How To: Accelerate?
 

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.

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.

if you're delivering my pizza, I agree. :)

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).

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

accelerate quickly, get it over with
 

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.

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%.

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?

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.

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.

Oh yea, I forgot the OP has a 98 which means OBD-2. OP get a scan gauge or ultragauge depending on what you can afford.

yes.
accelerate (Reasonably) hard, and short shift. keep the rpm between 1500 and 3000 (assuming that the area your make has the best BSFC)

It's like Chess or golf, think of where you want to be and the speed you want when you get there. Use the highest gears that allow you to reach that point with the lowest RPM possible, without going to close to full throttle where you get enrichment. It takes less throttle than you might think to get to 70-80% load when you are shifting at 2000-2500 RPM. Keep the load fairly high but shifting earlier than most people. You want to use the engines torque to get there, not high RPM horsepower. A lot like the big rig drivers.

regards
Mech

No, no gauge yet. I'd love one, but it's not in the budget yet.

Aside from keeping the engine around bsfc peak, the biggest acceleration tip is to know when to stop accelerating (and coast if traffic/time allow it). If you accelerate then mash the brakes then you just turned all that gas into heat, better to accelerate just enough to coast to the next stop/obstacle. If the obstacle is temporary (light, geese crossing, train, etc) it is better yet to stop accelerating sooner so you can "time" the clearing of the obstacle and keep your momentum.

Yeah, bump starting and throwing in another pulse beats having to brake.

For scenario #1 (0-35mph), my (admittedly back of the envelope) calculations say that if I keep it in 2nd up to 35mph (3k rpm) before shifting to 5th, I turn a total of 7875* revolutions; whereas if I shift at 1,350rpm** and accelerate more slowly, I only turn 5400* revolutions. So I guess I'll be accelerating more slowly. Oh well.

*Over and above the 12.5 rev/sec that the engine turns at an assumed 750rpm idle, which it'd be turning anyway as long as it is running.

**I'm pulling these RPM levels from a calculation based on gear ratios, wheel diameter, and mph shift points, since I have no tach...

Recheck the math on the first calculation. How many seconds does it take you to get to 35 MPH? How many seconds in each gear?
regards
Mech

I timed myself the other night, and it took 7 seconds to hit 35mph in 2nd; it took 18 seconds when I shifted at 1350rpm every gear.

I did assume (among other things...) a straight slope for my acceleration curve, so that I didn't need calculus to solve for the area under the curve.

Oh...Hang on. I think I forgot to convert revs/minute to revs/second. Oops.

After converting minutes to seconds so that my units match....

Revving it up to 35mph in 2nd uses 131.25 revolutions, while slogging it out only uses 90.

You also need engine load in there somewhere. The total engine revolutions isn't much of the story.

I'm pretty sure I'm close to a 80-90% load in both scenarios. Accelerating as fast as the engine will allow, without overrichening the A/F mix by too much throttle. Then again, all this is seat of my pants, since I have no gauges...

The real magic comes in the glide. Any reasonable acceleration method will get you 90+% of the way there. By reasonable I mean not actually flooring it and not revving past 3000 or so. Focus on "no brakes" and coasting all the time. What I mean is, my standard driving mode is neutral EOC, only starting the engine and engaging the gears when I need more speed and then I'm back to coasting.

it doesn't matter how you accelerate as long as its followed by a good period of coasting.

Usually the harder I accelerate the longer I coast after. I have found no significant difference from using say 60% engine load to accelerate to cruising speed on highway than 80-90%.

in your case, if the parking lot is fairly short and you have to come to a stop early on again, I would use as little throttle as possible. But if you are accelerating on a long open road with no stops any time soon use as much throttle without inducing open loop enrichment (this varies with cars).

So driving like a grandma is not always the most fuel efficient? So with a standard tranny its ok to pull hard up to 2500-3500 rpm depending on the Fuel enrichment mapping on you car? In other words rpms kill mpg not engine load?
sorry for all the questions, but I am a newbie at this stuff :P

Had to chime in, sorry for my first post being in mid thread, but as a instructor and a tuner this is something I get asked alot.

The short an simple answer is that disregarding air resistance or losses due to great average speed, the ideal eco-acceleration should be the maximum acceleration available prior to entering open loop, or pulling ignition timing due to knock, in the highest gear possible.

I tend to accelerate under heavy load in 5th gear (as soon as its reasonable to take), but reduce the acceleration enough to stay in closed loop.

This minimizes frictional losses in the engine by using the piston stroke, valve duration and pumping losses to their maximum ability and since rotational losses are not load dependent the greater power output increases the efficiency of each engine power stroke by reducing the loss %.

Its the same idea as having to withdraw $400 from a ATM, would you do it all in one hit or in 10 little hits? Your gonna get dinged $1.00 for each transaction so might as well minimize the number of transactions by maximizing the available amount in each one.

Its the same reason why the same engine in a turbo diesel variation can get better mileage then in that engine in non turbo diesel variation. The extra RPM needed in the non turbo causes unnecessary frictional losses.

Hope that helps,

Steve

ok cool !

I concur, but with a clarification. High throttle is good for acceleration, but as BV1 stated, low rpm. Higher rpm will cost you more. I usually limit my rpm to about 2200 max, except when I need the extra power (freeway merge or something like that). Even then, I'll push it to 3000 but not much more.

... except when my speed forces me to rev higher thanks to Honda's poorly chosen gear ratios. Limiting myself to 2200 rpm requires keeping my speed below 48 mph. Faster than that and the revs just keep climbing.

At higher speeds I learned that you can accelerate very gradually with practically no loss in mileage, then glide and your mileage skyrockets. I'm not so sure it would work well on perfectly flat roads but if there is any elevation change then you can use the technique to climb the upslope and then greatly extend your glide on the downslope.

To the east of my home the greatest grades are overpasses, but even those grades that are less than half of an overpass offer opportunities to stay close to the same speed while pulse and gliding. I also look for drafting opportunities when traffic is so heavy that you have no choice, like it is regularly around here.

I'll stay in the right lane and if there is traffic entering the Interstate, in most cases I stay in the right lane and coast, even up the overpass to allow the traffic to merge safely. I might move over, but in heavy traffic here those in the left lane like to speed up to block you from being courteous and allowing those entering to merge safely, which is absolutely stupid and causes most accidents. On the Fort Eustis Interstate cloverleaf, there is one section of guardrail that is replaced almost every month due to idiots who exit at too high a speed, and those who try to move from the left lane to the exit ramp at much too high a speed, too late. You have to watch for those Morons all the time if you stay in the right lane and they don't know what a turn signal is, in many cases when I turn on miy signal the car in the lane I am trying to get into will rapidly accelerate to block me from safely changing lanes.

That may not be the case for many here, but I suspect "lane blocking" is common in higher traffic areas.

regards
Mech

The lane blockers are predictable and can be worked. I've got some horribly concealed on ramps to deal with- you don't know if someone's coming up them until they're right there because of hills and curves. If I'm not in traffic there's no problem and I can move left as needed. If I'm in traffic I move left no matter what- it's safer to leave room just in case than to not be able to leave room and find out you have to.

I have a set point in the road where I want to be in the left, and I'm watching for a gap coming up as I approach my point. If the gap comes early and there won't be another one, I move left early. What's really funny is that the lane blockers aren't expecting that move until we're on top of the on ramp, so moving over is easier when they're not ready to block.

Despite their desire to pin people in the right lane, most of them seem to agree that it's unsafe to approach those on ramps in the right lane because they almost never cut into the right to pass me.

Cool so in other words you can pull hard up to 2500 in each gear?

Depending on the slope, traffic and cargo, I pull hard until I think I can upshift and hit the next gear with enough revs to keep accelerating. It's more of a seat of the pants feeling than a set number.

Agreed. It's easier to say, "shift at x rpm", but the better approach is to shift when you'll land at the right rpm in the next gear. That varies with each shift, though.

One thing to remember is that spinning up the stock flywheel is like accelerating a 150-200lb mass in 1st gear, and that adds up! So shift out of 1st earlier if you can so the drop in rpm doesn't represent as much lost energy. In the rest of the gears I push the engine to 2500-3000 depending on situation.

@PaleMelanesian, Honda's "poorly chosen gearing" is still a lot better than Toyota's poorly chosen gearing. 2200rpm is 40mph flat for me :P I think I say this every time though lol, and how you get such good mpg with the 5 speed transmission that the penny pinchers at Honda gave you is beyond impressive.

One, I have a commute that doesn't require sustained high speeds.
Two, I'm stupidly dedicated and refuse to give up.

With my subaru and it's rather short gears it's fairly impossible to short shift. As soon as I get on the throttle a bit in first gear the RPMs hit 2000-2500.

However I noticed how I accelerate has far less effect on my daily consumption than how I decelerate. I just don't use brakes. The whole 25 km drive home in 20km/h average speed maybe I tap the brakes lightly here and there. In regular city driving with light traffic I accelerate to 3000rpm getting up to 70kmh with gears 1-3 and then I drop to neutral and coast down to 50kmh. But my next pulse is followed by a fifth gear acceleration at 1500rpm instead. This gives me about 30mpg on a stretch of road. I can get same or worst results if I use all gears off the red light but at a slower arrival time.

Here's how to determine the best engine load for accelerating.