2st Vs 4st: Actual fuel Data!

Hi All,

Noticed this on the tohatsu OB site that lists actual fuel consumtion comparisons between their 2st and 4st outboards.

Makes for some very interesting reading!

http://www.tohatsu.com/tech_info/fuel_consumpt.html

 

Just a few observations.

The 115hp 2str Tdli can exactly match the 4str 115hp at 36 ltr/hr, And the md90 Tdli2str only uses 5 ltr/hr more (30ltr) than bft90hp 4str (25ltr).

That's very impressive for 2str engines: although the 90hp 2str uses 20% more fuel.

50A 4str 15.7ltr/hr, to the md50 2str 16.5ltr/hr: all but indentical!

Would you buy one?

 

Those two strokes are "direct injection" engines ? That type of motor ( e.g. e-tec, optimax) does compete on pretty much level terms, fuel use wise, but you have also to factor in there is oil usage, and the oil isn't cheap. But service costs with 4 strokes are higher. Some say prohibitively so.

 

...and the data isn't exactly scientific; what about props, boat weight, hull form etc etc....

Is a wot fuel consumtion test relevant?

 

DFI 2-stroke outboards get pretty well the same fuel use figures as 4 strokes at virtually any part of the rev band. Which is pretty well to be expected, as they don't have any fuel entering the combustion chamber until the piston is near TDC, as with the 4-cycle, though there may be a slight loss of efficiency toward the bottom of the stroke, as the exhaust ports are exposed. Against that, though, every down stroke is a power stroke. That most manufacturers are now drifting toward 4S, says they are not falling short of market demands.

 

I think that's a good analysis of the combustion process. I'm just wondering if it's getting rid of the exhaust gasses at high revs in a 2str that might reduce the efficiency?

A quick google of 'Etec problems' brings up hundreds of complaints with major issues; I'm aware that the internet often skews bad news over good news, but there seems to be a lot of unsatisfied customers out there.

I'm not familiar with 4s?

 

I have taken the data from the link in the OP, divided motors into two groups (2-stroke and 4-stroke ones) and plotted fuel-burn indicators for both groups.

Since fuel burn (LPH - liters per hour) is proportional to HP and RPM according to equation LPH = K HP RPM, the factor K contains the info about the efficiency of the combustion.

Hence, the graphs show the K (expressed as LPH / (HP RPM) plotted against HP. Two things appear evident from these graphs:

1. for any given engine size (HP), a 4-stroke engine has a much lower fuel burn, hence higher fuel efficiency.
2. big motors perform much better than small ones, and it is true for both 2-s and 4-s motors.

Hence, it is not so true that modern 2-stroke engines compare pretty well to 4-strokes. If the motor weight was included into the analysis, things would probably get better for 2-strokes. But that kind of analysis would have to take into account the hydrodynamics of the boat the motors are mounted on. The analysis would have to include a wide variety of boats for each motor (planing boats react very differently to variations of displacement than semi-planing or displacement ones) - hence, too complicated.

Cheers

 

Thanks; the graphs show very clearly the differences.

The tests were performed at Wot, which is where 2str engines are less efficient because there is less time for the burnt gas to evacuate the cylinder without a scavage pump (blower)???

Although the Etec OB say that their motor is approved for use on lake Garda in Italy which has the strictest emissions in Europe, which would suggest that there is very little unburnt fuel in the exhaust.

Taking everything into account is very complex.

 

daiquiri,

Thanks, much easier to see the results in your graphs.

I also wonder how much different the results might be a lower throttle settings.

 

There is bugger-all difference between 4 stroke and 2 stroke DFI (direct fuel injection) fuel consumption at any speed, including idle speeds. If it were otherwise, it would beg the question why the big investment in developing 2S DFI was ever made, it would have been a knockout win to 4S. I have heard opinions that 4S will last much longer because the bores are free of porting, but also other opinion that some 4S are based on automotive designs that were not originally designed for raw salt water cooling, and show corrosion issues. But there is a massive difference between a 2S that draws/pushes a fuel/air mix into the cylinder on the downstroke, and a DFI 2S that only has air being introduced, until the last moment.

 

There is some burnt oil odour associated with 2S DFI, that may bother some people. One advantage that 2S DFI had over 4S is/was weight, but there seems to be a concerted effort to reduce 4S engine weights, which will reduce that issue.

 

Cant wait to see some comparisons on the new g2 e-tecs
They are cleaner than a 4s and now should be clearly ahead in economy

 

Talking of all things 'Drives', have you seen the latest Volvo front facing outdrive .....
http://www.volvopentaforwarddrive.com



Who's been smoking exactly what....?

 

looks like the lower off an IPS
Front facing props are more efficient but....

 

Could be 'interesting' winching that onto a trailer.