Engine Revs question

[JDR]

Well bang goes that theory.Found paperwork for engine from 1991-peak torque 85Nm approx at 2000rpm (approx).Max power 35bhp at 3000rpm only making 18bhp at 2000rpm.However same graphs show fuel consumption 3.5ltr p.h. at 1500 revs which is plain wrong.

 

So I have an engine which is nominally 35bhp but which for 95% of its life never gets above 18bhp.

Edited August 22, 2011 by JDR

[Tony Brooks]

Well bang goes that theory.Found paperwork for engine from 1991-peak torque 85Nm approx at 2000rpm (approx).Max power 35bhp at 3000rpm only making 18bhp at 2000rpm.However same graphs show fuel consumption 3.5ltr p.h. at 1500 revs which is plain wrong.

 

So I have an engine which is nominally 35bhp but which for 95% of its life never gets above 18bhp.

 

The graph is not wrong, it is a common misunderstanding of consumption graphs that cause confusion.

 

Engines are tested with the "throttle" wide open and the speed is controlled by loading the engine on the dynamometer. The fuel flow and torque at a series of speeds are taken and the hp calculated at each speed.

 

If you loaded your engine so it would only reach 2000rpm at full throttle you would find something very close to the consumption quoted. Extrapolating from typical experience one can say that your throttle is probably only about 1/3 open at 1500 RPM with the speed being limited by the fuel available. Hence the typical consumption of between 1 and 1.5 litres an hour.

[Robin2]

Well bang goes that theory.Found paperwork for engine from 1991-peak torque 85Nm approx at 2000rpm (approx).Max power 35bhp at 3000rpm only making 18bhp at 2000rpm.However same graphs show fuel consumption 3.5ltr p.h. at 1500 revs which is plain wrong.

 

So I have an engine which is nominally 35bhp but which for 95% of its life never gets above 18bhp.

 

Something wrong with the maths here.

 

If the maximum torque is at 2000 rpm and there is 18hp at 2000 rpm then there can't be more that 27hp at 3000 rpm.

 

Also (I hope my maths is right) 85Nm of torque at 2000 rpm = 8.9kW or 11.9 hp.

[AlanH]

I'm a bit confused here. Are we talking about MPG or hours/litre? I can see that in mpg terms consumption will be lower at lower revs. However if I were to run at tickover for an hour I would use less fuel than if I ran at 1500 rpm but I wouldn't get as far and would need to travel for longer in order to travel the same distance as I would at 1500 rpm thus using more fuel.

Edited August 22, 2011 by AlanH

[JDR]

Something wrong with the maths here.

 

If the maximum torque is at 2000 rpm and there is 18hp at 2000 rpm then there can't be more that 27hp at 3000 rpm.

 

Also (I hope my maths is right) 85Nm of torque at 2000 rpm = 8.9kW or 11.9 hp.

I dont have access to Betamarine website at the moment so figures are approximations based on very small inaccurate graphs.Peak torque may be a couple of hundred revs either way. Looking at older threads a figure of 2200rpm was quoted.

[nb Innisfree]

I'm a bit confused here. Are we talking about MPG or hours/litre? I can see that in mpg terms consumption will be lower at lower revs. However if I were to run at tickover for an hour I would use less fuel than if I ran at 1500 rpm but I wouldn't get as far and would need to travel for longer in order to travel the same distance as I would at 1500 rpm.

 

That's it, trying to equate modern car engines with canal boat engines is missing the point, we are trying to minimise consumption at one specific point, i.e. walking pace, not the wide range of speeds in a car, that is from walking pace to 70mph or more which involves a lot more parameters.

[casper ghost]

I checked the fuel efficiency of my little 10hp Sabb on a long cruise from Llangollen to Oxford and back. It averaged over 10 hours to a gallon. I admit it didn't go very fast but I doubt there are many engines that can match that, old or new. I've not checked my new engine yet, which is actually older!

Casp'

[JDR]

If you are calculating power and torque what are the formulas? I always thought there was a direct relationship where power=torque x revs.Power in kw, torque in Nm but do you have to convert revs to an SI unit as well?

[JDR]

The graph is not wrong, it is a common misunderstanding of consumption graphs that cause confusion.

 

Engines are tested with the "throttle" wide open and the speed is controlled by loading the engine on the dynamometer. The fuel flow and torque at a series of speeds are taken and the hp calculated at each speed.

 

If you loaded your engine so it would only reach 2000rpm at full throttle you would find something very close to the consumption quoted. Extrapolating from typical experience one can say that your throttle is probably only about 1/3 open at 1500 RPM with the speed being limited by the fuel available. Hence the typical consumption of between 1 and 1.5 litres an hour.

 

Thanks for explaining that so clearly.I have to declare no technical quallies at all just find it a very interesting subject and enjoy learning stuff.

 

PS:Apologies Dean for hijacking your thread.

[JDR]

Something wrong with the maths here.

 

If the maximum torque is at 2000 rpm and there is 18hp at 2000 rpm then there can't be more that 27hp at 3000 rpm.

 

Also (I hope my maths is right) 85Nm of torque at 2000 rpm = 8.9kW or 11.9 hp.

I am probably (almost certainly) wrong but this is what I make it.

Power=T x w

T=85Nm

w=angular velocity

 

60rpm=2pi radians p/s

60rpm=6.28318 rps

1000rpm=104.72rps

 

P=85 x 209.44

P=17802 watts

1hp=746w

hp=23.86

 

Which means i was quoting kw (18) instead of bhp.Sorry for the confusion.

Edited August 22, 2011 by JDR

[DeanS]

I suspect that most of these replies to Deans topic have confussed him even more

 

Hi Mick...been a while

 

I'm actually not too fussed...my Lister growls like a lion and stings like a bee.. It can move when it needs to..and purrs when it's happy.

[Robin2]

I am probably (almost certainly) wrong but this is what I make it.

Power=T x w

T=85Nm

w=angular velocity

 

60rpm=2pi radians p/s

60rpm=6.28318 rps

1000rpm=104.72rps

 

P=85 x 209.44

P=17802 watts

1hp=746w

hp=23.86

 

Which means i was quoting kw (18) instead of bhp.Sorry for the confusion.

 

 

The way I calculate it is as follows (my earler calcs were wrong by a factor of 1/2 )

 

2000 rpm = 33.3 rps

circumference of circle of radius 1m = 2x3.142 = 6.28m

M/s = 209.26

Nm/s = 17787 = 17.8 kW = 23.8 hp

 

Max Hp at 3000 rpm (assuming the same torque) would be 35.7

 

I'm a bit confused here. Are we talking about MPG or hours/litre? I can see that in mpg terms consumption will be lower at lower revs. However if I were to run at tickover for an hour I would use less fuel than if I ran at 1500 rpm but I wouldn't get as far and would need to travel for longer in order to travel the same distance as I would at 1500 rpm thus using more fuel.

 

Because fuel consumption rises more sharply than speed mainly due to propeller inefficiency the consumption per mile will be lower at lower speeds. A car gets a firm grip on the ground and only suffers from increased air resistance as it goes faster.

[steelaway]

All engines have sweet spots where they are happy to sit all day.I find that as a rule of thumb this is just below the point where secondary vibration and noise first begin to become irritating.You have probably sussed out where this is for your boat so stick with it.If you looked at a torque curve for your engine I would be very surprised if maximum relaxed cruising revs didn't bear a strong correlation to peak torque (and probably peak fuel economy).

 

Peak torque is the sweet spot for any given engine, where it is most efficient and supplies the highest MPG. See you engine specification - torque curve

All engines are different, depending on compression, valve size, camshafts etc. Basically the amount of through flow of the fuel/air mixture and the bigger and the frequency of the bang. If you force more air in, be it by supercharging / turbo charging / NO2 and the correct amount of fuel to suit you will get a bigger bang and subsequently BHP.

Diesel engines produce a big bang (due their very high compression) more torque at lower revs, but in a smaller power/revs band. Because of this high compression and low revs diesel engines have a very strong construction, adding to their long life

 

Alex

[JDR]

Peak torque is the sweet spot for any given engine, where it is most efficient and supplies the highest MPG. See you engine specification - torque curve

All engines are different, depending on compression, valve size, camshafts etc.

 

 

Alex

 

That has been my experience on many bike engines-singles,twins,triples and fours.After half an hour on a motorway you can tell where peak torque is being made.The numbers don't add up for my boat engine. Peak torque is somewhere around 2k revs but from experience fuel economy improves as revs are reduced with 1200rpm or less consistently returning best results.

 

Boats are so different I think you would have to compare a few similarly engined boats of similar size/setup to get a clear picture.

Edited August 22, 2011 by JDR

[steelaway]

That has been my experience on many bike engines-singles,twins,triples and fours.After half an hour on a motorway you can tell where peak torque is being made.The numbers don't add up for my boat engine. Peak torque is somewhere around 2k revs but from experience fuel economy improves as revs are reduced with 1200rpm or less consistently returning best results.

 

Boats are so different I think you would have to compare a few similarly engined boats of similar size/setup to get a clear picture.

 

 

 

It still amazes me how many boater, like me, are bikers

 

Alex

[DeanS]

It still amazes me how many boater, like me, are bikers

 

Alex

 

especially since bikes are so fast, and boats are so slow?