Ask Joe Mechanic: Supercharging verus Turbocharging

For the last several weeks, we have examined turbochargers and superchargers regarding the history, methods of operation, design, innovation and performance characteristics of each. Today we will compare the two systems as far as advantages and disadvantages.  Both supercharging and turbocharging have significant disadvantages in their inherent designs, but the gain in horsepower outweighs those disadvantages. When you can achieve the same horsepower from a four cylinder as a V-6, or the same power from a V-6 as a V-8, it is possible to overlook some other factors.

            The first major disadvantage of supercharging is the amount of power that it takes from the engine to operate it. On some large engines such as the single stage supercharged Rolls Royce Merlin; the supercharger requires about 150 horsepower (HP) at maximum operating pressure.  However, the benefit outweighs the power demand because the supercharger generates an extra 400HP, so there is a net gain of 250HP. The other problem that is created is that the engine must be built to withstand the stress of the extra 400HP, not the resulting gained 250HP. While centrifugal superchargers such as those the Rolls Royce Merlin use operate at about a 65-80 percent efficiency, a Roots blower by comparison often only results in an efficiency of about 40-50 percent at maximum boost.

            The second major disadvantage is the amount of heat generated. In technical terms, it is referred to as lower adiabatic efficiency. Adiabatic efficiency is the measure of a compressor’s ability to compress air without adding excess heat to the charge air. A compressor will always generate heat as a byproduct of the compression process however; more efficient compressors generate less heat. Turbochargers generate the least amount of heat, next best are the centrifugal superchargers, and while the Roots type superchargers have the poorest efficiency, thereby generating the highest temperature spike. For a given volume of air, the turbocharged air is cooler, more dense and containing a greater concentration of oxygen. This means that it will generate more potential power, all other things being equal, than a supercharged engine. In practical application, that power difference can be as much as 15-30 percent depending on the designs and types of units being tested.

            This is the main reason why most manufacturers using supercharging or turbocharging install an intercooler (an air to air radiator) in the system. An intercooler can often restore between 20-30 percent of the power lost through turbocharging and supercharging. But, even with the cooling, the turbocharger will retain its higher efficiency rating, which usually means a gain in fuel economy and more power.

            The greatest drawback to turbocharging is throttle response. Termed “lag,” there is a time that lapses between when the throttle is depressed and when the turbo “spools up” to the point where it is generating boost. This is because at idle or low engine speed, there is little exhaust pressure to spin the turbocharger. There is a brief delay until there is enough pressure in the exhaust to spin the turbo fast enough to generate the desired amount of boost. This lag is reduced by the use of smaller, lighter weight turbochargers and by using variable pitch or twin turbochargers, but it cannot be eliminated. This is the one distinct advantage of supercharging. With supercharging, the response to throttle demand is almost instantaneous due to the fact that it is directly driven off the engine.

            In places where instant power is demanded, such as drag racing or tractor pulling, supercharging is the method of choice. Increasingly, for vehicle use, turbocharging has become the choice for most applications. Even Mercedes Benz, whose “Kompressor” models are well known and have been around since the early days of supercharging, has started using turbocharging on some of its gasoline-powered vehicles as well as its diesel models over the last few years.  No matter which method you choose, you can derive more power from a smaller engine, which equates to less fuel used.  In today’s world, that is an important factor to consider.

Material for this post was sourced from www.wikipedia.org.

Ask Joe Mechanic: Turbocharging Part II

We discussed in part 1 that turbocharging lately has become very advanced due to the governmental requirements to achieve higher gas mileage ratings. We now are finding turbochargers installed on V-6 engines which either necessitates some special designing to use only one turbo, or the use of twin-turbochargers.  With twin turbos on a V-6 engine, each manifold has a turbocharger installed on it and both feed into a single plenum on the intake manifold. This system is also used on boxer or flat engines such as Subaru uses. 

There are also manufacturers who are using twin-turbochargers in series to create higher boost at higher road speed, but eliminate turbo lag at low speeds. To accomplish this, a small turbo charger is installed first which will spool up quickly at low speeds. Then, there are specially designed piping leading to a second larger turbocharger for road speed. This type system is most commonly used on diesel engines, but some exotic car builders also use it.

            Another design is the twin-scroll turbocharger where there are two exhaust inlets in one turbocharger, with a smaller angled one designed for quick response and a second less angled larger inlet for peak performance. Usually, these twin turbos will pair cylinders 1 and 4 along with pairing 2 and 3 to more efficiently burn the fuel mixture and to reduce engine manifold temperatures. It will also greatly reduce turbo lag.

            Variable geometry or variable nozzle turbos adjust the amount of air entering the intake side of the turbocharger with a set of adjustable vanes. This will cause the turbocharger to operate at optimum pressure and efficiency based on the demand placed on it. There is an actuator which is computer controlled to move the vanes to increase or decrease airflow. By doing so, it will maintain the correct exhaust velocity throughout the engine’s power range and limit turbo lag.

            The center housing/hub rotating assembly (CHRA) is the most highly engineered and probably the most important part of the turbocharger. This section contains the lubrication, cooling and the turbine impellers and their mounting. The housing has ports for engine coolant to run throughout, and also oil passages to the bearing system. The bearings in most automotive turbochargers are either high-speed ball bearings or thrust bearings. In older turbochargers, the oil would sometimes become so hot that it would actually harden around the bearing, called coking, and this would cause the turbo to fail. This risk has been greatly reduced with better bearings, cooling designs and synthetic oils, which are more resistant to heat.

            One of the technologies that has been most effective in improving turbocharger performance is intercooling. The process of intercooling is basically forcing the air from the intake side of the turbo through a radiator in an effort to cool it as much as possible. The reason for this is that hot air is less dense than cool air and that loss of density means loss of power. When you force air through the turbocharger it builds up heat, plus it absorbs some from heat transfer from the exhaust side, so by going through the intercooler, it gives the air a chance to cool down before entering the engine.

            Another application that is used often by performance tuners is water injection where a spray of water is injected into the air charge to further cool it. A variation of this is to actually alter the air/fuel ratio by richening the mixture. The extra fuel does not actually get burned, but by turning the fuel from a liquid to a gas, it absorbs heat.

            The final add on feature to a turbocharger is a waste-gate. The waste-gate’s purpose is to regulate the pressure built in the turbocharger by regulating the amount of exhaust gas passing through the turbo. A pressure sensor sensing that the engine is reaching optimum boost pressure does this. The sensor sends a signal to the engine computer, which in turn sends a signal to a vacuum valve that opens and pulls vacuum, opening the waste-gate and allowing the exhaust gas to bypass the turbo.

Some information for this post was sourced from www.wikipedia.org.

Ask Joe Mechanic: Turbocharging Part 1

Over the next several weeks, we are going to discuss turbocharging and supercharging.

Compressor Section of an Automobile

We’ll start with the history of turbocharging, followed by how it works and its main parts. We will then do the same for supercharging. After completing these, we will discuss the advantages and disadvantages of each in a comparison.

            Alfred Buchi of Switzerland who developed a compressor driven by exhaust gas to force air into the intake of a diesel engine to create more power patented the first turbocharger in 1905. It still took another twenty years though before an actual operating turbocharger was built for vehicular use. There were several attempts by the French to turbocharge some types of airplane engines in World War I with limited success. Turbocharging of aircraft engines was perfected by the early 1920s and a short time later the same thing took place on diesel engines on ships. 

The two biggest problems to developing a turbocharger for automotive use were the ability to scale down the size and manufacturing a seal that could be small enough but withstand the pressure and heat inherent to turbocharging. There were some applications to racecars during the 1940s and 1950s, but many of these were adapted aircraft turbochargers. The first manufacturer to produce a production built vehicle with turbocharging was Saab in 1977. Other manufacturers followed, unfortunately, many of the early turbochargers failed due to heat and seal problems. Saab started using a turbocharger which was cooled by antifreeze in 1986, and this proved much more reliable. Since that time, and especially in the last few years, turbocharging has become very popular because of the ability to derive the same or more power from a much smaller displacement engine, thereby achieving a much higher gas mileage without sacrificing performance, and in many cases bettering it.

The theory behind turbocharging is actually quite simple. In most internal combustion engines, the intake mixture of gas and air is actually drawn into the engine by the downward movement of the piston. In a turbocharged engine, that intake charge is forced into the engine by the turbocharger, resulting in a much larger volume of intake charge, which when ignited by the spark plug, creates much more power. The pressure to force that charge into the intake comes from the other half of the turbocharger, which is spun by the exhaust gases that are escaping from the engine. Also, the use of pressurizing the charge causes it to burn more fully, which increases the fuel efficiency of the vehicle.

The control of turbocharging has evolved dramatically in the last few years and is now quite complex. Many manufacturers now use knock sensors, all use waste-gates and blow off valves. And many use variable geometry and intercooling. 

            Boost is the term applied to the amount of pressure created by the turbocharger above normal atmospheric pressure. The level of boost is normally indicated on a pressure gauge in bar, psi or kPa. Boost pressure must be controlled so that the design of the engine is not exceeded which would cause it to fail prematurely. Over-boosting can damage the engine by overheating, over-stressing of parts or by detonation. Detonation or preignition means that due to the amount of heat and pressure, the intake charge ignites before the piston is near the top of its cycle. This exerts undue stress and heat on the internal parts. This is controlled with a knock sensor, which if it detects detonation, signals the computer to open to blow-off valve, which will release the boost pressure. The same thing can be achieved by the waste-gate which is vacuum controlled.

            The main components of the turbocharger are the turbine, which is a radial flow design to build pressure on the intake side. The compressor section is where the exhaust gas passes through to build the pressure. The center housing is where the seals, lubrication and cooling are contained. The size and design of the compressor components dictate how much boost it will create and how quickly it will build to maximum boost. In some turbochargers, it is possible for the impellor to spin at speeds of up to 250,000 rpm. This is the reason that seal design, heat dissipation and lubrication are so important.

Next week we will discuss the types of turbochargers and the other related technologies. Some information for this article was sourced from www.wikipedia.org.