Turbo or Supercharger?

Forced Induction Basics and Turbos vs. Superchargers

 

This is the age old discussion that everybody on a boosted internet forum pretends to be an expert on.  Clearly, we are supporters of turbocharging technology, specifically in this day and age of “green” where fuel economy and performance can no longer be mutually exclusive terms.  With that said, we offer up the explanation below in support of our position and we think you’ll agree that it is a clearly objective position.

 

There are many ways to increase the power output of an engine, but all of them work on the same principle, that being increase the airflow into the engine, spray fuel to match that airflow and make power.  It sounds simple, doesn’t it?  Very clearly a normally aspirated engine makes power by filling the cylinder with as much air as possible.  This amount of fill of the cylinder is called volumetric efficiency.  On a normally aspirated engine, this cannot exceed 100%.  At 100% volumetric efficiency, the entire cylinder is filled with air which is essentially at the same pressure as the atmosphere in which that engine is operating.  It’s a simple calculation of the volume of each cylinder times the number of cylinders times the engines operating speed.  Clearly, all engines have a sweet spot where the cam offers the best ability to fill the cylinder as completely as possible in search of the elusive 100% volumetric efficiency.  With normally aspirated engines there are many ways to increase the power output.  In most situations, this means increasing the engine’s displacement and turning the engine to higher engine speeds to increase the amount of air pumped through the engine.  While sounding simple enough, doing all of that work normally results in a pretty large expense and pushes the power band further up on the scale, making for a less than streetable car.  Then there are issues like rod ratios which are too low, significantly increased piston speed, and the issues caused by the high compression ratios typically used to produce the high power out of a high-performance normally aspirated engine.  All in all a reasonable approach to attempting to get the most power out of the engine, but not one without many sleepless nights on figuring out how to pull it all together and wondering what it will cost when you’re done.

 

Anybody who has operated a normally aspirated car in higher altitudes knows the feeling the engine has at 10,000 ft as compared to how the engine feels at sea level.  This is no different than watching a skier hike at altitude as compared to as sea level…he or she is quite a bit slower at 12,500 ft than at 3,000 ft!  Quite simply, a normally aspirated engine cannot make the same power at altitude that it can make at sea level.  This is directly related to the atmospheric pressures under those two conditions.  Very simply, there is less air at altitude per unit volume as compared to at sea level.  That’s the world we live in and there’s no way around it.

 

The question begged for many years…how does one increase the output of the engine without having to turn the engine at ultra-high RPM and use exotic fuels to make the power?  Here entered the solution of forced induction.  Forced induction is very simply the act of increasing the manifold pressure into the engine.  If we increase the manifold pressure into the engine by twice what the atmospheric levels at that point are, then, in theory we can have 2X the power at that RPM point (all other factors equal).  In practice this is quite a bit more complicated as there are variables at play here which make it difficult to get that exact relationship, but for talking purposes the relationship still holds true.

 

So how do we go about making the air that we introduce into the engine more dense?  We can’t carry around a huge tank of compressed air to meet these needs, as it would run out in a matter of seconds.  Remember we’re talking about high mass flowrates here, not air with which we plan to inflate a low tire or basketball.  The answer to the question is simple, we fit the engine with some form of a compressor.  Therefore, the compressor’s key duty is to increase the density of the air we introduce into the engine.  The ideal compressor would:

 

Require zero power to compress the air charge

Not heat the air charge to anything above atmospheric conditions

Have few or no moving parts

Not impede the engine’s fuel economy whatsoever

Allow civilized engine performance when not in use

Make manifold pressure when needed (i.e. on demand)

 

That list is a pretty tall one to accomplish and has yet to be solved 100%.  Until a genius type comes along, we are stuck with the current state of the art.  While there are many different types of compressors, for all practical purposes we can divide these up into three specific types:

Belt Driven Centrifugal Types (Centrifugal Supercharger)

Belt Driven Positive Displacement Types (Positive Displacement Supercharger)

Exhaust Gas Driven Centrifugal Types (Turbos)

 

Before the details of each type of compressor are covered, it is clearly necessary to explain the fundamental differences in the types of compressors used.  From out list we can see that the turbo and centrifugal supercharger both use the same type of compressor, yet on the road and in the real world they don’t drive anything like each other.  Likewise, the positive displacement pump makes for yet another whole different driving experience.  We explain this in detail so you can understand what makes each of these systems clearly different from the other.

 


 

A centrifugal compressor makes its airflow through high shaft speed, which in turn allows the compressor to output pressurized air as a function of shaft speed.  It is very important to understand that the centrifugal style compressor requires a very high shaft speed to put out high airflow rates and in addition that it is a non-linear device.  What that means is that if a centrifugal compressor puts out 10 psig of air (at a certain mass flow rate) at a given shaft speed we will call X, there is no way for that same compressor to put out twice the airflow at two times the baseline shaft speed 2X.  Therefore, the mass airflow output of a centrifugal style compressor is non-linear.  It’s very important to understand this when you are comparing different types of systems, because it is so very fundamental to the way a system performs in the real world.  In addition it is important to realize that shafts speeds on exhaust gas driven centrifugal compressors regularly exceed 100,000 RPM in most cases.  Keep in mind when we talk about centrifugal style compressors, we are discussing the style of compressors used in centrifugal superchargers and exhaust gas driven turbochargers.  The compressor sides of these devices are very similar in design and construction (though belt driven centrifugal compressors tend to be much larger since they spin at a lower RPM).  Centrifugal compressors have the highest adiabatic efficiency of any style of compressor used for these purposes.  That means they heat the compressed air less than the other types of compressors.

 

In contrast, a positive displacement type compressor puts out a certain amount of airflow per revolution.  What this means is that at a given pump speed, there is a certain amount of mass airflow output and at twice that speed, for all practical purposes, there is twice the mass airflow rate.  In addition to this, positive displacement pumps make near-immediate (it’s not totally immediate as most positive displacement supporters would like you to believe) manifold pressure from off-idle all the way to maximum engine speed.  “Nearly instant and linear” would be a good observation.  While old school blowers have a low adiabatic efficiency the modern versions (while still lower than centrifugal compressors) are actually quite good.  These pumps, when coupled to a piston engine offer a good overall or average improvement in horsepower.  The positive displacement pump is a very different animal next to a centrifugal compressor.  On the flip side, the turbo is yet again different than the other two.

 

The above descriptions are a generalization of what makes each style of pump different in the way they deliver airflow to the engine.  In addition to understanding the general characteristics of airflow, one has to also understand the basic means of driving the compressors.  Remember, there is no free lunch as all three forms of forced induction compressor setups must have a way to provide input power to the compressor.  If you were asked to hand crank an air pump to keep a 5.0 liter V8 fed with one atmosphere of manifold pressure do you think you could do it?  The engine on your riding lawnmower is not even capable of that task, as it takes a large amount of power just to run any of these pumps.  So here we are stuck with one of the ill effects of a self driven air pump (compressor).  We have to drive it!  Fortunately we have some options on how we can drive the pump. These being with a mechanical drive (belt, gears, etc.) or with a gas turbine (turbocharger).  Don’t for a minute believe the sales pitch that an electric motor is a viable option because there is simply nowhere near the power needed from an automotive electrical system to make an appreciable difference.

 

Let’s start with the case of the positive displacement blower.  These compact and tidy devices usually sit atop the engine and have a small pulley which is driven off of a poly-V or cogged belt which is coupled directly to the crankshaft.  Air is forced into the intake manifold and the amount of air output is basically linear, meaning that if we double the shaft speed then we double the output of the compressor.  These are the pumps people refer to when they talk about a zero-lag supercharger system, because for all practical purposes on the street the have no appreciable waiting time to make full manifold pressure.  It is important to note that having full manifold pressure does not necessarily mean having the most torque or power as there are a lot of other factors involved.  These pumps require the most power to run because they have large lobes with significant inertia and mechanical mass.  These setups can typically be on full boost by 2300 RPM on an engine setup to run to 6,500 RPM and if sized properly can maintain that boost to redline.  But understand that boost is not necessarily always proportional to power production.  In many instances these setups require a very short intake runner which can really hold the mid range and top end power back because the velocity stacking ability of the original long runner is lost.  Some designs attempt to compensate by mounting the pump lower in the engine bay or on the engine and then creating an intake runner with some length to help.  The driving characteristics of a vehicle equipped with a positive displacement pump can best be described as near to immediate boost response with a lot of tip in torque.  These setups are great for low and mid range performance which is why they are favored by stoplight to stoplight racers everywhere.  They work very well on larger engines where getting every last ounce of power out of the engine isn’t the ultimate goal.  Most positive displacement pumps tend to lack the top end power numbers which are easily achieved with a turbocharger running less manifold pressure.  While they also don’t make the same peak HP as a centrifugal they are a better overall choice when compared to that option for many reasons.  All in all, these pumps make for a fun ride with good predictability.  If you like the whine of a belt driven pump, most modern units have a nice precision sound to them.

 

The belt driven centrifugal supercharger has very different driving characteristics than the positive displacement type, which is contrary to what most centrifugal salesmen will attempt to tell you.  The pump is still driven directly off of the crankshaft typically via a belt and while these pumps do require less power than a positive displacement pump, there are a few real drawbacks.  Supporters of these systems love to tell you how they have zero lag, but they’re only telling you part of the story.  Lag to build manifold pressure is defined from the time the system needs to build boost from when the driver called for boost.  So, yes by that definition centrifugal superchargers have zero lag.  What they fail to mention to you is that the centrifugal supercharger has a very odd and undesirable boost curve.  If you take a centrifugal supercharger setup made to run 10 psig.  By definition of the type of compressor used, the peak manifold pressure will be reached at peak engine RPM, never before!  This is a mathematical certainty.  Sounds great, peak manifold pressure at peak engine RPM, right?  Hold on a minute there, because you have to take a look at the entire manifold pressure curve and realize that torque is proportional to manifold pressure and power is the product of torque and speed.  Back up to half the engine RPM and you’ll be lucky to see 3 psig on that same centrifugal setup.  This is because the output of the centrifugal compressor is non linear, but in the centrifugal supercharger we are driving it with a linear input (i.e. twice engine speed results in twice supercharger speed…but not twice the airflow)  At that same midrange point the twin screw pump is on full song, making full manifold pressure and the turbo is likely coming on very hard if not already making full boost in many cases.  So guess what?  You’re stuck at that manifold pressure until engine RPM changes.  There is no way around this inherent limitation to the system without an elaborate drive system or over-boosting and bleeding off (the most foolish thing you’d ever want to do).  Yes, these pumps do require less power to drive than a positive displacement pump but they simply don’t have the instant linear feel of a positive displacement system and they’re a far cry from a well sorted turbocharger system.  In most cases a twin screw pump is a better choice for a street or track driven car.  Additionally, while some of these units have belt drives which are quiet, most have internal gear drives which can be quite loud.  Remember, you’re talking about having to speed the compressor to 40,000+ RPM with an input speed of say 6,000 RPM.  Sounds like a lot of moving parts, doesn’t it?  In most cases, these pumps are the least desirable form of forced induction.   The positive displacement setup, while not as powerful on the top side will typically prove for an all around better driving vehicle.  Additional useable power is not all that great in a centrifugal system.  The Achilles Heel of these setups for a piston engine is that you have fitted a compressor that needs a high shaft speed to match the desired airflow requirements of your forced induction system, but you have no way to speed the compressor up independently of the engine speed.  In this case manifold pressure is a function of engine speed, not engine load.  With all of that said, these pumps do offer an increase in performance and are typically at a lower price point as compared to a positive displacement or turbocharger system.  They also typically offer a very simple installation and are easy to remove should you so desire.

 

Turbocharging is a proven method for reliably increasing the performance of an engine.  While the compressor of the turbocharger is practically identical to that of a centrifugal supercharger (but smaller because it spins at a higher RPM), there is one very large difference.  The turbocharger uses a gas turbine to drive the compressor which is completely independent of the crankshaft speed.  Here we have the case for boost as a function of engine load, not just a function of engine speed.  This offers some huge advantages including, the ability to increase or decrease manifold pressure at the touch of a button, the ability to retain very docile street manners on the vehicle if you so desire, and the ability to correct for lost altitude density.  In the case of the turbocharger, the compressor speed is a function of the engine load and the compressor gets its driving power from the turbine which is being driven directly from the 1/3 of the raw heat energy that the typical piston engine throws out the exhaust as wasted energy.  Unfortunately the internal combustion engine is a horribly inefficient device.  Herein resides the single largest advantage the turbocharger offers, explicitly stated the turbocharger does not penalize the system by robbing power off the crankshaft to drive the pump, as this energy is coming directly from the heat and pressure in the exhaust system which is otherwise simply wasted out the tailpipe.  While there is some pumping loss associated to fitting a turbo system to a piston engine (as compared to its normally aspirated counterpart), this is nowhere near the losses you will see with either style of belt driven supercharger.  There’s more to like about turbocharging as well.  Since you can make more power with the same manifold pressure, it is therefore a certainty that you can make the same power with less manifold pressure.  What this means is that for the same torque and power output you put less stress on the engine and burn less fuel than the belt driven competition (either one).  More efficient with the most power potential per unit of manifold pressure.  Additionally, you have the option to set a turbocharger system up to run a modest amount of boost for around town driving and at the flip of a switch pull in more manifold pressure for your time at the track or when you want to run race fuel.  There are many ways to accomplish this from a simple wastegate signal line bleed off to an all out pulse width modulated boost control system which allows full manifold pressure control as a function of engine speed and gear selection.  This is not possible with a belt driven pump without an elaborate drive system or bleed off and even if you did achieve it reliably on one of those systems you are still stuck with the requirement to drive the pump mechanically from the crankshaft and that will cost you appreciable power. 

 

Any supercharger salesman will immediately start pointing to turbo lag as the reason to stay away from a turbocharger system.  Please understand there is a difference between turbo lag and boost threshold.  Turbo lag is the time it takes to make manifold pressure from the point at which you called for the manifold pressure.  Boost threshold is the point at which the turbo starts making manifold pressure.  So, yes a turbocharger has a definite boost threshold, but in most cases a properly sized turbo will start to build appreciable manifold pressure at 30% of the engine’s operating speed range and can be sized such that by the midrange point it will be on full song.  With street and track driven cars we are typically concerned with the power band from midrange to max engine RPM and this is where the turbocharger outshines the other pumps; hands down.  In comparison, yes a positive displacement pump will make more immediate manifold pressure (but only down low) but this does not necessarily mean more power.  On the flip side the centrifugal supercharger setup will be making far less manifold pressure and consequently much less power than the turbo setup or the positive displacement pump.  We always laugh when a supporter of a centrifugal supercharger starts telling us about turbo lag.  True he has zero lag when he tips the throttle in but it’s a certainty that he’ll be waiting until maximum engine speed to make his peak manifold pressure (and overboosting if he goes past maximum engine speed!).   Modern turbos have little to no turbo lag and a very good boost threshold that doesn’t tax the top end power, specifically if you start working with variable nozzle geometry systems as are present on the new 911 turbo cars from Porsche.

 

Additionally, the supercharger salesman always likes to point to a red hot turbo exhaust manifold and tells you that the heat of the hotside heats the air on the cold side, so therefore the turbo makes a hotter intake charge temperature than a supercharger.  If you peel back the skin on that onion you will quickly realize that the mass airflow rates out of either compressor are so high that the air simple has no time to pick up that heat and put in into the intake charge (remember, air is an insulator).  In most cases, the compressor discharge temperature is a function of the pressure ratio and the adiabatic efficiency of the compressor.  Don’t fall into the mindset that a centrifugal supercharger puts out cold air while the turbo’s discharge air is hot enough to melt the piston, because it simply isn’t true.  In most cases, the turbo will have the same or slightly lower compressor discharge temperatures, specifically if the turbo is very large.

 

The turbo isn’t without its challenges.  Properly engineered turbocharger systems typically cost more than supercharger systems because there are more parts required, specifically in an aftermarket scenario because you have to typically make new exhaust manifolds and in many cases an entirely new exhaust system.  Additionally, a properly engineered turbocharger system will likely take slightly longer to install than a supercharger system and is more tricky to tune in properly because you have to account for the fact that the compressor can (and will) operate over a wide range of pressures at many different speed points.  But remember, these are one time deals.  We’d like to think the extra cost and effort is well worth it when you drop your right foot for the remainder of the time you own the vehicle.

 

In an effort to make this all make sense, we have included some charts and graphical information which should help explain most of what is stated above in perhaps a more user friendly format.

 

Forced Induction Chart:

 

The chart below attempts to weigh the three basic forms of forced induction relative to each other.  Where possible, the systems have been rated with a 1, 2, or 3 with a one representing the best score in that category.

 

Torque and Power Comparison:

 

The graph below tells the story on where things play out in terms of real torque and power production with all three forms of forced induction.

 

 

 

 

© 2011 Kuhn Performance Technologies.