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-   -   How to slow down? (https://ecomodder.com/forum/showthread.php/how-slow-down-27454.html)

krrcan 11-05-2013 04:16 AM

How to slow down?
 
I often find myself in the situation where my engine does 3000rpm, and I want to shift up to reach the sweet spot at around 2300-2700rpms.

But I also want to drive slower, to decrease aerodynamic drag!

How to drive slower, in a efficient way?
Should I just drive at 2300-2700 in third gear, and stick to the speed that gets me?

RedDevil 11-05-2013 04:46 AM

In a manual?
Never mind the sweet spot, select the highest gear you can and drive as slow as you dare.
The sweet spot is important under high load. Under light load more RPM just means more engine drag.

My 1.3 liter Insight has a CVT, but it runs at just 1800 RPM at 90 km/h and 60-70% engine load.
Would it rev to 3000 RPM to do the same then it would have about 40% engine load and lose al lot of power to pumping losses and friction. No doubt the same is true for manuals.

Buy an UltraGauge or ScanGauge so you can see the instant fuel consumption while maintaining a steady speed. No doubt you will see it improve with the higher gears.

MetroMPG 11-05-2013 10:55 AM

Highest gear, lowest reasonable road speed.

People often mistake the "sweet spot" on the BSFC (Brake Specific Fuel Consumption) map as meaning that's where they should always try to operate the engine.

Not so. As RedDevil explains, that's where you want to operate the engine when moderate/heavy load is required (ie. when accelerating or climbing a steep grade).

But not when cruising at light load. Go for the highest gear/lowest RPM.

krrcan 11-05-2013 02:33 PM

Thanks guys for clarifying this for me.
Indeed I have mistaken the "sweet spot" on the BSFC.
I'll adjust my driving accordingly.

But it's better to accelerate to say.. 70km/h, and then do a EOC down to 40km/h and repeat.
Rather than cruising with low RPM on a high gear constantly on 50-60km/h?

I have some places in my regular commute where EOC is bothersome though, I'll let the engine hum along nicely at those parts then. :)

euromodder 11-05-2013 04:17 PM

Quote:

Originally Posted by krrcan (Post 398349)
But it's better to accelerate to say.. 70km/h, and then do a EOC down to 40km/h and repeat.
Rather than cruising with low RPM on a high gear constantly on 50-60km/h?

You're describing the Pulse (accelerate) and glide (EOC) technique.
Surprisingly, P&G burns less fuel than running at a constant speed.

The benefit is reduced when the car is already quite efficient @ constant cruising speeds though - think direct injection diesels.

Quote:

I have some places in my regular commute where EOC is bothersome though, I'll let the engine hum along nicely at those parts then. :)
P&G is a hassle ...
Not all cars take it easily - some don't like being bump started, steering can get very heavy, the ignition key system might be weak ... my Volvo had all 3 issues.

Keeping the engine running reduces the effectiveness of the technique - but you might still see a positive effect.

euromodder 11-05-2013 04:27 PM

Quote:

Originally Posted by krrcan (Post 398288)
How to drive slower, in a efficient way?
Should I just drive at 2300-2700 in third gear, and stick to the speed that gets me?

The idea is that you use the sweet rpm range while accelerating, and in top gear, if possible (that is : if it doesn't mean you'd be going too fast).


You may have noticed that it's possible to get more or less the same fuel consumption at different speeds, despite being in the same gear ?

You might get a nice fuel consumption at a lower speed, and lower rpm than the sweet spot - then get the same fuel consumption at a higher speed but with rpm in the sweet spot.

That's when the better BSFC offsets the extra drag due to higher speed.


With the up! , I need to be around 2500 rpm on the motorway - that's around 85 - 90 kph.
Going slower doesn't mean the fuel consumption drops along with the speed.
FC might actually go up.

If I can follow a truck, I can go slower and have better fuel consumption as there is less drag to overcome, even at the distance that I follow.

As it's colder now, strangely enough I can't go as slow as I used to be able to in summer (warmer = lower density air = less drag).

roosterk0031 11-05-2013 05:06 PM

SentraSE-R experimented with different loads with engine off P&G, and if you don't do it right, almost no savings, at 70% load during accelerating, vs just leaving it in 5th gear at the same average speed his MPG was practically the same at 55mpg(32.5 mph). At 82% load 66 mpg, and 90% load 72 mpg.

Depends a lot on the car, only way to find out is to try yourself using others experience to save some time.

http://ecomodder.com/forum/showthrea...d-19594-2.html


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