Fuel Consumption Data

[Robin2]

Yesterday I measured the fuel consumption of my Isuzu 35 engine and here are the figures for anyone who may be interested. I ran out of time so I have not yet done tests to see what the effect on fuel consumption of different alternator loads. However I will spend some time studying these figures before carrying out more tests. On interesting finding is that (with the engine in neutral) the amount of fuel injected in each stroke is almost 40% higher at 935 rpm compared to 2700 rpm. Presumably this reflects combustion inefficiency at low rpm.

 

Anyway, here are the data. Comments, suggestions and data from other users are very welcome.

 

____RPM_____L/hr

 

Neutral

____935____1.02

___1200____1.14

___1500____1.37

___2070____1.63

___2700____2.13

 

In Gear

_____870___1.06

____1278___1.74

____2070___4.93

[AlanH]

Yesterday I measured the fuel consumption of my Isuzu 35 engine and here are the figures for anyone who may be interested. I ran out of time so I have not yet done tests to see what the effect on fuel consumption of different alternator loads. However I will spend some time studying these figures before carrying out more tests. On interesting finding is that (with the engine in neutral) the amount of fuel injected in each stroke is almost 40% higher at 935 rpm compared to 2700 rpm. Presumably this reflects combustion inefficiency at low rpm.

 

Anyway, here are the data. Comments, suggestions and data from other users are very welcome.

 

____RPM_____L/hr

 

Neutral

____935____1.02

___1200____1.14

___1500____1.37

___2070____1.63

___2700____2.13

 

In Gear

_____870___1.06

____1278___1.74

____2070___4.93

Oh dear - Analworld again! What on earth are you going to do with this information?

[RLWP]

Oh dear - Analworld again! What on earth are you going to do with this information?

 

Worry about it when he's boating

 

Richard

[JonL]

Oh dear - Analworld again! What on earth are you going to do with this information?

 

That's a little harsh

I actually find this sort of information interesting.

There are a huge amount of threads about fuel costs so someone actually taking time and I assume money to give accurate consumption information on a common used engine will help people understand there boating costs and make informed decisions.

 

It's very easy to criticise.

[Ex- Member]

That's a little harsh

I actually find this sort of information interesting.

There are a huge amount of threads about fuel costs so someone actually taking time and I assume money to give accurate consumption information on a common used engine will help people understand there boating costs and make informed decisions.

 

It's very easy to criticise.

 

Agreed trying to get accurate costs when comparing different energy sources the fuel efficiency figure is king. I'm currently trying to calculate engine running cost against solar panels and was using 1 litre per hour as an average for my 50hp perkins, well if the 35hp Izuzu is doing around that then my Perky 50 is probably drinking more than a litre per hour when charging.

 

Anyone done any fuel consumption figures on a Perkins 50hp Max revs 2800

 

 

 

[alan_fincher]

I almost hate to ask.....

 

But how do you really run your engine in neutral at 2,700 RPM for long enough to measure with any accuracy that you are consuming 2.13 litres an hour ?

 

Can you really know to two places of decimals, and how on earth are you measuring it ?

 

And "in gear" ?

 

Is that moving ? Is so, doesn't canal conditions affect it ?

 

Or tied up ? In which case if it were on BW waters, you really shouldn't be!

 

No idea what revs you normally boat at, but (say) 1278 doesn't sound a lot. If you were really using a genuine 1.74 litres an hour at those revs, it really doesn't sound good enough for a modern engine like an Isuzu 35.

 

We don't exactly try and save the fuel, and seldom do worse than 1.5 litres per hour for a very elderly BMC 1800, and generally a lot better than that. I'd be very disappointed with our presumably less efficient engine to be using that much fuel at under 1300 RPM! Sounds plain wrong, TBH!

 

And how are you measuring 1278 RPM - seems a very precise number!

[RLWP]

Robin, how did you measure these figures?

 

Richard

[Proper Job]

......No idea what revs you normally boat at, but (say) 1278 doesn't sound a lot.

 

1278! I'd be positively planing with two water skiers behind me

[RLWP]

1278! I'd be positively planing with two water skiers behind me

 

And getting ready to dodge flying bits of exploded engine?

 

Richard

[Robin2]

Thanks for the comments.

 

Yes, it may be anal, but I wanted to have a good basis for considering the economics of alternatives including a separate genny or solar panels.

 

Measuring at 2,700 rpm is simple - push the throttle all the way forward !

 

I agree the 2 places of decimals is spurious accuracy but I have given you the numbers as recorded.

 

I set up an apparatus with a section of clear plastic hose marked at 0, 5, 10 and 15cc and timed how long it took the engine to consume that amount of fuel.

 

The RPM is measured accurately - though I had intended to round off the 1278 figure.

 

My recent cruising practice is to use 1278 rpm (which looks like 1100 on my tachometer and gives just over 3mph most of the time.

 

I probably should not have done "in gear" tests while tied up but each was only for a very short time. Also there would almost certainly be a difference when the boat is moving - probably less load on the propeller. But it would be very difficult to do tests while the boat was moving without a more elaborate test setup and an assistant.

[alan_fincher]

1278! I'd be positively planing with two water skiers behind me

Suspect "cruising revs" for an Isuzu are a lot higher than a JP though.....

 

(But then you know that, don't you!.....)

 

1278 would probably even be a bit speedy for Sickle (HR3), but I'd expect to use more than that in clear water with Chalice (BMC).

 

Quoted Isuzu 35 data:

 

35HP at 3,500 RPM

65 ft lbs at 1,800 RPM

850 RPM Tick-over

1,499 CC.

[Proper Job]

And getting ready to dodge flying bits of exploded engine?

 

Richard

They're happy when you work 'em hard

[RLWP]

Thanks for the comments.

 

Yes, it may be anal, but I wanted to have a good basis for considering the economics of alternatives including a separate genny or solar panels.

 

Measuring at 2,700 rpm is simple - push the throttle all the way forward !

 

I agree the 2 places of decimals is spurious accuracy but I have given you the numbers as recorded.

 

I set up an apparatus with a section of clear plastic hose marked at 0, 5, 10 and 15cc and timed how long it took the engine to consume that amount of fuel.

 

The RPM is measured accurately - though I had intended to round off the 1278 figure.

 

My recent cruising practice is to use 1278 rpm (which looks like 1100 on my tachometer and gives just over 3mph most of the time.

 

I probably should not have done "in gear" tests while tied up but each was only for a very short time. Also there would almost certainly be a difference when the boat is moving - probably less load on the propeller. But it would be very difficult to do tests while the boat was moving without a more elaborate test setup and an assistant.

 

What sort of accuracy do you think you have on the fuel volume and time readings Robin? +/- 0.5 seconds on timing I guess, what about the fuel consumption?

 

And, does that include fuel spilled back to the fuel tank?

 

Richard

Edited July 5, 2011 by RLWP

[alan_fincher]

Measuring at 2,700 rpm is simple - push the throttle all the way forward !

 

That's a lot less than the off-load RPM that you might expect to get from quoted numbers.

 

Perhaps there is some kind of end-stop set somewhere in the linkage ?

 

My recent cruising practice is to use 1278 rpm (which looks like 1100 on my tachometer and gives just over 3mph most of the time.

 

If only cruising at those revs, and getting just over 3mph, I'd be disappointed to be using 1.74 litres an hour. Instinctively, it doesn't sound right.

 

Mind you, I don't know how big your boat is, what draught, or where you are boating it.

[Robin2]

What sort of accuracy do you think you have on the fuel volume and time readings Robin? +/- 0.5 seconds on timing I guess, what about the fuel consumption?

 

And, does that include fuel spilled back to the fuel tank?

 

Richard

 

I created the measuring points by filling the tube with a small syringe so I reckon they are accurate to better than 0.5 cc

 

If, by fuel spilled back to the tank, you mean the overflow from the injectors - yes I have taken that into account - that fuel fed back to my measuring equipment. In fact that flow seems to be far greater than actual consumption.

[RLWP]

I created the measuring points by filling the tube with a small syringe so I reckon they are accurate to better than 0.5 cc

 

If, by fuel spilled back to the tank, you mean the overflow from the injectors - yes I have taken that into account - that fuel fed back to my measuring equipment. In fact that flow seems to be far greater than actual consumption.

 

I'm struggling to understand exactly what you did, and I'm interested to see if I can repeat it with our engine

 

Could you knock up a sketch of your method Robin?

 

Richard

[Robin2]

That's a lot less than the off-load RPM that you might expect to get from quoted numbers.

 

Perhaps there is some kind of end-stop set somewhere in the linkage ?

 

 

 

If only cruising at those revs, and getting just over 3mph, I'd be disappointed to be using 1.74 litres an hour. Instinctively, it doesn't sound right.

 

Mind you, I don't know how big your boat is, what draught, or where you are boating it.

 

I, also, suspect there is an end-stop. But it is probably academic because the engine would only go to 2070 rpm in gear before it started smoking. The air intake arrangement is unusual - almost non-existent so I doubt that 35hp is possible.

 

I have no comparative figures for cruising fuel consumption so I am interested to hear from others. Mine is a 57ft boat with quite shallow draught. As I said above the fuel consumption when actually moving might be lower.

 

I believe my engine is in good internal condition. It does not smoke and it starts in the first or second revolution even in cold weather.

[bottle]

Having a Beta 43, I just went to the web site and looked at the graph for fuel consumption.

 

Oh! and I have a fuel gauge fitted, when it gets near 1 (scale 0 to 10) I fill it up.

[Robin2]

Here's a diagram of my apparatus - much less embarrassing than a photo.

 

 

 

Having a Beta 43, I just went to the web site and looked at the graph for fuel consumption.

 

Oh! and I have a fuel gauge fitted, when it gets near 1 (scale 0 to 10) I fill it up.

 

Have you a link to the fuel consumption graph? I would be surprised if it shows part-load figures.

[nb Innisfree]

I've been meaning to do an accurate record in exactly the same way as Robin, but assessing as accurately as I can I came up with 1 litre per hour at 1200 rpm cruising with our LPWS4 which is very handy for calculating fuel split as we do 3 mph at those revs and as we have an hour counter actuated when we are in gear we can easily work out the ratio of cruising to generation.

 

Don't knock Robin for doing stuff like this, after all he's only playing like the rest most of us.

Edited July 5, 2011 by nb Innisfree

[RLWP]

By my calculations, 15cc is about 30mm of 8mm diameter tube, and it takes 22 seconds to use that up at 4.93 litres/hour. If you could read the 30mm to +/- 0.5 mm and the time to +/- 0.5 seconds you've got your volume to about +/- 1.6% and your time to +/- 2.3%

 

That's pretty good for what you are trying to work out

 

Richard

 

<someone will show me where I got my sums wrong soon>

[steelaway]

Thanks for the comments.

 

Yes, it may be anal, but I wanted to have a good basis for considering the economics of alternatives including a separate genny or solar panels.

 

Measuring at 2,700 rpm is simple - push the throttle all the way forward !

 

I agree the 2 places of decimals is spurious accuracy but I have given you the numbers as recorded.

 

I set up an apparatus with a section of clear plastic hose marked at 0, 5, 10 and 15cc and timed how long it took the engine to consume that amount of fuel.

 

The RPM is measured accurately - though I had intended to round off the 1278 figure.

 

My recent cruising practice is to use 1278 rpm (which looks like 1100 on my tachometer and gives just over 3mph most of the time.

 

I probably should not have done "in gear" tests while tied up but each was only for a very short time. Also there would almost certainly be a difference when the boat is moving - probably less load on the propeller. But it would be very difficult to do tests while the boat was moving without a more elaborate test setup and an assistant.

 

 

 

 

Well I think its a good idea

 

My experience has always been that peak MPG is always at peak torque on any engine. I would expect you will find that the highest efficiency will be at 1800 rpm with your Beta.

 

My maximum torque (Barrus 45) is between 1600 and 1800rpm my prop was chosen to allow me to cruise at these revs.

 

Alex

[nb Innisfree]

The best fuel economy for cruising is to propel the boat as slow as possible to reduce drag though that needs to coincide with a low economical engine speed, if a small enough engine can idle and at the same time propel a boat which is still steerable then that's the most economical you can get IMO.

 

Drag is the killer, it increases at an alarming rate with speed.

Edited July 5, 2011 by nb Innisfree

[koukouvagia]

I must admire all these scientific calculations and the energy and ingenuity expended. Way over my head.

 

All I need to know on my Kelvin K2 is how much extra fuel I've used if I've opened it up a bit – say on a river. I never used to bother, but with the price of fuel these days I like to see how much I've saved by going more slowly. Flat out on a river I'm doing about 450 – 550 rpm; on a canal it's more like 250 – 300.

 

At the end of a day's boating I fill up the day tank. Eight hours cruising will mean that I pump the hand pump 80 times. On canals no matter what the conditions – locks, moored boats etc. steady progress always means 10 pumps per hour that the engine's been running. If I count up to hundred pumps, I've been hammering it a bit and anything over 100 means I've used up to a third more diesel than I need have done.

Edited July 5, 2011 by koukouvagia

[bottle]

Link to page with all Beta engines:

 

http://www.betamarine.co.uk/Templates/inland.htm

 

They ae PDF documents, it does say

 

" Fuel Consumption based on theoretical

propeller loading - matched at full speed"