vol effiency?????

[yiddo72]

can anyone please tell me the factors limiting the volumetric efficiency of a normal aspirated CI engine and xplain how pressure charging can increase the volumetric efficiency and hence the torque and power output.

[theFREAKnasty82]

a naturally aspirated engine relies on vacuum from the manifold to induce air into its cylinders. Because air is not forced fed or crammed by any other means, volumetric efficiency will only be 90%, tops. Supercharging or turbocharging increases that b/c now you are force feeding air into the cylinders, thus increasing efficiency to 100%. Engines run on this basic principle: suck, squeeze, bang, blow. If you recall from the muscle car days, those engines pushed out compression ratios as high as 14:1. High compression means more power. But w/ a forced fed engine, you need to lower compression ratios significantly, as low as 8.5:1, b/c again, you increase volumetric efficiency, and those high pressures will blow the heads of the engines.

For example, say for instance your pumping air into a bicycle tire w/ one of those little pumps. What will get air into the tire faster, pumper it faster or slower? Obviously faster b/c your exerting more pressure to fill the tire. Same principle applies w/ force feeding engines; the more air you can jam in, the more it can push out and that equals more power. But there are advantages and disadvantages to force feeding engines.

Supercharging requires a direct drive from the engine itself. Though there is no lag or waiting for the power to kick in, it's always there, it does create a frictional horsepower loss b/c it now depends upon the engine to make it go. Turbocharging doesn't use that direct drive like a supercharger does, it uses exhaust gases to drive it. The downside is that it takes it a while to build up pressure and power, aka turbo lag. It runs extremely hot and hot air means less power, thus most racers will run intercoolers. It also requires oil to keep it running, most times the oil comes from the engine itself and some have had a tendency to leak. There is one common thing that all supers and turbos have in common, they both use a waste gate to regulate pressure. If not kept in check, supers and turbos can boost enough pressure that it'll blow the engine apart. So basically in a nutshell, N/A engines rely on vacuum to get air in, thus not allowing it to take in a lot of air. Force fed engines use supers or turbos and cram air in and can create higher volume within the cylinders.

[Stevusc]

nice read