eluzion

With our ridiculous gas prices, I figured I would make a post that might help some with improving their fuel economy. There seems to be a common misconception that lower RPMs = less fuel used, so driving at lower RPMs means better gas mileage. False. Let me explain:

This is a fuel map of a 94-01 Acura Integra GSR (P72). With VTEC, the car actually runs two fuel maps (low and high) for when VTEC is and is not engaged. This is just the low fuel map since this is where you should be while daily driving. Ignore the red and green boxes for now.



The left column indicates the engine RPMs and the top row indicates load measured in inches of mercury (in/HG). You can see once you reach 0 in/HG, it reads positive pressure (PSI) which is what we call boost on forced induction applications. The values you see in the table are raw fuel values. Bigger number means more fuel, simple as that. Now take any load row column, such as 12.6 in/HG, and look at the raw fuel values as the RPMs increase. Notice how the change is not very significant? Now take any RPM row, such as 2500 RPMs, and look how much the fuel values increase as the load increases -- significant change.

Here is an example. Driver A drives with a "heavier" foot but keeps their RPMs low. The area of the fuel map that would likely be used from is outlined in red in the picture above. Driver B drives with light throttle and a little higher RPMs. The area of the fuel map that would likely be used is outlined in green. Which person is using more fuel?

This is why stop-and-go (city) gas mileage is significantly worse than cruising (freeway) gas mileage. When you're accelerating, you're venturing into the higher load parts of the map versus cruising which you might be higher RPMs, but are in the lesser load parts of the map.

So what's the bottom line? Light throttle, slow acceleration = the hippies will love you. Heavy foot, fast acceleration = more gas consumed. That is common sense, but what I'm really trying to show here is higher RPMs isn't really what eats up gas. Trying to keep your RPMs really low and you'll have no choice but to press the pedal further to accelerate versus giving your engines a little more RPM and use less smash on the pedal.

Hopefully this makes some sense. I guess it's a lot easier to understand when you are able to drive a car hooked up to a computer with it tracing the exact fuel values you're using.

EDIT: I just realized using in/HG was a bad idea as it might cause confusion. The larger the in/HG value, the lower the load. Basically, look at the fuel map left to right (left being less load to right being more load).

Dads05TL

That makes perfect sense! My old 93 BMW 318 got much better gas mileage at 3K rpm's in 5th gear than 2500rpm's in 5th...but that also had to do with dual intakes, and at 3k the larger intake opened up

mishar

This makes sense, but only if you don’t know too much about internal combustion engines. Things are actually exactly opposite when you are talking about acceleration and very different when you are talking about cruising.

To understand this you need a basic knowledge of physics and internal combustion engine thermodynamics. For the beginning: engine efficiency is higher as average cylinder pressure is higher. So to produce SAME POWER you will use less fuel if you are at lower RPM with more throttle (higher cylinder pressure) than if you are at higher RPM and less throttle (lower cylinder pressure). That solves cruising: higher gear = lower consumption. Extremes excluded.

When we are talking acceleration thinks are a bit more complicated. We should start with basic physics. To accelerate car to the certain speed you need same energy no matter how fast acceleration is. So you need same energy to accelerate from 0 – 60 whether you do it in 6 or 30 seconds. Now back to engine efficiency. To produce that amount of energy you will use less fuel if you do it with more throttle. So, to accelerate 0 - 60 you will use less fuel if you do it in 6 seconds than if you do it in 30 seconds. In the real world the least fuel you will use if you do it with 2/3 of throttle. TL would probably do it in 10 seconds or so. It is same for any acceleration. Do it with 2/3 of throttle and you will save gas. Tiptoeing is waste of gas, time and other people nerves. Period.

eluzion

"To accelerate car to the certain speed you need same energy no matter how fast acceleration is."

No doubt. I guess what I'm getting at is there's a point where too much load at a low RPM will just result in a bunch of unburnt fuel. Like driving up a hill at 40mph in 5th gear compared to 3rd gear. You'll get there in the same amount of time, use the same amount of energy, but at the expense of a higher duty cycle (more unburnt fuel). I guess a hill is kind of extreme compared to driving in a straight line though.

If I drive around with a "heavy" foot (2/3 throttle) every time I accelerate compared to (1/3 throttle), I get far worse gas mileage in the long run. Basically I guess I'm getting at there's a point where more fuel is unburnt fuel is being dumped than is actually being used.

I'm no physics major though but just a limited understanding from observations I made while tuning, so your knowledge is appreciated.

eluzion

Gah the damn 5 minute edit rule.

Let me re-word that sentence.

"Basically I guess I'm getting at there's a point where more fuel is being dumped than is being used (actual energy)".

mishar

It is just the opposite. Running uphill in 5th gear, providing it can maintain speed, will consume less fuel than running in 3rd gear, because engine works at higher pressure thou higher efficiency.

By the way, at lower RPM combustion is better because there is more time for it compared to shorter cycles at higher RPM.

No. It is just opposite. That’s exactly what I am trying to explain here.

I am not major physicist either, but I am automotive engineer.

Timeless

Excellent post! and I agree with both of you. Optimal efficiency is a happy median between having a light foot and a lead foot. I've been having this argument with myself about what the most efficient way to accelerate is and what the ideal engine rpm is depending on what type of driving i'm doing.

That chart is some pretty good info.

eluzion

Even better! Automotive engineer.

But here's the thing, which is why I guess I'm not following along. When monitoring a car (through some type of management system), I've put my car (not the TL, this was older Hondas) through that scenario. Running up a hill in 5th gear at 40mph and the duty cycle of the injectors is way up there (say 60%+) and AFRs are very low. Same scenario in 3rd gear, 40mph, and the duty cycle is far less and the AFRs are much higher. Based on that it's telling me more fuel is being dumped (higher duty cycle), but a lot of it is being unused (low AFRs).

Now I guess what I'm getting from your post is that higher pressure = higher efficiency. Again, I was always under the impression that increased cylinder pressure, to a point is good (maximum brake torque). This has more to do with another post I made in the "What happens if I run 87 octane in my car" about ignition timing and the combustion process.

But again, I'm no automotive engineer. I hope I don't come off as trying to question your knowledge but I'm just trying to understand. Good info though!

TLTrance

In layman's terms, which is not perfect -- but works, I usually keep the MPH/RPM ratio as high as possible, which is what maximizes distance traveled per gallon. And its quite high (around 3.7) around 60-65mph.

This doesn't work all the time, especially if you're coasting downhill -- in this case the engine would maintain RPM and the gas used would actually fall.

01tl4tl

If you are driving up hills in 5th, go read the owner book and rework your charts
Thats behind the torque/powerband, and is hard on the engine
Use 4th - if you need 3rd and are in 5th (overdrive gear) you have serious problems looming~

eluzion

Haha, no worries. It's just an example of a low RPM / high load scenario, not something I actually do.

Surprised you didn't tell me to SeaFoam though. ;P

(but you did mention the owners manual!)

Just giving you a hard time.

mishar

Your scenario is a bit loose. You are mentioning very low A/F ratio. That means you were very close to wide opened throttle. Remember that I said 2/3 throttle. When you get to wide opened throttle A/F ratio is significantly lower in order to get maximum power. On 2/3 or even 3/4 throttle it is not the case. A/F ratio is high for lower consumption. Don’t forget that in 3rd gear you have more cycles than in 5th.

Higher cylinder pressure is good for consumption, but it is limited by detonation. Higher octane fuel is more resistant to it. By retarding ignition and A/F ratio computer can fight it to the point with lower octane fuel. Just remember that it is based on a knock sensor which means that you will have detonation every now and then in order to detect that point. You might not hear it but your pistons will for sure.

Legend2TL

Two factors that will greatly affect the fuel economy of the engine are the pumping looses (losses associated with the throttle, intake plenum, air filter,...) and internal engine friction from moving components. Lower RPM for crusing results in better gas mileage since both losses are less at lower engine speeds. For acceleration this also holds true when you accelerate with heavy (2/3-3/4 throttle) but short shifting at low RPM (2500-3000RPM). This was the basis of a study and "eta" sereies of engines BMW did in the early 80's. So for cruising I select as high a gear as possible but use heavy throttle in acceleration with short shifting. For cruising I on occasion have had 6th gear turning 2K RPM.

THe one problem with the intial graph is that it does not show the potential torque output of the engine at that point. Knowing both the RPM and respective intake plenum pressure is needed to really contrast the two numbers on the fuel map.

Oh one last thing some for those reading this may not know a actual fuel map may have anywhere from 16K to over 1M points on it using the many variables to address and modify the results (RPM, intake plenum pressure, intake air temperature,throttle postion, engine temperature, O2 exhause level,....)

mishar

You are right about short shifting if it is manual transmission. How short depends on engine. TL would be probably round 3000 rpm. General rule is 2/3 throttle to the desired speed and than cruising in highest gear that doesn’t require more than 2/3 throttle.

sr1sws

As Spock would say "Fascinating."

However, I'm not Mr. Spock (or Dr. Spock either) so can youse guys net it out for me?

So, if I get this, I should not drive my '06 TL 5AT in city traffic as if I had a raw egg between the bottom of my foot and the accelerator pedal? I should push the car more to ramp up to speed, but don't exceed 2/3 throttle? Did I get the point correctly? And this will (potentially) yield better gas mileage?

Thanks,
Steve

TeknoKing

^ I love your signature, almost fell off my chair laughing, (3) Custom DVD-As

Forget the gas worries, just enjoy your TL

sr1sws

I love my TL

I like the DVD-As 'cause I haven't seen anyone be real happy with an iPod interface. I did use the USA Spec device in my Explorer - worked fine. In TLs they complain about skipping.

So, I took my favorite play list from the iPod and created DVD-As from the songs.

Steve

Walshstl

Geez I have no idea what you guys are talking about... but I've always wondered:

Is it better to go fast up a big hill to get over it quickly, or keep the rpms lower but take longer to get over the hill?