Re: Грижи за Корадо / Техника / Автомобили / VW

Eto .. murzi me da prevezhdam

Turbo:
The advantage of turbocharging is obvious - instead of wasting thermal energy through exhaust, we can make use of such energy to increase engine power. By directing exhaust gas to rotate a turbine, which drives another turbine to pump fresh air into the combustion chambers at a pressure higher than normal atmosphere, a small capacity engine can deliver power comparable with much bigger opponents. For example, if a 2.0-litre turbocharged engine works at 1.5 bar boost pressure, it actually equals to a 3.0-litre naturally aspirated engine. As a result, engine size and weight can be much reduced, thus leads to better acceleration, handling and braking, though fuel consumption is not necessarily better.

LPT (Light Pressure Turbo)
Light pressure turbocharging is one of the most popular power boosting technology in recent years. Saab, the pioneer of turbo in saloons, is the first car maker put it into mass production. In 1992, it surprised many by introducing the Saab 9000 2.3 turbo Ecopower. The engine had only 170 hp, that is, just 20 hp more than the normally aspirated version and 30 hp below the standard 2.3 turbo. Basically, it was just the standard engine with a smaller turbo and lighter boost pressure.

While other car makers were still pursuing "on paper" peak power, Saab's clever engineers realised that less equals to more. Despite of lower peak power, light turbo engine remains to be strong in torque, thus aids acceleration. Most important, it has very much better drivability due to the inexistence of turbo lag. Throttle response is nearly instant. Besides, Saab proved that the better torque curve enables taller gearing, thus actually delivering better fuel economy that a normally aspirated engine of the same size !

In the past, poor drivability and fuel consumption prevent turbocharging from adopting in main stream sedans. Now the trend is reversed - due to the increasing requirement of safety and comfort, modern cars are growing every year. Heavier weight asks for more power. For many four-cylinder sedans, they have 2 choices: either upgrade to six-cylinder or add a light pressure turbo. Of course the latter is more cost effective. It need no more space, adds little manufacturing cost, and burns less fuel than a 6-pot engine, therefore many other car makers also adopted it.

Advantage: Improve torque without adding much cost; furgal
Disadvantage: Nil

Variable Turbine Geometry technology is mostly used in turbo diesel engines, but there is no evidence that it could not benefit petrol engine. It is said that (don't ask me why): turbine makes best use of exhaust gas flow if the latter hit the blades at right angle under low speed, and at narrow angle under high speed. Variable Turbine Geometry mechanism therefore varies the direction of the exhaust nozzle according to speed, thus improve the acceleration of turbine.

Another Variable Turbine Geometry alters the cross-sectional area through which the exhaust gas flows, thus controls the amount of boost pressure. This is implemented by adjusting the position of guide vanes inside the turbocharger. At lower engine speeds, they restrict the flow and therefore increase boost pressure; at higher engine speeds they open wide and reduce the exhaust back-pressure.

Advantage: Improve turbine response without altering maximum boost pressure
Disadvantage: Nil

Twin-Turbo: Parallel or Sequential ?
The use of twin-turbocharger is a question of both efficiency and packaging. For larger engines, say, 2500 c.c. or above, it is better to use 2 smaller turbochargers instead of a big one, as small turbines reduce turbo lag. Today, performance cars no longer employ a large single turbo like the early 911 Turbo.

For V-shape and boxer engines, it is also recommended to use twin-turbo, because one turbo serves each bank shorten the turbo pipes and save a lot of space. Moreover, the shorter the pipes, the less turbo lag generates.

Some twin-turbo engines have the turbos arranged such that exhaust flow from one bank of cylinders drives a turbo which boost the intake of another bank. This is actually the concept of "feedback loop", which helps reaching power balance between two banks.

Most twin-turbo engines have the turbochargers arranged to operate independently, each serves one bank of cylinders. This is so-called "Parallel Twin-Turbo". An alternative arrangement, "Sequential Twin-Turbo", was designed to improve response and further reduce turbo lag. The turbos operate sequentially, that is, at low speed, all the limited amount of exhaust gas is directed to drive one of the small turbines, leaving another idle. Therefore the first turbine will accelerate quickly. When the exhaust flow reaches sufficient amount to drive both turbos, the second turbo intervenes and helps reaching the maximum boost pressure. Unfortunately, sequential twin-turbo requires very complicated connection of pipes (exhaust from both banks should reach both turbos; so do the intake pipes from both banks), thus is now losing interest from car makers. Porsche 959, Mazda 3rd generation RX7, Toyota Supra and Subaru Legacy are the only applicants as I know.

Supercharger = Kompressor

GM is one of the keen customers of supercharger. Most of its mid / full size sedans, such as the Pontiac Grand Prix GPX shown in here, have a 3.8 litres supercharged V6 to choose.
Before turbocharging arrived in the 60s, supercharging used to dominate the forced induction world. Supercharging, also called mechanical charging, appeared in around early 20s in Grand Prix racing cars in order to increase power. Since the compressor is driven directly by the engine crankshaft, it has the advantage of instant response (no lag). But the charger itself is rather heavy and energy inefficient, thus cannot produce as much power as turbocharger. Especially at high rev, it generates a lot of friction thus energy loss and prevent the engine from revving high.

A typical supercharger transforms the engine very much - very torquey at low and mid range rpm, but red line and peak power appear much earlier. That means the engine becomes lazy to rev (and to thrill you), but at any time you have a lot of torque to access, without needing to change gears frequently. For these reasons, supercharging is quite well suited to nowadays heavy sedans, espeically those mated with automatic transmission. On the other hand, sports cars rarely use it.

The noise, friction and vibration generated by supercharger are the main reasons prevent it from using in highly refined luxurious cars. Although Mercedes-Benz has introduced a couple of supercharged four into the C-class, they are regarded as too unrefined compare with the V6 serving other versions.

The introduction of light-pressure turbochargers also threathen the survival of supercharger. Volkswagen group, for example, dropped its long-standing G-supercharger and chose light-pressure turbo. Now supercharger is completely disappeared in budget cars, leaving just a few GT or sports sedans which pursue high torque without much additional to employ it. General Motors is perhaps the only real supporter to supercharger. It offers a 3.8-litre supercharged V6 for most of its budget mid to full-size sedans.

Advantage: Torquey and cheap
Disadvantage: Lack top end power, ruin revability, unrefined noise and vibration.

WWell BE COOL