The function of supercharger and turbocharger in automobiles

 

Valeo calls its device, left, an electric supercharger. It uses an electric motor — not exhaust gasses — to spin the impeller. A traditional supercharger looks like the one from Eaton, middle. It is driven by a fan belt connected to the front of the engine. Turbocharges like the one made by Honeywell, right, have an exhaust gas driven turbine on one side and an impeller on the other.

How a Car engine works?

When your body needs fuel, you feed it food. When your car needs fuel, you “feed” it gasoline. Just like your body converts food into energy, a car engine converts gas into motion. Some newer cars, known as hybrids, also use electricity from batteries to help propel a vehicle.

The process of converting gasoline into motion is called “internal combustion.” Internal combustion engines use small, controlled explosions to generate the power needed to move your car all the places it needs to go.

If you create an explosion in a tiny, enclosed space, such as a piston in an engine, a huge amount of energy is released as expanding gas. A typical car engine creates such explosions hundreds of times per minute. The engine harnesses the energy and uses it to propel your car.

The explosions force pistons in the engine to move. When the energy from the first explosion has almost run out, another explosion occurs. This forces the pistons to move again. The cycle continues again and again, giving the car the power needed to run.

Car engines use a four-stroke combustion cycle. The four strokes are intakecompressioncombustion, and exhaust. The strokes are repeated over and over, generating power. Let’s take a closer look at what happens during each phase of the combustion cycle.

Intake: During the intake cycle, the intake valve opens, and the piston moves down. This begins the cycle by bringing air and gas into the engine.

Compression: As the compression cycle begins, the piston moves up and pushes the air and gas into a smaller space. A smaller space means a more powerful explosion.

Combustion: Next, the spark plug creates a spark that ignites and explodes the gas. The power of the explosion forces the piston back down.

Exhaust: During the last part of the cycle, the exhaust valve opens to release waste gas created by the explosion. This gas is moved to the catalytic converter, where it is cleaned, and then through the muffler before it exits the vehicle through the tailpipe.

 

To increase the output of any engine more fuel can be burned and make bigger explosion in every cycle. One way to add power is to build a bigger engine. But bigger engine, which weigh more and cost more to build and maintain are not always better.

Another way to add power is to make a normal sized engine more efficient. This can be accomplish by forcing more air into the combustion chamber . More air means more fuel can be added and more fuel means a bigger
explosion and greater horsepower.

This can be done with the help of turbocharger and supercharger.

 

The function of supercharger and turbocharger in automobiles

 

The function of supercharger and turbocharger in automobiles

 

What is a Supercharger?

A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine. Supercharger allows more fuel to be burned and more work to be done per cycle, increasing the power output of the engine. Power for the unit can come mechanically by a belt, gear, shaft, or chain connected to the engine’s crankshaft.

superchargersupercharger

 

Purpose of supercharging
  1. To raise the density of the air charge, before it enters the cylinders.
  2. To raise engines power output.
  3. To increase the volumetric efficiency since the utilization of air is what going to determine the power output of the engine.
    Hence, an engine must be able to take in as much as air as possible.

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Points to be noted on supercharging an engine

1. Supercharging increase power output of an engine, doesn’t increase fuel consumption.
2. Engine should be designed to withstand the higher forces due to supercharging.
3. The increased pressure & temperature as a result of supercharging may lead to detonation.
4. Therefore, the fuel used must have a better anti-knock characteristics.
5. Certain % of power is consumed from engine itself in compressing the air, which leads to power loss.
6. However, it’s seen that the net power output will be more than the power out of an engine without supercharging.

Types of supercharger

There are two main types of superchargers defined according to the method of compression

i. Positive displacement
ii. Dynamic compressors

Positive displacement deliver a fairly constant level of pressure increase at all engine speeds (RPM), whereas the latter deliver increasing pressure with increasing engine speed.
Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for pressure by diffusing or slowing it down.

Supercharger advantages and disadvantages

Advantages

  • Increased horsepower: adding a supercharger to any engine is a quick solution to boosting power.
  • No lag: the supercharger’s biggest advantage over a turbocharger is that it does not have any lag. Power delivery is immediate because the supercharger is driven by the engine’s crankshaft.
  • Low RPM boost: good power at low RPM in comparison with turbochargers.
  • Price: cost effective way of increasing horsepower.

Disadvantages

  • Less efficient: the biggest disadvantage of superchargers is that they suck engine power simply to produce engine power. They’re run off an engine belt connected to the crankshaft, so you’re essentially powering an air pump with another air pump. Because of this, superchargers are significantly less efficient than turbochargers.
  • Reliability: with all forced induction systems (including turbochargers), the engine internals will be exposed to higher pressures and temperatures, which will of course affect the longevity of the engine. It’s best to build the engine from the bottom up to handle these pressures, rather than relying on stock internals.

 

What is a Turbocharger?

A turbocharger, or turbo is a centrifugal compressor powered by a turbine that is driven by an engine’s exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine (forced induction), thereby resulting in greater performance (for either, or both, power and efficiency). They are popularly used with internal combustion engines.

 

turbocharger

 

The turbocharger has three main components
1. A turbine, which is almost always a radial inflow turbine.
2. A compressor, which is almost always a centrifugal compressor.
3. The center housing/hub rotating assembly.

 

Objectives of turbocharger

The objective of a turbocharger, just as that of a supercharger, is to improve an engine’s volumetric efficiency by increasing the intake density .
The compressor draws in ambient air and compresses it before it enters into the intake manifold at increased pressure, that results in a greater mass of air entering the cylinders on each intake stroke.
The power needed to spin the centrifugal compressor is derived from the high pressure and temperature of the engine’s exhaust gases.
The turbine converts the engine exhaust’s potential pressure energy and kinetic velocity energy into rotational power, which is in turn used to drive the compressor.

Turbocharger advantages and disadvantages

Advantages

  • Significant increase in horsepower.
  • Power vs size: allows for smaller engine displacements to produce much more power relative to their size.
  • Better fuel economy: smaller engines use less fuel to idle, and have less rotational and reciprocating mass, which improves fuel economy.
  • Higher efficiency: turbochargers run off energy that is typically lost in naturally-aspirated and supercharged engines (exhaust gases), thus the recovery of this energy improves the overall efficiency of the engine.

Disadvantages

  • Turbo lag: turbochargers, especially large turbochargers, take time to spool up and provide useful boost.
  • Boost threshold: for traditional turbochargers, they are often sized for a certain RPM range where the exhaust gas flow is adequate to provide additional boost for the engine. They typically do not operate across as wide an RPM range as superchargers.
  • Power surge: in some turbocharger applications, especially with larger turbos, reaching the boost threshold can provide an almost instantaneous surge in power, which could compromise tyre traction or cause some instability of the car.
  • Oil requirement: turbochargers get very hot and often tap into the engine’s oil supply. This calls for additional plumbing, and is more demanding on the engine oil. Superchargers typically do not require engine oil lubrication.

Twincharger

Twincharger refers to a compound forced induction system used on some piston-type internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other. A belt-driven or shaft-driven supercharger offers exceptional response and low-rpm performance as it has no lag time between the application of throttle and pressurization of the manifold (assuming that it is a positive-displacement supercharger such as a Roots type or twin-screw and not a Centrifugal compressor supercharger, which does not provide boost until the engine has reached higher RPMs). When combined with a large turbocharger — if the “turbo” was used by itself, it would offer unacceptable lag and poor response in the low-rpm range — the proper combination of the two can offer a zero-lag powerband with high torque at lower engine speeds and increased power at the higher end. Twincharging is therefore desirable for small-displacement motors (such as VW’s 1.4TSI), especially those with a large operating rpm, since they can take advantage of an artificially broad torque band over a large speed range.

 

twincharger

 

Twincharging does not refer to a twin-turbo arrangement, but rather when two different kinds of compressors are used.

Technical description

A twincharging system combines a supercharger and turbocharger in a complementary arrangement, with the intent of one component’s advantage compensating for the other component’s disadvantage. There are two common types of twincharger systems: series and parallel.
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Series

The series arrangement, the more common arrangement of twinchargers, is set up such that one compressor’s (turbo or supercharger) output feeds the inlet of another. A sequentially-organized supercharger is connected to a medium- to large-sized turbocharger. The supercharger provides near-instant manifold pressure (eliminating turbo lag, which would otherwise result when the turbocharger is not up to its operating speed). Once the turbocharger has reached operating speed, the supercharger can either continue compounding the pressurized air to the turbocharger inlet (yielding elevated intake pressures), or it can be bypassed and/or mechanically decoupled from the drivetrain via an electromagnetic clutch and bypass valve (increasing efficiency of the induction system).

Other series configurations exist where no bypass system is employed and both compressors are in continuous duty. As a result, compounded boost is always produced as the pressure ratios of the two compressors are multiplied, not added. In other words, if a turbocharger which produced 10 psi (0.7 bar) (pressure ratio = 1.7) alone blew into a supercharger which also produced 10 psi alone, the resultant manifold pressure would be 27 psi (1.9 bar) (PR=2.8) rather than 20 psi (1.4 bar) (PR=2.3). This form of series twincharging allows for the production of boost pressures that would otherwise be unachievable with other compressor arrangements and would be inefficient.

However, the efficiencies of the turbo and supercharger are also multiplied, and since the efficiency of the supercharger is often much lower than that of large turbochargers, this can lead to extremely high manifold temperatures unless very powerful charge cooling is employed. For example, if a turbocharger with an efficiency of 70% blew into a Roots blower with an efficiency of 60%, the overall compression efficiency would be only 42% — at 2.8 pressure ratio as shown above and 20 °C (68 °F) ambient temperature, which means that air exiting the turbocharger would be 263 °C (505 °F), which is enough to melt most rubber couplers and nearly enough to melt expensive silicone couplers. A large turbocharger producing 27 psi (1.9 bar) by itself, with an adiabatic efficiency of around 70%, would produce air at just 166 °C (331 °F). Additionally, the energy cost to drive a supercharger is higher than that of a turbocharger; if it is bypassed, the load of performing compression is removed, leaving only slight parasitic losses from spinning the working parts of the supercharger. The supercharger can further be disconnected electrically (using an electromagnetic clutch such as those used on the VW 1.4TSI or Toyota’s 4A-GZE, although this is not because it is a twin charged engine; it is intended only to bypass the supercharger under low-load conditions) which eliminates this small parasitic loss.

With series twincharging, the turbocharger can be of a less expensive and more durable journal bearing variety, and the sacrifice in boost response is more than made up for by the instant-on nature of displacement superchargers. While the weight and cost of the supercharger assembly are always a factor, the inefficiency and power consumption of the supercharger are almost totally eliminated as the turbocharger reaches operating rpm and the supercharge is effectively disconnected by the bypass valve.

Parallel

Parallel arrangements typically require the use of a bypass or diverted valve to allow one or both compressors to feed the engine. If no valve were employed and both compressors were merely routed directly to the intake manifold, the supercharger would blow backwards through the turbocharger compressor rather than pressurize the intake manifold, as that would be the path of least resistance. Thus a diverter valve must be employed to vent turbocharger air until it has reached the pressure in the intake manifold. Complex or expensive electronic controls are usually necessary to ensure smooth power delivery.

Disadvantages

The main disadvantage of twincharging is the complexity and expense of components. Usually, to provide acceptable response, smoothness of power delivery, and adequate power gain over a single-compressor system, expensive electronic and/or mechanical controls must be used. In a spark-ignition engine, a low compression ratio must also be used if the supercharger produces high boost levels, negating some of the efficiency benefit of low displacement.

 

References
  1. http://www.autonews.com/article/20141014/BLOG06/141019954/
  2. http://auto.howstuffworks.com/question122.htm
  3. https://wonderopolis.org/wonder/how-does-an-engine-work
  4. http://www.autozine.org/technical_school/engine/Forced_Induction_5.html

 

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