Fuel Consumption - Maths

[Diesel Dave]

OK I need a maths check here, I've been doing a bit of spreadsheet analysis comparing fuel consumption from the engine graphs to what I'm actually getting. I looks like I'm close, but one of the side benefits is the ability to find the most economical engine speed.

Here's the Yanmar L100 fuel graph.



50 mph for old Sluggy is about 2700 rpm so spot on the curve for best fuel consumption:

Here's the maths bit - it's MPG so apologies for out continental members who work in Litres per 100km

g per Hp hour = 180
50mph needs 8hp (ok this is a best guesstimate)
So to travel 50 miles you need 8 x 180g = 1440g or 1.44kg of fuel
Diesel weighs 3.87kg per Imp gallon (now I've lost out US members too)
So 1.44kg / 3.87kg = 0.3721 gals/hour

So 1 gallon will take you 50/0.3721 = 134.37 miles - Ta Da!

Now that's pretty close to what I'm actually getting.

Now moving up the theoretical chain

3600 rpm for me is 66.65 mph at which point we are up to 200 g per HP Hour.

If we accept that 10hp will power you to 66mph..... then MPG dips down to 97 - quite a reduction.

[Tetronator]

Interesting, so if I calculate the approximate fuel use for my bike using the D722 and custom gearing it will be...

195 - 205 g/HP-h
Density Diesel = 0,84 kg/dm3 @ 20 Degrees (0.84 kg/l)
Using top gear on all calculations





Using Dave's guesstimate @ ~2300 RPM
85 km/h ~ 9HP (Heavy bike and rider)
9hp * 195 g/HP-h = 3218g
3.218kg/0.84 kg/l = 3.83 l/h
85km/2.09 l/h = 40.67 km/l = 114.9mpg



Using all available power @ ~2300 RPM
85 km/h ~ 14 HP
14 hp * 195 g/HP-h = 2730 g
2.730 kg/0.84 kg/l = 3.95 l/h
85km/3.25 l/h = 26.15 km/l = 73.9mpg

Using all available power @ 2600 RPM
111 km/h ~ 16.5 HP
16.5 * 195 g/HP-h = 3218 g
3.218kg/0.84 kg/l = 2.64 l/h
111km/2.64 l/h = 42.05 km/l = 118.8mpg

Using all available power @ 3600 RPM
132 km/h ~ 20 HP
20 * 205 g/HP-h = 4100 g
4.100kg/0.84 kg/l = 4.88 l/h
132km/4.88 l/h = 27.05 km/l = 76.4mpg

Tell me if I did something wrong here!

[Diesel Dave]

I hope you don't mind but I added the MPG equivalents.

Looks about right to me.

Just goes to show if you can reduce your drag by 1hp you can make big fuel savings.- Must put Sluggy's screen back on.

[Tetronator]

Well if you are going to do that make sure you add the american one as-well.

Yeah, saving fuel by reducing drag... Adding a sidecar might pose a bit of a problem then...

It does put into perspective that the thing really will need more power if its going to lugg a sidecar around.

[sbrumby]

HP, speed, weight, fuel consumption, drag, are all interconnected, but what is not so obvious and is worth considering is a naked bike is quieter than a faired bike. My big bike gets noisy after 70mph this is wind noise, reading that a naked bike is quieter I didn't believe it , so riding along I stood on the foot pegs and all went quiet. Next day removed 6" of screen much better.