ok ok ok ok I can't resist any longer.
Posted by Parshall on 2000-05-10 14:34:49:
In Reply to: What's the difference between a turbocharged and a supercharged engine? posted by Agent002 on 2000-05-09 18:43:03
The "turbo charger" actually used to be known as a turbo supercharger. In other words they are all 'superchargers' There are many different types of superchargers driven in many different ways. What is commonly referred to as a 'turbo charger' is two centrifugal blowers linked by a common shaft. One 'blower' (known as a turbine hence 'turbine supercharging aka turbocharging) is driven by the engine exhaust gases. This causes the second blower (known as the compressor or supercharger) to turn which forces more air into the engine intake than would be there without the blower. Some companies take the exact same blower and turn it by a belt rather than a second blower in the exhaust. This would have a similar response to a turbocharger but be easier to install as an aftermarket kit. Centrifugal blowers are not 'positive displacement.' The result of this is that their response is very non-linear. If the blower spins twice as fast, it pumps more than twice as much air. This could blow up an engine very quickly. In the days before computerized bypass valves, the centrifugal blower was sized so that the blower reached peak boost close to redline. This would result in the engine feeling normal for the first few thousand rpm then launch as the blower really started getting in on the act. This is what the uninformed refer to as 'turbo lag'. It is NOT a turbo lag. It is just that the motor torque really increases as the blower spins faster at the higher rpm. So when you floor the pedal at 1000 rpm in a porsche turbo, it is not that the turbo isn't as fast as it is going to spin at 1000rpm, it's just that the blower isn't spinning fast enough to give any boost yet. This was compounded in the old pre-computer/bypass days because designers felt that the compression ratio had to be reduced to prevent knocking/pinging. This meant that at low rpms, the turbo'd engine had less torque than the normally aspirated. This of course drives americans crazy because they are used to big V8 low end grunt and don't want to wait.
The trick to reducing what people think of as turbo lag is to size a blower so that it is spinning fast enough to give useable boost at low rpms. In the old days this would blow up an engine because as you got into the higher rpms there would be too much boost. In the TT and all modern turbo Audi, there is a bypass valve which regulates the boost pressure. So now they can have low rpm boost. At higher rpms some of the exhaust gas bypasses the turbine to keep it from spinning the compressor too fast. Audis also have an adjustable inlet vane to the turbine to vary the gas velocity to the turbine which allows it to spin up faster. Drawbacks: The turbine and compressor have to turn up to 100,000 rpm in a very hot environment. Before water and oil coiling turbocharger bearings used to fail all the time. Turbochargers are more difficult to put on in the aftermarket because a new exhaust manifold and possibly intake manifold have to be designed. Pro's: marginally more efficient than a non-exhaust driven supercharger.
Positive displacement superchargers such as the 'Eaton' roots type blower used by Mercedes and Ford, have different flow versus rpm characteristics. The Eaton blower runs at only 1 to 3 times the speed of the engine and is driven by a belt from the crank. This puts it under much less stress than a turbine. The Eaton blows about the same amount of air at low rpms as high. This makes for a more uniform torque curve and good acceleration right off the line. At high rpms the eaton blower starts to loose efficiency and heat up the air more. So for V8 feel and ease of installation an Eaton supercharger is good. For peak efficiency and power, a turbo charger is probably best.
Intercoolers: Any high boost supercharging system is going to need an intercooler. It is not just the density of the air. In simple terms, engines knock or ping because as the air in the cylinder is compressed, it gets so hot that the fuel ignites due to the heat in the cylinder and not the spark plug. Ways to prevent this are: use higher octane fuel which requires more heat to ignite, keep the cylinder cooler through better water flow around it, start out with cooler air! So the fact that the intercooler lets more air fit in the cylinder is just a positive byproduct of trying to make sure that the cylinder/air stays cool enough to prevent spontaneous ignition/knocking.
Why do I know this? Because my goal this year is to put an Eaton supercharger on my Audi 4000CS and boost the power to around 250hp. Then have a big smile before it blows up.
green^3 225 TTR