Is there a limit for a S/C

[marcusus]

On a side note, which one would be more fuel efficient assuming you drove them both the same? Turbo or super?

[Gordo]

Turbo thats why all the Germans use them in there smaller engines

[Kain]

On a side note, which one would be more fuel efficient assuming you drove them both the same? Turbo or super?

Assuming they're dríven the same, a turbo would be more fuel efficient, as when highway cruising you are able to be out of boost (with careful driving), and unless your turbo spools fairly instantly you can be off boost around town as well. A supercharger has the same boost at X rpm no matter how you're driving it, but a turbo will only develop full boost if you're nailing it (it's a positive feedback system). The amount of fuel you're dumping into the combustion chamber is entirely dependant on how much air you're forcing in, so less boost = less fuel.

*EDIT* - and by dríven the same, I pretty much mean dríven by your granny.

[marcusus]

Hmmm, fair enough. I always think of fuel efficiency as a function of your foot, so if you were applying the same amount of pressure on the throttle, I would've thought that you would (in theory) be using the same amount of fuel.

[Guran]

Turbo thats why all the Germans use them in there smaller engines

BMW prefer turbos, true ... but only in their 3L in-line 6cyl and V8. Their smaller engines are all naturally-aspirated or turbo-diesel.

Audi prefer turbos in their 4s, but have taken to superchargers for their 3L V6 in the S4, S5 and A6.

Mercedes-Benz still have some models with supercharged smaller engines (CLC200K, SLK200K).

Volkswagen is having a bet each way with the 118TSI twincharger.

[Casey]

Hmmm, fair enough. I always think of fuel efficiency as a function of your foot, so if you were applying the same amount of pressure on the throttle, I would've thought that you would (in theory) be using the same amount of fuel.

Exhaust gas superchargers (turbos) work off load, not engine RPM. So if you are traveling at the speed limit on the freeway at 3000rpm with only cruise throttle the turbo would be making no boost (the boost gauge will be showing vacuum) as there is minimal load. And you will be getting very similar fuel economy to an N/A engine. However floor the throttle and you have almost instantaneous boost, power, and fuel consumption to match.

A belt dríven S/C (aside from the Toyota ones with clutches) is always be dríven off the engine and will always present a load. A bit like running the air conditioning all the time.

FWIW on our clubs first Targa Tasmania run our SP had better fuel consumption then all the N/A MX5's (NA6, NB8A, NB8B) and did far better then a M48 supercharged NB8A. A bit like the Top Gear Prius vs M3 fuel consumption comparison - the SP only had to keep up

[Benny]

Turbochargers do not consume any power and in fact, actually use the heat and pressure of the exhaust, which is usually wasted energy, to produce boost so they are inheritantly more efficient.

Actually that is not true Benny. The fact that the turbocharger uses exhaust pressure and flow to compress and pump the air intake charge means that the engine is loaded by the turbocharger and is therefore a parasitic load....and when in positive pressure the parasitic power consumption between a supercharger and a turbocharger become somewhat closer. However, when the engine air intake is in vacuum the parasitic load of a turbocharger is substantially less than a supercharger.

Regards,
Danny

However, the losses using a turbocharger at high boost levels are far less than the losses incurred by a supercharger.
Superchargers ALWAYS use power, even at low revs, but turbochargers do not.
In relation to each other, turbochargers consume far, far less power than superchargers do.

What you also forget Danny, is that a lot of the rotational force of a turbocharger is not caused just by exhaust pressure, but by exhaust heat, and it is this heat that actually causes most of the rotation of the turbocharger impellor.
If not used this way, the energy in the heat of the exhaust is just wasted.
Boyle's Law says that an increase in the temperature of a gas will also mean an increase in pressure, and it is the heat generated by the exhaust that causes this increase in pressure which turns the impellor.
So with a turbocharger, you do get something for nothing!

He he he ... I bet if I put a turbo charger in an oven and heated it up ther impeller wouldn't turn. Heat on its own is the most useless form of energy.
Fuel burning with oxygen in the cylinder creates heat.
The heat is directly related to pressure when the volume is constrained. {Insert gas equation name here to appear smart }
The pressure drives the piston.
Then, on the exhaust stroke the piston drives the gas (still at pressure but lower than at TDC) out of the cylinder and into the exhaust.
If there is a restriction in the exhaust, say a turbine wheel turning slower than it wants to because it is connected to a compressor wheel that is busy pumping, then the piston must push harder to expel the exhaust air.
Yes there is resdual pressure in the cylinder at the end of the power stroke that we can extract more useful work out of, but as we do, we load up the piston on the exhaust stroke. You have also lost the ability to tune the exhaust so that active scavenging occurs.
You would only be getting something for nothing if you were using the exhaust gas velocity for thrust.

M

Be careful Mokesta.
Your lack of a physics education is showing....................................

Go and read up about Boyle's Laws.

[Mokesta]

OK

CALL

I've got an honours degree in mechanical engineering including better than pass results in all thermodynamics and thermo-fluids subject and distinctions (7) in dynamics, am a registered professional engineer. Lay down your cards...

True, my beer consumption at and after uni means I don't know much about most stuff these days, but I reckon it trumps self-taught Wikie or internet collective-wisdom eductation any time.

Have a beer and accept that your text above about heat from the exhaust is what makes a turbo spin for free is just nonesense

M

[Dweezle]

Cmon guys..
We all know mines bigger than yours!! :p


Sent from my iPhone using Tapatalk

[Kain]

Be careful Mokesta.
Your lack of a physics education is showing....................................

Go and read up about Boyle's Laws.

Just for your reference, Boyle's Law correlates temperature, pressure and volume, so long as one is held CONSTANT. In your example of a turbocharger volume is not a constant.

Also, have a read up on how a turbine works (such as the hot-side of a turbo). It's dríven entirely by pressure differential, not by temperature.

In summary - It pays to understand your own argument before you start calling people out.

[safetyman]

Doesn't the bypass valve on the MP45 and MP62 superchargers reduce the amount of parasitic load placed on the engine when off boost?

[safetyman]

It stops it producing boost at idle or low throttle openings but by doing this it should also be reducing the load placed on the engine as the engine is now only spinning a 'relatively' freewheeling pulley.
When producing boost is when the parasitic load is at its greatest.

[Benny]

But what causes the pressure?

It isn't whistling Dixie.

As the compressed air/fuel mixture is ignited (increasing temperature), the pressure of the gases from this heating is used to power the turbocharger (after it pushes the piston down and is exhausted out the exhaust valve), hence a turbo is powered by the pressure of the heated gasses.
Is that correct or not?

The last time I checked my turbo, there weren't any mice running around in there.

I don't profess to be an expert on fluid dynamics and such, but have a basic grasp of how things work, and IMHO, turbos are powered by energy that would otherwise be wasted, rather than in the case of a supercharger which is powered directly from the torque and revs of the motor, which is a far greater parasitic load on the engine than using the spent gasses which would otherwise be just sent out of the exhaust.

[rob323]

Here is an interesting take on things for what it is worth.
http://www.hiperformancestore.com/MagnusonSuperchargers.htm
I've owned and tinkered with turbo cars for the last 15 years, now looking forward to playing with my first supercharger at Christmas time.

[Gordo]

thats a good find rob.

For somone who doesn't know alot about Turbo's and S/C it helped alot.

thanks