I have a technical question hopefully martin or one of the pro's answer ... does a supercharge engine suffer from a Lag as in a turbo engine ? As far as I am concerned the supercharge fitting is different from the turbo where it is directly powered from the engine main belt (which means instant spooling of the charger) .... I could be wrong but I hope some one answer ..
Nice question Mazzer... sorry to get back only now - yesterday was a public holiday down here.
In short, the quickest way to answer your question is to say this:
No. Supercharged engines do not suffer from lag
in the way that a turbocharged engine does.
Unfortunately there are a few provisors to this statement. So we need to go into a bit more detail around engine charging. (Those of you who are bored with this stuff by now should stop reading here coz here goes:
)
For instance, the concept of "lag" is the driver's perception of the time taken for an engine to respond after depressing the accelerator pedal. Before we go into a discussion around engine response, let me briefly cover the basics again.
In turbo'd engines this lag is prevalent because of the way a turbocharger works in conjunction with exhaust gas. A turbo only begins to work optimally when there is sufficient exhaust gas volumetric flow to turn the turbine fast enough for the compressor side to start generating the desired inlet compression or boost pressure. Along with this, a turbo's output is directly related to not only engine rpm but also throttle openings. With light throttle loads even at good rpm there is a period of lag where the turbo needs to spool up. The nice thing with a supercharger is that is being driven purely as a result of engine rpm - irrespective of the throttle position - so boost is readily available even at light throttle loads.
A supercharger is a compressor that is connected to the crankshaft via dedicated pulleys and a belt. So, even at idle the compressor within the supercharger is turning and producing some inlet pressure. However, some superchargers are designed to be, wait for it, more responsive at low rpm and others to be deliver more power at higher rpm. So, if you want your supercharger to deliver a bigger wallop then you need to deliver a greater volume of compressed air. To do this you need either a larger or faster spinning compressor (which can be achieved with gearing).
A supercharger's greatest drawback is that it consumes almost 1/5th of engine power as a result of being driven off the crank. So, theoretically, if you bolt a giant supercharger to your little 1.4 litre engine, the engine will labour to get that supercharger delivering enough power to overcome the mechanical drag of the supercharger itself. So whilst throttle response is instantaneous and the power delivery of the engine is linear, there most certainly is a perception more forceful acceleration at higher rpm. This is definitely perceptible in the old MINI Cooper S for example. A classic example of a quick responding supercharger is of that employed in the VW 1.4 TSI engine which makes use of a small supercharger driven off the crank and turbocharger driven by exhaust gas. At low rpm the supercharger immediately starts delivering compressed inlet air and the turbo sits idle. As the revs rise the turbo starts to spin and contributes to the inlet boost pressure. As efficiency in the turbo reaches the optimum, the less efficient supercharger is then decoupled from the crank via a clutch.
You'll notice that this new supercharged 3.0 TFSI engine delivers maximum torque at 2200 rpm - higher than that of the 335i (+- 1500 rpm iirc) - and this is directly as a result of the supercharger rpm having to reach a certain speed in order for the engine to produce the desired amount of torque. So, in effect, it is the 335i's turbo engine that is more responsive, hence the driver's perception of lag is significantly diminished.
So in such cases we can see that turbocharging technology has come a long way (small, twin-turbo'd configurations; twin-scroll turbos) in the interests of reducing lag almost entirely. But, as long as turbo's are spun by exhaust gases - there will always be some small measure of lag.
I mean the superchargers hooked up to the V8 in the SLR can't be eating 80-120hp or so, no?
