Questions about turbochargers

[brownzilla786]

Hey guys,

I am giving a short presentation on turbochargers for one of my classes, and I got all of the basics down of how it works but I still have some lingering questions

1. Nowadays you turbocharge something to increase MPG, but from what I understand a turbocharger simply allows more air (oxygen) to enter the cylinder so you can add more fuel (this would lower MPG). I am guessing that instead of a v6 you have a turbocharged 4 cylinder, and instead of a v-8 you have a turbocharged v-6. BUT all things being equal, a N/A v-6 will have better MPG than a turbocharged v-6 So ultimately you get fuel savings by having a smaller engine with less cylinders with same power output = +++MPG?

2. What affect does turbocharging have on compression ratios?

3. What is the difference between a waste-gate and a blow off valve?

4. When they mention boost pressure does that mean the pressure of air entering the cylinder? So an increase in pressure would mean more air entering? Is this separate from the density of air entering the cylinder?

Thanks guys <3

 

[skyking]

1) Not a simple answer here. Turbos allow greater horsepower density per pound of engine. That can increase efficiency a bit. Otherwise there is no direct correlation between turboed vs non turboed. Diesel engines are an exception and a totally different subject due to the differences.
2)To maximize effect, lower compression pistons are commonly used in turbo engines.
3) A wastegate vents some of the exhaust drive pressure from behind the hot section of the turbo. It is vented downstream into the exhaust piping in most cases.
A blow of valve vents air from the intake system. both are used to regulate the boost. A BOV is not as efficient as a wastegate, since the engine had to create that pressurized air in the first place. The wastegate reduces drive pressure to accomplish the same thing. Reduced drive pressure = lower temperatures, more power.
4) yes. The density of the air charge is not only related to pressure, it is related to the temperature of the air. That is why intercoolers will increase available power, by increasing density.

 

[futuristicmonkey]

Hey guys,

I am giving a short presentation on turbochargers for one of my classes, and I got all of the basics down of how it works but I still have some lingering questions

1. Nowadays you turbocharge something to increase MPG, but from what I understand a turbocharger simply allows more air (oxygen) to enter the cylinder so you can add more fuel (this would lower MPG). I am guessing that instead of a v6 you have a turbocharged 4 cylinder, and instead of a v-8 you have a turbocharged v-6. BUT all things being equal, a N/A v-6 will have better MPG than a turbocharged v-6 So ultimately you get fuel savings by having a smaller engine with less cylinders with same power output = +++MPG?

2. What affect does turbocharging have on compression ratios?

3. What is the difference between a waste-gate and a blow off valve?

4. When they mention boost pressure does that mean the pressure of air entering the cylinder? So an increase in pressure would mean more air entering? Is this separate from the density of air entering the cylinder?

Thanks guys <3

1. You reduce frictional losses indirectly because you can generate more torque at lower speeds. (By using a higher gear ratio you can reduce the speed of the engine for a given output at a given speed)

2.Turbocharging has no direct effect on the compression ratio. Whether you select components i.e. larger combustion chamber, shorter connecting rods to reduce the compression ratio (to combat "knock" or premature detonation) of an engine I suppose could be considered an indirect effect.

3. See skyking's post

4. The average density of the air would be affected by temperature, moisture content, pressure as well as effects due to the geometry of the space through which the air is flowing. By increasing the pressure you can force more air into the cylinder. Natural aspiration relies on the piston moving down on the intake stroke to suck air in. Boost is used to push more air in.

 

[Fenixgoon]

1) Not a simple answer here. Turbos allow greater horsepower density per pound of engine. That can increase efficiency a bit. Otherwise there is no direct correlation between turboed vs non turboed. Diesel engines are an exception and a totally different subject due to the differences.
2)To maximize effect, lower compression pistons are commonly used in turbo engines.
3) A wastegate vents some of the exhaust drive pressure from behind the hot section of the turbo. It is vented downstream into the exhaust piping in most cases.
A blow of valve vents air from the intake system. both are used to regulate the boost. A BOV is not as efficient as a wastegate, since the engine had to create that pressurized air in the first place. The wastegate reduces drive pressure to accomplish the same thing. Reduced drive pressure = lower temperatures, more power.
4) yes. The density of the air charge is not only related to pressure, it is related to the temperature of the air. That is why intercoolers will increase available power, by increasing density.

1. You reduce frictional losses indirectly because you can generate more torque at lower speeds. (By using a higher gear ratio you can reduce the speed of the engine for a given output at a given speed)

2.Turbocharging has no direct effect on the compression ratio. Whether you select components i.e. larger combustion chamber, shorter connecting rods to reduce the compression ratio (to combat "knock" or premature detonation) of an engine I suppose could be considered an indirect effect.

3. See skyking's post

4. The average density of the air would be affected by temperature, moisture content, pressure as well as effects due to the geometry of the space through which the air is flowing. By increasing the pressure you can force more air into the cylinder. Natural aspiration relies on the piston moving down on the intake stroke to suck air in. Boost is used to push more air in.

to answer more of the "why" part of your questions:

1. turbocharging does require more fuel due to more air. the increase in power generally means the engine doesn't have to work as hard under low loads. but if you really boost the crap out of an engine to get lots of power, you will still end up with low mileage (mitsubishi lancer evo gets like 25mpg highway since it's such a high strung turbo I4).

2. turbocharged engines have lower compression ratios because turbochargers are compressors. when you compress the air, you also heat it up. with hotter air, it will increase the tendency to have premature detonation of the fuel/air mixture when the mixture is compressed as the cylinder rises in the compression stroke of the engine. so to combat this, you dont compress the mixture as much before ignition. you can also use an intercooler (heat exchanger) to try and cool down the compressed air some before it mixes with the fuel.

3. not sure about the difference between a wastegate and BOV. i know a BOV will simply vent the excess compressed air to the outside. there are also recirculators (cant think of the exact name offhand) that will keep the compressed air in the engine loop. if you have a recirculator, the ECU will expect that air to be in the system, but if you slap a BOV on without retuning the ECU, things will get all sorts of messed up.

4. boost pressure is the pressure above atmospheric pressure (gage pressure) that the turbocharger creates. the density of dry air will be a function of temperature and pressure (but in reality, you also have to take into account moisture as previously mentioned). more pressure will always mean more air.

PV = NRT.

pressure*volume = number (or mass)*gas constant*temperature

it explains a lot of gas behavior, even if doesn't get the relation perfectly

 

[skyking]

This round flap in the picture is the wastegate.

It is opened up to do porting work. The exhaust manifold is bolted on at the top of the picture, by the wax stick.
It is normally closed and connected to an actuator, an air cylinder connected to the boost side of the compressor section. When boost pressure opens that valve, it bypasses the turbine wheel, reducing boost.
The perspective is from the exhaust outlet side of the turbo housing.

 

[exdeath]

BOV on the compressor side is to protect the turbo from damaging pressure spikes known as compressor stall, compressed air forcing its way back into the compressor, when the throttle is suddenly shut under boost. Wastegate is to regulate maximum boost by bleeding off input energy (read exhaust gas) around the turbine. Two completely unrelated functions. Wastegate is a vital part which without you would get "infinite" runaway boost and destroy your engine. BOV or a recirculation valve will help with turbo longevity under excessive use of high boost but isn't vital.

Forced induction requires lower static compression (engine CR) to prevent compounding to catastrophic levels, but the turbo effectively increases dynamic compression ratio under boost. Boost + low compression is higher effective total CR than a higher CR NA engine.

You get better mileage with a smaller turbo engine than a larger n/a engine making the same power because you only achieve the highest air flow volume under boost while accelerating, almost like having variable displacement. The NA engine displaces the same volume all the time, while the turbo engine provides power when you need it but displaces less the other 99% of the time. This has to be a design goal targeting a specific power level with a smaller engine. Obviously its not the only possibility, eg max power from a big engine with no regard for fuel economy is another option.

Boost is pressure above and beyond the 14.7 psi atmosphere pressure a normal engine sees.

For your presentation make sure you discuss how a turbo does not harm an engine because the peak cylinder pressure is never exceeded over the n/a engine but rather the mixture burns at a finite rate, thus more mixture can burn longer throughout to complete stroke rather than dying off sharply. A turbo engine has the same peak cylinder pressure as a NA engine, but a turbo engine is still burning and pushing on the piston all the way to BDC, because it still burns at the same rate, but longer because there is more. Of particular interest is the combustion pressure at 90 degs crank throw, explaining the spectacular torque of a turbo engine.

In fact, lowering of static compression ratio when turbocharging is exactly for the purpose of keeping peak cylinder pressure the same.

 

[skyking]

Wastegate is a vital part which without you would get "infinite" runaway boost and destroy your engine.

My engine is fuel limited to about 37 PSI. I could weld it shut and it won't go above that. It is a quirk of the mechanical injection system. IF I dropped in bigger injectors.....

 

[soccerballtux]

My engine is fuel limited to about 37 PSI. I could weld it shut and it won't go above that. It is a quirk of the mechanical injection system. IF I dropped in bigger injectors.....

generally you don't want to be running a high duty cycle on the injectors. Much better to get the fuel in asap.

 

[skyking]

generally you don't want to be running a high duty cycle on the injectors. Much better to get the fuel in asap.

they are mechanical. The injection event is all about barrels and racks and plungers, no electricity involved.

 

[brownzilla786]

Wow thanks for the informative posts everyone, gonna have to tweak my presentation
and now I finally understand BOV/wastegate (thanks for the pic)

So the wastegate protects the turbine by routing excess exhaust. The BOV protects the compressor if the throttle is suddenly closed.

For your presentation make sure you discuss how a turbo does not harm an engine because the peak cylinder pressure is never exceeded over the n/a engine but rather the mixture burns at a finite rate, thus more mixture can burn longer throughout to complete stroke rather than dying off sharply. A turbo engine has the same peak cylinder pressure as a NA engine, but a turbo engine is still burning and pushing on the piston all the way to BDC, because it still burns at the same rate, but longer because there is more. Of particular interest is the combustion pressure at 90 degs crank throw, explaining the spectacular torque of a turbo engine.

In fact, lowering of static compression ratio when turbocharging is exactly for the purpose of keeping peak cylinder pressure the same.

Hmmm very interesting. I always thought that a turbo would cause a larger explosion so the piston would be pushed down harder, giving way to larger stress on the engine.

If I understood correctly, a turbocharger (because it provides better breathing) allows the cylinder to achieve high efficiency by completing the full intended stroke, where a N/A engine due to variable air intake can sometimes fall short?

A turbocharger provides a better, longer burn, not necessarily a more explosive one?

 

[exdeath]

The air fuel mixture burns subsonic. It takes a finite amount of time to burn at a fixed mass rate. If you double the air fuel it doesn't all burn at once instantly, it just takes twice as long to burn, meaning there is a more constant force throughout the entire stroke of the piston. Lowering static compression ratio ensures that peak cylinder pressure is never exceeded at ignition/TDC. Exceeding peak cylinder pressure spec will destroy the engine materials.

If you graphed cylinder pressure vs piston position of na vs turbo, you'd see that na peaks on ignition then sharply falls off from TDC to BDC like a spike, where the turbo graph would peak just the same, but carry through like a plateau rather than a sharp drop. There is still fuel burning and producing pressure on the piston near BDC long after a na engine has finished producing useful work. Nothing to do with "variable air intake", there is simply more mixture in the cylinder under boost so it can burn longer and keep pressure up as cylinder volume increases on the down stroke.

PS wastegate doesn't necessarily protect the turbine, it protects everything. Without a wastegate, boost would climb indefinitely in a feedback loop and you'd have no control over boost level. More exhaust = faster spool = more boost = more exhaust = faster spool = etc etc etc. Wastegate is a feedback controlled system that allows enough exhaust energy through the turbine to achieve the target boots level and hold it there, venting excess exhaust around the turbine. Waste gate sets your boost level. When you have a boost controller to adjust boost from the driver seat, you are manipulating the wastegate to open and close at a certain intake PSI. If you want 17 psi, you measure intake pressure, keeping the wastegate closed. When you hit 17 psi on the intake side, you open the wastegate and modulate it so you hold at 17 psi regardless of RPM, throttle position, etc. If you hit 17 psi and lift a little bit, the wastegate closes a little bit to maintain 17 psi, and if you get in it harder and exhaust flow spikes, wastegate opens more, all it wants to do is keep intake at 17 psi.

 

[Throckmorton]

exdeath, are you saying it's more efficient to have X amount of air/fuel burning in a smaller cylinder because in a NA engine the combustion process fizzles out long before BDC? I always assumed it was more efficient to have a larger space for expanding hot air to providing power for longer, and that a smaller cylinder ends up wasting energy.

 

[exdeath]

My engine is fuel limited to about 37 PSI. I could weld it shut and it won't go above that. It is a quirk of the mechanical injection system. IF I dropped in bigger injectors.....

Diesel is cheating, go away.



:awe:

 

[exdeath]

exdeath, are you saying it's more efficient to have X amount of air/fuel burning in a smaller cylinder because in a NA engine the combustion process fizzles out long before BDC? I always assumed it was more efficient to have a larger space for expanding hot air to providing power for longer, and that a smaller cylinder ends up wasting energy.

Cylinder size is irrelevant. 17 psi is 17 psi regardless if it's a .5L cylinder or a 1L cylinder. It's still twice as much air in twice the cylinder volume under the same pressure. It's all proportional. Even n/a if you graphed pressure vs crank angle of a 2L engine vs a 7L engine they would be identical provided all else equal (compression ratio, etc).

Of course if you took a turbo making 17 psi on a .5L cylinder and put it on a 1L cylinder you would see less than 17 psi, but we don't build engines that way, we size the turbo and boost target for the application.

It doesn't fizzle out, it's just that the pressure drops drastically as the cylinder volume increases, the fuel is still burning and expanding, just not enough to keep up with the piston anymore, and this is in fact the very wasted energy that is captured to power the turbocharger for "free" in the first place.

Technically, the larger cylinder wastes more energy as there is more surface area to bleed energy into the cooling system, but you're making so much more power it doesn't matter.

 

[Throckmorton]

What about at low rpm? Is there any piston speed where combustion can keep up with the piston, allowing a bigger cylinder to extract more energy from the same amount of combustibles than a smaller one?

 

[Bartman39]

Diesel is cheating, go away.



:awe:

Exdeath... You beat me to it... But your information is really good but why not give him the low down on VGT turbo's as well...? Also wasnt the reason for BOV due to the EGR system and its operation which can cause the turbo to stall...?

Just about to go to Tier 4 school for the engines in our equipment and did learn they (Komatsu) did a little change to the Cummins engine setup with the VGT's... They use an electrical over Hyd. control of the VGT instead of just an electrical servo... They added a high pressure oil pump to the engine to supply the Hyd. power, also for the EGR valve as well... This all seems to be overkill but with a heavy duty application its more for longevity & reliability as I was told...

 

[manimal]

when talking about turbochargers its important to use the word moar moar often


Great thread!

Isnt it great when you read a thread and every single point you would have made was already made.

 

[exdeath]

What about at low rpm? Is there any piston speed where combustion can keep up with the piston, allowing a bigger cylinder to extract more energy from the same amount of combustibles than a smaller one?

Nope, its a function of the geometry of a cylinder and min vs max volume. Cylindrical cylinders and pistons just aren't the most efficient geometry for capturing expansion energy, the volume just increases too fast and doesn't allow for fully capturing the energy. Even early steam engines attempted to get around this by routing used steam through a series of progressively smaller cylinders to capture the remaining energy.

For pistion engines we have turbochargers that can capture that energy.

 

[jlee]

Incidentally, not all turbocharged gasoline engines have blow off or bypass valves. Certain Toyota Celicas with the 3SGTE came without one, and the Toyota shop manual refers to the BPV in at least one case as a method of reducing noise. There are a few guys running modified MR2 Turbos without a BOV/BPV suffering no ill effects. I may be blocking mine off in the near future to eliminate the "fall on face" effect between shifts.

It's hard to confirm that long-term, because we're typically rebuilding or upgrading well before we'd likely see failure anyway. Just to throw another wrench in the works.