Boost question 8

[panelman]

Guys, might sound a stupid question but stay with me

can someone explain what causes a turbo to generate boost, more specifically...

Subaru Impreza, 4000 rpm standing still generates no boost, same rpm when driving and it's scalded cat time

Is it something the ecu does?

Cheers

 

[SBBlue]

For a turbo to make power the pressure of the exhaust manifold must be higher than atmospheric pressure. If the pressure is below atmospheric a turbocharger will be a net power consumer, since it will take extra work by the engine to compress the exhaust gas during the exhaust stroke and push it through the turbine.

That brings us to the question -- when would the exhaust manifold pressure be less than atmospheric?

As a rough approximation, for a gas (spark ignition) engine with a compression ratio of ~ 10:1, the pressure of the exhaust gas at the end of the power stroke will be about 4 times the pressure of the intake air in the cylinder at the beginning of the compression stroke.

Consider when the car is going at 4000 rpm and scooting the car down the road at 60 mph. What percent throttle will that be? 50%? If it's 50% throttle than the pressure in the IM will be about one-half atmosphere, and the pressure of the exhaust gas at the end of the power stroke will be about two atmospheres of pressure. Hence, plenty of pressure (and heat) to have the turbine make power.

But at no load conditions -- even if the engine is racing at 4000 rpm -- what percent throttle are we looking at? Maybe 25%? If it is 25%, than the IM atmospheric pressure in one-fourth atmosphere and the pressure in the cylinder at the end of the power stroke will only be one atmosphere or so. Hence, there is no excess power to be recovered from the turbocharger.

Hope this helps

 

[panelman]

Thanks for that

you say four to one for spark engines, what's the figure for oil burners?

 

[SBBlue]

Since spark engines are basically stoichometric, the exhaust temperature can roughly be calculated -- in this case, about 1800 deg F.

By "oil burners" I'm assuming you are referring to Diesel engines. Diesel engines are not throttled; however, the intake manifold pressure can vary according to the degree of supercharging. In addition, Diesel's are excessive lean -- with a air/fuel ratio of 30:1 or so.

If fuel consumption is held constant, the higher the boost the lower the exhaust gas temperature, since there is more air to absorb the same amount of heat energy. And since the exhaust gas pressure in the cylinder when the exhaust valve opens is dependent upon temperature, the relationship is not quite as straightforward as with a gas engine.

Having said that, the Diesel's I'm familiar with have a 3:1 or 4:1 pressure increase between the intake valve closing and the exhaust valve opening. The difference is that the diesel might be running at a boost of 2.5 atmospheres and have a exhaust gas opening pressure of 7 atmospheres.

 

[rmw]

While SBBlue is correct in stating that it is the pressure difference across the turbo that drives the gasses through it, it is the heat energy in the gasses that imparts work to the turbo shaft, as the gasses flow through the turbine blades, just like a steam or gas turbine.

Take the same gas flow at the same pressure drop, but pre-cool it to say, atmospheric temperature prior to it entering the turbo, and you will get precious little boost from the turbo, if any at all.

All the rest of the discussion regarding the pumping work of the engine to create the pressure, etc., is still true, but the work out, which is done on the opposite side of the wheel where the inlet air is, is a function of the heat in.

When you run your engine up to 4K rpm at in neutral, you are doing no really significant work with the engine, and the exhaust is "cool". When driving at the same RPM, and even more so, if that rpm is while climbing a mountain or such like, you are asking your engine to work hard, and part of the engines losses is exhaust loss, so a higher fraction of the fuel energy you burn shows up in the exhaust gas as exhaust loss, and the turbo then converts some of that heat back into useful work to "boost" the combustion air.

Pure first and second law thermodynamics. It is just a little miniture gas turbine.

rmw

 

[SBBlue]

What RMW says is absolutely true. It is the heat, and not the pressure, that provides the energy for the turbine.

Here's an example. Suppose that we had a tank of compressed air that supplied a constant two atmospheres of pressure and the air was routed through a turbine and discharged into the atmosphere.

Let's assume that we have a 100% efficient turbine. If the temperature of the air entering the turbine was 80 deg F, the turbine would produce about 23 BTUs of energy per pound of air. If the air entering the turbine was 1800 deg F (gas engine exhaust temperature), the turbine would produce about 99 BTUs of energy per pound of air.

Obviously, the higher temperature gas will produce more energy. And the interesting part is the temperature of the air exiting the turbine. For the hot gas setup, the temperature of the gas exiting the turbine is about 1445 deg F. For the ambient gas setup, the gas exiting the turbine will have a temperature of -17 deg F.

The way to consider this is that the turbine extracts heat energy from the gas, but it is the pressure of the gas that enables the turbine to do the extraction.

What happens if we increase the pressure of the gas going into the turbine, to say, three atmospheres? For the hot gas setup, the amount of recovered energy will increase to 149 BTUs/lb, and the temperature of the exit gas will be 1261 deg F. For the ambient gas setup, the amount of recovered energy will increase to 35 BTU and the temperature of the exit gas will be -66 deg.

So the converted energy comes from the heat, or temperature of the gas. But the pressure of the gas is what enables the turbine to extract the energy.

 

[Brandnew]

Very interesting SBBlue!

Is the power the turbine can produce effected if the temperautre is "restricted" from dropping after the turbo??

Specifically, I'm wondering about those that thermally wrap the downpipes AFTER the turbo. In theory, does this hinder the power the turbo can produce, help, or have no effect??

TIA!

 

[rmw]

To my way of thinking, the insulation downstream of a turbo, unless, of course, there is another turbo downstream of the first one, done, is counterproductive.

Is this done just to sheild other engine compartment components or cab components from exhaust heat??

If the cooler, but still quite hot exhaust gas exiting from the turbo, greatly expanded at this point, because the pressure has dropped through the turbo, (and the volume of the gas has expanded,) cool off to any extent by transferring its remaining heat to the atmosphere through the walls of the exhaust pipe, it shrinks in volume somewhat, and reduces the flow losses of exhaust system, and hence, the backpressure on the turbine, which does have a big effect on turbine performance.

By blanketing that piping, the heat is held in, and the gas remains at the volume at which it leaves the turbo.

I have been contemplating blanketing the piping between the exhaust manifolds and the turbo on a diesel bus I have, both to retain the heat in the gasses upstream of the turbo, to be able to produce more work in the turbo, as discussed above, and, to try to reduce the 'underhood' heat under the floor of the bus at the rear. But, I never comtemplated blanketing the downstream tubing. Hmmmm.....

rmw

 

[Brandnew]

That's what I was thinking too. I can understand holding the heat in the exhaust manifold but in the downpipe it seems counterproductive. I bring it up because I recently saw a car with the "downpipe thermally wrapped".

Curious what the exprets thought on this.

 

[SBBlue]

Brandnew asked:

Is the power the turbine can produce effected if the temperature is "restricted" from dropping after the turbo??

Specifically, I'm wondering about those that thermally wrap the downpipes AFTER the turbo. In theory, does this hinder the power the turbo can produce, help, or have no effect??

My thoughts:

RMW nailed it on the head.

Insulating the exhaust distal to a turbocharger will have a slight deleterous effect. Given an adequate exhaust system, the increase in pressure should be trivial, however.

I certainly don't see any advantage in insultating the exhaust system once you are past the turbocharger.

I also agree with RMW about insulating the exhaust manifold and exhaust pipes proximal to the turbocharger. This is much more significant for a diesel engine as opposed to a gas engine, because a diesel turbocharger essentially operates all the time, while a gas engine turbocharger is only functional with the throttle wide open.

 

[Brandnew]

Thank you for your thoughts. Glad to see I was thinking along the correct lines.

 

[liviu2004]

For a turbine to make boost pressure it is necessary a flow of exhaust gases to pass through it. It is the only way that the exhaust gases on a turbine-equipped engine can go out. So, exhaust gases goes out and the turbine rotates. If it rotates then it generates pressure in the intake manifold.
What the ECU (Engine Control Unit) has to do with this? Apparently nothing. ABSOLUTELY WRONG. Example: BMW 530d 2003 common-rail diesel engine with a lot of engine power loss. With diagnosis software I sow this error code: boost pressure control – deviation. On actual value data it shows: at 850 rpm boost pressure was 1000 mbar, at 3500 rpm it was 2300 mbar. And the vehicle was standing still. On drive tests boost pressure raised very quickly and ECU forced to keep it at 2200 mbar by cutting out fuel injection (loss of power).
So, we come back to the basic: on low and partial engine rpm (not to be confused with engine load) the volume of exhaust gases is relatively low and the turbine generates a normal pressure between 1000 mbar-2200 mbar. On high rpm the volume of gases is very high and the turbine would rotate so quickly that it could produce so much pressure that it could explode the hoses and inter-cooler radiator.
The solution: deviate a part of exhaust gases from the turbine and lead them outside. Deviation is performed with the help from a vacuum controlled actuator. The vacuum applied to the actuator is controlled by the ECU through a vacuum control solenoid. To keep boost pressure to a normal operating condition ECU receives information from air mass meter, injectors, engine temperature, oil temperature sensor, air temperature sensor, gear selected (automatic transmission only), climatronic, pedal position sensor, etc.
So, Panelman: your Subaru Impreza has no need to generate boost pressure because the load is very low when the car is standing still, BUT due to a malfunction in boost pressure control it could generate big boost pressure at 4000 rpm.
PS: the vacuum actuator on BMW was mechanical blocked in max. supercharge position.

 

[ruggedscot]

I know this happened once, a guy goes to a garage and complains of the car going really really fast and the engine being rough....

Mechanic takes it out comes back and then hoists it up and has a look around, finds the wastegate was jammed thus allowing the car to generate boost all the time and not regulate it, the only regulation being the amount of fuel being injected - hence race cars with higher boost use multiple injectors on the fuel delivery.

Id tend to think that you need to check out the boost situation and re think what you are saying. reving a car up when no load is on engine ie sitting with clutch in does not use a lot of fuel, engine will spin up quickly. Now problem is that as the engine is not buring more fuel the gas produced is not that hot relatively speaking. Bring in some load, an uphill section of road pulling a trailer with a low gear selected, the engine revs climb and as the throttle is down the butterflies on the engine are wide open and the engine 'puter is putting in as much fuel as possible. This way the turbo is getting really hot exhaust gases and this is spooling up the turbo to produce pressure in the intake, which is in turn causing more hot gas to come out the exhaust and into the turbo. The boost quickly climbs and the engine starts to produce more horses. The engine has to have a load on it to do this and you see this by reving it up with no load makes no boost. If the engine was supercharged this would be different and you would see boost being produced as it comes from a mechanical source rather than from hot exhaust gases. You need to read up on thermal dynamics to fully understand how an engine starts to provide boost though a turbo charger.

 

[tooleboy]

Panelman your first instinct was correct YES it is something to do with the ECU. Many of the posts have speculated that while the vehicle is in neutral thermodynamics will not allow the engine to produce boost. This is incorrect. In neutral it is still possible to load an engine to it's full extent. It will simply accelerate VERY fast as the only resistance is the Inertia and frictional loads of the engine. Many drivers of turbocharged vehicles will clutch the vehicle prior to exiting a corner down shift and jump on the throttle to get the turbo generating boost before releasing the clutch thus having 0 turbo lag on corner exit.

As liviu2004 suggested what you are likely noticing is the wastegate control. Typically on turbocharged vehicles, gear indicated and vehicle speed are tied into the waste gate to control boost. For example if you are in first gear at 5mph there is likely a lookup table limiting boost. Why waste this boost you ask? It can be used as an openloop traction aide to maximise acceleration. If you had high boost (torque) at low speeds in low gears the only product would be tire smoke.

A funny side note: I knew a guy who bought a turbo charged vehicle and thought he would be really smart by disconnecting his vehicle speed sensor (ie odometer fraud) well much to his dismay he was left with a very very sad engine running 8.0:1 compression, NO BOOST and had to pay 75 bucks to get his Check engine light cleared.

Pure Karma!!!

 

[SBBlue]

Toleboy wrote:

"Many of the posts have speculated that while the vehicle is in neutral thermodynamics will not allow the engine to produce boost. This is incorrect. In neutral it is still possible to load an engine to it's full extent."

Okay, I'll bite. How is it possible to load an engine to it's "full extent" if it's not connected to a load (ie, in neutral)?

I will agree that it's possible to have the engine race at full throttle while it's in neutral. But's that not quite the same as "loading an engine to it's full extent." There may be engines out there in which it's okay to run unloaded on a wide open throttle, but I'm not aware of them.

I will point out that running at an engine at wide open throttle will have the effect of increasing the exhaust valve opening pressure in the cylinder under any conditions.

Once again, on a gas engine the turbocharger is really only beneficial when the throttle is wide open.

 

[ruggedscot]

when you work the clutch in a corner as you said you are only producing rotation in the turbo, keeping the vanes moving fast, this has the effect of reducing the period of time or turbo lag that it takes to produce boost when the engine starts to work again. I still say that a turbo cant boost unless it is working, on idle there can be no boost only rotation of the vanes. The boost guage will show no boost present when in this state. But will rapidly develop boost when throttle is applied as the rotor has been spinning at a higher speed than what it would have normally.
Boost is only a function of the thermodynamics of the engine.

 

[moparmark]

or my 2 cents, petrol engines use a throttle butterfly, and any boost developed by the turbo is controlled by this butterfly. As the engine in the wrx only requires a minimal amount of air flow to free rev to 4000rpm the inlet tract after the butterfly will be in a state of vacuum still, and given that boost gauges are plumbed into the plenum after the throttle valve a state of vacuum will show on the gauge while the engine free revs. Its quite possible that the turbo is making boost, but to measure this you would have to plumb the gauge into the pipe work leading to the throttle. As for the ecu controling the wastgate to achieve this, not so, the ecu can only decide at which point to allow boost pressure to go to the wastegate, the ecu cannot generate boost by itself to open the gate, it only uses the boost generated and at its set limit sends that boost to open the valve. This only realy applies to petrol engines with butterflys though.

And yes toolboy, some companies do employ torque reducing measures , but mainly to aid the driveline meet durability requirements, but again this is only basic control over max psi seen by the engine, and is not done in nuetral as there is no need. Consider a non turbo efi engine, it will rev of the clock and also make max power without boost, so why would a turbo engine require boost to free rev to 4000rpm, it wont so why would the manufacturer create boost control for something that will never be a problem?

 

[crysta1c1ear]

http://www.eng.fsu.edu/~shih/eml4421/student presentation EML 4421/turbo & supercharger.pdf

page 9
The turbine power can be calculated by
Power = mass flow rate*cP*(T3-T4)

So air flow and temperature drop are important, as we have read in previous postings. Now both petrol and diesel engines are able to run pretty quick in neutral without pressing hard on the accelerator. What happens is different in each case.

Petrol engine:
light throttle restricts airflow, good mix of air and petrol produces hot exhaust gases but low airflow, and the turbo doesn't do much, as one poster suggested. Mass flow rate is low.

Diesel engine:
small fuel charge on low accelerator results in cooler exhaust and smaller temperature drop. Remember SSBlue's posting? What RMW says is absolutely true. It is the heat, and not the pressure, that provides the energy for the turbine. In this case, it will be the temperature drop that is low.

So it looks to me like the turbo isn't going to do much while your sitting at the traffic lights - even at high RPM, the original question - and for different reasons, depending on what car you've got.

 

[Wilsonman02]

"I certainly don't see any advantage in insultating the exhaust system once you are past the turbocharger."
If the downpipe is wrap with insulator, the downpipe traps that heat from the exhaust gas in the downpipe, the hotter the exhaust gas the faster it moves, so the faster u can get that gas out the faster the spool and the better the power. Same reason they wrap the Exhaust Manifold, to increase exhaust flow. But most guys just opt for a bigger exhaust, because on turbo charged cars, the best exhuast is no exhaust, or the bigger the better, unlike NA cars.

"Okay, I'll bite. How is it possible to load an engine to it's "full extent" if it's not connected to a load (ie, in neutral)? "
Well to my knowledge its not possible to load an engine to its full extent in neutral, but if your peak boost is set to 20psi, and u hold it at rev limiter you can still build 8,10, or even higher psi, which makes for one hell of a launch. Also many guys use a small shot of nitrous to load the engine and create more exhaust and to spool the turbo further before a launch.

"when you work the clutch in a corner as you said you are only producing rotation in the turbo, keeping the vanes moving fast, this has the effect of reducing the period of time or turbo lag that it takes to produce boost when the engine starts to work again. I still say that a turbo cant boost unless it is working, on idle there can be no boost only rotation of the vanes. The boost guage will show no boost present when in this state. But will rapidly develop boost when throttle is applied as the rotor has been spinning at a higher speed than what it would have normally.
Boost is only a function of the thermodynamics of the engine."
Most guys either use the heel toe shift technique to keep RPM's so there's less turbine inertia. Or a two stage rev limiter, so they can use the "no lift to shift" technique, they just keep the throttle pedal to the floor at all times. Or in old school F1 racing, they put another injector into the exhaust mani or directly into the turbine housing, and when the clutch was depressed the injector would shoot fuel in the turbine, igniting the fuel and causing the turbo to prespool before the next gear was selected. And even at idle, 750rpms the fins of a large turbine housing will be spinning rather slowly, if you hold the engine at rev limiter or close to it, your not producing a load like driving the car would, but you're still creating more exhaust gas than at idle and the gas is moving faster. So you can pre spool the turbo, but you are limited to what PSI you can boost to.

 

[SBBlue]

Okay, I'll bite on Wilsonman02's comments.

"the hotter the exhaust gas the faster it moves, so the faster u can get that gas out the faster the spool and the better the power. Same reason they wrap the Exhaust Manifold, to increase exhaust flow."

Well, I will agree that the hotter the exhaust gas the faster it moves -- but that's the reason that insulating the exhaust pipe is a bad idea.

Here's why. If you increase the heat of a gas, you will increase the volume of the gas. If you have an increase in the volume of the gas, and if the exhaust pipes remains the same capacity, the gas will have to flow at an increased velocity -- just to maintain the same mass flow rate.

What will cause the gas to move faster? The engine. The engine has to work harder to push the same mass of exhaust gas out.

For all practical purposes, though, the increase in engine work will probably be trivial for an insulated tailpipe.

"Also many guys use a small shot of nitrous to load the engine and create more exhaust and to spool the turbo further before a launch."

Nitrous doesn't increase the load on an engine. It just provides more oxygen so more fuel can be burned on each stroke.

I think the phrase you should be using is "increase the rpm", because that is what you are describing. And that does increase the engine gas flow.