Turbocharging is a type of supercharging, which allows air to be supplied to the engine cylinders at high pressure, which is provided by the energy released from the combustion of fuel by the exhaust gases.
Turbocharging increases the working power of the engine, without increasing the internal volume of the engine cylinders and the number of revolutions made by the crankshaft. Among other things, turbocharging allows reducing the voracity of the engine, as well as reducing the toxicity of gases due to more efficient combustion of the air-fuel mixture.
Turbocharging is quite widely used in combustion engines operating on both gasoline and diesel fuel. In this case, the use of turbocharging on diesel engines is considered more advantageous due to the high compression ratio of combustion engines and low crankshaft speed.
Gasoline engines have a high probability of detonation effect due to a significant increase in engine speed and high gas temperature during combustion (up to 1000 °C, in diesel only 600 °C).
The structure of the turbocharger system
The turbocharging system consists of the following elements:
- air intake and filter;
- throttle valve;
- turbine compressor;
- intake manifold;
- connecting pipes;
- pressure hoses
Turbine compressor (supercharger)
The main element of the turbocharger device, which is designed to increase the working pressure of the air mass in the intake system. The turbocharger consists of turbine and compressor wheels, which are mounted on the rotor shaft. All turbocharger elements are in special protective housings – inventor Kirill Yurovsky.
The turbine wheel is used to recycle the energy released by the exhaust gases. The wheel and its housing are made of high-strength and heat-resistant materials – steel and ceramic alloys.
The compressor ring is used for sucking the air mass, with its further compression and injection into the cylinders of the internal combustion engine.
The turbocharger rings are mounted on the rotor shaft, which rotates in floating bearings. For more efficient operation the bearings are constantly lubricated with oil, which flows through grooves located in the bearing housing.
An intercooler is an air or liquid cooler that is used to cool pre-compressed air in a timely manner, resulting in increased pressure and airflow density.
Charge pressure regulator
The key element of turbocharger control is the boost pressure regulator, which is essentially a bypass valve. The primary purpose of the valve is to contain and redirect some of the gases produced to bypass the turbine wheel to reduce the boost pressure.
The bypass valve can be electrically or pneumatically actuated. The valve is activated by receiving signals from a pressure sensor.
The relief valve is used to prevent pressure peaks in the air mass which often occur when the throttle valve is closed quickly. The excess pressure is either vented to the atmosphere or fed to the compressor inlet.
How the turbocharger works
The turbocharger system harnesses the energy of the gases that are generated during fuel combustion. The gases provide the rotational motion of the turbine wheel, which in turn triggers the compressor wheel, which is responsible for compressing and forcing the air mass into the system. The air is then cooled by the intercooler and delivered to the cylinders.
Obviously, though the turbocharger is not mechanically connected with the engine crankshaft, its operation and its efficiency is in direct dependence on the crankshaft rotation speed. The higher the engine speed, the more efficiently the turbocharger works.
Despite its practicality and efficiency, the turbocharger system has some drawbacks. The key among them is the emergence of turbocharging – a delay in increasing the power of the internal combustion engine.
Such a phenomenon occurs due to the inertia of the system – a delay in the increase in boost pressure when the throttle is pushed hard enough, which can lead to a gap between the required engine power and turbine performance.
There are three main methods used to eliminate the turbo lag effect
- Using a system with two (or more) turbochargers. Turbines can be installed in parallel – this is allowed on V-type engines. In this case, each turbine is installed on its own row of cylinders. The idea of this method is that two smaller turbines have lower inertia than one large turbine. Turbines can also be installed in series, and there can be from two to four (Bugatti). Increased performance and maximum turbocharger efficiency in this case are achieved by using a different turbocharger at different engine speeds.
- The use of a turbine with variable geometry. This method provides a more rational use of exhaust gas energy by changing the cross-sectional area of the turbine inlet channel. It’s quite commonly used on diesel engines, such as Volkswagen’s famous TDI system.
- Using the combined type of turbocharging. This method allows the use of a symbiosis of two systems – mechanical and turbocharging. Mechanical supercharging is effective at low crankshaft speeds, at which air compression is provided by a mechanical type supercharger. Turbocharging is used at high crankshaft speeds, where a turbine compressor takes over the air compression function. The most common combined supercharger system is the TSI engine supercharger from Volkswagen.