Types of Supercharging

There are far more ways to supercharge an engine than the handful of methods in common use on airplanes, but in aviation, turbosupercharging is easily the most popular, with some centrifugal superchargers here and there.

A third method is nitrous oxide injection, which we’ll lightly detail, as air racers are fond of the stuff. But let’s start with the centrifugal supercharger, as it is perhaps the easiest to understand.

Centrifugal Supercharging

Centrifugal supercharger. (Photo: Tom Wilson)

A centrifugal supercharger looks and moves air similarly to a pistol-style hairdryer. A snail-like housing (the volute) contains the impeller, which is a small paddle wheel. The paddles, or vanes, on the spinning wheel are what scoop the air and speed it downstream through the volute.

The impeller is mechanically connected to the engine’s crankshaft by a belt or gears, so if the engine is turning, the impeller is turning. And when the impeller is turning, a great mass of air is pumped into the engine—much more than the engine would have naturally ingested. This raises the manifold pressure significantly, allowing more power to be made.

Besides the drive belt or gears, a supercharger will also have a set of “step-up” gears to increase impeller speed to anywhere from three to 10 times crankshaft speed. These gears are packaged into the supercharger assembly and ensure the impeller spins at anywhere from 25,000 to 80,000 rpm (approximately), depending on the supercharger’s size and the engine’s altitude needs.

As noted, centrifugal superchargers can be designed as an integral part of the engine and be driven directly by gears (or a fluid coupling) off the crankshaft. Or a centrifugal supercharger—often called a blower—could be an add-on accessory, in which case it is belt-driven by the crankshaft.

In any case, the power to turn the blower can be substantial. On smaller general aviation engines, it may take 40 to 100 hp to turn the supercharger at takeoff power; on old, large radials, it could take several hundred horsepower to drive the supercharger. That means the engine has to be sufficiently robust to make its rated power plus the power to drive the supercharger. This also means the engine will burn the fuel and suffer the wear associated and heat generated with the total power being made, not just its rated power.

Image: ATI ProCharger – www.procharger.com, CC BY-SA 3.0, Link

For this reason, supercharged engines have a gas-guzzling reputation and are typically not rated for long TBO (time between overhauls). But they typically make 25% or more power than if naturally aspirated, so if a rapid climb or heavy payloads are the need, a supercharged engine can do the job.

Furthermore, compared to other types of superchargers, centrifugal supercharging installations can be quite efficient for the amount of air they move, plus they are easier and simpler to package on an engine than a turbo. They also do not restrict the exhaust stream, and for lightly supercharged or normalized engines running at moderate altitudes, say 12,000 to 20,000 feet, a blower offers power, simplicity, and fewer heat-related issues than a turbocharger installation.

Turbosupercharging

A turbosupercharger—colloquially called a turbo—has two housings and two vaned wheels packaged into a common assembly. The one volute and impeller are essentially a centrifugal supercharger. But the other volute and wheel are plumbed into the engine’s exhaust pipes. The hot, pressurized exhaust gases spin the wheel on the “hot” side of the turbo as they pass through. That exhaust wheel shares the same shaft as the impeller on the cold side, so when the exhaust side spins, the cold or intake side impeller spins at the same speed. Again, air is blown into the engine’s intake system in great quantities.

Unlike a supercharger, a turbo is driven by the engine exhaust gasses.

Turbos tend to have smaller impellers and turn at higher speeds than superchargers. Some small turbos on motorcycles and cars rotate at 250,000 rpm; half that is more likely in an aviation application.

Mechanics and engineers refer to the cold-side wheel as the impeller (sometimes as the compressor) and the hot-side wheel as the turbine. You might hear “turbine wheel” in conversation.

So, loosely speaking, a centrifugal supercharger is “engine-driven” by power taken from the crankshaft, while the turbosupercharger is driven by the energy in the exhaust system. As there is copious energy otherwise dumped overboard in the exhaust, a turbocharger does recoup some of this otherwise wasted energy. But it is not completely free energy, as some might tell you. A turbocharger restricts the flow of exhaust gas, as some energy is consumed turning the turbine/compressor wheels. All told, however, a turbocharger uses less fuel than a mechanically driven supercharger and is the most efficient form of supercharging for aircraft.

A real issue with turbo systems is packaging the tangle of plumbing—half of it very hot—around the engine and its accessories, and still keeping it all under the cowling. This adds much expense during production (high-grade stainless steel is required) and slows normal maintenance, plus it adds maintenance keeping the crack-prone exhaust plumbing airworthy. The turbos themselves can wear out or fail, as they run hot and fast. The extensive exhaust piping under the cowl sheds plenty of heat onto rubber parts, such as the engine mount cushions, hoses, and so on. This can lead to early replacement of those parts.

Supercharger Complexity

Today, most supercharging involves a single centrifugal or a pair of turbos running in parallel. But supercharging can involve multiple-speed gearboxes, infinite-speed viscous couplings, and big centrifugals blowing into smaller centrifugals; some big radials had two blowers running at three different speeds. Likewise, turbo systems can have two stages (one big turbo blowing into a smaller turbo to step up boost pressure), although this is rare in aviation.

A legacy single-turbo installation on a Continental six-cylinder engine. (Photo: Tom Wilson)

Some of the vintage supercharging systems had the pilot select blower speed manually; others were automatic. Today, you won’t be tasked with that.

What you might be required to adjust manually is limiting boost pressure to safe levels. Too much manifold pressure is the fast track to engine-destroying detonation, and on older general aviation turbo systems, this is a job for the pilot. Either setting throttle position in reference to manifold pressure or adjusting a vernier control on a wastegate could be required.

A wastegate is a throttle-like flap valve limiting the amount of exhaust gas routed to a turbocharger. A fully closed wastegate routes all exhaust through the turbo; when fully or partially opened, the wastegate allows bypassing the turbo, limiting its speed and thus boost output.

Supercharging Controls

Pilot interaction with supercharging varies from none to busy. Simple centrifugal systems require no pilot input; these systems are sized—the blower speed and dimensions chosen by the designing engineer—to never make more boost than the engine can handle. There is no wastegate.

A Hartzell Engine Technologies turbocharger wastegate and actuator assembly.

On older aircraft turbo systems, the wastegate is manually controlled by the pilot—another important knob to twiddle with during takeoff, climb, and landing. More modern systems automate the wastegate to reduce pilot workload. If automated, the electronic box making the decisions is typically called the boost controller. Naturally, fuel requirements rise with increasing boost, and this can keep the pilot busy juggling the throttle, mixture, and wastegate controls on old manual systems.

Importantly, pilots should remember a supercharged engine has distinctly less detonation margin than a naturally aspirated version. Also, when things crack and leak on turbo systems, dangerously hot exhaust gas could be loose under the cowling. That’s why boost problems require abundant caution, and prompt, preemptive landings are smart should things go wrong with turbocharging.

Charge Cooling

Because supercharging (centrifugal or turbo) compresses the engine’s intake air, that air gains heat. At low boost levels, this is not a concern, but with meaningful boost, the inlet air can zoom to 350° F and must be cooled to avoid detonation. A charge cooler—think of a radiator but with air inside instead of water—is then plumbed into the intake air tract. If the charge cooler is mounted downstream of a single supercharger, then it is an aftercooler. If the cooler is between two blowers or turbos, it is an intercooler; same piece, but the name denotes where it fits in the system. Unfortunately, lazy speech around the airport and racetrack means everyone calls everything an intercooler when they are almost always aftercoolers. Using the umbrella term “charge cooler” avoids this inaccuracy.

While most charge coolers you’re likely to encounter are air-to-air designs, on liquid-cooled engines, charge cooling can be air-to-water. The P-51 Mustang uses air-to-water charge cooling, for example.

A limitation to charge cooling—and overall engine cooling—is altitude. The thinner, less dense atmosphere aloft doesn’t offer the same cooling as denser air down low, and cooling highly boosted engines in the upper teens or higher can be temperature-limited. Of course, the higher the airplane flies, the harder the supercharging must work to deliver increased air density—and the hotter the intake air gets. This phenomenon can lead to supercharger systems sized to provide good boost at altitude but easily too much boost at sea level, ultimately leading to more engine management workload for the pilot.

Another form of charge cooling is ADI, or anti-detonation injection. This is an add-on system (tank, pump, filter, nozzle) spritzing water into the inlet tract; evaporation drops the air temperature remarkably. Alcohol is added to the water to avoid freezing at altitude.

A requirement of ADI is leaning the air/fuel mixture when the “water” is turned on. In the old days, a mechanical linkage did this, while modern systems are digitally controlled. Because only so much ADI can be reasonably carried, ADI is reserved for limited maximum-power efforts, such as takeoff, initial climb, or racing.

Nitrous Oxide

And finally, “nitrous,” a molecule of two parts nitrogen and one part oxygen. The nitrogen is inert, but the oxygen is ready to burn fuel to make heat. A gas at room temperature, nitrous is a liquid when compressed and bottled. A nitrous system thus contains a heavy bottle (or bottles) of nitrous oxide at approximately 1,900 psi, along with some method of supplying extra gasoline in proportion to the amount of nitrous injected.

High-pressure lines carry the nitrous from the bottle to the intake manifold (or supercharger inlet), the whole controlled by solenoid valves. The extra fuel can be easily supplied by electronic fuel injection with proper software.

Nitrous is a very powerful power adder and is ideal for delivering short bursts of extra oomph. It’s somewhat popular with backwoods STOL fans and pylon racers, but it is not used by mainstream aviators because only limited nitrous can be carried, and there’s much bottle temperature and pressure management required.

The post Types of Supercharging appeared first on AvBrief.com.