low RPM + low MPH = bad for car?

NYSpeed Automotive
1

so on the way into work today, I was super low on gas and the closest open gas station was around 10 miles away (car said that I had 22miles until empty)… so anyway on the way there I was driving it super easy… not giving it too much throttle.

when rt.5 dropped down to 40, from 55 I just kept the car in 6th gear at 45-ish mph and it was sitting right around 1K-1100 RPM. I was really supprisind to find that I was getting like 38 mpg on average. Even if I had to give it gas to speed up it was only dropping to around 19-20mpg, which is what I usually get when I drive thru that area normally in 4th gear @ 2200 RPM.

with the price of gas these days… this probably isnt bad practice, but is this bad for the engine or trans by somewhat bogging it? I’m sure nailing the throttle probably isnt the best thing to do without downshifting. I’m also sure that the reading of mpg probably isnt totally accurate, but it’s probably close.

any ideas?

2

As long as you don’t push the accelerator pedal down too far its okay. With the car in say 5th or 6th gear, the engine is turning at very low RPM’s. When you push the gas pedal down too much, the engine cannot take in the amount of air that its trying to force in to it, because its not spinning fast enough.

Some of the airplanes I use to fly had 2 levers in the cockpit. One controlled manifold pressure (throttle), and the other controlled engine rpm. Having the engine rpm lever set in a low rpm configuration, along with a high manifold pressure setting was always considered to be a bad practice.

3

As long as you aren’t lugging it and putting a lot of load, I wouldn’t think it’d be bad.

4

Think about how an automatic works. It will shift into a high gear when you’re at a steady speed, even when you’re going quite slow. But when you give it some gas, it downshifts.

If you do the same in your car, you’ll be fine.

5

umm, how can it hurt it? As far as not being able to move out the air thats being pushed in, thats not the case. The faster a N/A motor turns the more air it SUCKS in. A NA motor SUCKS air. Boosted cars are where air is PUSHED in. Only down side is if the transmission has weaker parts on the higher gears.

6

I always shift up pretty quickly in my saturn.

The thing has an economy light that comes on super early. At 40mph it wants me to lug the car around in 5th gear.

I don’t think its hurting anything.

If your really low on gas, you might want to try rolling in neutral as far as possible, and avoid using the brake.

So long as your rolling down a very slight grade, you can get some insane mileage, as the motor is only spinning at 900rpms.

If your going down a steep grade, then obviously you want it in gear, applying some engine brake.

7

Coasting downhill will actually depend on your cars injection system. I know my DSM while coasting at RPMs above around 1200, the O2 sesons would actually read a voltage of 0, which leads me to believe that the injection actually shut down while coasting with zero throttle input.

However, this may vary car to car.

8

pretty much what they all said, as long as your not putting too much load on it, your fine. i wouldn’t recommend say flooring it at 1000rpms in 6th :stuck_out_tongue:

9

[quote=“Bigairskier1580,post:7,topic:29309"”]

Coasting downhill will actually depend on your cars injection system. I know my DSM while coasting at RPMs above around 1200, the O2 sesons would actually read a voltage of 0, which leads me to believe that the injection actually shut down while coasting with zero throttle input.

However, this may vary car to car.

[/quote]

I’ve noticed the same in my GTO. If I lift off the throttle and coast in gear, the instant MPG thing maxes out, but if I coast in neutral it doesn’t. That leads me to believe there is no gas going through the injectors when coasting in gear, or at a minimum there is less.

10

its either proper amount of fuel or NO fuel

LESS fuel then the engine needs = running uber lean, and thats bad

decel fuel shutoff is a great thing, and the mpg gauge maxes because your using NO fuel, meaning you have an infinite mpg rating

it doesnt do it in nuetral because if it killed the fuel, you would have to restart the car using the key, where as when the ecu kiks out of decel fuel shutoff the engine is still spinning rombeing in gear, apply fuel and spark and the engine is once again running

11

the reason why your consuming more gas in a higher numerical gear at low speeds is becasue you don’t have the mechanical advantage with higher numerical gears when at lower speeds…the motor has to work harder.

more work requires more power. more power requries more fuel.

12

^^
exactly… or to put it another way. low RPMs does not mean high MPG. low load on motor = high MPG.

13

^ Thank you captain science… but he’s using less gas in a high gear at low speeds. :wink:

14

low load on on the motor doesnt mean high mpg either

the best mileage comes from running the car at the most efficient pointpossible

this means the most rpm with the least fuel, least rolling resistance, least wind resistance, least heat loss via the radiator, least heat loss out the exhaust, least power loss in the engine, least power loss in the transmision

this is why some cars get the best mpg at 40, and some get the best mpg at 80 … its all about finding the point where all the factors involved allow the car to use a majority of the combustion to actualy push the car along

15

Since this particular forum is about “Theory and Application” let me try putting this into a situation we can all relate to.

http://photos.e46fanatics.com/data/500/ROTATING_PENIS.JPG

Say there was a giant (100 ft long penis), heavy (3000lbs) green penis and it was rotating around it’s balls at a constant RPM. For dick’s sake, lets just say the tip was moving at a constant velocity of 50mph.

Farther down the shaft, closer to the balls, on that same green penis (about the start of the ‘dorsal’ vein) the speed is only 20mph

The little stick figure is you (the motor), capable of a constant pushing power, trying to keep up with the speed of the penis to maintain it’s constant velocity.

All of a sudden, you decide you want the penis to rotate faster (increase velocity, acceleration, and RPM)

3 scenerios:

(high gear, 6th …at the balls)
equivalent to standing at the balls and trying to roate the penis, it’s going to require a lot more pushing “power” and therefore more energy (or fuel), but you don’t have to run nearly as fast around.

(middle gear, 3nd…blue arrow)
You’re standing at the center of the shaft and it’s easier to accelerate because it requires less torque (you have more mechanical advantage then standing at the balls)…also you’re at a much more comfortable running speed. This requires less power then 6th gear and less engergy consumption

low gear (1st…red arrow)

This is like standing at the cranium of the penis, much easier to accelerate a giant green penis around an axis because of an incredibly long moment arm (high mechanical advantage)… as a result it’s easier to accelerate. HOWEVER, you are human and like a motor, you have a max speed. SO, you try to accelerate, yet you can’t because you have “rev limited”…consuming your energy through speed, not power.

CONCLUSION: Just like your drivetrain, there is an optimal posistion on the hairy green drive shaft (or gear) to push.

16

^awesome

17

nice diagram…

If you want efficiency you should drive by a vaccum gauge to keep your economy up. The vacuum idea was more important with a carburetor, but it still applies.

In the high gear low rpm scenario the engine is moving a small volume of air so you need a very small throttle opening to maintain proper running. Thus you can do this cruising on the flat, but opening the throttle to go up a hill won’t do anything for you.

There are two conflicting factors that affect your overall fuel consumption.

First, at lower rpm the water pump, alternator, ps pump, and other ancillaries are turning at lower rpm using less power (except the alternator, it can compensate). At lower rpm you also lose less energy to friction in the engine.

However, at lower rpm the engine is less efficient. Gasoline engines are typically most efficient at 80% of rated power. That’s why an engine that uses at or near 80% most of the time (think v6 ram) is much more efficient than a vehicle that uses a fraction of rated power most of the time (think Hemi). That’s why an engine large for the vehicle usually gets worse mileage. That’s why a Hemi Ram gets 12 mpg loaded or not. Some people have gotten better mileage by going to 4.56 gears in a Hemi with 20’s. This raises the rpm to make the engine more efficient ( better velocity in the big hemi head ports, better scavenging, yada yada yada).

How these two factors interact will give you your most efficient place to be. For you engineers out there you, if you have the data you can plot hp-hrs/gal against friction loss (typically nearly linear) across the rpm range to get a true representation of where your best net efficiency is.

On flat ground, the vehicle will require X hp for Y speed. If you have multiple gears you can figure the which gear is most efficient for a certain speed and load. If you are going 40 on a flat road at 1100 rpm in sixth you may be a little slow rpm wise. You might be better in fifth, bringing the engine into better efficiency, but it depends on gear spacing. On the other hand you may have increased friction without increasing efficiency. It all depends on your scenario. That’s why the more gears you have, generally the more efficient you are (with weight and cost being the limiting factor).

In terms of the simple answer, if you aren’t lugging the engine and the engine isn’t overheating, it won’t hurt it anything.