How To: Accelerate?

[96CX]

Two questions:


First: I deliver pizza, and every time I pull out of the store parking lot onto the main street, I have a dilemma:

I have to go from 0mph to 35-40.

Is it more efficient to accelerate slowly, hitting every. single. gear. so that I'm in 5th gear asap, and spend 20ish seconds in a long "pulse"?

Or is it more efficient to hit wide-open-throttle for 5ish seconds in 2nd gear up to 35mph, and then immediately skip up to 5th, so as to have as short of a "pulse" as possible, and be cruising (possibly "gliding") for longer?

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Second: When I'm merging onto the freeway, I normally round the corner onto the on-ramp at about 25mph in 5th (top) gear. The on-ramp is downhill, the freeway is more or less level. Is it better to accelerate at wide-open throttle up to full-speed while still on the on-ramp, or is it better to accelerate more slowly, and possibly still be accelerating by the time I've merged onto the freeway?

I want to say it's better to floor it and use the hill while accelerating, but I don't know if maybe it's still somehow better to accelerate slowly without flooring it.

[serialk11r]

Your car isn't that fast, so I'd say use all the gears, shifting so that you land at a bit over 2000, and then drop straight into 5th once you're up to speed. Also I wouldn't use wide open throttle before 3000rpm, try 80-90% load.

For on-ramps I accelerate just for fun (I put up with the harsh ride and scraping from my lowered suspension for a reason right?) but I think to save fuel again you'd want efficient acceleration, if you can get up to speed in time using 4th or 5th then maybe just coast down the onramp and build up a little bit of speed with the clutch in, and if the merge is shorter I'd still coast down but give it some gas towards the end of the ramp, or earlier if necessary.

[markweatherill]

Originally Posted by 96CX

I want to say it's better to floor it

if you're delivering my pizza, I agree.

[Occasionally6]

The answer will depend on how long you will be travelling at the higher speed.

For a given target speed the Kinetic Energy (KE) embodied in the car at that speed is the same whether you accelerate slowly or rapidly. If you accelerate quickly, there is some small (extra) loss due to the higher average velocity. This diminishes in relative magnitude the longer the time spent at the target speed.

Against that the engine is more efficient when loaded up to the point you get into closed loop fuel enrichment. That points towards the faster acceleration being more efficient.

Inlet manifold vacuum is the best way to determine engine load, rather than throttle opening. At low engine power output (i.e. rpm) the engine can be the ultimate restriction on airflow, not the throttle i.e. full engine load is reached at some fraction of WOT.

Further opening the throttle after that point is reached will not increase the engine power output and you will be in open loop mixture control at significantly less than WOT.

Using all of the gears allows the engine to be operated near its peak efficiency for more of the time than does skipping gears.

If you wish to accelerate - on average - more slowly, accelerate fast and shift slow (coast in between gears).

[user removed]

Get to speed as quickly as practical with the fewest number of engine revolutions. As posted previously no WOT enrichment (never floor it). Pick a peak RPM in each gear and stick with it for a tank of fuel. I generally accelerate with the flow of traffic, shift as early as I can while maintaining that rate of acceleration.
Not only is it very close to best efficiency, but it also creates the least disruption in traffic.

regards
Mech

[kennybobby]

i would use a lot of throttle (but not WOT enrichment) to let the air flow and skip over the gears. When you accelerate you want the engine using as much air as necessary to burn the fuel most efficiently (the region of volumetric efficiency). Extending the acceleration phase just prolongs higher fuel consumption if the engine is not working at the more efficient higher-load region of the BSFC chart. With no chart you might have to experiment to determine that optimal load region, but the basic pattern and trend is common to all IC motors.

On the downhill ramp, by all means use gravity as your friend and accelerate up to the highway speed before trying to merge. Safest merging occurs when all vehicles are moving at the same speed.

For best fuel use, get up to speed quickly and then coast/glide in highest gear, time the lights and stay off the brakes.

[digital rules]

I short shifted w/ WOT frequently before joining here & was still getting great MPG. (Of course it is even better now) My floormats are 2 deep, (OEM & aftermarket on top) so probably was not truly @ full throttle & more like 80 to 90%.

[96CX]

So on the on-ramp (25mph up to 55)-- I should use just enough throttle to achieve the maximum rate of acceleration (any throttle more than that merely enriches the A/F ratio?), and get up to 55mph while still on the downhill?

[Baltothewolf]

Originally Posted by Old Mechanic

Get to speed as quickly as practical with the fewest number of engine revolutions. As posted previously no WOT enrichment (never floor it). Pick a peak RPM in each gear and stick with it for a tank of fuel. I generally accelerate with the flow of traffic, shift as early as I can while maintaining that rate of acceleration.
Not only is it very close to best efficiency, but it also creates the least disruption in traffic.

regards
Mech

Agreed. In my 95 camry, when she was still alive, I managed to get about 29 average in the city. I found that whether I accelerated using 2750rpm and 2250rpm I got the same FE, mind you this was an automatic. I would honestly suggest testing. Because for me, accelerating using 3000rpm resulted in 24mpg, yet just 250rpm less resulted in 5 mpg more. I figure that anything over 2750 my car riches the mixture, causing more fuel to burn. I'm just talking out my ass, but it's a good estimated guess.

[Fat Charlie]

It depends, and this is where a gauge comes in handy- did you get one yet? Pick a route that won't have much traffic interference and log the short trip mpg when you get there. After a few runs, switch to another method for a few runs. Let us know.