pietro
Ex. Club Member
Electric Superchargers, do they work?
http://autospeed.drive.com.au/cms/A_0237/article.html
Boost:
An engine inhales air that - when mixed with petrol - is combusted inside the engine. The greater the mass of air that the engine can inhale, the higher the pressures that will result from this combustion. Higher pressures result in more force on the piston during the power stroke, giving more torque output at the crankshaft. And a higher torque at a given rpm equals more power.
So that explains the impact of having more combustion pressure during the power stroke - but what about the other three strokes? Prior to the power stroke is the compression stroke, and prior to that again, is the intake stroke. On the intake stroke, the piston descends, creating a lower pressure than atmospheric. The pressure of the air outside of the cylinder is higher than the pressure in the cylinder, so air flows into the cylinder. And as we've already seen, the greater the amount of air that flows in, the higher will be the combustion pressures a few strokes later.
The amount of air that flows into the cylinder, compared with the cylinder volume, is called the engine's breathing - or volumetric - efficiency. In a 3 litre six cylinder engine, each cylinder has a swept volume of 500cc. If the cylinder breathes in only 400cc on the intake stroke, the engine is said to have an 80 per cent volumetric efficiency (ie 400/500 = 0.8 or 80 per cent). Volumetric efficiency will depend on lots of factors (including how well the ports flow), but let's say that the VE of the example engine is in fact 80 per cent. If this 3 litre engine is revving at 6000 rpm full throttle, this means that it inhales 7200 litres of air per minute (remember, one intake stroke per two rpm), or 120 litres per second. To put it in different units, each minute this engine consumes 254 cubic feet of air. To put that into context, a little 60mm diameter PC cooling fan flows only about 18 cubic feet per minute. So, just to flow the amount of air that this naturally aspirated, 3 litre engine needs, you'd need an array of fourteen 60mm fans working flat-out. And that's without creating any boost at all....
Talking about boost, how can we improve an engine's VE? One way is to actively force the air into the engine, pushing it in with more than atmospheric pressure. If you shove in more air than the engine can consume, a boost pressure is developed. When the engine is being fed boosted air, VE can rise to 120 or 150 or even 200 per cent. And that spells good increases in power!
http://autospeed.drive.com.au/cms/A_0237/article.html
Boost:
An engine inhales air that - when mixed with petrol - is combusted inside the engine. The greater the mass of air that the engine can inhale, the higher the pressures that will result from this combustion. Higher pressures result in more force on the piston during the power stroke, giving more torque output at the crankshaft. And a higher torque at a given rpm equals more power.
So that explains the impact of having more combustion pressure during the power stroke - but what about the other three strokes? Prior to the power stroke is the compression stroke, and prior to that again, is the intake stroke. On the intake stroke, the piston descends, creating a lower pressure than atmospheric. The pressure of the air outside of the cylinder is higher than the pressure in the cylinder, so air flows into the cylinder. And as we've already seen, the greater the amount of air that flows in, the higher will be the combustion pressures a few strokes later.
The amount of air that flows into the cylinder, compared with the cylinder volume, is called the engine's breathing - or volumetric - efficiency. In a 3 litre six cylinder engine, each cylinder has a swept volume of 500cc. If the cylinder breathes in only 400cc on the intake stroke, the engine is said to have an 80 per cent volumetric efficiency (ie 400/500 = 0.8 or 80 per cent). Volumetric efficiency will depend on lots of factors (including how well the ports flow), but let's say that the VE of the example engine is in fact 80 per cent. If this 3 litre engine is revving at 6000 rpm full throttle, this means that it inhales 7200 litres of air per minute (remember, one intake stroke per two rpm), or 120 litres per second. To put it in different units, each minute this engine consumes 254 cubic feet of air. To put that into context, a little 60mm diameter PC cooling fan flows only about 18 cubic feet per minute. So, just to flow the amount of air that this naturally aspirated, 3 litre engine needs, you'd need an array of fourteen 60mm fans working flat-out. And that's without creating any boost at all....
Talking about boost, how can we improve an engine's VE? One way is to actively force the air into the engine, pushing it in with more than atmospheric pressure. If you shove in more air than the engine can consume, a boost pressure is developed. When the engine is being fed boosted air, VE can rise to 120 or 150 or even 200 per cent. And that spells good increases in power!
