The performance of an internal combustion engine (ICE) can be described through its output torque. Low speed engine torque has significant impact on the vehicle driveability and high speed engine torque determines the maximum vehicle speed and the arrangement of the gear ratios.
Engine torque can be increased by several methods:
Turbocharging is the right method to increase the intake air density. It requires additional work on the air intake side, on top of the pumping work of a atmospheric (naturally aspirated) engine, in order to force extra air mass in the cylinders. This additional work is supplied by the turbocharger, where a turbine uses the energy of the exhaust gases to spin an intake air compressor (impeller).
Historically, turbochargers were first fitted to compression ignition (diesel) engines, mainly for these reasons:
- the specific power output of a naturally aspired diesel engine is poor
- the power output of a diesel engine is limited by smoke emissions and more air mass added in the cylinder can reduce the smoke formation
- (compared to a spark-ignition, gasoline engine) diesel engine knock is not possible because the fuel is injected at the end of the compression cycle
- (compared to a spark-ignition, gasoline engine) diesel engines are more expensive to manufacture, therefore the cost of a turbocharger has a lower impact on the overall cost of the engine
On a spark-ignition (gasoline) engine, the main reason for fitting a turbocharger is to boost the output torque/power from a restricted (volumetric) engine capacity. The main limit of a turbocharged gasoline engine, in terms of how high the boost pressure can rise, is engine knock. The additional boost air within the cylinders causes the temperature of the air-fuel mixture to increase significantly at the end of the combustion, which can lead to engine knock. To prevent knocking, turbocharged engines usually have lower compression ratios than naturally aspirated (atmospheric) engines.
Turbocharging can be summarized as a specific method of supercharging, where the energy of the hot exhaust gases is used to drive the intake air compressor. The advantage is that, instead of being wasted, the energy of the exhaust gases is used to power up the compressor.
By putting the turbine in the exhaust manifold, the exhaust gas pressure builds up before (upstream) the turbine. This forces the engine to consume more energy to expel burnt gases from the cylinders during the exhaust stroke. The turbine converts both the flow and thermal energy of the exhaust gas into compression energy. Therefore, the air intake pressure build up is greater than the rise of the exhaust gas pressure, which means that the overall engine efficiency is increased.
Automotive (pneumatic, fixed geometry) turbochargers are made up from four main parts:
- compressor housing
- core (central) housing
- turbine housing
- wastegate actuator
The compressor housing (usually made from aluminium) contains an axial-inflow, radial-outflow compressor (also known as impeller). The turbine housing contains a radial-inflow, axial-outflow turbine, connected with the compressor through a shaft.
The speed of the turbine-compressor assembly can easily reach 120 000 rpm or even 300 000 rpm. To withstand such high speeds, the shaft rotates in hydrodynamic oil film low friction plain bearings, which are hosted in the core (central) housing.
There are two types of plain bearings: radial and axial. There are usually two radial bearings (bush) and one axial (thrust) bearing. The bearings have lubrication channels which allows the oil to reach the interior of the bearings and form a hydrodynamic oil film between the bearing and the shaft. These kind of bearings are also called fully floating bearings. The lubrication circuit of the turbocharger is connected to the main lubrication system of the internal combustion engine.
The oil temperature can vary widely, between a minimum temperature (e.g. -30 °C) and nominal engine operating temperature (which is around 90 °C). In order to ensure oil flow for cooling, in any temperature condition, a clearance between bearings and shaft must be ensured.
- compressor wheel
- axial (thrust) bearing
- radial (bushing) bearings
- turbine wheel
The turbocharger bearings can be plain (as in the picture above) or roller-bearings. Exhaust-gas turbochargers with roller-bearings are more efficient than plain bearings, have a better transient performance (they accelerate faster) and can provide higher boost pressure at partial engine loads. The main disadvantages of roller-bearings are long-life reliability and acoustic performance (more noisy). Roller-bearings are mainly used in motor-sport high performance turbochargers.
The bearings can operate properly if exhaust gas temperatures is below 800 °C, oil cooling being sufficient to maintain nominal operation. On gasoline engines, where the exhaust gas temperature can exceed 1000 °C, water-cooled central (bearing) housing are necessary.
The core housing contains also some sealing elements, which prevent oil from spilling into the exhaust or intake manifold and also reduce as much as possible the entry of intake air and exhaust gases (blow-by gasses).
The compressor assembly consists of an axial-inflow, radial-outflow impeller (compressor wheel) and an aluminium cast housing. In order to avoid any air leaks between the compressor and housing, the gap must be kept to a minimum value.
The compressor wheel (impeller) is usually made up from cast aluminium alloy. Modern turbochargers have the impeller milled from an aluminium forging alloy. To avoid compressor surge (air flow reversal in case of throttle closing), the compressor housing is equipped with a blow-off (pop-off) valve.
Some of the commercial vehicles, with very long service life requirements for the components, have the compressor wheel (impeller) milled from a titanium alloy.
Turbocharged gasoline engine compressors have blow-off (pop-off) valves, which have to prevent compressor surge when the engine load drops sharply (e.g. throttle valve goes from fully open to fully closed in a very short time). Most of the modern blow-off valves are electrically actuated, the opening and closing events being controlled by the Powertrain Control Module (PCM).
The turbine side of the turbocharger consists of:
The purpose of the diffuser is accelerate the flow of the exhaust gas and distribute it evenly in the turbine blades (wheel). The diffuser is integrated in the spiral housing of the turbine.
The housing of the turbine has to withstand very high temperature and it’s made from high-alloy cast iron. There are two types of turbine housing, function of the type of exhaust gas pressure build-up:
- pulse supercharging housing
- constant-pressure housing
In the case of pulse supercharging, the exhaust gas pipes coming from each cylinder are routed separately into the turbine housing. The turbine housing is designed is such a way that prevent the mixing of the exhaust gas streams as much as possible before entering the turbine wheel.
In the case of constant-pressure supercharging, the exhaust pipes of all cylinders are connected to a high volume exhaust manifold, which filters out the individual pressure pulses.
The standard turbine wheel has radial-inflow and axial-outflow design. Since it has to operate in very high temperature environment, the turbine wheel is manufactured from steel alloy containing high quantities of nickel.
In order to minimize the turbo-lag (delay in engine acceleration) the mass moment of inertia of the compressor wheel, turbine wheel and shaft should be as low as possible. For this reason, high strength, low density materials are being researched to be introduced in future turbocharger applications.
- compressor housing
- compressor wheel (impeller)
- pneumatic actuator
- central (bearing) housing
- wastegate control arm
- turbine housing
- turbine wheel
The boost pressure is controlled by regulating the amount of exhaust gas flowing through the turbine wheel. The exhaust gas flow in the turbine is controlled by the wastegate, which can be actuated by a pneumatic unit or an electric actuator.
The air supply for the wastegate pneumatic unit control can be supplied by the boost pressure itself or by a vacuum pressure (from the vehicle’s vacuum pump). The disadvantage of using the boost pressure is that the control of the wastegate depends on the engine load (boost pressure). By using the vacuum pump, the boost pressure can be controlled independently of the engine operating state.
Latest turbocharger technologies have direct electrical actuation of the wastegate. This brings faster and more precise actuation of the wastegate, independently of the engine operating state.
High performance turbocharges – EFRTM from BorgWarner
- forged milled compressor wheel
- Gamma-Ti turbine wheel and shaft
- stainless steel turbine housing
- high flow wastegate
- turbine back-disk
- dual-row ball bearing cartridge with ceramic balls and metal cage
- compressor housing
- integrated compressor recirculation valve (CVR)
- boost control solenoid valve (BCSV)
- speed sensor
Continental’s RAAXTM turbocharger
RAAXTM (which stands for “radial-axial”) is the new Continental turbocharger with the most important innovation of the turbine design. Unlike the most common type of gasoline turbocharger today, the radial turbocharger, which features a radial exhaust gas inlet, the new Continental turbocharger has a radial-axial (semi-radial/semi-axial) inlet path.
The associated special blade design allows a substantial reduction of approximately 40 % in the rotational moment of inertia of the turbine wheels. This means the turbocharger responds faster to engine load changes, so boost pressure is developed more quickly and turbo lag is minimized. In addition to this significant improvement in response, RAAXTM technology also results in up to 3 % greater efficiency in the engine relevant operating range, leading to reduced emissions.
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