Quote:
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Originally Posted by lexuspilot
... Jim 99 from what you are saying it would take a Honda CRX just to turn over the supercharger on the xkr???
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Hi,
Well, not
exactly, but essentially, correct. All the ancillaries on an engine cause a
Parasitic draw on the Crankshaft which powers them.
For many, this draw is variable (on some, like the PS Pump and AC Compressor(when on), the draw is more or less constant). The Waterpump and Alternator for instance, draw varying amounts of Crank HP depending on the load put upon them.
A waterpump draws around .5-1 HP at idle, but may jump to 6HP when it is really churning due to the increased resistence of the coolant to the impellor blades.
An alternator draws maybe 0.1 HP at idle w/ no electrical demand on it, but can draw up to 10 HP under full load.
The same is true of the Supercharger. At idle, it draws next to nothing because it has a variable clutch to allow it to
Freewheel, but the power draw increases at it spins faster and experiences more resistance by the air it's compressing. On the example I used, the XKR makes 400 HP at full boost. Compare this to the 290 HP produced by the NA XK8. But, this HP gain is a
Net gain of 110HP. The Supercharger draws 86HP in making this additonal 110 Net HP, so in total, it makes 196HP. It makes about 2.5 times what it draws, so it is a positive compromise. This is one of the advantages of a Turbocharger, that it doesn't consume Crank HP, the energy it makes is essentially Free. And, unlike a Supercharger, which is limited to the Crank HP available, the faster a Turbo turns, the more power it makes, allowing it to turn even faster, allowing it to make even more power - the limiting factor here is turbine blade efficiency. But, of course Spool-up time (Lag) and excess heat are the most apparent trade-offs, or disadvantages.
Motive force is fairly easily calculated. 745.5 Watts is equivalent to 1 HP of
Work. In my example, I stated that the electric motor driven fan
may be twice as efficient as the mechanical Supercharger (this isn't so, but I wanted to add some perspective to the example).
Anyway, twice as efficient would mean it would draw only 43 HP to make the same amount of boost. Doing the math, this means that it would take 34,293 Watts of power to do the same work. In converting the Watts to Amps (not an exact conversion, but an
accepted way of averaging it) the equation is Watts/Volts=Amps. So, 34293/12= 2,857.75 Amps.
There is only so much
Work available from the 120 Amps the Alternator creates. And, since the Maximum Output of the Alternator is 120 Amps, you would need the equivalent of almost
24 Alternators, dedicated to only the Supercharger, to accomplish the same work (and remember, this assumes a 2X efficiency) as a mechanical Supercharger. Now, if the Alternator drew 10HP at max Load, and you needed 24 Alternators (all at Max Load), these alternators combined would draw 240 Crank HP to spin them, while the Machanical SC only draws 86HP, so clearly this is not nearly so efficient (and remember we assumed a 2X efficiency). This is why Electric Superchargers are not really feasible. So, there really is no way that an Electric Supercharger can support the claims made by the manufacturer.
Happy Motoring!...Jim'99
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