Overview
The turbocharger was developed by Dr. Alfred J. Buchi between 1909 and 1911 and the first real use of it was on aircraft in World War 1.
By World War 2 it use in aircraft engines was an accepted norm as a way of maintaining performance at high altitude where its ability to force air into the engine could combat the thin de-pressurized nature of the sky.
More recently turbochargers have been incorporated into the engines of road vehicles.
A major use of Turbo's is on diesel trucks. Since these vehicles travel many miles and often need very powerful engines turbochargers are a good addition. The most interesting thing about turbochargers is that they weigh only around 20 pounds can be fitted to almost any fuel injected engine and increase power without a exponential sacrifice of fuel economy.
Recently many major car manufacturers have been using this to their benefit. It allows them to manufacture cars with smaller engines with power outputs equivalent to larger displacement, less fuel efficient engines. From a purely prosaic point of view the manufacturer can utilize a say 130bhp Two litre engine standard engine, and then reuse a large proportion of the parts in a 250bhp Turbo motor. The possible development cost savings to be had are huge.
Volkswagens r&d people claim that their turbocharged diesel engines get 11% better real world fuel mileage than their nonturbo equivalents. This is a remarkable achievement for a device that weighs only around 10-20 pounds and also increases power.
Basic Theory
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 - starting 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 overheating to the 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 or worse than an ordinary saloon. Open full throttle and raise the engine speed, counting from 1, 2, 3, 4 .... suddenly the power surges 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 loss 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. Flooring the throttle cannot result in instant power rise expected by the driver - all reactions appear several seconds later, no matter whether depressing or releasing throttle. You can imagine how difficult to drive fast in city or twisted roads.
For a view on how difficult a Turbo car can be look at some reviews of early 911 turbos, which still have a reputation for wild behaviour when the boost arrives.
Porsche’s solution to turbo lag
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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. NB: The early 911 turbo are still regarded as wild on off beasts despite the attention of the finest German brain power. |
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 (the intake air became of higher density) but also allowed the compression ratio to be raised without worrying about heat too the cylinder head. Of course, higher compression led to improved low-speed output.
Continuous development
During the 80s, Turbocharging continued to evolve for better road manners. As the material and production technology improved, the turbine's weight and inertia were greatly reduced, hence improving response and reduce turbo lag. To handle the tremendous heat in the exhaust flow, turbines are mostly made of stainless steel or ceramic (the latter is especially favored by the Japanese IHI). Occasionally there are some cars employ titanium turbines, 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-pressurising 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 revs, the pressure
would have been too high, causing too much stress and heat to the combustion chamber, and
thus damage the engine. The Wastegate is a valve added to the intake pipe. Whenever the
pressure exceed a certain value, the 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 the 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 contributes to refinement. Basically, Electronic Boost Control is just a wastegate activated by engine management system rather than atmospheric pressure.