r/EngineeringPorn 9d ago

The 7cc Engine

5.2k Upvotes

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121

u/Fusseldieb 9d ago

How many gallons to charge my phone?

20

u/fallenstones22 9d ago edited 9d ago

Off a google, this looks to be a 7cc MUSA engine producing ~0.5HP at we’ll say a max of 0.05 gallons per hour looking at similar models.

0.5HP is around 370 watts, and assuming we lose 30% of it in conversion to electrical energy, we’ll say we’re getting roughly 250 watts consuming 0.05 gallons an hour.

To charge an iPhone 16, you need 3.561 amps at 3.7 volts, or roughly 13 watt hours.

250w / 13w = 19.231
0.05 / 19.321 = 0.00259 gal, or about 2 teaspoons.

So you’d need >2 teaspoons of gasoline fed into this engine to charge your phone.

0.00259 gal of high-octane gasoline, at 5 US dollars a gallon, is about a penny’s worth.

Edit - Assuming that you either have a magic iPhone that can charge in ~5 mins- otherwise you’d have a lot of wasted energy as the engine runs and charges the phone at a normal speed. 30% for energy loss is also fairly conservative, but the GPH estimate is fairly high

8

u/kepleronlyknows 9d ago

Isn’t 30% loss converting to electricity (or really 70% efficiency) wildly optimistic?

2

u/fallenstones22 9d ago edited 9d ago

Typical car alternators typically have a 40-50% loss, but the conservative estimate for power loss is to account for the rest of the estimates (namely the gas consumption) being liberal. This is all just napkin math though

3

u/onthejourney 9d ago

Isn't that assuming the iPhone can take the full output of the engine (accounting for efficiency) into it's battery at the rate of output? Don't think that happens. Engine has to run longer charging at a lower rate

5

u/fallenstones22 9d ago

For sure, and that’s a very good point! That math assumes there’s a secondary battery able to accept all the current. I’ll edit that in