Monday, October 31, 2011

BAF and LAF Part 2

Let’s start with LAF.
No particular reason other than it’s my ‘Blog’ and I choose to start with LAF. OK?

LAF is short for ‘Little Aeroplane Fuel’ and is petrol. Also known in some, less developed, parts as ‘gasoline’. For this reason the petrol they put in aeroplanes is known as AvGas.
AvGas is ‘Aviation Gasoline’.
AvGas is very similar to the stuff you put in your car to make it go—unless your car runs on diesel or NGV in which case it is very different.
Putting petrol in aeroplanes gives several problems.
Let’s start with the idea that as you ascend up Mount Everest the boiling point of your water is going down. By the time you get to the summit your water will boil off by pouring some in your hand.
Similarly, the water in your car radiator will not boil until it gets to 110°C, or thereabouts, because the radiator and cooling system is pressurised.
This change in boiling point is all due to the different pressures on the surface of the water.
To say that water boils at 100°C is only true if we add “at ISA SL” atmospheric pressure.
Note: ISA SL = International Standard Atmosphere at Sea Level.

Caution: If you remove the cap from the radiator of a hot engine the pressure on the coolant will suddenly reduce—thus lowering the boiling point dramatically. Equally dramatically, every molecule of coolant will now wish to turn to vapour, which will cause it to occupy a much greater space. Since there is no more space in its immediate surroundings the only place it can move to is outside the system that contains it. It will do this by attempting to pass through the hole that you have just made.
Result? A jet of superheated steam in excess of 100°C will now come out of the radiator filling hole at huge velocities. Contact with your skin will scald the skin and the velocity of the escaping gas will rip it off.
Important to know this, isn’t it?

Note: The boiling point of a substance is the temperature at which it can change state from a liquid to a gas throughout the bulk of the liquid.

Now let’s think about petrol. Petrol is lighter than water. It evolves into vapour more readily than water. Its boiling point is lower than water (95°C at ISA SL). Like water, the boiling point will reduce with altitude.
Aeroplanes fly higher than Mount Everest but, in World War Two, the American high level bombers, flying lower than Mount Everest, were losing up to 10% (Ten Per Cent!!!) of their fuel load because the gasoline was boiling off—it was turning into vapour.
Although the figures will vary considerably we could say that the RVP (Reid Vapour Pressure) for petrol is around 10-12psi.
So what is RVP?
Reid Vapour Pressure is defined as the absolute vapor pressure exerted by a liquid at 100 °F (37.8 °C) as determined by the test method ASTM-D-323.
What does this mean in practical terms?
There is a surface pressure at which hydrocarbon fuels will begin turning to vapour at a specific fuel temperature. Of course, gasoline and other fuels will vaporise over a period of time at any pressure and temperature but there comes a point at which vapour evolution is critical and measurable because of the reduction in boiling point.
The cure? Pressurise the fuel storage system (fuel tanks). This may create a heavier structure and a complex system to control it—also heavier.

What other problems are there?

In the old days there were lots of different petrols (Avgas) available. They were engineered to tolerate different compression ratios.
This leads us straight into a triumvirate of conditions.
Piston engines that use gasoline all operate as a ‘Modified Otto Cycle’ They are four stroke (for the most part) engines. The four strokes are:
1.             Induction. The piston moves down to allow a fresh mixture (fuel and air) into the cylinder head.
2.            Compression. The piston moves upwards to increase the pressure on the mixture.
3.            Power. Just before the piston reaches the top of the compression stroke there will be a spark to ignite the mixture. The burning mixture now adds heat energy to the system at constant volume. The increase in pressure moves the piston down on the power stroke.
4.            Exhaust. The piston moves back up to allow the burnt gases out of the cylinder head, thus making room for fresh mixture.
At 3,000 rpm each stroke takes 1/50th of a second. Not much time, then.
Gasoline vapour burns at about 100’ (feet) per second. It takes time to burn and more time to transfer the heat to the air and then the air needs time to expand.
This is why the spark has to be before the piston gets to the top, to allow for a delay in the burning, transfer of heat, expansion in the cylinder head.
Another factor is mixture strength.
Complicated, isn’t it?
The stoichiometric ratio (sorry about that!) for gasoline is 14.6:1 (Natural Gas and Diesel are about 14.5:1; compare with Hydrogen at 34.3:1.).

Note: A stoichiometric ratio is the ratio at which all the fuel and all the available oxygen are completely burnt. A perfect mixture, if you like.

So you have 14.6 parts of oxygen and 1 part of fuel vapour. It will now burn nicely.
About 12:1 will also burn but that is called a ‘rich’ mixture. You are leaving a trail of black smoke (unburnt fuel turns to carbon as opposed to oil that burns blue-ish or white). A weak mixture extreme might be around 17:1. We like to burn a tiny bit weak because that saves us money on fuel but too weak creates a hole in the piston, valves and our bank account.

So what if we have a high compression ratio, a weak mixture and an advanced ignition?
Whoa? A what?
The faster that an engine is running the less time there is for the fuel to burn. For this reason the spark needs to be initiated even earlier. Then, as the rpm of the engine reduces, the spark can be moved closer to TDC (Top Dead Centre—the point where the piston changes direction at the top of the cylinder as opposed to BDC—Bottom Dead Centre which is the same thing at the other end of the piston’s travel).
Moving the spark away from TDC is advancing the ignition and moving it back towards TDC is retarding the ignition.
If we advance the spark too far the air will expand and push down on the piston too soon. We need this maximum pressure—called BMEP (Brake Mean Effective Pressure), around 22° after TDC for best results.
Too soon and the mixture in the cylinder head will cease to burn rapidly and will, instead, explode. This is called detonation or ‘pinking’ or ‘knocking’. It is very damaging to the engine.
Aero Piston engines needed increased compression ratios to develop more and more power. This increase in CR (Compression Ratio) creates a higher possibility of having detonation.

Note: Many piston engines have bimetal washers under the cylinder head wired to a gauge. This acts as a temperature sensor. A sudden increase in cylinder head temperature indicates that detonation is occurring.

Petrol is engineered to deal with detonation—up to a point.
Most petrols have an ‘octane’ rating. For your car it will be either 95 or 97.
For aeroplanes it is normally, now, 100LL.
The octane rating of petrol was increased by the addition of Tetra Ethyl Lead. This is a bad thing.
Now it is done with other additives and hence we have 100LL or 100 Low Lead.
An octane rating is found by increasing the compression ratio of an engine using the fuel at a specified mixture strength until detonation occurs and then comparing it with an iso-octane fuel. The 95 or 97 is a percentage of that figure—the iso-octane is always 100%.
You cannot get more than 100% so a higher number will be a ‘Performance Number’ and not an octane rating.

100/130 AvGas, now commonly called Avgas 100, is dyed green. 100LL has replaced 100/130 in most places, but AvGas 100/130 is still sold in Australia and New Zealand, I’m told.
In the past other grades were also available particularly for military use, such as AvGas 115/145 (dyed purple) and 91/96 (dyed brown).
Limited batches of 115/145, commonly called AvGas 115, are produced for special events such as unlimited air races; in the past 115/145 was used as the primary fuel for radial engines.
The second number is the Performance Number at ‘Rich Mixture’ conditions. On most of these engines the mixture strength can be adjusted for ‘warm up’, acceleration (Take off) and ‘Economical Cruise’.

So now you see that Little Aeroplane Fuel is quite complicated.  You should also note that it catches fire more easily and is more difficult to extinguish than BAF. It is also more expensive.

We shall look at BAF in Part 3.

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