BogMonster Posted November 19, 2008 Share Posted November 19, 2008 If you have a diesel engine, e.g. a Tdi (though this isn't), and wind up the fuelling on the pump so the engine is running richer for the same amount of air going in, it increases the exhaust gas temperature. If you take the same engine, and put a restriction in the exhaust system so there is some back pressure in the exhaust, would it also increase the EGT on load due to less oxygen getting into the engine? I think so, but can't quite get my head around it. The other option in my mind is that because the same amount of fuel is going in, the combustion temperature is the same, but I'm not sure.... I don't know if the increase in EGT with tuning is due to the extra fuel going in, or the richer mixture (or maybe both)? Informed comments only please, I have Google Quote Link to comment Share on other sites More sharing options...
steffUK Posted November 19, 2008 Share Posted November 19, 2008 When I was running my 300tdi we took it to a Bosch service station in Germany and spend 4 hours on a rolling road to set up turbo boost and fuel pump. I had an EGT fitted to monitor exhaust temperatures. When we were running it at the standard 0.9bar pressure but put up the fuel to black smoke level (so running a rich mixture) the engine started to run cooler, not hotter like you write in your post. When playing with the waste gate and getting the pressure up (I could get mine to 1.7bar) the EGT showed hotter tempratures. So more diesel cools down the EGT. We found that we got the best torque curve at 1.2 bar and the fuel pump turned to the stage just before black smoke. The same subject was also discussed here: http://fsae.com/eve/forums/a/tpc/f/125607348/m/56310846841 We also played with different exhaust systems from standard 300tdi to an open 2.5" pipe. Best performance was with the open system, I could not see a lot of impact on the EGT when we changed over but that is maybe due to not restricting the system enough to measure. stef Quote Link to comment Share on other sites More sharing options...
angusb Posted November 19, 2008 Share Posted November 19, 2008 With a richer mix you have a lower temperature, a leaner mix gives a higher temperature. So all things being equal, lower fuelling or higher boost will result in a leaner mix and a higher temperature. If you restrict the exhaust so that back pressure rises the mixture may not be completely removed from the cylinder before the next load of fuel and air is injected. This makes the mixture richer resulting in lower temperature exhaust gas. Quote Link to comment Share on other sites More sharing options...
DaveSIIA Posted November 19, 2008 Share Posted November 19, 2008 As diesels run with fully open air intakes and increase fuelling to increase power, wouldn't the above cause the EGT to be higher under light loads than at full throttle? Quite the reverse of what actually happens! There may be some confusion between petrol and diesel EGT's vs fuel/air ratio. Quote Link to comment Share on other sites More sharing options...
steffUK Posted November 19, 2008 Share Posted November 19, 2008 Did some more research and found a good physics page: http://hyperphysics.phy-astr.gsu.edu/Hbase...rmo/diesel.html If you go down the page you can play with the air ratio y and also the compression ratio rc and it will show the efficiency of the engine as well as the temperature. Increasing y from optimal 1.4 to 1.6 increases the temperature. Quote Link to comment Share on other sites More sharing options...
DaveSIIA Posted November 19, 2008 Share Posted November 19, 2008 If you have a diesel engine, e.g. a Tdi (though this isn't), and wind up the fuelling on the pump so the engine is running richer for the same amount of air going in, it increases the exhaust gas temperature.If you take the same engine, and put a restriction in the exhaust system so there is some back pressure in the exhaust, would it also increase the EGT on load due to less oxygen getting into the engine? I think so, but can't quite get my head around it. The other option in my mind is that because the same amount of fuel is going in, the combustion temperature is the same, but I'm not sure.... I don't know if the increase in EGT with tuning is due to the extra fuel going in, or the richer mixture (or maybe both)? Informed comments only please, I have Google IIRC, a restrictive exhaust causes EGT to increase as the cylinder cannot fully exhaust and hence a full volume of air cannot be drawn on the intake cycle. The engine then runs more fuel rich, raising EGT. @steffUK: The term your refer to as 'air ratio y' is the ratio of specific heats and is generally not a tuneable commodity. It has a widely accepted value of 1.4 for air, but this is dependent on the specific characteristics - temperature; pressure; humidity; etc. Quote Link to comment Share on other sites More sharing options...
David Sparkes Posted November 19, 2008 Share Posted November 19, 2008 There may be some confusion between petrol and diesel EGT's vs fuel/air ratio. Now there's diplomacy for you :-) There's scope in the Moderators Corp for you!! I do agree, I cringed when I saw the topic, not because of the questions raised, but because they were bound to raise simplistic answers to a complex situation. There are just so many interrelated factors when discussing either Spark Ignition or Compression Ignition engines in detail. They is a limit to how simple the answers can be expressed, without leaving so many openings for misunderstandings. I'm not sure I'm going to even try and explain the detail. Yep, retired, defeated! Cheers. Quote Link to comment Share on other sites More sharing options...
BogMonster Posted November 19, 2008 Author Share Posted November 19, 2008 IIRC, a restrictive exhaust causes EGT to increase as the cylinder cannot fully exhaust and hence a full volume of air cannot be drawn on the intake cycle. The engine then runs more fuel rich, raising EGT. That's what I thought, ta. Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted November 20, 2008 Share Posted November 20, 2008 I have heard of one diesel engine where the air intake was partly blocked with sand that turned into a sort of concrete in Namibia. The engine melted its pistons and was completely FUBAR. It was a G-wagen by the way. If it had been a petrol engine it would have run cooler with all that nice petrol evaporating in the intake. Yes, diesels are different, because the fuel is different in specific heat value and calorific content. Quote Link to comment Share on other sites More sharing options...
angusb Posted November 20, 2008 Share Posted November 20, 2008 Looks like I got this totally the wrong way round Found this whilst trying to work out why. Quote Link to comment Share on other sites More sharing options...
Lara Posted November 20, 2008 Share Posted November 20, 2008 Have to say that there are far too many, assumptions and guesses mixed in with hear say and miss-information here for anyone to get any reliable information from, The EGT of any engine be it SI or CI is related directly to the inlet air temperature and fuel / air ratio and the ignition timing, so supposing that just because one observed engine's EGT goes up or down when boost or fuel was added or an exhaust was blocked etc then all engines will respond the same way is just plan ignorance. And I am not blaming anyone for this as observation of testing is very good practice, but limited experience given out on forums by people with limited experience is the way to disaster. Real research and then years and years of testing is the "only answer" and even then generalisations must be very very carefully made! It is possible for a Diesel turbo engine, if engineered well, to keep on giving more power all the way to 15+Bar without exceeding 750 degrees C, EGT as long as the injection timing (and therefore the Ignition timing) is altered in connection with the fuel quantities injected and the inlet air is cooled correctly. The bottom will probably blow out first rather than the EGT's being too high. Look at tractor pulling for an example. Problems usually occur for the following most common reasons. 1. Fuel added to an otherwise lean running engine, Extended full throttle use. Obviously will increase EGTs as you will be making more power from the same air and the excess air that would have helped reduce EGTs will now be producing more power and therefore more heat. 2. Running on the "rich limit" Potentially the most power produced from a given air quantity, but no excess air for piston / cylinder cooling / stabilizing, (most potential for problems) 3. Increasing fuel without thought for injection timing. (the combustion flame travels at very differing speeds dependant on many criteria, eg lean, rich, hot inlet air, cold inlet air, etc etc) Maximum cylinder combustion pressure should be reached a few degrees (depending on different engines) before TDC, too late and EGTs go up dramatically without increasing power, too soon and power drops dramatically allong with raising EGTs and in bad cases, bearing / piston destruction. 4. Boost increased beyond 1.4/5 bar, fuel increased, inlet air cooling not increased. Not only does this not actually increase the potential oxygen inside your combustion chamber because the air is actually so much less dense than previously (diminishing returns etc) but the hot air causes ignition problems and needs the fuel to be injected at an abnormal time to combat pre-ignition (yes, see 3 above) Power does not increase, smoke usually increases, / EGTs increase people get confused by results. These are just a very few of hundreds of reasons not to go on with this thread, Without being rude and not meaning to upset anyone, but If you have to ask on a forum you need to do your own research, read more books and gain a lot more experience on this subject before you can figure out the carp from the genuine info, and all being given as "informed" answers, so little point asking There are no "rules" bar one, Do your own thing on your own engine, Do not listen to others unless they back up their theory with a VERY good argument. (and even then don't completely trust them) Be prepared for many destroyed engine parts and a lot of expenditure. Don't be put off by someone saying "you can't" and 'impossible" if you think you can and the theory stacks up then "you can" until you find out why you can't ;o) probably too late anyway Good luck, Lara. Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted November 20, 2008 Share Posted November 20, 2008 Do your own thing on your own engine, Don't be put off by someone saying "you can't" and 'impossible" if you think you can and the theory stacks up then "you can" until you find out why you can't ;o) probably too late anyway Good luck, Lara. "Do your own thing on your own engine" is exactly what I have done with water injection to my basically standard 300Tdi. It gives me a 3% improvement on MPG and all members ignore it. What they want is a 25% snake oil improvement so they can throw money away on it. Jim Quote Link to comment Share on other sites More sharing options...
Lara Posted November 20, 2008 Share Posted November 20, 2008 Of course Jim, We Only want 25% you think? Lara Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted November 20, 2008 Share Posted November 20, 2008 Well, I drive 6000 kms and get a free jerrycan of diesel. I reckon that it is worth it. It is interesting that nobody seems to be interested in this method of saving a few €s . Oh well, soe is die lieve. Quote Link to comment Share on other sites More sharing options...
white90 Posted November 20, 2008 Share Posted November 20, 2008 Nice to see some Facts Lara rather than hearsay(crappy band) and mis-information Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted November 20, 2008 Share Posted November 20, 2008 Yes Lara talks good stuff. As an ex-RAF apprenticed engine fitter I can spot the good stuff when I read it. But unfortunately we only were trained on supercharged piston engines and jet engines, so my theoretical knowledge of diesels is non-existent. In those days the only diesels were the fuel tankers. Quote Link to comment Share on other sites More sharing options...
BogMonster Posted November 20, 2008 Author Share Posted November 20, 2008 Without being rude and not meaning to upset anyone, but If you have to ask on a forum you need to do your own research, read more books and gain a lot more experience on this subject before you can figure out the carp from the genuine info, and all being given as "informed" answers, so little point asking That's all right, the original reason I asked the question has now been rendered irrelevant by something else that has come to light explaining the expiry of the motor in question, so it doesn't matter if all the above was carp. However the question was quite simple; does a restriction in the exhaust system cause an increase in EGT with all other factors remaining unchanged, I didn't throw a bigger turbo on, wind up the injection pump alter the timing and blah, I just wanted to know if it could have made an engine go bang but now I know it didn't anyway. It could happen in a number of ways; collapsed silencer or an exhaust pipe flattened on a rock being just two (though if you flattened the exhaust on this motor on a rock it would be bad news). They do say free advice is often worth every penny and by the way I make that four rules not one supercharged piston engines ooOOhhh Quote Link to comment Share on other sites More sharing options...
JimAttrill Posted November 20, 2008 Share Posted November 20, 2008 Bristol Hercules engines on Vickers Varsity aeroplanes, 14 cylinder 2-row radials, 28 spark plugs to change when it misfires a bit. A very oily job. The oil was SAE100 very thick stuff. A terribly dirty job, but fun in a way. I worked on them 66-68. And I loved every moment Quote Link to comment Share on other sites More sharing options...
Lara Posted November 21, 2008 Share Posted November 21, 2008 BogMonster, sorry if the answer sounded a bit offish but was just trying to explain why I thought the question was unanswerable accurately. Jim, Wonderful engine that Hercules, you are lucky to have worked on such a masterpiece! I love plane engines, have a mate with a collection of old war planes, and regularly drool over them, and especially the engines! Love the power additives they were playing with in the war years! 66-68 Must have been a good time for you. Lara Quote Link to comment Share on other sites More sharing options...
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