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Comments
A great site! It explaines FI in great detail, both gas & diesel. We should have had you post this site much sooner. It would have saved a lot of bickering about fuel economy! I am always ready to learn about new technology, but only when it is explained in a civil manner, and backed up with proof.
My last post was deleted by the host, and probably just as well as it would have extended the discussion about fuel Injection and fuel economy.
I stand corrected, the fuel is shut off when decellerating (as when coasting downhill). At least in Citroens'. Probably true in most other systems too.
It's just too bad that it had to be a FRENCH site to explain it in detail! (just joking)
Regards
Boxwrench
2004 Mazda MPV...25.1 mpg mostly hwy 180 mile trip. Light luggage...very little use of a/c.
Very pleased overall with my Sedona.
Get one. Or a couple.
this guy should top off, go on a 250-300 mile HWY-only trip at 60-65 relatively constant speed, exit the highway, fillup, and divide the gallons pumped into miles actually driven.
I agree. Dividing the miles pumped into the miles driven is the only sure way to know your total MPG usage. Even that can very a little, depending on the cut off of the pump, but it is small.
The computer can giive you total different readings depending on whether you set it before getting on the freeway, or if you reset it after getting on the freeway. I have had the computer tell me I was getting as much as 36MPG highway at times, driving a combo of city/hiway. But when gassing it up, it's usually always around 17-21. Most of my driving is highway. I have a 2005 DGC. If you want to know exactly what your getting to a tank full, do it by hand or use a hand computer at fillup.
The trip computer shows very accurately where the fuel is being used. At the end of a 1380 mile round trip, the overall average will drop from 28.3 to 28.2 MPG while waiting at a stop light.
Vehicles burn a considerate amount of fuel just starting and warming up to operating temperature. The trip computer vividly indicates this fact.
One more reason is adaptive control technology on these vehicles which alters air-gas ratio depending on driving habbits and road condition. I hope my ody starts giving better mileage soooooooooon.
Of what possible difference would SOHC make vs. DOHC to mileage? They are both 4-valve/cylinder designs, they both use some form of variable valve timing. I'm really curious about why SOHC would have some inherent advantage on highway mileage but disadvantage on city mileage?
Just what is the advantage of DOHC over SOHC anyway? All I know is that all the Racing engines tout DOHC for increased power. Seems like there is more valve train gear to drive, robbing some of the increased power! It must have something to do with more Valves per cylinder.
Just how many more valves per cylinder can they keep putting into the Head anyway??
How about it Averigejoe -- what's the ansewer!
Boxwrench
SOHC are lighter and simpler design
DOHC are complicated and have weight disadvantage.
I am not an engine gure but at higher speed, SOHC gives better mileage and i am sure there are engine gurus out there who knows exactly why. I would suggest the moderator to start a new thread and it will be very interesting one. since sienna and all toyota SUVs are DOHC Vs Honda's SOHC in VTEC.
Here is an excerpt explaining SOHC Vs DOHC.
SOHC and DOHC are basically two different ways to configure the valvetrain. In the whole valve train setup there are camshafts, lifters, rockers, valves and springs. It is best that the valvetrain is light. Valves open at intervals of 25 times a second at 3000rpm. If the valvetrain is heavy, when the camshaft lifts it the valvetrain will just keep going up until the valve spring catches it. This is called valve float. You can overcome this with stronger (which means heavier) springs, but this extra weight increases the power requirements to open the valve.
The cams are driven by the crankshaft, using either a belt or chain called a timing belt or timing chain. If the timing belt/chain snaps and the camshaft stops spinning, the piston coming back up the combustion chamber might hit the open valves. This is very costly. That’s why you should always change your timing belt/chain at the interval specified by your car manufacturer.
SOHC refers to Single Overhead Camshaft. In the days before DOHC, it was known as OHC, with no need to differentiate between a single or double camshaft. In SOHC, the camshaft is situated in the cylinder head, above the valves. The valves are opened and closed either directly with a shim between the cam lobe and the valve stem, or via a rocker arm. SOHC engine valve configurations typically have 2 or 3 valves per cylinder. It is also possible to have 4 valves per cylinder using SOHC but this translates into a complicated combination of rocker arms and cam lobe shapes.
DOHC refers to Double Overhead Camshaft. This arrangement uses two camshafts in each cylinder head. Two cams per cylinder head means that a DOHC V engine has 4 camshafts because it has 2 banks of cylinder heads. This allows the manufacturer to easily implement a 4 valve per cylinder setup. Most of the time it also allows the engine to rev higher. It also allows better placement of the valves in an optimized setup that gives you maximum performance. But the disadvantage of such a setup is more weight, more cost and more complexity. It takes more stuff to drive two camshafts. The main reason to use DOHC is to drive more valves per cylinder. If a SOHC setup can allow 4 valves per cylinder, having a DOHC engine will not bring that much benefits over SOHC and the additional weight becomes a burden instead. DOHC engines also allows the spark plug to be placed right in the middle of the combustion chamber. This promotes efficient combustion. With SOHC, the camshaft is usually in the middle of the head because it has to drive both the intake and exhaust valves, robbing the sparkplug of it’s optimal location.
In the end, a SOHC 16 valve engine would have better torque on the low end where the DOHC valvetrain’s weight results in lower torque. But at high engine speeds, the 16 valve DOHC engine’s peak torque and horsepower would be greater. That’s the trade-off. With the amount of valves being equal, SOHC has better low-end torque because the valvetrain package is lighter while DOHC has better top-end power.
Other benefits of DOHC would be making it easier to implement variable valve timing technologies (which I will cover in another blog post) and also you can tweak it better with adjustable cam pulleys. If you were to put high-profile cams in your DOHC engine, the cam lobe profile can also be more optimized than a SOHC engine because you can play around with the lobe shape easier with separate camshafts for the intake and exhaust valves.
Why more valves per cylinder? Why not just make 1 huge intake valve and 1 huge exhaust valve? Bigger valves weigh more than the smaller one, so controlling the extra weight as it gets flung open and close becomes difficult. The spring has to be stiffer. A stiffer spring means more energy has to be spent overcoming the valve pressure. This partially oversets the gains which a bigger valve has to offer. Another problem with a single big valve is at lower RPMs the intake velocity will be lower. I’m sure you guys know this the same amount of air going through a big pipe will have lower pressure than the same air going through a smaller opening. Think of how you can control the water pressure of your garden hose by adjusting the opening size with your finger. Because of the velocity drop, low RPM torque and driveability will suffer. Although two smaller valves weigh the same as 1 big valve, and with the extra rocker arms and springs they can actually end up weighing more, this is offset by less mass to be overcome when opening and closing the valves.
So to sum it all up, SOHC has better low-end power, DOHC has better high-end power and overall maximum power. 4 valves per cylinder is much better than 2 valves per cylinder and it doesn’t matter whether 4-valves is achieved via SOHC or DOHC.
The torque at lower RPM in the Sienna is probably why the Sienna I test drove had MUCH quicker acceleration than the Odd.
SOHC was originally used when there was only 2 valves per cylinder (1 intake, 1 exhaust). DOHC was originally used when the move was made to 4 valves per cylinder. The thoughts regarding low end torque for SOHC engines is strictly due to the 2-valve configuration.
Also, it was easier to implement different valve lift/duration timing on the intake vs. exhaust valves when 1 cam drove the intake side and a seperate cam the exhaust side. The high rpm/power nature of DOHC engines is due more to the better breathing ability of 4-valves/cylinder rather than the DOHC design.
I still think that the torque and power characteristics of an engine is due MUCH more to valve configuration (size and location of valves, lift and duration of the timing, etc.) than whether the valves are controlled by a Single overhead cam per head or two.
Ummmm, you ARE aware that the Sienna has peak torque of 242ft-lbs (@ 3600rpm), while the Ody has a LOWER peak torque of 240ft-lbs (@ 4500rpm).
Sienna: HIGHER torque AND it arrives earlier, despite a displacement disadvantage (3.3l V6 vs. the 3.5l V6 in the Ody).
But I think the performance difference hansienna detected was NOT necessarily due to the higher torque output. I think it's probably due to the few hundred pounds of extra mass the Ody must lug around......at ALL rpms! :P
I did not test drive a Sienna, but if it is faster off the line than a 2006 Odyssey it must really really go. I would like to see some 0-60 and 1/4 mile comparisons. That would be very interesting. Mine Oddy is so fast that if I am not careful I will be doing 80 mph by the end of some of the merge ramps around here. The gearing in the transmission can also help with acceleration performance. And what is up with the Odyssey weighing 400 lbs more that the Sienna. That is 10% more. What does the Honda have that the Toyota does not?
Not sure on the weight issue just WHY the Sienna is so much lighter; I also don't know how much of a mileage penalty (if any) the Ody has because of the additional weight.
But here it is in a nutshell.
It's a physics thing. You know, mass, inertia, fluid mechanics. Stuff like that.
DOHC allows smaller valves and lighter total valve weight for the same flow characteristics, while a similarly breathing SOHC engine would have heavier total valve weight.
An engine is basically an air pump. You want to get as much air/fuel mixture into and and exhaust gases out of the combustion chamber as quickly as you can. The more and faster you can do it, the greater the potential for power is.
Heavier valves limits the RPM of the motor. Valve float occurs at a lower rpm with heavier valves than with lighter valves.
MPG is not necessarily better or worse depending on the number of camshafts. Many other things affect fuel efficiency.
Hahahaha.
If your trip computer is very accurate, as you say, you'll be sitting there a long time. Everyone behind you will be honking and someone might even call a tow truck to have you hauled away.
Nobody likes a traffic jam.
But as for the stoplight mpg change after 1380 miles, it's simple math. Use paper and pencil or a calculator.
Or better yet, do the test. 2/10 mpg. Report back and tell us how long that takes after 1380 miles.
Hahahaha.
Although I did not spell it out in that previous post, you need to understand that when I say DOHC engine, I mean multi (4) valve per cylinder engine. SOHC engines generally have less than 4 valves per cylinder. (Uh-oh, I just learned the Odyssey engine is SOHC with 4 valves per cylinder. Oh well, it does not really change my statement about 2 smaller lighter valves being better for power production compared to a single larger valve.)
To restate though, the DOHC multi-valve motor is typically higher revving and better breathing than a similar displacement SOHC engine having the typical less than 4 valves per cylinder. This is chiefly due to the lighter total weight of its valves and the greater airflow allowed by two smaller valves as compared to that afforded by a larger single valve. So long as each engine is otherwise technically up to date, the DOHC design has the greater horsepower potential of the two.
Bottom line: paraphrasing another poster, count the valves, not the camshafts.
My question is... when ECO mode, Oddy have 3 cylinders of and 3 running... correct?
Then, should be the milage close to double? Maybe not double but a little less.
This not the case... then does the working three cylinders are working harder and in more stress. Wouldn't this be bad for those 3 operative cylinders?
Are the same 3 cylinders turned off each time by VCM or are done on rotating basis?
Is this VCM a gimmick and could be bad thing for the engine?
Is VCM worth-it??
For reference, I had/have a first generation Odyssey (4-cylinder) that gets between 22 and 32 mpg highway (depending on speed and conditions). I got 26 mpg for a trip from SD to NYC travelling between 70 and 80 mph.
On the way back from the dealer (about 400+ miles) we had both vans driving identical conditons and filling up at the same gas stations. Because we were varying speed and stopping frequently to help the break in, our old van only got about 23.5 mpg. The amazing thing is that the Sienna (despite not being broken in yet) got 25.5 mpg (calculated - and on the trip computer).
If extrapolation works at all, then this new Sienna will get high 20's to near 30 once broken in, and driven under the conditions that I usually drive. Very happy with this, especially considering I have more room and 50% more power.
BTW to answer the above post to some extent. VCM will not come close to doubling mileage. VCM vehicles actually rev higher on the freeway to make it easier for the 3-cylinders to pull the load. I discounted VCM because the speed limit is 75 in SD and we always drive 80 on the highway. I am positive that a van the size of the Ody cannot run on 3-cylinders going 80 mph (especially with the wind blowing, which it always does when you don't have trees). Therefore VCM would be a waste in my situation.
Yes it is the same 3 cylinders.
VCM is not a gimmick, and does help somewhat, but no more than just using a slightly smaller engine in a lighter vehicle like Toyota did.
FYI: in my last car I and pretty-darn-good TC. It was rarely correct tank to tank, but after 22k miles (when I traded it) the over-all milage average (it had an MPG average from day one) was surprisingly close to the spreadsheet I keep like and O/C idiot. . . lol. . . look at the TC every 20k miles for a fairly accurate calculation.
Thanks,
A calculator computes 28.482 MPG. He reset the Trip Computer more than once and does not have the trip computer calculated mileage for the 986 miles.
On one round trip test to determine maximum possible mileage, his Sienna got 34.6 MPG. ( An early saturday morning on a rural Interstate before there was much traffic, very little wind, moderate temperature, and kept speed below 60 MPH).
The best my 2002 T&C LX 3.3L V6 got was 28.1 MPG on a 1409 mile round trip (dividing miles driven by gasoline consumed) but I did not keep speed at 60 MPH or less.
My time is worth more than the lower cost of fuel needed by driving below the posted speed limits.
You must be in the Mountains of Colorado, or elsewhere at High altitude to be using 85 Octane. I always wondered if the MPG was better or worse in High altitude with lower octane fuel?
I think it is due to less congested roads with less stop and go. If I drive to lower altitude like Las Vegas or California, I use 91 octane for first tank to bring up the average and then use 87 octane until back into higher altitude.
I would suggest looking at the torque values (lower max rpm) and the lower mass of the Toyota that it would probably accelerate faster than the Honda under "normal" driving conditions. However, if you really get on it in the Honda and get the rpm over 5000 quickly and keep it there, I think that the Honda might win.
Also, the Honda has 235/65/R16 tires while the Toyota has 215/65/R16 tires which have a 3.65% smaller circumference, better for acceleration but worse for handling. I could not get a list of gear ratios for Toyota to compare with those available for the Honda.
Please distinguish number of valves per cylinder as well as number of cams.
1) Each cam in a DOHC set-up will rotate at half the speed as an equivalent SOHC set-up (good), but will probably cause more friction (more bearings; bad).
2) You get to a point where two (or more) smaller valves give more effective area than one larger valve because you are limited by the diameter of the cylinder (more or less). Thus with multiple smaller valves you can actually have more intake and exhaust area than with two larger valves.
3) For clarification, most OHC engines (SOHC and DOHC) do not have lifters, but have the rocker arms riding directly on the cam lobes (good?).
I don' think you can sum it up quite the way you did since the cam profile and how the engine is designed around it is what primarily controls the power and torque and not just the number of cams or valves per cylinder.
I don't think it does.
Theoretically, lighter valves in an engine will permit higher engine RPM and likely higher peak horsepower as a result.
During the last 20 years or so, most DOHC engines have had more valves per cylinder than have most SOHC (and OHV) designs. And those valves in the DOHC multi-valve (4 valves per cylinder) engines were typically smaller and lighter than the valves in the average SOHC or OHV motor which had fewer than 4 valves per cylinder.
Assuming all other factors were practically the same, those multi-valve DOHC motors had the potential of higher RPM and horsepower because of their smaller and lighter weight valves. It is because valve float (with lightweight valves) can be postponed until higher RPMs are reached. Heavier valves would begin to float at lower RPM.
Valve float is a main limiting factor in the RPM an engine can attain.
I am aware that a camshaft in a 4-cycle reciprocating piston engine rotates at one half the speed of the crankshaft.
But I can't figure out why you would think the RPM of a camshaft in a DOHC motor could rotate at any other speed.
Any camshaft, in a flathead, a pushrod operated overhead valve design, a SOHC motor or a DOHC one turns at 1/2 crankshaft speed.
Both exhaust valves open together in a 4 valve per cylinder motor. Then they close together and the intake valves open and close together too.
Were you thinking the valves took turns individually?
Or maybe you thought there were two cam lobes operating each valve?
DOHC simply means two camshafts are arrranged over the tops of the cylinders. It does not mean that two cam lobes operate each valve. So if it is a V-motor, it would have 4 camshafts total. An inline DOHC motor has only 2 total.
Each valve has to open and close one time during the four stroke cycle of the engine. One cam lobe on a camshaft forces its corresponding valve open every time the camshaft turns 360 degrees. The four stokes of the piston (up, down, up, down) cause the crankshaft to spin around two times. So, the camshaft has to spin at exactly 1/2 the speed of the crankshaft so each valve will only open every other revolution of the crankshaft.
And who knows if the EPA test procedure takes account of wind resistance?
Oh! Oh! I do! I do! :shades:
.
.
.
.
http://www.fueleconomy.gov/feg/how_tested.shtml
Because the amount of fuel consumed in the test is NOT meaured with a fuel gauge or flow meter of some sort (the fuel consumed in the test is determined by measuring the amount of carbon in the vehicle's exhaust), the tests can NOT be done on the road.
The vehicle is run on a chassis dyno following a very specific procedure which mandates vehicle speed and rate of acceleration. According to that website, "The energy required to move the rollers can be adjusted to account for aerodynamic forces and the vehicle's weight."
How they determine how to adjust the dyno for aerodynamic drag seems a bit iffy to me, since I don't think they stick the vehicle in question in a wind tunnel to independently determine the coefficient of drag or the vehicles frontal area. They probably just take whatever numbers the manufacturer gives them.
I would think it would be more straight-forward (and accurate) to simply run the vehicles in question over a test road and actually measure the amount of fuel consumed. ... :confuse:
I was recently in the junkyard getting the electric rear window cranks. And I picked up the trip computer for my van.
$10 for both parts.
Simple install, and quickly noted that we only got 16 MPG. :sick: Although it ran fine.
Well with a trip to Orlando in the coming weeks. And $2.79 a gallon for gas. I felt a checkup was in order. The van recently had plugs, oil and filters.
Further investigation found a few small problems.
A new O2 sensor,
proper inflated tires 35 PSI ,
removing the unused roofracks reduced air noise and drag.
I managed to get it up to 18.7MPG. Mostly suburbs style driving short trips.
Orlando came and went. The van was great. 2533 miles.
averaging 25.5 MPG peaking at 27MPG.
This was 5 passengers, fully loaded. Avoiding the unsafe posted speed limit signs.
I found the trip computer to be a great tool.
It allows me to check for instantaneous variations.
If it has been removed on newest vans (as read here)
It's a great loss.
So far the TC has taught me to keep the RPM's low when starting off.
Approx 2K RPM will allow all 4 gears to shift.
After getting up to speed, Let off the gas slightly.
You can keep the same speed, But increase mileage.
Biggest supprise was that drafting behind tractor trailers.
22MPG went to 32MPG instantaneously and consistantly.
Approx 3 car lengths back.
I am looking for further savings.
I want to remove the air intake silencer, next.
Any other suggestions?
By the way lowest priced gas in GA. $1.89 reg
Yes.
Don't draft tractor trailors by 3 car lengths. Since you can't see in front of the tractor trailer, the only warning you'd have if they had to slam on the brakes is his locked up wheels and clouds of tire smoke. You'll eat up that 3 car lengths before you can react by hitting your own brakes.
It ain't worth the extra mpg.
And it generally ticks them off. They cannot see you.
But when they pull into your lane and you see the difference. You realize what a difference air resistance can make.
I wish there was a safe way to utilize it.
If you don't have anything better to do...how about giving us some mpg figures for 5 car lengths and then 8 vagabonebuy?
Caution though...semi's kick up a lot of road debris(pebbles etc). I have some chips in my front hood to prove it.
I cannot vouch for accuracy.
I figure that after 4-5 you lose the draft effect.
I know that you can feel turbulance if your not in the sweet spot.
By 8 it's gone.
Sorry, I wont run tests.
But it was a few minutes of entertainment on a otherwise long boring trip.
Even more now, I see no benefit to DOHC vs. SOHC if each has 4 valves per cylinder and the Honda and Toyota each have 4 valves per cylinder.
I agree with the reader's comment that this discussion is off track. It is still interesting.