![Reply](https://www.horizonsunlimited.com/hubb/images/buttons/reply.gif) |
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
Registered Users
HUBB regular
|
|
Join Date: Mar 2007
Location: Sussex, UK
Posts: 80
|
|
Quote:
Originally Posted by oothef
So does 0.5 atm become 12.25 atm at 10:1 (or there about)
There's much less initial pressure for a flame front (or two) to work against to create detonation.
|
Yep, that's right...
__________________
brighton2expeditions
'02 Africa Twin (sold), GSX-R 1000 K5 (sold), '97 TL1000S, '08 DRZ400 SM/S, '92 CRM250
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
Registered Users
HUBB regular
|
|
Join Date: Mar 2007
Location: Sussex, UK
Posts: 80
|
|
Quote:
Originally Posted by colebatch
This is fantastic stuff. And has my brain working overtime to keep up ![Smile](https://www.horizonsunlimited.com/hubb/images/smilies/smile.gif) Its 25 years since I last looked at Physics
I had forgotten about the heating of the air under compression impacting additionally on the pressure ...
So let me see if I understand this correctly ... temp being constant, 1 ATM compressed 10 times will be at 10 ATM ... but the fact that temp isnt constant (due to rising pressure) and it would reach 24.5 ATM means the Kelvin temp of that air has risen 2.45 times? Is that right?
But that would give me a temp of 462 C, rather than the 477 C you mentioned above (initial air temp 27 C)
|
Sorry buddy, didn't spot this as you were posting it as I was posting another...
The formula for temperature is similar to the one for pressure, but whilst
Pressure x Volume-to-the-power-gamma = constant
Temperature x Volume-to-the-power-gamma-minus-one = constant
So if we assume gamma for air is 1.4, a compression of 10:1 starting at 1 bar and 25 degrees C (298K) gives us a pressure of:
1 bar x 10^1.4 = 25.11 bar, which as we're working relative to a volume of 1 needs no further calc to give the pressure (for any other resulting volume along the way, you'd need to divide by that volume raised to the power of 1.4)
The temperature, would be
298K x 10^0.4 which is 748K, or 475 deg C
Similarly, the temp for any intermediate volume along the way would be given by dividing that result by the volume raised to the power 0.4...
Of course, this is all slightly approximate anyway as there is some heat lost to the cylinder walls etc during the compression - not a lot, but enough to distort the cylinder pressure trace noticeably in our data...
Geek out.
__________________
brighton2expeditions
'02 Africa Twin (sold), GSX-R 1000 K5 (sold), '97 TL1000S, '08 DRZ400 SM/S, '92 CRM250
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
![Chris Scott's Avatar](https://www.horizonsunlimited.com/hubb/customavatars/avatar10468_1.gif) |
Super Moderator
Veteran HUBBer
|
|
Join Date: Jun 2000
Posts: 4,840
|
|
Thanks dja, your original explanation spells it out most convincingly for me.
Other things mentioned:
standing up on your bike at higher altitudes is definitely easier than at sea level: decreased air resistance
Is this something others have noticed too, or could it just be an impression (possibly caused by lack of oxygen to the brain? ;-)
Put that way it does make sense, but I've freewheeled down from as much as 5300m and can't say I noticed I was hurtling down effortlessly with no drag due to the thin air. But I was probably not thinking about that - like most I was trying to keep on the road. And a day or 2 later, crossing a 4000m plain into a strong headwind didn't feel any less of a drag than normal.
One small correction about intercoolers. AFAIU it's not so much about cooler air increasing compression and detonation resistance (though they may be side benefits). Cooler air is denser and so carries more oxygen. With more oxygen you can up the fuel rate (with a screwdriver or chip) to get a broad spread of extra power (but also more heat to the point where I've thought of fitting an EGT gauge).
Fitting a bigger IC is commonly done on TD 4x4s.
Another thing I wondering is whether petrol-engine efi manages a perfect mixture at 4000m just as it does at normal elevations. No reason why it shouldn't I suppose, and the XCountry mpg in the Pamirs seems to prove it.
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
Registered Users
HUBB regular
|
|
Join Date: Mar 2007
Location: Sussex, UK
Posts: 80
|
|
More geek fodder, thanks Chris...
Yes, there will be less air resistance at altitude, though this is most noticeable for airliners (and their cruising altitude is approximately the optimum compromise between reduced air resistance and reduced engine performance - but their engines are optimised for that altitude, and could be re-engineered to produce more power at sea level if the materials had been developed that could take the pressures and temperatures).
The compromise for motorcycles is not so great cos the engine has not been optimised to spin faster at altitude to make up for the less dense air (you just spin it at that speed at sea level and enjoy more performance!)
Meanwhile, intercoolers cool the charge so:
- Increase charge density - more oxygen means more fuel can be burnt means more power
- Reduce charge temperature - in a diesel, more fuel always equals higher exhaust temperatures and high speed performance is commonly limited by turbine inlet temperature limits
There still comes a point of course at which you crash into the hardware limits (or straight through the hardware limits if you're "chipped"), but it all helps. The bits the general public are less interested in are reduced NOx emissions (which is formed by the same high pressures and temperatures that cause knock)
As to whether a fuel injection system will result in a perfect mixture at 4000m, that depends on the system and how well it's been calibrated during development. In theory, most are capable of perfectly adjusting for the change in barometric pressure, but only those running a lambda sensor and closed loop fuelling will get it spot on, and only while they're operating closed loop (i.e. in the part of the operating map that gets used on the emissions test).
__________________
brighton2expeditions
'02 Africa Twin (sold), GSX-R 1000 K5 (sold), '97 TL1000S, '08 DRZ400 SM/S, '92 CRM250
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
Registered Users
HUBB regular
|
|
Join Date: Mar 2007
Location: Sussex, UK
Posts: 80
|
|
Oh - and while I remember, the improved fuel economy at altitude...
One of the most significant losses in IC engines is the power used by the engine to pump the air into and out of the engine. In the case of a gasoline engine, the throttle restricts the airflow into the engine to reduce the power output to that required to just cruise along at constant speed (as opposed to accelerate full bore). That means the engine is continuously pumping air from a low pressure (the pressure after the throttle) to a high pressure (atmospheric pressure/exhaust back pressure). That takes a lot of work when you add it up for the whole time the engine is running.
At altitude, the absolute pressure at the inlet port for a given power is the same as at sea level, but the exhaust is being pumped out into a lower atmospheric pressure. So the pumping losses are greatly reduced...
So, so long as you have a fuel system that doesn't over fuel at altitude (i.e. a carb set up for sea level) you will see better fuel economy when up a mountain.
__________________
brighton2expeditions
'02 Africa Twin (sold), GSX-R 1000 K5 (sold), '97 TL1000S, '08 DRZ400 SM/S, '92 CRM250
|
![Old](https://www.horizonsunlimited.com/hubb/images/statusicon/post_old.gif)
18 Nov 2010
|
![oothef's Avatar](https://www.horizonsunlimited.com/hubb/images/avatars/Animal004.gif) |
Registered Users
Veteran HUBBer
|
|
Join Date: Feb 2010
Location: york
Posts: 265
|
|
Introducing the exhaust into it opens up a whole new kettle of worms........!
__________________
Anything can happen in the next half hour
|
Currently Active Users Viewing This Thread: 1 (0 Registered Users and/or Members and 1 guests)
|
|
Posting Rules
|
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts
HTML code is Off
|
|
|
|
Check the RAW segments; Grant, your HU host is on every month!
Episodes below to listen to while you, err, pretend to do something or other...
2020 Edition of Chris Scott's Adventure Motorcycling Handbook.
"Ultimate global guide for red-blooded bikers planning overseas exploration. Covers choice & preparation of best bike, shipping overseas, baggage design, riding techniques, travel health, visas, documentation, safety and useful addresses." Recommended. (Grant)
![Ripcord Rescue Travel Insurance.](/stuff/ripcord/ripcord-logo-250.jpg)
Ripcord Rescue Travel Insurance™ combines into a single integrated program the best evacuation and rescue with the premier travel insurance coverages designed for adventurers.
Led by special operations veterans, Stanford Medicine affiliated physicians, paramedics and other travel experts, Ripcord is perfect for adventure seekers, climbers, skiers, sports enthusiasts, hunters, international travelers, humanitarian efforts, expeditions and more.
Ripcord travel protection is now available for ALL nationalities, and travel is covered on motorcycles of all sizes!
What others say about HU...
"This site is the BIBLE for international bike travelers." Greg, Australia
"Thank you! The web site, The travels, The insight, The inspiration, Everything, just thanks." Colin, UK
"My friend and I are planning a trip from Singapore to England... We found (the HU) site invaluable as an aid to planning and have based a lot of our purchases (bikes, riding gear, etc.) on what we have learned from this site." Phil, Australia
"I for one always had an adventurous spirit, but you and Susan lit the fire for my trip and I'll be forever grateful for what you two do to inspire others to just do it." Brent, USA
"Your website is a mecca of valuable information and the (video) series is informative, entertaining, and inspiring!" Jennifer, Canada
"Your worldwide organisation and events are the Go To places to for all serious touring and aspiring touring bikers." Trevor, South Africa
"This is the answer to all my questions." Haydn, Australia
"Keep going the excellent work you are doing for Horizons Unlimited - I love it!" Thomas, Germany
Lots more comments here!
![Five books by Graham Field!](/stuff/grahamfield/graham-field-2021-280.jpg)
Diaries of a compulsive traveller
by Graham Field
Book, eBook, Audiobook
"A compelling, honest, inspiring and entertaining writing style with a built-in feel-good factor" Get them NOW from the authors' website and Amazon.com, Amazon.ca, Amazon.co.uk.
Back Road Map Books and Backroad GPS Maps for all of Canada - a must have!
New to Horizons Unlimited?
New to motorcycle travelling? New to the HU site? Confused? Too many options? It's really very simple - just 4 easy steps!
Horizons Unlimited was founded in 1997 by Grant and Susan Johnson following their journey around the world on a BMW R80G/S.
Read more about Grant & Susan's story
Membership - help keep us going!
Horizons Unlimited is not a big multi-national company, just two people who love motorcycle travel and have grown what started as a hobby in 1997 into a full time job (usually 8-10 hours per day and 7 days a week) and a labour of love. To keep it going and a roof over our heads, we run events all over the world with the help of volunteers; we sell inspirational and informative DVDs; we have a few selected advertisers; and we make a small amount from memberships.
You don't have to be a Member to come to an HU meeting, access the website, or ask questions on the HUBB. What you get for your membership contribution is our sincere gratitude, good karma and knowing that you're helping to keep the motorcycle travel dream alive. Contributing Members and Gold Members do get additional features on the HUBB. Here's a list of all the Member benefits on the HUBB.
|
|
|