Turbocharging

Overview

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.

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.

 
Intercooler

The 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.

Twin-Turbo: Parallel or Sequential ?

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.

Light Pressure Turbo (LPT)

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
Who use it ?	Volkswagen group 1.8T (150hp) PSA 2.0-litre turbo Saab 2.0, 2.3 and 3.0 Ecopower Volvo 1.9 and 2.4LPT.

 

Variable Turbine Geometry (VTG)

Advantage:	Improve turbine response without altering maximum boost pressure
Disadvantage:	Nil
Who use it?	- Many turbo diesel engines - 1989 Honda Legend Wing Turbo - 1989 Shelby CSX (Garrett) - Porsche 997 Turbo (BorgWarner)

 

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.

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.
Who use it ?	Aston Martin DB7 3.2 six and Vantage 5.3 V8 GM 3.8-litre V6 Jaguar 4.0 V8 for XKR and XJR Mercedes 2.0 and 2.3 four Kompressor Mazda Miller Cycle V6 Subaru Pleo 0.66 four

 

Supercharger + Turbo: Volkswagen Twincharger

Ram Air

You can clearly see ram air inlet in the bonnet of Ferrari 550 Maranello. Don't confuse it with inlet for intercooler, this car is not turbocharged !

In fact, you can see ram air devices whenever you watch motor racing. The air box in every formula 1 race cars and the roof air inlet of GT race cars are all ram air devices. A Formula 1 engineer said a typical air box can gain 20 horse power when the car is running at 200 kph.
 

Advantage:	Little additional cost
Disadvantage:	Also little additional power, available in high speed only.
Who use it ?	Ferrari 550 Maranello Lamborghini Diablo SV and GT McLaren F1 GM Pontiac Firebird WS6 and Chevrolet Camaro SS