Most passenger car gas engines around the world are built as normally aspirated, that is reliant completely on atmospheric air pressure, not boosted air pressure to feed combustion air into the cylinders. By contrast, all diesels for light vehicles are boosted today. Traditionally, the boosted gasoline engine market for highway-licensed cars has been performance-oriented. In recent years, the boosting penetration in new U.S. market automotive gasoline engines had been running around 5 percent. However, boosting penetration of new vehicles is sharply rising and new suppliers (such as Bosch and Mahle) have entered the boosting business to help meet demand. Such boosting has relied on either exhaust-driven turbo-superchargers, also known as turbochargers or turbos (see below) or mechanical belt-driven superchargers (such as the Roots-type blower—see below, or more-costly screw-type superchargers offering higher pressure ratios). In the aftermarket, centrifugal-type belt-driven superchargers are widely offered for retrofit.
Toughened fuel economy regulations with dramatically higher mpg targets and strict limitation on global warming gases that include carbon dioxide from engine exhaust emissions have pushed boosting from a performance niche toward a mainstream fuel-saving presence. All around the world car builders have strongly pursued gasoline engine downsizing (and to a lesser extent diesel downsizing). In the U.S., that would typically mean replacing a naturally aspirated 3.5 liter V-6 with a 2.0 liter I-4 gas engine. In Europe, a 2.0 liter I-4 could be replaced with perhaps a 1.4 liter I-4, or 1.0 liter I-3. To restore the peak power and torque lost in the displacement downsizing process, forced intake air induction is applied, usually with an added intercooler (also known as charge air cooler) to remove much of the heat of compression. This enables more boost pressure and thus more power before running into destructive detonation. It also turns out that boosting works very well with another increasingly popular engine enhancement strategy: gasoline direct injection (GDI). The downsize/boost strategy is fairly expensive (enhanced fuel economy is rarely free), as the boosted/intercooled and strengthened smaller I-4 engine may cost the vehicle manufacturer up to $1,500 more per installation than larger non-boosted V-6 engines, in high volume.
Drivers may retain the expected overall performance and acceleration ability, but must realize that the sound and feel (such as launch feel) of boosted small engines will be different. The smaller boosted engine will likely be turning at higher revolutions per minute (RPM) and have more of a “buzzy” sound and feel. The smaller boosted engine will be working harder, run hotter, and exhibit higher specific power ratings; horsepower (hp) or kilowatt (kW) per liter of displacement. That set of circumstances has implications for engine lubricating oil, which may be more stressed and break down faster requiring more frequent oil change intervals and the use of synthetic engine oil. Further, there will probably be a need for a dedicated engine oil cooling circuit to keep lube oil at tolerable temperatures.
To enjoy the full-power benefit, owners of boosted engines must expect to pay for higher-octane gas at each filling. Gasoline that is 92 octane typically costs $0.20/gallon more than standard 87 octane. With the use of lower-octane gasoline, the engine controller via knock sensors will retard spark timing cutting power.
Original Equipment Manufacturers (OEMs) have a choice of turbo or mechanical superchargers. The latter offers better low-end performance and excellent longevity in service, but costs more and is less-flexible than the turbocharger in terms of packaging within tight engine compartments. Therefore, the turbo is more prevalent in the marketplace. Turbos, however, often suffer from poor boost performance at low RPM and marginal transient response, which encourages the use of countermeasures. In the diesel engine world, the variable-geometry turbo (VGT) is commonly used to improve the low-end response. However, in the gasoline engine world, where there is a much higher exhaust gas temperature (EGT) that exceeds 1,000 degrees Celsius, it has proven rather difficult to engineer a variable-geometry turbo that will last. There is only one such OEM production program (at Porsche) for variable-geometry turbo gas engines. Another means to improve response is to close-couple the exhaust manifold with the hot side of the turbo, making it an integral component to the system. Turbochargers with twin hot scrolls further optimize performance over a wider range of operation, low to high RPM.
Although a mechanical Roots-type supercharger can easily last the life of the gas engine (say 150,000 miles), that is not quite the case with turbos, which may need at least one replacement at considerable expense. No OEM offers a lifetime warranty on turbos. In the early days, turbo bearings failed prematurely because lubricant oil coked up (pyrolized) with heat soak after shut down, freezing the rotor. Now the use of high performance synthetic lubricating oil and in most cases continuous water/glycol cooling of turbo bearings is common. In higher-pressure ratio turbo applications (as seen in diesels), fatigue failure of aluminum impeller blades due to cyclic bending is the leading failure mode. However, for gasoline engines in passenger cars, manifold boost pressures are modest (around 8-12 psi above atmospheric), with short-term boost to perhaps 15 psi—not enough to worry about, in terms of impeller fatigue.
Turbos are often claimed to provide a boost by harnessing “free energy”, otherwise wasted out the exhaust. Nonetheless, all turbos introduce some added exhaust back pressure that can diminish power, but this loss is fortunately offset nicely by the added intake air which raises power.
