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Fuel-air mix pressurization - works great, but beware...Jul 16 '00 Write an essay on this topic.The first thing we need to understand is just what a turbocharger or supercharger does to improve performance. Both do the same thing, they just do it differently. The key is pressure...an engine normally creates suction as the aggregate effect of each piston moving downward during an intake stroke, creating a partial vacuum below one or more intake valves. The valves open a passage from the fuel system and manifolding into the cylinder. So, as the piston moves down, it "sucks" the fuel vapor and air mixture being provided by the fuel system - be it a carburetor or injection system. If you could get that air-fuel mixture to be under pressure, it would rush into the cylinder much more quickly...and you get more in there to burn and generate the energy needed to push down on the piston when ignited. Turbo and Super chargers do that - they pressurize the air-fuel mixture, forcing more into each cylinder during the piston's intake downstroke. The key difference is where the force needed to do the compressing comes from. In a supercharger there are vanes on a shaft that is coupled to the engine's crankshaft - like the water pump, or air-conditioning compressor. Some kind of belt system is used to turn the supercharger. The vane configuration and pulley ratios determine the revolution speed of the supercharger and hence the amount of pressure it generates. In a turbo charger, the energy comes from the flow of exhaust gases through a set of vanes on a shaft that drives yet another set of vanes that pressurized the air-fuel mix. It's a "soft" linkage, and works only when the engine is turning at higher RPM, generating a significant amount of exhaust gas flow. If you've ever driven a turbocharged car, you will remember that the turbo effect does not kick in until high engine RPM is achieved. With the supercharger the link is a "hard" one, and the pressure builds as RPM increases with a linear relationship. Double the RPM, double the pressure. There are fancy controls that can vary effective delivery pressure as required to maximize performance under different engine loading conditions and so on. For total performance increase over unpressurized fuel deliver, the supercharger is more effective. It takes some of the engine power to turn it, but higher pressures can be achieved. The turbocharger cannot develop the same pressures and increases exhaust system backpressure and in that way robs some engine power. Both systems rob less power than they provide - hence the overall increase. The supercharger is capable of a greater overall increase. There is a big caveat here...if you are adding either to an engine that was not originally designed to handle the higher forces, you need to do a lot of work inside the engine. First - you had better be certain that the low end - crankshaft main and rod bearings, wristpin and pistons are up to it. Hang a supercharger on a stock engine, pump it up, and watch the bottom end fall out. For example...GM produced a series of turbocharged engines and put ROLLER bearings in the main and rod bearings. Big block engines had cross-bolted main bearing caps - all done to take the added force being put on the crank and related components. If you are buying new, make certain that the manufacturer really has done its homework inside the engine. Overall, use of fuel-air pressurization is truly effective - but engine wear can be greater. The care and feeding of such an engine is a little more rigorous - more frequent oil changes, use of expensive racing oil formulations - both are important. You don't get 100,000 miles plus out of turbochargers and superchargers without overhauls...this adds significant expense to your total cost of ownership. There is really only one valid use for such power enhancement - competition. For the highway, you can get a vehicle whose overall power to weight ratio is sufficient for anything you will need to do on the road.  |
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