Should You  Turbo Charge or  Super Charge?

TURBO VS. SUPERCHARGING

PICTURES COURTESY OF www.turbotechnics.com - Turbo Technics, based in the United Kingdom, specialize in automotive turbochargers and associated turbo equipment. 

The short answer: There are differences and similarities in Turbo charging and super charging - - - Turbo charging is actually a form of super charging - - the term supercharging refers to the air/fuel mixture that the cylinder is "charged " with each time the piston oscillates in the bore of the engine.

A normally aspirated engine is normally charged when the piston moves down the bore and the volume it displaces is filled by the atmosphere through the open intake valve. If in some way you increase the amount of charge that you get into the empty cylinder you are in effect "super" charging the cylinder. Some supercharging can be accomplished by porting and timing of the valves but these are minor effects used for tuning purposes. When you here someone talking about supercharging they are usually talking about a mechanical device that is installed on the engine to accomplish a much larger effect. These devices are essentially a sophisticated air pump that is mechanically driven and can deliver a huge volume of air to the intake of the engine - - sort of force feeding the engine. When you make more air available to the engine it of course needs more fuel to go with it as the more air means more oxygen to burn the fuel. Now turbo chargers do the same thing (make denser air available to the engine) the difference with the turbo is that it is driven by a turbine that is powered by the exhaust gases exiting the engine not by a belt or some sort of gear box or other mechanical means. 

A little more detail on turbo charging/ supercharging VS normally aspiration

INTRODUCTION 1

Engine Basics - -aspiration 
Whether an engine is a of the two or four stroke cycle variety the basic principle is the same. A piston in a bore moving up and down , back and forth or in and out of a cylinder. The end of the cylinder is closed or capped and the other end open with a rod connecting the piston to a crank mechanism that converts the linear motion into rotating motion. Every time the piston vacates the cylinder volume a negative pressure is created. If we have a means of opening an access port to the cylinder through some sort of valve, the atmosphere tries to equalize itself and a flow of air takes place into the cylinder through the opening. If the cylinder is then sealed off when the piston moves back up the cylinder bore it will squeeze the air in the cylinder into the tiny chamber formed at the capped end of the bore. 

If the air has fuel vapour in it that more or less matches the amount of oxygen in the air to support the burning of the fuel and you introduce a method of igniting this air fuel mixture it will burn very rapidly and there is a resulting gas expansion that is several times the volume of the original air. The pressure increase from the gas expansion pushes the piston back down the bore. At this point we now open another access port to the cylinder and allow the remains of the expanded gases to escape. In a two stroke cycle engine this cycle occurs every time the cylinder moves up and down the bore and in a four stroke cycle engine this cycle is spread out over two complete strokes of the piston. 

There are a couple of key points to remember here. One is the mixture of fuel particles and air. If the ratio gets to be around 14 pounds of air for every 1 pound of fuel then the mixture is ripe for combustion. The second point is that the compression of the mixture is not necessary for the fuel to burn, but the more you compress the mixture before you ignite it the more useful work you get from the resultant gas expansion. 

The measurement of the amount of compression that a piston-cylinder combination will exert on the mixture is called the compression ratio and is simply the measurement of the total volume of the cylinder with the piston out of the way compared to the remaining volume when the piston is as far as possible toward the capped end of the cylinder. 

Now we need to know this ratio when building an engine for two reasons. The first is that when the engine is made up of more than one cylinder we want each of the cylinders to produce equal power pulses. So by measurement and calculation we can make a start at this by at least getting the dimensions of the squish volume the same for each combustion chamber. The second reason for knowing the compression ratio is that when we compress the air in the cylinders it gets hot; in fact so hot that it can ignite the air/fuel mixture. So during the build of the engine we want to make sure that we keep the compression ratio at a level where the fuel has to wait for the spark to be introduced by our ignition system and not light off before at some inappropriate time. Fuels have chemicals in them to make them less volatile. The specific volatility of the fuel is indicated by the octane rating. The higher the octane rating the higher the resistance to ignition by compression or the slower burning the fuel is. Higher compression engines therefore need higher octane fuel. 

What we glean from this is the fuel that the engine uses has a huge impact on the mechanical makeup of the engine. Inside an engine is a collection of parts that are all designed and built to do completely different specific jobs but they have to all work together in a relationship. Sound familiar - - - "Let the Eternal Compromise Begin" Every time the work demand goes up certain components have to change in order to meet this output demand. Every time a component needs to change in some way it effects the entire system. Higher compression means more resultant expansion but it also means that the components that do the compressing and receive the power pulses need to be stronger or better in some way. More horsepower means higher strait line speeds which means bigger brakes which of course require larger wheels which means wider fenders which means different aerodynamics - - not to mention the larger fuel tank to go the same distance which means more weight - -oh yah ! The aero stuff weighed more too - - good job we went for the better brakes - - but the weight change and center of gravity change means different springs which means different shocks to control them etc. etc. You could literally go on for ever. 

The point is - - when changing any mechanical system the compromises have to be a consideration. In case of a vehicle, enhancing the performance of any of the onboard systems, especially in the engine, tends to take away the designed-in diversity and make it more specialized. So it is important to have the ultimate use in mind. 

This talk or discussion is aimed at the aspiration of an engine in particular that of a turbocharged engine as this is an anniversary of turbo's for Porsche . I will try and illustrate some interesting information about turbo's by comparing turbo and non turbo engine characteristics and by comparing some of those differences on two different very special turbocharged cars that only a company like Porsche could build - - The 935 - -and the Gt1

THE CRANK HOUSING
Up until now we have mentioned the aspiration that takes place in the business end of the cylinder with the pistons pumping all that air and expanded gases in and out of the engine through the ports in the heads - of which we will talk more about later - But we sometimes forget that the air underneath the piston is being moved around as well. The easier we make it for this air to flow around inside the block or crankshaft housing, the less energy is required for moving it and we reduce the pumping losses of the engine, thus increasing performance. In these slides are depicted two samples of what we are talking about. You will notice that the earlier generation engine housing from the 935 has a much more dramatic treatment than the newer Gt1 housing, Both engines produce more or less equivalent horsepower - - the smaller displacement 935 housing is at the limit of its development and every aspect is under consideration for horsepower gain -- the larger displacement Gt1 engine doesn't need to work so hard to make the same power so the treatment to the housing is mild in comparison 

However there are some other factors to consider. We know that materials are weakened by flaws in their structure or surface so a smooth piece of material is stronger than a rough sample due to the lack of stress raising flaws. So the other benefit of massaging the inside of the housing is in the increase in strength attained by the smoothing of the cast aluminum, especially around areas that are left sharp after the machining to final dimensions.

Turbo charging an engine in really broad terms can have the effect of a 70% increase in horsepower output of and engine per displacement as a straight bolt-on addition with no other changes. However we know it isn't that simple - - but if we add up the compromises and we compare a smaller displacement turbo engine to a large displacement normally aspirated engine it usually comes out a better deal if you look at cost of development and life expectancy in a race scenario. Normally aspirated engines have to be much larger to produce the same power. Larger means heavier overall, but also the weight of the reciprocating parts increase. Here is the problem: When a piston is larger and heavier the connecting rod has to be stronger and heavier (if we have the same materials available for both components ) because it has to hang on to the heavier piston and change its direction on every stroke, and since the piston is heavier and the rod is heavier the crankshaft has to be heavier. If you have a situation where the displacement is limited by the rules then to get more power you have to crank up (excuse the pun ) the Rpms. With higher revs different vibration frequencies are uncovered that have different harmonics that can cause different failures etc. 

The turbocharged engine in comparison can be lower revving with smaller, lighter components so the vibration problem, which is a major material failure catalyst, is substantially less. But there is of course the compromise - - turbocharged engines have higher combustion pressures and temperatures therefore development hurdles are mainly thermal management issues. 

The piston is the part that contains the combustion and therefore is exposed to the highest peak temperature in the engine. But the piston itself it is not the hottest part of the engine. The greatest amount of heat energy is absorbed by the connecting rod bearings. If you look at the cumulative effect of the gas expansion, plus the inertial energy of the components, and the energy direction change from linear to rotary motion. Just think all that energy absorbed in a few square millimeters of contact area between the bearing, a thin film of oil, and the surface of the crankshaft journal. This is one of the reasons that race engines have high capacity oil pumps and oil storage capacities an order of magnitude larger than any street car. The 935 carried 20 litres of oil and had an almost 50 mm in diameter oil feed line to its pump. Race engines also use harder bearing materials for the con rods that is necessary for any street car application. The harder material is more resistant to hydraulic deformation under the force that they see. These things are common to both normally aspirated and turbo race engines. In the turbo engine we have higher pressures from the gas expansion. In the normally aspirated engine we have higher inertia energies and higher pressure from the compression.

Now back to the piston. The piston designed for the turbo will have aspects that reflect the fact that it will be exposed to higher combustion temperatures and higher cylinder pressures. First of all the turbo pistons tend to be flat tops ; a shape typical of lower compression engines. The crown area will tend to be thicker in order to withstand the higher pressures and temperatures. The first ring will be farther down the piston in a more dimensionally stable area away from the heat. The alloy used may have a lower coefficient of thermal expansion and this will have a direct influence on the dimensional shape of the piston. In the case of the air-cooled 935 and the liquid cooled Gt1 the pistons will be different due to the totally different thermal cycle. The liquid cooled engine can run a much tighter fit tolerance because of the more consistent operating temperatures. 

The piston from a normally aspirated engine will have a shaped top that will fit into the combustion chamber in order to get the smaller squish volume. The other aspects of its design will be tied to higher velocities. For reduced friction, thinner (and lighter) rings and smaller piston skirt areas are used.

HEADS 
The cylinder heads of the two aspiration types can look virtually the same the differences in the turbo cylinder heads are all heat management related. In the 935 cylinder heads a special cast steel alloy ring was used as a seal and heat transfer aid between the heads and the tops of the cylinders. The next level to this arrangement was electron beam welding the heads to the tops of the cylinders. This arrangement was actually done for the stillborn Porsche Indy car that Danny Ongias was to drive long before the Holbert project. That fixed the heat transfer and cylinder pressure problems but then the valve angles had to change in order that the valves could be installed and removed. Other significant differences are in the materials that the valves, guides, and seats, are made of ; all capable of handling the elevated temperatures. The 935 exhaust valve guides actually had lubrication ports for additional cooling. Some turbo cylinder heads like the 944 turbo have a cast in ceramic port for the exhaust.

The similarities between the two heads are in the way that the air/fuel mixture enters and the exhaust leaves the combustion area. When designing or building a turbo engine we must not think of air being blown into the engine. We must think in terms of the compressor supplying higher density air. Flow is Flow and whether it is water, thin air or dense air, with different Reynolds Numbers, the rules are all the same. All of the things that interrupt flow are just as prevalent in a turbo engine as in a normally aspirated engine. The Port shape, if we look at a this slide of the Gt1 combustion chamber is no different than it would be if the engine was normally aspirated. Note the transition in the bowl area from the port entrance. The shape of the port is probably the most critical factor in how much power a given engine can produce and more importantly where in the rpm band does the power start and peter out. Remember , the higher intake manifold pressure developed by a turbo leads to higher velocity flow into the combustion chamber. In my own experience I have found that getting the port to flow at low valve lift seems to go hand in hand with a broad torque band and that translates into better drivability.

Of course there are other factors that influence the width of the torque band of an engine but they are all tied to the breathing. The intake manifold for instance is very key as it is upstream from the ports of the cylinder head. Turbo charging, because of the huge benefit of the denser air can sometimes allow engine designers to get away with a lot. If we looked at the street 930 the manifold has literally sharp 90 degree corners in it but the engine makes (for most people) ample power. The bottom line is that every time the air has to change direction we need to gently convince it to do so, such that the amount of energy it looses through pumping losses is minimized. If we look at the slides of the Gt1 engine intake system we can see the smooth trumpet shapes that are used to get the air to enter the runners with the least amount of turbulence developed. Notice the straight smooth runners to minimize the pumping losses. These are very typical characteristics of what you would see on any sophisticated normally aspirated engine. The difference in the normally aspirated engine might be in the runner length as it would be tuned for optimum performance at a different Rpm range. 

INTER COOLING
When air is compressed it heats up. We talked about this earlier when we were discussing the compression taking place in the cylinder. The air compressed by the turbocharger acts no differently. If our goal is to feed the engine a denser air charge then the logical thing to do is to cool the air after the compression. This process is called inter cooling and simply involves cooling the air after being compressed by the Turbo to try and retain and increase the density. 

Back in the days of the 917's the charged air wasn't cooled, in the 935era the cooling was initially accomplished through the use of liquid to charged air coolers. The coolers could be quite compact and efficient because of the high heat absorption rate of the liquid. However that was a short lived effect, as when the coolers came up to a constant operating temperature the power of the engine dropped off as the heat absorption rate went down. The other drawback was the complication of the additional liquid cooling system that was needed on board the vehicle and the associated weight gain. The later 935's were equipped with air to charged air coolers which were a bit bulkier for the same cooling performance but over all the system got simpler and more versatile. The cooler, initially located over top of the engine, was moved to the rear fenders where the ambient cooling air could be easily ducted through them. In the slide we have a Gt1 inter cooler that is located back over top of the engine in a similar packaging arrangement as the original 935, but this time they are fed by a huge duct on the roof of the greenhouse and not by the engine cooling fan sucking its air through the cooler. When looking at this picture notice the smooth transitions for the charged air in and out of the cooler core.

THE TURBOCHARGER 
Over the last 60 or so years the centrifugal supercharger or compressor has evolved a great deal from its early gear driven beginning's running on old Offy's at the brickyard and pumping up the power on second World War aircraft. But even in the aircraft industry back then the exhaust driven turbine was being tried. The famous American fighter, the Lockheed Lightning had turbos on its "V"12 Allison power plants. These turbos looked more like a jet engine hot housing coupled to the cold housing of a Turbocharger of today. The turbine engine and the turbocharger are in fact very close relatives. Cold air is compressed and pushed into a burn chamber where ignition takes place and the expanded gases escape through a turbine blade set in a housing. The exhaust turbine or hot side is coupled by shaft to the intake side so as the exhaust side is pushed faster the intake side pumps more and so on. You just have to keep adding fuel to increase the output. In the case of a Turbocharger we replace the burn chamber with a positive displacement air pump called an engine where we burn our fuel. There is actually a guy that I know of that builds turbines out of turbochargers - - I think his web site is Ney turbine systems or something like that. 

Here in the pictures we have a Gt1 turbocharger visually it looks no different than the type of turbo you would see on the 935. Since the 935 most of the big advancements in the last 30 years are more subtle and internal. Things like ceramic impeller blades and ball bearing center housings are the types of things possible with today's material technology. Other changes to the shape of the cold housing and how it feeds the air into the impellers or the shape of the impeller wheel blades are all not necessarily discern able by the untrained eye. 

Sizing a turbo for an application requires a look at three different aspects that must be considered coincidentally because guess what ! : there is a compromise and the turbo must be considered as a system unto itself once the direction of application is established for the engine as a whole. The compressor map of the cold housing and the torque development of the hot housing have to be considered along side with the engine performance profile. On the 935 we used to change turbos to two smaller K26's if we were going to a tight twisty race course and we would go to two K27's or larger for tracks with higher sustained speeds or long straitaway's like Brainard Minnisota or Road America Elk hart Lake Wisconsin. On a champ car the turbo impellers are limited by rules in diameter so they change only the hot side housing. For the tight twisty road courses they run smaller diameter headers and a small housing to get higher exhaust velocity entering the turbo to make it spool up more quickly. For the big ovals they run a large hot housing and a set of larger tube diameter headers for the high sustained speeds and high air demand of the engine. This sounds quite simple but the design of the original map was completed prior to the track side changes. These compressor specs and turbine characteristics have a direct effect on when the engine will respond to the foot. 

However if we get the porting in the head right and the intake manifold design correct we can run a larger turbo to get the air volume at high engine speeds and also get torque development at low engine Rpms or at least reduce the lag that we would normally get if we just bolted on a bigger turbo. 

EXHAUST
The exhaust of a turbo engine is a very special component made from special high heat materials. Exhaust headers for turbo race cars are made from high alloy nickel steels or high nickel alloys that aren't called steel anymore. Alloys like Inconel or Hastalloy are used and incidentally these are the same alloys used in jet turbines for the same high heat capabilities. These materials literally can be so hot in operation that they glow and be stronger in this condition than at room temperature. Most normal automotive applications utilize 300 series stainless steels which have good high heat capabilities and good cold ductility . In our picture we have a header from a Gt1 these are made from Inconel, this one in particular has a problem. This failure is stress related and very typical of the type of material. It is incidentally 100 % weld repairable, the key would be to look for the cause of the induced stress in the area so the failure doesn't repeat. 

As in the design of the Intake system the same things that are important to the normally aspirated engine are also important to the turbo. The headers don't have to be equal length but it helps if each of the pressure pulses going into the turbo impellers are equal. But here there is a difference from the normally aspirated engine which responds to a tuned length. The tuned length and proper collector actually help the flow of the normal engine by creating a negative pressure in the collector to help the next pulse flow out. The turbo has this hot housing and impeller stuck in the way so the equal pulses help but the pressure build prior to the Turbo negates any benefit derived from the a tuned length and collector. 

DREAMS AND ASPIRATION