Splash & Go writes:
"Green Flag: Turbochargers"
Posted by Uptight Motorsports Nerd on March 15, 2012
Viewed 253 times
Author's note: air pressure can be measured by two scales: absolute and gauge. Absolute pressure uses zero to represent a perfect vacuum (the kind you find in deep space). Gauge pressure uses zero to represent the Earth's normal atmospheric pressure at sea level (that is 101,325 Pa, 14.7 psi, 29.9 inHg, or 760 mmHg on an absolute scale). For the duration of this article, I will be using gauge pressure.
With INDYCAR and F1 adopting turbochargers in their new engine formulae, I thought it important to explain the merits and flaws of turbochargers. In the most basic sense, a turbocharger uses waste heat from an engine's exhaust to compress air in the engine's intake. Not only does this make an engine more powerful, it does so with energy that would otherwise have gone to waste.
Two engines, both alike in horsepower; in fair Daytona, we lay our scene. Engine A is a 5 liter V8, and Engine B is a 2.5 liter I4 with a turbocharger that produces 1 atmosphere of boost. At full throttle both engines are producing the same amount of power and consuming the same amount of fuel. This does not seem advantageous, but if we slow down to a cruising speed, Engine B is only consuming half the fuel Engine A needs.
I would be remiss if I did not stop to mention cylinder deactivation. Cylinder deactivation requires using the PCM to divert engine oil pressure away from selected tappets. This action forces the valves to stay closed and leaves air pressure in the intake manifold higher to feed the cylinders that are still firing. Routing these extra oil lines in a dual overhead cam engine with four valves per cylinder gets complicated; therefore, cylinder deactivation is better reserved for pushrod engines with two valves per cylinder. Of all racing series, it seems this technology best lends itself to Sprint Cup, and that's a long shot as NASCAR was so reluctant to allow fuel injection; furthermore, I don't think NASCAR allows hydraulic tappets.
The flaw most associated with turbochargers is turbo lag. This is that moment of hesitation between holding the throttle wide open and the action of the turbocharger applying full boost. Bigger turbochargers produce more boost and thus more lag. The classic solution is to have a low pressure turbo feeding into a high pressure turbo making for a smooth power curve. A more modern solution is the variable geometry turbocharger, but these are less reliable. Tales have been told of anti-lag systems in rally cars used that retard ignition until the exhaust valve opens and apply the flame front directly to the turbine; however, turbochargers subjected to this level of repeated punishment are not long for this world.
The merit most associated with turbochargers is immunity to altitude. If we revisit the question of Engines A and B at a high altitude location such as Denver, atmospheric pressure is reduced to -2.1psi (or -14,500 Pa, -4.28 inHg, -109 mmHg, etc.). At wide open throttle, Engine A is down on power because its manifold pressure can't exceed ambient pressure; however, Engine B is producing 1 sea level atmosphere worth of boost and has all the power it did before (though it may see slightly more turbo lag).
The lesser merit of turbochargers is that they muffle exhaust noise. It may not seem impressive, but F1 and INDYCAR are hosting a lot of street races and cities often have noise ordinances. There is forward thinking at work here.
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