A good friend of mine recently
posed a follow on question to the watts article that appeared here some time
ago. He had recently read about a new small car that had an engine power output
of 6.5hp. Now, Tom is keen to know if that kind of output could be supplied by
pedals and a couple of willing and fit passengers. So this post is for you Tom.

A good place to start is by looking at the definition of horsepower. Basically
the answer is in the name. Back when the steam engine had just been invented
there was born a need to be able to measure power. What was this boiler shaped
thing, puffing steam capable of? It was all very well turning up at a mine with
one in hand but unless you could say to the mine owner that this had the power
of something that they already knew, it was always going to be a tough sell.
Thus horsepower was born. A guy named Thomas Savery came up with the horse
comparison around 1702. He used it to good effect and sold many a steam unit by
saying how it did the work of 15 horses. A problem arose quite quickly however,
with competing manufacturers adding an extra horse on the figures here and
there. All I can fathom is either some out and out monstrous sized horses were
around back in the day or the figures were being fudged a little. So along came
a standardized measurement of an average horse's potential, devised by Mr.
James Watt the Scottish engineer. Using a gathering of fit dray horses, a mill
wheel and a measured weight they ended up with the following. It is still the
standard today. They fathomed that the average horse could produce 33,000 foot
pounds per minute. Anyone that knows horses will tell you that those figures
are a little optimistic, especially for a longer, period but those are the
numbers.

So that is the horsepower figure and Toms little car is producing about six and
a half cart horses worth. The more astute of you are probably seeing a problem
already, but I will continue. How does a human stack up with a horse? And can
we fit them all in the car?

There is a basic conversion for watts into hp., I won’t bore you with it,
suffice to say that 1hp is equivalent to roughly 745.5watts, sustained,
therefore the 6.5hp car is equivalent to 4845.75 watts. Going back to the
original post we can see that a guy of reasonable recreational rider fitness,
weighing in around 185lbs will be able to sustain approximately 196.8 watts
over a period of an hour or so. Looking at these figures we can see that the
car will need to be the size of a school bus to accommodate our human engine.
The figures look a lot better if we can convince a dozen or so tour guys to be
the engine but even then it is going to get cramped and gas, although
expensive, is still cheaper than steroids.

Tom had already fathomed this out and his main question is. Would it be
possible to power batteries by pedal power that in turn powers the car with the
necessary wattage and equivalent hp? Answer; yes, hooray. Not so fast, yes it
would be possible for pedals to charge the battery bank but it would take a
long time.

Using Ohm's law (amps=power
divided by voltage) we can see that the average person could quite reasonably
produce a 10amp charge rate, requiring about 120watts output, this is about the
same as your average car battery charger. So, looking around at some electric
motors it seems that the average requirement for a 1hp output motor is around
1000 watts. I know I said that 1hp = 745 watts but there is no such thing as a
100percent efficient motor.

Watts ÷ volts = amps. Therefore the
above example would mean 1000 ÷ 12 =83.3. Your average car battery can supply
about 50 to 80 amp hours of capacity each battery would then require 5 to 8
hours of charge time at your pedal power 10 amp charge rate (divide battery
capacity in amp hours by the charge rate).

Our 6.5 hp car would need
approximately 8 batteries. I will leave you to work out your charge routine… Now
I shall go and lay down, all this math has given me a headache.