A Site About Fast, Expensive Exotic Cars

200 MILES PER HOUR - SHOWCASING STREET LEGAL RACING MACHINES: Forced Induction The Real Story

Forced Induction The Real Story

Forced Induction

    Turbocharging

    Overview

    Basic TheoryThe 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.
    Problems - Turbo Lag
    Turbocharging was first introduced to production car by GM in the early 60s, using in Chevrolet Corvair. This car had very bad reputation about poor low-speed output and excessive turbo lag which made fluent driving impossible.
    Turbo Lag was really the biggest problem preventing the early turbo cars from being accepted as practical. Although turbocharging had been extensively and successfully used in motor racing - started from BMW 2002 turbo and then spread to endurance racing and eventually Formula One - road cars always require a more user-friendly power delivery. Contemporary turbines were large and heavy, thus could not start spinning until about 3,500 rpm crank speed. As a result, low-speed output remained weak. Besides, since the contemporary turbocharging required compression ratio to be decreased to about 6.5:1 in order to avoid overheat to cylinder head, the pre-charged output was even weaker than a normally-aspirated engine of the same capacity !
    Turbo lag can cause trouble in daily driving. Before the turbo intervenes, the car performs like an ordinary sedan. Open full throttle and raise the engine speed, counting from 1, 2, 3, 4 .... suddenly the power surge at 3,500 rpm and the car becomes a wild beast. On wet surfaces or tight bends this might result in wheel spin or even lost of control. In the presence of turbo lag, it is very difficult to drive a car fluently.
    Besides, turbo lag ruins the refinement of a car very much. Floor the throttle cannot result in instant power rise expected by the driver - all reactions appear several seconds later, no matter acceleration or releasing throttle. You can imagine how difficult to drive fast in city or twisted roads.
    Porsche’s solution to turbo lag
     
    The first “practical” turbocharged road car eventually appeared in 1975, that’s the Porsche 911 Turbo 3.0. To reduce turbo lag, Porsche engineers designed a mechanism allowing the turbine to "pre-spin" before boosting. The secret was a recirculating pipe and valve: before the exhaust gas attains enough pressure for driving the turbine, a recirculating path is established between the fresh-air-charging turbine's inlet and outlet, thus the turbine can spin freely without slow down by boost pressure. When the exhaust gas becomes sufficient to turbocharge, a valve will close the recirculating path, then the already-spinning turbine will be able to charge fresh air into the engine quickly. Therefore turbo lag is greatly reduced while power transition becomes smoother. 
     
    IntercoolerThe 3.3-litre version 911 Turbo superseded the Turbo 3.0 in 1978. It introduced an intercooler at between the compressor and the engine. It reduced the air temperature for 50-60°C, thus not only improved the volumetric efficiency (in other words, the intake air became of higher density) but also allowed the compression ratio to be raised without worrying over heat to cylinder head. Of course, higher compression led to improved low-speed output.
    Continuous development
    During the 80s, turbocharging continued to evolve for better road manner. As the material and production technology improved, turbine's weight and inertia were greatly reduced, hence improved response and reduce turbo lag a lot. To handle the tremendous heat in exhaust flow, turbines are mostly made of stainless steel or ceramic (the latter is especially favoured by the Japanese IHI). Occasionally there are some cars employ titanium turbine, which is even lighter but very expensive.
    A Titanium turbine from Mitsubishi Lancer GSR
     
    Another area of improvement was boost control. The early turbo engines employed mechanical wastegate to avoid over-pressurised the combustion chamber. Without wastegate, the boost pressure would have been proportional to the engine speed (because the speed of turbine depends on the amount of exhaust flow, hence the engine speed). At high rev, the pressure would have been too high, causing too much stressed and heat to the combustion chamber, thus may damage the engine. Wastegate is a valve added to the exhaust pipe. Whenever the pressure exceed a certain value, wastegate opens and release the boost pressure.
    The introduction of boost control in the late 80s took a great step forward from mechanical wastegate. While wastegate just set the upper limit of boost pressure, Electronic Boost Control governs the boost pressure throughout the whole rev range. For example, it may limit the boost to 1.4 bar for below 3,000 rpm, then 1.6 bar for 3,000 to 4,500 rpm and then 1.8 bar for over 4,500 rpm. This helps achieving a linear power delivery and contribute to refinement. Basically, Electronic Boost Control is just a wastegate activated by engine management system.

      Supercharging

 
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.