For internal combustion to take place, fuel needs oxygen, the source of which is atmospheric air. During the intake stroke (as the piston descends within the cylinder), the mass of air inducted is strictly at ambient pressure. Cylinder volume is a physical constant, but the mass of air that fills any space is a function of pressure. Hence, the higher the pressure, the greater the mass of air that can occupy any given volume, simply because air is compressible.
A device that “blows” air into the cylinder would enable more air-mass to be squeezed inside than by natural aspiration alone. This concept of forcing air into the cylinder to achieve greater than 100 percent volumetric efficiency at a given ambient pressure is termed “supercharging”.
The device mentioned in the previous paragraph is called a compressor. It can be driven by an electric motor or mechanically by a belt off the crankshaft. The turbocharger, however, relies neither on a motor nor a belt. Instead, a turbo compressor is driven by a shaft-connected turbine, which is made to spin by the hot, fast-flowing exhaust gases of combustion. In theory then, turbo-supercharging (to use the “correct” technical term) consumes no energy since exhaust gases are waste products of the internal combustion process.
By the 1990s, most automakers were playing the turbo game. From Audi to Volvo, any respectable performance car had to carry a “Turbo” badge. Even aftermarket tuners developed turbo kits that could be bolted on to a naturally aspirated engine. Although some of these “boosted” engines eventually disintegrated from excessive thermal and mechanical stresses (that they were not designed for in the first place), their performance was devastating when they did deliver.
HOT AND BOTHERED
Turbocharging, though highly effective, is not as simple as it sounds. Heat is turbocharging’s biggest complication. Compressed air, especially if it flows from an exhaust gas-driven device, experiences a significant rise in temperature. Not only does this mean a drop in the density of said air, it also causes pre-ignition of the air-fuel mixture in the combustion chamber – a phenomenon that frequently leads to stress failure in the cylinder head and, sometimes, even the engine block.
It was essentially for this reason that the grand-daddies of turbo engines were designed with ridiculously low compression ratios – 6.5 to 1 in the case of Porsche’s 930 Turbo. This largely alleviated the pre-ignition problems, but the downside was lethargic pre-boost performance, better known as turbo lag. The turbo motor would be relatively lethargic till about 3000rpm, but the surge that came thereafter more than made up for lost time.
Porsche later incorporated something called an intercooler into the 930’s engine plumbing. It is basically a heat exchanger, which works like a radiator to cool the compressed air as it flows into the intake manifold. This allows the engine to run a slightly higher compression ratio and increases the density of intake air.
THE TURBO TODAY
While performance continues to be a major incentive to force-feed engines using a turbocharger, tremendous progress in the technology of both hardware and software has realised huge gains in fuel consumption and exhaust emissions, too. Arguably the greatest effect the turbo has had on the car industry is “downsizing”, or the reduction of engine capacity.
Volkswagen’s 1.4-litre TSI engine is a perfect example of the modern, small-capacity turbocharged engine. There’s no turbo lag or overheating, just plenty of smooth torquey performance that belies the quoted 122bhp, thanks to a full 200Nm of torque available between 1500rpm and 4000rpm. The 1.4-litre VW’s sprightly mid-range acceleration feels more like that of a 2-litre, but when it comes to average mileage, the figures are closer to those of a naturally aspirated 1.2-litre. It’s the proverbial best of both worlds, on wheels.
THE TURBO TOMORROW
Today’s most capable turbo engines deliver performance, economy and driveability that their naturally aspirated cousins of similar capacity cannot match. With turbo technology continuing to improve, future turbocharged engines are likely to be even better than the ones we have right now.
