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: Supercharging

Before I start this week’s article, I want to publicly thank Stephanie, who collaborated with me on the Meet Joe Mechanic article, for stepping in at a late time last week when for medical reasons I could not put my column together and found a suitable and timely replacement for me. I am now on the mend and am resuming my duties on a slightly limited basis.

            Supercharging in theory is very similar in what it does to turbocharging. In fact, turbocharging was originally called turbo-supercharging. The main difference between the two is where the power is created that forces the intake charge into the engine. In turbocharging, we already saw that that force was the use of the exhaust escaping the engine, which forced the intake charge into the engine. In supercharging, it is by external mechanical means, usually a belt, a chain or direct drive from the crankshaft of the engine.

            Ironically, the first effective use of supercharging had nothing at all to do with an engine. In 1860, Brothers Francis and Philander Roots of Connersville, Indiana, were exploring the means to get more air into blast furnaces for making steel and other industrial applications. Their company, Roots Blower Company, developed the first operating supercharger, which forced air into those blast furnaces.  It was that design which became the most commonly used supercharger design for many years.

           

The first use of a supercharger on a functioning engine was by Dugald Clerk in 1878 on a two-stroke engine. Gottlieb Daimler, a name well known in early automotive history, received a German patent for a supercharger on an internal combustion engine in 1885. Louis Renault, another well-known name of the Renault automotive family, received a French patent for a centrifugal supercharger in 1902.

            Ironically, the first known application of a supercharger took place in Pottstown, Pennsylvania by Lee Chadwick in 1908 and his car achieved an almost unheard of speed for that time of 100 miles per hour. The Chadwick factory still stands in Pottstown, in recent years in was home to the Bill Pollack, of Pollack Steel Company, automobile collection. That collection included a couple very rare Chadwick automobiles. The Chadwick will be a subject of a future Automotive History article, as I knew the late Bill Pollack personally and have been to his museum a number of times.

            The first production road cars with superchargers were built in 1921 by whom else, but Mercedes. The models were the 6/25/40HP and the 10/40/65HP and had Roots superchargers. They were designated “Kompressor,”, a designation that Mercedes uses to this day on all supercharged cars they build.

            Another design for a supercharger received a patent in 1878 by Heimrick Krigar of Germany. This was for a screw type supercharger, which is actually a much more efficient design, but its parts are very difficult to machine. The design utilizes two opposing screws with a 180 degree twist along their length which combines to create a full 360 degree turn. The design was so advanced for its time that it took until 1935 to be able to create the first working model by Alf Lysholm of Sweden working for Ljungstroms Angturbin AB, which became the Svenska Aeroplaten Company, which eventually became Saab. Who ironically became the first mass-produced successful turbocharged automobile and at the time the only company which produced an all turbocharged model line.

            There are two main types of superchargers, which are designated by the method of air transfer. The two types are positive displacement and dynamic compressors. Positive displacement blowers deliver an almost constant pressure increase at all engine speeds. Dynamic compressors do not build pressure at idle or low speeds; above a certain threshold speed pressure increases with engine speed. Positive displacement pumps deliver a nearly fixed volume of air per engine revolution at all speeds, minus leakage, which means its importance decreases at higher speeds.

            Positive displacement pumps are further subdivided into two groups, internal and external compression. Roots superchargers are of the external type, although the high-helix roots blowers try to have the same effectiveness as the Lysholm screw. All other superchargers have some degree of internal compression. Internal compression means that the compression of the air charge takes place inside the body of the supercharger. This is more effective and efficient than the backflow compression of the Roots type and means a smoother transition to boost than the other type. Internal compression superchargers usually have a fixed boost pressure which equals the compression pressure of the supercharger, thus back flow is zero. If the boost pressure is higher, back flow will occur, but if properly matched, they achieve a very high factor of efficiency.

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