Power

[markw]

Can a techy out there please answer/explain the following:

 

1) The difference between indicated horsepower, brake horsepower and effective horsepower, and which is usually expressed.

 

2) The power of continental cars is expressed in kilowatts, which version of horsepower is this comparable to as stated in the car's user manual.

 

3) What is the conversion factor between horsepower(s) and kilowatts.

 

If this wasn't confusing enough

 

4) Is brake horsepower per ton(ne) expressed in metric or imperial ton(ne)s

 

Mark

[Peter Carmichael]

Indicated is to do with "indicator diagrams" - pressure vs. volume in the cylinder. It is theoretical at best.

 

Brake horsepower is measured with a brake, but essentially means the output at the flywheel.

 

Don't know about "effective" - sounds like after losses.

 

1)Manufacturers quote Brake horsepower.

 

2) The DIN kw figures are equivalent.

 

3) One horsepower is about 750watts.

 

4) Always metric tonne.

 

[edmandsd]

The conversion factor of horsepower to kilowatts is 1.34 i.e. 100kw = 134bhp (Inverse is 0.75)

 

When car's are weighed in kilos then bhp per tonne (i.e. Metric - 1,000 kgs) would seem sensible - and easier to calculate.

 

[Simos]

Imperial ton = 1,016kg

 

i.e. not a great deal of difference anyway.

 

U.S. ton = 907kg

 

Metric tonne = 1,000kg

 

Edited by - simos on 23 May 2002 14:20:44

[scooby dooby doo]

You'll see PS as well. This is PferdSchrieber (ie german for horse power (spellt incorrectly no doubt in this case)). I think 1 PS = 1.015 hp. ie its the same really.

 

You're going to ask me to convert between NM and ft lb soon aren't you?

 

the losses that some have spoken of occur between the engine and the road (ie in the gearbox, diff, tyres etc etc). There have been very heated arguments about this but over 30% loss is possible. A dyno reading will measure the flywheel output, a rolling road records the output at the wheels. You can then work back by looking at how it is slowed down by the transmission once you put it in neutral.

 

Dave Hooper - North London

dmch2@lineone.net

[murph7355]

Don't use the % sign Hoops, PC now has a trophy to throw at you if you do teeth.gif

[murph7355]

Sh*te

 

 

Edited by - andy murphy on 24 May 2002 00:54:38

[scooby dooby doo]

er? what? me?

 

um.... er... the losses may be more than you think? will that do?

 

um... my car made 100 bhp at the rollers at one place but at dave walkers with the coast down they reckoned 144. this from a 1.6 K SS. um, draw your own conclusions - i'm off to bed!

 

smile.gifsmile.gifsmile.gif

 

 

Dave Hooper - North London

dmch2@lineone.net

[Seven in SA]

Hoopy,

 

The coastdown adjustment will enable you to calculate your power at the flywheel instead of the wheels. We need to know the flywheel one for boasting rights (as its always bigger).

 

As for the different power units I found this.

 

One SAE/British Horsepower is 746 watts/0.746 kilowatts.

One DIN/JIS HP/PS is 735.5 watts.

SAE = Society of Automotive Engineers.

DIN = International Standards Organisation.

PS = Abbreviation for German term for horsepower.

JIS = Japanese Standards Institute.

1 DIN/JIS hp = 0.986 SAE/British hp

 

 

Edited by - Seven in SA on 24 May 2002 08:18:17

 

Edited by - Seven in SA on 24 May 2002 13:08:26

[markw]

Thanks for the enlightenment!

 

So when comparing the performance of cars would quoting figures on torque and power available at the wheels be preferential, thus taking into acount the transmission ratios and the transmission losses respectively?

 

Mark

[Peter Carmichael]

Are you trying to wind me upquestion.gif wink.gif

 

"No" is the short answer. [pre-emptive mode]I invite contradictions of the laws of physics, but you're all wrong.[/pre-emptive mode]

 

People correctly, but imprecisely talk about "torquey" engines. Magazines quote maximum power and maximum torque - this misleads people into thinking that it is a simple matter of how big the numbers are, which is only half the story.

 

You do not run your car at maximum power all the time. When you floor the throttle for an overtake, the engine develops the most amount of power it can as it runs through the rev range. There is more than one power number - the peak power figure is just one point on the power curve. What matters is how fat the power curve is, but this is almost impossible to present in a neat table at the back of Autocar, so they don't bother. They tell you peak torque and the peak power - two points in the rev range.

 

First I will convince you that torque is a pointless abstract unit of engine performance.

 

Torque is twisting force. When you torque up the various nuts and bolts that hold the car together you use a torque wrench, so you are very familiar with the concept and the numbers involved. My engine produces a peak of 167lbft of torque. I torque my rear hubs up to 200lbft, using a torque wrench. If I took my engine out and attached that big torque wrench to the input shaft of the gearbox, I could apply more torque to the gearbox than the engine can. Why then don't we have little men under the bonnets of our cars with torque wrenches? Why do we have engines instead?

 

The answer is that the engine develops 167lbft of torque at 6000rpm - it is torque that is moving fast . The 200lbft that I can supply with a torque wrench is static . If that gearbox input shaft was rotating, I would struggle to keep up with it with my torque wrench and the actual torque I could supply would diminish to nothing.

 

So torque is only useful if we know how fast it is moving. We need to read the whole description of the peak torque figure before it has any meaning. We cannot separate the torque figure from the revs figure at which it is developed. Now having done the intuitive bit, here is the physics bit:

 

Torque(lbft) * rpm /5252 = Power (BHP)

 

Try it out:

 

167lbft @6000rpm?

 

167 x 6000/5252 = 190.78 BHP. At 6000 rpm my engine produces ~191 horsepower. Hey presto! We have a second point on the power curve.

 

There's more to come, but I will let you digest this much and see if it prompts a few questions.

 

Peterid=blue>

eek.gifSecond loser, Class 5, Curborough May 2002id=red>

[scooby dooby doo]

I always thought that a better measurement of a gearbox / engine system would be something like:

integrate the torque over the revrange between the optimum change up point and the point at which the revs will fall to in the next gear.

 

Obviously this varies for each gear, so we need to choose, eg the one which takes you through 60.

 

Also if the engin is VERY peaky and you have loads of ratios its no good as you spend all your time changing gear.

 

so perhaps the total figure should be adjusted:

integrated torque divided by (the time the engine takes to accelerate the car over the range plus the shift time).

 

OTOH thats so complicated that a simple acceleration curve using optimum change points would be more useful. In fact matching up in gear acceleration graphs is how you find the optimum change point isn't it?

 

I'll go away again now i think...

 

Dave Hooper - North London

dmch2@lineone.net

[Peter Carmichael]

Almost right hoopy, but it is the power curve you need to integrate. This is because between final drive ratios and gearboxes, nobody really gives a damn what the acceleration is at "5000rpm in third gear", for instance, but people do care about how much acceleration they can achieve when pulling out to overtake a truck at 50mph. You hinted at this, but the power curve consideration gives you everything you need to know. Take my engine for instance. It peaks at 253 horsepower, but gearing means I can always find a gear to keep it on the boil with more than 235 horsepower - I can do all my balls-out acceleration calculations based on a figure of ~240bhp and ignore the gearing.

 

You also plumped on the idea of the combined engine/gearbox unit, but if you plug an engine with fatter power delivery into the same gearbox, then you get more out the other side. I would suggest it is valid to compare engines on their own when you know they are going to end up attached to similar gearboxes.

 

There is still the issue of "torquey" engines vs. peaky, which again you acknowledge. A mathematical integral will only readily tell you the answer for balls out acceleration through the gears. Instead, you can get the same comparison graphically by plotting the power curves against a horizontal scale from zero (engine stationary) to one (change up point). This gives the very telling comparisons that you have supposed.

 

I have all the curves from the three Sevens list Emerald rolling road days and I have a spreadsheet for comparing them in this manner. It means that you can legitimately compare Ian Crocker's 300bhp Rover V8 with Graham Ford's screaming Vauxhall. I can email this to interested parties, but be aware that you will have to RTFM because it is a working tool for my own engine developments rather than a polished product.

 

Peterid=blue>

eek.gifSecond loser, Class 5, Curborough May 2002id=red>

 

Edited by - Peter Carmichael on 30 May 2002 13:49:32

[wg_mulholland]

So torque is directly proportional to power at 5000 odd rpm? Is this not a rather unfair way of comparing bike engines with cars then? If a good k revs to 9000, and torque is measured at 5000 odd- then that gives an idea of the shape of the graph for the top 3.5 K revs. So for a bike running to 11K revs, should we look at torque at 11-3.5= 7500 rpm? What about slow revving engines? 500 odd might be near the peak power of a slow revving engine giving a peaky slow revver a really good torque figure?

 

Is this right? What about diesels? can they reach 5500 for measurement? What about tractors?

[V7 SLR]

Peter I would be interested in your comparisons-spreadsheet please. I've been struggling to add a number of other people's results over mine. I'm trying to determine whether the characteristics of other engines, similar in spec to where I'm aiming, are similar in terms of power/torque curves.

 

My next upgrade will most certainly be a set of 1227 cams. I wish to see whether everyone who already have moved to this spec develop more power at revs below 8000rpm which is my M.O. If 1227 cams don't make much difference until after 8000rpm I probably won't bother... I don't believe that this is the case though...

 

If it's particularly difficult to use, don't suppose and quick bulletted user guide is possible? I tried simply adding other people's figures into the same worksheet as my own, but I think they reference difference revs... they don't seem to line up right anyway.

 

Thanks.

[oilyhands]

Nudger,

 

Looking at your power curve here, a similarly specced engine with 1227 cams starts to go ahead at about 5800 where your curve starts to soften, your has very good torque but this starts to fall quite rapidly above 6300 where a 1227 equipped engine continues on a plateau until around 7900-8000, from this I think I can safely say that you will get appreciable gains from 1227 cams and some decent throttle bodies.

 

Oily

[V7 SLR]

I had no doubt, but rather fancied being able to overlay Mick's, Steve Butts, Robert Walkers...etc... just for comparison. Not much else to do other than to write another damned quote for someone who'll just complain that it's too expensive.

 

I like the sound of the torque still climbing in the areas you state. That's about where I keep the engine.

 

Gotta say though, that it's absolutely fabulous as it is. Problem I have is leaving it alone if it could be better... and then finding the cash to make it better.

[Peter Carmichael]

Bill,

 

You are right that bike engines get a raw deal in the simple scheme of comparisons. Apart from peak power, the only other piece of information you tend to get told about is peak torque - I have explained how you can turn that into another point on the power curve. What goes on between those two points is anyone's guess, but it will be going upwards fairly consistently and curving off at the top. Ideally you want the whole curve to compare.

 

Torque does not tend to get measured at any fixed rpm, as you suppose - the industry just brags about the peak torque figures as though they have some pertinent meaning and reports them wherever they occur in the rev range.

 

A more sensible comparison that you could tabulate in Autocar would be:

Column 1: Peak power @ peak rpm

Column 2: % of peak power available at 75% of peak rpm

Column 3: % of peak power available at 50% of peak rpm

 

For my engine these values would be: 253BHP, 87%, 47%

For a good SLR the figures I have on a graph show (V7 SLR): 195BHP, 87%, 50%

For a Rover V8 graph that I have (Ian Crocker): 300BHP, 85%, 50%

For a good Rover 1.6 Supersport (Pie Boy's): 144BHP, 85%, 53%

For Bill Ayre's BDX: 201BHP, 80%, 36%

 

Sorry, I kept on going until I found a good example of peakiness. Bill's engine is peaky compared to the others I selected. You can see that my engine doesn't give much away in flexibility terms compared to big V8s and lesser tuned Ks.

 

You wrote "so torque is directly proportional to power at 5000rpm". This is a bit of a muddled way of looking at it. It is just a curiosity of units that 5252 rpm is the magic number with horsepower and lbft as the units. The power is proportional to the torque at any rpm - but it is proportional to the rpm value too!

 

In SI units, the fudge constant of 5252 disappears:

 

Power (Watts) = Torque (Nm) x angular velocity (radians/second)

 

 

Peterid=blue>

eek.gifSecond loser, Class 5, Curborough May 2002id=red>

[Simos]

It's perhaps worth noting where the 5250 figure comes from.

 

1 horsepower is defined as 550 ft lb / sec.

Since we're dealing with rpm and not linear measure the length part of the units is divided by 6.283 (a 1lb weight on the end of a virtual bar 1 ft long rotated through 1 revolution travels 6.283 feet (Pi * diameter of 2 feet))

So our 1 lb weight needs to do 87.5 revs (550/6.283) per second to be doing 1 hp (550ftlb/sec) or 87.5*60sec = 5252 rpm.

 

This is where the "relative to 5252 rpm" comes from - it's the defining point of 1 horsepower - and why all imperial torque/horsepower curves cross or touch at 5252 rpm.

[Simos]

Bill's is a peakier engine, suspect that is because it is on carbs which are tuned to the top end and that the common denominator for the others is injection and ecu map giving better tune at the 50% revs. With injection i suspect Bill's engine would show a not dissimilar spread of power.

[scooby dooby doo]

Peter - I'd love the a copy of the spreadsheet please!

from you're figures the V8 is not as torquey (in specific terms) as is imagined. is it highly tuned then?

 

now - two molotovs to fan the flames in this discussion...

 

1) the power versus torque debate is raising its head here. specifically as you get faster, ie in later gears, should you change earlier or later?

my (incorrect / random / open for correction) thoughts are that power is what pushes the air out the way but torque is what accelerates you as it pushes on the road. so at low speed (assuming symmetrical torque and power curve (hmm... thats not actually possible is it?), instantaneous changes etc) you change around peak torque in 1st gear. ie so that the torque is the same before and after the change but you're the other side of the peak.

but when changing into top you need to change around peak POWER as almost all the engines effort is going into pushes air out the way.

is this right?

 

2) slightly further off the topic. I've managed to get hold of some steel rods (cheers oilyhands!) so my main reason for sticking to piper 740 cams has evapourated as i can now be safe from the engine letting go at up to 8500 instead of 8000 (are those numbers about right?). so the question is whether 1227 cams would be worth getting given that its a std (but ported) head? or should i delay the build a bit further and look for a VVC head? (which will mean getting a new set of larger valves again etc).

the alternative is to stick with the std head, 740s and have an engine that can safely rev much higher than peak power so should be able to hold gears on the exits of corners should i not quite have the right gearing availalbe?

 

Dave Hooper - North London

dmch2@lineone.net

[oilyhands]

Dave,

 

The 1227s when properly timed will give best results with a VVC head, peak power will be higher and torque will be better, the difference is not huge, but worth having.

 

Dave

[wg_mulholland]

Aaaaah- so power is torque x angular velocity,

 

So peak torque is simply the point where the force of the explosion in the cylinder is at its maximum- but the power is not as increacing the revs more than compensates for the reduction in efficiency and force of this explosion to give more power up to the maximum power point- after which efficiency falls away?

 

So what is more important for acceleration- to be at max torque or max power? Initially I think max torque as Angular acceleration= torque/moment of inertia. But I suppose you should really think of it as a change in kinetic energy- in which case max power would give the best acceleration. Am I right?

[wg_mulholland]

Aaaaah- so power is torque x angular velocity,

 

So peak torque is simply the point where the force of the explosion in the cylinder is at its maximum- but the power is not as increacing the revs more than compensates for the reduction in efficiency and force of this explosion to give more power up to the maximum power point- after which efficiency falls away?

 

So what is more important for acceleration- to be at max torque or max power? Initially I think max torque as Angular acceleration= torque/moment of inertia. But I suppose you should really think of it as a change in kinetic energy- in which case max power would give the best acceleration. Am I right?

[scooby dooby doo]

dave - are you saying that 1227s on a std head will give benefits but they'll be small? will i lose much lower down with them? or should i just build it as it is and in a while i can get a VVC head, 1227 cams (my 740s were s/h so were half price) and a steel crank and do it all properly? hmm- thats the end of 2003 bonus spent...

 

wg_m - power id force times velocity. or if you are rotation its torque (angular force) x rotational velocity. change in kinetic energy requires an impulse which is force times time.

hence my thoughts that torque accelerates you.

 

but power pushes air out the way with a 3rd power relationship with velocity. so you need power for top speed.

 

Dave Hooper - North London

dmch2@lineone.net