Consumption at low engine revs.

[1.4 hdi men]

First. Greetings all. I found this forum and though not in my language try to expose my query. I hope you understand me

Is it true that the use of a very low engine speed is counterproductive to fuel consumption with respect to use more speed?. Sorry for my English. (I'm Belgian). My question is based on my car consumes less so in fourth gear to fifth cruise, when I have the feeling that in fifth gear would consume less. Thanks for your time in this great forum.

[Gealii]

in my own thoughts it depends on how low your engine speed is if it is so low that your engine is lugging then I could see it being counterproductive but as far as my car you wanted to be in the highest gear possible if holding a steady speed without the engine lugging.

[1.4 hdi men]

Originally Posted by Gealii

in my own thoughts it depends on how low your engine speed is if it is so low that your engine is lugging then I could see it being counterproductive but as far as my car you wanted to be in the highest gear possible if holding a steady speed without the engine lugging.

That was my theory, the ideal point is to use cruise top gear that allows traffic in a minimum revs where the engine does not vibrate, pressing the accelerator slightly. In my case the fifth with 43.7 km / h per 1000 revs will be too long to move above the minimum engine speed at 50 km / h, where the fourth is above 1200 rpm. (Below 1200 revs, appears a slight vibration in fifth and consumption grows in fifth).

[ecomodded]

I find 5th gear to be the most economical, but i have heard that 4th gear at slower speeds is more economical. Could be that high gear put added strain on the motor / drive-train and robs some hp / torque causing it to use more fuel to do the same work.

[1.4 hdi men]

Originally Posted by ecomodded

I find 5th gear to be the most economical, but i have heard that 4th gear at slower speeds is more economical. Could be that high gear put added strain on the motor / drive-train and robs some hp / torque causing it to use more fuel to do the same work.

In my experience, I confirm: In the fourth of about 50 km / h my instantaneous consumption ranges (a steady speed) 2.5 - 2.7 Km Lts/100 But at the same speed in fifth, no less than 3. And I do believe that in my case, it happens as you say, the fifth steals "too much" power to the motor (more power than in the fourth). I guess it's like riding a bike a pinion too high forces us to give more force than is reasonably necessary for driving at sustained speed.

[1.4 hdi men]

Originally Posted by ecomodded

I find 5th gear to be the most economical, but i have heard that 4th gear at slower speeds is more economical. Could be that high gear put added strain on the motor / drive-train and robs some hp / torque causing it to use more fuel to do the same work.

In my experience, I confirm: In the fourth of about 50 km / h my instantaneous consumption ranges (a steady speed) 2.5 - 2.7 Km Lts/100 But at the same speed in fifth, no less than 3. And I do believe that in my case, it happens as you say, the fifth steals "too much" power to the motor (more power than in the fourth). I guess it's like riding a bike a pinion too high forces us to give more force than is reasonably necessary for driving at sustained speed.

[user removed]

The vibration you feel is the engine going below its best minimum RPM for the load applied whne you are in 5th. My Civic VX would do 1000 RPM in 5th just fine but it was designed to run at that low RPM. If you are recording better FE in 4th then 1200 is probably your best minimum RPM. That may change depending on the vehicle load, like when you are climbing or descending a grade (small hill). At too low RPM an engine will loose the harmonics of the induction pulse, which hurts the AF mixture and efficiency.

regards
Mech

regards
Mech

[1.4 hdi men]

Ok, I understand, definitely resolved. Thank you all. Clearly, the minimum speed of my motor is a 1250 for fourth gear, and 1500 revs to fifth gear. If I circle in fourth gear over 1250, my instantaneous consumption rises. Also rises if in fifth I circle below 1500 rpm.