Fuel Properties

Two facts:

1. Petrol is made up of a blend of many chemicals which generally fall under the category hydrocarbons. (chemicals that have hydrogen and carbon atoms in them)

2. Liquid fuel does not burn. To burn it must be a vapour and mixed with oxygen (air) at a ratio between approximately 9:1 and 20:1. (EQR 0.8 and 1.4)

You may know that fuel comes in 91, 95 and 98 octane and while this is just a small part of what fuel is about I will explain this first. Octane is also referred to as RON (Research Octane Number) and rather than an actual chemical measurement it is defined as the resistance to knock* or detonation. So to determine the RON of a fuel it is actually combusted in a special test engine. How resistant the test fuel is to knock is compared to a reference fuel of iso-octane. Iso-octane has a RON value of 100. Some fuels can be more knock resistant than iso-octane and thus can have RON ratings greater than 100. It’s important to note that the fuel does not have to contain iso-octane in any particular proportion; all that is being compared is the resistance to knock.

* Knock is uncontrolled burning of the air-fuel mixture, causing high cylinder pressures and heat which damages the engine. I will talk more about knock and engine design later. But for now, believe me, if your engine does not require high octane fuel to prevent knock, then there is absolutely no advantage to using it! Some engines self-adjust to learn what RON fuel is in it using knock sensors, but this is not common on bike engines.

The MOST important property of fuel isn’t shown at all on the pump. It is fuel “volatility”. I’m sure you have all heard of “a bad batch of fuel” or the phrase “the fuel has gone off.” Or even experienced your bike being hard to start if you have left it a long time.

Fuel is made up of many hydrocarbons of different molecular weights. The lighter molecules evaporate more easily at low temperatures. Now remember that only fuel that is a vapour will burn. When you start your engine and everything is cold, only the lightest hydrocarbons in the fuel will vaporise for that first spark plug firing. So how many light hydrocarbons are present in the fuel at low temperatures is critical for starting and cold temperature running. We call this fuel volatility.

The graph below shows a distillation curve for a high volatility and a low volatility fuel. (The fuel is slowly heated and how much has evaporated at each temperature is measured). There are other measurements of fuel volatility but we will stick with this for now.

You can see that there can be quite a difference in fuel volatility. On a 45deg C day, if you left the high vol fuel in an open container, 60% of it would eventually evaporate! Whereas only 5% of the low volatility fuel will evaporate.

This is one of the reasons why your fuel tank is sealed, and why when you open it, sometimes there is pressure inside it: the light hydrocarbons have vaporised in the heat. If your tank is not sealed very well the light hydrocarbons are lost and you slowly turn a high volatility fuel into a low volatility fuel!

As you can imagine, there is a need to change the fuel volatility from summer to winter. Using the fuels shown above, in the middle of winter the low volatility fuel would be very hard to start since very little will vaporise in a cold engine, where the high volatility fuel in summer will boil off very quickly and can lead to vapour lock in the fuel lines and very rich starts. Also high volatility fuel generally costs more to produce.

All this is very well but your engine ECU doesn’t know what fuel you have put in. It is tuned to a nominal volatility worked out by the manufacturer for temperatures associated with different times of the year. Also fuel sold at Omeo in the mountains will be higher volatility than fuel sold in Darwin. The problem occurs when you say fill a bike up in summer then don’t ride for a long time or ride up a mountain and do a cold start with low volatility summer fuel. Other problems happen at servos where not a lot of fuel is sold and they can sell the wrong fuel in the wrong season. People get very upset when their car/bike doesn’t start so manufacturers spend a lot of money testing and calibrating the startabiltiy of an engine at a range of temperatures and on a range of fuels. Cold start is a problem with low volatility fuels (running too lean) and hot start is a problem with high volatility fuels (running too rich).

But it’s not just starting that is the problem. Transient fuel control is also very much impacted by fuel volatility. Transient fuelling is fuelling correction for rapid changes in engine load (or throttle position if you prefer).

Imagine your engine is at a constant speed and load (say cruising at 100 km/h). The injectors are injecting fuel that is being sucked into the engine and burnt at EQR 1. However, not all the fuel that is injected into the runner is sucked into the engine on every cycle. Some of it remains in the intake. This is not a problem at steady state as the fuel will simply be sucked in on the next cycle and replaced by more fuel. But suddenly crank the throttle open and the air pressure (manifold pressure) in the runner rises. And just like steam in a pressure cooker the fuel can’t vaporise as easily as air pressure rises. Vaporised fuel falls out of the air and condenses in the runner. Remember that liquid fuel won’t burn, so the engine will run lean and stumble. In a fuel injected bike you have to add more fuel to make up for the fuel that has condensed. The amount you have to add depends very strongly on the temperature of the engine, the air temperature and the fuel volatility. When the engine is cold you might need three or four times extra fuel than you would when warm. Bikes with carburettors can’t do this and this is why carburettored bikes run so badly when it’s cold. New direct injection engines (SIDI) that inject fuel directly into the combustion chambers don’t encounter these transient problems.

In summary, fuel volatility has a huge impact on how your engine starts and runs when cold. But once warmed up to 100 deg C or so, there is almost no difference in how different volatilities affect how your bike runs.

Next time a discussion on engine torque, spark advance and knock.

Henry Wright