Go Back   EcoModder Forum > EcoModding > Instrumentation
Register Now
 Register Now
 

Reply  Post New Thread
 
Submit Tools LinkBack Thread Tools
Old 12-18-2009, 12:29 AM   #1 (permalink)
epic stock master
 
luvit's Avatar
 
Join Date: Oct 2009
Location: US
Posts: 377

86 Accord LX (carb) - '86 Honda Accord LX
90 day: 35.57 mpg (US)
Thanks: 19
Thanked 17 Times in 15 Posts
Turtle Priorities: RPMs vs VAC

  • sometimes when i climb a hill in high gear i get low vacuum.
  • if i shift down, my vac goes up, but so do my rpms.

which is better? -- i think i know.

__________________
.
single person car thread
.
  Reply With Quote
Alt Today
Popular topics

Other popular topics in this forum...

   
Old 12-18-2009, 04:22 PM   #2 (permalink)
...beats walking...
 
Join Date: Jul 2009
Location: .
Posts: 6,191
Thanks: 179
Thanked 1,521 Times in 1,122 Posts
...it's all about "loading" on the engine.

...switching to a lower gear allows the engine to rev higher which reduces its "load" and is shown by the higher vacuum reading, because the vacuum gauge closely parallels the engines "loading".

...best example, is the older non-smog engines, where coasting down a hill in high gear would almost "peg-out" the vacuum gauge...an almost perfect example of "no load."

...conversely, "flooring" the gas pedal would likewise "zero" the vacuum gauge...an almost perfect example of "maximum load."

...the answer to your question is to "mentally optimize" the two by aiming (throttling/shifting?) for highest vacuum reading at lowest RPM that you're comfortable with driving (the people behind you might not agree, but what the heck)...and that sometimes means driving UNDER the posted speed limit.

Last edited by gone-ot; 12-18-2009 at 06:02 PM..
  Reply With Quote
Old 12-18-2009, 10:24 PM   #3 (permalink)
epic stock master
 
luvit's Avatar
 
Join Date: Oct 2009
Location: US
Posts: 377

86 Accord LX (carb) - '86 Honda Accord LX
90 day: 35.57 mpg (US)
Thanks: 19
Thanked 17 Times in 15 Posts
so puttering around town tonight (max 35mph) i found i always get much higher vacumm if i leave the car in 3rd gear instead of 4th gear overdrive. -- i have an automatic transmission.
__________________
.
single person car thread
.
  Reply With Quote
Old 12-20-2009, 02:57 PM   #4 (permalink)
...beats walking...
 
Join Date: Jul 2009
Location: .
Posts: 6,191
Thanks: 179
Thanked 1,521 Times in 1,122 Posts
...optimally, you'd want the tranny in the highest gear (4th) possible, but most don't "shift" until you pass 40 mph, so the trick is to get up to where it shifts from 3rd into 4th and then back down to 35 mph.

...of course, as soon as you encounter a slight rise or grade increase which places more load on the engine, the transmission will automatically downshift to 3rd.

...an automatic is not as easy for the driver to control as a manual, that's why many of the "new" multi-speed transmissions are bascially "manually shifted automatics" although using real "gears" rather than "bands" and sungears.
  Reply With Quote
Old 12-20-2009, 03:53 PM   #5 (permalink)
Hydrogen Nut
 
Ptero's Avatar
 
Join Date: Oct 2008
Location: California
Posts: 151

Smart Car ForTwo Pure - '08 Smart Fortwo Pure stripped
90 day: 51.35 mpg (US)

BMW 750iL V12 - '90 BMW V12
90 day: 26.4 mpg (US)

Wildfire 250C - '08 Shandong Pioneer 250C
Thanks: 2
Thanked 33 Times in 22 Posts
Find the torque curve graph for your engine. Now find a hill with a constant grade and try several gears at he torque peak rpm. One of those gears will give you the best mpg near the torque peak.

The torque peak represents the rpm where your engine delivers the most useable power. Depending on the compromises made by the manufacturer, your fuel/air mix at this rpm is close to ideal for power.

But we sre not seeking to optimize horsepower in hypermiling. We want efficiency. Maximum efficiency is obtained by leaning the mixture found at the peak of the torque curve down to the point where you flirt with predetonation on gas engines.

Most gas engines are designed to run slightly rich in normal use. This keeps them from being damaged from too hot a burn temperature or too extreme downforce generated too close to TDC - or both, which is really the case. This overly-lean mix results in melting of the piston dome.

Knock sensors, exhaust gas temperature gauges and realtime exhaust gas monitors are useful to people playing on the edge of this envelope. People messing around with turbochargers and superchargers tend to discover these limits quicker.

Ceramic piston head coatings are a way we can push these limits because they can withstand higher temperatures. Federal-Mogul makes these for various engines.

Ceramic coatings are mostly used for racing. It would be interesting to see ultra-lean mpg applications using them. (I think some Japanese engines have already incorporated ceramic coatings - does anyone know?)

For more info, see Inside Secrets of Racing Coatings by Swain Tech for Automotive Coatings including engine piston coatings, ceramic coating services engine, automotive ceramic coating, ceramic header coating, dry film lubricant, race engine coatings, exhaust header co

Last edited by Ptero; 12-20-2009 at 04:06 PM..
  Reply With Quote
Old 12-21-2009, 05:03 PM   #6 (permalink)
Batman Junior
 
MetroMPG's Avatar
 
Join Date: Nov 2007
Location: 1000 Islands, Ontario, Canada
Posts: 21,391

Blackfly - '98 Geo Metro
Team Metro
Last 3: 70.09 mpg (US)

MPGiata - '90 Mazda Miata
90 day: 52.8 mpg (US)

Winter beater Metro - '00 Chevrolet Metro
90 day: 73.57 mpg (US)

Fancy Metro - '14 Mitsubishi Mirage top spec
90 day: 58.72 mpg (US)
Thanks: 3,166
Thanked 6,020 Times in 3,121 Posts
To answer the original question:

My experience in every vehicle I've driven is that you always increase fuel economy by going with the highest possible gear you can drive in without lugging.
__________________
Latest mods test: 15 mods = 15% MPG improvement: A-B test, 2007 Honda Civic 1.8L, 5-speed
Ecodriving test:
Manual vs. automatic transmission MPG showdown: Nissan Micra 1.6L



EcoModder
has launched a forum for the efficient new Mitsubishi Mirage
www.MetroMPG.com - fuel efficiency info for Geo Metro owners
www.ForkenSwift.com - electric car conversion on a beer budget
  Reply With Quote
The Following User Says Thank You to MetroMPG For This Useful Post:
Gasoline Fumes (12-22-2009)
Old 12-21-2009, 11:49 PM   #7 (permalink)
epic stock master
 
luvit's Avatar
 
Join Date: Oct 2009
Location: US
Posts: 377

86 Accord LX (carb) - '86 Honda Accord LX
90 day: 35.57 mpg (US)
Thanks: 19
Thanked 17 Times in 15 Posts
Quote:
Originally Posted by MetroMPG View Post
To answer the original question:

My experience in every vehicle I've driven is that you always increase fuel economy by going with the highest possible gear you can drive in without lugging.
yeah, you're right... i'm reading so much i don't want to forget the fundamentals prior to the instrumentation that makes me read so much...
that's the answer that got out from under my finger. thanks!
__________________
.
single person car thread
.
  Reply With Quote
Old 12-26-2009, 04:56 AM   #8 (permalink)
Master EcoModder
 
Join Date: Dec 2009
Location: Leavittsburg, Ohio
Posts: 364

Mater - '98 Dodge 3500 Dually

Wilber - '99 Toyota Corolla LX

Minyatta - '93 Mazda Miata MX-5
Thanks: 102
Thanked 11 Times in 11 Posts
Quote:
Originally Posted by MetroMPG View Post
To answer the original question:

My experience in every vehicle I've driven is that you always increase fuel economy by going with the highest possible gear you can drive in without lugging.
Yep, well written!

Dave
  Reply With Quote
Old 01-18-2010, 11:09 PM   #9 (permalink)
halos.com
 
Join Date: Oct 2009
Location: Oklahoma City, OK
Posts: 528

ECONORAM - '07 Dodge RAM 1500 QC SLT flex-fuel
90 day: 18.16 mpg (US)

the Avenger - '08 Dodge Avenger SXT
90 day: 27.06 mpg (US)
Thanks: 385
Thanked 92 Times in 78 Posts
Send a message via Yahoo to ECONORAM
Quote:
Originally Posted by Ptero View Post
Find the torque curve graph for your engine. Now find a hill with a constant grade and try several gears at he torque peak rpm. One of those gears will give you the best mpg near the torque peak.

The torque peak represents the rpm where your engine delivers the most useable power. Depending on the compromises made by the manufacturer, your fuel/air mix at this rpm is close to ideal for power.

But we sre not seeking to optimize horsepower in hypermiling. We want efficiency. Maximum efficiency is obtained by leaning the mixture found at the peak of the torque curve down to the point where you flirt with predetonation on gas engines.

Most gas engines are designed to run slightly rich in normal use. This keeps them from being damaged from too hot a burn temperature or too extreme downforce generated too close to TDC - or both, which is really the case. This overly-lean mix results in melting of the piston dome.

Knock sensors, exhaust gas temperature gauges and realtime exhaust gas monitors are useful to people playing on the edge of this envelope. People messing around with turbochargers and superchargers tend to discover these limits quicker.

Ceramic piston head coatings are a way we can push these limits because they can withstand higher temperatures. Federal-Mogul makes these for various engines.

Ceramic coatings are mostly used for racing. It would be interesting to see ultra-lean mpg applications using them. (I think some Japanese engines have already incorporated ceramic coatings - does anyone know?)

For more info, see Inside Secrets of Racing Coatings by Swain Tech for Automotive Coatings including engine piston coatings, ceramic coating services engine, automotive ceramic coating, ceramic header coating, dry film lubricant, race engine coatings, exhaust header co
I had thought about altering my truck's air/fuel ratio, but your post makes me wonder if I would melt something down. With coated pistons and combustion chamber portions of cylinder heads (I am guessing both should be done), will this prevent this from occurring since the heat is reflected back into the combustion chamber?
  Reply With Quote
Old 01-22-2010, 02:01 PM   #10 (permalink)
EcoModding Lurker
 
Join Date: Jan 2010
Location: Los Angeles, CA
Posts: 54
Thanks: 0
Thanked 20 Times in 8 Posts
Quote:
Originally Posted by ECONORAM View Post
I had thought about altering my truck's air/fuel ratio, but your post makes me wonder if I would melt something down. With coated pistons and combustion chamber portions of cylinder heads (I am guessing both should be done), will this prevent this from occurring since the heat is reflected back into the combustion chamber?
I'm sorry, but as a long time lurker I thought I would chime in one this one, because lean and rich are sometimes used as relative terms ("too lean...") and that can be easily misunderstood in terms of what is good and bad for the engine.

A modern car spends much of the time running at stoichiometric ratio, or 'stoich'. This is nominally an AFR (air/fuel ratio) of about 14.7:1, but it varies with exact fuel mix. The vehicle doesn't spend all its time here, that's why OBD-II 'MPG' readings have to be fudged so much to match actual fuel usage, but it spends a lot of it.

Stoich is the optimum *thermal* reaction. It is the balance of air and fuel that releases the most heat. So, it also coincides with peak EGT. The only reason vehicles spend so much time running here is emissions. Stoich is a point where CO, HC, and NOx are all relatively low. If you go richer, CO and HC soar, if you go leaner, NOx soars. The peak exhaust gas temps are also needed for the catalytic converter to operate.

The car normally finds stoich using O2 sensors. Based on the name, you'd think they measure AFR, but they actually measure 'equivalency ratio', or how much O2 must be added or removed from a sample of the exhaust gas for stoich. Normally, we use the reciprocal of this and call it "lambda". So, stoich is 1.0. A smaller number is richer, a larger number is leaner. To get an approximate AFR, multiply lambda by the typical stoichiometric ratio for the fuel being used. So, with gasoline, that would be lambda x 14.7.

Stoich is good for emissions, but not for everything else. It is reasonable to think that peak thermal reaction is best for power, but the engine is not a steam engine. We are looking for the optimum combination of gas pressure inside the chamber, transferring optimum mechanical advantage. In other words, we aren't looking for the most heat from the fuel, but the highest mechanical advantage.

This is a point richer than stoich. It varies with engine design, but let's say .86 lambda, or roughly there about, on average. Although it is not a precise technical term, this is sometimes referred to as "Best Power". If we were looking at EGT temps, this would be roughly 125 to 150 degrees 'rich of peak' or 'enrichened until the temp is 125-150 degrees cooler than peak'.

Similarly, best economy is also not at stoich. That point is leaner than stoich, say 1.05 lambda, perhaps '25-75 degrees lean of peak' (lean until 25-75 cooler). The reasons are actually surprisingly complicated, but suffice to say that excess air helps insure that all the fuel is used and it is a point that still yields good mechanical advantage.

Sorry to be so long winded, but I wanted to make sure that my use of 'lean' and 'rich' that follows is clear.

There is a long standing MYTH that running an engine lean of stoich, or lean of peak EGT, is bad for it. It is clearly, and utterly a myth. Lindbergh disproved it in WW-II with P-38s, and helped turn the war in the Pacific. And we have seen it analyzed again, and again, and again, ever since. You are not going to detonate without heat, fuel, and pressure, and lean of peak, or lean of stoich, all three are at their lowest.

Likewise, you aren't going to burn anything up without heat, and CHT plummets lean of peak. The problems with running this lean are a) it is hard to keep the fuel distribution even to the cyls with such small amounts, so you can get rough engine operation and b) your emissions soar.

There is also a widespread believe that engines can't really run at Best Power because of heat. But thermal energy from the fuel goes basically three places, exhaust, cyl heads, and 'work'. At best power, the engine is doing a good job at 'work', so it is probably not surprising that both CHT and EGT are relatively low. The problems with running here guzzling fuel, and mechanical torques and stresses. Again, one of the issues is just how evenly you can distribute fuel.

The problem area for burning things up is just rich of peak, between stoichiometric and Best Power. This is where peak pressures and CHT's soar. In other words, this is when everything detonation needs is in plentiful supply.

So, when we are talking about the consequences of 'leaning out the engine', we need to be clear about which types of operations we are leaning. If we are leaning out 'closed loop' operation, or when the engine is running at 14.7:1, our primary concern is the environment. We can get so lean that we run rough, but you'll hear it.

It is possible to put in a wideband sensor and send a simulated narrow band O2 signal to an ECU and fool it into running leaner, but I think that this is counterproductive and a waste of time. You get into issues with long and short fuel trims, simulating cat efficiency, etc. and you ruin your cat while being bad for the environment. I think a more useful application is to use the wideband to 'tighten' the ECU's adherence to stoich. ECU's chase stoich, rich, lean, rich, lean. Following the narrow band sensors. The sensors have a little curve in their response, so the oscillations can get wide.

With a wideband (at least a fast one), you can simulate the speed of the response for the 'narrow band' simulation however you want. I've found that by adjusting the simulated angle of the narrow band, I can simulate a faster narrow band that the ECU will track. The swings are still there (they need to be for the cat to operate), but they hover closer to stoich. This can save a little fuel, but more importantly, it seems to lower my emissions when I do a test smog check. Cat efficiency is very dependent on peak temp. It plummets as we move off stoich. So, by fooling my car to run tighter to stoich, I can keep my cat operating at a higher efficiency. I could get it tighter if I could modify the control loop parameters in the ECU, but that is a whole different level of intervention.

If we are talking about leaning out things when we are 'open loop', or when the vehicle ECU is going rich because we are stomping on the gas to pass, etc., then going 'too lean' (being caught between stoich and best power) is a real concern. Although we are seeing more wideband technology in stock vehicles, most cars still just pour on a guess in excess fuel. And, to be safe, they generally error on the rich side.

From an eco point of view, I think that the answer is the same as in performance automotive. Tuning to as close to best power, closed loop with an add on wideband if you can, seems to make the most sense. Think about it, you are stomping on the gas for a reason. Tuning for max performance in these cases means that you will get the needed performance result down as quickly as possible, while using less fuel than a stock tune. Tuning any leaner than this is counter productive. You'll have to stomp harder and longer to get whatever you want done. It seems better to me to get the performance when you need it, and get back into closed loop operation as quickly as you can.

Sorry for the long post, but it is sort of 'my area'.

Regards,
-jjf

  Reply With Quote
The Following 3 Users Say Thank You to jfitzpat For This Useful Post:
ECONORAM (01-26-2010), j12piprius (02-26-2011), Nels (01-30-2010)
Reply  Post New Thread


Thread Tools


Similar Threads
Thread Thread Starter Forum Replies Last Post
welded body dropped rpms..and that... bgd73 Success Stories 27 06-24-2009 09:31 PM
Lower idle RPMs save gas? Xringer EcoModding Central 5 06-14-2008 10:19 PM
Accelerating and shift points bhazard Hypermiling / EcoDriver's Ed 26 04-25-2008 04:52 PM



Powered by vBulletin® Version 3.8.11
Copyright ©2000 - 2020, vBulletin Solutions Inc.
Content Relevant URLs by vBSEO 3.5.2
All content copyright EcoModder.com