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At Burning Man, I see a lot of "art cars" based on internal combustion engine platforms using generators separate to the original vehicle's engine to power their LED displays and sound systems.

Why do they do this? A typical truck engine might produce 200-400kW and even an extremely large LED array and sound system would only consume a few kW at most. Wouldn't it be simpler and more efficient to upgrade the original vehicle's alternator to supply the required power rather than adding a whole additional engine?

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A typical truck engine might produce 200-400kW [...]

That is mechanical power. Electrical power is generated in the alternator, and thouse typically generate power in the order of 1 kW, as that is enough to power the lights and charge the battery etc.

The dedicated power generators are also more fuel-efficient, and can produce higher voltages - a big truck operates at only 24 Volts. Light and sound installations in the multi-kW range will usually need 110 or 230 Volts to operate.

EDIT:

Wouldn't it be simpler and more efficient to upgrade the original vehicle's alternator

It would be simpler only when there is space available for a bigger alternator and when there is a bigger one actually available.

But it would never be more efficent in fuel: The generators engine is highly optimized for its task (hopefully), and will be operated at or near its point of maximum efficiency. It will also be a lot smaller, as you need only a few kW instead of 100 or more. This has a big effect in fuel consumption.

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  • \$\begingroup\$ Agreed the 200-400kW the engine produces is mechanical power but I'd have thought pushing more of that through a beefy alternator would be more efficient than a totally separate engine. Most LED lighting systems operate on 5-12V DC, so the 24V would be an advantage. The sound you might need 110-230V for though. \$\endgroup\$
    – Robert Atkins
    Commented Sep 6, 2012 at 17:06
  • \$\begingroup\$ I've clarified the question slightly to acknowledge the original alternator might need to be beefed up. \$\endgroup\$
    – Robert Atkins
    Commented Sep 6, 2012 at 17:10
  • \$\begingroup\$ @Robert: It's not just beefing up the alternator, if you can even find one. What about the pulleys, belts, etc? And then you're stuck with a larger and heavier alternator the rest of the time when you're not trying to produce a lot of electricity. A separate generator more easily lets the system be tested from wall power before moving it to a truck too. At a few kW, I'd rather deal with 110 V than the very high current and associated waste and large cables at 12 V or even 24 V. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 6, 2012 at 20:26
  • \$\begingroup\$ It's actually quite common in high-load situations to simply install a second alternator. For example, most ambulances have a second alternator to run all the equipment. \$\endgroup\$
    – Connor Wolf
    Commented Sep 6, 2012 at 20:44
  • \$\begingroup\$ I understand it gets complex quickly, but my intuition was that having a whole separate engine would be even more complex. Guess it's one of those things where theory is just different to practice. \$\endgroup\$
    – Robert Atkins
    Commented Sep 6, 2012 at 20:46

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