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CATEGORIES (articles) > Engines > Technical > Supercharger for your engine

Supercharger for your engine


A supercharger (also known as a blower, a positive displacement pump or a centrifugal pumper) is a gas compressor used to pump air into the cylinders of an internal combustion engine. In gasoline engines this is the fuel/ air mixture (the charge) when the fuel has been added. This increases the mass of oxygen and of fuel in the cylinder, thus improving the volumetric efficiency of the engine making it more powerful. It is similar in purpose to the closely related turbocharger, but with a clear difference. Whilst a turbocharger is powered by the mass-flow of exhaust gases driving a turbine, a supercharger is powered mechanically, by belt- or by chain-drive from the engine's crankshaft . A supercharger may absorb as much as a third of the total crankshaft power of the engine, but at the same time may increase total engine output by over 100 percent. It is commonly seen in high performance cars such as Top Fuel Dragsters.

The supercharger is used as a power-boosting device in aircraft and cars, particularly for operation at altitude, although the turbocharger is more commonly used in both roles. Another use is in the Miller cycle engine that uses a supercharger to alter the normal four-stroke engine cycle to operate more efficiently.

Basic concept

The amount of power a particular engine can produce depends on the amount of fuel and air that can enter the cylinder, or the "charge." With any internal combustion engine, one way to produce more power is to burn more fuel. In order to burn a given amount of fuel, an exact amount of oxygen is required if the mixture is to be consumed without leaving excess fuel or oxygen behind. This chemically correct mixture (14 parts air to each part fuel) is called stoichiometric; most engines must operate at or near this chemically correct mixture.

The simplest way to produce more power is to build a larger engine with bigger cylinders, but this is not always practical for a variety of reasons. For aviation use, the main problem with increasing size is increasing weight out of proportion to the capacity. For automobile use, the fuel consumption at all power settings increases with capacity.

Designers are left with a choice of supercharging or turbocharging.

Automobiles

Hot Rod magazine cover, featuring Offenhauser engine with large Roots supercharger
1929 "Blower" Bentley from the Ralph Lauren collection.

In cars, the device is used to increase the "effective displacement" and volumetric efficiency of an engine, and is often referred to as a blower. By pushing the air into the cylinders, it's as if the engine had larger valves and cylinders, resulting in a "larger" engine that weighs less. Turbochargers are more commonly used in this role because they use otherwise "wasted" heat energy instead of using up power from the crank, but the supercharger reacts more quickly to power application and thus outaccelerates a car with the same amount of boost being provided by a turbo.

In 1900 Gottlieb Daimler (of Daimler-Benz / Daimler-Chrysler fame) became the first person to patent a forced-induction system for internal combustion engines. His first superchargers were based on a twin-rotor air-pump design first patented by American Francis Roots in 1860. This design is the basis for the modern Roots type supercharger.

It wasn't long after its invention before the supercharger was applied to custom racing cars, with the first supercharged production vehicles being built by Mercedes and Bentley in the 1920s. Since then superchargers (as well as turbochargers) have been widely applied to racing and production cars, although their complexity and cost has largely relegated the supercharger to the world of pricey performance cars.

Boosting has made something of a comeback in recent years due largely to the increased quality of the alloys and machining of modern engines. Boosting used to be an effective way to dramatically shorten an engine's life but, today, there is considerable overdesign possible with modern materials and boosting is no longer a serious reliability concern. For this reason boosting is commonly used in smaller cars, where the added weight of the supercharger is smaller than the weight of a larger engine delivering the same amount of power. This also results in better gas mileage, as mileage is often a function of the overall weight of the car and that is based, to some degree, on the weight of the engine.

There are three commonly used types used in today's automotive world: Roots type supercharger, and Eaton or twin-screw type supercharger, and Centrifugal type supercharger.

For example, GM cars use the Eaton-type supercharger in the Chevrolet Monte Carlo SS, Chevrolet Impala SS, Pontiac Bonneville SSEi, Pontiac Grand Prix GTP, and Buick Regal GS. These cars use the venerable GM Series II, or newer Series III 3.8 L V6.

Aircraft

A more natural use of the supercharger is with aircraft engines. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off—at 6000 m (18,000 ft) the air is at half the pressure of sea level. Since the charge in the cylinders is being pushed in by this air pressure it means that the engine will normally produce half-power at full throttle at this altitude.

Altitude effects

A supercharger remedies this problem by compressing the air back to sea-level pressures, or even much higher. This can take some effort. On the single-stage single-speed supercharged Rolls Royce Merlin engine for instance, the supercharger uses up about 150 horsepower (100 kW). Yet the benefits are huge, for that 150 horsepower (100 kW) lost, the engine is delivering 1000 hp (750 kW) when it would otherwise deliver 750 hp (500 kW). And while the engine might be fooled into thinking it's at sea level, the airframe is quite aware of the halved air density and the plane thus has half the drag. For this reason supercharged planes fly much faster at higher altitudes.

A supercharger is only able to supply so much pressure because the compression increases the air temperature and the engine is limited in maximum charge-air temperature before pre-ignition occurs. The boost is typically measured as the altitude at which the supercharger can still supply sea level pressure (1 bar) and is referred to as the critical altitude. Throughout WWII British superchargers generally had higher critical altitudes than their German counterparts and, when combined with higher octane fuels that the Americans supplied, that allowed for higher boost levels. British engines were generally able to outperform German ones.

Altitude efficiency

Below the critical altitude the supercharger is capable of delivering too much boost and must therefore be restricted lest the engine be damaged. Unless other measures are taken, this means that at least some of the power driving the supercharger is wasted. Also, due to the denser air at lower altitudes, the supercharger is not operating at its best efficiency, and this can cause an additional load on the engine.

For the early years of the war this was simply how it was and this led to the seemingly odd fact that many early-war engines actually delivered less power at lower altitudes, because the supercharger was still using up power to compress air that was not delivering any power back. As the war progressed two-speed superchargers were introduced using better controllers and, notably, hydraulic clutches, that allowed the boost to be managed over a wide range of altitudes by operating at low rpm down low and at high rpm at higher altitudes. This generally "flattened out" the power below the critical altitude.

Improving octane rating

In 1940 a batch of 100 octane fuel was delivered from the USA to the RAF. This allowed the boost on Merlin engines to be increased to 48 inches Hg (1.6 Bar) and the horsepower to rise by more than 10% (from 1030 to 1160 hp). By mid-1940 another increased boost yielded 1310 hp. Supercharging, by itself, could not have achieved these improvements, but married with fuel improvements the engine could respond to both.

Multiple stages

In the 1930s two-speed drives were developed for superchargers. These provided more flexibility for the operation of the aircraft although they also entailed more complexity of manufacturing and maintenance. Ultimately it was found that for most engines (excepting those in high-performance fighters) a single-stage two-speed setup was most suitable.

A final improvement was the use of two compressors in series, which were introduced to solve the pre-ignition problem. Compressing a gas always causes its temperature to rise, and an overcompressed fuel-air mixture may therefore prematurely ignite. In order to avoid pre-ignition the "two stage" design was used. After being compressed "half-way" in the low pressure stage the air flowed through an intercooler radiator where it was partially cooled down before being compressed the rest of the way in the high pressure stage and then aftercooled in another air/air or coolant/air radiator ( heat exchanger). At low altitudes one stage could be turned off completely. The two-stage Merlin was losing 400 hp (300kW) to turn the supercharger but developing between 1500 and 1700 hp (1125 to 1275 kW) at the propeller shaft, depending on model.

It is interesting to compare all of this complexity to the same system implemented with a turbocharger. Since the turbo is driven off of the exhaust gases, simply dumping some of the exhaust pressure is sufficient to drive the compressor at almost any desired speed. In addition the power in the exhaust would otherwise be wasted (except to the extent that the exhaust itself provided thrust) whereas in the supercharger that power is being taken directly from the engine. Thus at low altitudes the turbo robs nothing and, as the altitude increases, it can use just as much power as it needs and no more. Better yet the amount of power in the gas is the difference between the exhaust pressure and air pressure, which increases with altitude, so turbochargers generally have much better altitude performance.

Yet the vast majority of WWII engines used superchargers, because they maintained three significant manufacturing advantages over turbochargers, which were larger, involved extra piping, and required exotic high-temperature materials in the turbine. The size of the piping alone is a serious issue; consider that the Vought F4U and Republic P-47 used the same engine but the huge barrel-like fuselage of the latter was, in part, needed to hold the piping to and from the turbocharger in the rear of the plane.

More cons of supercharging

Superchargers are often considered inferior to turbochargers for several reasons. Firstly, a turbocharger is more efficient than a supercharger whilst requiring only 50% of the supercharger's manufacturing cost. In addition, turbochargers are able to reach efficient operating speed much more quickly than a supercharger.

The physical space occupied by a turbocharger is significantly less than it's direct-drive counterpart. This gives the opportunity of fitting multiple turbochargers to a single engine, such as in a "sequential turbo", where one turbo is tuned to give increased performance at low engine speed and another turbo is tuned to increase the high-speed engine performance.

An alternative arrangement utilises two turbochargers of the same type, known as a "twin turbo". This gives a large power increase for a given engine speed at the cost of increasing the lag-time for the exhaust to heat up sufficiently to drive the turbochargers. This lag can be addressed by reducing the size of each individual unit such that the combined output is still as great as a single large turbocharger without having to suffer the lag-time required to reach operating speed.

A final benefit of the turbocharger over the supercharger is the operating speed; It is not unheard of for a turbocharger to reach 200,000rpm. This is well beyond the operating specification of a supercharger.




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