[doc] Horsepower

[Martin Gutkowski - DMC Ltd <martin@xxxxxxxxxxxxxx>]

I recently had a comversation across the atlantic about the term 
"horsepower" and decided to try and find out once and for all, the 
differences.....

There're several "versions" of the term that need to be defined

1) First off, James Watt's original 1782 definition of the HP is the one 
based on the work a horse could do and works out to be 746W. Today our 
European friends have decided that it's actually 735W and this is often 
quoted as "ps" (Acronym for the german for "horse strength") and 
recognised as the DIN unit of Horsepower. Just to confuse us, the French 
call it "ch"which I'm guessing might have a little to do with the french 
word for horse which is cheval.... The Japanese also use the PS 
notation, and it's measured at the flywheel, engine running, everything 
connected

2) Brake Horse Power (bhp) is the one we're most familiar with. It's 
defined as the power outputted by the engine at the flywheel, driving 
nothing more than the internals of the engine - so water pump and 
alternator are excluded. In this case, it's James Watt's "proper" 768W. 
This is a measured figure. This ALSO pretty much defines the older 
pre-1972 definition of hp gross (SAE) horsepower in the US, except that 
this represented an engine running under superb ideal conditions and is 
the theoretical maximum for that engine

3) hp net (SAE) Horsepower (US standard today post 1972, not including 
the new voluntary post 2005 SAE certification) and ALSO 768W is defined 
as the power outputted by the engine with all engine ancillaries 
connected but EXCLUDING the drivetrain AND emissions controls - ie 
exhaust, catalytics etc. This is a measured figure

We're in a wierd place in the UK because we still like our BHP figures 
but all the manufacturers everywhere else are working in a unit that's 
slightly smaller (110bhp is more powerful than 110 PS) and you can never 
guarantee that someone hasn't mixed up the terms (apparently the Veyron 
has been a headache because it -doesn't- produce 1000+ bhp).

Wattage measured at the wheels is the only measurement that you can a) 
compare reliably with other cars and b) give you a rough idea of how 
fast it'll go

Clear as mud, innit?

Martin




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