Accelerating and shift points
 

The way I think about it is, you can accelerate slowly and have to be in the throttle for a longer period of time, or you can accelerate quicker and get to cruising speed much sooner. Ive also got two different cars to consider, and the only thing they have in common is they're turbo 4 cyls. One is a 5 speed and one is automatic, so for the 5 speed another thing to consider is shift points (and you can also consider that on the auto to an extent).

The shadow has a stock vac/boost gauge whereas the tbird only reads boost, Ive been wanting to pick up an aftermarket vac/boost gauge for it. Ive always assumed that accelerating with the most vacuum is best, and try to keep it between 7-10in of vac when accelerating (cruises at 14-15). However, Ive noticed that acceleration doesn't really improve much with more pedal until you get to around 2 to 0 inches (remember its a turbo car so 0 vac doesnt mean wide open throttle. At that point, close to 0 inches the car seems to accelerate a good bit better, even though you arent giving it hardly much more pedal. Between 0 vac/boost and 2psi of boost is where I would consider "normal" non-hypermiler acceleration.

Basically I just wonder if it would be better to accelerate slowly (7 or more inches of vac for me) and have to stay accelerating for a much longer time, or to accelerate at a more "normal" pace (2in vac to 2psi boost) and get up to speed much sooner.

Its a pretty different story for my Thunderbird. Since its a 5 speed theres no torque converter to slip around and keep the engine from lugging. Its happy accelerating from at least 2000 rpms, and anything under 1500 is a no-no, except in 1st or 2nd gear. In the upper 3 gears its like a semi truck jake braking under 1500, sure to rattle the bearings and such pieces apart in time. So its not only a matter of how far to open the throttle, but what rpm range to use as well. Should I accelerate in the range that seems "easiest" on the engine or try to keep the rpms as low as possible without rattling the car apart. Theres different combinations of rpms and throttle to try too, low rpms and low throttle, low rpms and more throttle, high rpms and low throttle, etc.

So what are you peoples opinions? Accelerate slow or quickly? Keep rpms as low as possible or use more of the engines efficiency range?

Careful - correct conclusion for the wrong reasons.... In the miles per gallon equation, there is no "time" variable. mpg is independent of time. If your engine efficiency stays the same all the time - it would take the same amount of fuel regardless of how fast you accelerate... But, efficiency changes with load :p

As for your question - I'll let someone else answer for a turbo :p My explanation won't be very eloquent :D

Feel free to speak your mind. Im open to opinions.

Quicker acceleration is better as the engine will operate where it is most efficient. Driving for FE can also be fun sometimes. The goal is to get to your cruising speed in the shortest reasonable distance without completely putting the pedal to the metal.

Don't be affraid to use boost when accelerating, it just makes your engine more efficient under load.

This is true as long as it isn't kicking into open loop.

I saw somewhere else on the forum a discussion of specific fuel consumption and an "island of efficiency" related to torque loading of the engine - the load and speed where it is making the best power for the fuel it burns. Find a BSFC map for your engine (good luck - I can't find anything like that for mine), then do the math for your drivetrain to determine what that equates to for your gears. I'm thinking that if you do all your accelerating in that sweet spot, you'll be hitting the best possible load/fuel burn.

Just guessing.

I know that the ECU in the tbird goes to open loop at 3000 rpms, so I would have to keep the rpms under that, but Im not sure at what point on the throttle side of things that it goes to open loop, obviously at wide open throttle, but Im not sure if theres a certain amount of throttle that causes open loop as well.

I dont really know anything about the Shadow as far as open loop goes, not sure if theres a certain rpm or throttle opening.

So could you say as long as I'm staying in closed loop its the same or even better to accelerate at normal brisk pace from a stoplight instead of an agonizing crawl? That would sure make alot of folks behind me less annoyed.

Thats my guess.

Could try say, 3 tanks of gas accelerating slowly, and then 3 tanks accelerating more normally and compare.

It can't be worse for sure. But give it a try, it's really easy to test. It can be as short as a 1 mile test. Start at a stop, pick a target cruising speed at which you'll stop accelerating, reset your trip on the SG and do runs accelerating like a granny and runs giving it a lot of pedal.

I usually set my target at 40 mph in the city. I go 1-2-3-5 shifting a bit over 3k rpm.

From a stop, I use light-light-light acceleration. Maybe 1 second per 1 mph. Also, I'm shifting BEFORE 2000 rpm, often at 1800. That drops into the next gear at 1300-1400 rpm.

Now when I'm at speed and doing Pulse & Glide, it's a different story. Then I'm using up to 3/4 throttle and up to 2500 rpm before shifting. (although, I usually do the pulse in a single gear, no shift needed)

I there any reason the acceleration coming off a stop should be any different than a P&G acceleration?

Well, if your MPG during that acceleration stays high, there's no hit to the average. Yes, the engine is in a different, less-efficient mode than a higher rate. But overall, you're minimizing the fuel used. Just use no more than the absolute minimum. Running the engine at it's peak BSFC point is secondary.

It was a suggestion from Wayne on our cross-country hybrid vs diesel shootout. He uses this style all the time, and I've been playing with it since. Right after that trip, I posted my best tank ever, and this one is looking to be even better.

While cruising, keeping around BSFC is secondary (unless your engine is small enough to do so)... While accelerating, it's primary.

Don't look at your average mpg while trying to figure out acceleration (effectively instantaneous) optimal. Miles/Gallon does not include a time variable - so trying to incorporate time (by watching average) will only bring false confidence. My average will stay really high if I've been driving for a long time compared to when I accelerate just after start up - but in both of those cases, it doesn't change my instantaneous fuel efficiency in the acceleration event.

Shoot for peak thermal efficiency while accelerating - then peak specific consumption while cruising (which will likely not be peak thermal). Then later, when you consider the time variable - your figure will be higher.

Nope :thumbup: Pulse and glide just sounds better than accelerate and coast (I guess :P).

And none of our engines are that small, so... P&G is all about keeping the engine at its peak BSFC, or not running at all. The average is higher than running at steady-state and lower BSFC efficiency.

Of course, the same acceleration will affect your average less if you're averaging over a longer distance. That doesn't change the actual fuel consumption during that accel. I'm comparing the same points on my daily drive from one day to the next using different methods - apples to apples. At this one point, about 5 miles into my drive:

Light acceleration will bring the average down by maybe 2mpg vs 4mpg at heavier accel. The faster accel will get you up to speed 100-200 yards sooner. After 5 miles, you just can't make up that 2mpg difference in 200 yards, even if you EOC.

Here's my theory: If you could have the ideal situation and coast all the way to zero at the end of the drive, or the next light, or whatever you stop, then the higher accel rate would work. But if you use your brakes at the other end, at all, then you can't make up the deficit.

I think we're trying to say the same thing here. ;)

I did a quick test on a .6 mile flat stretch between a red light and a stop sign. Every run was made in the same direction of the stretch.

"A" runs (Granny runs): Slow pace, egg under foot acceleration, shifting at 2000 rpm and finally resuming CC at 40 mph in 5th gear.

"B" runs (Jackrabbit runs): Quick acceleration (75% LOD), shifting just over 3000 rpm and finally resuming CC at 40 mph in 5th gear.

A1 39.1 mpg
B1 45 mpg
A2 39.8 mpg
B2 44.7 mpg
A3 39.5 mpg
B3 45.1 mpg

"A" runs average: 39.47 mpg
"B" runs average: 44.93 mpg

Test for yourself, you'll see.

Thanks for that. I like seeing hard evidence. I'll try it out myself and see what I find. Was that in the Elantra or the TDI?

One thing about that test is that you combined a different load and different rpm shift point. I'd be interested in separating those out.

I did the test with the elantra.

I tested low rpm / low load and high rpm / high load. I agree should have tested low rpm / high load and high rpm / low load also, but I ran out of time.

In 5 miles you sure can :D But, I don't know what "after 5 miles...200 yards" means....

That is ideal - but the recovery is much faster than 1:1 as suggested.. It's situations like that that makes me hate the mpg metric - it's not really appropriate here and just leads to confusion....

I wrote and rewrote that, and it still came out confusing. Let me try again:

After 5 miles of driving and building up an average, then I have this stop and an acceleration. The faster accel drops the average more, but arrives at speed sooner. Once arriving at the point where the slower accel would have been up to speed (200 yards farther along), the average is still lower than the slow acceleration would yield at that same point.

But I want to test some more to be sure...

I think the discussing needs to be quantified between standard and automatic transmission.

Mileage sucks until you're in top gear. There is an optimal acceleration rate and it might be surprisingly quick on some vehicles for what the owners might actually think it is.

I still haven't tested this. A accelerate at a LOD of 50 or so, although I'm using LPH instead but it really gets impacted by the cold. I need more than 1! ;)

In terms of simple physics, one would assume that the extra energy needed to propel at a higher velocity, within a short time frame would be exceptionally costly, but maybe I'm incorrect.

I agree -- without full control in an automatic, one would expect to get to the desired speed at a high load / lowest RPM possible -- or relatively quickly. Depending on the engine. I'd need to see the BSFC, but mine is about half of where max torque is made (5200/2 = 2600) 2500-3000 seems to be the best operating range.

Of course tests are required to confirm.

So how about a slap-shift auto? On the TSX I can force upshifts in the 2300 RPM range and keep the accelerator mashed (not fully, but 75% or so). I almost always drive with the manu-matic -- prevents the unexpected downshift until it computes that it absolutely has to based on speed.

RH77

http://forum.ecomodder.com/em-fuel-log.php?vehicleid=16

Dang .. you've got that thing tweaked!

I guess I will change the name on that. It's a bike not a car:)

Done.

Here is what I learned from my last car, which I got quite a bit into both mileage and performance enhancement, was an AWD Turbo, this should pertain quite a bit to you:

Open-loop = horrible, horrible for mileage. I got anywhere from 2-6 mpg in open loop at WOT (wide-open-throttle). Turbocharged cars, especially older ones can go very very rich (8:1 was possible) under open loop, which kills mileage in a hurry. This is the easiest way to save (and also waste) fuel.

Somewhat counter-intuitively, low throttle position = bad for mileage. The throttle plate is a restriction to the engine. The pistons have to work hard to suck air through a closed throttle plate, which leads to large parasitic losses. This is a big reason why diesels get better mileage than gasoline engines - no throttle plate. The less resistance to movement you can get on the engine, the better. This includes things like a more open intake and exhaust to get rid of the pumping losses, windage trays in the oil pan to get rid of oil slinging around the crank, using good oil, etc. The easier it is for the engine to turn, the more efficient it is. This is difficult to do, because usually open throttle = open-loop, which ends up being less mileage overall, see above.

The trick is finding a balance between open-throttle and closed-loop. If you can use external methods to force both of these conditions, that's optimal for mileage, but can come with problems of its own (turbocharged cars run rich under wot for a reason, to protect the engine from knock).

The turbocharger also improves efficiency (theoretically). You are scavenging otherwise-wasted energy from the exhaust gas. The problem is, as stated above, that when the turbo spools up under WOT, the car runs rich to keep everything safe, which takes any efficiencies the turbo gives you and throws them on the ground angrily.

What I was just starting to get into on my last car before it broke was a way to have the throttle fully open, the turbo fully spooled, but yet running extremely lean so I didn't keep accelerating. It was technically open-loop, but I was manually keeping an eye on the EGT, the knock, the temps etc, and running way leaner than 14:1 (which is where closed-loop keeps you). Didn't have time to get much into it before an unrelated event broke my car and I decided I was sick of it.

My 3 cents.

-Jesse