Torque

[Supermalc]

Yes ........ of course I should have mentioned 'engine' as in what we are taking about regarding Torque and Horse Power.

 

That's the problem with language, especially English. Too many words have too many meaning, however what was I just saying about taking things out of context.

 

Naughty naughty ..........

[Gibbo]

Yes ........ of course I should have mentioned 'engine' as in what we are taking about regarding Torque and Horse Power.

 

That's the problem with language, especially English. Too many words have too many meaning, however what was I just saying about taking things out of context.

 

Naughty naughty ..........

 

Ok to keep it in context.....

 

An engine is the big lump of metal, usually at the back, that drinks diesel, gets hot, spits oil out, has been "marinised" (painted and a nought added to the price), and moves the boat.

 

That's in context.

 

Gibbo

[Supermalc]

Very droll

 

Why not just say 'I give up' .........

[Gibbo]

Very droll

 

Why not just say 'I give up' .........

 

That would have been too easy. Where would all the equations be if I'd done that?

 

Ok..........

 

I give up.

 

And as nobody else has replied let's assume they do too.

 

So.....

 

What is an engine?

 

Gibbo

[Supermalc]

Very simple ...... a heat converter.

 

It converts heat energy into mechanical energy.

 

Brakes convert mechanical energy back to heat. So to drive economically, don't use your brakes, or only brake gently.

 

The object of the exercise btw is hopefully to help others understand, just as I was by a good teacher at Ag Eng college at Thame near Oxford.

 

I learned more from him about physics and science in only 2 terms, than I did for the rest of my education ........ even to the point I can comment on some of the electrical questons. I distinctly remember drawing the sine wave graph to illustrate how with a 3 phase alternator, the combined current (if that is the expession to use) is always 0 so the neutral and earth can become one, therefor only 4 wires are required instead of the 6 that would at first appear necessary.

[Simondrains]

What is an engine?

 

Very simple ...... a heat converter.

 

It converts heat energy into mechanical energy.

 

I think Malcolm, (if you're talking internal combustion) you must mean it converts chemical energy into mechanical energy?

 

Mind you, I only did woodwork at Rycotewood, so what would I know?

 

Si

Edited September 23, 2007 by Simondrains

[Supermalc]

chemical energy

 

What's chemical energy?

[Guest]

Engine

Definition: A compound machine by which any physical power is applied to produce a given physical effect.

[Simondrains]

What's chemical energy?

 

It's, er, thing...

 

Potential energy tied up in a chemical substance.

 

Well whatever's tied up in diesel/petrol/coal/hydrogen isn't heat, that's for sure. If you set up an exothermic reaction, you'll get heat, of course. But in an internal combustion engine the heat is just a loss. It's the change of state from liquid/atomised fuel + air to a very large quantity of gas which is the important bit. Heat is a by-product, effectively.

[Supermalc]

Yes .... ok. I'll give you that one.

 

But by burning you get energy that requires converting.

 

Never thought about it in depth before, just took it at face value.

 

So energy from burning i.e. heat energy is converted into mechanical energy, that can be utilized.

 

I can understand, and agree with both sides of the argument. So would say both are correct.

[Amicus]

What seems like centuries ago me and a mate were chatting to a guy who was repairing a cough huge excavator.

My mate asked the hp/ft lb question. "Well, it's like this", he then described a scene where we had two guys and two wheel-barrows, one guy was an Olympic weight lifting champ and the other was an Olympic 100 metre sprint champ and........................and I cant remember anymore anymore.

At the time I thought it a good simile, can anybody finish the story?

[BigRoj]

What seems like centuries ago me and a mate were chatting to a guy who was repairing a cough huge excavator.

My mate asked the hp/ft lb question. "Well, it's like this", he then described a scene where we had two guys and two wheel-barrows, one guy was an Olympic weight lifting champ and the other was an Olympic 100 metre sprint champ and........................and I cant remember anymore anymore.

At the time I thought it a good simile, can anybody finish the story?

 

I suspect it goes something like......' they can both shift a ton of coal in the same amount of time, but the olympic runner makes 3 times more journeys than the weightlifter'

 

Yes? No? Maybe?

 

Back to the original subject, some answers may be found here, www.carbibles.com

This is a very good website for all things engine!!

Edited September 24, 2007 by BigRoj

[Timleech]

The crucial thing to remember is that the engine will only produce the hp & torque that it's called upon for, up to the level of which it's capable. Also the torque required to turn the propellor goes up roughly with the square of the rpm (and the power by the cube). This means that, unless you have a fouled prop or some such, maximum torque and maximum power will both be at the maximum rpm for that particular engine/prop combination.

Suppose, for the sake of argument, that your engine is using 1 bhp to turn your prop at 1000 rpm, at 2000 rpm (double the speed) it will need 2 x 2 x 2bhp = 8 bhp (two cubed) to turn the propellor, and at 4000 rpm it will need 64 bhp (eight x [two cubed]). If it can't manage the 64 bhp, it won't reach the 4000 rpm.

 

I hope that's right, I was horrified to realise how rusty my maths has become

 

Tim

[chris w]

The crucial thing to remember is that the engine will only produce the hp & torque that it's called upon for, up to the level of which it's capable. Also the torque required to turn the propellor goes up roughly with the square of the rpm (and the power by the cube). This means that, unless you have a fouled prop or some such, maximum torque and maximum power will both be at the maximum rpm for that particular engine/prop combination.

Suppose, for the sake of argument, that your engine is using 1 bhp to turn your prop at 1000 rpm, at 2000 rpm (double the speed) it will need 2 x 2 x 2bhp = 8 bhp (two cubed) to turn the propellor, and at 4000 rpm it will need 64 bhp (eight x [two cubed]). If it can't manage the 64 bhp, it won't reach the 4000 rpm.

 

I hope that's right, I was horrified to realise how rusty my maths has become

 

Tim

 

Torque and HP will not have their maximums at the same rpm since, as a few of us have stated above:

 

BHP = (Torque x RPM)/5252 (with torque measured in ft.lbs)

 

If you run the calculus of dHP/dRPM (and remembering that HP is a function of torque, so it's not simple) you will see that the two can never peak at the same RPM. They can be numerically identical (ie: when the RPM = 5252 revs) but this is not the maximum point.

 

Chris

[ChrisPy]

Torque and HP will not have their maximums at the same rpm since, as a few of us have stated above:

 

BHP = (Torque x RPM)/5252 (with torque measured in ft.lbs)

 

If you run the calculus of dHP/dRPM (and remembering that HP is a function of torque, so it's not simple) you will see that the two can never peak at the same RPM. They can be numerically identical (ie: when the RPM = 5252 revs) but this is not the maximum point.

 

Chris

right and wrong. the brake test measures the maximum torque at any speed.

 

but putting a propellor on the end of the drive shaft and trying to displace water, the engine will run at say 2000rpm at full throttle. It is producing the maximum torque it can at that speed. Reduce the throttle and at lower speeds the engine is not workling so hard, and not producing as much torque, and even less power.

 

To simulate the brake test you would need a series of propellors, each larger than the one before. In the real world we have one propellor and one rev-point of maximum torque which coincides with maximum power.

 

I guess the best illustration is that the brake test is like going up a series of hills in a car, each steeper than the last. It will reach a different final speed for each hill.

 

In a boat it is like driving on a level road with one gear ratio. When the car will go no faster the engine is producing the maximum torque possible in that situation. At any lower speed it's just not working very hard.

[chris w]

In a boat it is like driving on a level road with one gear ratio. When the car will go no faster the engine is producing the maximum torque possible in that situation. At any lower speed it's just not working very hard.

 

Chris

 

The maximum speed is a function of the gear ratios (or in the case of the boat) of the gearbox and propellor pitch. (Before anyone jumps on me, the maximum possible speed of a displacement hull is related to its length and even a jet engine won't make it go faster).

 

But, ignoring the latter point for the moment and concentrating on the engine, the maximum torque at the propellor will still be at the point where the engine is producing its maximum torque. The actual torque at the propellor will be a factor of G x the engine torque where G is the gearbox ratio (eg: 2:1).

 

Torque does not continually increase with engine speed or, at least it does at first then flattens off and then decreases as revs are raised. For example my Isuzu 35 produces maximum torque (65ft.lbs) @ 1800rpm but maximum power (35HP) @ 3000rpm.

 

Torque will be felt as the acceleration of the boat since torque = mass x acceleration (ie: a lighter boat or more torque will both result in increased acceleration) and the gearing will affect the maximum speed as this determines what revs are applied at the propellor before maximum permitted engine revs are reached.

 

Chris

[Timleech]

Chris

 

The maximum speed is a function of the gear ratios (or in the case of the boat) of the gearbox and propellor pitch. (Before anyone jumps on me, the maximum possible speed of a displacement hull is related to its length and even a jet engine won't make it go faster).

 

But, ignoring the latter point for the moment and concentrating on the engine, the maximum torque at the propellor will still be at the point where the engine is producing its maximum torque. The actual torque at the propellor will be a factor of G x the engine torque where G is the gearbox ratio (eg: 2:1).

 

Torque does not continually increase with engine speed or, at least it does at first then flattens off and then decreases as revs are raised. For example my Isuzu 35 produces maximum torque (65ft.lbs) @ 1800rpm but maximum power (35HP) @ 3000rpm.

 

Torque will be felt as the acceleration of the boat since torque = mass x acceleration (ie: a lighter boat or more torque will both result in increased acceleration) and the gearing will affect the maximum speed as this determines what revs are applied at the propellor before maximum permitted engine revs are reached.

 

Chris

 

 

The torque available from the engine does indeed usually reach its maximum below max rpm.

The torque required to turn the propellor, however, increases with the square of the rpm and therefore must, absolutely, be at its max at max rpm.

 

Tim

[ChrisPy]

The torque available from the engine does indeed usually reach its maximum below max rpm.

The torque required to turn the propellor, however, increases with the square of the rpm and therefore must, absolutely, be at its max at max rpm.

 

Tim

ferzakkerley

 

and to give an example. let us say our engine is geared to drive the propellor at it's maximum power point, say at 3,500rpm. the load (torque) is greatest at the greatest propellor speed. so at any lower rpm the engine is not required to, and therefore does not, produce as much torque. so the maximum torque produced by the engine in the real boating situation coincides with the maximum power rpm.

Edited September 26, 2007 by chris polley

[Guest]

ferzakkerley

 

and to give an example. let us say our engine is geared to drive the propellor at it's maximum power point, say at 3,500rpm. the load (torque) is greatest at the greatest propellor speed. so at any lower rpm the engine is not required to, and therefore does not, produce as much torque. so the maximum torque produced by the engine in the real boating situation coincides with the maximum power rpm.

I regret that I can't remember the full story, but this sounds familiar to what the tuning guys used to try and explain when testing racing cars on a dyno. This of course measured BHP and torque at the wheels at various revs and under load in a given gear. Wish I'd listend more, then again it is difficult with ear defenders on.

[Supermalc]

OK ......... the engine tuning guys.

 

They tune 3 engines of the same type, all are EXACTLY the same, as near as can be measured. 2 of the engines produce the expected HP, the third does not. Why?

[Mr Baites]

OK ......... the engine tuning guys.

 

They tune 3 engines of the same type, all are EXACTLY the same, as near as can be measured. 2 of the engines produce the expected HP, the third does not. Why?

 

mi hors cant tork burav herd o mr ed he can, he got no brakes on im eever so no bhp

[Gibbo]

The confusion and disagreement is arising because some people are talking about the actual torque produced (which may or may not be the maximum it is capable of) when the engine is producing a certain BHP ie moving a boat and others are talking about what torque the engine is capable of producing at a certain RPM

 

If one extracts the maximum BHP from an engine at a particular RPM then that will coincide with the maximum torque the engine can produce AT THAT RPM.

 

That won't necessarily be the maximum torque the engine is capable of producing at any other RPM. It may be capable of producing more torque at a lower RPM. But the increased revs means more BHP at a higher RPM.

 

On a graph, the maximum torque USUALLY starts to taper off well before the maximum BHP.

 

Gibbo

[Sir Nibble]

Power is directly dependant on the amount of air burned in a given time.

Torque is directly dependant on the bmep developed in the cylinder.

 

It is easy to deduce that the air burned in any given time is directly proportional to how many induction strokes take place in the same time.

Similarly, it is apparent that bmep is independant of speed.

[anhar]

...On a graph, the maximum torque USUALLY starts to taper off well before the maximum BHP.

Gibbo

Indeed. Here's an article which explains the relationship between torque and hp quite well and includes some graphs. It's from a motorcycle site but the principles are presumably widely applicable. I have seen many motorcycle torque/hp graphs and this is the pattern they all exhibit in that max. torque is achieved before max. BHP and then falls off at higher revs. I presume the same goes for cars too.

 

http://www.totalmotorcycle.com/school-SectionSixB.htm

 

regards

Steve

Edited September 27, 2007 by anhar

[Timleech]

Indeed. Here's an article which explains the relationship between torque and hp quite well and includes some graphs. It's from a motorcycle site but the principles are presumably widely applicable. I have seen many motorcycle torque/hp graphs and this is the pattern they all exhibit in that max. torque is achieved before max. BHP and then falls off at higher revs. I presume the same goes for cars too.

 

http://www.totalmotorcycle.com/school-SectionSixB.htm

 

regards

Steve

 

These graphs are relevant in the motor vehicle situation, where they give a fair indication of what the engine will produce at different speeds when the throttle is wide open.

Their only relevance to the marine propulsion situation is in determining propellor size. Once a given prop is fitted, any torque at speeds below max is irrelevant for reasons already discussed.

 

Tim