Power/Torque for those who care.

[jwalker]

To prevent more hijacking of The Beard's thread re.two motors or one I've started this thread separately.

I feel there is a bit of confusion regarding the concept of power and torque, especially related to boat engines.

As said in the other thread, boat engines can rarely make use of their maximum power at revs less than WOTat rated rpm because they are propped to allow WOT to be achieved at their rated rpm and they only have a single gear. Comparison with a road vehicle is not valid since in a road vehicle it is perfectly possible to find and incline and choose a gear which will allow full throttle to be applied without acceleration being achieved. A road vehicle overcomes this by having a selection of gears the driver may choose from, our boats don't have this luxury.

Only in a few circumstances will a boat engine be so loaded as to allow full throttle operation at low revs without accelerating.

The picture is a page from a Volvo Penta manual and the top graph's upper trace shows the maximum power a particular engine is capable of producing when artificially loaded. However, to be more realistic for a typical boat application the lower trace depicts the expected power the engine will be producing when the correct props have been selected for the boat's application. As can clearly be seen, there is a big difference between actual power and possible maximum power. However, if, say, the boat is climbing the back of a wave the props will become a mismatch for that operational load and therefore the power produced when the throttle is opened will be somewhere between the two traces or, in an extreme situation, at the upper trace if the load is high enough. We cannot know what that power will be while operating the boat so the best we can do is prop the engine to suit how we are likely to use the boat. The graphs can guide us.

Now, torque. Related to an engine this is a turning force not a measure of power and whilst the notion of having more turning force seems a good thing it may not be achievable within the constraints of a particular design of engine. Each engine will be designed for a range of application and compromises will be made during its design. Further, in a boat application there may be no worthwhile gain by increasing the torque available at lower revs. To illustrate this let's look at the torque curve of the lower graph. It can be seen that there is a levelling of the curve in the mid rpm range. (There is good reason why this occurs but let's ignore that for this example.) Now imagine that the engine was designed so that flattening of the engine torque did not occur in the mid range. This would result in the maximum power trace in the upper graph also being higher. However, since the boat is propped at WOT at its rated rpm the lower power trace would still be valid since that represents the power available at that propeller load. So, unless we happen to be in a transient boating situation where the conditions load the boat at just that area of revs where our hypothetical torque, and hence power, was increased, there would be no gain in performance. Whether that would ever happen, we can only surmise.

There are plenty of modifiers to the possible use of a particular engine with particular characteristics to a particular boat or boating application but the above is not attempting to address these, nor could it, but it is hoped that power, torque and their graphs are a little better understood when applied to a boating application.

Oh aye, apologies for the scrawl on the graphs, it was necessary when calculating the correct gear ratio for my boat.

[Searider]

But surely the difference between the upper and lower lines represents the "excess" power available and hence the ability to accelerate the boat?

Obviously up to a point - too much power will overcome the propeller and cause cavitation.

Strange torque curve on the KAD300 - maximum torque not far off maximum power.

Would be good to see the same curves for outboards.

[jwalker]

Originally Posted by Searider

But surely the difference between the upper and lower lines represents the "excess" power available and hence the ability to accelerate the boat?

Obviously up to a point - too much power will overcome the propeller and cause cavitation.

Strange torque curve on the KAD300 - maximum torque not far off maximum power.

Would be good to see the same curves for outboards.

Well yes to all your points. The lower trace is at exp.2.5 and is just a typical prop load reference.
With regard to the torque curve, I believe the engines are fuel starved in the mid range to meet pollution regulations. They're also air starved to some extent around the cross over from the charger to the turbo at approx 2800rpm.

[9D280]

Originally Posted by jwalker

If you've got the manufacturer's power curve you can plot your own wee graph of torque from that using the formula given in a previous post.

Originally Posted by martini

HP = Torque x RPM ÷ 5252

Errrr that's a linear relationship. Have a look at the power & torque curves for any internal combustion powered anything. It's not a continually rising figure with rpm. It WILL tail off, whether it does so at 3000 RPM or 6500 RPM, depends on a bucketload of things in the engine design & set-up. The bottom line is it isn't a simple linear calc.


Now, to throw even more confusuion into the pot, with EFI, the torque curve can be modified further by programming the ECU - look at the Ford Puma 1.7. Perfectly flat torque curve between about 1000 & 6000 RPM. Then look at the 1.7 (same engine) in the Focus. Much peakier curve, and also much more torque. But it's the same engine! Different gearbox in the Focus. Now, is the flat torque curve on the Puma maybe down to it being capped to not destroy the gearbox? (and as a handy aside it also makes for a much more usable engine as you have the torque available at any RPM.)

Bottom line - you need a torque curve to have this discussion sensibly.

[jwalker]

Horse power.

[9D280]

That's converting torque to power at a given RPM, which as shown is easy, because by definition torque is a derivative of power. I thought this discussion was about finding the torque?

According to that formula, mathematically they should follow each other, which they don't -There's too many thermodynamic & physical variables in an engine as the revs increase to be able to interpolate with one multiplier.

As I've had a quiet day, I decided to use the formula to compute power from torque & vice versa for JW's engine, the graphs of which I assume were from a dyno test. Tq- Power the shape is about right although the values are well out. As for Power to Torque......

I rest my case.

(Admin, if you want to movce this to JW's thread, feel free to do so.......)

[Searider]

Need to be careful with units and scales.

Just had my car remapped today.

Got rid of that stupid low flat torque curve and upped it by 100Nm between 3000rpm and 5500rpm.

Big smiles

:-)

[9D280]

Originally Posted by Searider

Need to be careful with units and scales.

Don't worry - I worked it all out in lb-ft & converted so the graphs were easily comparable with the original.