/article...onomy&A=109931



At the end of the day there are several cited articles on google scholar which present that turbocharging is increasing economy. And none to few that resist this idea.

Since most of the modern research is done on "new" subjects, such as Lean Burn with DI and high CR, or variable van turbocharging, EGR reduction of EGT, and so forth, it is difficult to find such a phrase as "turbocharging has been shown to improve fuel economy" and much more likely to find phrases as "it is well known that turbocharging improves economy..."

Kingtal0n

Oh NO that is not what I meant exactly. Auto manufacturers are adding turbos to improve economy- by using various methods WITH the turbo, such as engine downsizing or lean burn DI. No auto manufacturer is adding ONLY a turbo specifically for fuel economy. It would be silly, expensive, and pointless, dare I say dangerous, not to modify an engine to accept a turbo, as people trying to buy those cars expect higher output and raise boost pressure with high hopes. Which means stronger parts, heavier drivetrain, etc... are required.


Pressure here is relative. It was just to give you an example to imagine. If the pressure in the cylinder is higher than what exists below it, or not, is all that matters. To us, 3" Hg or 10" Hg is a "VACUUM" not a pressure. Yet, 3" of Hg could still "push down on a piston" if the pressure below the piston were lower. Think of a DRY sump or VACUUM pump type of situation. Vacuum under the piston is able to "suck it down" the same as pressure above it would "push it down". So we must be careful to look both above, and below, a piston, when comparing what pressure (or vacuum) will be applying force and where. During cruise, an engine PCV system typically applies a vacuum to the crankcase around 3" Hg as it were. So 1" Hg above the piston would help move it down. "HELP" it move, not necessarily move it completely. You have to imagine the engine is spinning anyways; It will move down even if something breaks at that instant. So we compare forces, knowing that ultimately piston will move down no matter what as the connecting rod pulls it down eventually, but that it can also get "help" moving down from pressure changes above and below the piston.

Pressure is relative, so I ask you merely measure pressure from one end to the other. When both valves are open the pressure entering the intake manifold can escape into the exhaust. And visca versa. So valve timing, has a monstrous role in whether the pressure of an exhaust system or pressure of a compressor wheel, what will happen next can vary.

NSFW

One of these things is not like the others:


But I think we're on the same page now.

Kingtal0n

Engines where a turbo was added is any engine platform where they altered it to include a turbo to achieve a higher output rather than going to a larger engine. So yes we are on the same page.

Kingtal0n

For example 2jz-gte and sr20det both posses turbo and non turbo varieties. The 2.0Turbo can output what a 10L engine could output, if it wanted, by adding a turbo.

So the point is moot, as this is the goal and obvious choice for every manufacturer of combustion engine.

I think what you are trying to say is that I meant a turbo will add fuel economy in place of a well designed exhaust by re-capturing kinetic energy and filling the air molecules with it. As I said before the act of intercooling dissipates internal energy and presents a pathway of friction. Therefore the turbo need be size small enough to spin fast enough at a steady state cruise speed to impart enough kinetic energy to overcome the restriction of the intercooler, etc... and meet the throttle body.

This versus having to draw it through some length of intake tube, no matter how small, how short, WITH an air filter,

The delivery in the turbo example costs less of the engine. Maybe this wasn't "WHY" they wanted 10L of power from a 2L; but it still means better economy with than without.

Kingtal0n

If the engine is cruising at 15" Hg. That means the manifold is 7.5psi of pressure inside it. That means when the intake valve opens, it only "pays" 15" of Hg worth of syringe effect in vacuum.

If you disconnect the gate and the engine slumps now to 12" of Hg. Why is that change in the intake manifold when now the wastegate is open, and exhaust find an even easier pathway? Why doest thou fuel economy decrease in this situation? For surely the vehicle has not gotten heavier, thus the amount of work needed to push it has not increased, and by all right I have given exhaust an easier pathway to bypass the imposing turbine wheel.

Kingtal0n

higher intake manifold vacuum is associated with higher economy. If the piston cylinder intake valve opens and it has 1psi in it, the air flows out of the cylinder and into the intake manifold.

Therefore, to enter the piston cylinder, pressure has to be even lower than the intake manifold vacuum.

I think where you are getting stuck is that the pressure in the cylinder need not equal the pressure of the intake manifold. Surely you can see that it ultimately the pressure drop of the piston cylinder which pulls air in a traditional sense. 14.5psi of atmospheric pressure sitting at the throttle body only looks like positive pressure to the engine because it is running. If you shut it off, the pressure is equal all around, and there is no FLOW through the engine. there is no kinetic energy of moving column of air mass present. Which is an extremely active, present ideaology of combustion engines, Imperitive to their performance aspects. Most engines work to generate the incoming column of air, and then rely on the exhaust column for some "payback". Adding a turbo merely preps or pre-conditions the IN coming column of air, so that instead of having to generate a wave of pressure drop across the manifold that leads TOWARDS the throttle body, FROM the intake valve, and OUT past the throttle body, THROUGH the air filter. Normally an engine WORKS HARD to pull all this column of air into motion through these passages towards itself. The more torturous the passage way the more work it does and fuel is wasted. That is why intake manifold pressure increases when we disconnect the turbine on a turbo car and try to cruise.

If instead, I stand at the throttle body and fire molecules through the cracks like supercharged cats, pointed through the throttle body, pointed towards the intake valves. Now you have this FORCE of airflow in the opposite direction, pressing INTO the intake passages, forcing itself INTO the engine intake manifold. I don't care what the pressure is in the intake manifold because I can assure you that it is going DOWN as I fire my cats harder and harder into those passages, and down means like from 15" to 18" to 20". I am getting MORE vacuum in the intake manifold now because the engine is no longer doing WORK to produce that incoming column of air.

Kingtal0n

manifold vacuum is inversely preportional to throttle opening. more opening = less vacuum.

so, more opening = more pressure.

SO if you reduce throttle opening, you have less pressure, more vacuum.

Less pressure means fewer molecules per unit space. aka density of air. The engine's piston cylinders are getting less air now than before. Less air because,

because the following explosion can be smaller. We need less energy from combustion reaction. Injecting less fuel too because, Something got better somewhere.

Was it energy extraction from combustion model? no. Was it an easier exhaust stroke? Well I put a turbine in the way of the exhaust valve, so no, initially the engine had to work harder when I connected the turbine, so technically we should be needing more air and more fuel, not less now. Was it an easier compression stroke? Hmm less air in the cylinder NOW means less compression ratio, compression got easier recently. But what happened before this that caused us to back off on the throttle in the first place? What was the initiating cause for our decrease in throttle position after connecting the turbine and causing the turbo to spin? It wasn't the exhaust stroke that got better, it got worse. It wasn't the power stroke that got better. And the compression stroke is only now suddenly getting better, now that I just backed off the throttle position. All else being equal, we can't just suddenly have an easier compression stroke out of no where. Thats impossible to happen all by itself and it isn't the cause for us to back off the throttle to begin with.

And there is only one stroke left to examine, for I doubled checked, the turbine is more difficult to move past than the open gate, and there is no way my power stroke suddenly yielded more power out of no where, driving down the road nobody touched the tune. All I did was connect the turbine.

Kingtal0n

These statements ring true. 2jz-gte a turbo(s) was added to improve power while adding potential for economy at an even lower compression ratio, which would otherwise not be possible without the turbo(s) in place. They used a "sequential" design where exhaust was fed to only 1 turbo at first, to boost response, so the wheel of that turbo would be moving twice as fast at cruise/idle/launch than if both turbines were fed simultaneously all the time. Strictly economy purposes? Okay maybe I left out the power part. But thats the obvious-to-everybody part, do I really need to say "oh, Also, turbos were added for power output increase...." I mean, you need to be told that?

I can see how you might think I was being purposefully misleading "as if I wanted people to install turbos solely to boost their economy" (which, it would, all else being equal, if done right) but that wasn't the case as I am not selling or profiting from anything here. Furthermore, if the decision was between supercharging and turbocharging, then the auto manufacturers are choosing turbos for economy purposes after all. Since its literally one, or the other, if they want to keep the same engine. So it was added to the engine for economy purposes, no matter what we spin this, even if you still think turbos don't or can't boost economy.

Turbos do add economy, they know it, and they used it because of that fact, maybe not 100% that reason but it sure didn't hurt in their decision making process. Think again to the 4-strokes and the change in manifold vacuum upon deactivating a turbine while cruising. Ask yourself for each one, did I gain something here? Did I lose something here?

nutbuster

I just love the noise! It gives me a chubby!