Lab quiz tomorrow! need diesel engine help!
 

hey i need help!!! i have a thermodynamics lab test tomorrow and i dont understand how a diesel engine is able to increase its rpms. i know that the intake volume is constant, so is there just more fuel injected in order to increase rpm? if this is the case is the engine always running in a lean condition other than at maximum rpm?

I don't know the correct answer, but wouldn't any engine with more torque than internal drag be able to increase RPM's?

That is, if the movement of the internals uses 4lb/ft, and the engine produces 350lb/ft, then there's loads of extra energy to increase rotational speed.

Or did I misunderstand the question?

umm sorta....firstly its not torque that would make the engine speed increase, its horsepower. all fuel has a measurement of its potential power usually in BTU/lbm. therefore we know that for a given amount of fuel injected into the cylinder we'll get a net work output of the potential energy/lbm * x lbm that is injected. so if a certain engine speed is desired one need to input a certain amout of fuel. this can only be done up to a point know as the stoichiometric ratio because the volume of air in teh cylinder is constant. after this point teh engine runs rich which means that more fuel is injected than combusted and therefore the work out remains the same as at teh stoichiometric point and the engine will come to equilibrium not not accelerate any further.

so to restate my question; is a diesel engine always running in a lean condition other than at maximum rpm?

Not sure of the correct answer either, but good luck with the test deth... I hate those things.. :(

diesel motor workings


i read through to make sure it wil answer your questions and then some. :D

^^^ good job man! i liked the link. intresting :):)

thanx for the link, but it didnt address the lean/rich issue i'm looking for.

I dont' know for 100% sure, but I believe that Diesel engines, like most other IC engines run lean in steady-state conditions, but go rich as you wish to accelerate. Your statement that the volume of air in the cylinder is not necessarily accurate either. Many diesels are TD's, so there is a different volume of air forced into the combustion chamber during heavy throttle applications. I know for gasoline, stoich is achieved at 14.7 to 1, so that is what manufacturers strive to set their steady-state conditions to. Under heavy throttle applications, the air/fuel ratio generally dips to around 11.7-12.2 to 1 with a significant increase in emissions and unburnt fuel. Even though the combustion process in a diesel is a bit different, I would imagine that in terms of stoichiometry and volumetric efficiency, they are set up much the same way as IC gasoline engines. (That's kind of a long-winded and not very good answer, but I think it sort of reaffirms what you're asking.)

Hope it helped :-D

yup, u're right....PennFSAE just PMed me with a simialr answer....i'm tired, its late, all leading to me being an idiot, but thanx to all that helped!

They just add more diesel. A diesel engine is unthrottled.
Besides the energy content of diesel being higher then gas this is yet another reason why diesel tends to be more efficient. Gasoline engines tend to run rich at idle as opposed to a diesel engine which doesn't.

ur gonna get fucked!! but good luck man, i just know jack shit about diesel eng.

So when I floor it in a disel more fuel is injected, instead of the valves opening more?

yessum...

An interesting thing to note. Diesel engines do not enging brake(well unless you count the drivetrain losses.

so are you're saying that the diesel engine is always running a lean mixture unless its a max throttle?

I guess.(makes sense given the above facts) I'm not an me or a diesel engine person. I only know what I've read and observed. Particularly I had a class in economics of the environment which touched from a high level the benefits of diesel over gas. I also read an article in road and track and some internet stuff.

I'll find you a link if you want... but itd prolly just be something off google since the road and track article wasnt posted online.

ok.. found it..

". Diesel engines are quality controlled while gasoline engines are quantity controlled. This means a diesel engine runs with an unrestricted airflow and only the injected fuel volume controls the power setting. Diesel engines run lean their whole life without any negative influence.
"
http://info.thielert.com/centurion/m...rchive&wpid=20

So yes.

"A diesel has no spark plugs. It has a very high compression ratio--about twice that of a gasoline engine. So the compression stroke builds up tremendous heat and pressure. A readily combustible fuel is injected and the heat of compression instantly ignites it. Unlike a gasoline engine with a throttle plate to regulate airflow, the diesel is unthrottled. It draws in all the air it can, all the time, without wasting energy pulling air past a partly closed throttle. Performance is determined (within combustion limits) by the amount of fuel injected, so the diesel can produce a great deal of torque at low rpm. Step on the accelerator and a variable turbocharger adds still more air, the injectors spray more fuel, and the diesel develops tree-stump-pulling torque before the tach hits two grand. A gasoline engine needs an air-fuel mixture of about 15:1. The diesel can run on mixtures as lean as 100:1."
http://www.popularmechanics.com/auto...es/print.phtml

Thats as close is im gonna get for links. As I said, diesels are not my fortay. Beyond my economic studies and random automotive tech I don't see them often in the US.:)

Um, i always thought it was purely the rate at which the fuel is injected, and the quantity which is that determined the 'throttle' effect in a diesel engine. I dont know of many variable geometry turbo'd diesels that have bin in production for more than a couple of years, whereas the diesel engine has been around centuaries!
They have no throttle butterfly to restrict the flow, as has been said, and as you assumed deth i think they run lean up to where peak power is created. The peak RPM is determined by a rev limiter on the fuel injection pump is it not?
How'd the test go?

its was pretty good...ended up not being tested on teh diesel engine. the way it worked was that we did three labs, dielse engine, air compressor(fricking huge machine probably about 20'x20'x15'), and Parsons steam engine. then on the test day they make us write up the lab for one of them with a few surprise questions. its kinda evil, but it saves u from writing individual reports.

btw i still have ICE, gas turbine, HVAC, steam generator labs to go. i'm actually pretty happy with this course, its quite involvong :mrgreen: i'll try to get some pic up of teh equiptment, its all pretty cool.

I realize. That particular article was discussing a special type of diesel setup. I posted it to demonstrate the amount of disel idea as a relation to rpm. The variable turbo idea is actually a turboed diesel engine.

fair enuff mate. I just felt it was a tad misleading. :wink: