How a Turbo Works Details and Principles of Design – AET Turbos
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How a Turbo Works Details and Principles of Design

Posted by Andy Taylor on

How a turbo works is down to the principles of design which we will now explain in this detailled and efficient explanation:

A turbocharger consists of a turbine and a compressor linked by a shared axis. The turbine inlet receives exhaust gases from the engine exhaust manifold causing the turbine wheel to rotate. This rotation drives the compressor, compressing ambient air and delivering it to the air intake of the engine. 

The objective of a turbocharger is to improve upon the size-to-output efficiency of an engine by solving for one of its cardinal limitations. A naturally aspirated car engine uses only the downward stroke of a piston to create an area of low pressure in order to draw air into the cylinder. Since the number of air and fuel molecules determines the potential energy available to force the piston down on the combustion stroke, and because of the relatively constant pressure of the atmosphere, there ultimately will be a limit to the amount of air and consequently fuel filling the combustion chamber. This ability to fill the cylinder with air is its volumetric efficiency. Since the turbocharger increases the pressure at the point where air is entering the cylinder, and the amount of air brought into the cylinder is largely a function of time and pressure, more air will be drawn in as the pressure increases. The intake pressure, in the absence of the turbocharger determined by the atmosphere, can be controllably increased with the turbocharger.

The application of a compressor to increase pressure at the point of cylinder air intake is often referred to as forced induction. Centrifugal superchargers operate in the same fashion as a turbo; however, the energy to spin the compressor is taken from the rotating output energy of the engine’s crankshaft as opposed to exhaust gas. For this reason turbochargers are ideally more efficient, since their turbines are actually heat engines, converting some of the kinetic energy from the exhaust gas that would otherwise be wasted, into useful work. Superchargers use output energy to achieve a net gain, which is at the expense of some of the engine’s total output.

Components

The turbocharger has four main components. The turbine and impeller wheels are each contained within their own folded conical housing on opposite sides of the third component, the centre hub rotating assembly (CHRA).

The housings fitted around the compressor impeller and turbine collect and direct the gas flow through the wheels as they spin. The size and shape can dictate some performance characteristics of the overall turbocharger. The area of the cone to radius from centre hub is expressed as a ratio (AR, A/R, or A:R). Often the same basic turbocharger assembly will be available from the manufacturer with multiple AR choices for the turbine housing and sometimes the compressor cover as well. This allows the designer of the engine system to tailor the compromises between performance, response, and efficiency to application or preference. Both housings resemble snail shells, and thus turbochargers are sometimes referred to in slang as angry snails.

The turbine and impeller wheel sizes also dictate the amount of air or exhaust that can be flowed through the system, and the relative efficiency at which they operate. Generally, the larger the turbine wheel and compressor wheel, the larger the flow capacity. Measurements and shapes can vary, as well as curvature and number of blades on the wheels.

The centre hub rotating assembly houses the shaft that connects the compressor impeller and turbine. It also must contain a bearing system to suspend the shaft, allowing it to rotate at very high speed with minimal friction. For instance, in automotive applications the CHRA typically uses a thrust bearing or ball bearing lubricated by a constant supply of pressurized engine oil. The CHRA may also be considered “water cooled” by having an entry and exit point for engine coolant to be cycled. Water-cooled models allow engine coolant to be used to keep the lubricating oil cooler, avoiding possible oil coking from the extreme heat found in the turbine.

Boost

Boost refers to the increase in manifold pressure that is generated by the turbocharger in the intake path or specifically intake manifold that exceeds normal atmospheric pressure. This is also the level of boost as shown on a pressure gauge, usually in bar, psi or possibly kPa. This is representative of the extra air pressure that is achieved over what would be achieved without the forced induction. Manifold pressure should not be confused with the amount, or “weight” of air that a turbo can flow.

Boost pressure is limited to keep the entire engine system including the turbo inside its design operating range by controlling the wastegate, which shunts the exhaust gases away from the exhaust side turbine. In some cars the maximum boost depends on the fuel’s octane rating and is electronically regulated using a knock sensor, see Automatic Performance Control (APC).

Many diesel engines do not have any wastegate because the amount of exhaust energy is controlled directly by the amount of fuel injected into the engine and slight variations in boost pressure do not make a difference for the engine.

Wastegate

By spinning at a relatively high speed the compressor turbine draws in a large volume of air and forces it into the engine. As the turbocharger’s output flow volume exceeds the engine’s volumetric flow, air pressure in the intake system begins to build, often called boost. The speed at which the assembly spins is proportional to the pressure of the compressed air and total mass of airflow being moved. Since a turbo can spin to RPMs far beyond what is needed, or of what it is safely capable of, the speed must be controlled. A wastegate is the most common mechanical speed control system, and is often further augmented by an electronic boost controller. The main function of a wastegate is to allow some of the exhaust to bypass the turbine when the set intake pressure is achieved.

Fuel efficiency

Since a turbocharger increases the specific horsepower output of an engine, the engine will also produce increased amounts of waste heat. This can sometimes be a problem when fitting a turbocharger to a car that was not designed to cope with high heat loads. This extra waste heat combined with the lower compression ratio (more specifically, expansion ratio) of turbocharged engines contributes to slightly lower thermal efficiency, which has a small but direct impact on overall fuel efficiency.

It is another form of cooling that has the largest impact on fuel efficiency: charge cooling. Even with the benefits of intercooling, the total compression in the combustion chamber is greater than that in a naturally aspirated engine. To avoid knock while still extracting maximum power from the engine, it is common practice to introduce extra fuel into the charge for the sole purpose of cooling. While this seems counterintuitive, this fuel is not burned. Instead, it absorbs and carries away heat when it changes phase from liquid mist to gas vapour. Also, because it is denser than the other inert substance in the combustion chamber, nitrogen, it has a higher specific heat and more heat capacitance. It “holds” this heat until it is released in the exhaust stream, preventing destructive knock. This thermodynamic property allows manufacturers to achieve good power output with common pump fuel at the expense of fuel economy and emissions. The optimum Air-to-Fuel ratio (A/F) for complete combustion of petrol is 14.7:1. A common A/F in a turbocharged engine while under full design boost is approximately 12:1. Richer mixtures are sometimes run when the design of the system has flaws in it such as a catalytic converter which has limited endurance of high exhaust temperatures or the engine has a compression ratio that is too high for efficient operation with the fuel given.

Lastly, the efficiency of the turbocharger itself can have an impact on fuel efficiency. Using a small turbocharger will give quick response and low lag at low to mid RPMs, but can choke the engine on the exhaust side and generate huge amounts of pumping-related heat on the intake side as RPMs rise. A large turbocharger will be very efficient at high RPMs, but is not a realistic application for a street driven car. Variable vane and ball bearing technologies can make a turbo more efficient across a wider operating range, however, other problems have prevented this technology from appearing in more road cars (see Variable geometry turbocharger). Currently, the Porsche 911 (997) Turbo is the only petrol car in production with this kind of turbocharger. One way to take advantage of the different operating regimes of the two types of turbocharger is sequential turbocharging, which uses a small turbocharger at low RPMs and a larger one at high RPMs.

The engine management systems of most modern vehicles can control boost and fuel delivery according to charge temperature, fuel quality, and altitude, among other factors. Some systems are more sophisticated and aim to deliver fuel even more precisely based on combustion quality. For example, the Trionic-7 system from Saab provides immediate feedback on the combustion while it is occurring using an electrical charge.

The new 2.0L FSI turbo engine from Volkswagen/Audi incorporates lean burn and direct injection technology to conserve fuel under low load conditions. It is a very complex system that involves many moving parts and sensors in order to manage airflow characteristics inside the chamber itself, allowing it to use a stratified charge with excellent atomization. The direct injection also has a tremendous charge cooling effect, which enables engines to use higher compression ratios and boost pressures than a typical port-injection turbo engine.

Automotive design details

The ideal gas law states that when all other variables are held constant, if pressure is increased in a system so is the temperature. Here exists one of the negative consequences of turbocharging, the increase in the temperature of air entering the engine due to compression.

A turbo spins very fast; most peak between 80,000 and 200,000 RPM (using low inertia turbos, 150,000-250,000 RPM) depending on size, weight of the rotating parts, boost pressure developed and compressor design. Such high rotation speeds would cause problems for standard ball bearings leading to failure so most turbo-chargers use fluid bearings. These feature a flowing layer of oil that suspends and cools the moving parts. The oil is usually taken from the engine-oil circuit. Some turbochargers use incredibly precise ball bearings that offer less friction than a fluid bearing but these are also suspended in fluid-dampened cavities. Lower friction means the turbo shaft can be made of lighter materials, reducing so-called turbo lag or boost lag. Some carmakers use water-cooled turbochargers for added bearing life. This can also account for why many tuners upgrade their standard journal bearing turbos (such as a T25) which use a 270 degree thrust bearing and a brass journal bearing which only has 3 oil passages, to a 360 degree bearing which has a beefier thrust bearing and washer having 6 oil passages to enable better flow, response and cooling efficiency. Turbochargers with foil bearings are in development. These will eliminate the need for bearing cooling or oil delivery systems, thereby eliminating the most common cause of failure, while also significantly reducing turbo lag.

To manage the upper-deck air pressure, the turbocharger’s exhaust gas flow is regulated with a wastegate that bypasses excess exhaust gas entering the turbocharger’s turbine. This regulates the rotational speed of the turbine and the output of the compressor. The wastegate is opened and closed by the compressed air from turbo (the upper-deck pressure) and can be raised by using a solenoid to regulate the pressure fed to the wastegate membrane. This solenoid can be controlled by Automatic Performance Control, the engine’s electronic control unit or an after market boost control computer. Another method of raising the boost pressure is through the use of check and bleed valves to keep the pressure at the membrane lower than the pressure within the system. Some turbochargers (normally called variable geometry turbochargers) utilise a set of vanes in the exhaust housing to maintain a constant gas velocity across the turbine, the same kind of control as used on power plant turbines. These turbochargers have minimal amount of lag, have a low boost threshold (with full boost as low as 1,500 rpm), and are efficient at higher engine speeds; they are also used in diesel engines. [2] In many setups these turbos don’t even need a wastegate. A membrane identical to the one on a wastegate controls the vanes but the level of control required is a bit different.

The first production car to use these turbos was the limited-production 1989 Shelby CSX-VNT, in essence a Dodge Shadow equipped with a 2.2L petrol engine. The Shelby CSX-VNT utilised a turbo from Garrett, called the VNT-25 because it uses the same compressor and shaft as the more common Garrett T-25. This type of turbine is called a Variable Nozzle Turbine (VNT). Turbocharger manufacturer Aerocharger uses the term ‘Variable Area Turbine Nozzle’ (VATN) to describe this type of turbine nozzle. Other common terms include Variable Turbine Geometry (VTG), Variable Geometry Turbo (VGT) and Variable Vane Turbine (VVT). A number of other Chrysler Corporation vehicles used this turbocharger in 1990, including the Dodge Daytona and Dodge Shadow. These engines produced 174 horsepower and 225 pound-feet of torque, the same horsepower as the standard intercooled 2.2 litre engines but with 25 more pound-feet of torque and a faster onset (less turbo lag). However, the Turbo III engine, without a VATN or VNT, produced 224 horsepower. The reasons for Chrysler’s not continuing to use variable geometry turbochargers are unknown, but the main reason was probably public desire for V6 engines coupled with increased availability of Chrysler-engineered V6 engines. [3] The 2006 Porsche 911 Turbo has a twin turbocharged 3.6-litre flat six, and the turbos used are BorgWarner’s Variable Geometry Turbos (VGTs). This is significant because although VGTs have been used on advanced diesel engines for a few years and on the Shelby CSX-VNT, this is the first time the technology has been implemented on a production petrol car since the 1,250 Dodge engines were produced in 1989-90. Some have argued this is because in petrol cars exhaust temperatures are much higher (than in diesel cars), and this can have adverse effects on the delicate, moveable vanes of the turbocharger; these units are also more expensive than conventional turbochargers. Porsche engineers claim to have managed this problem with the new 911 Turbo.

There is also a type of turbo called centrifugal (or simply belt-driven), this functions in some ways similar to a standard turbo and in some ways similar to a supercharger. Since it’s belt driven (no exhaust is used) there is never any lag, however the boost isn’t “free” like with a standard turbo. The “cost” is extra drag on the crank, thus a loss in efficiency. The benefits are no lag, easier to setup – since no exhaust modifications are needed, and likely easier maintenance access.


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