Anybody who has watched Top Gear, the popular British motoring show, must know of Jeremy Clarkson, a belligerent but charming chap known for screaming “Poweeeeer!” when driving something expensive very fast on a runway.
So, how do the makers of the power machines he spins squeeze more power out of their engines? The easiest (and laziest) way is to go for cubic capacity: bigger engines develop more power, naturally, but the payoff is increased weight and dimensions, making it tricky to design good handling into the car’s chassis. Americans are notorious for this.
Another way is to adopt boffinry like variable valve timing, redesigning the combustion chambers, minimising friction and such, but this involves too much brainwork and requires a lot of resources in prototype form before one gets it right.
There is, however, a third way that does not require the brute force approach of supersizing or the Einstein-esque chipping and techno-frippery model of improvement: Forced induction.
Supercharging: Supercharging is forced induction by means of rotating fans. To run, a typical internal combustion engine requires air (oxygen) and fuel (petrol or diesel), mixed in varying proportions. The resultant chemical reaction, triggered by a spark, causes a violent explosion that releases tremendous amounts of energy that is then converted into noise, heat and motion.
If you paid attention in high school during Chemistry, you must have heard of the Mole concept. More moles of a given substance involved in a chemical reaction mean more energy transactions conducted. So in an engine, if you wanted more power, you have to add one or both of the reactants to get more energy, and thus more power.
Adding fuel is easy. Fit bigger injectors, or more carburetors and higher capacity pumps. Getting more air into the engine is, however, trickier since air goes in by atmospheric pressure, which you cannot adjust no matter how hard you tried. The air thus needs a pump of its own, and it is this pump that is known as a supercharger.
The supercharger is an air pump driven by the car’s engine mechanically, just like the power steering system or the alternator. Early superchargers were first invented for use in blast furnaces (!), from which combining innovations resulted in their motor vehicle application.
Turbocharging: Turbocharging is an easier pronunciation of “turbo-supercharging”, which is itself derived from “turbine supercharging”. The reason it is called thus is that, unlike supercharging, the system uses turbines to force the air into the cylinders. The name is a bit confusing since some aircraft companies still say “turbosupercharging” when talking about turbocharging, while petrolheads refer to turbosupercharging as using both a turbo and a supercharger, like the Lancia Delta Integrale Group B rally car.
The turbo is made of two fans, the turbine mounted in the exhaust manifold; and the compressor, or the impeller, in the intake manifold, connected by a shaft. When exhaust gases are pushed out of the cylinders, they turn the main turbine at high speed, which, in turn, drives the compressor, forcing more air into the cylinder.
Turbochargers spin at speeds of up to 200,000 rpm. The two fans are hidden inside conical housings whose size and shape varies according to manufacturer and application. The centre housing/hub rotating assembly (CHRA) is what conceals the shaft connecting the two fans. One common feature of turbocharger units is the wastegate, a device used to control the boost pressure. It is the one that goes ‘pffft’ every time you come off the throttle in a turbocharged car.
Sometimes the output flow volume of the turbo exceeds engine volumetric flow, and thus pressure builds. Should the turbo speed up beyond its recommended setting, a control method is needed, the wastegate.
To prevent detonation/pre-ignition of the intake mixture due to excess pressure and thus heat, the wastegate is used, and it vents excess exhaust gases to bypass the turbine. The wastegate actuator is connected to the compressor by a signal hose, which is itself controlled by a solenoid run by the engine control unit.
Turbo lag is the time needed to bring the turbo up to speed, and is manifested as hesitation in throttle response. Most auto reviewers complain about turbo lag in boosted cars, and typically warn fast drivers against being caught “off-boost”. Lag can be dealt with by lowering the rotational inertia of turbine by using lightweight material to reduce spool up time, such as ceramic. However, fragility is a limiting factor.
Better wastegate response also helps to reduce lag, but is costly and unreliable. Using foil bearings instead of oil bearings reduces friction and kills lag, too.
The last method of anti-lag effort is application of different sizes of boosters: small ones spool faster, so they work at low engine speeds, but at high rpm the bigger one takes over to provide large masses of air into the cylinder.
One other key feature of a turbocharger is the anti-dump, or blow-off, valve. When hard under power, closing the throttle suddenly will cause the compressed air to stop at the throttle plate and start decompressing backwards towards the impeller. Compressor stall can easily occur, causing turbo failure because the shaft connecting the two fans will be forced to slow down suddenly.
A valve fitted between the turbo charger and the intake vents excess air, either to the atmosphere (diverter) or back into the turbo (blow-off valve, BOV).
Engines that use MAF sensors demand the use of BOVs because dumping the air into the atmosphere causes the burning of a rich mixture, seeing as the sensor already sent the signal that excess air was getting into the engine, thus injecting more fuel, but the air gets diverted.
There is a lot to say about turbocharging, but I’ll try to keep it short and simple for now. One of its biggest advantages is that it is immune to the sort of parasitic losses that plague other types of forced induction. Turbocharging also allows vast amounts of horsepower to be derived from a relatively small engine. Toyota Supras and Nissan Skylines have been known to push 1,500hp from heavy boosting.
Turbos also give car manufacturers the chance to sell two versions of the same engine, cutting R&D costs instead of developing a whole new engine.
I had already talked about maintaining a turbocharged car, but just a quick reminder. To prevent thermal shock, where the sudden and uneven cooling of the turbos components causes fractures, a brief cool-down period is needed, about 3-5 minutes.
The sudden stop also causes the accumulated heat to be dumped into the lubricating oil, causing coking, a destructive distillation of that oil. A turbo timer is usually used here: the timer is a device that keeps the engine running briefly after cutting out to allow the turbo to cool normally, before shutting down the engine.
It is necessary to make frequent checks on one’s turbo to prevent a nasty surprise some time in the future. For reliability, it is recommended that clean (very clean) synthetic oil be used. Dirty oil is lethal, considering the stratospheric rotational speeds that turbos sometimes attain. Synthetic oils are known for their heat capacities, and they can withstand the mechanical violence that comes with components spinning at up to 200,000 rpm.
Twin Turbo Technology: Speed freaks and Subaru lovers always talk of twin turbos. Twin turbos are exactly as the name says: two turbos working together. The layout, however, varies a little. They could either work side by side, each blower boosting its own bank of cylinders in what we call parallel turbocharging, common to V engines; or they could work in tandem, in a setup referred to as sequential turbocharging.
In this scenario, the first turbo compresses the air taken in and then sends it to the next turbo, which compresses the already compressed air even further, delivering it into the cylinders at extremely high pressure.
Another way of using sequential turbos is having one acting across the entire rev range and another one checking in only at high rpm, below which the secondary turbo does not work but the first one does. Porsche’s monstrous 959 is an early subscriber to this school of thought.
They are, however, more complex and, thus, costly. Two small turbos work like one big turbo, but they reach their optimal rpm speed and deliver boost faster. The first car to use parallel turbos was the Maserati Biturbo, later followed by the Audi TT, the Toyota Supra (nice) and the Nissan GT-R (even nicer).
Asymmetric turbocharging: Saab aficionados will agree with me that the Swedes are a quirky lot. With the 9-5 Griffin car, Saab introduced the world to asymmetric turbocharging. They started with a run-of-the-mill 2.5-litre V6 engine from General Motors. The turbo in it is driven by both banks of cylinders, yes, but the compressor works on only half the cylinders; on only one bank of the V6. Odd, very odd, but it works. Somehow.
Intercooler: Because of the temperatures involved in a turbocharged engine, pre-ignition is a bit of an issue, as well as the self-defeating concept of density reducing (Chemistry: high pressures mean high temperatures, a phenomenon which tries to expand the gases and reduce density).
To prevent this, a heat exchanger is used. The exchanger cools the intake charge, thereby reducing the risk of pre-ignition, and simultaneously increasing the charge density to create an effect similar to turbocharging since a greater mass of air finds its way into the cylinders. This heat exchanger is the intercooler, and will be discussed elsewhere.