ok, so I cleaned the PCV valve...

As a rough and ready engineer I love approximations, and so for me 1 bar
is about 1 atmosphere.
Same as 10N is about 1Kgf when doing rough and ready maths (I know it
isn't, and if doing calculations I use the accurate figures).

or the velocity is higher, hence the same mass flow.


As I said before, I was only working in approximations, however, if you
wish to measure the smallest amount of compression, then everything is
compressible.
Even better is that in fluidics a gas is considered a fluid, and that is
most certainly compressible.


Anyway, I accept that are correct.

 

I don't know what you mean by "rough and ready engineer," but I assure you,
bachelor's degreed and higher engineers involved in design do not use
approximations that are over 1% in error (as this one is) while designing.

(Not that "bachelor's degreed and higher engineers" are all necessarily
qualified to talk about automotive repairs and maintenance. I'd say most these
days are not.)

Are you in the United States?

I am. Never in my years of engineering (including teaching in colleges) have I
used the Kgf. Doing so seems to defeat the purpose of the metric system. Plus, I
personally wouldn't assume 10 N = 1 Kgf in design. It's off by about 2%.

These errors propagate and "mathematically grow."

Manned spaceships have crashed due to such silly, seemingly trivial mistakes.

You're ignoring my comment about material concerns.

Velocity is a big deal when it comes to preserving material integrity. Pump,
pipe, and most any design involving fluid flow will take velocities into
account.

Nope. I wish to point out that a car's power steering pump inlet and outlet
lines are different sizes for a good reason.

This is not some pie-in-the-sky nitpicky point. The compressibility of liquids
has immediate practical implications, though I grant also that in many, but not
all, applications it is perfectly fine to assume a liquid is "essentially
incompressible."

For the layperson: If you're ever trying to figure out the inlet and outlet on
any sort of device that moves fluid and raises its pressure substantially, then
the line that is smaller will be the outlet, for all the aforementioned
reasons...

 

ok i'll be serious - I too work to many decimal places when required,
however for newsgroup chatter I use approximations - This time a Big
mistake

Certainly not - UK

Nor would I for design etc, but if somebody asked me how large a 100
newton force is, I would say about 10kg - I know this is wrong.

Agreed

Anyway one thing I eventually learned at school was never disagree with
the teacher, and in this case you are right - smile it's meant as a joke

 

i dont think the OP was designing anything, and they admitted its not an
exact number.

called "tolerance stacking", innit?

 

Hadn't heard that one. Nice.

 

Nah. I was just "observating" with my original comment. It was only a
persnickety "note"... Or so it was meant to be.

Now that's not wrong. Ten kg in your hand is *about* a 10 newton force. Um, on
earth, anyway.

But you know this.

I'm not the teacher but just some babe wondering last week which hose was the
inlet and which was the outlet on my power steering pump.

 

Uh oh...

 

"Guess why the outlet hose's diameter is smaller."

My guess is it is because of the differences in pressures and has nothing to
do with compressibility of the medium. For each atmosphere increase in
pressure, the volume of the water would only decrease by 46.4 parts per
million.

 

Uh oh indeed.....................

 

Umm, here's a good reference site:
http://www.ex.ac.uk/Projects/trol/dictunit/dictunit.htm

BTW on your comment about never using the Kgf, it appears all the time in
workshop manuals, even Honda ones and you'll find Kgf/cm² on many pressure
gauges in the U.S. Yes we are a backward country as far as SI units go but
mechanics and plumbers etc. can relate to a Kg or a cm² - they know what it
feels like and they like that and if they're going to be forced to metrify,
they prefer something err, tangible. You know the chassis design specs for
the Grandam Rolex racing series are all in inches and lbs - keeps them
furriners away... and none of the suckers are going to stray off into the
LeMans series.

We're also not alone in bastardized systems - all the Japanese and European
auto mfrs are now quoting engine brake power in pferdestärkes (ps) and one
of those is exactly 75 kilogram-force meters of work per second which
fortuitously works out to .9862bhp. So in this case, at one time, the
metric folks bent to the ubiquity of the imperial unit based bhp and
invented a metric measure which closely corresponded to it - ironic!

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

No, but hang on...

True. After running some numbers and seeking the explanation for the difference
in pipe diameter, I stand mostly corrected. The numbers:

Power steering fluid is "petroleum oil." (That's what it says on the back of my
Honda PS fluid bottle.) Its compressibility is around 1.5 to 2 times that of
water, from my searching on the 'net. Power steering pump pressures max out at
between 1000 psi and 2000 psi, roughly, from my reading, with pressures being
highest when, say, one is parallel parking. The inlet pump pressure is about
atmospheric. Let's assume the PS fluid compressibility is closer to 2, because
of entrained air in the PS system. Let's assume a PS system pressure of 1000
psi. Then the % volume change from inlet to outlet is

2 * (46.4 / 1,000,000) * (1000 / 14.7) *100% = 0.63%

The outlet specific volume is thus 100% - 0.63% = 99.4% of the inlet specific
volume. This is the same as

(inlet density) / (outlet density) = 0.994

From the continuity equation for fluid flow, (mass flow in) = (mass flow out),
or

Ro1 * A1 * v1 = Ro2 * A2 * v2

where
Ro = density
A = cross-sectional area of pipe
v = fluid velocity in pipe
1 denotes inlet. 2 denotes outlet.

Assuming pipe velocities are about equal (due to material considerations), and
given that Ro1 /Ro2 = 0.994 (from above), then

A2 / A1 = 0.994

The ratio of the inlet pipe's radius to the outlet pipe's radius is the square
root of the above, or

r2 / r1 = 0.997

That's not going to be visible to the naked eye. On my 91 Civic's steering
system, I'm estimating an actual ratio of more like around 0.8 . (I am about to
start rebuilding my power steering pump and was going to give more precise
numbers, but I decided to put this off a few days.)

I am confident that the density of PS fluid at 1000 psi (or so) is used in the
outlet pipe design, but the inlet pipe design depends on other factors. Why in
particular is the inlet pipe diameter larger than the outlet pipe diameter?

Cavitation concerns.

At all times rotary pumps (as well as some other types of pumps) need to have
pressure at the pump inlet high enough so that the liquid does not vaporize.
Cavitation reduces pump efficiency, and the PS system therefore won't work as
designed. Cavitation especially tends to happen at higher pump RPMs. The fix is
to have an adequate head on the pump inlet at all times. Hence the PS reservoir
being above the pump. Hence a pipe design with minimal pressure losses. Larger
diameter piping, among other things, reduces the pressure loss.

A nice summary of cavitation appears at:
http://www.mcnallyinstitute.com/01-html/1-3.html (focuses on centrifugal pumps,
but rotary pumps, like the PS pump, also may experience cavitation)

For rotary pumps, see http://www.mcnallyinstitute.com/CDweb/r-html/r019.htm

I didn't pull these sites out of thin air, so to speak. I did forget, from years
ago, that "net positive suction head" is the driving force behind pump inlet
pipe diameter design. Only this morning after reviewing some reference books on
pump design did the bell ring.

 

George, I bet these manuals are exclusively for cars whose country of design
origin is not the United States.

I don't recall seeing Kgf in my Honda's Chilton's or at the UK site's Honda
manuals. But I believe you that it does pop up. I know I haven't read as many
manuals as you and many of the regulars here.

What I do see in my Honda Chilton's and at the UK site is torques specified in
"lb-ft" and "kg-m."

As you probably know, the "lb-ft" is supposed to be lbf-ft." I imagine the kg is
supposed to be kgf, too. (Not gonna explain all the notation. People who've
worked with both American and metric know about pounds(mass) and pounds(force),
etc. They get this or they don't.)

I certainly wouldn't bet against Kgf/cm² cropping up. Indeed, it's on my
rinky-dink tire air compressor in the garage right now.

But for any application in U.S. power plants, U.S. military equipment (from
ships to tanks), etc., with signficant non-vacuum pressure., I know psi is
usual. Newport News shipyard does not design using Kgf/cm² .

I don't feel still using mostly English units makes the U.S. backward.

If a wannabee engineer is uncomfortable with unit conversions, he/she won't be
anymore comfortable with an "all metric" system than with an all English system,
IMO. There are just so many types of units out there. You either get used to
carrying around a little booklet of conversion factors, or you get out of the
business.

Obviously the yahoos with NASA circa 1998 spilled coffee on their conversion
booklets the day they were designing for thrust for the Mars Climate Orbiter.
They forgot to convert pounds(force) to newtons. The rest is history (crash;
millions of bucks down the drain).

I'm going to respectfully disagree with this. I think most U.S. mechanics and
plumbers are far more comfortable with pounds and inches, and they spend most of
their lives with these units as opposed to kilograms

Ha ha ha...

George, this little review is almost as fun as reading the white pages, but...

Aw, c'mon. Little gems come out of conversions all the time.

A kilowatt is very close to 3/4 of a horsepower, for example.

Anyway, I need to go study up for my brake caliper rebuild... ;-)

 

Caroline wrote

NASA specified and used metric units so there was no need in the
design for conversion. Unfortunately Lockheed Martin failed to follow
the specification.

The official report says:
www.hq.nasa.gov/office/codeq/risk/mco_mib_report.pdf
"the root cause for the loss of the MCO spacecraft was the failure to
use metric units... of Newton-seconds (N-s). Instead, the data was
reported in English units of pound-seconds (lbf-s). ... The SIS, which
was not followed, defines both the format and units"

 

Unit conversions of one kind or another are inevitable in any large-scale
engineering project. Manufacturers, consultants, etc. may sell their product
using pounds or other English units. Should NASA not contract with such
businesses, thus reducing competition, in violation of its "Faster, Better,
Cheaper" pledge?

NASA specifying "all metric" would certainly not preclude a metric conversion
error, either.

This in no way, shape, or form absolves NASA of some of the blame. Those
employed directly by NASA liaison extensively with sub-contractors. See below
for a prime example of how NASA engineers could have, and, according to the
"Mars Climate Orbiter Mishap Investigation Board," should have detected this
error.

It also points a heavy finger at NASA itself. For example, from the Executive
Summary, page 6:
_____
The Mars Climate Orbiter mission was conducted under NASA's "Faster, Better,
Cheaper" philosophy, developed in recent years to enhance innovation,
productivity and cost-effectiveness of America's space program. The "Faster,
Better, Cheaper" paradigm has successfully challenged project teams to infuse
new technologies and processes that allow NASA to do more with less. The success
of "Faster, Better, Cheaper" is tempered by the fact that some projects and
programs have put too much emphasis on cost and schedule reduction (the "Faster"
and "Cheaper" elements of the paradigm). At the same time, they have failed to
instill sufficient rigor in risk management throughout the mission lifecycle.
These actions have increased risk to an unacceptable level on these projects.
_____


From page 80:
_____
The Board found several instances of inadequate training in the MCO project. The
[NASA controlled, Cal Tech Jet Propulsion Laboratory www.jpl.nasa.gov]
operations navigation team had not received adequate training on the MCO
spacecraft design and its operations. Some members of the MCO team did not
recognize the purpose and the use of the ISA. The small forces software
development team needed additional training in the ground software development
process and in the use and importance of following the Mission Operations
Software Interface Specification (SIS). There was inadequate training of the MCO
team on the importance of an acceptable approach to end to end testing of the
small forces ground software. There was also inadequate training on the
recognition and treatment of mission critical small forces ground software.
____

I'm one of those former government contracted, defense industry engineers who
did not look the other way when our people did wrong. It was always embarrassing
when one met some kid engineer or some middle-aged engineer who thought it was
okay to cut corners. They are out there. This mishap is just one example of how
engineers screw up royally.

 

I like your numbers, and seems we now agree.
By the way, cavitation also wears out the pump, so it's important for a
pump that runs continiously (as does the power steering pump does) to have
a good inlet flow, otherwise it will have a short life.

(It is also not very fuel efficient to have the pump running all the time,
as much of the time the power steering is not required - but it does !!!!)

 

I don't follow. When do you want the power steering off?

Only when the car is stopped at a light for a few minutes can I see maybe having
a design where the pump is not rotating (one way or another; electric or
disconnected from the crankshaft, somehow). Such a design I suppose would turn
the power steering pump off for a few minutes, while the car is idling at a
stoplight, and then, when the driver presses the gas, the PS pump would start
again.

Seems like there would be too much delay for safety.

Otherwise, I want that power steering while cruising on the highway or moving
anytime at all, for quick response; for safety.

I did read in the Usenet archives about at least one person who disconnected the
PS pump (by removing the PS belt) to gain more performance from his engine. Said
he felt the performance (hmm; I think we're talking an extra 2 hp or so, tops)
but steering was exhausting.

Interesting discussion, anyway.

 

It's not required when ever it's not doing any work, for example driving
along a straight road, or even at high speeds where the steering input is
small.
Of course you want it available all the time, as you may need to make an
unexpected lane change or what ever, so it must run.

However to have a pump and accumulator and clutch would be expensive.

Another solution is electric power steering, it's more fuel efficient
(motor only runs when moving the rack) but some people do not like the
feel, and it's not cheap.

As to being able to notice a difference of a couple of horsepower, I
assume it's about the same effect as turning on or off the A/C compressor,
as that is also a couple of hp. (I'm using approximations again !!!).

 

Okay. I see from my _Marks' Standard Handbook for Mechanical Engineers_ that
normal, straight highway driving relies on a mechanical, spring-action pre-load
mechanism (you may know more about this than I) and *no* hydraulic assistance.
This gives the driver an excellent "feel of the road." So Marks says.

So am I.

I think that 2 Hp "tops" estimate I made may be off by a factor of 10 or so for
normal driving. It's only when doing something like parallel parking that
serious power steering pump Hp is required.

Thanks for checking the calculations I made before.

 

I'm talking about the Helm manuals which are the *only "Honda" manuals
which, AFAIK, are country agnostic though there is usually both the Kg
(usually as Kgf but not always) and the lb.

Yeah well it seems that we (my contemps and I) wasted a lot of time
learning stuff which was obsolescent: how lbf-ft was torque and ft-lbf was
work. I dunno whether to mourn the loss of the poundal or curse my science
teacher, his curriculum and the unnecessary grief it all gave us.

As far as falling into line with the rest of the world - yes... and in a

Oh sure but if they are to be forced to metrify.........

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??

 

Hmm, I've wondered about the direct electric systems - haven't driven one
yet so can't comment on feel. Another system which is in some G.M. Euro
models (Vauxhall, Opel etc.) is a hydraulic system with an electric pump.
I drove a rental car with it and it felt OK but obviously I didn't "live
with it" for long.

Rgds, George Macdonald

"Just because they're paranoid doesn't mean you're not psychotic" - Who, me??