Fortunately, the methodology is terribly sloppy anyway, so there's nothing serious to discuss. The researchers directly measured 30 trucks. Then they measured the total cloud of particles downwind of the traffic. There was more carbon than they'd expect given the measured value for trucks and the estimated value for cars. Therefore the cars must be emitting much more on average. Oddly, they never directly measured any cars. The idea that the additional black carbon might be due to some other source besides the cars was apparently not considered.
Knowing one of the authors (I wrote the software that runs one of the instruments), I asked him for a comment on this comment (above). Here is his response:
Actually, the black carbon was measured upwind and downwind of the highway, along with many, many other parameters (including wind speed and direction). We were able to correct the results for back ground black carbon levels (other sources). The measurements were conducted very close to the highway, so nothing else other than traffic could have generated the increase over the background. The traffic flow along the highway was monitored (with licence plate identification to determine the number of cars vs. trucks). The part about chasing 30 trucks on the highway was just a corollary study, and was not essential to the conclusions.
Other papers have addressed the use of LII (Laser Induced Incandescence) on both gasoline and diesel cars and trucks (both on-road and on test stands).
I think we probably agree more than disagree - any turbofan for high speed applications will be a low bypass setup. At the moment though, I don't know of any flying aircraft that cruise at Mach 2+, turbojet or turbofan. I think the noise issues from a "pure" turbojet tend to push it out of the running (for civillian applications).
This isn't something I've looked into much lately, but did some reading - interestingly enough NASA was (is?) pushing a mixed-flow turbofan for supersonic (Mach 2+) cruise.
Some good diagrams for recent ideas on supersonic airliner propulsion can be found in [PDF file]. Most of these are centered around a low-bypass turbofan, although tweaked around to deal with the issues of high-speed flight and low-speed noise.
Personally, I don't think the engine type is the main issue to this type of aircraft - I suspect the environmental issues will be more critical (not just talking about sonic booms, but the exhaust products from the engines and the takeoff noise).
Most everything you've said is true, but most turbojet installations also have an inlet geometry designed to slow the incoming air via shock wave(s). Thus, the turbine (whether turbofan or turbojet) sees incoming air at less than supersonic speeds. For example, the F-15 (now about 33 yrs old) has been using turbofan (albeit with a bypass ratio of about 0.7) - it's capable of Mach 2.5 or so at altitude. I don't know of an installation designed for supersonic flight that exposes the fan (or compressor) to M > 1 (not to say they aren't out there - just that it's not the conventional design).
A turbojet has the same problems with supersonic incident air as a turbofan (although a fan should be affected more, everything else remaining the same). Not all turbofans have the high-bypass configuration - the fighters listed above don't when compared to what you see on a passenger jet. For example, the bypass ratio on the Rolls Royce Trent 900 on the Airbus 380 is about 8.5 - I agree , you won't be using these for supersonic flight. However, the assertion that turbofans don't work well at higher Mach numbers (I'm talking 1-2+) isn't correct, unless we're talking about a "naked" jet hanging out on a pylon and not an installed engine.
I think if you look at most recent (last 30 years) supersonic fighter jets, you'll find they use turbofan engines (from some quick googling):
F-14A: (2) TF30-414A Afterburning Turbofans with over 40,000 lb Total Thrust F-14B/D: (2) F110-GE400 Afterburning Turbofans with over 54,000 lb Total Thrust F-16 Pratt & Whitney F100-PW-200 Turbofans F-18 Hornet: Two F414-GE-400 turbofan engines
Supersonic jets in the past used real jets, not turbofans, afterburners if you like. Basically just burning raw jet fuel and using the hot gas to accelerate the plane. Fast, but VERY inefficient. Normal jets get theri efficiency from their turbofan engines.
Turbofans are "real" jets. Turbojets can be thought of as turbofans with zero bypass air. The overall layout of both is the same, and they have the same components. Turbofans can have afterburners also - that's really a separate part of the engine (conceptually speaking).
>Fans, like all propellors, drastically lose efficiency...
Not quite true - the engines for the F-22 Raptor are F119 Turbofans (http://www.pratt-whitney.com/prod_mil_f119.asp) which enable the aircraft to achieve supersonic flight without afterburner (quite an achievement).
A turbofan and turbojet are both turbine engines with a set of fan blades as the first thing the incoming air "sees" after the inlet. These aren't like propeller blades - the solidity ratio is much higher for the turbofan (unless you are talking about the unducted fans, which were really super-turboprops, and still didn't have that many blades compared to a typical turbofan).
The difference between a turbofan and a turbojet is that a turbofan has some air that bypasses the "core" of the engine (the combustors and turbine section). This (generally) results in a more efficient design, depending on altitude, conditions, etc. Overall, turbofans and turbojets are very similar in concept - the turbojet is really a turbofan with zero bypass air.
Don't pollute? Look again - diesels, whether vegetable oil, biodiesel or low-sulphur all produce soot (made of carbon). Engine manufacturers have done a good job at reducing the amount so it's not so visible, but it is still there. Particulate traps and filters may go a long way to reducing or eliminating particulate matter, but these are still in development and not in widespread use.
Not to mention that biodiesel and ethanol are only efficient in small quantities - the overall cycle results in energy loss when you consider the amount of energy needed to produce these fuels.
Except that your meatspace filing system has another attribute - location. People are good at remembering that last quarter's reports are down in the bottom left drawer, etc.
Also, unlike computer filesystems, the different "directories" in your meatspace filing system *look* different. A file folder doesn't look like a filing cabinet, and you typically don't stuff filing folders inside other filing folders - you use a hanging folder for that. In a computer, all directories/folders look the same to the user.
Not true. In the above case, the bounds checking can be easily optimized out. From Sun:
Compiler Optimizations
Range check elimination: The Java programming language specification requires array bounds checking to be performed with each array access. An index bounds check can be eliminated when the compiler can prove that an index used for an array access is within bounds.
Slashdot Mirror
Knowing one of the authors (I wrote the software that runs one of the instruments), I asked him for a comment on this comment (above). Here is his response:
Other papers have addressed the use of LII (Laser Induced Incandescence) on both gasoline and diesel cars and trucks (both on-road and on test stands).
Two thoughts:
1) You must not have kids
2) Divide the DVD cost by the number of people that will watch it - If I buy a $15 2-hour DVD and watch it with my wife, that's $3.75/hour per person.
I think we probably agree more than disagree - any turbofan for high speed applications will be a low bypass setup. At the moment though, I don't know of any flying aircraft that cruise at Mach 2+, turbojet or turbofan. I think the noise issues from a "pure" turbojet tend to push it out of the running (for civillian applications).
This isn't something I've looked into much lately, but did some reading - interestingly enough NASA was (is?) pushing a mixed-flow turbofan for supersonic (Mach 2+) cruise.
Some good diagrams for recent ideas on supersonic airliner propulsion can be found in [PDF file]. Most of these are centered around a low-bypass turbofan, although tweaked around to deal with the issues of high-speed flight and low-speed noise.
Personally, I don't think the engine type is the main issue to this type of aircraft - I suspect the environmental issues will be more critical (not just talking about sonic booms, but the exhaust products from the engines and the takeoff noise).
Most everything you've said is true, but most turbojet installations also have an inlet geometry designed to slow the incoming air via shock wave(s). Thus, the turbine (whether turbofan or turbojet) sees incoming air at less than supersonic speeds. For example, the F-15 (now about 33 yrs old) has been using turbofan (albeit with a bypass ratio of about 0.7) - it's capable of Mach 2.5 or so at altitude. I don't know of an installation designed for supersonic flight that exposes the fan (or compressor) to M > 1 (not to say they aren't out there - just that it's not the conventional design).
A turbojet has the same problems with supersonic incident air as a turbofan (although a fan should be affected more, everything else remaining the same). Not all turbofans have the high-bypass configuration - the fighters listed above don't when compared to what you see on a passenger jet. For example, the bypass ratio on the Rolls Royce Trent 900 on the Airbus 380 is about 8.5 - I agree , you won't be using these for supersonic flight. However, the assertion that turbofans don't work well at higher Mach numbers (I'm talking 1-2+) isn't correct, unless we're talking about a "naked" jet hanging out on a pylon and not an installed engine.
I think if you look at most recent (last 30 years) supersonic fighter jets, you'll find they use turbofan engines (from some quick googling):
F-14A: (2) TF30-414A Afterburning Turbofans with over 40,000 lb Total Thrust
F-14B/D: (2) F110-GE400 Afterburning Turbofans with over 54,000 lb Total Thrust
F-16 Pratt & Whitney F100-PW-200 Turbofans
F-18 Hornet: Two F414-GE-400 turbofan engines
MiG-29 2 RD-33 turbofans
And the list goes on.
From the parent post:
Supersonic jets in the past used real jets, not turbofans, afterburners if you like. Basically just burning raw jet fuel and using the hot gas to accelerate the plane. Fast, but VERY inefficient. Normal jets get theri efficiency from their turbofan engines.
Turbofans are "real" jets. Turbojets can be thought of as turbofans with zero bypass air. The overall layout of both is the same, and they have the same components. Turbofans can have afterburners also - that's really a separate part of the engine (conceptually speaking).
This page has a good explanation of the various types of "jet" engines.
>Fans, like all propellors, drastically lose efficiency...
which enable the aircraft to achieve supersonic flight without afterburner (quite an achievement).
Not quite true - the engines for the F-22 Raptor are F119 Turbofans (http://www.pratt-whitney.com/prod_mil_f119.asp)
A turbofan and turbojet are both turbine engines with a set of fan blades as the first thing the incoming air "sees" after the inlet. These aren't like propeller blades - the solidity ratio is much higher for the turbofan (unless you are talking about the unducted fans, which were really super-turboprops, and still didn't have that many blades compared to a typical turbofan).
The difference between a turbofan and a turbojet is that a turbofan has some air that bypasses the "core" of the engine (the combustors and turbine section). This (generally) results in a more efficient design, depending on altitude, conditions, etc. Overall, turbofans and turbojets are very similar in concept - the turbojet is really a turbofan with zero bypass air.
Don't take my word for it - here's a good explanation of the differences (with illustrations).
Don't pollute? Look again - diesels, whether vegetable oil, biodiesel or low-sulphur all produce soot (made of carbon). Engine manufacturers have done a good job at reducing the amount so it's not so visible, but it is still there. Particulate traps and filters may go a long way to reducing or eliminating particulate matter, but these are still in development and not in widespread use.
Not to mention that biodiesel and ethanol are only efficient in small quantities - the overall cycle results in energy loss when you consider the amount of energy needed to produce these fuels.
Except that your meatspace filing system has another attribute - location. People are good at remembering that last quarter's reports are down in the bottom left drawer, etc.
Also, unlike computer filesystems, the different "directories" in your meatspace filing system *look* different. A file folder doesn't look like a filing cabinet, and you typically don't stuff filing folders inside other filing folders - you use a hanging folder for that. In a computer, all directories/folders look the same to the user.
Not true. In the above case, the bounds checking can be easily optimized out. From Sun:
Compiler Optimizations
Range check elimination: The Java programming language specification requires array bounds checking to be performed with each array access. An index bounds check can be eliminated when the compiler can prove that an index used for an array access is within bounds.
(from Hotpot Documentation)
Which would be trivial in the case supplied.