Stale Fuel?

[Daffyd]

I always understood that meths in the fuel was to help deal with water (in the fuel) - the water droplets are too big to fit through the jets so blocked jets = baggy running motor. The meths emulsifies the water (makes the droplets smaller) which will pass through the jets. As far as 'stale" fuel goes, it would be stale as all the useful volatiles have evaporated and whats left is probably closer to kerosene. Once its got to that stage I cant see meths making any difference.

That is why I used it. When I bought the bike it had been standing for some time and I guessed that with the high humidity here there would be water in the fuel.

[Old Steve]

Stale petrol is most often a case of volatility. Petrol has to evaporate to burn completely in the combustion chamber. Petrol has 3 general components, called 'light ends', "mid cut' and 'top ends', the light ends are around the butane, pentane range of hydrocarbons. Petrol's volatility is measured as its Reid Vapour Pressure and there are limits set in the Ministry of Energy fuel specifications. Of course this volatility varies by season (winter petrol has to have higher volatility) and region (southern South Island petrol has to have more volatility than northern North Island petrol)

When left in storage the light ends evaporate, leaving a mixture which is more middle cut/top ends. Stale fuel exhibits as starting problems and rough running problems.

What has been said about getting on your bike and riding it is exactly right. Use some of the fuel up and top it up with new fuel suitable for the season. Run the tank down and refill it.

[Cosmik de Bris]

Stale petrol is most often a case of volatility. Petrol has to evaporate to burn completely in the combustion chamber. Petrol has 3 general components, called 'light ends', "mid cut' and 'top ends', the light ends are around the butane, pentane range of hydrocarbons. Petrol's volatility is measured as its Reid Vapour Pressure and there are limits set in the Ministry of Energy fuel specifications. Of course this volatility varies by season (winter petrol has to have higher volatility) and region (southern South Island petrol has to have more volatility than northern North Island petrol)

When left in storage the light ends evaporate, leaving a mixture which is more middle cut/top ends. Stale fuel exhibits as starting problems and rough running problems.

What has been said about getting on your bike and riding it is exactly right. Use some of the fuel up and top it up with new fuel suitable for the season. Run the tank down and refill it.

You seem to know a lot about this Steve. Do they really blend winter petrol for different areas here in NZ?

Cheers

[Old Steve]

You seem to know a lot about this Steve. Do they really blend winter petrol for different areas here in NZ?

Yep, I was one of four Technical Managers from the Fuel Companies that put together the specifications for unleaded petrol for the Ministry of Energy back in the early 90s.

Have a look at "Engine Fuel Specifications Regulations 2011" (Google 'engine fuel specifications') or go to this website:

[url]www.legislation.govt.nz/regulation/public/2011/0352/latest/DLM4044783.html?search=ts_regulation_engine+fuel_r esel&p=1[url]

In that, you'll see the section on vapour pressure (volatility). Diesel fuel has a similar seasonal and regional variation on cloud point (the formation of free wax which will block fuel filters).


Vapour Pressure4 (DVPE) (kPa) Maxima: Auckland and Northland: 65 kPa summer, 80 kPa autumn and spring, 90 kPa winter; ASTM D5191
Rest of North Island: 70 kPa summer, 80 kPa autumn and spring, 90 kPa winter
South Island: 75 kPa summer, 85 kPa autumn and spring, 95 kPa winter
Minimum: 45 kPa all year


So you see that petrol volatility varies by season and region between summer, autumn and spring and winter, and through Auckland and Northland, Rest of North Island, and South Island.

[Old Steve]

And incidentally, petrol octane is determined by Research Octane number (RON) and Motor octane number (MON), the difference between the two numbers is called the petrol's 'sensitivity'. So 'regular' petrol is 91 RON/81 MON and 'premium' petrol is 95 RON/85 MON, both specs set a sensitivity of 10.

Marsden Point refinery is MON limited, it has trouble getting a petrol's MON up, so they have to give away RON. Sensitivity of premium 95 petrol from Marsden Point may be around 12 or even 13 instead of the sensitivity of 10 for regular 91. It may well come out of the refinery as 98 RON/85 MON.

Now, looking at the engine fuel specifications you'll see there is no spec at all or 98 RON petrol, this is just a marketing grade. I believe that the first 98 RON petrol marketed in NZ was just the wide sensitivity premium 95/85 spec petrol from Marsden Point which had a RON of 98 to get the MON up to 85.

If you want to have some fun with your local service station, fill the tank with 98 and then ask what the MON (Motor octane number) of the fuel is, is it 88 cos your engine needs 98 RON/88 MON petrol. If it's not 88 MON, can they drain the petrol out of the tank?

Incidentally, the USA uses an average octane, RON + MON/2. So while the manual of a Harley states "use 91 octane fuel" they mean use a fuel with an average RON + MON of 91, so in your Harley use premium 95 RON/ 85 MON petrol. By the way, that 98 RON/85 MON petrol has a US average octane of 91.5 so would be best for your Harley.