Thermodynamics Help plz (Diesel Engine)

gurusan · 7 Nov 2009 at 14:26

So I'm doing a thermo report on a diesel engine and can't for the life of me think of why volumetric efficiency changes with the engine at CONSTANT rpm and the only thing we are varying is fuel.

VE goes down as you increase fuel flow but then goes up a little bit at the end as you can see from the table below. Why does airflow change at all? I can't figure it out...I mean the fuel is injected just before the end of the compression stroke so the inlet valve has obviously closed ages ago for the beginning of compression and the cylinder has already ingested all the air it's going to see til the next cycle.

dalin80 · 7 Nov 2009 at 14:39

gurusan said:
VE goes down as you increase fuel flow but then goes up a little bit at the end as you can see from the table below. Why does airflow change at all?

complete guess mode-

The fuel itself is much cooler then the compressed air (compression heats the fuel to about 80c in the pipes) so as fuel quantity/flow is raised the evaporating fuel pulls more heat in its change of state eventually to the point of briefly acting like a air cooler.

could this change airflow?

its a known effect and has been linked to creation of NoX

gurusan · 7 Nov 2009 at 14:42

Thanks.....yeah someone else also suggested more fuel evaporation but I still can't see how it could effect airflow :/

I think I'm just going to ask the lecturer on monday

Dogbreath · 7 Nov 2009 at 18:32

A slightly restrictive exhaust could reduce VE since some exhaust gasses will remain in the cylinder at the end of the exhaust stroke; this would obviously be more pronounced with more fuel injected (and therefore more exhaust gasses to pass).

gurusan · 7 Nov 2009 at 21:16

The exhaust stroke is completely separate from the intake stroke though...so that doesn't make much sense.

Ricochet J · 7 Nov 2009 at 21:23

Wouldn't there be a constriction in the compression stroke, momentarily causing high pressure low velocity air intake resulting in higher mass flow rate which may explain your slightly fluctuating air flow at the end of the stroke.

gurusan · 7 Nov 2009 at 22:23

I'm pretty sure it's a 1 piston system so exhaust should not have any influence over intake volume.

I'm quite drunk now though, so responses after this may not make sense.

gurusan · 7 Nov 2009 at 22:29

also just curious, I used Willan's Line to calculate friction power loss at the constant RPM (2000)...is Willan's line meant to be taken before smoke point (as I did)? If not, when do you stop in terms of fuel flow?

Col_M · 7 Nov 2009 at 23:52

A little off topic, is the lecturer Dr Pickering?

I seem to remember doing something very similar. Unfortunnately my lecture/lab notes are back in the UK and I can't remember this stuff off the top of my head

Dogbreath · 8 Nov 2009 at 08:00

gurusan said:
I'm pretty sure it's a 1 piston system so exhaust should not have any influence over intake volume.

I'm quite drunk now though, so responses after this may not make sense.

The exhaust can have significant influence over the inlet, though more so on higher revving petrol engines than diesels where pulse tuning is involved.

However, back pressure in the exhaust could reduce VE; imagine the piston is forcing out the exhaust gasses on the exhaust stroke, as the piston nears the top and the exhaust valve starts to close, there will still be some exhaust gas left in the cylinder under pressure. When the inlet valve opens the remaining gasses can exit into the inlet manifold, where it will be drawn in again on the inlet stroke. Since the cylinder now contains less air than it could, VE is reduced.

Fuzz · 8 Nov 2009 at 17:19

At a guess I'd say it was the valve overlap at the end of the exhaust stroke. It will drag in more fresh air as the energy of gases leaving the exhaust has more suction effect on the inlet.

gurusan · 8 Nov 2009 at 17:40

Thanks for the replies all.

Col_M said:
A little off topic, is the lecturer Dr Pickering?

I seem to remember doing something very similar. Unfortunnately my lecture/lab notes are back in the UK and I can't remember this stuff off the top of my head

Nope, I'm at brookes uni, lecturer is Dr. Stephen Samuel

Col_M · 8 Nov 2009 at 21:21

gurusan said:
Nope, I'm at brookes uni, lecturer is Dr. Stephen Samuel

Yeah I went to Brookes too, I guessed he may well have moved on as it was 9 years ago but just wondered if he still taught this

gurusan · 8 Nov 2009 at 21:31

ah yeah he must have retired. I do know that the coursework has stayed the same through a few professors though in a few subjects.

Did you do the dynamics 4 bar mechanism coursework? WHAT A NIGHTMARE!!!!!!!!!!!!!!! OMG OMG OMG OMG OMG

Also just curious, what have you gone on to with your degree?

Jonnycoupe · 8 Nov 2009 at 21:53

gurusan said:
The exhaust stroke is completely separate from the intake stroke though...so that doesn't make much sense.

Inert exhaust residual gases do though.

Col_M · 9 Nov 2009 at 15:40

gurusan said:
ah yeah he must have retired. I do know that the coursework has stayed the same through a few professors though in a few subjects.

Did you do the dynamics 4 bar mechanism coursework? WHAT A NIGHTMARE!!!!!!!!!!!!!!! OMG OMG OMG OMG OMG

Also just curious, what have you gone on to with your degree?

Yes the 4 bar mechanism coursework was a absolute pain in the ass the lecturer wasn't too helpful from what I remember, I'm trying to remember the name of the guy who taught us Dr Balkwill?

I remember liking the electronics labs with Dr Cordery, there was a hot as post grad student who helped out in the labs called Svetlana

I didn't finish my degree at Brookes, some problem with my student loan meant I transferred to Plymouth uni where I'm from. I'm a Global Reporting Analyst for a software company now, quite easy with an engineering background as it can be applied to anything that requires maths and an analytical mind.