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?
Not necessarily. Depends on the efficiency of the engine.
The actual power of an engine comes from exploding the fuel/air mixture and converting that into mechanical movement via the pistons etc.
It thus follows that an engine which is more efficient can convert more of the latent energy in the fuel/air misture into mechanical power and thus the more efficient engine is more powerful.
Now, when it comes to engine size what you are actually measuring is the amount of space in the cylinders to contain the fuel air mixture and explode it.
The bigger the engine, the more fuel/air misture you can put in it and thus the more energy there is available when it explodes.
However an inefficient big engine can be less powerful than an efficient small engine. This being the historical difference between european/japanese engines and american engines. Not a whole lot of difference across the spectrum in actual power to the wheels but eu/japanese do it with a more efficient 2litre (or less) engine while the americans do it with a less efficient 4litre engine (ok, gross simplification but you get the idea I hope).

So mentioning horsepower (hp or bhp) in relation to the vehicle gives some idea of its performance whereas just mentioning engine size does not.


"The horsepower of a vehicle tends to be measured by its ability to turn the wheels;"
"This can be affected by many things, including the vehicle weight, efficiency of the differential and grip of the tires. "
Whilst this is true it does not affect the hp of the engine. What it affects is the hp at the wheel.
This is a not widely known fact that many manufacturers measure engine power at the crank.
As B mentions, power is lost through all sorts of things - the clutch, the gearbox, the differential, the tyres, the heater, the air con etc etc.
So what you actually get as a force between the tyre and the road is less than that quoted in the figures.
Some manufacturers measure at the wheel - Ducati do this and that is why their hp ratings for their bikes are always lower than japenese manufs. yet they still manage to beat them in racing.
So the more complex the transmission chances are the more power it takes up. And also why turning on the air con increases fuel consumption - cos you are using some of the power that otherwise would go to the road.

"A 3.5 ton van with a 2 litre engine will haul heavier loads than a sports car with same size engine "
Only true if the engine/gearboxes are different.
Put the same engine and transmission in the van and it will not only be slower but use more fuel and carry less. Because the van weight is taking up a load of the power that is spare in the sports car.
If, however, you retuned the engine for torque and low down power OR gave it a low(er) ratio gearbox then whilst the hp would be reduced (at the crank for retuning, at the wheel for gearbox change), the torque would be increased.
And torque is the rotational force at the wheel/crank. More torque means more ability to turn the wheels thus if you have a heavy load you need more power to turn the wheels round thus need more torque.
Typically though you'd retune a bit for torque and change the gearbox for a lower ratio since it is easier that way.

"Either way, the output power of the engine itself, not what comes through the wheels, is what matters as far as hp is concerned."
mmmmm as above both are valid and if you really want to know how thevehicle will perform then both bhp and torque at the wheels is what you want to know.
But at the crank is what you usually get since the figures will be higher and look more impressive.

I've also seen them use PS as a measure as well, just to confuse everyone.

A better measure is BMEP which is Brake Mean Effective Pressure and can be used to better compare vehicles/engines with differing characteristics.

OK, let me put it another way:

If I need a vehicle that is heavily loaded and I also want to tow with it, what should I be thinking of? Engine size? HP? Something else?

kea ~ asks: 'Something else?'

Yes, you should look at the torque figure. High torque at low revs is best in a load lugger or towing vehicle. Many high power engines produce their peak power and torque high up the rev range: you would have to keep the engine screaming at high revs in low gear to pull a load, and it would be hard to start off from a standstill.

This is why towing vehicles are typically turbodiesels or big, lazy petrol engines which produce lots of torque at low revs.

Note also that a smaller engine producing more power is not necessariy more efficient in the sense of fuel consumption: if it revs twice as fast, it gets through twice as much fuel/air mix.

Zube

indeed, torque is what you need, and low down. For which you really need a big engine.

The simplest way to think of it is from the design of the engine.
The explosion of the fual/air mixture acts against the piston crown pushing it down and rotating the crank and ultimately the wheels.

A bigger piston has a bigger surface area on which this force can act, and since it is wider the force can be applied further from the centre of the piston.

With a larger surface area to push against it is easier to turn the crank and thus the wheels.

In fact it is a bit like a lever. In the same way that a longer lever with a greater distance between the force (your hand) and the fulcrum can lift a heavier weight, so a larger piston surface means a you can apply the force further from the centre of the piston and get greater leverage for turning the crank.

Of course, a larger piston area doesn;t have to mean a larger engine displacement (you could have a small but wide cylinder) but the way things are made to get a larger piston you need a larger engine.

Well really you'll want Newton-Metres of moment.

But if you must use imperial, then it'll be foot-pounds .

The difference between this and horsepower: power is like how fast you can run or how fast your kettle boils. Moment is turning force - force applied at 90 degrees to an axis or pivot point and some distance away from it, so I suppose something like an arm wrestle might be a good example. Basically leverage.

Gah, I got that wrong. Teach me to post before finishing my research.

Torque isn't an imperial-specific term, nor is it the same as moment. Moment applies to leverage on a pivot. Torque is around an axis. Both are measured in Newton-Metres though.

What they said. Lots of variables; the weight of the engine itself being another one. You won't see many big, lazy diesel engines in motorbikes for example, or small, powerful high-revving motorbike engines in lorries. Engines are usually pretty well matched to the vehicle and what it is expected to do.

If you're main priority is towing ability and not outright performance, go for high torque and ignore the bhp figures. Big torquey slow noisy dirty diesel engine is what you need.

Can't we please just keep this simple and accurate?

In a rotational system, torque is the analogue of what we know as force in a linear system.
So F = m.a (force = mass x acceleration) from linear dynamics, becomes
T = I.alpha (torque = moment of inertia x angular acceleration) in rotational dynamics.

While force is linked to power in linear dynamics as:
Power = force x speed (ie rate of working = force x distance moved by force in unit time)
then, in the rotational system:
Power = torque x angular velocity
or, for the petrol-heads:
HP = Torque x rpm (times some constant to get those grotesque units right)
So most of the above follows from these simple equations, eg for high power at low revs you need lots of torque. It's not at all difficult, provided you don't mystify it.

On a more practical note, and relating to the question of towing...

There's one other thing you've got to look at when buying a vehicle to be used for towing...

Each manufacturer will put a maximum towing weight restriction on their vehicle, completely independently of the power/torque/number of hamsters/etc in the engine...

This has more to do with the physical strength of the metal making up the structure of the vehicle...

Pick the wrong type of vehicle (typically a front wheel drive family car), and even if it's got a monster diesel engine in it, if you try to tow too much weight the doors will all fly open as you set off because you'll have physically stretched the thing.

That, of course, is the extreme case, but towing more than the manufacturer says is safe is a good way of invalidating any warranty on the vehicle and, under certain circumstances, giving your insurance company a good reason to tell you to get stuffed if you have to make a claim.

"Pick the wrong type of vehicle (typically a front wheel drive family car)"

Not that I know anything about towing, but I'd guess that'd be a big factor as well? Presumably, with the additional weight of the towed item, you want either a rear wheel drive or a 4x4? And something which is designed for it so that the rear shocks can take the additional weight (therefore not a sports car which may well have the torque and the rear wheel drive but not the shocks/body strength to take the weight)?

And whilst we're on the subject, on of my favourite signs seen on a broken down vehicle many years ago, scrawled on a bit of card and attached to it was 'On Toe'
Still not sure if it was deliberate or not

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

I think what I was trying to say was - ignore the maths; the engineers have done all that for you. There is no hard and fast relationship between engine size and horsepower, and horsepower alone is not a good yardstick of practical real-world performance. Power-to-weight ratio is far more important, as is gearing. Manufacturers will adjust all the variables to match an engine to the vehicle according to the required performance and efficiency. Judge a vehicle by its performance figures.

>> (Ictoan) "Of course, a larger piston area doesn;t have to mean a larger engine displacement (you could have a small but wide cylinder) but the way things are made to get a larger piston you need a larger engine."

But for a given swept volume, if you had a wide bore, that would mean a short stroke, which would largely cancel out the torque effect of a larger piston surface area, since the shorter the stroke, the less leverage there must necessarily be on the crankshaft for the increased piston force to work with.

That's assuming there is a fixed leverage moment. Which is extremely unlikely.

B

Could you expand on that?

T h a t ' s a s s u m i n g t h e r e i s a f i x e d l e v e r a g e m o m e n t . W h i c h i s e x t r e m e l y u n l i k e l y .

Well, excuse me for thinking people might actually be interested in the topic.

oh come on, lighten up - I left it a good while for there to be a serious reply and as none was forth coming I didn't see the harm.

And my (rather weak I'll admit) reply in no way precludes B from coming back and responding seriously.