YACT: optimal MPG

[RU482]

I was out driving my son (who has been sick) around as a last resort trying (successfully) to get him to go to sleep. I was on a deserted rural road, so I drove by the tachometer instead of the speedometer.

I noticed that at around 42MPH, I could maintain about 1200RPM. this was about as low as I could keep the RPM and maintain speed in top gear (other gears were just too damn slow).

It got me to thinking...was I getting the vehicle's (03 Pontiac Montana van) optimal MPG.

to elaborate...in absense of wind resistance, is RPM the macro determining factor in how many Miles per gallon I can achieve?

just thinking

 

[CycloWizard]

Not really. Keeping RPMs low keeps the power output of the engine lower, thereby decreasing the amount of fuel required to supply that power. However, power equals RPMs times torque, where torque typically depends on the RPMs and other factors. I believe this is why the speed limit was set to 55 mph in the US in the 60's-70's, because this was the optimum mileage speed for most vehicles.

So, I guess the answer is RPMs have an effect, but they're not the whole story. As always, I'm sure someone here can give a better explanation.

 

[Sahakiel]

If I remember correctly, government imposed speed limit to cut down on gas usage.

 

[The Boston Dangler]

Most US vehicles get their optimum fuel efficiency at about 55 mph because designers shoot for that target.
The speed limit was set to 55 mph because of the big "death on the highway" campaign.

Using minimum revs in top gear will usually bring good efficiency, but the amount of throttle used is a big factor too. Because the equation isn't constant, it's possible to get better efficiency in a lower gear sometimes, i.e. going up a large incline, in top gear, you need to use more throttle to keep the motor in the power band. In a lower gear, the revs would be higher, but the throttle lower.

Constant speed is also key to maximum mileage.

 

[fbrdphreak]

Using minimum revs in top gear will usually bring good efficiency, but the amount of throttle used is a big factor too. Because the equation isn't constant, it's possible to get better efficiency in a lower gear sometimes, i.e. going up a large incline, in top gear, you need to use more throttle to keep the motor in the power band. In a lower gear, the revs would be higher, but the throttle lower.

Winnar!

I found this out first hand with my car. A '93 Concorde, it has a 3.3L V6 that is all low-end torque (and little at that). Forever I would just accel to like 2000-2200RPM in each gear and when I hit 3rd & 4th I would give it more throttle to keep going. Revving to 2500-3000 in 1st and 2nd with lower RPMs/throttle in 3rd/4th gives me noticeable better gas mileage and better acceleration.

It is definitely a combination of throttle and RPM, the optimal amount changes depending on your engine/tranny. Mine is all low-end torque, but a high-tuned V6 or I-4 will have to be driven differently. And of course a manual changes things, makes it a lot easier!

 

[deveraux]

I remember performing some experiments on simple motors for power, torque and efficiency. It was quite a while ago so I can't remember the exact results. But I vaguely recalled that there is an optimum RPM for the engine itself which is irrespective of gear ratio's. And to reduce MPG, it usually involves accelerating to that RPM in the fastest possible manner to reduce throttling. Of course, that was using a carburetor powered engine IIRC, so won't really apply directly to EFI engines.

I think that max efficiency is around 2500 RPM (I could be wrong though).

 

[CrispyFried]

Yes, all gas engines have a sweet spot. The sooner you get there and the longer you stay there the better the mpg. If you have a fuel data center (MPG display) you can play around and find it. To a point anyway. My V8 has lots of low end grunt so it like the low rpm, high gear situations.

Also changes in speed are the killer. Cruise control is the best MPG getter around, unless its set to 120 mph

 

[BDawg]

Doesn't your speed play a role too? Sure, you're revving high at 5000 rpm, but if you're going 120 mph, that helps your mpg.

 

[deveraux]

Speed is a very relative term. The component that actual uses fuel is your engine. And your engine doesn't really care what the external speed is. It only cares about what torque its producing. Therefore, I think the most important attribute is the engine's RPM. The biggest role I can think of that speed plays is with wind/air resistance (I could be wrong though -- someone might come up with a better explanation for speed).

 

[infosponge]

One thing that seems to be overlooked so far is engine LOAD. That is "the" basis for fuel economy. Everything else comes into play along with engine load, but it is the amount of load that has the most dramatic impact in an automotive environment. Load consists of many variables...inclines, running the a/c, vehicle weight, inconsistant pedal travel(if ya mash the gas the engine is being loaded), wind resistance, etc...etc... The less an engine is loaded, the more horsepower there is available to motivate the vehicle, therefore less fuel is required to maintain the desired speed. A very informative approach to this subject is to look at the variety of sensors typically used to control engine parameters. Airflow sensors--MAP or MAF, temperature sensors--oxygen, IAT, CTS. 'could be many more depending on the system being studied, all allow the Electronic Control Module to "see" what the engine needs/wants.

MAP --manifold absolute pressure
MAF-- mass air flow
IAT--inlet air temp.
CTS--coolant temp. sensor

 

[fbrdphreak]

Well, the more resistance your car faces the more fuel your engine will need to use to produce the same amount of work to keep the car at a certain speed. The engine will produce the same torque at a given RPM regardless of wind resistance, but the car will go at a slower speed due to the increased resistance.
Thus if you have to use more fuel to keep the same speed, of course your MPG will go down.

Sure, you're revving high at 5000 rpm, but if you're going 120 mph, that helps your mpg.

You're on the right track. I would say it is more correct to say that at a higher speed, you end up spending less time driving to your destination and less time using gas. So a higher speed is beneficial to a degree, but as previously discussed the optimal travelling speed is like 55-60.

 

[fbrdphreak]

Originally posted by: infosponge
One thing that seems to be overlooked so far is engine LOAD. That is "the" basis for fuel economy. Everything else comes into play along with engine load, but it is the amount of load that has the most dramatic impact in an automotive environment. Load consists of many variables...inclines, running the a/c, vehicle weight, inconsistant pedal travel(if ya mash the gas the engine is being loaded), wind resistance, etc...etc... The less an engine is loaded, the more horsepower there is available to motivate the vehicle, therefore less fuel is required to maintain the desired speed. A very informative approach to this subject is to look at the variety of sensors typically used to control engine parameters. Airflow sensors--MAP or MAF, temperature sensors--oxygen, IAT, CTS. 'could be many more depending on the system being studied, all allow the Electronic Control Module to "see" what the engine needs/wants.

MAP --manifold absolute pressure
MAF-- mass air flow
IAT--inlet air temp.
CTS--coolant temp. sensor

:thumbsup:

 

[Calin]

Originally posted by: deveraux
I remember performing some experiments on simple motors for power, torque and efficiency. It was quite a while ago so I can't remember the exact results. But I vaguely recalled that there is an optimum RPM for the engine itself which is irrespective of gear ratio's. And to reduce MPG, it usually involves accelerating to that RPM in the fastest possible manner to reduce throttling. Of course, that was using a carburetor powered engine IIRC, so won't really apply directly to EFI engines.

I think that max efficiency is around 2500 RPM (I could be wrong though).

This great improvement came from the very fact that the carburetor is designed with a certain airflow in mind - the air flow that gives it the maximum efficiency (or the best fuel atomisation). In an gasoline engine the only fuel that is useful is the one vaporised or in very small droplets, bigger droplets don't burn completely.
The fuel injected engines give about the same fuel atomisation in most every conditions, and the oxygen sensor helps decrease the unburnt fuel.

 

[Calin]

Originally posted by: deveraux
Speed is a very relative term. The component that actual uses fuel is your engine. And your engine doesn't really care what the external speed is. It only cares about what torque its producing. Therefore, I think the most important attribute is the engine's RPM. The biggest role I can think of that speed plays is with wind/air resistance (I could be wrong though -- someone might come up with a better explanation for speed).

There are many things that increase drag when speed increase - air resistance, all the rotating parts connected to the wheels. There is also tires resistance (however, how this changes with speed I don't know. Anyway, stiffer tires or higher tire pressure is better for fuel efficiency - that's the reason trains have iron wheels).
Usually the best fuel efficiency of an engine is at the RPM where the highest torque is (but there might be many exceptions). The main idea is to keep all the rotating parts at their minimum speed (so use highest gears and lowest RPM possible), and hope that the engine's efficiency at that RPM is good enough

 

[joe4324]

I know there are many factors involved here, I once read up on it quite extensively, and unfortunately it confused me even more then it cleared things up!

You have the factors of vehicle weight (load) engine type and design (optimal operating effeciency) Drag characteristics, Incline, etc, etc.

I know that many cars on the road now, actually could possible reduce MPG by sticking to 55, many cars now actually are so performance tuned/designed that they reach there peak effeciency MUCH faster then that, I beleieve a new Porche 911 was somewere around 88mph, a jetta desiel sedan was 74mph? (this is from a recent article in popular mechanics)

I wonder if there are ways to test this, I wonder what my 93, toyota pickup's optimal speed and rpm is, Or my wifes Saturn SC2, I also wonder what modifcations I can make, will increase my toyota's abysmal mpg (16mpg..gah) Would a aftermarket intake increase power/effeciency? without sucking more fuel? what about a electric fan?

so many questions!

 

[fbrdphreak]

I wonder if there are ways to test this, I wonder what my 93, toyota pickup's optimal speed and rpm is, Or my wifes Saturn SC2, I also wonder what modifcations I can make, will increase my toyota's abysmal mpg (16mpg..gah) Would a aftermarket intake increase power/effeciency? without sucking more fuel? what about a electric fan?

I think the biggest issue you're facing with your pickup is aerodynamics. Trucks/SUVs lose a substantial amt of mileage due to size & weight, then of course you have to have a bigger engine to move that larger vehicle. I would honestly say there isn't much to do about your Toyota. A new intake will increase the amt of power the engine makes at a given RPM, but probly won't even give you 5% IMO, unless you do an exhaust system also & probly some other mods.
The engine is a bit old, a rebuild would probly help but I doubt would be worth the money it would cost.

 

[Concillian]

Originally posted by: joe4324
I know there are many factors involved here, I once read up on it quite extensively, and unfortunately it confused me even more then it cleared things up!

This is accurate. There are a HUGE number of factors involved.

I was part of the super high milage vehicle team of the SAE club (Society of Automotive Engineers) when I was in college, so I feel somewhat qualified to discuss the subject, though our vehicle conditions were way different than normal usage (we only had to maintain 15 MPH, and our frictional losses due to wind resistance and rolling resistance were far less than a real car).

stepping on the gas does NOT control gas. It controls the amount of resistance/restriction that air has to go into the engine. Gas and air are not forced into the engine, air is 'let' in because the combustion process draws a vacuum on the intake side. Gas is metered in with the air depending on how much air is being 'let' into the engine.

Because of this you can't compare throttle positions to figure your ideal MPG.

The engine also has an efficiency curve. Efficiency changes (for a given engine design) primarily as a fuction of load on the engine and RPM. It's a full surface, with changes due to both factors. Volumetric efficiency is generally highest at the torque peak, and will fall off considerably above that. Lower RPMs offer lower volumetric efficiency, but there is a large advantage in terms of wasted energy to offset the reduction in efficiency.

Wasted energy in an engine is partially due to frictional losses, but some of the largest losses are due to the piston velocity. The pistons accelerate and decelerate. It takes a lot of energy to reverse the direction of pistion travel, especially when they are being reversed in direction several thousand times per minute.

Another factor is the mixture of Air to Fuel (AFR or Air Fuel Ratio). At high load, you can induce pre-ignition in the engine. At high loads, engine computers will enrich the AFR to provide a cooling medium (fuel used exclusively as a coolant). Stoichiometric ratio is 14.7:1, but best power is generally acheived on normally aspirated engines at around 12.5:1 (12.5 parts air to 1 part fuel). This means that a certain amount of fuel is quite simply thrown out the exhaust because there is not enough air to react with it. So running in a loaded condition may cause a fairly severe penalty in terms of fuel efficiency. Power efficiency and fuel efficiency are not the same.

Now getting away from theory for a while, we did several tests with our "car" (it didn't look much like a car) as far as how to program the fuel curves and how best to drive it. Again not a direct comparison to a real car because we had complete control over the fuel curves in the engine computer.

Basically we got a several hundred mile per gallon gain by going flat out pedal to the metal then shutting off the engine and coasting. So we had to maintain 15MPH average, but we would run it full bore up to 30-35 then coast way down slow. This was FAR more efficient than any constant usage scenario we had devised. I say several hundred MPH, that got us from in the 400s to in the 700s, but in the 700s we were still well behind the leaders who were in the 1000 range. It's important to understand that our fuel maps and engine were also optimized for fuel efficiency and gave up a significant amount of power to get that efficiency.

By the way the engine we had to use were handout 1HP Briggs and Stratton lawnmower engines. But we could modify them ANY WAY we wanted as long as the original case was used. We replaced every damn piece of that engine and bored a hole through where the original cam was and ran a dual overhead cam setup on a hand-made aluminum head and fuel injection. Those were some really sweet times spending my weekends at the machine shop on campus grinding different cam profiles by hand (we were quite low budget, no CNC for us... our yearly expenditures were on the order of a few hundred bucks for raw materials, most of our stuff was donated.)

Even though the cars are not directly comparable, I believe the same thing to be true in terms of how fuel efficiency can be gained. For a given engine design, you make a big difference just in behavior of driving.

For example, I see people lugging the engine up to speed on the freeway very slowly. They are running the engine under more load and at low speed. Generally it's best to be at a medium load, get up to speed and cruise. And try to anticipate slowdowns so you don't brake too much. Braking is pure waste energy. Realistically though, with a given engine design you aren't going to change things drastically with behavior given the constraints of normal traffic (you can't go through traffic speeding up to 80, then coasting down to 30, for example.) You'll get a couple MPG tops, but not a 50% improvement or anything.

 

[Concillian]

Originally posted by: joe4324
I also wonder what modifcations I can make, will increase my toyota's abysmal mpg (16mpg..gah) Would a aftermarket intake increase power/effeciency? without sucking more fuel? what about a electric fan?

Most aftermarket type modifications will only imporve consumption under heavy load. Most driving is not at heavy load. As a result most aftermarket modifications won't affect fuel economy very much (unless you drive at high load often).

I mentioned AFR becoming richer as load is induced. In production vehicles this is true to the point of over-doing it. Pre-ignition is very bad, and can cause significant engine wear and tear. Manufacturers wiill run slightly over-rich in order to provide additional margin against pre-ignition.

an intake will sometimes be made in such a way to change the airflow around the air sensor in the intake so that there is an offset in AFR... the car runs leaner, closer to ideal, and power and fule efficiency are both gained. But you are potentially losing margin. Not all intakes work like this, but some do.

But the important thing to realize for an intake is that there can only ever really be an volumetric efficiency benefit at WIDE OPEN Throttle (or very near it). Otherwise there is very little change in efficiency... I mean, when you are not at WOT, you are purposefully restricting the intake. Your foot controls a plate in the throttle body that opens more the further down you press the gas. If you press the gas just a little, then you only just crack that plate open, presenting a large restriction on the intake anyway... much larger than whatever restriction the intake piping and air filter provide. Improving efficiency of the intake tract only helps when your piping and filter are a significant restriction, and that is only the case at or very near WOT. If you examine your driving style, it's probably not hard to tell that you won't get a huge gain in MPG by these kinds of mods, merely because most people aren't at WOT more than a few percent of the time.

They are generally single digit gains at WOT. Multiplied by the percentage of time you're actually at WOT and it becomes negligible. More of the fuel economy gain from intakes comes from the induced inaccuracy of air readings that change the AFR and make the engine run leaner. But this is also only for a small fraction of total operation time, and also a small percentage gain.

 

[infosponge]

Just another quick note on mods for MPG. If you have plenty of money to throw at the subject( your toyota) then you can expect to make "some" gains in MPG. This is an efficiency game here. Most notably, how you fill the cylinder, then how you empty it. i.e. cam/valve timing. Valve overlap is the period in which both the intake and exhaust valves are open at the same time to allow for cylinder scavenging. What happens is the outgoing exhaust gases' velocity creates a vacuum in the cylinder. When the intake valve is opened, the air is drawn in and immediately begins to fill the space that has just opened due to the exhaust gases having left. Technically, what's taking place is the pressure is being equalized within the volume of the cylinder. Anyway...if you concentrate on optimizing the cam/ valve timing events, some noticable improvements can be made.
But, as impressively stated by concillian, the gains per dollar spent, will be small. If money IS an object, the best thing you can do is attempt to find out where where in the rpm band, that your engine is at its peak efficiency, then change the gears in the rearend, to keep the engine nearest that rpm point without going over it, while at highway speed--if highway speed is in fact where you spend most of your time driving. Doing this and attempting to make the exhaust more free-flowing(to a point) will give you a decent boost in MPG. What would I expect: somewhere between 5 and 15 MPG depending upon what your setup is now.

 

[RU482]

Originally posted by: fbrdphreak

Originally posted by: infosponge
One thing that seems to be overlooked so far is engine LOAD. That is "the" basis for fuel economy. Everything else comes into play along with engine load, but it is the amount of load that has the most dramatic impact in an automotive environment. Load consists of many variables...inclines, running the a/c, vehicle weight, inconsistant pedal travel(if ya mash the gas the engine is being loaded), wind resistance, etc...etc... The less an engine is loaded, the more horsepower there is available to motivate the vehicle, therefore less fuel is required to maintain the desired speed. A very informative approach to this subject is to look at the variety of sensors typically used to control engine parameters. Airflow sensors--MAP or MAF, temperature sensors--oxygen, IAT, CTS. 'could be many more depending on the system being studied, all allow the Electronic Control Module to "see" what the engine needs/wants.

MAP --manifold absolute pressure
MAF-- mass air flow
IAT--inlet air temp.
CTS--coolant temp. sensor

:thumbsup:

hey...it just so happens I have an OBDII reader that interfaces with my PC. I'm having some issues with the com port and the software I want to use, but as soon as that gets ironed out, I'll have to do some logging

 

[RU482]

Originally posted by: joe4324
I know there are many factors involved here, I once read up on it quite extensively, and unfortunately it confused me even more then it cleared things up!

You have the factors of vehicle weight (load) engine type and design (optimal operating effeciency) Drag characteristics, Incline, etc, etc.

I know that many cars on the road now, actually could possible reduce MPG by sticking to 55, many cars now actually are so performance tuned/designed that they reach there peak effeciency MUCH faster then that, I beleieve a new Porche 911 was somewere around 88mph, a jetta desiel sedan was 74mph? (this is from a recent article in popular mechanics)

I wonder if there are ways to test this, I wonder what my 93, toyota pickup's optimal speed and rpm is, Or my wifes Saturn SC2, I also wonder what modifcations I can make, will increase my toyota's abysmal mpg (16mpg..gah) Would a aftermarket intake increase power/effeciency? without sucking more fuel? what about a electric fan?

so many questions!

the bad thing, I assume, is that these figures change over the life of the vehicle and it's sensors and drivertrain

 

[RU482]

hmmm, anyone want to poke a stab at what RPM is the most efficient?

EDIT: So let me get this straight, to maximize fuel economy, I should accelerate as fast as possible to just above my desired cruising speed, and then let off to a realistic cruising RPM and set the cruise control.

 

[Concillian]

You should accelerate at a reasonable rate, not floored, but not lugging the engine. Faster than most. If you accelerate too quickly, the fueling will switch into a 'power mode' and the AFR will go richer. You want to accelerate quickly, but not switch the computer over to giving you rich fueling.

But yes, essentially accelerate briskly to cruising speed, then maintain is about all you can do.

Your efficient RPM is somewhere under 4000. (yeah, that helps )
Given the torque curve, you're probably pretty efficient down pretty low, but I assume your vehicle has significant weight too. Curves like this have so much 'averaging' that you can't really tell a whole lot from them. Just go as low as you can that the engine doesn't feel like it's struggling. If the car starts shaking, like it's really struggling, then you are too low. A little above that point should be the most efficient for 'cruising' speed.

 

[Calin]

Originally posted by: joe4324
I know there are many factors involved here, I once read up on it quite extensively, and unfortunately it confused me even more then it cleared things up!

You have the factors of vehicle weight (load) engine type and design (optimal operating effeciency) Drag characteristics, Incline, etc, etc.

I know that many cars on the road now, actually could possible reduce MPG by sticking to 55, many cars now actually are so performance tuned/designed that they reach there peak effeciency MUCH faster then that, I beleieve a new Porche 911 was somewere around 88mph, a jetta desiel sedan was 74mph? (this is from a recent article in popular mechanics)

I wonder if there are ways to test this, I wonder what my 93, toyota pickup's optimal speed and rpm is, Or my wifes Saturn SC2, I also wonder what modifcations I can make, will increase my toyota's abysmal mpg (16mpg..gah) Would a aftermarket intake increase power/effeciency? without sucking more fuel? what about a electric fan?

so many questions!

You could improve the aerodinamics by using a "soft top" for the pickup bed. Preferably one that goes from the cabin height near the cabin to bed margin height at the end. A "waterdrop" shape would be best, but it isn't easy to obtain.
An electric fan would help somewhat decrease the power sucked - however, it might need a bigger radiator. Anyway, the power rating for a cooling fan isn't very much, and it will reduce the consumption at most in the same ratio as the fan motor power/pickup engine power ratio.
Use high air pressure for the tires, decrease the load as much as possible, brake early, accelerate slowly. Change (if needed) the oxygen sensor. Have the sparks adjusted. This all modified intakes/exhausts are for other reasons (like better airflow at max RPM, and better sound output).

 

[Calin]

Originally posted by: infosponge
Just another quick note on mods for MPG. If you have plenty of money to throw at the subject( your toyota) then you can expect to make "some" gains in MPG. This is an efficiency game here. Most notably, how you fill the cylinder, then how you empty it. i.e. cam/valve timing. Valve overlap is the period in which both the intake and exhaust valves are open at the same time to allow for cylinder scavenging. What happens is the outgoing exhaust gases' velocity creates a vacuum in the cylinder. When the intake valve is opened, the air is drawn in and immediately begins to fill the space that has just opened due to the exhaust gases having left. Technically, what's taking place is the pressure is being equalized within the volume of the cylinder. Anyway...if you concentrate on optimizing the cam/ valve timing events, some noticable improvements can be made.
But, as impressively stated by concillian, the gains per dollar spent, will be small. If money IS an object, the best thing you can do is attempt to find out where where in the rpm band, that your engine is at its peak efficiency, then change the gears in the rearend, to keep the engine nearest that rpm point without going over it, while at highway speed--if highway speed is in fact where you spend most of your time driving. Doing this and attempting to make the exhaust more free-flowing(to a point) will give you a decent boost in MPG. What would I expect: somewhere between 5 and 15 MPG depending upon what your setup is now.

I don't think doubling the mpg is possible without big changes in engine. While the exhaust improvements are good, they won't help so much