h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Can someone explain to me what horsepower is (in really simple terms) and how it relates to engine size in vehicles. eg a 4 litre vehicle has more power than a 2 litre but how does hp come into that?

1 horsepower = 746ish Watts

Both represent energy/time, or force*distance/time (these are the same thing).

Horsepower is the old-fashioned unit of power, equivalent to about 750 Watts, where a Watt is the ability to lift 1 Newton by 1 metre in 1 second.

The horsepower of a vehicle tends to be measured by its ability to turn the wheels; it's measured on a set of those standing roller thingies (I think there's a proper name for them). This can be affected by many things, including the vehicle weight, efficiency of the differential and grip of the tires. So while engine size obviously has an effect, there is no direct relation.

B

It's called a rolling road.

We have bar blenders at work that are rated at 2Hp and 3.5Hp. That's some blender And we still mange to burn them out making milk shakes

>>This can be affected by many things, including the vehicle weight, efficiency of the differential and grip of the tires. <<

So there is no such thing as a set hp for a vehicle as something like a changing weight will change the hp?

>>
So while engine size obviously has an effect, there is no direct relation.
<<

What is the effect?

What I'm trying to understand is the relevance of hp when looking at a vehicle.

I think B might be confusing things somewhat with talk of rolling roads etc. The power generated by the engine of a vehicle is what's rated in horsepower, but what you do with the power that comes out of the engine can affect the efficiency, so it's not entirely accurate to say that a certain model is rated at so many hp. A 3.5 ton van with a 2 litre engine will haul heavier loads than a sports car with same size engine but it won't accelerate anywhere near as fast nor go as fast. Something to do with torque I think - someone else can probably explain that better than I. Either way, the output power of the engine itself, not what comes through the wheels, is what matters as far as hp is concerned.

Probably.

Oh, and it's both that get Jeremy Clarkson into a lather. Some people are easily pleased

Ok, that explains why trucks get talked about in terms of hp but cars usually don't (Am talking about general public here, not Jeremy Clarkson ) - because with a car fast acceleration is more important than carrying a heavy load.

Is that right?

The power rating of any device measures the rate at which it does work.
If device A can move a certain load a certain distance faster than device B, then device A is more powerful.
When people first started getting interested in this attribute of machinery, their benchmark was the horse. A decent horse could pull a certain load a certain distance in a certain time, and that was a measure that everyone could relate to.
So people started measuring the power of machines relative to horses, and the term horsepower is still used today (albeit only in reactionary branches of engineering like the motor industry)

Oh, petrolheads love horsepower ratings as much as truck owners. You should see some of the TV ads for cars here in the US Horsepower is always one of the most prominent selling points. Why a road car should need a 400hp engine, when that of a Formula One car is around 700hp, is beyond me. The engine in my first, 28cwt van (which weighed in at 2.5 tons, fully laden) was a mere 75hp, and then there's that line from Flanders and Swann's 'A Transport of Delight' - "Behind that monarch of the road, observer of the highway code, that big six-wheeler, Scarlet painted, London Transport, diesel engine, ninety-seven horsepower omnibus.", a vehicle that came in at about 7.5 tons, fully laden if I remember rightly from the official message painted on the side, down by the running boards.

The French use a different definition of horsepower from the British. The old Citroen Diane 2CV was known in France as the "deux chevaux" (two-horse) because it had a power of 2 horsepower. Any car has a power of at least 50 in British units, so it had to be a different unit.

And in Ireland for many years cars were taxed based on the horsepower of the engine, but the figure bore no relation to the British units. I think a typical saloon car had a horsepower of about 10 in "tax units".

Like all non-metric units, horsepower should be avoided.

>Like all non-metric units, horsepower should be avoided.<

Can someone please tell that to American scientists. Learning the Joule as a unit of energy (as we all do in the UK at least) is almost pointless as US chemists insist upon publishing everything in calories (or more often kcal) and since they dominate everything so much we are all forced into it too.

So what should I use when looking at the power of a vehicle?

But the kcal is _so_ much more useful if you want to heat water through one degree, isn't it?

Unless of course you use bloody degrees Fahrenheit.

Car manufacturers almost always list the power of their cars in kilowatts.

Ah, I see it's not limited to chemistry circles this grumble then.

I suspected as much.

Ireland is very slightly more metricised than the UK, and a lot more so than the USA.

I have to say that I measure my height in feet and inches, my weight in stones and pounds and I even do air temperatures in the dreaded () Farenheit (although I'm in celsius in the lab ). For your ordinary Joe I'm not sure it matters all that much other than it may be a bit easier on them at school.

It's a bit absurd to do scientific calculations in non-SI units though and that's where I draw the line. You really do need to have a sensible convention there I think.

Really Gnomon? I just looked at the specs for a Toyota RAV and a Subaru Impreza and both had hp but not watts (on the manufacturer websites).

When I went to the official Toyota site, all the power was given in both bhp and kW.

But then, I went to the official Irish site http://www.toyota.ie/

Perhaps sites for other parts of the world are different.

I was using dot coms. But I have to say I have never heard anyone in NZ talk about cars in terms of watts.