Guest Lecture: Engine & Fuel Combustion (by Professor Alison Ferris)
The guest lecture focused on combustion science and how it applies to engines and propulsion systems. Professor Ferris works with chemical kinetics, which looks at how gas-phase molecules react under extreme conditions such as high temperature and pressure.
She talked about the difference between air-breathing engines (like turbojets), which take in oxygen from the surrounding air, and rocket engines, which carry both the fuel and the oxidizer. Her research is specifically dealing with hypersonic flight, where they test at temperatures from 400 K up to several thousand Kelvin in low-pressure conditions.
One major focus of her work is fuel design. Jet fuel is made up of a mixture or smoothie of hydrocarbons of different sizes. Some of these molecules tend to produce soot, which contributes to contrails, which is the visible trails left behind aircraft when they fly. While most/temporary contrails are fairly harmless, long-lasting contrails can form cirrus clouds which trap heat and contribute to global warming. Her research looks into how fuel composition can be tweaked to reduce these persistent contrails and soot formation to mitigate these environmental deteriments.
To study combustion, Professor Ferris uses shock tubes, which is a very narrow and long tube, to simulate engine-like conditions. These devices generate high-pressure shock waves (using helium) which heat up a fuel-air mixture. This allows researchers to observe combustion processes over a longer period of time, when usually this process occurs in milliseconds.
Professor Ferris also talked about engine knock. Engine knock occurs when fuel-air mixture ignites automatically too early due to high pressure and temperature. Instead of a smooth outward flame front (deflagration), knock creates pressure spikes which reduces efficiency and can damage the engine. Resistance to knock is measured by a fuel’s octane number. Octane number measures a fuels’ resistance to autoignition. Higher-performance engines (higher compression) need higher-octane fuels.
She also explained thermal efficiency, which measures how much of the fuel’s chemical energy is converted into work. In general, increasing the compression ratio improves efficiency, but only up to the point where knock becomes limiting. In the 1960s, additives like tetraethyllead were used to decrease knock, but these additives contained lead and were later phased out due to severe environmental and health consequences.
Finally, she compared internal combustion engines (like our motorcycle engine), which operate in cycles (intake, compression, combustion, exhaust), with gas turbines, which use continuous combustion. Gas turbines (used in jet engines and power plants) typically achieve only around 35% efficiency.
Lab Work
In lab this week, we continued restoring the motorcycle. We worked with a gasket, which is used to seal engine components and prevent oil leaks. We installed or prepared a gasket for the engine cover (unsure?).
We also started working on the electrical system, specifically with wiring and switches. We worked on connecting a wiring harness and looking at past examples of wiring and the Triumph manual.
Additionally, we talked about engine wear and measurement. We learned about cylinder reboring, which is when the cylinder diameter is slightly increased in increments (0.010 inches at a time) to restore a worn engine. This process can only be done a few times before having to insert a sleeve. We learned about tools such as a Mitutoyo bore gauge to measure cylinder diameter accurately.
We also talked about piston rings, which are needed to maintain compression. These rings seal the gap between the piston and cylinder wall, which prevents air and fuel from escaping. Bad or leaky sealing can reduce engine efficiency and cause blue smoke (oil burning) or white smoke (bad combustion). The third ring, the oil ring, specifically prevents oil from entering the combustion chamber.
Precept Discussion: Shop Class as Soulcraft (Chapter 1)
We talked about themes from chapter 1 of the book. We talked about the difference between economic value and moral value in different types of work. One idea was that trades like plumbing or mechanics are often highly valued economically but are not given the same social status/prestige as more “intellectual” professions.
We also talked about the concept of agency, which Crawford defined as the ability to act independently and take control of one’s work. In manual trades, this is very similar to competence, because there is a standard to when one knows they have done a satisfactory job. Knowing what you are doing and being able to solve real-world problems is part of agency. Mechanical work specifically often requires effort to identify problems first before solving them, as opposed to textbook math problems for example. Then we talked about the cognitive demands of manual labor, where, contrary to popular belief, manual labor is equally if not more intellectually rigorous. We also connected these ideas to automation/assembly lines and the idea of outsourcing labor costs.
Some motorcycle terminology:
Backfire: combustion occurs at the wrong time. Flames sent through the intake (often due to incorrect timing or air-fuel mixture).
Afterfire: unburned fuel ignites in the exhaust system which leads to popping sounds or flames from the exhaust, usually during deceleration.