Radiator size calculation

[flatter4]

Sorry, wrong button press above.

Ref. the comment  "ISTR that the rule of thumb is that the rad needs to be able to dissipate 30% of the max bhp"

I think what you're remembering is that for a naturally aspirated spark ignited engine (petrol / no turbos) the fuel energy goes (approximately) 33% to flywheel;  33% to exhaust;  33% to coolant
...but I don't think anyone would sell a radiator on kW heat rejection capability as it's influenced by too many other parameters.

 

[flatter4]

 

Just remembered too, if you're using a brand new aluminium radiator be careful as the initial use will deplete the additive packs in the coolant - as they are trying to compensate for the oxidation of the aluminium.  I think it's connected with the aluminimum passivating itself (I think) - any material scientists.........

The important thing is to budget for a flush and coolant change after a few hundred miles.

 

 

 

[JohnD]

That's a new one in me!   Engineer aware?

WhAt the relation between but and heat outout?    But 30% sounds low.   Only the latest IC engines approach 50% efficiency, so 'ours' lose 60+'% as noise and heat.  Some goes down the exhausr, but at least 40% will be hot coolant.

Edited December 14, 2020 by JohnD

[JohnD]

Ooooops! "Engineer aware" was an autoedit foul up!    I meant "Anyone else aware?"

JOhn

[flatter4]

 

I work with industrial Diesel engines.  I've pulled out some notes to check.  The aluminium radiator issue is really only associated with the OAT (Organic Acid Technology) coolants.  Some brands have nitrites in them (they are great for cavitation resistance).  These nitrites are what are depleted as the new aluminium passivates itself.  If it happens the coolant loses colour and goes smelly (ammonia).  So you could watch out for that.

....shouldn't really run the OAT coolants in our engines unless you have completely cleaned the system out.  There can be some bad reactions with conventional and OAT coolants, that lead to a gel being formed that clogs stuff up.

[flatter4]

 

50% thermal efficiency for Petrol is stretching things.  The best Diesel engines can get to 40% at specific operating points.  Petrol engines are throttled (unless you're talking crazy technologies that don't exist on normal automotive - yet).  They can see higher values at wide open throttle and specific operating points, but in general 35% or less would be about right.

These are efficiencies across the engine only (to the flywheel), if you take the rest of the losses into account (running air con; power steering; running through a gearbox / diff and tyres) the thermal efficiency will drop to the low 20's I guess.

 

[JohnD]

flatter,

You can see the cowling on the fans I used, and whcih I intened to use again on the new project.  They are enclosed in a duct four inches deep.   Put those circular grilles  over them for fear for fingers in the paddock.  The new project will omit them, but the frotn openein g will need some sort of grille.

What is 'best', pull or push?

And you forgot the reference?   Please?

John

Edited December 15, 2020 by JohnD

[flatter4]

 

John,

What have I committed to giving a reference for?  Sorry, juggling too many things here!

Push or pull shouldn't really matter.
Key is to minimse radial tip clearance - so run the fan as close to the cowl as you dare diametrially.  You're in a good position as there is no relative movement between fan and cowl (not like an engine rocking on its mounts with a rigid chassis mounted radiator and cowl).  Only thermal expansion and a bit of vibration to deal with.

With a puller fan (air from outside; across rad; then to fan) the rule of thumb is that 2/3 of the fan blade width should be inside the cowl:

 

For a pusher fan (air going the other way) the rule is only 1/3 of the fan width to be inside the cowl.

Sorry sketches are not to scale!

 

I'm not an expert in fan design but practical experience shows these rules stand to reality and produce near optimum air flow (hence cooling).

 

[flatter4]

..that's for a fan to work, but what happens to the aerodynamics and the pressures in the car and in the 'wash' behind is beyond me - as GT6Steve says; getting a large delta P across the rad. (hence large air flow) is key.  Both with fan switched on and at 155 mph......

[JohnD]

Flatter, previous page you posted "Oh, and ref.yes, typical for a naturally aspirated spark ignited engine (petrol / no turbos) is"

That was all - I was reminding you, ans asking if yiou wnated to post a reference.  I', not disputing you, I wnat to educate myself!

Thanks for thr 'rules' on fan cowling!    Those truck fans area close fit inside and are ALL inside the cowl, why less?

 

I retrospect, my error with Silverback was to think thatair would find its own way into the back of the car and than be pulled through the matrix by those big fans.      If I had ducted it from the start - I tried later, but not very well - it might have worked.     40 project will have the advantage of a front rad, and I WILL duct it!    

JOhn

[flatter4]

 

Arrrrhhh, no that was just sent too early.  See first message on this page for a retyped reply.

I would guess that behind the car would be a negative pressure when you're running at speed.  It would be an interesting science project to measure pressures in front of and behind the rad. to see what happens.  Probably a simple U tube manometer would do it (though you need a pitot tube to measure properly) - but you'd need a passenger / lab assistant to take readings while you're driving!
The rad. at the front of the car is better - lots of forced air feed.  Perhaps someone on here could loan you a wind tunnel for half a day!

 

[JohnD]

That was certainly my guess, but as said, it can only be partially correct.    I know that the Kamm tail and blunt estate rear leave a maelstrom of vortices behind.    They are behind the car from a drag PoV, and their velocity will reduce pressure, but they are very turbulent.