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Re: Supercharging - Saab LPS? Posted by Ari [Email]  (#2847) [Profile/Gallery]
(more from Ari) on Fri, 22 Apr 2005 12:44:06
In Reply to: Supercharging - Saab LPS?, Shaun  [Profile/Gallery]
, Thu, 21 Apr 2005 19:49:40
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Supercharging presents a load on the engine - it takes horsepower to drive the supercharger.
The good part about the supercharger is that it provides forced induction at low rpm, so you get great off-the-line performance. But as the engine speed increases, the supercharger goes faster and faster, so you need to blow off that pressure. That's easy, but it creases a parasitic drag all through the speed range.
Turbochargers use the 'free' energy of the exhaust. Yes, it is not totally free because it produces some measure of backpressure that consumes horsepower. However, it is more efficient than a supercharger, because most of the energy is consumed by reducing exhaust gas temperature. It's a thermodynamic thing, but the exhaust leaving the tail pipe has less energy, and that recovered energy is used in the compression cycle. Very generally, the losses on a turbocharger are less than a supercharger.
Most probably the biggest advantage turbos have over superchargers is noise and packaging. The always-driven supercharger makes noise, and even more noise at higher rpm. Packaging is another issue - the supercharger has to be driven as an accessory, and it's not small. It's hard enough finding space for one A/C compressor - now find room for two. And you can't always share belts, because belts are power transmission devices. It becomes a packaging /engineering issue. Yes, the turbo has to be in the exhaust stream, but you've got a few more options. Also, turbo tend to be physically smaller than superchargers.
Lastly, turbos provide nice breathing at higher RPM. Superchargers do all their work at lower RPM, and are pretty much out of the picture at high RPM. If you're looking to make a small engine seem bigger and still keep good fuel economy, you go with a turbo, and live with the reduced low-end performance.
Is one better than another? No - each is an engineering trade-off with tons of pros and cons. What I can say is that there have been a few supercharged cars around, but they're in the vast minority. Primarily noise, packaging, and cost.
posted by 192.249....
, Thu, 21 Apr 2005 19:49:40
, Fri, 22 Apr 2005 15:10:31, Fri, 22 Apr 2005 12:44:06 <-- Viewing This Message, Fri, 22 Apr 2005 11:30:44
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