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I thought there was also an issue relating to the lack of control of ignition in diesels? They ignite under compresion, but the exact point of ignition is difficult to control - it goes when the condition are right, not when you appply a spark, so unless you can get the conditions spot-on every time you will have variance in the exact point of ignition. Once igniting, more fuel can be added through the pre-combustion chamber to give the required power, however this all takes time and is somewhat dependant upon when it first ignites. I'd suggest this couldn't be done at higher speeds. Or is all that rubbish?
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Or is all that rubbish? >>
No, it makes perfect sense.
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Thanks cheddar! Great explanation - it now makes perfect sense.
By the way, I also have a ZX7R. The noise is a great advert for the petrol way! :-)
Cheers again.
DP
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By the way, I also have a ZX7R. The noise is a great advert for the petrol way! :-)
There's a coincidence! Re the noise are you talking about induction or do you have an aftermarket pipe? I have an Akropovic system, sounds great.
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Diesels don't just fire when the compressed mixture feels like it. I thought the point was that only the air is compressed, and it reaches such a high temperature because of the compression ratio used that the instant the fuel is injected, it ignites. The timing of the injection is absolutely critical, as is the amount of fuel, which is why diesel injectors are such precision instruments and need such care and maintenance.
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Hi Cliff
I'm missing something here - if there's no fuel in the air at all, and the fuel ignites the instant it is injected, how do they get any air/fuel mixing? There will just be a flame front immediately at the exit of the fuel injector, in which case how does 'fresh' air get to the fuel, and past the products of combustion?
I thought that a lot of early deisels were so noisy precisely because the prcise point of ignition was not perfectly controlled.
I understand that the diesel injectors are impressive bits of kit that require great maintenance, but I thought that was more to do with the fact that they are operating in a hostile environment where the inject fuel into systems under huge pressure and temperature, especiaaly considering fuel will be added during combustion - unlike petrol, which merely injects into a cold air flow prior to compression/combustion. In terms of injecting fuel, they require such precision as the actual power output of the engine is entirely dependent upon how much is injected, unlike a petrol engine which only requires accurate injection to get a good a/f ratio and where power is regulated by limiting the volume of mixture in the cylinders (quality governed vs. quantity governed).
Please note this isn't intended to refute your point, it's just that the process is still not clear in my head. In the mists of time I attended a basic course on internal combustion engines, this was more to do with the heat cycles than the precise mechanics, but some of these points were raised - however, I feel I'm forgetting something important.
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I don't know Mattie, I'm just trotting out what I have always (half) understood. I know that diesel injectors work at an enormous pressure, so I imagined that the injected globlet of fuel instantly vapourised and burnt. It goes with more of a bang than petrol, hence the noise.
The difference in injection pressure is illustrated by the dire warnings to people testing injectors. Whereas a petrol injector just gives a harmless little squirt, diesel is injected at such a velocity that it easily penetrates the skin, and can kill.
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The basic answer given at the top of the thread is fundamentally the right one - diesel engine speeds are limited by the combustion process. If you run them faster, you would get high emissions of partially and unburnt fuel, and a corresponding drop in efficiency.
The combustion process has also been given in outline, however, the importance of the delay period has not been discussed.
The process goes a bit like this:
The piston comes up on compression, the air gets hot, and via various aerodynamic devices is swirling, swishing, and being generally turbulent.
The fuel is injected, at high pressure. The sprays of fuel are fast, and powerful in order to penetrate the turbulent air. The liquid fuel, is, however, cool and doesn't burn immediately.
Nothing much happens for a bit, while mixing and preliminary chemical processes get underway.
After this 'delay period', a fair proportion of the injected fuel has heated through, begun to dissociate into radicals, mixed with some air, and suddenly begins to burn. This sudden commencement of combustion, after the delay period, gives rise to the characteristic diesel knock.
The distributed flame fronts spread combusting the remaining fuel in the cylinder.
Modern diesel engines with electronic control sometimes use a pre-injection to reduce the sudden pressure spike and associated noise of the delay period.
number_cruncher
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I'm finding this thread fascinating and enlightening.
Could I ask if someone can explain how it is possible that "rechipping" a diesel can increase the power? Surely all that rechipping can do is to get the injectors to squirt more fuel and/or do that at slightly different times in the engine's cycle - yet claims are made that fuel economy is in fact enhanced along with power output. If it's such a good idea, why doesn't the manufacturer do it anyway?
No-one has mentioned turbochargers so far, which I assume radically affects what happens in the combustion chamber.
My apologies if this question reveals depths of ignorance - I guess I still don't really understand diesels.
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One great website for finding out about all manner of things work is HowStuffWorks.
Here's the diesel engine link:
auto.howstuffworks.com/diesel.htm
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What\'s for you won\'t pass you by
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PS
Make sure your speakers are on....:-)
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What\'s for you won\'t pass you by
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PPS
Turbochargers info is at:
auto.howstuffworks.com/turbo.htm
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What\'s for you won\'t pass you by
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Diesels are ideally suited to turbo charging. The deisel combustion process opearates at constant pressure (volume increases), unlike petrols which operate at constant volume (pressure increases). This means you can easily increase the starting pressure of the cycle (which is what a turbo charger does) without worrying about the compression ratio going completely to pot and the head exploding (OK it's not that easy, but the potential is there!).
Most modern TDs run about 1 bar boost pressure. This effectively means you put twice as much air in as running it at atmospheric pressure, and could in theory get twice as much power out. Obviously more power requires more fuel - BUT (and here's the really good bit!) as you raise the boost pressure the cycle efficiency goes UP, and you get more useful power out of the fuel burnt. So, when you chip a TD engine, the boost pressure is increased and the fuelling increased to match. The effect of higher boost pressure is that under the same driving conditions the potential exists to run the engine at higher efficiency and it uses less fuel. Nice! Of course if you chip the engine and then give it a bootful the whole time, economy is likely to suffer....
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RichardW
Is it illogical? It must be Citroen....
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> I should say that I am a great fan of both having a turbo diesel car, a Mondeo TDCi, and a Kawasaki ZX7R that redlines at 13000.
IIRC, Honda's small (50cc?) four stroke motorcycle engine raced in the sixties span to 20,000 RPM, on petrol! Pretty amazing.
Rant on/
BTW, ref Akropovic exhausts elsewhere in this thread: Please put the original can back on! I am a keen biker too, but the complaints my parent's neighbours keep making about race piped bikes (they are on a certain very popular 'circuit' in the Cotswolds) leaving their small village and howling up the NSL hill outside on a peaceful Sunday afternoon, is getting tiresome! For some reason, they think I represent all "Bikers", even though the exhaust on my Hornet is as stock as they come. Complaints to Plod from these neighbours about aforementioned bikers has already resulted in a frequent SCAMera van presence that otherwise wouldn't be there. Blend in with the crowd, and you'll get away with a lot. Stand out, and it seems to ruin it for everyone.
/Rant off
Happy biking! :-)
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IIRC, Honda's small (50cc?) four stroke motorcycle engine raced in the sixties span to 20,000+ RPM >>
They had a 5 cylinder 125cc that reved to over 20'000.
BTW, ref Akropovic exhausts elsewhere in this thread: Please put >> the original can back on! I am a keen biker too, but the complaints my neighbour's parents keep making about race piped bikes (they are on a certain very popular 'circuit' in the Cotswolds) leaving their small village and howling up the NSL
hill outside on a peaceful Sunday afternoon, is getting tiresome! >>
My Akro is in effect a race pipe with a removable baffle making it road legal, not much noisier than standard.
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What an educational thread - thanks
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My ZX7R has the standard pipe on it, and it still sounds awesome to my ears. The standard pipe is actually quite loud as these things go, and my mate did the honours with a video camera as I did a "fly by" once and I'm pretty happy with it. The Akrapovic systems are lovely though, and always work well on Kwaks.
I used a Quill T3 on my old ZZR 600. Very clever design with a spring loaded ball valve forcing the exhaust gas through the baffles until the pressure under heavy throttle / high rev conditions forces it open and it uncovers a straight through exit like a full on race can. With some re-jetting it gave 5 bhp on the ZZR, and sounded awesome at 14,000 RPM! Quiet as you like on tickover and around town though.
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My ZX7R redlined at 11500rpm Cheddar.
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\"Nothing less than 8 cylinders will do\"
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Oh yes, mine had a Phase One system on and it was very loud but sounded gorgeous! My current steed, a ZX12R has a Scorpion end can and produces a deep growl which is magnificent in the Dartford Tunnel!
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\"Nothing less than 8 cylinders will do\"
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My ZX7R redlined at 11500rpm Cheddar.
I think I said 13000, in fact the red area starts at 12500.
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