Turbo Charger

The turbocharger has become an integral component for modern internal combustion engines. Almost all directly injecting diesel engines of the passenger car up to the ship diesel are charged. Due to its early starting and limitation in the boost pressure an old dream of driving becomes true. To be able to powerfully accelerate from relatively low revs. In the meantime, the turbocharger starts to conquer a larger market share than presently 10% of the petrol engines. The turbocharger could either compete against the compressor and probably win, or establish a combination. Also the variable compression seems obsolete, because, in the meantime, the boost pressure is regulated electronically.

The turbocharger consists of the turbine (to the right) and compressor (to the left) on one common shaft. The shaft bearing manages far more than 100,000 1/min provided it is connected to the oil cycle of the engine. Sometimes there is a coolant connection. The turbine is driven by the exhaust gas of the engine. The compressor provides for a pre-compression of the fresh air. It should generate possibly already for low revs a high boost pressure. This must be controlled by a bypass valve (waste gate). It releases from a certain boost pressure onwards the direct way to the exhaust system, avoiding the turbine wheel. This valve can be headed pneumatically by the pressure in the inlet manifold or electrically.

Because of the lower exhaust gas temperature (diesel engine) the blades of the compressor wheel are adjustable. Therefore, the valves are sometimes optional. To avoid, e.g., while closing the throttle valve and/or too strong a deceleration of the compressor wheel, a bypass valve can also be inserted on the compressor side.

The number of engines with more than one turbocharger increases. These are mostly arranged parallel, however, can be arranged also in series. Diesel engines (particularly in utility vehicles) operate with higher pressures of maximum 1.5 to 2.5 bar, standard petrol cars rather below 1 bar. In series, pressures of 6 - 7 bar are (however, not in standard cars!) possible. High temperatures while compressing are reduced by an air-to-air heat exchanger (intercooler.) In the second picture you can see how the exhaust gas pressure is steered onto the blades of the turbine wheel. The fresh gases are accelerated from the middle and pressed through a narrow gap outwardly.

The maximum pressure is determined for the diesel engine primarily by the load of the engine, for the petrol engine, however, it is determined by the antiknock properties of the fuel. Because there is (2006) still no standard engine whose compression ratio can be adapted mechanically to the operating conditions, the petrol engine operates with solidly fixed waste gate, causing too low a boost pressure and thus lower efficiency. This can be absorbed by ignition characteristic diagrams only insufficiently, yet much better with an electronic boost pressure control. This allows substantially higher compression ratios. In this characteristic, in any case, a modern petrol turbocharged engine can hardly be distinguished from a normally aspired engine what becomes apparent positively in terms of fuel consumption. In combination with the stratified charge operation of a petrol direct injection and the variable valve lift still more is possible.

An essential disadvantage accompanies the turbocharged engine from the outset. It differs, e.g., from the compressor due to its lagging response time. Although a diesel engine with turbocharger is quite powerful in the low revs, you must pay attention with the initial drive, otherwise the engine shuts off. This is not typical, actually, for a diesel engine. Among its former, not charged congeners this was almost impossible what made them especially suitable as vehicles for driving schools. It takes a moment, until the charger has equalised the revs according to the accelerator pedal position. The old turbo lag remained partly. An additional electrically powered turbocharger could maybe help, opening up the bucket wheels in neutral with open boost pressure valve.