Does a "cold engine" burn more fuel(carby)?

[dom14]

Hey Guys,

I've been wondering about the above question with regards to RB30 & TB42 carby engines.
I know in EFI engines it is the case(low engine temperature means more fuel consumption).

Is it the same in carby petrol/LPG engines?!!

I'm considering a scenarios where,
1)a faulty thermostat opens up too early, effectively causing engine temperature to not reach the optimum level
2)thermostat stays stuck open
3)thermostat is removed.

I'm not generally referring to the scenario where the choke valve not opening fully & causing the air/fuel mixture to run
too rich all the time.

In my troll(RB30 carby), choke valve is fully electric, controlled by a bimetal heating element. So, it doesn't respond to engine temperature at all. It's a timed valve that gradually opens up in few minutes as the bimetal heating spring expands.

Thanx in advance for any suggestions, theories, explanations, etc etc.

[mudnut]

Just a hijack. I know of a few cases where a faulty thermostat was removed and replaced with a plate (restricter) with a certain sized hole drilled in it. Not ideal but it does work.

[dom14]

Just a hijack. I know of a few cases where a faulty thermostat was removed and replaced with a plate (restricter) with a certain sized hole drilled in it. Not ideal but it does work.

No kidding! I've had no idea that's doable.
Why do you think some blokes do that?
It can't be to save money on a thermostat for sure?!!

[UncleFrosty]

Hi Dom
Back onto Thermo! (with a username change for me...)
Vague memory of it being better with large delta-T between engine and intake air. (other than air density considerations)
Will dust off my Eastop n McConkey and see if I can find the theoretical basis. Won't necessarily apply in practice tho'...

[UncleFrosty]

Try Eastop sections 5.5, 5.6. (pp133-136) plus later sections for diesels.
Try scribd.com if you don't want the whole book - sometimes the chapters are available.

[dom14]

Hi Dom
Back onto Thermo! (with a username change for me...)
Vague memory of it being better with large delta-T between engine and intake air. (other than air density considerations)
Will dust off my Eastop n McConkey and see if I can find the theoretical basis. Won't necessarily apply in practice tho'...

I'm also considering changing my username. :D

I couldn't understand what you meant by "large delta-T between engine and intake air", mate.

If you're planning to refer to Eastop & McConkey for the answers, you might be looking at spending lot of time on it. ;)

[mudnut]

No kidding! I've had no idea that's doable.
Why do you think some blokes do that?
It can't be to save money on a thermostat for sure?!!

Probably as a quick fix that they never got around to doing properly.

[UncleFrosty]

Large temp difference (delta-T) seems to increase efficiency from a skim read, but I'll need to go over it a few times to understand why...
Luckily E and M put in some simpler explanations between all the maths n technical stuff for people like me...

[UncleFrosty]

If you're planning to refer to Eastop & McConkey for the answers, you might be looking at spending lot of time on it. ;)

Yep, there's two hours just gone that I'll never get back, although I'm beginning to remember why I failed it a couple/few times!
From what I could glean from the book, you want to maximise the amount of energy from the fuel burn going towards pushing the piston instead of being lost through the cylinder walls as heat (warmer block = lower temp gradient to inside of cylinder = more energy available to piston - or less energy loss to cyl wall, whichever...). All subject to the metallurgical limits of the components (think lost coolant...)
So, long-story-short: yes, warmer engine should be more efficient regardless of EFI, carby etc etc. thus the thermostat. I'd guess those-in-the-know design thermostats to operate at the temperatures that keep it all in balance...
Now, back to my morning coffee...

[dom14]

Yep, there's two hours just gone that I'll never get back, although I'm beginning to remember why I failed it a couple/few times!
From what I could glean from the book, you want to maximise the amount of energy from the fuel burn going towards pushing the piston instead of being lost through the cylinder walls as heat (warmer block = lower temp gradient to inside of cylinder = more energy available to piston - or less energy loss to cyl wall, whichever...). All subject to the metallurgical limits of the components (think lost coolant...)
So, long-story-short: yes, warmer engine should be more efficient regardless of EFI, carby etc etc. thus the thermostat. I'd guess those-in-the-know design thermostats to operate at the temperatures that keep it all in balance...

Perfect explanation mate. Now, it makes all sense to me.
BTW, at least you studied engineering thermo while failing it few times.
I've never got into it. I'm a pretty late start when it comes to mechanical & automotive stuff.
BTW, you failed it few times means you remember and understand it heaps better than the blokes(and chicks :) ) who passed it in one go. I used to cram for exams and manage to pass them, then three days later I wouldn't remember a damn thing. :D


Now, back to my morning coffee...

What?!!! Don't tell me you're a graveyarder! :thumbup::D

[dom14]

Try Eastop sections 5.5, 5.6. (pp133-136) plus later sections for diesels.
Try scribd.com if you don't want the whole book - sometimes the chapters are available.

I have the whole book, but I won't understand what's in page 133-136 without having some understanding about the earlier theory and equations. I do have some basic understanding of thermodynamics in terms of concepts like entropy & temperature gradients.
Explanation you gave above is spot on. I didn't need to understand thermodynamics at all. Basic behaviour of heat is enough.
Are you good with integral calculus? I wouldn't get anywhere without some brushup. :)

You should consider teaching this stuff, or are you already? :D

[dom14]

Large temp difference (delta-T) seems to increase efficiency from a skim read, but I'll need to go over it a few times to understand why...
Luckily E and M put in some simpler explanations between all the maths n technical stuff for people like me...

I think you put the engineering thermo bug in me. ;)
I find the book interesting! :D

[UncleFrosty]

Are you good with integral calculus?

...and the only thing I failed on more occasions was... you guessed it!
No teaching duties other than working at getting our 15yo thru high school!
Not a graveyarder either (I assume u mean permanent N/shift, rather than reeeeeaaaallly old). I just like my coffee. :)

Cheers for the feedback tho!

[UncleFrosty]

I think you put the engineering thermo bug in me. ;)
I find the book interesting! :D
Yeah, it's weird that bug. Logically, my copy of the book should've gone a long time ago. Maybe kept it just for this forum??? Dunno...