Page 11 of 100 FirstFirst ... 9101112132161 ... LastLast
Results 151 to 165 of 1498

Thread: The Technical Questions Thread

  1. #151
    Join Date
    Oct 2006
    Posts
    10,217
    Quote Originally Posted by hightower99 View Post
    I think you mean to ask why NA diesel engines make less peak power than similarly sized NA petrol engines right? The simple answer is that because diesel engines are designed to be stronger they cannot rev as high (ever noticed that diesel engines almost never rev past 5000RPM?). There are ofcourse other factors like: because they are designed with a higher compression ratio the valve system cannot be properly optimized for high RPM flow ect. ect.
    So according to you, diesels engines can't rev high because of their strength, a high compression ratio interferes with valve timing...

    Quote Originally Posted by jediali View Post
    well yes. but because diesel takes longer to burn it cannot allow high revs.
    ...but according to jediali, it is because diesel gas takes longer to burn. Is it a bit of both, or is one of you wrong?

    If diesel takes longer to burn, would it help the situation if there was a spark plug?

    Quote Originally Posted by hightower99 View Post
    I think you have misunderstood something if you have to ask this question. A diesel engine is not called a diesel engine simply because it burns diesel fuel but because it is a Heterogenous Compression Ignition engine that works according to the Diesel cycle (as opposed to the Otto cycle that petrol engines use).
    The fundamental difference being that the Otto cycle includes spark plugs, yes?

    Quote Originally Posted by hightower99 View Post
    Yes it is less efficient. Higher compression ratio = higher efficiency.
    According to jediali, higher compression ratios aren't used in gasoline engines as the air/fuel mixture will prematurely ignite and cause knocking?

    Quote Originally Posted by hightower99 View Post
    This is actually being done it is called a HCCI engine (Homogenous Charge Compression Ignition).
    This new engine then somehow counters the fact that gasoline is volatile and likely to explode before full compression is completed then?

    Quote Originally Posted by hightower99 View Post
    Yes ofcourse but why?
    In an automotive setting, would not a two-stroke with (an) overhead cam(s) be superior to the standard two-stroke layout due to superior efficiency?

    Quote Originally Posted by hightower99 View Post
    I am not sure why two-stroke diesel engines don't appear in cars I would imagine it has something to do with emissions?
    On the shitty little article I read about diesel two-strokes on how things work, they said that the air need be compressed via a supercharger or turbocharger. Maybe this would not be the case if a diesel two-stroke was designed with a camshaft? Am I making any sense?



    Quote Originally Posted by jediali View Post
    heterogeneous - occurs in petrols and throttles air supply and adds appropriate fuel quantity (ideally stoichiometric, 14.7:1) therefore cylinder size does not necesarily represent air/fuel mixture volume entering hence lesser volumetric efficiency.

    homogeneus - always allows fuel provision of air to cylinders, this time accelerator adjusts fuel deliverd through injectors (usually direct into cylinders) therefore maximising volumetric efficiency and reducing unburnt fuel.
    So a homogeneous setup is superior then?

    Quote Originally Posted by jediali View Post
    clever stuff this...worth looking out for. vauxhall are running this as we speak. this has been allowable through advancements in ECU and engine tolerances. I also think it has a lot to with good combustion chamber where there are no hot spots (hottest point of combustion chamber which sets of fuel to early - usually the exhaust valve or something else sticking out).
    This will then increase horsepower somewhat then yes?
    Last edited by Kitdy; 12-16-2007 at 04:57 PM. Reason: error in quoting

  2. #152
    Join Date
    Nov 2006
    Location
    Aberdeen
    Posts
    2,975
    Quote Originally Posted by Kitdy View Post
    So according to you, diesels engines can't rev high because of their strength, a high compression ratio interferes with valve timing......but according to jediali, it is because diesel gas takes longer to burn. Is it a bit of both, or is one of you wrong?
    both! a high compression ratio means piston gets closer to valves. I feel personally that the fuel burn time is to blame most.

    Quote Originally Posted by Kitdy View Post
    If diesel takes longer to burn, would it help the situation if there was a spark plug?
    Because diesel fuel droplets burn from the outside inwards, fuel burn time is reduced by creating smaller droplets (hence higher common rail pressures + high tech injectors). I dont know how diesel would react to a spark, it would have less effect i believe.

    Petrol fuel burning is not about equal burning of fuel droplets around the combustioon chamber like diesel is. Instead petrol relies on a turbulent movement of an air/fuel mixture which burns according to the time it takes for the flame front to spread from spark plug(s) to cylinder walls - this time is less than the diesel time.

    Quote Originally Posted by Kitdy View Post
    The fundamental difference being that the Otto cycle includes spark plugs, yes?
    yes

    Quote Originally Posted by Kitdy View Post
    According to jediali, higher compression ratios aren't used in gasoline engines as the air/fuel mixture will prematurely ignite and cause knocking?
    i think thats fairly agreeable

    Quote Originally Posted by Kitdy View Post
    This new engine then somehow counters the fact that gasoline is volatile and likely to explode before full compression is completed then?
    exactly. I have little knowledge on how its done, i guess its just a very precise engine

    Quote Originally Posted by Kitdy View Post
    So a homogeneous setup is superior then?
    I think so yes.

    Quote Originally Posted by Kitdy View Post
    This will then increase horsepower somewhat then yes?
    if a better compression ratio can be found then yes, more power can be made. Its noteworthy that in engines like that found in the rs4 the FSI (its homogeneous) is useful for cylinder cooling - fuel is introduced (direct injection) in such a way that temperature build up eliminates knocking. Same with the highly efficient veyron engine
    Last edited by jediali; 12-17-2007 at 01:38 AM.
    autozine.org

  3. #153
    Join Date
    Mar 2004
    Location
    Eindhoven, The Netherlands
    Posts
    7,833
    Quote Originally Posted by jediali View Post
    both! a high compression ratio means piston gets closer to valves. I feel personally that the fuel burn time is to blame most.
    Because of the very high compression in the diesel engine, the fuel burns equally fast or faster, has been thought to me. The main blame is the inertia needed to make the very high compression in the cilinder possible. It is also very difficult to make injector and their components able to reach 2000 bar, so fast.

    Because diesel fuel droplets burn from the outside inwards, fuel burn time is reduced by creating smaller droplets (hence higher common rail pressures + high tech injectors). I dont know how diesel would react to a spark, it would have less effect i believe.

    First of all, when droplets occur in your engine, you have a very serious problem. The fuel is injected under such a high pressure, that a dense haze is created.

    They don't neccesarily burn from outside-inside. Using all kinds of techniques, like multiple injections/stroke the mixture is made homogenously (sp ?). This means it will burn evenly, if it doesn't the vibration would cause serious mechanical problems.

    Adding a spark plug would be rather useless. The mixture would burn anyway, without it. The spark plug would probably cause an unbalanced ignition. Also the added electrics, would take away some of output.

    Petrol fuel burning is not about equal burning of fuel droplets around the combustioon chamber like diesel is. Instead petrol relies on a turbulent movement of an air/fuel mixture which burns according to the time it takes for the flame front to spread from spark plug(s) to cylinder walls - this time is less than the diesel time.
    Droplets ----> haze.

    This time is not neccesarily shorter. I would have to look up the exact values at same density mixtures. Will do this, when i have time and dont forget..

    And since I am at it, stoichiometric burn is NOT 14,7 air : 1 fuel. It is actually 14,83 : 1

    if a better compression ratio can be found then yes, more power can be made. Its noteworthy that in engines like that found in the rs4 the FSI (its homogeneous) is useful for cylinder cooling - fuel is introduced (direct injection) in such a way that temperature build up eliminates knocking. Same with the highly efficient veyron engine
    The advantages of higher compression ratio's end somewhere though. IIRC the gain made from a ratio of 25 or up, doesn't weigh up against the benefits.

    The cylinder cooling by the fuel is a entirely different thing. It is done by almost any modern engine out there. The cooling function of the injected fuel is minimum though. Perhaps it takes over 1-2% of the total cooling function. It can't stick to the cilinder wall, to prevent wall hugging and it's effects.. So it can only cool by absorbed radiated heat. The engine oil plays a much more important role here, cooling and cleaning the engine's internals.

    I think you can clearly see I had a long line of college's on these subjects recently

  4. #154
    Join Date
    Nov 2006
    Location
    Aberdeen
    Posts
    2,975
    Quote Originally Posted by drakkie View Post
    First of all, when droplets occur in your engine, you have a very serious problem. The fuel is injected under such a high pressure, that a dense haze is created.
    ok! i mean droplets as in the tiny individual masses of fuel that the injector delivers.

    Quote Originally Posted by drakkie View Post
    They don't neccesarily burn from outside-inside. Using all kinds of techniques, like multiple injections/stroke the mixture is made homogenously (sp ?). This means it will burn evenly, if it doesn't the vibration would cause serious mechanical problems.
    again miscommunication. yes combustion occurs across entire mixture simultaneously. The said fuel droplets, micro-diesel fuel quantitys - burn from outer surface and into core. we are talking microns here....not about the cylinder overall.

    Quote Originally Posted by drakkie View Post
    Droplets ----> haze.
    again, the haze is made up of tiny diesel fuel volumes in a mass of air, which i call many small droplets, you can use a different name.

    Quote Originally Posted by drakkie View Post
    This time is not neccesarily shorter. I would have to look up the exact values at same density mixtures. Will do this, when i have time and dont forget...
    the typical diesel combustion process lasts quite long (relatively) and focuses on providing pressure during much of the downstroke of piston. The petrol aims to combust at a particular degree after TDC in order to "nail" the burst of energy that occurs over a lesser amount of degrees of crank angle movement compared to mr diesel (not Vin)

    Quote Originally Posted by drakkie View Post
    And since I am at it, stoichiometric burn is NOT 14,7 air : 1 fuel. It is actually 14,83 : 1 .
    interesting...thanks for enlightening my day , sorry Kitdy, you better redo those calcs!

    by the way: (Stoichiometry - Wikipedia, the free encyclopedia)

    Fuel By weight By volume
    Gasoline 14.7 : 1
    Natural Gas 17.2 : 1
    Propane (LP) 15.5 : 1
    Ethanol 9 : 1
    Methanol 6.4 : 1
    Hydrogen 34 : 1
    Diesel 14.6 : 1
    might be wrong

    Quote Originally Posted by drakkie View Post
    The advantages of higher compression ratio's end somewhere though. IIRC the gain made from a ratio of 25 or up, doesn't weigh up against the benefits.
    mechanical frictions of injectors, piston seals and compression force do eventually limit compression ratio and i understand this is a current limitation of making diesels more efficient. The advancemement of diesel engine has relied an awful lot on how engineers have made high precision, strong diesel engines mass preoduction and relatively reliable.

    Quote Originally Posted by drakkie View Post
    The cylinder cooling by the fuel is a entirely different thing. It is done by almost any modern engine out there. The cooling function of the injected fuel is minimum though. Perhaps it takes over 1-2% of the total cooling function. It can't stick to the cilinder wall, to prevent wall hugging and it's effects.. So it can only cool by absorbed radiated heat. The engine oil plays a much more important role here, cooling and cleaning the engine's internals.
    agreed...but it helps.

    Quote Originally Posted by drakkie View Post
    I think you can clearly see I had a long line of college's on these subjects recently
    yep...keep us right will you! I wish i had done your sort of course. IMHO you should have submitted the post before typing this line.
    Last edited by jediali; 12-17-2007 at 04:34 AM.
    autozine.org

  5. #155
    Join Date
    Mar 2005
    Posts
    1,508
    This has all been a good discussion. I just thought I would add some clarity to something. I’ve seen homogeneous and heterogeneous thrown out but I wasn’t sure if the supplied explanations. For that reason I thought I would add these explanations.

    Homogeneous charge simply means the charge in the combustion chamber is uniform (same F/A ratio throughout). Most gasoline engines are homogeneous charge engines. The fuel and air are mixed before entering the cylinders. There are exceptions such as direct injection gasoline motors as well as motors that try to create a “rich-lean” condition. The Honda CVCC is an example. The small sub-combustion chamber was rich while the primary chamber was lean. I should note that if the fuel is directly injected early enough it may also be considered a homogeneous charge motor. The important thing is that the charge is basically uniform at the time of ignition. Diesels basically ignite the moment the fuel is pushed into the cylinder thus conventional diesels can’t be homogeneous charge.

    I can’t think of a homogeneous diesel motor but they might exist.

    Heterogeneous means that the F/A ratio changes throughout the combustion chamber. This shouldn’t be a surprise when we are talking about direct injection motors (gasoline or diesel) as the area right in front of the injector is very rich.

    Several companies are working on homogeneous charge compression ignition motors. These sound very promising. The claim is diesel like efficiency out of a gasoline motor. For fear of messing up the details I won’t try to hunt them down. Hopefully next month I will be able to dig up a bit more information on the subject. I’m headed back for a doctorate. One of my old profs is a expert in the field of combustion and would know a great deal about all these details.

  6. #156
    Join Date
    Nov 2006
    Location
    Aberdeen
    Posts
    2,975
    Quote Originally Posted by jediali View Post
    heterogeneous - occurs in petrols and throttles air supply and adds appropriate fuel quantity (ideally stoichiometric, 14.7:1) therefore cylinder size does not necesarily represent air/fuel mixture volume entering hence lesser volumetric efficiency.

    homogeneus - always allows fuel provision of air to cylinders, this time accelerator adjusts fuel deliverd through injectors (usually direct into cylinders) therefore maximising volumetric efficiency and reducing unburnt fuel.
    doh', these are the wrong way round!
    autozine.org

  7. #157
    Join Date
    Dec 2005
    Posts
    1,200
    Quote Originally Posted by Kitdy
    So according to you, diesels engines can't rev high because of their strength, a high compression ratio interferes with valve timing...

    ...but according to jediali, it is because diesel gas takes longer to burn. Is it a bit of both, or is one of you wrong?

    If diesel takes longer to burn, would it help the situation if there was a spark plug?
    First I mentioned that there are several reasons I didn't mention them all. Yes the relatively low flame front speed of diesel fuel keeps the redline speed down but if you do the math then it should only limit a 2L Inline 4 diesel engine to about 6000-6500RPM yet the fastest stop at 5000RPM or less. Rather then the low flame speed of diesel I would think that Jediali's explaination of how Diesel cycle combustion differs from Otto cycle (ie constant pressure versus constant volume) is vital in understanding the difference. Also I was implying that because the internal componants of a diesel engine have to be made strong they are also heavy and the high compression ratio normally means a longer stroke aswell as difficulties with high RPM valvetrain optimization.

    Quote Originally Posted by Kitdy
    The fundamental difference being that the Otto cycle includes spark plugs, yes?
    Well that isn't the only fundamental difference there is also the different combustion (like Jediali said) and the fact that they are wholly optimized for high compression (even at the expense of high RPM operation) as Mr. Diesel saw compression ratio as something that couldn't be optimized enough when using a Otto cycle engine.


    BTW:

    Jediali: I am happy you realised that you switched the meanings of homogeneous and heterogeneous mixtures. Also I would like to point out that Kitdy was right about energy content of diesel fuel. Diesel is more dense (850g/L vs 720g/L for petrol) and has a higher energy content per volume (40.9MJ/L vs 34.8MJ/L for petrol). However you are right about the caloric value (45.3MJ/kg vs 45.8MJ/kg for petrol).

    Culver: Thank you for your clear explaination on homo- hetero-geneous mixtures (I was writing up the same thing before I realised you beat me to it!)

    I am looking forward to what you Prof. has to say about HCCI technology. Personally I have been following progress on this subject for awhile now and it certainly seems interesting. Apparently instead of stopping detonation this type of engine is designed to create a smaller combustion that is like detonation (sudden almost instantaneous and complete combustion of the injected fuel). It is able to combust a lean homogeneous mixture. It seems the biggest problem is how to throttle (control) the engine properly and get it to work under high load conditions.
    Last edited by hightower99; 12-17-2007 at 12:10 PM.
    Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
    Engine torque is an illusion.

  8. #158
    Join Date
    Mar 2004
    Location
    Eindhoven, The Netherlands
    Posts
    7,833
    Quote Originally Posted by jediali View Post
    ok! i mean droplets as in the tiny individual masses of fuel that the injector delivers.
    But still strictly speaking, you don't have droplets We have this really funy teacher, doing the colleges on it. Each time we said droplets in the earliest lessons, he would count it up on the whiteboard. When we would reach 20, we would own him a crate of beer

    the typical diesel combustion process lasts quite long (relatively) and focuses on providing pressure during much of the downstroke of piston. The petrol aims to combust at a particular degree after TDC in order to "nail" the burst of energy that occurs over a lesser amount of degrees of crank angle movement compared to mr diesel (not Vin)
    It is almost hammered into us, that combustion occurs BEFORE TDC in both petrol and diesel. It is done for various reason, of which the main one is emissions. I'll attach some graphs shown in the man's powerpoints.


    interesting...thanks for enlightening my day , sorry Kitdy, you better redo those calcs!

    by the way: (Stoichiometry - Wikipedia, the free encyclopedia)



    might be wrong
    I have no idea where the tiny deviations come from. We use 14,83:1 as a standard value in all our calculations. Wednesday I am gonna ask.

    yep...keep us right will you! I wish i had done your sort of course. IMHO you should have submitted the post before typing this line.
    Only if you keep me on the right track too Some criticism is never wrong, especially if you can learn from it ! There is now a Masters degree in the English language starting in The Netherlands. If you are interested, I can find some info for you !

  9. #159
    Join Date
    Nov 2006
    Location
    Aberdeen
    Posts
    2,975
    Quote Originally Posted by drakkie View Post
    But still strictly speaking, you don't have droplets We have this really funy teacher, doing the colleges on it. Each time we said droplets in the earliest lessons, he would count it up on the whiteboard. When we would reach 20, we would own him a crate of beer
    so what do you call the little bits of diesel inside your mist?..see attatchement for one of my sources regarding this precise topic->

    Quote Originally Posted by drakkie View Post
    It is almost hammered into us, that combustion occurs BEFORE TDC in both petrol and diesel. It is done for various reason, of which the main one is emissions. I'll attach some graphs shown in the man's powerpoints.
    do you mean combustion begins before TDC due to time taken for fuel to begin releasing energy i order to power piston?

    Quote Originally Posted by drakkie View Post
    Only if you keep me on the right track too Some criticism is never wrong, especially if you can learn from it ! There is now a Masters degree in the English language starting in The Netherlands. If you are interested, I can find some info for you !
    Well i apologise for my sarcasm but i am unsure if this is sarcasm. I respect your intent and grasp on engines if thats any use....
    Attached Images Attached Images
    autozine.org

  10. #160
    Join Date
    Jun 2005
    Location
    East Coast of the United States
    Posts
    11,967
    Quote Originally Posted by Kitdy View Post
    Why don't we see any two-stroke diesels in the automobile market? They would produce nearly twice the power of a four-stroke diesel and would still return a relatively good fuel economy, would they not?

    That's all I can think of for now, if any of my questions sound stupid, keep in mind I am only learning about engines and have a somewhat limited grasp on how they function.
    None of these questions are stupid. I've thought about them before, I just never put it in this thread.

    I can't answer the other questions because they've been answered already, but 2 stroke engines have absolutely horrible emissions. They're the least efficient of the bunch and they actually have no engine oil to lubricate it- thus, the fuel that it burns also lubricates it, iirc. So, the fuel that lubricates it is also burned. Imagine how bad the smog must be if all cars were like that?

    2 strokes are used in leaf blowers and lawn mowers. They stink for a reason. That's the fuel they're burning.

    Also, they're very loud. Imagine all the cars out there as loud as that?

  11. #161
    Join Date
    Oct 2006
    Posts
    10,217
    Quote Originally Posted by hightower99 View Post
    Jediali: I am happy you realised that you switched the meanings of homogeneous and heterogeneous mixtures. Also I would like to point out that Kitdy was right about energy content of diesel fuel. Diesel is more dense (850g/L vs 720g/L for petrol) and has a higher energy content per volume (40.9MJ/L vs 34.8MJ/L for petrol). However you are right about the caloric value (45.3MJ/kg vs 45.8MJ/kg for petrol).
    What is the difference between energy content per volume and caloric value?

    If I follow you guys straight, then direct injection engines are heterogeneous, and non-direct injection engines are homogeneous, yes?

    As far as I have heard, direct injection engines are superior (for some reason) why is this, and is this true? In what way are they superior if it is true?

    EDIT:

    Quote Originally Posted by NSXType-R View Post
    None of these questions are stupid. I've thought about them before, I just never put it in this thread.
    Great minds think alike!

    Quote Originally Posted by NSXType-R View Post
    I can't answer the other questions because they've been answered already, but 2 stroke engines have absolutely horrible emissions. They're the least efficient of the bunch and they actually have no engine oil to lubricate it- thus, the fuel that it burns also lubricates it, iirc. So, the fuel that lubricates it is also burned. Imagine how bad the smog must be if all cars were like that?

    2 strokes are used in leaf blowers and lawn mowers. They stink for a reason. That's the fuel they're burning.

    Also, they're very loud. Imagine all the cars out there as loud as that?
    Why not then design a 2-stroke diesel with an overhead camshaft? It's emissions may be worse, but it would likely return average fuel economy - I am thinking of a performance vehicle as well where emissions, noise and fuel economy are less important.

    Are two-stroke car engines even legal?

    Rather then the low flame speed of diesel I would think that Jediali's explaination of how Diesel cycle combustion differs from Otto cycle (ie constant pressure versus constant volume) is vital in understanding the difference.
    What is the difference between constant pressure and constant volume in an engine then?
    Last edited by Kitdy; 12-17-2007 at 03:09 PM.

  12. #162
    Join Date
    Dec 2005
    Posts
    1,200
    Quote Originally Posted by Kitdy View Post
    What is the difference between energy content per volume and caloric value?
    Energy content per volume is what it says it is. It is the total thermal energy you get from burning a liter of fuel. Caloric values are normally energy per unit mass (MJ/kg) or the energy you get from burning 1 kg of fuel.

    Quote Originally Posted by Kitdy
    If I follow you guys straight, then direct injection engines are heterogeneous, and non-direct injection engines are homogeneous, yes?
    The great thing about direct injection petrol engines is that they run heterogeneous mixtures under light and medium load (this is called stratified charge) and under heavy load they can run homogeneous mixtures.

    Quote Originally Posted by Kitdy
    As far as I have heard, direct injection engines are superior (for some reason) why is this, and is this true? In what way are they superior if it is true?
    Direct petrol injection is superior because of several qualities. First the system is more accurate at metering fuel to each individual cylinder according to it's needs and your demands (meaning it does not waste as much fuel as other systems do). To be more accurate they operate with much higher injection pressures which has several advantages including finer atomization (which means less unburnt fuel and a quicker cleaner burn). Also because the fuel is injected directly into the cylinder and because it is an endothermic process to evaporate the fuel, this means that the air in the cylinder is cooled down meaning the engine can run higher compression ratio which makes it more efficient. Also in stratified charge mode it allows a lean mixture to be ignited because it can focus the fuel around the spark plug. Really there is alot of information out there about direct injection and I suggest you read up on it.


    Quote Originally Posted by Kitdy
    Why not then design a 2-stroke diesel with an overhead camshaft? It's emissions may be worse, but it would likely return average fuel economy - I am thinking of a performance vehicle as well where emissions, noise and fuel economy are less important.
    the majority of 2 stroke diesel engines are incredibly huge engines (several thousand liter displacement) and they do have camshafts because they have exhaust valves at the top (intake is through a transfer port like a normal 2 stroke engine). Also the problem of lubrication doesn't exist in a 2 stroke diesel because the crank assembly is seperated from the cylinder and is lubricated with a pressurized oil system like a normal car engine. However a 2 stroke diesel would have to be incredibly strong and it probably wouldn't be able to rev past 3000RPM.

    Quote Originally Posted by Kitdy
    Are two-stroke car engines even legal?
    manufacturers couldn't get them to pass emissions testing so no they aren't.



    Quote Originally Posted by Kitdy
    What is the difference between constant pressure and constant volume in an engine then?
    I was talking about the different ways that the combustion event happens in the different engines. In petrol engines the combustion happens very quickly before the piston has a chance to change volume much hence they are considered constant volume combustion engines. Diesel engines are designed to burn fuel through most of the power stroke trying to maintain a high pressure over most of the power stroke hence they are constant pressure combustion engines.

    It seems to me that you need to start reading up on more basic subjects in order to help you understand the answers to the questions you are asking.
    Power, whether measured as HP, PS, or KW is what accelerates cars and gets it up to top speed. Power also determines how far you take a wall when you hit it
    Engine torque is an illusion.

  13. #163
    Join Date
    Nov 2006
    Location
    Aberdeen
    Posts
    2,975
    Quote Originally Posted by Kitdy View Post
    What is the difference between energy content per volume and caloric value?
    good question, can someone clear that up. g/L is a volume density right? so would it not depend on what those 'grams' are?

    Quote Originally Posted by Kitdy View Post
    If I follow you guys straight, then direct injection engines are heterogeneous, and non-direct injection engines are homogeneous, yes?
    Thats a good general rule of thumb. Some smaty pants might bring up odd examples to disprove the assumption, but its a fairly trustable modern trend.

    Quote Originally Posted by Kitdy View Post
    As far as I have heard, direct injection engines are superior (for some reason) why is this, and is this true? In what way are they superior if it is true?
    I think direct inj. engines are better for road going engines because its far more benefical at partial load - the most used engine loads in town and when cruising. It isnt really mainstream yet i guess because the implications of all the competitiors mass production and R&D. BMW's valvetronic is one way to see how you can start to reduce the inneficencies of the otto cycle - it replaces the throttle butterfly (which causes suction losses) with variable lift intake valves.

    Because at partial load (or accelerator position) there is always a full amount of air measure in the cylinder:
    - unburnt fuel emsions are reduced due to excess oxygen at boundarys of combustion (fuel stilll has to burn in local stoichiometric region for petrol)
    -torque is increased due to higher volumetric efficiency (more gas in cylinder increasing pressure on piston)

    Maybe others can think of advantages or disadvantages too.

    Quote Originally Posted by Kitdy View Post
    What is the difference between constant pressure and constant volume in an engine then?
    its a theretcall idea but:
    - otto cycle (thats petrol) assumes a constant volume combustion. ie the fuel burns instantly and therefore assumes no time change thus no crank angle change and thus during combustion the combustion chamber is the same volume. This of course is not realised practically and combustion does take a minimal finite time according to turbulence of mixture etc.
    -diesel cycle (er...diesel fuel) assumes that the diesel combustion will take time and will provide heat release all through the power stroke by performing early fuel injection to allow fuel time to begin releaing energy at top of stroke and gradual reinjection to maintain heat release as fuel in chamber runs down at the end of the stroke. Thus diesel is providing a constant pressure in the entire power stroke. This of course is also difficult in practise and even Rudolf Diesel admitted sacrifices in cycle efficiency had to be made in order to allow better mechancial efficicncy.

    pic 1: an ideal diesel cycle (plot cyl. vol (related to crank angle) and pressure) where combustion occurs between points 2 and 3.
    pic 2: an ideal otto petrol combustion where combustion occures between points 2 and 3. same dort of thing again but notice how in both graphs the line 2-3 is constant pressure or volume for diesel or otto respectively.

    p.s. i am quite tired so if anyone spots anymore of my mistakes just say, id hate to misinform others blindly.
    Attached Images Attached Images
    Last edited by jediali; 12-17-2007 at 04:24 PM.
    autozine.org

  14. #164
    Join Date
    Jun 2005
    Location
    East Coast of the United States
    Posts
    11,967
    Quote Originally Posted by Kitdy View Post
    Great minds think alike!


    Why not then design a 2-stroke diesel with an overhead camshaft? It's emissions may be worse, but it would likely return average fuel economy - I am thinking of a performance vehicle as well where emissions, noise and fuel economy are less important.

    Are two-stroke car engines even legal?


    A 2 stroke engine doesn't look anything like a 4 stroke engine.

    You may not like Howstuffworks, but it's the best way to show it to you.

    Howstuffworks "How Two-stroke Engines Work"

    A 2 stroke engine with overhead valves.......where would you put them?

    The cylinder itself acts as the intake and exhaust valves. So you wouldn't be able to put them anywhere.

    And to answer you question about putting a 2 stroker in a car....

    Howstuffworks "How Two-stroke Engines Work"



    Also, I didn't know 2 stroke diesels existed. They work the same way I guess, without the spark plug?

  15. #165
    Join Date
    Mar 2005
    Posts
    1,508
    As this link shows, a 2 stroke diesel has overhead valves. It can even have a 4 valve head where all valves are exhaust valves.
    Howstuffworks "How Diesel Two-Stroke Engines Work"

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. The random picture thread
    By Mustang in forum Multimedia
    Replies: 489
    Last Post: 05-16-2014, 02:19 PM
  2. The "What car is it?" thread
    By The_Canuck in forum Miscellaneous
    Replies: 16
    Last Post: 10-07-2005, 01:28 PM
  3. lukehow and Robb Mann thread
    By Matra et Alpine in forum Miscellaneous
    Replies: 43
    Last Post: 12-12-2004, 06:54 PM
  4. About the enzo thread
    By werty in forum Website discussion
    Replies: 22
    Last Post: 09-19-2004, 04:03 PM
  5. Changing thread name
    By Rijoh in forum Website discussion
    Replies: 2
    Last Post: 07-11-2004, 07:33 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •