Why no fast acceleration?

[ninj4geek]

Nick666 brings up a good point that I'd like to riff on:

Ib like to think of everything in terms of energy, and use comparisons that easily demonstrate that idea.

Acceleration: faster acceleration takes more energy, and therefore more gas (engineering of your car notwithstanding). Think of riding a bicycle and accelerating to 20 mph from a dead stop.

It can be done easily if done slowly, but your legs would be on fire if you went flat-out to get to 20 mph. Same goes for the car.

This also applies for coasting, braking, conserving momentum, etc.

[UFO]

Originally Posted by ninj4geek

Nick666 brings up a good point that I'd like to riff on:

Ib like to think of everything in terms of energy, and use comparisons that easily demonstrate that idea.

Acceleration: faster acceleration takes more energy, and therefore more gas (engineering of your car notwithstanding). Think of riding a bicycle and accelerating to 20 mph from a dead stop.

It can be done easily if done slowly, but your legs would be on fire if you went flat-out to get to 20 mph. Same goes for the car.

This also applies for coasting, braking, conserving momentum, etc.

Of course faster acceleration takes more energy, but less time to reach the target velocity. So the most efficient way to get to the target velocity is to operate the engine in the part of the BSFC where its more efficient, and that is generally at higher loads and torque near the torque peak. So "brisk" acceleration with short shifting at the torque peak is usually more efficient, avoiding fuel enrichment and higher rpm.

[Fat Charlie]

Moving with a sense of purpose isn't drag racing.

[IamIan]

Faster Acceleration ... reaches higher amounts of wind resistance sooner.

If traveling the same distance .. and the same cruising speed .. The faster acceleration will have a higher average speed .. will require more energy.

If traveling the same distance .. and the same average speed .. the faster acceleration would than have to travel at a slower cruising speed in order to average the same speed .. a slower cruising speed would reduce the energy needed.

I suspect there is also a psychological aspect to the average type of person who accelerates quickly ... vs the type that does so slowly .. and I suspect that the net overall beneficial psychological type for energy or fuel efficiency .. is the slower accelerating psychology... Even if the specific causes of that net benefit may not necessarily be in the acceleration rate itself .. But in the overall method that type of psychology approaches numerous types of driving conditions/options.

- - - -
I would Ditto the BSFC points made previously .. but with the caveat .. there can be variation from that +/- caused by the differences between energy efficiency vs fuel efficiency .. for different contexts.

[Fat Charlie]

Faster Acceleration... reaches gliding speed sooner.

You're exactly right about the difference between energy efficiency and fuel efficiency. All of this is vehicle specific and situation specific, and pointless once the driver has a gauge and can learn the car and how best to apply it.

Overall, I'm with the "accelerate harder" crowd in most situations because pulsing longer than is called for is burning gas longer than is called for. Even "harder" is realtive- this isn't exactly a crowd that bounces off the rev limiter.

[JRMichler]

I ran some tests with my truck. Starting from work, pulling out onto the road, and accelerating to 35 MPH (the speed limit) at various rates and shifting at different RPM's. The Scangauge trip MPG started at zero, and I read it at the same point down the road. Accelerating like the proverbial grandma got me to 35 MPH just before the reference point, while accelerating harder got me to 35 MPH sooner. If I got to 35 MPH before the reference point, I would maintain 35 MPH to the reference point. One test per day.

Results: My truck does best if I tromp the gas pedal about 3/4 the way down, and shift at 2500 RPM. Coincidentally, that has me accelerating at about the same rate as most people.

[Hersbird]

With my manual Forester I like to accelerate at about 3/4 throttle and shift at 2500 rpm until I get in 5th and then just finish accelerating at 3/4 throttle until at desired speed. With my Town and Country and automatic, I use the "economy" button on and hold about 3/4 throttle which seems to let it naturally shift at 2500-3000 rpm. Without that "economy" button which most drivers seem to hate, it lets the van rev higher and use it's almost 300 hp which is great for drag racing but bad for economy. On my Hemi Aspen, any throttle and the thing accelerates fast. My key there is to get to the speed and back off the throttle so it will go into 4 cylinder deactivation mode faster. Then it's key to hold acceleration slow enough not to kick it into 8 cylinder mode which I wish it were possible to lock in 4 cylinder mode. In all cases I seem to accelerate with most traffic, faster then some, slower then others.

[serialk11r]

Originally Posted by PaleMelanesian

High-throttle enrichment varies widely from car to car. Some you have to actually have the pedal to the floor and high rpm, like 4000+. Some cars, anything above 80% throttle will enrich the mix. So to be safe, I'd say keep the gas pedal at or below 3/4 if you don't know otherwise.

On newer cars the enrichment kicks in frighteningly early. My FR-S' stock fuel mapping enriches at 60% load for most rpms. I changed the map, but it's very easy to accidentally kick it into enrichment.

On cars with no wideband sensor you need a bit more load to get enrichment, and old school cable throttle engines would just not enrich the mix until higher rpm (you also get very crap to nonexistent power output below 2000rpm as a result).

[oldtamiyaphile]

Originally Posted by serialk11r

On newer cars the enrichment kicks in frighteningly early.

You mean on sports and or turbo cars.

My (turbo) Fiat will do enrichment pretty early. My 2014 Renault is very reluctant to go into enrichment and my old school cable throttled Jeep has heaps of power below 2000 and won't do enrichment at all unless road speed is over 50mph.

[BenArcher006]

Since I red that thread I tried with my Vue short brisk accel (1-3 Vac = 60-65% throttle) vs steady accel (5-10 vac= 40-45% throttle) vs slow accel (12-15 vac=35-40% throttle) always shifting at 1800-2000 RPM and I got the best results with steady acceleration.

With short brisk accel my fuel economy went south about 1mpg, so I started with Lighter loads and my it went back up. With slow accel, the problem is that every acceleration takes forever to get to speed with barely any mpg improvement.

Mpgs now hoovering near 27mpg. Outside cool températures and high winds are killing my Mpgs on the highway.

Im doing every thing I can to get better Mpgs but cant seem to get anything better than 27mpg.

Oh well...