Outboard fuel efficiency: 2-strokes vs. 4-strokes

[rbmatthews]

We always hear that four-stroke outboards are much more fuel efficient that two-strokes, but are they? I have been doing a little bit of research to see how the more modern 2-strokes compare to 4-strokes. I gathered performance data on two engines – a Mariner Optimax 75 hp (two-stroke) and a Honda BF75 (four-stroke) from the web-sites of the manufacturers (I chose the Optimax because that is what we have on our boat). The graph below shows the fuel consumption figures for those engines with a range of boats – red ones for the Optimax, blue ones for the Honda. (Note that this is fuel consumption, the inverse of fuel efficiency.)



Up to about 3000 RPM there is very little difference between the two types of engine. Between about 3000-4000 RPM, it looks like the four-stroke consumes a bit less than the two-stroke, but at higher revs above 4000 RPM, the two-stroke is using less than the four-stroke.

In normal use, a range of engine speeds would be used, so my guess is that the differences between the mid- and high-range consumption figures would cancel out, and there would hardly be any difference between the two. If both engines are used at WOT continuously however, it would seem that the two-stroke might be even more efficient than the four-stroke!

I know that it is not much of a sample with just two engines, and also the weight of the boats used in the Honda tests seem to have been slightly heavier than those used in the Optimax ones, but I was just wondering if other forumites have found this in practice?

 

[landlockedpirate]

Your original statement that 2 strokes are less efficient than 4 strokes is based old 2 stroke technology. As you have realised modern 2 strokes have very little difference in economy to the 4 strokes. The family had a 90hp Etec on a center console boat and the economy was simply staggering, in fact it was better than the Honda 4 stoke outboard we currently have.

 

[rustybarge]

We always hear that four-stroke outboards are much more fuel efficient that two-strokes, but are they? I have been doing a little bit of research to see how the more modern 2-strokes compare to 4-strokes. I gathered performance data on two engines – a Mariner Optimax 75 hp (two-stroke) and a Honda BF75 (four-stroke) from the web-sites of the manufacturers (I chose the Optimax because that is what we have on our boat). The graph below shows the fuel consumption figures for those engines with a range of boats – red ones for the Optimax, blue ones for the Honda. (Note that this is fuel consumption, the inverse of fuel efficiency.)

View attachment 35957

Up to about 3000 RPM there is very little difference between the two types of engine. Between about 3000-4000 RPM, it looks like the four-stroke consumes a bit less than the two-stroke, but at higher revs above 4000 RPM, the two-stroke is using less than the four-stroke.

In normal use, a range of engine speeds would be used, so my guess is that the differences between the mid- and high-range consumption figures would cancel out, and there would hardly be any difference between the two. If both engines are used at WOT continuously however, it would seem that the two-stroke might be even more efficient than the four-stroke!

I know that it is not much of a sample with just two engines, and also the weight of the boats used in the Honda tests seem to have been slightly heavier than those used in the Optimax ones, but I was just wondering if other forumites have found this in practice?

The new Suzuki leanburn seems to be getting the same economy with a 1.5 ton boat/80 hp/4500rev...http://www.suzuki-marine.co.uk/docs/Sea Rover 21D - DF80A (Lean Burn).pdf

That's impressive for a boat 3 times heavier than on your graph!

 

[rbmatthews]

Your original statement that 2 strokes are less efficient than 4 strokes is based old 2 stroke technology.

To some extent, but I still keep hearing from others that even with all the new DFI technology etc., that 4-strokes are still about 20% more efficient that 2-strokes. The data in the graph and your own experience would suggest that this is not the case and that there is little difference.

We used to have a 1988 Mariner 75 hp which did an average of 1.1 NM/litre on the trips we did; we replaced it with a 2011 Mariner Optimax 75 hp last year (i.e. same engine, just 23 years later), which now does an average of 2.2 NM/litre on similar trips. That's an increase in efficiency of 100%! It's more than academic for us as it gives us twice the range before refueling.

 

[rbmatthews]

The new Suzuki leanburn seems to be getting the same economy with a 1.5 ton boat/80 hp/4500rev...http://www.suzuki-marine.co.uk/docs/Sea Rover 21D - DF80A (Lean Burn).pdf

That's impressive for a boat 3 times heavier than on your graph!

I think that the boat weights in the graph are for boat only and not including engine and gear, but all the same, it is impressive.

So if 2-strokes are more fuel efficient, similar or less in weight, less complicated, cheaper, and more immediate power available than 4-strokes, what is the advantage in the latter at all?

More reliable?

Rob.

 

[rustybarge]

I think that the boat weights in the graph are for boat only and not including engine and gear, but all the same, it is impressive.

So if 2-strokes are more fuel efficient, similar or less in weight, less complicated, cheaper, and more immediate power available than 4-strokes, what is the advantage in the latter at all?

More reliable?

Rob.

The new generation of 4str outboards use lean-burn tech. I'm not sure that this technology will work on 2 str engines.
Multi-sequental fuel injection give a 4str lots of time to burn the mixture, so overheating on a lean mixture doesn't affect the engine. On a 2 str the engine cycles overlaps, so perhaps it's not possible to run as lean a mixture.

Single cylinder 2str rule supreme, twice the torque, twice twice the power but twice the fuel consumption. Well nearly sort of....!

Please make corrections if this is BS.

 

[spannerman]

Try the consumption figures on the 300hp Mercury Verado, below 3000rpm they aren't too bad around 55 lph, go up to 5000 rpm and they go up to 130 lph! and we have a customer with 3 of the things on the back! at 40kts its burning 400 lph...!
Makes my low tech 5.7 V8 with a carb positively economical at 33lph@3000rpm and 65lph@4500rpm.

 

[RobWales]

Try the consumption figures on the 300hp Mercury Verado, below 3000rpm they aren't too bad around 55 lph, go up to 5000 rpm and they go up to 130 lph! and we have a customer with 3 of the things on the back! at 40kts its burning 400 lph...!
Makes my low tech 5.7 V8 with a carb positively economical at 33lph@3000rpm and 65lph@4500rpm.

Interested to know what boat your customer has with the three hanging off the back?

 

[ontheplane]

Try the consumption figures on the 300hp Mercury Verado, below 3000rpm they aren't too bad around 55 lph, go up to 5000 rpm and they go up to 130 lph! and we have a customer with 3 of the things on the back! at 40kts its burning 400 lph...!
Makes my low tech 5.7 V8 with a carb positively economical at 33lph@3000rpm and 65lph@4500rpm.

Yep..... your carb 5.7 is delivering 250-260hp - his Verado's are giving 900hp.... So 400lph for 900hp is 2.25hp per litre per hr - your engine is giving you 4hp per litre per hr - nearly twice as efficient.... I thought these new outboards were supposed to be good??

 

[rbmatthews]

Yep..... your carb 5.7 is delivering 250-260hp - his Verado's are giving 900hp.... So 400lph for 900hp is 2.25hp per litre per hr - your engine is giving you 4hp per litre per hr - nearly twice as efficient.... I thought these new outboards were supposed to be good??

You guys are in a different league to us, but just for comparison, our Optimax 75hp uses 22 lph at 5000 rpm which would make 3.4 hp/lph. Fuel consumption was measured on the SmartCraft display that comes with the engine.

If you plot out this data from Tohatsu http://www.tohatsu-outboards.com/fuel-consumption.htm, it gives 2.4 hp/lph across their range of engines, which is not far off the Verado figure above. I don't know if the Tohatsu data includes their older two-strokes, but the Optimax 75 at least seems to be doing quite a bit better than this.

Are you sure the 4 hp/lph figure is correct? It seems a bit high to me.

R.

 

[spannerman]

Yep..... your carb 5.7 is delivering 250-260hp - his Verado's are giving 900hp.... So 400lph for 900hp is 2.25hp per litre per hr - your engine is giving you 4hp per litre per hr - nearly twice as efficient.... I thought these new outboards were supposed to be good??

Yep, they don't make 'em like they used to! The Verado fuel consumption is horrific once you go over 3500rpm, you watch the numbers on the Smartcraft display emptying your wallet, they use the extra fuel for cooling at higher RPMs, plus it has a compressor so it needs high fuel flow to keep the Air Fuel Ratio correct
A 3.6 liter block x 5000rpm = 18000 ltr divide by 2 as its a 4 stroke = 9000liters of air per minute going through the engine! Thats without any boost from the compressor so the figure will be higher.

 

[spannerman]

Its a Boston Whaler Outrage, its about 42ft if I remember correctly.

 

[rustybarge]

Try the consumption figures on the 300hp Mercury Verado, below 3000rpm they aren't too bad around 55 lph, go up to 5000 rpm and they go up to 130 lph! and we have a customer with 3 of the things on the back! at 40kts its burning 400 lph...!
Makes my low tech 5.7 V8 with a carb positively economical at 33lph@3000rpm and 65lph@4500rpm.

The 300hp Suzuki does 18ltrs/18kts/3000revs/1.5tons..........http://www.suzuki-marine.co.uk/docs/Ribcraft 7.8 - DF300A (Lean Burn).pdf

Several people on the forum have bought the new MF 8.80 with this engine.....no quite sure of fuel no's.?