As I learn more, more questions present themselves...

[Guest tony78_280z]

From "How Things Work" http://auto.howstuffworks.com/turbo2.htm

Since normal atmospheric pressure is 14.7 psi at sea level, you can see that you are getting about 50 percent more air into the engine. Therefore, you would expect to get 50 percent more power. It's not perfectly efficient, so you might get a 30- to 40-percent improvement instead.

One cause of the inefficiency comes from the fact that the power to spin the turbine is not free. Having a turbine in the exhaust flow increases the restriction in the exhaust. This means that on the exhaust stroke, the engine has to push against a higher back-pressure. This subtracts a little bit of power from the cylinders that are firing at the same time.

So what is the efficiency loss with a supercharger in a percent format?

 

It then goes on to say

Older cars with carburetors automatically increase the fuel rate to match the increased airflow going into the cylinders. Modern cars with fuel injection will also do this to a point. The fuel-injection system relies on oxygen sensors in the exhaust to determine if the air-to-fuel ratio is correct, so these systems will automatically increase the fuel flow if a turbo is added.

Then how come everyone says you gotta overjet a carb if it is going to be turbo charged? This does make sense, more air flow over the venturi would equal more gas pulled off of it. But if this is the case, then why do people install a single injector over their carb to inject more fuel when the boost kicks in. (I've read these tricks and stuff on peoples personal webpages) One would think that when the boost kicks in it would simply pull more fuel because of the air flow.

 

The more I learn the more confused I am.

[cygnusx1]

A carburetor can increase fuel flow ONLY up to a point as well. That point is less defined in a carb than it is in a fuel injection system but the limit still exists. You are not confused. You are asking logical questions.

[mobythevan]

So what is the efficiency loss with a supercharger in a percent format?

I am going to get a side question in on this one, I am still learning too. Can we generalize and say that the two biggest losses on any turbo or super are parisitic and adiabatic? Parasitic being the power lost to drive the compressor(by belt or exhaust) and adiabatic being the power lost because the air is heated when it is compressed so less air makes it in the combustion chamber than the theoretical PSI number leads you to believe. The efficiency number given to my B&M 250 supercharge is 55% IIRC, and the turbo on my 240z runs around 70% in the compressor map. Just to throw out number to compare.

[Drax240z]

Overjetting a carb for forced induction typically is only necessary in a draw through design, not in a blow through design.

[zuhow]

Moby has it mostly right.

 

 

 

Let’s look at it this way: your engine doesn’t store any energy, so the energy in (fuel) has to equal the energy out. Now what is the energy out? Well, it is the work done to turn the crankshaft and all the accessories- water pump, AC compressor and so forth- and the heat that is sent out of the tail pipe.

 

 

 

The work done on the accessories is indeed a parasitic loss. Adiabatic is a whole ‘nother can of worms. I think you understand it right, you just have the terminology wrong. Adiabatic losses are losses due to heat transfer, basically (gross oversimplification here!) all the heat that goes out of the radiator (and oil cooler) are adiabatic losses. Adiabatic losses are loses due undesired heat transfer, instead of heating the engine block, that energy, in principle could be used to do work- ie turn the crank and put a smile on your face!

 

 

 

Inefficiencies in the compressor are called isentropic losses. Basically you can never operate at 100% efficiency, so you don’t get the actual compression (in psi) out of the compressor that the math (and engineering) says you should, hence you get a 70% efficiency on the compressor as you mentioned. Clear as mud? I hope so!

 

 

 

So, the blower you mentioned, and I’m assuming it’s a roots type blower, does indeed operate around 50% isentropic efficiency. The other 50% of that efficiency goes into making heat, not compressing the air. These blowers are what are called “positive displacement†a compressor like the one on the turbo is a “centrifugal†compressor. These are much more efficient- typically 70-80%. Still with me?

 

 

 

So, you can have a centrifugal compressor driven by a belt- like a vortech super charger, or by a turbo.

 

 

 

This brings us back to the energy in= energy out. The reason a turbocharged engine is more efficient is because it manages to pick up some of the energy that normally would be lost in the heat in the exhaust gas and use it to drive a compressor. A belt driven supercharger, sucks up some of the work that otherwise would go to the crank to drive the compressor.

 

FYI, power plants do the same thing we do to our engines. A large natural gas fired turbine power plant will take the exhaust gas and use it to make steam to drive a second turbine to scavenge some of the energy and boost the overall efficiency, not exactly turbocharged, but I think you get the drift.

 

 

 

Sorry for the long post, hope this helps clear it up for you. If not, by all means, ask away. I’ve been a long time lurker, and I’ve learned tons, so I’d love to be able to give some back.

[mobythevan]

that answers my questions, thanks

[Tony D]

Drax has it dead on with the drawthrough/blowthrough differentiation.

On my Corvair, I needed to run a main jet size somewhere near .145" diameter, just so the jet area will flow enough fuel in a VACUUM situation to supply a proper fuel mix to the engine under boost.

 

In my Turbo Z with triple Mikuinis, it was jetted a bit fat initially, before I learned how Maserati used Modulator Rings. After finding out how Maserati used a modulator ring on the front of the carburettor to cause a differential between the float bowl blanketing pressure, and the air pressure in the main throat (not to mention the delta caused by main and booster venturis) to force more fuel out under boost so you can run RICH on-boost, but properly jetted for a comparable N/A engine size, my life was much easier.

Having a 2.8L 275 to 350HP Blow-Through setup that was driveable in daily commuting and returning 17+mpg kinda hooked me on that system... Until EFI became affordable!

 

Even without the modulator rings, though, the main jets were never as large comparably as what I had to run in the Corvairs' SU, or even the Holley 650 Secondaries on the draw-through drag Z I helped build up.

 

Clear as mud now? LOL

 

As for the efficiencies the turbo map is a series of "islands" of varying efficiencies. So a turbocharger, or a centrifugal belt-driven blower will pass through the same islands on the way to the peak efficiencies...

 

But the same goes for the Blower! The blower will have it's efficiencies at a rated "tip speed" for the lobes/screws. Ideally, you will match the peak tip speed efficiency for the flow/pressure curves to coincide where the theoretical torque peak is on the engine you have selected, for the best boost in power. It will make for a REALLY large bump in peak torque. If you skew the blower higher or lower in tip speed efficiency, you can broaden the torque curve on the engine somewhat because you have less losses (more efficient compression) at a point different that peak torque, making for an artificial VE Boost in those ranges...

 

Like mentioned above, a stationary powerplant will use the waste heat for a recovery turbine, and in most cases to heat incoming air etc etc etc.... The more efficiencies you recover, the more power you put down the line. Kinda like those cooking kits that pump exhaust gasses through a chamber in the engine compartment to cook food. More efficiencies! Mmmmm, roadkill stew cooked by exhaust gasses!

 

I'm blabbering again aren't I?

 

I'm not helping much, I'm shutting up now...

[Guest tony78_280z]

he efficiency number given to my B&M 250 supercharge is 55% IIRC, and the turbo on my 240z runs around 70% in the compressor map. Just to throw out number to compare.

So the Turbo in this example is 25% more efficient than the Supercharger?

 

A carburetor can increase fuel flow ONLY up to a point as well. That point is less defined in a carb than it is in a fuel injection system but the limit still exists.

So any idear what that point is? I'm thinking about running somewhere around 6psi-8psi of boost. One plan for this is to run two realy small turbos in a blow through set up. Two small turbos will spool quicker and hopefully give me the boost I'm looking for. Would the average carb (like mine) work well and deliver enough fuel without overjetting? In another post I'm trying to figure out how vacuum secondaries would work in a boosted manifold.

[Tony D]

So the Turbo in this example is 25% more efficient than the Supercharger?

...

I'm trying to figure out how vacuum secondaries would work in a boosted manifold.

 

At THAT point, the turbo is that more efficient. See above on tip speed for blowers... There are compromises in everything, and how you compromise when designing depends on how things respond...

 

As far as vacuum secondaries in a blow-through, they have to be modified to work on pressure, or totally manual conversion... For a blow-through, a non-progressive mechanical synchronous secondaries is what should be done, depending on pressure activated dashopts to overcome boost reliably is tennuous at best.

 

Generally vacuum secondaries work better on draw-through applications, and in that case, you can also modify them to actuate at a PSI rating rather than a vacuum setpoint for a more reliable operation.

[Turbo Meister]

Overjetting a carb for forced induction typically is only necessary in a draw through design, not in a blow through design.

 

And I read Tony D's clarification.

 

Although I'm still learning about carbs and turbos I do have my own set of personal experiences in that category. When racing my TT383 Chevy engine 240Z, overjetting was required in a blow through application, to keep pistons alive. Perhaps it is the amount of boost that dictates if overjetting is necessary. Racing with 17 psi and overjetting has saved me from having to replace melted pistons. Whenever I jetted the 750 DP down, even slightly from my 79 primary and 95 secondary setting, disaster resulted.

The Z is running well despite being rich but safe.

Hanns

[Drax240z]

Hanns I think in your case there is plenty of reason to overjet. Chances are you are running richer than 13:1 at 17psi, or whatever the target AFR happened to be for your NA setup. Certainly running that kind of power/boost I'd want to play it on the safe side and run richer than if I was running a similar non-boosted engine. There is also a charge cooling effect from running richer than necessary mixtures, which can keep pistons alive in situations where less fuel would have them melted.

 

However with that said, you might be overjetting 10% with a blow through design just for a bit of safety, where if you were running a drawthrough design you'd be overjetting in the neighbourhood of 120% in order to get the required fuel.

[johnc]

For the ultimate in energy recovery...

 

The Pratt and Whitney Wasp Major R4360VDT engine had a supercharger that fed the intake side of two turbochargers and the exhaust that exitted the turbo chargers was routed to the compressor section of a turbo jet engine. It never went into production (it was designed for the B36J) because of cooling issues and the repalcement of the B36 with the B52.

 

http://www.pratt-whitney.com/about_history_classic_r4360.asp