ONCE upon a time, frugal four-cylinder engines were to be found only in the most modest of motor cars. Today, they are being fitted to even luxury models. The big difference, of course, is that present four-bangers are turbocharged—that is, they are force-fed more air than normal, allowing them to burn proportionally more fuel. The result is a compact power unit that punches over its weight, yet gets good miles to the gallon while emitting less in the way of pollution.



A turbocharger works by tapping the hot exhaust gas from the engine to spin a small turbine which, in turn, drives an equally small air compressor housed in the same compact casing. Air sucked into the turbocharger is compressed, so more oxygen molecules can be packed into each cylinder's fixed volume. For higher performance, an intercooler is sometimes placed between the compressor and the inlet manifold. This lowers the temperature of the compressed air and raises its density still further.



Packing a greater mass of air into the cylinders allows more fuel to be added and burned, boosting the amount of power produced. In general, a turbocharged 1.8-litre four-cylinder petrol engine can deliver the power of a naturally aspirated 3-litre six-cylinder unit. By the same token, a turbocharged V-6 can be more than a match for a conventional V-8.



Turbochargers are not to be confused with superchargers, made famous by the 4.5-litre Blower Bentleys of the 1920s and military aircraft of the second world war. While they serve broadly the same purpose—to cram more air into an engine, so more fuel can be burned—they function differently. Superchargers are better in only one respect: they do not suffer from “turbo lag” (the time taken for a turbocharger to spool up to speed) for the simple reason that they have no turbine. The compressor in a supercharger is driven directly by the engine, rather than by “free” exhaust gases. And that is the problem: it robs the engine of too much power. In thermodynamic terms, superchargers are also nowhere near as efficient as turbochargers. Hence the preference these days for the latter.



That said, turbochargers are not exactly new. General Motors fitted one to an Oldsmobile back in 1962. BMW made a turbo version of its classic 2002 model in the early 1970s. Later that decade, the turbocharged Saab 99 was one of the fastest family cars around. Others followed, including most notably the Lotus Esprit from 1980 onwards. Practically all of the early turbocharged cars used four-cylinder engines.



It is not just the power density and compactness of turbocharged four-cylinder engines that has made them attractive to motor manufacturers. In general, a turbocharged-four consumes around 15% less fuel than a larger, naturally aspirated, six-cylinder motor of comparable output. Also, with the compressed air supplying plenty of oxygen to support combustion, the mixture in the cylinders tends to get more thoroughly burned. The result is a cleaner exhaust.



Detroit started to take turbocharging more seriously in 2010, after the federal government announced that the CAFE (corporate average fuel economy) target would be 35.5 miles per US gallon (6.63 litres/100km) by the 2016 model year. It has taken the past five years to get the new turbocharged models into the showrooms. J.D. Power and Associates, a market research firm based in Westlake Village, California, expects 25% of light vehicles sold in America in 2015 to be turbocharged, up from 8% in 2010.



In Europe, where half of all cars and light trucks sold have long been diesel models, the benefits of turbocharging are well understood. Because diesels ignite their fuel using the heat of compression (rather than spark plugs), they need much higher compression ratios to function. To cope with the greater internal loads, a diesel’s engine block and cylinder head, as well as all its reciprocating and rotating parts, have to be made much stronger, and thus heavier.



Unfortunately, heavy rotating masses do not like being spun rapidly. As a result, diesels tend to operate in a lower, more narrow band of engine speeds. And because they spin relatively slowly, they have trouble sucking in sufficient air to fill their cylinders during their intake strokes. That is why diesel engines—whether in trucks, trains, ships and generators as well as passenger cars—have long used turbochargers to overcome their inherent shortness of breath.



The modern turbocharged petrol engine owes much to its diesel equivalent. But there are significant differences that have called for design changes. For instance, petrol is more volatile than diesel—igniting faster, burning hotter and requiring a lower air/fuel ratio. Petrol engines are also expected to operate over a much wider range of crank speeds, and to respond much more rapidly when called upon by the driver to do so. If the turbo lag is longer than a few seconds, the vehicle can be tricky to drive—with nothing happening initially, and then the boost suddenly arriving with a wallop.



The reverse is also true. If the turbocharger does not come off boost quickly enough when the driver lifts his foot from the accelerator and causes the throttle to shut off the air flow to the engine, pressure waves can surge back to the turbocharger and damage the compressor’s thrust bearing. To prevent that, a “blow-off” valve which dumps surplus compressed air to the atmosphere is fitted between the turbocharger and the inlet manifold.



On the exhaust side, a “wastegate” regulates the turbocharger’s output by bleeding off some of the hot exhaust from the engine, so as to bypass the turbine wheel. Doing so makes it possible to match the amount of energy the turbine receives to the amount the compressor needs, so only as much boost is produced as is required. With their more sedentary nature, diesels avoid much of this complexity.



Numerous other tricks have been tried to make turbochargers more responsive. Obviously, the smaller and lighter the rotating parts in a turbocharger are, the faster it can respond to changes in the throttle setting. Unfortunately, small turbochargers quickly run out of puff. Bigger ones produce all the boost required, but are slow to spool up. A number of hybrid designs have emerged that combine the best of both worlds.



The most popular type today is the “twin-scroll” turbocharger. This works like a pair of turbochargers connected in parallel, one for each of two separate exhaust manifolds. However, the problem with using twin turbos is that, while they reduce turbo lag, they increase the cost and complexity disproportionately. The twin-scroll design gets round this by having two exhaust-gas inlets and two nozzles feeding a single turbocharger. One nozzle injects exhaust gas at a wider angle to the turbine blades, for quick response, while the other injects the exhaust gas at a shallower angle, for peak performance.



Having two separate exhaust manifolds on a four-cylinder engine adds, of course, to the cost. But by pairing cylinders so their power strokes do not interfere with one another, the two exhaust streams can be injected into separate spirals (scrolls) in the turbocharger, causing it to spin more smoothly. Apart from improving the turbine’s efficiency, this also helps improve the scavenging of burned gases from the cylinders, lowers the exhaust temperature (and thus emissions of nitrogen oxides) and reduces the turbo lag still further.



What more could one ask for? Your correspondent has long dreamed of upgrading the turbocharger fitted to one of his elderly motor cars. Of the two “Lots Of Trouble Usually Serious” vehicles stacked in his garage, he built the older (naturally-aspirated) one back in 1972, while the younger (turbocharged) one came fresh from the factory in 1988.



The Garrett T3 turbocharger fitted to the latter’s 2.2-litre four-cylinder engine was state-of-the-art a quarter of a century ago. Today, it is an anachronism, producing a modest 215 BHP (160 kilowatts) on a dynamometer—a paltry 170 BHP per ton of vehicle weight. Nowadays, even runabouts like the Mini Cooper operate in that sort of territory. Unfortunately, Lots of Trouble No 2 has a rather delicate Citroen gearbox. And now, your correspondent learns, its geriatric Bosch injection system is unable to supply anywhere near enough fuel to meet the demands of a modern turbocharger.



An idle thought has passed through his mind about adding a turbocharger to Lots Of Trouble No 1 instead. In mid-life, 43-year-old LOT-1 was rebuilt from the wheels up—with a stiffer frame, beefier engine and transfer-box, plus a modern fuel-injection system and programmmable engine management unit. Weighing in at less than 1,650 pounds, this tiny mid-engined car delivers around 280 BHP per ton as it is. Turbocharging it could easily raise that to over 350 BHP per ton. The only problem is that your correspondent might then be too nervous to drive it. Ah, well, back to the daydreams and the drawingboard...