Mixture control - Effect of Summer heat

[old man emu]

The background to my question is not really to do with aviation, but simply management of the air/fuel mixture that is being supplied to an engine.

 

The air/fuel mixture supplied to my motorcycle engine at low engine speed is easy to change because the slow speed mixture control is close at hand. I find that on cool to cold mornings, the engine seems to run better that it does later in the day when the air temperature has risen. In other words, during the 20 minute ride to work in the morning, the bike seems to have more power than the return ride in the afternoon.

 

I'm about the set the mixture as per the book, and the current QNH is about 1005 and temperature 15C. I'm about 75' AMSL.

 

If I get the bike running nicely in these conditions, come summer and air temperatures around 30C, should I be making the air/fuel mixture or leaner to get the same results as I'll get now?

 

OME

 

 

[jamdfingr]

Basic theory says you should lean it out but not by much...

 

Cold air is slightly denser so at higher temps the air would be less dense and therefor reduce the amount of air in the cylinder.

 

Thus, theoretically to maintain the same ratio, you would reduce the amount of fuel with the reduced amount of air.

 

But 10 degrees or so wouldn't make that much difference I would imagine...

 

 

[Nobody]

You can use a density altitude calculator to work this out.

 

QNH 1005

 

Temp 15 degrees C

 

Dew point 5 degrees C (I guessed this one)

 

75 feet amsl

 

Using this calculator Density Altitude Calculator - English/Metric gives a density altitude of 484' and an air density of 1.208kg/m3

 

If the temperature increased to 30 degrees C but everything else stayed the same then the Density altitude increases to 2204' and the air density drops to 1.148kg/m3.

 

You need to reduce the fuel flow to 1.148/1.208 or about 95% of the "cold" tune to maintain the same air fuel ratio. Even if you retune the bike you will be down on power compared to the "cold" temprature as there are less oxygen molecules in each m3 of air.