Why no fast acceleration? - Page 5 - Fuel Economy, Hypermiling, EcoModding News and Forum

renault_megane_dci · 11-05-2015, 06:52 AM

Hi,

Interresting reading all along.

I think power runs are made at WOT.
I think best FE is around peak torque.
That is the conjunction of peak torque and WOT.
This calls for P&G on its own.
But also that acceleration implying less pumping loss are inherently more efficient.

Now that is for gas engines.

Diesel engines I'm not so sure.
Didn't find the proper way to use mine so far.
A very efficient drive to holidays last year (4.3 liters for 100 km between 70 to 76 mph on the highway) had to be totally ruined the day after when the car really lacked power to even pull out of its parking space and I ended up driving it at peak torque for the rest of the tank to get decent performance back.
It is a 150 000 miles car so must be carboned up badly already ...

Angib · 11-05-2015, 06:53 AM

It is amazing how many people think that pressing the accelerator pedal less means higher fuel economy.

If you need energy, for example to reach your chosen cruising speed, then you should buy it on the best terms possible, when the engine is operating as near peak efficiency as possible. As the graph shown a long way above demonstrates, a petrol/gas engine is nowhere near peak fuel efficiency with the throttle nearly shut, whatever the revs. So the most fuel-efficient acceleration is with the throttle wide open at revs up to about peak torque. I was a kid when BMW confirmed this by measurements in real-world vehicles.

Another tip is don't get a Scanguage, as you can end up minimising current fuel economy which may increase trip fuel economy for exactly the same reason.

Ultimate (closed track) fuel efficiency competitors never operate a petrol/gas engine at part throttle openings: they use 'coast and burn', accelerating at maximum throttle up to a (low) maximum speed and then coasting with engine off until they have slowed to the minimum speed that will enable them to achieve the minimum average speed for the competition. And if they can get over 1000mpg (yes, 1000, not 100) from their specially-built competition vehicles, this behaviour is not because they don't know how to drive economically.

Acceleration is like removing a Band-Aid - all kids start out thinking it will hurt less to peel it off slowly, but all adults have learnt that minimising the duration of the pain is better.

As renault_megane suggests, the picture is nowhere near as clear with diesel engines, as they have reasonable part-load fuel efficiency.

ProDigit · 11-05-2015, 07:03 AM

My take would be, to stay on the torque band, as the most efficient means of acceleration.

It all depends on how fast you need to accelerate.
If you don't need to accelerate fast, usually normal acceleration is the best.
On most cars that means, keep the tach betqeen 2 to 3k rpm, with reason that the sooner you shift into higher gear, the better your mpg is, save for when you're lugging the engine (eg:flooring it in 5th at 1200rpm).
There are a lot of parameters that will determine the car's best acceleration, like weight, engine size and type, transmission, length of gears, how fast you need to accelerate, and from what speed to what speed, etc...

But I believe most of this is true for most econo cars sold today:

If you happen to have a cvt car, and the engine revs from 1500 to 7500rpm, and has its torque band between 3.5 and 4k rpm, powerband at 6k rpm
Your fastest acceleration will be at 6k rpm, but your most fuel efficient acceleration will be around 3.5 to 4k rpm.
Your cvt will best stay at those rpms, but might go higher when you need faster acceleration than the engine's optimal acceleration.
With everything, there is a maximum efficiency zone, and a power zone.
Even with the human body.
If you ever needed to manually transport and stack a pile of wood from a lower floor, to a higher floor, you'd know there are 3 ways on doing it.
1- either every day you take a log of wood up the stairs, but that would mean lots of walking.
2- Or, you take as much as you can carry, but you might get burned out of energy before the stack is halfway.
3- or, you take a comfortable amount of wood, knowing that the body uses energy going uphill, but replenishes energy as you walk back down, at a rate enough for you to take a small stack back up.

The latter part will be your efficiency range. The 2nd example, your power range.

Just like with the removal of the bandaid example, you can take long time, and long time pain, normal, or short intense pain, with the possibility of tearing open the wound, applying more force than the newly skin can handle.
The normal way, is the way to go.

I also believe that on most econo cars, acceleration should take up 50-80% of the engine's power (or 50-80% throttle).

For a faster, or slower acceleration, usually mpgs suffer, faster acceleration it suffers most. Slower acceleration it only suffers for staying in a lower gear for too long.

If we exaggerate, say you have a 1 mile stretch, and accelerate very slow, to the point of only getting to third gear by the mile, that means you've spent the majority of the length of the road, as well as the majority of the time, riding in 2nd or 1st gear!
We all know this to be very inefficient.
You have to accelerate normal to a little peppy, but not overly hard, to get to final gear.
Once you reach cruising speed, be it 30, 40 or 80 or whatever mph, you should get an as high as possible gear, without lugging the vehicle.
In most cases, 30mph is too slow for 6th gear, and depending from car to car, 5th gear might still be lugging at 1200-1500 rpm.
But if fifth gear runs smooth at 1500rpm, keep it in fifth. Otherwise 4th.
If your cruising speed is 40mph, it would mean on most cars that you can get into 6th gear, without problems.
Should you, for whatever reason, need to accelerate rapidly from 40mph to eg:75mph, on most economy cars, it would be best to shift to 4th gear, at around 3k rpm, and shift to fifth at 4k rpm, and accelerate to cruising speed before shifting to 6th, as your car will accelerate faster than one gear higher at a lower rpm, thus shorten the time mpg drops. However third gear might be too high in rpm, and mpg might suffer, as high rpm always include greater friction losses.
Should you need to accelerate moderately to eg: 55mph, 5th gear would probably be the best gear.
Should you need to accelerate slowly (eg: to 45mph), then staying in 6th is best.

I want to address early replies:
Flooring (wot) is NOT the solution to better mpg, explained below.

A modern car built after 2008, most of the time runs lean. That means a 16:1 ratio of air/fuel. This is because of emissions. The fuel burns more clean, and the engine runs more efficient (better mpg).
Should you need to accelerate fast, the fi system will shortly boost fuel to a 14.5:1 ratio, or 'perfect fuel ratio', for more power, but nox emissions will rise, which is bad for the environment. Also mpg will suffer here, because there is no extra air playing part of the detonation.
Excessive Air that doesn't have a chemical reaction with gasoline in the combustion process, just gets heated up. Hot air expands, so just that 2 parts of added air (from 14 to 16:1 a/f ratio), causes more efficient riding, without using any extra fuel.
In essence, your engine will run most efficient at around 16.5:1 ratios. Anything beyond that, and detonation is less efficient, engine loses power, and the throttle needs to be opened more, more fuel gets injected, the more air doesn't heat up as mich, to the point of around 20:1 where detonation no longer is possible, due to too much air, and too little fuel, and the spark can't ignite the mix.
These kinds of high af ratios may work for 125cc engines, but anything above a 250cc will end up with melted pistons, valves, sparkplug or worse, as high heat causes excessive wear. On small engines there is sufficient cooling to divert the heat, but not so on larger engines.

A good car's fuel injection will never get the a/f below the perfect ratio. If it does, it will dump unburned fuel through the exhaust port, as there's not enough oxygen in the cylinder to burn it up. This is really bad for the environment, and for your mpg, and cars nowadays aren't allowed to be sold today with these issues!

For these reasons, mpg drops when going WOT. It only makes sense that accelerating at WOT in the torque band, pushing a load from zero to hero mph, in x-amounts of seconds, is going to cost you fuel, as it takes more energy to accelerate a mass faster.
Going WOT in the torque band is the best mpg for the fastest acceleration, as there's no other way to accelerating fast.
The best mpg permanently, is acceleration between 50-80% of throttle, and make due with a slower-than-fastest acceleration.

The numbers are different for turbo engines, that have turbo lag and boost to take in account. Turbos often exaggerate mpgs of a NA engine, meaning, on some turbo cars (like the Chevrolet Cruze, or Trax), it is best to accelerate at 1/3rd of top rpm, because boost kicks in from 2k rpm, and revving beyond 3k rpm is not recommended (from an mpg point of view).
When pushing a turbo to its limits, with hard acceleration or fast top speed, it uses many times the fuel that a larger engine would use without turbo.

Swordsmith · 11-05-2015, 07:09 AM

My experience as a purely casual eco driver is that I get much better economy by following simple granny driving technique; I accelerate relatively slowly, get to the speed limit, and stay there until I see that I will have to slow down, which I do as early as possible, avoiding the need to apply brakes.

It's quite possible that I could do even better by accelerating harder, getting to speed earlier.

But one thing I can say for sure, driving like I normally do; accelerating like I'm on a quarter mile drag strip, zipping along 19 miles over the speed limit, braking late, and generally enjoying my power-modified little Sonic, I get almost exactly 2/3rds the mpg that I do driving "granny style."

I don't know how that breaks down, it seems unlikely to me that the acceleration part really matters much at all since it's only for a few seconds every now and then, whether jackrabbit or turtle style, a tiny portion of my overall drive distance. But it puts me in the mood to do the rest of the "drive gently" regimen, and -that- matters a lot.

user removed · 11-05-2015, 08:20 AM

It's important to understand the difference between load and throttle position. When accelerating a very small increase in throttle position can make a huge increase in load.

This happens when you are in too high a gear, typically climbing a grade, where you have to downshift to increase your speed. Before you downshifted, you were at 100% load, regardless of the throttle position.

I accelerate at the rate of the traffic around me, in the highest possible gear. This way I am not aggravating the other drivers around me, which can cause them to do stupid things and increase the risk of an accident to everyone around me.

I don't go wide open throttle unless there is something going on that I can shut down by getting out of an aggressive drivers way. Spreading out the group of drivers decreases proximity and risk. It makes almost no difference in overall mileage.

I tested this by accelerating and coasting to the same point on a deserted road without any traffic influences. Only if I crept up to the target speed would the mileage be significantly worse. Even WOT was better than too slow.

regards
mech