Professor Ju Q&A Session
Professor Ju
– Research Area: Transportation and Nanoparticles
– Hypersonics generates enormous heating of the aircraft since the ignition temp is very high
o Necessitates different cooling for the engine stability
Questions for Professor Ju
What is an ideal fuel for the engine?
– depends on the engine.
– For ground transportation, you don’t want a fuel that gets ignited too fast. You can’t have fuel that has more than 50% ethanol due to corrosion (lubrication issue).
– Aromatic fuel (benzene) is very lubricating.
– For aircraft, there is a lot of aromatic fuel due to the fact that aircraft engines go at a higher rotation
Hybrid Cars?
– The batter is super expensive
– Electric cars are heavily subsidized at the moment, so actual costs of these batteries are a lot more expensive.
– Hybrid will have a good future before electric cars.
o Gasoline has 50x higher power to weight compared to the latest lithium batteries
o Lithium batteries have an inherent limit.
o But Lithium Flouride batteries have a better future than current ones.
Diesel Engines vs Gasoline Engines?
– European : 50% diesel engine
– U.S. : 90% gasoline engine
– In terms of efficiency, diesel engines are 30% more efficient than gasoline engines.
o Diesel engines does not have a knocking process—we can ramp up the compression ratio much higher than gasoline
o Because of the high pressure, the engines are heavier though.
o Also, diesel engines have more emissions (NOx); thus, Japanese carmakers have steered away from the diesel engines.
- Diesel engines have higher emission due to higher ignition temperature à more NOx.
o In Europe, diesel engines are very popular.
– With electric cars coming in, diesel engines might face tougher competition.
EGR?
– the temperature of the exhaust coming from the exhaust pipes are very low
o Thus, it is very difficult to use this energy
– Another idea is to use the heat radiating from the engine into electrical energy.
HCCI
– trying to combine the merits of diesel engines and the gasoline engines
o higher compression with lower emissions
– Premix vs Diffusion
o Candle flame is a diffusion; they mix as they burn
o Premix -> oxygen and fuel are “premixed” before you burn
o Diesel is diffusion and gasoline is premixed
– Instead of using a spark plug, you ignite it by compression with a very low premixed fuel
o Temperature is very low
o But controlling the ignition is hard
– HCCI concept has been around for 30 years but perfecting is hard
– Diesel cannot be fully premixed
o If it is premixed then as soon as compression happens, it will ignite.
Gas Turbines
– ground turbine has duel design—one from the gas turbine and the other from steam turbine
– Most efficient gas turbine
o Supercritical steam turbine: the efficiency can be up to 40%
o Gas combined with this supercritical steam turbine: 62%
– Professor Ju thinks natural gas will stay due to the fact that the efficiency is so high.
Lead
– Octane number: indicator of how good the fuel is in anti-knocking
o Lead is very cheap that increases the octane number
o Now they use aromatics: Toulene
- They are expensive and they produce emissions as well
- So refineries don’t really want to add aromatics
- They add ethanol now for most part of it.
Valve Overlap
– when the intake valve and exhaust valve is open at the same time.
– What determines how much overlap you have? Why do you even have it at all?
o Because when the exhaust valve is open, the exhaust gas coming out still has inertia and creates a vacuum of sorts, which helps to intake the gas.
– In a high performance engine, you would see more overlap.
Geometry of the pistons
– depends on the cam shaft
What’s the future?
– With gas so cheap, he doesn’t think that the gasoline and diesel engine will go away
– Hybrid engines will continue to increase.
– Hydrogen has very low energy density
o To do the compression, you waste a lot of energy.