Charging of engines: Various principles compared

The supercharging of the engine has a long tradition. And a great future. Because by the downsizing of the engines of the turbo is becoming increasingly important.

02/09/2011 Joachim Deleker

Turbo - big and snappy like a fighting dog were displayed these five letters in the seventies to cars like the BMW 2002 or Renault 5. Everything was brushed on attack, the driving behavior with Canyon-deep turbo lag and suddenly it starts to push extremely delicate, the drinking customs excessive. Long gone, long since the turbocharger from Krawallbruder has blossomed into a beacon of hope. Ever since is the magic word downsizing on everyone's lips is almost nothing without charging.

And who invented it? Exactly, a Swiss. As early as 1905 Alfred Büchi applied for a patent for its congestion charging. Nevertheless, it would take more than 80 years before the first TDI marked the turbo breakthrough. The gasoline DI took another two decades for his triumph, only with the combination of direct injection, turbocharging and variable valve timing of the philosopher's stone was found, a motor with a TDI-like character: strong departure from the low engine speed, low consumption and less displacement - all attributes of an exemplary downsizing concept.

More air means more power

The principle of charging is simple. The engine requires oxygen for combustion, which he obtains in the intake stroke itself. More air means more power, but the delivery rate is physically limited. Unless a compressor or turbocharger blowing more air into the cylinders. By the plus oxygen more fuel can be injected, combustion pressure and torque increase.  

The art of charging is to use the extra pressure over the greatest possible speed range and to circumnavigate the turbo lag. This is only possible due to the simple charger. Drivers of small and vierventiliger diesel engines are aware of the impatience in turbo lag until at about 2000 / min finally uses the awaited boost. Just this shortcoming eradicates from the VTG turbocharger, the vanes can be adjusted. So that the turbine cross-section is variable, and the engine gets even in the low engine speed enough boost pressure to develop rich boost. Disadvantage of the VGT-unit: it has to be regulated more complex, more expensive, and the vanes must be resistant to high temperatures - for diesel to 800 degrees, the petrol over 1,000.

The goal is a high and uniform level of speed

Not quite as good, but much cheaper is the twin-scroll turbo for gasoline engines. In this double-flow exhaust gases are blown loaders each of a pair of cylinders separated into the turbine. This avoids interactions between the gas column, reducing the exhaust back pressure and optimizes the charge in the engine.

The goal of the developers is yet to realize the highest possible and consistent level of speed from idle speed is easier to do with a twin turbo. When bi-turbo or twin turbo two small loaders replace a large. the pressure build-up succeeds the smaller the turbine, the faster due to the lower moment of inertia. Even better, the one sequential twin-turbo, in which the small turbine even at low speeds for printing and the larger charger is switched on until later. Ford and Peugeot use this solution in 2.2-liter diesel engine with Garrett-loader; BMW, Opel, VW and Mercedes rely on the similar R2S turbo BorgWarner.

Turbochargers are combined

The most expensive solution provides BorgWarner 740d for the six-cylinder in the BMW. It combines the two-stage turbocharging with a small VTG turbine that neatly built shortly on the state gas pressure. Jaguar, however, reverses the principle at the 3.0-liter V6 diesel to, uses the large VTG turbocharger for speeds up to 2,800 / min and then switches to the small loader to. The arguments: lower exhaust back pressure and less pumping losses.  

An expensive special path goes VW with the 160-hp twin charger engine. A Roots-compressor provides rich thrust from idle speed - an area in which weaken VTG turbos themselves. The Turbo takes speed increases continuously into the action and takes over beyond 3,500 / min the entire job.  

Turbo for Diesel has long been standard

Engines without charging will soon belong to a dying breed. Diesel engines with forced induction have long been standard. The future for the petrol engines is one of the direct injection with turbocharging - compressors are becoming increasingly rare - and camshaft adjustment. They promise the maximum potential for downsizing, low consumption and strong power delivery.