Guest Expert in Combustion: Professor Ju:
Before getting into our planned question and answer session, Professor Littman gave a brief summary of some of the areas of interest of Professor Ju. He mentioned that Professor Ju was heavily involved in the engineering department at Princeton, especially on projects centered around combustion processes(within jet engines and ground vehicles including plasma ignition, supersonic combustion) the use of nanomaterials. Additionally Professor Ju is heavily involved in developing clean electricity sources and is involved in the sustainable energy program at Princeton.
Before opening to questions, Professor Ju gave a brief summary of the state of energy production in the world today. 85% of the world’s energy demand is met by means of combustion. 20% is met by Nuclear energy and a mere 0.0005% is met by solar energy. Commenting on Wind energy, Professor Ju reasoned that although wind provides a fair amount of energy during peak periods of wind, off-peak periods have to be heavily compensated for. Other technologies such as hydro-electric power were mentioned in passing, but special attention was drawn to the option of natural gas and in particular Fracking technology as a sustainable energy source to be developed.
After this introduction, the discussion was opened to some of the questions we had prepared before in class.
- Please explain the advantages and disadvantages of different gasoline additives and impurities (in particular: leaded and unleaded gasoline/petrol, ethanol, nitro-methane).
Please explain the significance of high octane rating and a high compression ratio with regard to engine performance.
Lead increases the octane rating of a fuel which is a measure of the pressure and temperature which a fuel may be brought to before it spontaneously combusts (homogenous ignition) causing knocking. The higher the octane rating of a fuel the more the higher the temperature and pressure the fuel may be brought to without spontaneously igniting. Higher performance engines require higher octane rating fuels in order to be able to put more pressure down on each power stroke of the engine.
Lead performs a similar function to other additives, such as alcohols, and aromatics like toluene and methyl benzene. Alcohol is more effective at increasing the compression ratio than lead, however lead is less expensive. Lead also produces toxic emissions, which is why most countries have banned the use of lead in petrol. Professor Littman also noted that people used to know not to grow plants near to roads as the lead oxides would be absorbed by the plants and inhibit growth.
When asked about octane ratings and what would happen if a high performance engine were to be fed with low octane fuel, Professor Lu explained that there would almost certainly be knocking. However he noted that high tech cars such as Mercedes currently have sensors which detect the octane rating of a fuel sample and automatically retard or advance the engine accordingly.
When asked what the substantial difference between diesel and gas is, Professor Lu explained that both are hydrocarbons(molecules composed of carbon and hydrogen) but diesel and gas have different chain lengths. Diesel is slightly longer than gasoline, meaning it combusts less readily than gasoline.
Nitromethane (or Nitro, CH3NO2), Professor Lu explained burns far more readily than most hydrocarbons because of the presence of oxygen in the compound. It therefore requires less atmospheric oxygen in order to fully combust. It is in a sense pre-oxygenated fuel.
- How does a turbo/super charger work?
A turbo charger increases the internal pressure of the reaction chamber and fuel, meaning that more fuel may be injected, and may combust per power stroke; the more fuel which combusts during each stroke the greater the power per stroke and the more powerful the engine.) Pressurized chambers are generally at pressures at or around 2 atmospheres.
- Please explain the recent developments in gasoline diesel mixtures resulting in an increase in efficiency.
Professor Ju gave a thermodynamic formula for calculating a measure of fuel efficiency
n = 1- (1/gamma^phi)
Where gamma is the compression ratio and phi is an experimentally determined value in the range of 1.2 -1.4 (meaning not specified.)
He explained that gasoline has a cylinder to wheel efficiency of approximately 30%, diesel 38% and new fuel mixtures containing alcohol have roughly 40% efficiency.
- What are the alternatives of gasoline combustion powered vehicles (gasoline/electricity hybrids, hydrogen, bio-fuels.)
Professor Ju was adamant that Hydrogen vehicles were not an alternative to gasoline vehicles. Although H2 combusts efficiently, it requires a pressure of 3000psi in order to cram enough fuel into an engine to yeild the same power as gasoline. Producing H2 from CH4 is extremely expensive and has a yeild of only 38%. H2 is also so reactive that it knocks very easily, meaning that H2 engines would have to be tuned incredibly carefully.
Professor Lu was a large proponent of Hybrid engines and praised the Prius for its efficiency and original approach to electrical engine power. He explained that the engine and electrical power systems run simultaneously in a Prius, complementing each other .
- Professor Littman inquired about the HCCI and RCCI projects which Professor Lu is working on.
HCCI technology combines gasoline and diesel engines in order to create an engine which is capable of plasma ignition/homogenous ignition.As mentioned above; Gasoline has a lower compression ratio than diesel and so is more prone to ignite at high temperatures and pressures. This fact is exploited in an HCCI engine where no spark is used to ignite the fuel, rather the fuel is brought to such high temperatures and pressures that the mixture reaches the activation energy for combustion without a spark.
RCCI – Reactivity Controlled Compression Ignition uses more gasoline than diesel.
One final note was that what determines how much an engine’s timing needs to be retarded or advanced is the flame propagation of the type and amount of fuel being used (namely how long it takes for the fuel to sufficiently combust.)
Discussion of Readings:
Some general bits of information were highlighted in the discussion of the first text, namely that the power of an engine is determined by the volumes of the cylinders and that the size of the throat on a carburetor is important as the breathing ability of an engine is proportional to the power yielded by the engine.
Summary of Tuesday’s Lab period:
Frame: The frame group did inventory, filed the welding around where the crack had been filled in and sent parts for powder coating.
Bottom end: The bottom end did inventory, cleaned and made a list of parts to buy.
Electrical: The electrical group continued to work on the peculiar wiring of the battery and alternator.
Top End: The top end measured the spring constant of the old and new springs and took pictures of the progress being made in lab.
Clutch: The clutch group were praised for their good work on the 58’s clutch, which is in good working condition now.
Reading assignment:
Zen and the Art of Motorcycle Maintenance – Chapters 12-14
In-Class Presentations:
Professor Littman specified that presentations ought to be 15-20 minutes long.
The schedule is as follows:
4/11 Bottom End
4/16 Top End
4/18 Electrical
4/23 Clutch
4/25 Frames, Forks and Wheels
F.Steere