Well, pardon me for this ignorant question but I've been speculating about the application for a supercharger in the G20. Is this at all feasible? I've read numerous talks on turbo but have not read a single thread about supercharger application on the SR20DE. What are the pros and cons? Can anyone educate me on this? Thank you in advance.

Cheers.
jIm

1) There are no supercharger kits made for the SR20DE. If you want one, you will have to do all of the development work yourself.

2) Roots-type superchargers like the Jackson Racing units cannot support a lot of power. They typically can only provide 50-80 hp over stock. You will have a hard time breaking 200 whp with a Roots blower.

3) Centrifugal-type superchargers like a Vortec can run a lot more boost and support a lot more power than a Roots-type blower. However, the boost response of a centrifugal supercharger is proportional to engine speed. This gives Centrifugal superchargers much worse boost response than turbos.

Engines run of a mixture of gas and oxygen. The more fuel/air you can get into the engine, the more power you can produce. Adding more fuel is easy, larger pump, rails, and injectors. Adding more air can be a slightly more difficult task. It takes an incredible amount of power to spin a compressor fast enough to produce boost. You've generally got two options to power this air pump: the crank, and the exhaust gases.

Powering anything off the crank incurs what is referred to as "parasitic loss." What this means is that the crank is expending energy to power something else, other than transferring that power to the flywheel, and eventually via the transmission to the wheels. Things like power steering pumps and air condensers also induce parasitic loss. Try removing your power steering belt sometime, bet ya notice a difference :)

Now exhaust gas, you're just wasting it anyway, so why not use it to power something? Well, back pressure, but thats not really a major issue. When you put the turbine in the path of the exhaust gas, it makes the engine do a little more work to force out the exhaust gases. This is easily recovered by the power generated by the turbo. It is, inarguably, a more efficient way to power the compressor. Think that 200hp car produces approximatly 60hp of exhaust energy :D

Now, there is one benefit of superchargers that I haven't discussed, and it pretty much explains why people use superchargers at all anymore. Since superchargers are crank driven, the power they produce is directionaly proportional to how fast the crank is spinning. Its very linier, and driveable. Superchargers also don't suffer from turbo "lag." Turbo lag (more accuratley, "boost threshold") is caused by the time it takes for the engine to produce enough exhaust gas to power the turbo. The bigger the turbo, the more exhaust gases must flow past the turbine wheel to spin the compressor wheel. Smaller turbos always spool faster, but big turbos are always going to produce more power.

As to why you don't see many supercharger setups, i'd guess mostly because of the efficiency of the turbo design. As well as its beautiful adaption to FWD vehicles, struggling for traction. Just my $0.02, hope this post helps.

.jon

Originally posted by jon3k
Think that 200hp car produces approximatly 60hp of exhaust energy :D


Way more. About 40% of fuel energy goes into useful work. About 40% goes into the exhaust. The other 20% goes into other forms of heat transfer (radiator, etc) and into frictional losses.

So, your 200 hp car dumps about 200 hp worth of power out the tailpipe. Don't believe me?

Power = M_dot*Cp*Delta T

A 200 hp car will need about 20 lb/min of air flow. Cp for exhaust is about .284 BTU/lbm-R. Engine Delta T for a NA gas engine is about 1200 degrees F (so a engine with a 100 degree F intake manifold temp will have an EGT in the 1300 F range).

Power = 20*.284*1200 = 6816 BTU/min = 160 hp.

Turbo cars have even higher delta T's, so you can see that even more energy will be wasted.

Even putting a turbine in there to drive the compressor will only recover a portion of that work. The turbocharger can only contribute to engine efficiency through pumping work increases. So, if your intake manifold pressure is higher than your exhaust manifold pressure, you will have positive pumping work, and a portion of the energy wasted from the exhaust will be recovered. If you have higher exhaust manifold pressure than intake manifold pressure (most turbo street cars do), then the turbo actually absorbs work from the system and decreases net engine system efficiency.

It's still better than a supercharger, though, which consumes far more work than it supplies.

A better solution is turbocompounding. This uses an exhaust-driven turbine to either power a hydralic pump, an electrical generator, or a reduction gearbox in order to extract energy from the exhaust and put it back into the engine's crankshaft.

Originally posted by jon3k
Since superchargers are crank driven, the power they produce is directionaly proportional to how fast the crank is spinning.
Its very linier, and driveable. Superchargers also don't suffer from turbo "lag." Turbo lag (more accuratley, "boost threshold") is caused by the time it takes for the engine to produce enough exhaust gas to power the turbo.


Lag and 'Boost Threshold' are two different things.

Lag is a delay in time that is caused by the turbocharger's inertia. Even if the engine were spinning at 6000 RPM at a very light load (therefore, low exhaust energy and next to no turbo speed), if you were to romp on it, there would be a perceptable delay between when you romped on it, and when the car 'went'. This is due to the amount of time it takes to accelerate the turbocharger shaft to a speed required to make boost. This is lag. It is only a transient phenomenon; meaning that if you were to run the engine on a steady-state dyno (like an engine dyno), you would see no 'lag'. Turbo lag is a strong function of intake and exhaust volume and turbocharger inertia.

'Boost Threshold' is exactly what you have described; it is a point in time where there is not enough exhaust volume to spin the turbo fast enough to make the desired amount of boost. This is primarily a steady-state phenomenon. If you were to run the engine at constant speed, full load on a dyno, you would still not get the desired boost at a low enough engine speed. This is primarily a function of the engine displacement, intake and exhaust restrictions, EGT, turbocharger efficiency, and the turbine flow curve.

Think of lag as the difference in torque that the engine produced as it is accelerating relative to the torque that it would produce at steady state.

Superchargers do not suffer as much from 'lag'. The still have to fill and pressurize the intake system, so they will have some lag. The fact that most superchargers are not intercooled, and are mounted close to the intake manifold with very little induction volume to fill masks a lot of this. Superchargers do, however, suffer from the boost threshold problem. This is especially a problem with centrifugal superchargers. Since a centrifugal compressor's boost pressure is dependent on its speed; and since a centrifugal compressor has a fixed compressor speed/engine speed ratio, you can see that a centrifugal compressor will have an engine speed-dependent boost curve. As the engine revs higher, you will make more boost. At low engine speeds, you will make nearly no boost. This is a horrible response situation, especially on a small-displacement 4 cylinder motor that doesn't have a lot of low-end torque on its own. Centrifugals are commonly used on big V-8's, where the boost curve on the supercharger nicely evens out the torque curve of the engine. Have you seen the power curve for the Vetec-supercharged B16A Honda motor? Not pretty. That's why a 220 whp Vortec-supercharged Civic still isn't that fast: plenty of peak power, but not much area under the curve.

Roots blowers tend to suffer from poor volumetric efficiency at low rotor speeds due to excessive dwell time of the gases in the rotor passages and due to tip clearance and rotor backside leakage. Typically, the low-speed volumetic efficiency of the supercharger cannot match the low speed volumetric efficiency of the engine. The end result is also a 'boost curve'. As the volumetric efficiency of the blower varies relative to the volumetric efficiency of the engine, you will get a boost curve. This leads to a 'boost threshold' problem on positive-displacment supercharges as well. It is much less noticable than it is on either a turbo or on a centrifugal supercharger, but it is still there. Roots blowers and modified Roots blowers (ie Eaton blowers) have very little or no internal compression. This results in poor adiabatic efficiency that results in high intake charge temperatures and an excessive power requirements for the blower.

A typical Eaton-style supercharger will have a peak adiabatic efficiency of only about 65%; over some of the engine's operating range, this can be as low as 50%.

Centrifugal compressors should have much higher efficiency; however, the maps I've seen for Vortec compressor's are not impressive; many of them struggle to break 70% efficiency, at least at higher pressure ratios.

By comparison, a Garrett-designed centrifugal turbocharger compressor can easily break 75% efficiency, and can maintain that to well over 30 psi of boost pressure.