It depends on what you mean by ideal. Stoichiometric AFR for diesel fuel is about the same as that for gasoline. "Ideal" AFR depends on what your goals are - but often AFR for a diesel is set in the neighborhood of 28-32 under "typical" part load conditions.
I don't get your question. The vast majority of diesel engines are not throttled, so a stoichiometric fuel mixture is never achieved during normal operation. Thus making the ideal AFR a moot point.
Here we go again Ivymike!
If you try and throttle a diesel engine, you will indeed chop power, but diesel fuel has a very wide air-fuel flammability limit, somewhere around 3 to 42:1. With diesel engines, the fuel can be ignited and additional air force fed for additional power, (add on turbo kits, bigger flowing turbos, exhaust systems). You can also have a fixed amount of air and increase fuel (power chips, which change the fuel delivery curve, or larger flow injectors, or modified pumps) which will give additional power, but with tons of smoke.
I played around with a diesel throttle once in an attempt to reduce NOx, but was not happy with the results. If I decreased the airflow without decreasing a proportionate amount of fuel, I had a weak engine with copious amounts of smoke. To my knowledge, there are no throttled diesel engines anywhere.
Franz
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Here we go again Ivymike!
Sorry, I don't have any idea what you're talking about... Do you disagree with what I've said above?
I've seen at least one throttled diesel, but it wasn't a production application. The things that come to my mind for influencing AFR in a more-typical diesel are:
Amount of fuel injected
Amount of boost allowed (assuming electronic wastegate control)
Fraction of EGR used
I've seen some smoky high-perf applications with AFR below 20:1.
I think I've figured out what your "again" comment was in reference to - still burning up about the compression ratio discussion from 6 months ago? I'd forgotten all about that. I was right, though (still am). nah nah na nah nah.
The lastest VW 16v 2.0 tdi engines have a motorised throttle body and lambda sensors to part control egr and fuel ratio.The reason for my question is to work out engine power after cylinder head flow before and after porting.I will be racing and using on the roads 1 of these engines in a mk 2 Golf,which i raced before with the 8 v tdi engine in.
Ivymike:
Me burned up? Here in Central Texas? Never! But I do thoroughly enjoy a lively discourse!
I seem to remember an SAE toptec discussion about 10 years ago about the use of a throttle on a diesel engine, to control NOx. This is the time when EGR's were just getting into the research arena. Now, since almost all of the major OEM diesel engine manufacturers use EGR and not throttles, even though EGR has a few drawbacks, it apparently was the more effective technology.
Franz
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I've read about and conducted some experiments relating to homogenous charge compression ignition diesel fueled engines. It would appear under these conditions an air fuel ratio in the neighborhood of 14.7:1 would be at or close to the ideal mixture. Getting the engine internals to live with that fuel load is another matter. As would be expected tha Nox #s go down as you increase the fuel load. The diesel converted model airplane engines seem to survive ok but the wrist pin seems to be the weak link in larger displacement trials.------Phil
I am pretty sure some small Japanese diesels had throttles back in the 70s. I am not sure why as it would effectively reduce cylinder pressures and could suppress ignition if taken to far.
As far as I know, power is controlled in a diesel by fuel metering, not air metering, up to the point where there is more fuel than the unthrottled airflow can use, ie excessive black smoke.
Regards
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The original question mentions 14.7 so it's pretty sure the intention was to find out the stoichiometric air fuel ratio.
CxHy + a(O2+0.79/0.21N2) => xCO2 + y/2H2O + a*0.79/0.21N2
where a=x+y/4 since a C can use up a whole O2 and an H can only use 1/4 of an O2. The 0.79 and 0.21 are the proportions of N2 and O2 in air.
If you take the formula for a hydrocarbon reacting with air you can easily work out how much of each is required.
molecular weight of O2 is 32 and N2 is 28.
Octane C8H18 has a=12.5 and molecular weight 114 ( 8 12's are 96 plus another 18).
Dodecane C12H26 has a=18.5 and molecular weight 170.
So if you crunch the numbers to get the AFR
AFR = a(MWO2+0.79/0.21*MWN2)/MWfuel
you get 15.06 for petrol and 14.95 diesel.
Since hydrocarbons are mainly H-C-H, two to one, and the odd hydrogens at the ends making the difference are light, all the hydrocarbons will have similar stoichiometric air fuel ratios, with the exception of the very light ones where the two extra hydrogen makes up a significant proportion of the weight. Take CH4, the H-C-H weighs 14 and the two H's weigh 2. That's a large portion compared to 2 in 114 or 2 in 170.
So AFR for methane calculates to 17.2 and Ethane to 16.1.
Then propane 15.6 and Butane 15.4. You can see the figures closing in on the figure 15 for petrol and diesel.
Now if you add oxygen things change drastically since oxygen in the fuel means you need less oxygen from the air. And since air is 4/5 nitrogen, if you can cut down on a certain amount of oxygen then you reduce the amount of air you need by a factor of about 5, (drop 4 N2s for each O2 dropped). So the AFR for a fuel like ethanol works out to be about 9.
Now if you say that US gasoline is like petrol but cut with about 5% ethanol you can calculate 5%*9 + 95%*15 = 0.45 + 14.25 = 14.7 and get anAFR for gasoline.
So providing the diesel is all hydrocarbon based the AFR will be around 15. Diesel of plant origin (biodiesel) has oxygen in it and the AFR will be lower and mixtures would be in between, in the same way that the AFR for gasoline is between that for petrol and ethanol, heavily weighted towards the petrol figure. I don't think the figures change much if you take air as being 78% Nitrogen and 1% Argon instead of 79% Nitrogen and as I said before, changing one heavy hydrocarbon for another won't make much difference, ie calculating diesel as a coctail of hydrocarbons would only complicate the calculations and not significantly change the result.
I'm sure that stoichiometry calculations are all good fun for engineering students, but wouldn't it be a whole lot simpler just to pick up an introductory book about IC engines (Taylor or Heywood or Pulkrabek) and look in the appendices for a table of the same? Perhaps we should have just suggested that initially.
Interesting "rule of thumb" about stoich AFR approaching 15:1 as the caterpillars get longer.
(I'm glad to see that your interpretation of the question and your calcs both agree with my initial response, though)
One thing I remember from "way back" while attending a class, is a diesel engine takes in air and allows just enough fuel to "get the job done". At idle, that is indeed just a smidgen of fuel, for a barely usable amount of power. The AF ratio there was very high, somewhere around 40:1 if I recall. On a non boosted engine, just add more fuel and power and rpm's increase.
Try and toss in 14.7:1 air to diesel at idle and you have a very, very fast idle!
Franz
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Diesel engines produce their maximum torque rich of stoichiometric, even though it gets smokey and inefficient that side of the line. To limit the amount of fuel available to each injection, rotary pump (e.g. the Lucas DP range) manufacturers fitted (I think the term was) scroll plates. These limited the travel of the pumping plungers so that the fuel accumulated could never exceed stoichiomentric requirements. Scroll plates were adjustable so that the same pump design could fit different engines.
In the days before turbocharging was the norm, the only way to get more torque out of your diesel engine was to open the bonnet and tamper with the scroll plates. If you loosened them off and turned them a bit you could turn your taxi from a slow clean vehicle to a fast dirty one.
Methinks many black cabs had "adjusted" scroll plates in those days.
Full bore, I am sorry to say, I think was 1 part in 25,000. Long time ago, I may be wrong
Sounds like you're talking about volume of fuel:volume of air. The rest of us are talking about mass of fuel:mass of air.
A Diesel has not quite enough energy to keep running at idle from the heat of compression. Just a smidge of fuel keeps it ticking over.
Out of curiousity, are you at all familiar with the first and second laws of thermodynamics? What would they say about an engine running at idle from the heat of compression?
you are indeed correct on throttled diesels, have seen them myself. they had a throttle in the intake manifold like an injected petrol engine. The big dfference was it controlled vacuum available to the injector pump for injection control.
Throttle pedal wasnt connected directly to injector pump.
My guess is it was a crude way of limiting the amount of fuel injected at lower engine speeds to reduce the exhaust smoke.
