Re: engine compartment heat

["hydee" <hydee@underground.prestel.co.uk>]

> Duh.  What does this have to do with the discussion?
 Well at standstill - no pressure difference, no forced airflow, 
convection only. (OK there are very slight pressures involved due to 
air density differences)

> > A greater than 1 atmosphere pressure drop will occur due to the
> > front grill acting like a scoop where the pressure is higher than
> > 1 atmosphere, i.e. the front of the car.
> 
> Man, you really need to start over!

If the pressure is not higher at the front of the car no air will 
flow under the bonnet. Maybe I tried to make it too simple? The front 
of the car is EXACTLY a 'scoop'.
 

> Perhaps, then, you should consider what is really going on.  The air
> entering the front of a moving car is brought to an effective
> standstill

Standstill!?!? The cooling is effected less by the speed of air flow than 
the mass flow rate (it is the mass of air that absorbs the heat) which MUST
 be the same in and out. The only way the air can come to a standstill is if 
the pressure is the same as on the outside! It can slow down due to an 
increase in pressure and associated increase in density...... and it starts to
 get too bloody complicated to work out and guess at in this way.

> within the engine compartment -- IOW, the pressure within
> the engine compartment is closer to the TOTAL pressure of the
> airstream, not the static.  It's therefore a slightly raised
> pressure (t/s at mach 0.1 is not real impressive), which is what
> forces air OUT of properly designed hood louvers.  
> 
> If NACA scoops are added, the airflow into the engine compartment
> will improve, while there is no improvement in outflow, so the
> pressure within the engine compartment will increase.

This increase in pressure therefore increases the outflow. If the 
air entering the front of the car produces a 'not real impressive' 
pressure increase (negligible?) then what effect will a couple of 
relatively low frontal area scoops make?

>  The increased
> pressure over the underside surface of the hood will cause a
> considerable lift -- the pressure differential is low, but the
> surface area is a LOT of square inches.  It will also cause a
> decrease in the flow through the radiator -- the flow through the
> radiator being a function of delta p across it.

Erm 1% increase in pressure will still only translate to a 1% 
increase in lift.
 True(?) Might be cause for concern in very hot climates. Not here 
though were cooling systems have a much easier time.

> OTOH, proper louvers will improve flow OUT of the engine
> compartment.  The result will be a reduction in pressure in the
> engine compartment -- resulting in reduced front end lift, and
> increased flow through the radiator.
> 
> As described before, either opening -- or any other opening, for
> that matter -- will help things when the car has been parked hot.
> 
> > ( I got a 2:1)
> 
> What's that?
 A degree with upper second class honours. (Average of 80-90% for all exams and 
practical projects during an extra year over an ordinary degree.)

> >Go to the library and get a book before you claim to be an expert.
> 
> This is far and away one of the stupidest concepts common to modern
> society -- that a person must have a shitload of credentials in
> order to be considered correct.  What is correct is what is correct,
> it doesn't matter who says it!  And what is wrong is simply wrong,
> regardless of the source.

 Due you mean credentials or qualifications. Credentials are pretty 
important. Qualifications less so. Wouldn't you give more credence to 
a lifetime Jag R&d man than a back street 'Fix-em-All' mechanic with 
no apprenticeship?
Anyway books aren't credentials or qualifications they are just bits 
of paper with the correct facts written on them.

> I have described the factors involved in hood openings above, and
> you'll note the issue of ambient atmospheric pressure never came
> into it.
It did you just never used those words by using atmospheric press as 
your reference, without stating it.

> I'll grant that it is an issue -- none of this would be
> important on the moon -- but the whole concept of inflow vs. outflow
> doesn't depend on what atmospheric pressure is.

Not overall because it's added and then subtracted again later on. 
But absolute zero pressure gives and undisputable reference point. 
You have simply used atmospheric pressure, a variable, as your 
reference.
 
> > If you were you'd appreciate little things
> > like bonnet angle will create a downforce which will counter act,
> > and may over-ride, any upward force created by air escaping
> > underneath.
> 
> So?  Regardless of what the CURRENT front-end lift situation is,
> adding hood scoops will increase the lift/decrease the downforce. 
> If there is some concern that we are getting TOO MUCH downforce on
> the front, maybe we'd consider hood scoops.

Granted it will increase it, I would say insignificantly. The air 
which flows in the small air scoop area has a vast area to escape 
from. Therefore it requires very little extra pressure to force this 
air out. All thats being done is the frontal area is increased 
slighty. The fact that part of this frontal area has been moved 
backwards slightly has no bearing on air flow amounts just where it 
flows.
 If your concerned about the added frontal area, block off the 
gaps around the outside of your radiator to compensate, then the 
overall effect is simply to move a part of the inlet to somewhere 
else, therefore no increased inflow.

> Here's a plan for you:  Go take the front spoiler off your car --
> it's only a few screws.  The effect is fairly similar to what we're
> discussing.  The car handles NOTICEABLY worse, especially at high
> speed.  You also may develop overheating problems, due to reduced
> airflow through the radiator at speed.

The effect is similar but on a vastly greater scale. I doubt I'll 
have heating problems, it's snowing!!
 
> Fortunately, the XJ-S fuel tank is fairly centrally located, and
> therefore has little effect on handling balance.  Being an engineer,
> you should also recognize that adding weight to a car adds to both
> the traction available and to the lateral cornering forces; since
> both are nearly linear, they almost cancel each other, making the
> handling of a car more affected by changes in suspension geometry
> due to lower ride height than by the actual weight difference
> itself.

I wasn't implying the weight itself was the problem, but that, like 
you say, suspension geometry changes and the fact that if you corner 
hard enough your 20 gals will be on one side of the car.

> > Fluid flow is an extremely complex subject not easily understood
> > by the layman. Perhaps I should keep my informed opinions to
> > myself in future.
> 
> Get them informed, first!
> 
> > If I find the time (which I doubt) I could try modelling it on the
> > finite element analysis fluid flow modeller in the lab then we
> > would now for sure.
> 
> Another stupid concept common to modern society -- you need a
> computer analysis to know what's going on!  Absolute BS -- if you
> improve the airflow into the engine compartment without improving
> the airflow out, you're gonna increase lift.  If your finite element
> analysis shows otherwise, it is wrong.

If you improve the airflow in without improving the airflow out, 
pretty soon you'll have a large bang and bits of shiny Jag flying 
everywhere. :-)
You don't need computer analysis to show what's happening. A breezy 
alley and a fat cigar will do a reasonable job. (Maybe) All computers 
do is what humans have been doing for years, only faster. They are 
therefore usually used to do more complex analysis. Are you a 
technophobe?
I dare say for this type of non-critical problem, wind tunnels and 
smoke would be used.
 
> > In summary scoops and outlets will BOTH work. Which you use 
> > isprobably less important than the correct positioning of them.
> 
> Wrong, wrong, wrong.
Right, right, right. You've agreed above they both work to some 
degree ( but you say scoops will have an adverse effect on the 
handling.)
But wouldn't it actually be best to have scoops to force air into the 
top of the engine compartment and vents to let it out again at the 
back? Or even remove the bonnet altogether? It's a nice enough looking 
engine.

> > Incidently is there anyone on this list who ever worked for Jaguar
> > designing their cars who could tell us any design secrets?
> 
> We have one member who works at Jaguar, but on the assembly line,
> not in the design department.  We also have one guy that works in
> design, but at Ford, not Jaguar.
 That's a shame. I noticed someone alse mentioned it would be nice if 
Jaguar would back up this list with official replies.

Now that I've worked through the incorrect terms and figured out what 
I think your trying to say I feel you've fallen into the old trap of 
doing the right things for the wrong reasons.
I'm wondering if some of the disagreements in the basic discussion 
here aren't due more to misunderstanding/ not clear enough wording?

I don't have an XJ-S handy. What would the increase in frontal 
opening be by adding scoops. From this we could start producing a few 
numbers and see how well our theories match up. I am interested in 
finding proper answers and reasons rather than gut reaction ones. 
After all like you said increased airflow in => greater pressures => 
lower airflow. Where do they balance out? and numerous other 
ponderables........

OK 
This system is a fairly complex shape but is directly analogous to a
nozzle. For flow, but not cooling effects, laminar and turbulent flow
are insignificant, supersonic conditions irrelevant.

Consider a stream of fluid at:
pressure  p1
enthalpy  h1
velocity  c1
Assuming heat loss negligible i.e. adiabatic flow, Q=0
assuming no done on or by the fluid, W=0

>From the first law of thermodynamice we develop the steady flow 
energy equation between the inlet section and any other section 
where:
pressure  p2
enthalpy  h2
velocity  c2
we have:

h1 + (c1^2)/2  =  (h2 + c2^2)/2

enthalpy h=u + pv
where u= internal energy of fluid
           p= pressure of fluid
           v= volume of fluid


The procedure will be effectively isentropic, i.e. entropy 
doesn't change. and for an ideal gas, which air will be under these 
conditions,  pv^g  is constant.(g=gamma).
 
Then for any given inlet conditions we can plot the Area, Volume, and 
velocity against the pressure at any point. We do not need actual 
numbers. We can assume unit values for one inlet condition and work 
on relative figures for the other.

It's hard to write equations in plain text isn't it, I hope you can 
read them.

To determine the overall effect we must consider not only what 
happens under the bonnet but also under the rest of the car.

Care to supply some figures. I don't have an XJ-S. I had a 1979 
series3 Daimler Sovereign until a month ago. (A bargain at 150 pounds 
that was, the tyres were worth more than that). I now have an '87 3.6 
Sov.

Cheers,
Richard.

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