Combustion Dynamics and Fuels—Part 3: Combustion Basics

With all this discussions on fuel flexibility, we would be remiss if we did not talk about what makes fuel flexibility difficult.  There are many factors that affect how something burns, such as the fuel being a liquid or a gas (or even a solid), the structure of the underlying hydrocarbons, the amount of oxygen present, and the geometry of the flame zone.  These are the big ones, but there are many other smaller factors which I will not have the chance to dig into here.

Automotive Liquid Fuel Injector

The state of a fuel has a lot to do with its combustibility.  Ultimately, for fuel to burn it must mix with oxygen (or some other oxidizer).  Gases mix very well and very evenly, which makes them easier to control in the combustion process.  Liquids on the other hand do not mix well, and often need to be premixed (such as with a carberator) or aerosoled into little droplets (like a high pressure fuel injector).  These processes take much more tuning to get right. Lastly, there are solids, which implicitly do not mix well.  Solids usually need to be pulverized into little bits, much like aerosoling, for them to be good combustion candidates.  More often than not, solid fuels are mixed with solid oxidizers in even proportions to enable more complete combustion.  This is most commonly seen in gunpowder or APCP found in solid rocket motors. 

With fuel, you need oxygen to react with hydrocarbons to enable combustion.  However, it may not be intuitive that having some of each is not enough to enable combustion.  Even with a spark, there will be no flame—this is a phenomenon known as the flammability limits.  Generally combustion is most efficient when there is just sufficient oxygen to burn with the fuel; as the amount of fuel gets cut in half the mixture ceases to combust properly.  Likewise, if the amount of oxygen is cut to a third, the mixture will fail to combust.  To contend with this, many modern engines will have sensors to balance and meter out fuel to match the air in the system—however even with careful tuning, poorly mixed fuels may have spots that lie outside the flammability limits.  

As you can imagine, combustion is a very complex process.  Energy is released as complex hydrocarbons are reduced to simple carbon dioxide & water.  The steps to get there can be rather complex. The chemical bonds between atoms are formed and reformed during the combustion process—with many intermediate molecules formed during the process.  The result is that combustion takes time, on the order of several milliseconds.  This isn’t a lot when you consider a car engine rev’s up to 7,000 RPM, each stroke (which must include compression, combustion, expansion, and evacuation of the gasses, can only take 8.5 ms.  The result for some systems that operate on these time scales is that combustion becomes hare to control, which can prevent it from coming to completion.  Generally, however, simpler hydrocarbons burn “faster” than more complex ones. 

To make this even more complex, different hydrocarbons carry different amounts of energy, independent of volume or weight.  To try and describe this effect, engineers developed something called the Wobbe Index, that allows for relatively simple mathematical scaling of fuel injection rates for a given fuel—assuming that fuel is known ahead of time.   Unfortunately, this makes simple fuel metering systems, like a carburetor, a poor solution when the fuel is unknown.   The ability to handle different fuels requires a more advanced fuel delivery system that is capable of providing fuel at different rates.

For reciprocating engines, an important factor to consider is the “knock” rating of the fuel.  It is essentially a measure of when a fuel will self ignite (assuming it is heated from the compression stroke of an engine).  Reciprocating engines become more efficient and more powerful if they have more compression, however the fuel limits the amount of compression that can be practically achieved.  To further complicate matters, the flammability limits of fuels change as they are compressed.  The result is that adding fuel flexibility often comes at the cost of engine performance and emissions.  As a real world example of this complication, many diesel manufacturers are trying to build engines that can run on both natural gas and diesel.   As it turns out natural gas has a much higher anti-knock index rating than diesel—due to this quirk of nature, manufacturers have developed bi-fuel generators.  In many of these solutions the products have to run on a 50/50 diesel & natural gas mixture, such that the diesel is compressed to ignition, burns, and in turn burns that natural gas in the combustion chamber.  While this is a good technique for reducing diesel dependency, it is not true fuel flexibility. 

From Lafebvre & Ballal, "Gas Turbine Combustion"

The last thing that really drives combustion is the physical location where combustion takes place.  The geometry, or shape can drive the local mixing of fuel and air; it also provides the combustion constituents with the time and space to burn to completion.  The combustion chamber and its aerodynamic interactions with the rest of the engine define a lot of how a combustion process performs. 

 These are only a sprinkling of the characteristics to consider when designing combustion systems.  As one can imagine, all of these things must be taken into account when trying to build a fuel flexible system.